AMORPHOUS METAL OXIDE THIN FILMS FROM AQUEOUS PRECURSORS: NEW ROUTES TO HIGH-Κ DIELECTRICS, IMPACT OF ANNEALING ATMOSPHERE HUMIDITY, AND ELUCIDATION OF NON-UNIFORM COMPOSITION PROFILES by KEENAN N. WOODS A DISSERTATION Presented to the Department of Chemistry and Biochemistry and the Graduate School of the University of Oregon in partial fulfillment of the requirements for the degree of Doctor of Philosophy December 2017 ii DISSERTATION APPROVAL PAGE Student: Keenan N. Woods Title: Amorphous Metal Oxide Thin Films from Aqueous Precursors: New Routes to High-κ Dielectrics, Impact of Annealing Atmosphere Humidity, and Elucidation of Non- uniform Composition Profiles This dissertation has been accepted and approved in partial fulfillment of the requirements for the Doctor of Philosophy degree in the Department of Chemistry and Biochemistry by: Shannon W. Boettcher Chairperson Catherine J. Page Advisor James E. Hutchison Core Member Benjamin J. McMorran Institutional Representative and Sara D. Hodges Interim Vice Provost and Dean of the Graduate School Original approval signatures are on file with the University of Oregon Graduate School. Degree awarded December 2017 iii © 2017 Keenan N. Woods iv DISSERTATION ABSTRACT Keenan N. Woods Doctor of Philosophy Department of Chemistry and Biochemistry December 2017 Title: Amorphous Metal Oxide Thin Films from Aqueous Precursors: New Routes to High-κ Dielectrics, Impact of Annealing Atmosphere Humidity, and Non-uniform Composition Profiles Metal oxide thin films serve as critical components in many modern technologies, including microelectronic devices. Industrial state-of-the-art production utilizes vapor- phase techniques to make high-quality (dense, smooth, uniform) thin film materials. However, vapor-phase techniques require large energy inputs and expensive equipment and precursors. Solution-phase routes to metal oxides have attracted great interest as cost- effective alternatives to vapor-phase methods and also offer the potential of large-area coverage, facile control of metal composition, and low-temperature processing. Solution deposition has previously been dominated by sol-gel routes, which utilize organic ligands, additives, and/or solvents. However, sol-gel films are often porous and contain residual carbon impurities, which can negatively impact device properties. All- inorganic aqueous routes produce dense, ultrasmooth films without carbon impurities, but the mechanisms involved in converting aqueous precursors to metal oxides are virtually unexplored. Understanding these mechanisms and the parameters that influence them is critical for widespread use of aqueous approaches to prepare microelectronic components. Additionally, understanding (and controlling) density and composition inhomogeneities is v important for optimizing electronic properties. An overview of deposition approaches and the challenges facing aqueous routes are presented in Chapter I. A summary of thin film characterization techniques central to this work is given in Chapter II. This dissertation contributes to the field of solution-phase deposition by focusing on three areas. First, an all-inorganic aqueous route to high-κ metal oxide dielectrics is developed for two ternary systems. Chapters III and IV detail the film formation chemistry and film properties of lanthanum zirconium oxide (LZO) and zirconium aluminum oxide (ZAO), respectively. The functionality of these dielectrics as device components is also demonstrated. Second, the impact of steam annealing on the evolution of aqueous-derived films is reported. Chapter V demonstrates that steam annealing lowers processing temperatures by effectively reducing residual counterion content, improving film stability with respect to water absorption, and enhancing dielectric properties of LZO films. Third, density and composition inhomogeneities in aqueous-derived films are investigated. Chapters VI and VII examine density inhomogeneities in single- and multi-metal component thin films, respectively, and show that these density inhomogeneities are related to inhomogeneous metal component distributions. This dissertation includes previously published coauthored material. vi CURRICULUM VITAE NAME OF AUTHOR: Keenan N. Woods GRADUATE AND UNDERGRADUATE SCHOOLS ATTENDED: University of Oregon, Eugene, OR California Lutheran University, Thousand Oaks, CA DEGREES AWARDED: Doctor of Philosophy, Chemistry, 2017, University of Oregon Master of Science, Chemistry, 2015, University of Oregon Bachelor of Science, Chemistry, 2013, California Lutheran University AREAS OF SPECIAL INTEREST: Materials Chemistry PROFESSIONAL EXPERIENCE: Research Assistant, Page Research Group, University of Oregon, 2013-2017 Research Assistant, Tannaci Research Group, California Lutheran University, 2009-2013 GRANTS, AWARDS, AND HONORS: Informal Science Education Fellowship, University of Oregon, 2016, 2015, and 2014 National Science Foundation Scholar, American Chemical Society Green Chemistry Institute, 2015 Graduate Research Fellowship Program Honorable Mention, National Science Foundation, 2015 Science Communication Fellowship, Oregon Museum of Science and Industry, 2014 vii CFS Science Scholarship, California Lutheran University, 2012 Academic Excellence Award, California Lutheran University, 2012 and 2011 Swenson Summer Research Fellowship, California Lutheran University, 2011 and 2010 Presidential Scholarship, California Lutheran University, 2009 PUBLICATIONS: Woods, K. N.; Hamann, D. M.; Page, C. J. “Composition-Property Relationships in High-κ LaxZr1-xOy Thin Films from Aqueous Solution.” Solid State Sci. Under review. Woods, K. N.; Thomas, M. C.; Mitchson, G.; Ditto, J.; Xu, C.; Kayal, D.; Frisella, K. C.; Gustafsson, T.; Garfunkel, E.; Chabal, Y. J.; Johnson, D. C.; Page, C. J. “Nonuniform Composition Profiles in Amorphous Multimetal Oxide Thin Films Deposited from Aqueous Solution.” ACS Appl. Mater. Interfaces. 2017, 9, 37476-37483. Woods, K. N.*; Plassmeyer, P. N.* Park, D.-H.; Enman, L. J.; Grealish, A. K.; Kirk, B. L.; Boettcher, S. W.; Keszler, D. A.; Page, C. J. “Low-Temperature Steam Annealing of Metal Oxide Thin Films from Aqueous Precursors: Enhanced Counterion Removal and Improved Resistance to Water Absorption.” Chem. Mater. 2017, 29, 8531- 8538. Woods, K. N.; Waddington, E. C.; Crump, C. A.; Bryan, E. A.; Gleckler, T. S.; Nellist, M. R.; Duell, B. A.; Nguyen, D. P.; Boettcher, S. W.; Page, C. J. “Tunable High- κ ZrxAl1-xOy Thin Film Dielectrics from All-Inorganic Aqueous Precursor Solutions.” RSC Adv. 2017, 7, 39147-39152. Plassmeyer, P. N.; Mitchson, G.; Woods, K. N.; Johnson, D. C.; Page, C. J. “Impact of Relative Humidity during Spin-Deposition of Metal Oxide Thin Films from Aqueous Solution Precursors.” Chem. Mater. 2017, 29, 2921-2926. Wood, S. R.; Woods, K. N.; Plassmeyer, P. N.; Marsh, D. A.; Johnson, D. W.; Page, C. J.; Jensen, K. M. Ø.; Johnson, D. C. “Same Precursor, Two Different Products: Comparing the Structural Evolution of In-Ga-O “Gel-Derived” Powders and Solution- Cast Films Using Pair Distribution Function Analysis.” J. Am. Chem. Soc. 2017, 139, 5607-5613. Woods, K. N.; Chiang, T.-H.; Plassmeyer, P. N.; Kast, M. G.; Lygo, A. C.; Grealish, A. K.; Boettcher, S. W.; Page, C. J. “High-κ Lanthanum Zirconium Oxide Thin Film Dielectrics from Aqueous Solution Precursors.” ACS Appl. Mater. Interfaces. 2017, 9, 10897-10903. viii Mitchson, G.; Ditto, J.; Woods, K. N.; Westover, R.; Page, C. J.; Johnson, D. C. “HAADF STEM Signal Intensity Analysis Method for Extracting Structural Information from Complex Thin Films.” Semicond. Sci. Tech. 2016, 31, 084003 Norelli, K. M.; Plassmeyer, P. N.; Woods, K. N.; Glassy, B. A.; Knutson, C. C.; Beekman, M.; Page, C. J. “Influence of Composition and Processing Parameters on the Properties of Solution-Processed Aluminum Phosphate Oxide (AlPO) Thin Films.” Solid State Sci. 2016, 55, 8-12. Fairley, K. C.; Merrill, D. R.; Woods, K. N.; Ditto, J.; Xu, C.; Oleksak, R. P.; Gustafsson, T.; Johnson, D. W.; Garfunkel, E. L.; Herman, G. S.; Johnson, D. C.; Page, C. J. “Non-uniform Composition Profiles in Inorganic Thin Films from Aqueous Solutions.” ACS Appl. Mater. Interfaces. 2016, 8, 667-672. ix ACKNOWLEDGMENTS I am deeply grateful to the University of Oregon and the Center for Sustainable Materials Chemistry (CSMC) for funding during the course of my graduate studies. I thank the staff of the Center for Advanced Materials Characterization in Oregon (CAMCOR) for use of instrumentation facilities and for their frequent assistance. I also thank the Department of Chemistry & Biochemistry and my committee members (Shannon W. Boettcher, James E. Hutchison, and Benjamin J. McMorran) for wonderful discussions, a welcoming environment, and general support. I especially thank my advisor, Catherine J. Page, for her mentorship and friendship. I would be remiss if I did not also thank the many collaborators and colleagues who helped make this work possible. This list of graduate students, professors, and high school and undergraduate mentees is a long one, and I express my deepest gratitude to you all. In particular, I would like to thank Paul N. Plassmeyer and Matthew G. Kast for their advice, laughter, and “scientific discussions.” Words cannot express my appreciation to my family for all of their support, guidance, and love. Each of you has influenced me in some way, and I am a better man for it. Thank you for believing in me and for making life worth living. I especially thank the love of my life, Jacklyn R. Whitehead, for all of her love, encouragement, and positivity that inspired me and helped me make it across the finish line. x To my family xi TABLE OF CONTENTS Chapter Page I. INTRODUCTION .................................................................................................... 1 Metal Oxide Properties and Applications .............................................................. 1 Thin Film Deposition Approaches ......................................................................... 1 Vapor-phase Deposition Methods.................................................................... 1 Solution-phase Deposition Methods ................................................................ 2 Challenges in Solution-phase Deposition ............................................................. 3 Addressing Challenges in Solution-phase Deposition: This Work........................ 5 Authorship Statement............................................................................................. 6 Bridge ..................................................................................................................... 7 II. THIN FILM CHARACTERIZATION .................................................................. 8 Challenges in Thin Film Analysis.......................................................................... 8 Physical Characterization....................................................................................... 8 Chemical Analysis ................................................................................................ 12 Electrical Characterization ..................................................................................... 14 Bridge ..................................................................................................................... 16 III. HIGH-κ LANTHANUM ZIRCONIUM OXIDE THIN FILM DIELECTRICS FROM AQUEOUS SOLUTION PRECURSORS................................................. 17 Authorship Statement............................................................................................. 17 Introduction ............................................................................................................ 17 Experimental .......................................................................................................... 19 Precursor Solution Preparation ....................................................................... 19 xii Chapter Page Thin Film Preparation ...................................................................................... 19 Precursor Solution Characterization ............................................................... 20 Thin Film Characterization .............................................................................. 20 Device Fabrication and Testing ...................................................................... 21 Results and Discussion ............................................................................................... 22 Conclusion .................................................................................................................. 30 Bridge ........................................................................................................................... 30 IV. TUNABLE HIGH-κ ZrxAl1-xOy THIN FILM DIELECTRICS FROM ALL- INORGANIC AQUEOUS SOLUTION PRECURSORS ..................................... 32 Authorship Statement............................................................................................. 32 Introduction ............................................................................................................ 32 Experimental .......................................................................................................... 34 Precursor Solution Preparation ....................................................................... 34 Thin Film Preparation ..................................................................................... 34 Precursor Solution Characterization ............................................................... 35 Thin Film Characterization ............................................................................. 36 Device Fabrication and Testing ...................................................................... 36 Results and Discussion ............................................................................................... 37 Conclusion .................................................................................................................. 45 Bridge ........................................................................................................................... 45 V. LOW-TEMPERATURE STEAM ANNEALING OF METAL OXIDE THIN FILMS FROM AQUEOUS PRECURSORS: ENHANCED COUNTERION REMOVAL, RESISTANCE TO WATER ABSORPTION, AND DIELECTRIC CONSTANT .................................................................................. 47 xiii Chapter Page Authorship Statement............................................................................................. 47 Introduction ............................................................................................................ 47 Experimental .......................................................................................................... 49 Precursor Solution Preparation ....................................................................... 49 Thin Film Preparation ..................................................................................... 50 Thin Film Characterization .............................................................................. 51 Device Fabrication and Testing ...................................................................... 52 Results and Discussion ............................................................................................... 53 Impact of Steam Annealing on Bulk Chemistry .............................................. 53 Impact of Steam Annealing on Water Absorption .......................................... 57 Impact of Steam Annealing on Morphology .................................................. 61 Impact of Steam Annealing on Device Properties ........................................... 64 Conclusion .................................................................................................................. 65 Bridge ........................................................................................................................... 66 VI. NON-UNIFORM COMPOSITION PROFILES IN INORGANIC THIN FILMS FROM AQUEOUS SOLUTIONS ........................................................... 68 Authorship Statement............................................................................................. 68 Introduction ............................................................................................................ 68 Experimental .......................................................................................................... 71 InGaOx and AlOx Precursor Solution Preparation ........................................... 71 Hf(OH)4-2x-2y(O2)x(SO4)y•zH2O (HafSOx) Precursor Solution Preparation ... 72 InGaOx, AlOx, and HafSOx Film Preparation ................................................ 72 xiv Chapter Page Thin Film Characterization .............................................................................. 73 Results and Discussion ............................................................................................... 74 Conclusion .................................................................................................................. 84 Bridge ........................................................................................................................... 85 VII. NON-UNIFORM COMPOSITION PROFILES IN AMORPHOUS MULTI- METAL OXIDE THIN FILMS DEPOSITED FROM AQUEOUS SOLUTION ........................................................................................................... 86 Authorship Statement ............................................................................................ 86 Introduction ............................................................................................................ 86 Experimental .......................................................................................................... 88 Precursor Solution Preparation ....................................................................... 88 Thin Film Preparation ...................................................................................... 89 Thin Film Characterization .............................................................................. 89 Device Fabrication and Testing ...................................................................... 91 Results and Discussion ............................................................................................... 92 Conclusion .................................................................................................................. 99 VIII. SUMMARY AND OUTLOOK .......................................................................... 101 APPENDICES ............................................................................................................ 103 A. CHAPTER III SUPPLEMENTARY INFORMATION .................................. 103 B. CHAPTER IV SUPPLEMENTARY INFORMATION .................................. 106 C. CHAPTER V SUPPLEMENTARY INFORMATION ................................... 110 D. CHAPTER VII SUPPLEMENTARY INFORMATION ................................. 125 REFERENCES CITED ................................................................................................ 134 xv LIST OF FIGURES Figure Page 2.1. Simulated XRR patterns for films of various composition and thickness on a Si substrate generated using Bede REFS modeling software . ........................ 9 3.1. TGA of La(NO3)3•xH2O, ZrO(NO3)2•xH2O, and LZO precursor powders derived from drying 1.0 M solutions . ............................................................. 23 3.2. FTIR spectra of LZO films annealed between 200 and 600 °C ...................... 24 3.3. GIXRD of LZO films annealed between 300 and 1100 °C ............................. 25 3.4. (a) XRR patterns of LZO films annealed between 300 and 600 °C and best- fit models. A cartoon inset depicts the two-layer model necessary for generating good fits to the data. (b) Total thickness and total density extracted from best-fit models of XRR data in panel (a) ................................. 26 3.5. (a) Representative AFM images of a 500 nm2 area and (b) comparison of surface roughness determined by XRR and AFM for LZO films annealed between 300 and 600 °C ................................................................................. 27 3.6. Cross-sectional SEM images of two-coat LZO films annealed at temperatures between 300 and 600 °C ........................................................... 28 3.7. (a) Dielectric constants and loss tangents, and (b) J-E data for MIS devices made from two-coat (~100 nm) LZO films .................................................... 29 4.1. (a) Mass loss percent and (b) derivative of mass loss as a function of annealing temperature for bulk ZAO precursor powders ................................ 38 4.2. FTIR spectra of two-coat ZAO films annealed at (a) 200 °C and (b) 500 °C ......................................................................................................... 39 4.3. GIXRD of single-coat ZAO films annealed at (a) 500 °C and (b) 600 °C ........... 40 4.4. Representative AFM images of single-coat (a) AlOy, (b) Zr0.25Al0.75Oy, (c) Zr0.50Al0.50Oy, (d) Zr0.75Al0.25Oy, (e) Zr0.90Al0.10Oy, and (f) ZrOy films annealed at 500 °C and (g) corresponding surface roughness values .............. 41 4.5. SEM cross-sectional images of two-coat (a) ZrOy, (b) Zr0.75Al0.25Oy, (c) Zr0.25Al0.75Oy, and (d) AlOy films annealed at 500 °C ................................ 42 xvi Figure Page 4.6. (a) Dielectric constants measured at 1 kHz and (b) corresponding loss tangents for MIS devices fabricated from two-coat (~100 nm) ZAO films annealed at various temperatures ..................................................................... 43 4.7. Representative JE data for MIS devices made from two-coat ZAO films (~100 nm) annealed at 500 °C ........................................................................ 44 5.1. FTIR spectra of LZO films deposited from 1.8 M solutions derived from (a) NO3 - precursors or (b) NO3 -/Cl- precursors. All films were annealed under humid or dry conditions at 200 °C ......................................................... 53 5.2. TPD data for dry- and humid-annealed LZO films deposited from a 1.5 M solution (NO3 -/Cl- precursors) and annealed at 200 °C prior to TPD experiments: (a) m/z 35, (c) m/z 30, and (d) m/z 32........................................ 55 5.3. FTIR spectra of LZO films deposited from a 1.8 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 200 °C. Solid lines are data taken immediately after annealing and dashed lines are data taken after 1 h in ambient lab air (~45% relative humidity) ............................ 58 5.4. XPS spectra of films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 500 °C ................................... 60 5.5. SEM images of LZO films deposited from a 1.5 M solution (NO3 -/Cl- precursors) and annealed at 500 °C under (a) dry or (b) humid conditions ..... 62 5.6. SEM images of LZO films deposited from (a) 0.5, (b) 1.0, or (c) 1.5 M solutions annealed using a standard (25 °C min–1) ramp, and LZO films deposited from a 1.5 M solution and annealed using (d) slow (0.25 °C min–1), (e) standard, or (f) fast (125 °C min–1) ramps. All films were deposited from NO3 -/Cl- precursors and annealed under dry conditions at 500 °C .................. 63 5.7. (a) Dielectric constants and loss tangents for MIS devices made from two- coat LZO (NO3 -, 1.0 M) films (~100 nm) annealed at various temperatures under dry or humid conditions ......................................................................... 65 6.1. XRR patterns of four-coat films of InGaOx, AlOx, and HafSOx without peroxide............................................................................................................ 75 6.2. (a) A schematic representation of two simple structures of a four-coat stack, one in which each coat is homogeneous and one where each coat has the identical inhomogeneous electron density. (b) Simulated XRR data for the two cases shown in panel (a) .......................................................................... 76 xvii Figure Page 6.3. Raw and modeled XRR data of a four-coat multilayer of HafSOx without peroxide annealed at 180 °C for 1 min ............................................................... 77 6.4. XRR data for a single-coat HafSOx film without peroxide annealed at 180 °C for 1 min ............................................................................................. 78 6.5. XRR data for a single-coat HafSOx film containing peroxide annealed at 80 °C for 1 min ................................................................................................ 80 6.6. (a) HAADF-STEM image of the cryo-FIB cross-section of a single-layer HafSOx film containing peroxide annealed at 80 °C for 1 min and (b) the heavy atom density profile of the film obtained by integrating the intensity of the pixels in the boxed region in panel (a) .................................................. 81 6.7. (a) MEIS data collected on a HafSOx film containing peroxide annealed for 3 min at 80 °C, 5 min at 150 °C, and 5 min at 300 °C. (b) An expansion of the Hf area ........................................................................................................ 83 7.1. XRR patterns of four, three, two, and one layer LZO films deposited from 0.25, 0.33, 0.50, and 1.00 M precursor solutions, respectively ....................... 93 7.2. HAADF-STEM images of one (a), two (b), three (c), and four (d) layer LZO films deposited from 1.00, 0.50, 0.33, and 0.25 M precursor solutions, respectively ..................................................................................................... 94 7.3. La:Zr atomic ratio as a function of distance from the film surface for a three layer film deposited from a 0.33 M precursor solution as determined by STEM-EDS elemental mapping ..................................................................... 96 7.4. (a) C 1s and (b) O 1s XPS spectra of a single-layer LZO film annealed under vacuum and (c) C 1s and (d) O 1s XPS spectra of a LZO film annealed in air for various Ar+ sputtering times ....................................................................... 97 7.5. (a) Dielectric constants and (b) loss tangent as a function of frequency for MIS devices made from four, three, two, and one layer LZO films deposited from 0.25, 0.33, 0.50, and 1.00 M precursor solutions, respectively............... 98 AA1. a) Dielectric constant as a function of frequency for LZO MIS devices and b) corresponding loss tangents ........................................................................ 104 AA2. J-E hysteresis test for a MIS device made from a two-coat (~100 nm) LZO film annealed at 500 °C ................................................................................... 105 xviii Figure Page AB1. FTIR spectra of two-coat ZAO films annealed at (a) 300 °C and (b) 400 °C ......................................................................................................... 106 AB2. Representative XRR spectra for single-coat ZAO films annealed at 500 °C and overlaid best fit models ............................................................................ 107 AB3. (a) Film thickness and (b) density of single-coat ZAO films extracted from XRR best fit, two-layer models ....................................................................... 108 AB4. (a) Dielectric constants as a function of frequency for ZAO-based MIS devices and (b) corresponding loss tangents .................................................... 109 AC1. FTIR spectra of (a) ZnO, (b) YAO, and (c) LZO films deposited from 1.8 M aqueous nitrate precursors and annealed under dry or humid conditions at various temperatures ................................................................... 111 AC2. Photographs of LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 200 °C. Films were photographed immediately after annealing (0 h) and then after 1, 2, and 72 h of exposure to ambient air. ................................................................ 112 AC3. Representative AFM images (1 X 1 μm2) taken of LZO films deposited from a 1.0 M solution (NO3 - precursors). Films were annealed at (a) 200 °C under dry conditions, (b) 200 °C under humid conditions, (c) 500 °C under dry conditions, and (d) 500 °C under humid conditions ........................................ 113 AC4. FTIR spectra of LZO films deposited from a 1.8 M solution (NO3 - precursors) and annealed under dry or humid conditions at 200 °C. Solid lines are data taken immediately after annealing and dashed lines are data taken after 1 h in ambient lab air (~45% relative humidity) ............................ 114 AC5. XPS spectra of LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 200 °C: C 1s (a, g), O 1s (b, h), Cl 2s (c, i), N 1s (d, j), La 3d (e, k), and Zr 3d (f, l) ........................................................................................................ 117 AC6. XPS spectra of LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 500 °C: Cl 2s (a, b), N 1s (c, d), and Zr 3d (e, f)........................................................... 120 AC7. GIXRD spectra for LZO films deposited from a 1.0 M solution (NO3 - precursors) and annealed at 500 °C under dry or humid conditions ................ 122 xix Figure Page AC8. SEM images of LZO films deposited from a 1.5 M solution (NO3 - precursors) and annealed at 500 °C under (a) dry or (b) humid conditions ..... 123 AC9. J-E data for MIS devices fabricated from two-coat LZO (NO3 -, 1.0 M) films (~100 nm) annealed under dry or humid conditions at various temperatures ..................................................................................................... 124 AD1. FTIR spectra of a single layer LZO film deposited from a 1.00 M precursor solution sequentially heated in situ to various annealing temperatures ........... 125 AD2. Single layer models fit to XRR data collected from (a) one, (b) two, (c) three, and (d) four coat films deposited from 1.00, 0.50, 0.33, and 0.25 M precursor solutions, respectively ......................................................... 127 AD3. Multilayer models fit to XRR data collected from (a) one, (b) two, (c) three, and (d) four coat films deposited from 1.00, 0.50, 0.33, and 0.25 M precursor solutions, respectively ......................................................... 129 AD4. Cross-sectional STEM-EDS profiles from a three layer LZO film deposited from a 0.33 M precursor solution. Intensity profiles plotted are the O Kα, La Lα, and Zr Lα signals vs. distance from the film surface ........................... 131 AD5. MEIS experimental data collected from a thin two layer (~8 nm) film overlaid with the best fit model ....................................................................... 133 xx LIST OF TABLES Table Page 5.1. C and O Atomic Percentages from XPS Peak Fitting in Figure 5.4a-d ........... 61 AA1. XRR best-fit modeling parameters ................................................................. 103 AC1. XPS atomic percentages for fitted C 1s and O 1s spectra for LZO (NO3 -/Cl-) films annealed at 200 °C ................................................................ 118 AC2. XPS atomic percentages for La, Zr, O, C, Cl, and N species for LZO (NO3 -/Cl-) films annealed at 200 °C ................................................................ 119 AC3. XPS atomic percentages for La, Zr, O, C, Cl, and N species for LZO (NO3 -/Cl-) films annealed at 500 °C ................................................................ 121 AD1. XRR single layer best fit model parameters ................................................... 128 AD2. XRR multilayer best fit model parameters ..................................................... 130 AD3. EPMA measurements....................................................................................... 132 AD4. XPS measurements .......................................................................................... 134 xxi LIST OF SCHEMES Scheme Page AC1. A cartoon schematic of the steam annealing apparatus .................................. 110 1 CHAPTER I INTRODUCTION Metal Oxide Properties and Applications Metal oxides display an extensive range of physical and electrical properties, making them ubiquitous in modern technologies.1 For example, metal oxides function as water splitting catalysts,2,3 cathode materials for Li ion batteries,4,5 and electrochromics6,7 in energy production, storage, and conservation technologies, respectively. Metal oxides also operate as dielectric and semiconductor components in integrated circuit and display technologies.8–12 For all of these applications, thin films of metal oxides are preferred. Thin Film Deposition Approaches Vapor-phase Deposition Methods Thin film materials are produced using vapor- or solution-phase methods. The most developed and widely employed methods are vapor-phase routes, which are generally considered the current industry standard.13 These techniques can be further designated as physical vapor deposition (PVD) or chemical vapor deposition (CVD) methods. PVD methods sublimate or evaporate pure materials into the gas phase, which are then transported to and condensed onto a substrate.14–16 CVD methods are similar but employ mixtures of volatile precursors that react with or decompose on a substrate.17 Perhaps the most well-known CVD method is atomic layer deposition (ALD), which takes advantage of self-limiting chemical reactions to synthesize films with atomic-level precision.13,18 In general, vapor-phase techniques produce high-quality (smooth, dense, defect-free) films, 2 but suffer from expensive precursors and equipment, large energy inputs, and significant waste (low-atom economy). Careful control of multi-metal composition is also challenging.19,20 Solution-phase Deposition Methods Solution-phase methods offer the potential to circumvent many of the drawbacks of vapor-phase methods,21,22 including significantly reduced precursor costs. A simple cost- analysis of vapor-phase and solution-phase La precursors illustrates this. (iPrCp)3La, a common liquid-injection ALD La precursor,23–25 is ~200 times the cost of La(NO3)3•6H2O, an analogous solution deposition precursor26,27 (approximately $8,445 vs. $42 per mole of La, respectively). Solution-phase methods also offer higher atom economies relative to vapor-phase methods; solution precursor waste has the potential to be recovered and recycled,28,29 while vapor-phase deposition precursors cannot be recycled because they are chemically altered during the deposition process. Furthermore, solution-phase methods employ relatively simple and inexpensive deposition equipment and lower energy inputs, while also offering the potential for high-throughput processing and large area coverage.30–33 Facile manipulation of precursor stoichiometry also enables careful control of multi-metal oxide composition and tuning of thin film properties.34 In the field of solution-phase thin film deposition, sol-gel methods have dominated.35 Traditional sol-gel methods utilize metal alkoxides or metal salts suspended in organic solvents, often with added chelating agents to produce homogeneous solutions. Controlled addition of water, acid, or base facilitates hydrolysis of the metal alkoxides/salts to metal hydroxides, which subsequently condense to form the 3 corresponding metal oxide during thermal processing.36–38 In general, sol-gel methods can produce films with tailored porosity and properties. However, elevated annealing temperatures required to fully remove residual carbon (which can negatively impact film properties39,40) can result in rough, non-uniform film morphologies.41 Aqueous solution-phase routes avoid the need for organic additives and solvents altogether, producing smooth, continuous films with competitive morphology and device properties.42 The simplest aqueous routes employ water soluble metal nitrate or metal chloride salts as precursors.43–51 Increasing the pH of these acidic solutions through the addition of base or electrochemical reduction of H+ ions can facilitate metal oxo-hydroxo cluster/oligomer formation for some metal cations.52–59 In the latter case, reduction of H+ ions to H2(g) at the working electrode drives NO3 - to the counter electrode and out of the main compartment as the metal oxo-hydroxo cluster/oligomer forms, effectively reducing nitrate in the precursor solution and subsequent films.55 Because nitrates and other counterions must be eliminated during film evolution (either through evaporation of the conjugate acid or thermal decomposition) to realize fully condensed oxides, this electrochemical reduction approach is an effective method for reducing residual nitrate content in aqueous-derived oxides. Challenges in Solution-phase Deposition Despite the potential of solution-phase deposition to address the deficiencies of vapor-phase deposition, the promise of low-temperature routes to fully-dense, defect-free metal oxide thin films has not been broadly realized. Many solution deposition methods require relatively high processing temperatures (≥400 °C) to fully remove residual 4 counterions and solvent. A number of strategies for reducing processing temperatures have been investigated, including deep-ultraviolet photochemical activation,60 ozone annealing,61 and microwave irradiation.62 Combustion synthesis is another widely investigated method for forming metal oxides at low temperatures,1,63–65 but there is evidence that combustion occurs only in bulk powders and not in thin film samples because of inherent differences in sample mass and thermal transport.66–68 In the case of sol-gel films, steam annealing has also been shown to promote condensation of metal organic precursors and formation of the subsequent metal oxide network.69–71 However, the effect of steam annealing on the evolution of aqueous-derived films has not been explored. High processing temperatures are also generally required to improve the stability of aqueous-derived films. Films processed at low temperatures (<400 °C) are often unstable with respect to ambient water absorption after annealing. Water absorption roughens the film surface, which negatively impacts device properties.72 Researchers are exploring a variety of approaches to ameliorate post-anneal water absorption to realize lower processing temperatures. For example, Keszler and coworkers deposited a thin (10-20 nm) HfO2 capping layer on aluminum phosphate oxide (AlPO) thin film dielectrics to create an AlPO-HfO2 dielectric stack, which eliminated the negative effects of water absorption in the AlPO films.73,74 Specifically, an AlPO-HfO2 stack annealed at 350 °C for 1 h was shown to have the same resistance to water absorption as an AlPO film annealed at 600 °C for 1 h. Although dielectric properties were enhanced in the case of the AlPO-HfO2 stack, this approach has a limited applicability; the deposition of an additional HfO2 dielectric layer may not always be advantageous for a given application. 5 Furthermore, this report did not investigate the general applicability of this approach for preventing water absorption in other metal oxide thin films. A more general approach for eliminating water absorption in films processed at low temperatures is still needed. It is generally assumed that homogeneous solution precursors produce homogeneous thin films.75 However, a few reports in the sol-gel and aqueous deposition literature show data suggesting the presence of inhomogeneities.73,76–79 These inhomogeneities may arise from density or compositional variations, but the nature of these non-uniformities and their implications on device properties have not been investigated. Addressing Challenges in Solution-phase Deposition: This Work To address these challenges, fundamental understanding of the processes involved in the evolution of aqueous precursors to metal oxide thin films is required. The elucidation of these processes is complicated by the inherently small volume of thin films samples, requiring the use of multiple complementary techniques to craft a complete picture of film formation. Chapter II surveys these techniques, providing a foundation for the subsequent chapters that are focused on advancing the field of aqueous solution deposition in three areas. First, all-inorganic aqueous deposition routes to high-κ lanthanum zirconium oxide (LZO) and zirconium aluminum oxide (ZAO) dielectrics are detailed in Chapters III and IV, respectively. The chemical evolution and morphologies of the LZO and ZAO thin films are studied as a function of annealing temperature and metal composition. Additionally, incorporating these dielectrics into metal-insulator- semiconductor (MIS) capacitors allowed for evaluation of their electrical properties. Second, a general route to lowering processing temperatures and improving resistance to 6 post-anneal water absorption using steam annealing is described in Chapter V. Specifically, the impact of steam annealing on the bulk chemistry of LZO, yttrium aluminum oxide, and zinc oxide thin films is reported. A detailed investigation of the effect of steam annealing on post-anneal water absorption, thin film morphology, and insulating properties of LZO dielectrics is also discussed. Lastly, the elucidation of non- uniform electron densities in single- and multi-metal aqueous-derived films are presented in Chapters VI and VII, respectively. These inhomogeneities are related to inhomogeneous metal distributions in hafnium oxide sulfate (HafSOx) and LZO films. Implications of these non-uniform metal distributions on dielectric properties are investigated for the LZO system. Authorship Statement This dissertation contains previously published coauthored material. Chapter III was previously published in ACS Applied Materials and Interfaces with Tsung-Han Chiang, Paul N. Plassmeyer, Matthew G. Kast, Alexander C. Lygo, Aidan K. Grealish, Shannon W. Boettcher, and Catherine J. Page. Chapter IV was previously published in RSC Advances with Emma C. Waddington, Connor A. Crump, Elizabeth A. Bryan, Tyler S. Gleckler, Michael R. Nellist, Brett A. Duell, Daniel P. Nguyen, Shannon W. Boettcher, and Catherine J. Page. Chapter V was previously published in Chemistry of Materials with Paul N. Plassmeyer, Deok-Hie Park, Lisa J. Enman, Aidan K. Grealish, Brenna L. Kirk, Shannon W. Boettcher, Douglas A. Keszler, and Catherine J. Page. Chapter VI was previously published in ACS Applied Materials and Interfaces with Kurtis C. Fairley, Devin R. Merrill, Keenan N. Woods, Jeffrey Ditto, Can Xu, Richard P. Oleksak, Torgny 7 Gustafsson, Darren W. Johnson, Eric L. Garfunkel, Gregory S. Herman, David C. Johnson, and Catherine J. Page. Chapter VII was previously published in ACS Applied Materials and Interfaces with Milana C. Thomas, Gavin Mitchson, Jeffrey Ditto, Can Xu, Donna Kayal, Kathleen C. Frisella, Torgny Gustafsson, Eric Garfunkel, Yves J. Chabal, David C. Johnson, and Catherine J. Page. Bridge Chapter I provided a broad overview of thin film deposition approaches, a summary of grand challenges in the field of aqueous deposition, and an outline of the contributions included in this work. Chapter II provides an overview of the thin film analytical techniques utilized in Chapters III, IV, V, VI, and VII. Many traditional material characterization techniques, such as thermogravimetric analysis, have insufficient sensitivities for thin film analysis because of the inherently small volume of these samples. Fortunately, there are a number of complementary characterization techniques that have been specifically developed for probing thin film samples. 8 CHAPTER II THIN FILM CHARACTERIZATION Challenges in Thin Film Analysis The elucidation of chemical reactions that occur during the evolution of aqueous solutions to metal oxide thin films is complicated by the inherently small volume of thin film samples, which makes characterization challenging. Bulk powders are often used as analogues for thin films, but differences in sample geometry and mass transport make direct comparisons difficult.1–4 Additionally, amorphous materials lack long-range atomic order, further complicating structural characterization.4 These amorphous materials are a major focus of this work and are preferred for microelectronic device applications because grain boundaries in crystalline materials provide pathways for current leakage.5 Because of these challenges, multiple complementary methods are generally required to fully characterize the evolution and properties of thin film materials. Physical Characterization Excellent thin film morphology (smooth, dense, uniform) is critical for the fabrication of functional microelectronic devices.6–8 X-ray reflectivity (XRR) is a non-destructive technique that provides a wealth of morphological information for thin film samples.9,10 In XRR, the incident beam and the detector are coupled (θ/2θ scanning geometry) and scans are performed at low incident angles (i.e., 0 to 10° 2θ). At angles above the critical angle (the angle of total reflectance), X-rays penetrate the sample and are diffracted from parallel surfaces with distinct electron densities (e.g., film surface and film-substrate 9 interface). Constructive and deconstructive addition of the diffracted X-rays result in an XRR pattern, which provides information on film thickness, electron density, and roughness (Figure 2.1a). Figure 2.1. Simulated XRR patterns for films of various composition and thickness on a Si substrate generated using Bede REFS modeling software. (a) Main features and corresponding extractable information, (b) La2Zr2O7 (LZO) films of various thickness, (c) 50 nm films of various composition and density, (d) 50 nm (total thickness) nanolaminates comprised of LZO and Al2O3 (AO) layers of equal thickness, (e) 50 nm LZO films with various surface roughness, and (f) 50 nm LZO films with various film- substrate interface roughness. Film thickness is inversely proportional to the spacing of the peaks, or Kiessig fringes, in XRR patterns (Figure 2.1b). The electron density of the sample is directly related to the square of the critical angle (Figure 2.1c). Additionally, the amplitude of the 10 Kiessig fringes increases with increasing electron density contrast; large differences between sample and substrate electron density improve the resolution of Kiessig fringes (Figure 2.1c). Stacks of thin films with different electron densities (e.g., nanolaminates) produce XRR patterns with modulated periodicity related to the number of deposited layers (n), with n - 1 low-intensity fringes between intense peaks or valleys (Figure 2.1d).11,12 Kiessig fringe amplitude is also heavily dependent on surface and interface roughness (e.g., film-substrate interface). Specifically, Kiessig fringe intensity (and the maximum angle in 2θ that they can be resolved) decreases as roughness increases (Figure 2.1e,f). Quantitative thickness, electron density, and roughness values can be extracted from XRR patterns using modeling software that generate a best fit based on an initial model input and a genetic algorithm to minimize residuals.13 It is important to use a complementary technique (such as electron microscopy, discussed below) to inform XRR models; complex fits with an unreasonable number of parameters may fit the raw data better, but are not always meaningful. XRR provides important insight into the surface and internal morphology of thin films, but the technique requires ultrasmooth surfaces and interfaces to obtain well- resolved Kiessig fringes for analysis. Atomic force microscopy (AFM) can probe the surface morphology of significantly rougher films.14 In AFM, a sharp cantilever (tip radius ~15 to 40 nm) probes a sample surface using one of a number of sample interaction modes (e.g., tapping mode). As the cantilever scans the sample, vertical and lateral deflections are registered by a reflected laser beam (detected by a position- sensitive photo diode). AFM can produce a topographical map of a specified surface area, and subsequent image processing can calculate a surface roughness (most commonly 11 root-mean-square roughness).8,15–18 Although AFM and XRR probe different types of roughness (physical vs. electron density roughness, respectively), the roughness determined by both techniques are complementary.9 The topographical maps obtained from AFM and electron density profiles from XRR can be further supported by electron microcopy techniques, which offer a method for visualizing the surface and bulk morphology of samples on a nanometer scale. In scanning electron microscopy (SEM) a high-energy beam of electrons is rastered along a sample (i.e., film surface or cross-section), causing the emission of secondary electrons that are detected to form a 3D image.19–24 Conductive samples are preferred for SEM imaging to prevent the buildup of static charge at the sample surface (charging effects) that cause anomalous brightness and prevent collection of high-quality images.25 Charging effects can be mitigated by applying a conductive coating to the film surface, decreasing the accelerating voltage of the electron beam, or applying a bias to the sample.26 In this work, insulating dielectric films were coated with Al for SEM analysis. Transmission electron microscopy (TEM) can also be used to visualize thin film samples.12,27–29 In TEM, an electron beam is passed through an extremely thin (~50 to 100 nm) sample, which is prepared by milling out a cross-section of the film using a focused ion beam in an SEM instrument.30,31 Although TEM can only produce 2D images, the resolution of TEM is superior to SEM; indeed, individual atoms can be imaged using TEM.32,33 TEM can also be operated in scanning mode (STEM), allowing analysis of thicker samples and access to dark field and secondary electron imaging modes.34 SEM and (S)TEM are powerful tools for visualizing nanomaterials, but they can alter samples during prolonged exposure to high-energy electron beams.35,36 Films 12 processed at low-annealing temperatures that are not fully condensed are particularly beam-sensitive. Chemical Analysis Metal oxide properties depend on the degree of oxide condensation and densification of the material and are often negatively impacted by residual counterion or solvent content. A common technique for investigating bulk film chemistry is Fourier transform infrared (FTIR) spectroscopy, which takes advantage of characteristic absorption of IR radiation at specific wavelengths related to vibrational modes with associated changes in dipole moments.4,37–41 Vibrational modes associated with water and nitrate are of particular interest to this work. Metal oxide vibrational modes generally occur at low wavenumbers and are difficult to detect and assign definitively. Films can be probed using attenuated total reflection (ATR) or transmission modes. The FTIR data presented in this work employ transmission FTIR, using an N2 atmosphere to remove contributions from ambient H2O and CO2 and double-side polished Si wafers as a substrate. FTIR is a convenient method for investigating residual counterion and solvent content in thin films, but not all species are FTIR “active.” Thermal programmed desorption (TPD) is a complementary technique for investigating residual counterion and solvent content. In TPD, a previously annealed sample is heated under ultrahigh vacuum (UHV) conditions and desorbed species are detected using a mass spectrometer. TPD has the additional benefit of providing insight into film evolution reaction pathways through detection of molecular fragments (although the influence of UHV conditions must be considered).37,38,42–44 13 Another analytical UHV technique is X-ray photoelectron spectroscopy (XPS). In XPS, a sample is irradiated with X-rays and the measured kinetic energy distribution of ejected electrons provides information about composition, formal oxidation states, and local chemical environments.45–48 This technique is surface sensitive (~10 nm) and can also probe bulk film chemistry by sputtering through the sample with an ion gun between XPS scans (depth-profiling analysis). To extract quantitative composition values, XPS peaks are modeled using a peak fitting program. Similar to XRR data fitting, a complex model with many peaks can always fit the data better but may not be meaningful; therefore, it is critical to inform XPS peak fitting with educated assumptions concerning the composition of the film. In addition to XPS, medium energy ion scattering (MEIS) provides a method for investigating surface composition. MEIS is a high-resolution version of Rutherford backscattering, which measures the number and energy of backscattered ions to create a quantitative areal density and compositional depth-profile of the surface region.49,50 For very thin films, a bulk density can be calculated if the film thickness is known (e.g., from XRR or TEM). Heavy atom distributions in the bulk film can be analyzed using high-angle annular dark-field (HAADF) STEM. In HAADF-STEM, intensity is proportional to the square of the atomic number, providing insight into the distribution of heavy atoms in samples with little to no long range order.51 More detailed elemental mapping can be performed using STEM energy-dispersive X-ray spectroscopy (STEM-EDS) by combining statistical analysis of STEM-EDS spectral images52 with average bulk composition determined using electron probe micro-analysis (EPMA).10,53 14 Electrical Characterization As discussed in Chapter I, metal oxides are critical microelectronic device components. For example, metal oxides are important gate dielectric components in thin- film transistors (TFTs, used in liquid crystal display technologies) and metal oxide field effect transistors (MOSFETs, used in integrated circuit technologies).54–58 The most common gate dielectric is SiO2 because of its excellent insulating properties (i.e., large band gap, low leakage current densities, high breakdown fields), high thermodynamic stability on Si, and low-defect density interface with Si.59 To keep up with demands for increased performance, transistors (and their dielectric components) have been scaled down to increase device capacitance, which results in higher drive currents and decreased switching time.59 This can be demonstrated by considering a parallel plate capacitor described by Eq. 2.1, where C is capacitance, κ is the dielectric constant (or relative permittivity) of the dielectric material, ε0 is the permittivity of free space (a constant), A is the area of the capacitor, and t is the thickness of the dielectric component. Eq. 2.1 C = κ ε0 A t-1 At t ≤ 1 nm, current leakage via electron tunneling through the dielectric layer becomes a significant issue and represents a fundamental scaling limit for high- performing devices. Replacement of SiO2 with a high-κ dielectric would allow for continued improvements in device C without shrinking t, thereby eliminating electron tunneling effects.57–60 For example, a dielectric material with κ = 16 and t = 4 nm would have an approximate equivalent C to a SiO2 (κ = 3.9) with a thickness of 1 nm. Although 15 device performance does not scale perfectly with κ because of quantum mechanical and depletion effects from a Si substrate and gate,59 this simplified picture illustrates the potential for high-κ materials to enable future improvements to transistor performance. Ideally, the functionality of a gate dielectric would be directly investigated by incorporation into a transistor device and measuring device performance. However, the fabrication of thin-film transistors is challenging; interfacial reactions and component compatibility with multiple processing steps are major issues that generally require lengthy optimization procedures.22 To avoid these difficulties, basic electrical characteristics of dielectrics can be characterized using relatively simple metal-insulator- semiconductor (MIS) devices prepared by stacking an insulating metal oxide between a Si substrate and a metal top contact.23,61 In this work, Al dots are thermally evaporated onto an annealed film using a shadow mask. The dot size determines the area of the MIS capacitor device, and smaller devices generally display improved behavior because fewer defects in the insulating layer are likely to be sampled. Contact to the Si substrate (semiconductor) is made by physically scratching through one corner of the film to the bare Si substrate using a scribe and then applying a conductive In/Ga eutectic. The dielectric constant of the insulating layer can be measured using impedance spectroscopy, in which an alternating current is applied at a specific frequency and the magnitude of the current and its phase are recorded.62 This process is repeated for a range of frequencies. Capacitance can be calculated from the measured impedance (Z) for a specific frequency (f) using Eq. 2.2. The dielectric constant can then be calculated using Eq. 2.3, a rearranged form of Eq. 2.1. 16 Eq. 2.2 C = (2 π f Z)-1 Eq. 2.3 κ = C t (ε0 A)-1 Additionally, the leakage current density through the dielectric layer and the breakdown field (the field at which all insulating behavior is lost) can be determined by measuring current as a function of applied voltage (electric field).23,63 Generally, low leakage currents and high breakdown fields are desirable for gate dielectric applications. Electrical characterization of dielectric materials will be discussed in Chapters III, IV, V, and VII. Bridge Chapter II surveyed a number of complementary thin film characterization techniques, which will be used extensively in the following Chapters. Together, these techniques provide insight into the evolution of aqueous precursors to metal oxides, which is critical for developing aqueous deposition chemistry for widespread industrial production of thin films materials. 17 CHAPTER III HIGH-κ LANTHANUM ZIRCONIUM OXIDE THIN FILM DIELECTRICS FROM AQUEOUS SOLUTION PRECURSORS Authorship Statement This chapter was previously published as a full article in ACS Applied Materials and Interfaces in 2017 (volume 9, pages 10897-10903) and was coauthored by Keenan N. Woods, Tsung-Han Chiang, Paul N. Plassmeyer, Matthew G. Kast, Alexander C. Lygo, Aidan K. Grealish, Shannon W. Boettcher, and Catherine J. Page. The manuscript was written by K.N.W. with contributions from T.-H.C., P.N.P., and M.G.K. Editorial assistance was provided by S.W.B. and C.J.P. Experiments were performed by K.N.W., T.-H.C., M.G.K., A.C.L., and A.K.G. with data analysis done by K.N.W., T.-H.C., P.N.P., and M.G.K. Introduction Amorphous metal oxide thin films are used in a range of technological applications, particularly as channel and/or gate dielectric layers in metal-insulator-semiconductor field-effect transistors (MISFETs)1 and thin-film transistors (TFTs).2 In the case of gate components, the incorporation of high-κ dielectrics into devices is attractive for reducing power consumption.3 Zirconium oxide (ZrO2) 4–11 and lanthanum oxide (La2O3) 4,12–15 are two high-κ materials (κ = 22 and 27, respectively) that have attracted interest as gate layers in MISFETs.16 However, ZrO2 crystallizes at relatively low-annealing temperatures (~500 °C), which results in undesirable grain boundary formation, and 18 La2O3 suffers from water and carbonate absorption, 17–19 both of which result in increased leakage currents and poor device properties.20 By combining the La and Zr components into ternary lanthanum zirconium oxides, it may be possible to take advantage of the inherent high-dielectric properties while suppressing crystallization and eliminating carbonate and water absorption. Ternary oxide phases are known for La-Zr-O,21 and the La2Zr2O7 crystalline pyrochlore has been extensively investigated as a substrate for epitaxial growth of YBCO superconductors.22–29 In the past decade, amorphous LaxZryOz has been studied as a dielectric material and has been synthesized via vacuum-based vapor-deposition routes30– 32 and sol-gel solution-deposition routes.33–36 Although vapor-deposition routes generally produce high-quality films,37–40 they are energy-intensive, require expensive starting materials and equipment, and have low atom economy. In contrast, solution-deposition routes are relatively inexpensive, require less-sophisticated equipment, and can produce less waste.41–56 However, common sol-gel routes rely on organic additives and solvents which must be expelled during film processing, leading to porosity57 and undesirable high leakage currents in devices.58,59 Many of these disadvantages can be circumvented by using aqueous precursors. We report an all-inorganic, aqueous route to LaxZryOz (where x = 2, y = 2, and z = 7). The method employed is similar to the prompt inorganic condensation (PIC) route developed by Keszler and coworkers,60,61 which has been used to synthesize a range of metal oxide dielectrics,47,52,62–65 in some cases producing films that rival those deposited by vapor-phase methods.66,67 The amorphous La2Zr2O7 (LZO) films reported herein are smooth on a subnanometer scale and are fully condensed and densified by 600 °C. 19 Crystallization of the LZO films does not occur until 800 °C, approximately 300 °C higher than for pure ZrO2. Electrical characterization of LZO-based metal-insulator- semiconductor (MIS) devices reveal high dielectric constants with low deviation from ideal capacitive behavior and current density characteristics competitive with other aqueous, solution-deposited metal oxide dielectrics.61,62,64–67 Experimental Precursor Solution Preparation A 1.00 M (total metal, 1:1 La:Zr) LZO precursor solution was prepared by dissolving ZrO(NO3)2•xH2O (Sigma Aldrich, 99%) in 18.2 MΩ cm Millipore H2O by gentle heating (~70 °C) and vigorous stirring of the solution. After complete dissolution, La(NO3)3•6H2O (Alfa Aesar, 99.9%) was added in small portions to the still stirring solution. The solution was then diluted to 1.00 M and filtered through a 0.45 μm PTFE syringe filter. Thin Film Preparation Si substrates (2 x 2 cm squares) were sonicated in 5% Decon Labs Contrad-70 solution for 5 min and thoroughly rinsed with 18.2 MΩ cm H2O. Substrates were then spin-dried using a spin-coater and subjected to a 1 min O2/N2 plasma etch using a PE-50 Benchtop Plasma Cleaner (Plasma Etch, Inc.) set to maximum power. Substrates were again rinsed with 18.2 MΩ cm H2O and spin-dried before film deposition. Five drops of LZO precursor solution were deposited onto cleaned Si substrates through a 0.2 μm PTFE syringe filter, and then immediately spun at 3000 RPM for 30 s. 20 Samples were directly transferred to a custom-built hot plate (consisting of an Al block with heating elements, a thermocouple, and a PID controller) set to 125 °C and ramped to a final annealing temperature of 200, 300, 400, or 500 °C at a rate of 25 °C min-1 and held for 1 h. Stacked, two-coat films for MIS device fabrication were annealed to 300 °C, 400 °C, or 500 °C for 10 min between coats and were annealed for 1 h at the desired final annealing temperature. Samples with a final annealing temperature over 500 °C were first heated to 500 °C for 10 min on the hot plate (for each layer), cooled to room temperature, and transferred to a box furnace which was ramped to the final annealing temperature (ramp rate 25 °C min-1). Precursor Solution Characterization Samples were prepared for thermogravimetric analysis (TGA) from individual 1.0 M solutions of ZrO(NO3)2•xH2O, La(NO3)3•6H2O, and LZO precursor that were dried at 50 °C for 12 h. TGA was performed using a TA Instruments Q500A with a ramp rate of 5 °C min-1 under a N2 atmosphere. Thin Film Characterization Fourier-transform infrared (FTIR) spectra on LZO films deposited on lightly doped double-side polished p-type Si substrates were collected using a Thermo Fisher Nicolet 6700 spectrometer. To increase signal-to-noise, FTIR analysis was performed on thicker films made from 1.8 M total metal concentration LZO precursor solutions. Background subtraction was performed by using bare Si substrates subjected to the same thermal treatment. 21 Grazing incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) analysis were conducted using a Rigaku SmartLab X-ray diffractometer (Cu Kα radiation, 40 kV, and 44 mA). X-ray reflectivity data was modeled using the Bede REFS v4.00 software package,68 and film thicknesses and densities were extracted from the generated best fits. Atomic force microscopy (AFM) measurements were carried out on a Digital Instruments Multimode Atomic Force Microscope with an IIIa Controller. Si tapping mode probes (Nanosensors, PPP-NCH) were used, and three measurements from different locations were taken per film over a 500 nm2 window at a scan rate of 1 μm s-1. The height and phase response were collected on the retrace sweep. After data collection, a first order flatten fit and third order plane fit was applied to remove bowing effects from the piezoelectric motors and deviations in the sample from parallel to the piezoelectric motors. NanoScope Analysis software was used to calculate the root-mean-square roughness (Rrms) over the area of the scan window. Cross-sectional scanning electron microscopy (SEM) was performed using a ZEISS Ultra-55 FESEM using a 20 μm aperture and 5 kV accelerating voltage. Films were first coated in thermally evaporated Al to prevent charging effects while imaging.69 Device Fabrication and Testing MIS devices were fabricated to determine LZO dielectric constants and current density-electric field characteristics (J-E). Two-coat LZO films (~100 nm) were deposited on degenerately doped p-type Si (0.008-0.020 Ω cm). Al top contacts (0.013 cm2, ~100 nm thick) were thermally evaporated onto the stacked LZO films through a shadow mask. A back contact was made by scribing through the film to the Si substrate 22 and applying an In/Ga eutectic. Dielectric constants were calculated from impedance measurements measured using an Agilent 4284A Precision LCR meter (20-100,000 Hz, 500 mV oscillation amplitude). Batch-to-batch variations were evaluated by making three separate samples per annealing temperature and measuring impedance of five MIS devices per sample. J-E characterization was accomplished using a Keithley 2400 SourceMeter (0.2 V steps, 0.2 s delay). J-E hysteresis tests were performed using an Agilent 4155B semiconductor parameter analyzer (0.1 V steps, 0.1 s delay). All device testing was performed in ambient atmosphere in a dark environment. Results and Discussion The decomposition pathways of dried La(NO3)3•xH2O, ZrO(NO3)2•xH2O, and LZO precursor powders to their corresponding oxides were examined by TGA (Figure 3.1). Thermal decomposition of La(NO3)3•xH2O begins with a small mass loss around 100 °C, likely from loss of excess (unbound) water, and proceeds through two large mass-loss events starting around 180 and 400 °C, attributable to the loss of bound water and nitrates, respectively. The final mass-loss event starting around 500 °C can be attributed to the expulsion of residual water and nitrates during the final condensation and densification of the oxide. The mass-loss profile of ZrO(NO3)2•xH2O is more gradual and appears as two broad mass-loss events from 100 to 220 °C and 220 to 400 °C. These events likely correspond to gradual water and nitrate loss, respectively. The mass loss profile of the LZO precursor appears to be an approximate average of the two metal nitrate salts and shows a gradual mass decrease from 100 to 160 °C (water loss), a mass- loss event around 220 °C (nitrate loss), and a more pronounced, gradual mass loss over 23 several hundred degrees (further nitrate loss, condensation, and film densification). Conversion of the LZO precursor to the corresponding oxide is complete by 500 °C, which occurs at a lower temperature than for either the La or Zr nitrate salts individually. Although thermal decomposition of the bulk powders is not necessarily representative of the decomposition pathways in thin films, the bulk mass-loss events offer insight into film formation phenomena and can be related to the infrared spectroscopy conducted on thin film samples heated to various temperatures. Figure 3.1. TGA of La(NO3)3•xH2O (blue), ZrO(NO3)2•xH2O (red), and LZO precursor (purple) powders derived from drying 1.0 M solutions. FTIR spectroscopy was used to probe the hydroxide, water, and nitrate content in LZO films after annealing to various temperatures (Figure 3.2). Residual hydroxide and water (monitored via the broad O-H stretching mode peak around 3500 cm-1 and the sharp H-O-H bending mode peak around 1650 cm-1)70 and NOx content (monitored via the N-O stretches centered around 1280 and 1460 cm-1)71 is dramatically reduced 24 between 200 and 300 °C. Loss of these species is important for film stability; films annealed at or below 200 °C were not stable, absorbing water and becoming cloudy after removal from the hot plate. Due to their instability, these low-temperature annealed films were not studied by other methods and are not discussed further. By 500 °C, water and hydroxides are completely removed from the films, indicating that complete condensation has occurred. Residual NOx appear to remain in the films up to 600 °C. The sharp peaks at 744 and 1032 cm-1 are present only in the spectrum for the 200 °C sample and indicate the presence of hydrated metal nitrate species.71,72 The peak centered around 1100 cm-1 corresponds to Si-O-Si73,74 vibrations and is the result of small differences in the subtraction of the sample spectra and the bare Si substrate reference. Figure 3.2. FTIR spectra of LZO films annealed between 200 and 600 °C. Generally, polycrystalline films are undesirable for gate dielectrics because grain boundaries provide pathways for current leakage.62 Amorphous films are thus preferred. GIXRD was used to assess the degree of crystallization in the films as a function of 25 annealing temperature (Figure 3.3). Films remained amorphous until 800 °C, at which temperature LZO begins to crystallize as the cubic pyrochlore phase (ICSD 154752).75 At temperatures above 1000 °C, a mixture of phases is observed. The films used in MIS devices discussed below were prepared at temperatures well below the crystallization temperature (≤600 °C). Figure 3.3. GIXRD of LZO films annealed between 300 and 1100 °C. LZO forms the cubic pyrochlore phase (ICSD 15475, peak positions shown in black at plot baseline) between 800 and 900 °C but decomposes at higher annealing temperatures. The morphology of LZO films annealed between 300 and 600 °C was examined using XRR (Figure 3.4a). In order to extract thickness, density, and surface roughness information for the LZO films, modeling of the XRR patterns was conducted using Bede REFS software.68 Best fits were obtained using two-layer models comprised of a bulk layer and a ~1-3 nm capping layer of different density (Figure 3.4 and Table AA1), consistent with XRR modeling performed on other aqueous solution-processed films.76 Total film thickness decreases with increasing temperature, consistent with FTIR results 26 demonstrating the loss of H2O and NOx species as temperature increases. Overall film density (calculated from a weighted average of the densities derived from the two layer models) increases from 4.35 g cm-3 (300 °C) to 5.27 g cm-3 (600 °C), approximately 90% of the bulk density of the pyrochlore La2Zr2O7 (6.05 g cm -3).77 Surface roughness was remarkably low (≤0.4 nm) for all samples (Figure 3.5b). Figure 3.4. (a) XRR patterns of LZO films annealed between 300 and 600 °C and best-fit models (dashed, black lines). A cartoon inset depicts the two-layer model necessary for generating good fits to the data. (b) Total thickness and total density (weighted average of two-layer model) extracted from best-fit models of XRR data in panel (a). AFM was used to corroborate surface roughness determined from XRR modeling (Figure 3.5). Rrms values of < 0.25 nm were calculated from AFM images (500 nm 2 area) 27 of LZO films annealed between 300 and 600 °C. Film roughness decreases from 300 to 400 °C but remains constant (within error) at higher annealing temperatures. Rrms values determined by AFM are smaller than surface roughness values determined by XRR data modeling, which can be ascribed to differences in how AFM and XRR measurements probe the samples.78 The subnanometer smoothness of these films makes them promising materials for a variety of electronic devices, including TFTs.79–81 Figure 3.5. (a) Representative AFM images of a 500 nm2 area and (b) comparison of surface roughness determined by XRR and AFM for LZO films annealed between 300 and 600 °C. 28 Cross-sectional SEM was used to visualize the morphology of stacked, two-coat LZO films used in electrical device fabrication (Figure 3.6). The films are uniform, dense, noncrystalline, and display no signs of interfacial roughness. Film thickness decreases with increasing annealing temperatures, which is in agreement with the trends observed by XRR. Figure 3.6. Cross-sectional SEM images of two-coat LZO films annealed at temperatures between 300 and 600 °C. The dielectric characteristics of LZO-based MIS devices were measured using impedance spectroscopy. Dielectric constants and loss tangents were measured at frequencies between 20 and 100,000 Hz (Figure AA1), and dielectric constants and loss tangents measured at 1 kHz are shown in Figure 3.7a. The dielectric constant increases steadily with increasing annealing temperature, starting around 12.2 at 300 °C and reaching 16.4 at 600 °C. Loss tangents decrease with annealing temperature, consistent with more fully condensed (and consequently denser) films at higher annealing temperatures. Loss tangents were below 0.6% in all cases, demonstrating low deviation from ideal capacitive behavior. MIS devices were re-examined after 10 months of 29 exposure to ambient air. Samples annealed at 300 °C showed slight decreases in dielectric constants and increased loss tangents, while films annealed ≥ 400 °C were stable and exhibited no change. J-E measurements were conducted to assess the effectiveness of LZO as a viable gate dielectric material (Figure 3.7b). Increasing the annealing temperature resulted in lower Figure 3.7. (a) Dielectric constants (κ) and loss tangents (loss δ), and (b) J-E data for MIS devices made from two-coat (~100 nm) LZO films. leakage currents at all electric fields. This trend is consistent with trends of decreasing water and nitrate content (FTIR), increasing film densification (XRR), and low loss tangent values. LZO MIS devices generally experienced catastrophic breakdown and loss of insulating behavior at ~4 MV cm-1. Stability of the LZO devices was tested by studying J-E hysteresis of successive cycles, as shown in Figure AA2. After an initial 30 burn-in sweep, it was observed that current density settled at 10-7-10-8 A cm-2. No discernable shift was observed in J-E behavior after the second sweep, indicating good stability. LZO films annealed ≥ 500°C have current density characteristics that meet or exceed those of other aqueous solution-deposited dielectric materials.61,62,64–67 Conclusion An aqueous route to high-κ lanthanum zirconium oxide dielectrics using simple nitrate salts was reported. Residual nitrate and water species are fully removed from the films by 600 °C and the pyrochlore phase forms upon annealing to T ≥ 800 °C. The remarkable smoothness of these films, the high dielectric constants with low deviations from ideal capacitive behavior, and low leakage currents make them promising gate components for thin-film transistor devices. Studies examining the effects of varying La:Zr composition on film properties and towards understanding density inhomogeneities (i.e., the less dense capping layer suggested by XRR modeling) are in progress. Bridge Chapter III detailed the preparation of ultrasmooth, dense, and uniform lanthanum zirconium oxide dielectric films from aqueous precursor solutions. These ternary films combined the advantageous dielectric properties of binary ZrO2 and La2O3 while suppressing crystallization of ZrO2 and mitigating carbonate and water absorption of La2O3. Complete characterization of thin film properties and evolution chemistry was achieved using multiple complementary analytical techniques, and the competitive device properties of lanthanum zirconium oxide were demonstrated. 31 Chapter IV builds on this initial study and makes use of the same aqueous route to explore ternary zirconium aluminum oxide dielectrics. This study takes advantage of the tunability offered by aqueous solution chemistry to investigate a complete range of Zr:Al compositions and corresponding film formation pathways and final properties. 32 CHAPTER IV TUNABLE HIGH-κ ZrxAl1-xOy THIN FILM DIELECTRICS FROM ALL-INORGANIC AQUEOUS PRECURSOR SOLUTIONS Authorship Statement This chapter was previously published as a full article in RSC Advances in 2017 (volume 7, pages 39147-39152) and was coauthored by Keenan N. Woods, Emma C. Waddington, Connor A. Crump, Elizabeth A. Bryan, Tyler S. Gleckler, Michael R. Nellist, Brett A. Duell, Daniel P. Nguyen, Shannon W. Boettcher, and Catherine J. Page. The manuscript was written by K.N.W. with contributions from E.C.W. Editorial assistance was provided by S.W.B. and C.J.P. Experiments were performed by K.N.W., E.C.W., C.A.C., E.A.B., T.S.G., M.R.N., B.A.D., and D.P.N. with analysis done by K.N.W., E.C.W., and M.R.N. Introduction Metal oxide thin films function as critical components in a number of technological applications, including as gate dielectrics in transistors.1,2 For these applications, amorphous high-κ materials are desirable for lowering power consumption and decreasing leakage current through the dielectric layer.3 Zirconium oxide (ZrOy) 4–14 and aluminum oxide (AlOy) 15–33 are two high-κ materials that have been investigated as alternatives to SiO2 gate dielectrics. ZrOy has a high dielectric constant (~25) but is incompatible with the processing temperatures required for integrated circuit manufacturing.3 This stems from an inherently low crystallization temperature (<500 °C), 33 which leads to grain boundaries and undesirable leakage currents.34 Additionally, facile diffusion of oxygen through ZrOy at elevated temperatures can result in uncontrolled interfacial growth at ZrO2-Si interfaces. 3 Although AlOy has a lower dielectric constant (~10), it remains amorphous until much higher temperatures (~800 °C) and exhibits high thermodynamic stability on Si.3 Previous reports of high-k zirconium aluminum oxide (ZrAlOy, ZAO) thin film dielectrics indicate that Al incorporation can effectively suppress the crystallization of ZrOy. 35–42 These reports utilized both vapor phase deposition routes (pulsed laser deposition35,36 and atomic layer deposition37,38) and solution deposition routes (spray pyrolysis39,40 and sol-gel41,42). Solution deposition routes are attractive because they generally employ inexpensive precursors and equipment, allow for facile variation of metal composition, and offer the promise of low-temperature processing.43,44 However, the previously reported solution deposition routes to ZAO films make use of organic ligands and solvents, which are often difficult to remove and can result in carbon impurities that can adversely affect dielectric properties.45,46 Only one previous study has investigated the effects of Zr:Al composition on dielectric properties. This study examined a limited composition range using a pyrosol method, but the authors were unable to establish a relationship between composition and dielectric properties.39 Herein, we report an all-inorganic aqueous solution deposition route to ZrxAl1-xOy (ZAO, 0 ≤ x ≤ 1) thin films. This route is distinct from the previously employed solution deposition routes in that no organic ligands or additives are employed. This all-inorganic aqueous synthetic approach has previously produced dense, ultra-smooth metal oxide dielectric films with a variety of metal components.34,47–57 Aqueous solution deposition 34 allows for precise control of film composition through the facile manipulation of precursor stoichiometry,58 which we exploit to investigate a complete range of Zr:Al compositions. The ZAO films prepared via this aqueous route are smooth, dense, and defect-free. Films annealed at temperatures ≤ 500 °C are amorphous when the Al content is ≥10%. Device testing of ZAO-based metal-insulator-semiconductor (MIS) capacitors shows that the dielectric and leakage current density characteristics can be systematically tuned by varying the Zr:Al composition. The facile control of film composition and corresponding electrical properties make ZAO films promising functional components for microelectronic applications. Experimental Precursor Solution Preparation A 1.0 M Zr stock solution was prepared by adding ZrO(NO3)2•xH2O (Sigma Aldrich, 99%) to 18.2 MΩ cm Millipore H2O heated to ~70 °C. The solution was stirred vigorously for 1 h at this temperature, then stirred overnight at room temperature (RT) to produce a stable solution. A separate 1.0 M Al stock solution was prepared by dissolving Al(NO3)3•9H2O (Alfa Aesar, 98.0-102.0%) in 18.2 MΩ cm H2O at RT and stirring vigorously for 1 h. Mixed ratio precursor solutions were prepared from the 1.0 M stock solutions for a range of Zr:Al ratios. The total metal concentration for all Zr:Al ratios was 1.0 M, and all solutions were filtered through a 0.45 μm PTFE syringe filter. Thin Film Preparation Si substrates (2 x 2 cm2 squares) were sonicated in 5% Contrad-70 solution (Decon 35 Laboratories) at RT for 1 h followed by rinsing with 18.2 MΩ cm H2O. Precursor solutions were deposited onto treated Si substrates through 0.2 μm PTFE filters and then spun at 3000 rpm for 30 s. For samples annealed at or below 500 °C, samples were immediately transferred to a hotplate at 50 °C. Samples were then ramped to 200, 300, 400, or 500 °C (12.5 °C min-1) and held for 1 h. Stacked, two-coat films (used for FTIR, SEM, and impedance measurements) were annealed to 200, 300, 400, or 500 °C for 10 min between coats with a final anneal at the desired temperature for 1 h. The maximum temperature that the hotplates could achieve was 500 °C. Therefore, samples with a final annealing temperature > 500 °C were first ramped to 500 °C for 10 min on a hotplate (for each layer), cooled to RT, and then transferred to a box furnace, which was then ramped to the desired final annealing temperature and held for 1 h. In the case of Al-rich ZAO films (ZrxAl1-xOy where x ≤ 0.15), sonicated Si substrates were also subjected to a 10 min O2/N2 plasma etch using a PE-50 Benchtop Plasma Cleaner (Plasma Etch, Inc.) set to maximum power, followed by rinsing with 18.2 MΩ cm H2O. Following deposition, these Al-rich samples were transferred to hotplates preheated to 150 °C instead of 50 °C. These modifications were necessary to ensure good substrate wettability and to prepare high-quality, Al-rich ZAO films. All other annealing parameters (ramp rate, final annealing time) were kept the same. Precursor Solution Characterization Bulk powders were prepared for thermogravimetric analysis (TGA) from 1.0 M precursor solutions that were dried at 50 °C for 12 h. TGA was done using a TA Instruments Q500A with a ramp rate of 5 °C min-1 under an N2 atmosphere. 36 Thin Film Characterization Fourier transform infrared (FTIR) spectra were collected using a Nicolet 6700 spectrometer in transmission mode. Samples were deposited on lightly doped double- sided polished p-type Si substrates and background subtraction was done using a spectrum collected from a blank Si substrate (heated to match the thermal processing of each measured sample). X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXRD) measurements were performed using a Rigaku SmartLab diffractometer with a Cu Kα radiation source (40 kV, 44 mA). XRR data were modeled using Bede REFS v4.00 software package,59 and film thickness and density were extracted from best fit models (see Appendix B). Atomic force microscopy (AFM) images were collected using a Dimension ICON AFM (Bruker, USA) in tapping mode. Standard tapping mode probes were used (42 N m- 1, 320 kHz). Images were collected at a scan rate of 1 Hz with 384 lines and 384 measurements per line. Image processing was performed using NanoScope Analysis 1.50 software, and a first order flattening compensated for any sample tilt in each image. Cross-sectional scanning electron microscopy (SEM) was done using a FEI Helios Dual Beam FIB (5.0 keV accelerating voltage, 86 pA current, immersion mode). Samples were coated with thermally evaporated Al prior to imaging to prevent charging effects.60 Device Fabrication and Testing Metal-insulator-semiconductor (MIS) devices were prepared by thermally evaporating Al top contacts (0.013 cm2, 100 nm thick) onto ZAO films deposited on 37 degenerately-doped n-type Si substrates (<0.005 Ω cm). Electrical contact to the Si substrate was achieved by physically scratching the film with a scribe and applying an In/Ga eutectic. Impedance measurements were taken using an Agilent 4284A Precision LCR meter at 0.1, 1, and 10 kHz with 500 mV oscillation amplitude. Batch-to-batch variations were evaluated by making three separate samples per annealing temperature and measuring impedance of five MIS devices per sample. A Keithley 2400 SourceMeter (0.2 V steps, 0.2 s delay) was used for current density-electric field (J-E) characterization. All MIS device testing was performed in ambient atmosphere in a dark environment. Results and Discussion The decomposition of bulk precursor powders to the corresponding oxides was investigated by TGA (Figure 4.1). Mass loss events can be related to the elimination of water and nitrate counterions from the bulk precursor powders (Figure 4.1a). The temperatures at which significant mass loss occurs can be better visualized by examining the mass loss derivatives (Figure 4.1b). In the case of the ZrOy precursor, gradual mass loss is observed between 50 and 150 °C (predominately water loss) followed by two large mass loss events between 150 °C and 250 °C (predominantly nitrate loss). Above 250 °C there is gradual mass loss (further water and nitrate loss) until ~500 °C when final densification and crystallization occurs. In contrast, the AlOy precursor fully condenses and densifies at a much lower temperature, and mass loss appears to be complete by ~350 °C with the largest mass losses occurring between 50 and 200 °C. 38 Figure 4.1. (a) Mass loss percent and (b) derivative of mass loss (absolute values) as a function of annealing temperature for bulk ZAO precursor powders. For the mixed ZAO precursor powders, increasing the Al content decreases both the decomposition temperature and the resulting oxide formation temperature. This trend is not surprising as it is known that metal nitrate decomposition temperatures decrease with increasing metal cation charge density.61 The higher charge density of Al3+ polarizes the nitrate ions, weakening N-O bond strengths and lowering decomposition temperatures for the Al-rich films. TGA provides insight into the decomposition of the bulk precursor powders, but these measurements do not necessarily relate to the corresponding evolution of thin films due to differences in sample geometry, mass transport, and evaporation rates.62,63 Nevertheless, TGA is useful for providing approximate temperature ranges at which decomposition events take place and for correlating how these change with metal composition. 39 FTIR allows a direct measurement of water and nitrate content in ZAO films as a function of annealing temperature (Figure 4.2). These spectra indicate that significant water (~3500 cm-1)64 and nitrate (~1280 and 1460 cm-1)31,65 are retained in films annealed at 200 °C (Figure 4.2a). Al-rich films generally contain less nitrate, consistent with TGA data of bulk powders. Water is fully removed from films of all compositions by 400 °C (Figure AB1), and nitrate is effectively removed by 500 °C (Figure 4.2b). Peaks indicative of Al-O (~600 to 900 cm-1)31 and Zr-O (~415 cm-1)41 are observed for the pure AlOy and ZrOy films, respectively. The intensity of these peaks increases for films annealed at higher temperatures, indicating oxide formation is more complete. Significant overlap and broadening of the metal oxide peaks makes identification of Al-O and Zr-O peaks in the mixed composition samples difficult.41 The loss of water and nitrate, coincident with growth of the oxide peak intensities, suggest that condensation is largely complete by 500 °C. XRR studies (which allow measurement of film thickness and density) also show that densification is complete by 500 °C (Figure AB2 and AB3). Figure 4.2. FTIR spectra of two-coat ZAO films annealed at (a) 200 °C and (b) 500 °C. FTIR spectra of ZAO films annealed at 300 and 400 °C can be found in Figure AB1. 40 As discussed above, amorphous metal oxides are preferred for gate oxide applications because grain boundaries in polycrystalline films negatively impact device performance.3,34 The extent of crystallization as a function of Zr:Al composition and annealing temperature was investigated using GIXRD (Figure 4.3a,b). All compositions were amorphous at 400 °C, while ZrOy is crystalline by 500 °C. ZAO films with compositions of ≥ 25% Al are amorphous at 500 °C and remain so up to 600 °C. The minimum amount of Al needed to prevent crystallization at 500 °C was 10% Al, although these films crystallize by 600 °C. Figure 4.3. GIXRD of single-coat ZAO films annealed at (a) 500 °C and (b) 600 °C. The tetragonal zirconia phase (ICSD 66781)66 is plotted for reference. For many microelectronic applications, subnanometer surface roughness is essential for optimum device functionality.67–69 AFM shows ZAO films annealed at 500 °C (Figure 41 4.4) are remarkably smooth (root-mean-square roughness (Rrms) < 1 nm). AlOy films, in particular, are exceptionally smooth with Rrms ≈ 0.1 nm (Figure 4.4a). ZAO films with varying Zr:Al ratios are still quite smooth with Rrms ≈ 0.2-0.4 nm (Figure 4.4b-e). Figure 4.4. Representative AFM images of single-coat (a) AlOy, (b) Zr0.25Al0.75Oy, (c) Zr0.50Al0.50Oy, (d) Zr0.75Al0.25Oy, (e) Zr0.90Al0.10Oy, and (f) ZrOy films annealed at 500 °C (1 μm2 area) and (g) corresponding surface roughness values. Error bars were determined using measurements taken from three separate 1 μm2 areas on the same film. 42 In contrast, ZrOy films are significantly rougher (Rrms ≈ 0.8 nm). This increased roughness is likely associated with crystallization observed by GIXRD; indeed, crystallites are apparent in AFM images (Figure 4.4f). Cross-sectional SEM shows the morphology of stacked, two-coat ZAO films incorporated into MIS devices (Figure 4.5). Amorphous ZAO films are dense and homogeneous, with no signs of interfacial roughness (Figure 4.5a-c). In the case of crystalline ZrOy, large columnar grains are observed (Figure 4.5d). Figure 4.5. SEM cross-sectional images of two-coat (a) ZrOy, (b) Zr0.75Al0.25Oy, (c) Zr0.25Al0.75Oy, and (d) AlOy films annealed at 500 °C. MIS devices were fabricated from two-coat ZAO films, and dielectric properties were examined by impedance spectroscopy (Figure 4.6 and Figure AB4). Dielectric constants (1 kHz) increase with increasing Zr content, ranging from 5.4 (AlOy) to 18.6 (ZrOy) for films annealed at 600 °C. We attribute the large increases in dielectric constants and batch-to batch variations for ZrOy films annealed at 500 °C and Zr0.90Al0.10Oy films annealed at 600 °C to the onset of crystallization (see Figure 4.4). Generally, loss tangents were low, indicating minimal deviation from ideal capacitance (Figure 4.6b). Although it is difficult to directly compare properties of materials deposited using 43 different precursors and thermal treatments,63 the dielectric constants reported here are within the range of values reported in the literature. In a previous sol-gel study, Zr0.10Al0.90Oy films annealed between 250 and 350 °C for 2 h exhibited dielectric constants of 8.4 to 11.8.42 Although these values are larger than those reported here, they compare well to dielectric constant of Zr0.25Al0.75Oy films we prepared at 200 °C (κ ≈ 12) and 300 °C (κ ≈ 10). We do not include data for samples annealed at these low temperatures in Figure 4.6 because FTIR data (Figure 4.2 and AB1) indicate that these films contain residual polarizable water and nitrate, which can inflate dielectric constant values and lead to sample-to-sample variations.53 In a separate ultrasonic spray pyrolysis study, ZAO films with similar compositions and annealing temperatures (475 °C) are also in the range of those reported here.39 Figure 4.6. (a) Dielectric constants (κ) measured at 1 kHz and (b) corresponding loss tangents (loss δ) for MIS devices fabricated from two-coat (~100 nm) ZAO films annealed at various temperatures. Error bars were determined using measurements taken from three separate batches of devices. A plot of κ and loss δ as a function of frequency for ZAO-based MIS devices fabricated at 500 °C can be found in Figure AB4. 44 J-E measurements were conducted to assess the effects of Zr:Al composition on breakdown fields and leakage current densities of ZAO-based MIS devices processed at 500 °C (Figure 4.7). In general, ZrxAl1-xOy devices experience catastrophic breakdown (defined as I > 100 mA) and loss of insulating behavior at lower electric fields when x ≥ 0.90 (2.0 to 2.5 MV cm-1). In addition to lower breakdown voltages, batch-to-batch variation for Zr0.90Al0.10Oy devices was high, suggesting the formation of microcrystallites beneath the detection limit of GIXRD for these films. In contrast, devices fabricated from more Al-rich ZrxAl1-xOy films (x ≤ 0.25) experience breakdown at higher electric fields (~3 MV cm-1), with AlOy devices breaking down at ~3.7 MV cm -1. Figure 4.7. Representative JE data for MIS devices made from two-coat ZAO films (~100 nm) annealed at 500 °C. Additionally, ZrOy-based MIS devices display the highest leakage current densities at all electric fields, in agreement with the relatively high loss δ measured for ZrOy-based devices annealed at 500 °C (Figure 4.6b). We attribute this to the formation of crystallites and the presence of grain boundaries in films annealed at this temperature.34 Notably, incorporation of Al significantly decreases the leakage current densities of ZAO-based 45 devices at all electric fields. ZrxAl1-xOy films with 0.25 ≤ x ≤ 0.90 show similar low leakage current densities, while AlOy films exhibit the lowest leakage current densities. Together, the MIS characterization suggests that Zr0.75Al0.25Oy displays the optimal balance of a high dielectric constant and low leakage current densities. Conclusion We report an all-inorganic aqueous deposition route to variable composition high-κ zirconium aluminum oxide films with subnanometer surface roughness and tunable properties. Residual nitrate and water are fully removed from ZAO films by 500 °C, and incorporation of Al ≥10% effectively suppresses crystallization of the tetragonal zirconia phase up to 500 °C. Dielectric constants increase with increasing Zr content, and films with Al >10% exhibit low leakage current densities. For the composition range studied, the combination of high dielectric constant and low leakage current densities makes Zr0.75Al0.25Oy a promising candidate for microelectronic applications. Bridge In Chapter IV, the impact of manipulating solution stoichiometry on thin film evolution chemistry and resulting thin film properties was investigated. Optimization of Zr:Al composition produced amorphous zirconium aluminum oxide dielectrics with excellent morphologies and competitive device performances. Both Chapters III and IV demonstrate the versatility and utility of all-inorganic aqueous approaches for preparing ultrasmooth, dense, and uniform metal oxide thin films. However, elevated processing temperatures (≥400 °C) were required to fully 46 decompose and remove residual counterions and solvent, which is a common issue for solution-deposited materials. Generally, films processed at low annealing temperatures also suffer from significant water absorption from the ambient after annealing, which prevents their use in functional microelectronic devices. Chapter V discusses a method for lowering processing temperatures by adding water to the annealing atmosphere. This steam annealing method effectively improves counterion removal, resistance to water absorption, and dielectric properties of aqueous-derived metal oxide thin films. 47 CHAPTER V LOW-TEMPERATURE STEAM ANNEALING OF METAL OXIDE THIN FILMS FROM AQUEOUS PRECURSORS: ENHANCED COUNTERION REMOVAL, RESISTANCE TO WATER ABSORPTION, AND DIELECTRIC CONSTANT Authorship Statement This chapter was previously published as a full article in Chemistry of Materials in 2017 (volume 29, pages 8531-8538) and was coauthored by Keenan N. Woods, Paul N. Plassmeyer, Deok-Hie Park, Lisa J. Enman, Aidan K. Grealish, Brenna L. Kirk, Shannon W. Boettcher, Douglas A. Keszler, and Catherine J. Page. The manuscript was written by K.N.W. and P.N.P. (equal contributions) with additional contributions from D.-H.P. and L.J.E. Editorial assistance was provided by S.W.B., D.A.K., and C.J.P. Experiments were performed by K.N.W., P.N.P., D.-H.P., L.J.E., A.K.G., and B.L.K. Introduction Metal oxide thin films function as critical device components in a wide range of electronic and renewable energy technologies. The need for high-volume manufacturing has fueled interest in new deposition methods with capabilities to produce complex compositions, control morphologies, and increase throughput. In particular, solution deposition presents a potentially scalable, inexpensive route to address these demands.1–7 Sol-gel and similar metal-organic methods, which utilize organic ligation to form stable solutions and control condensation reactions, currently dominate solution deposition.4–16 48 In contrast, inorganic aqueous solution deposition avoids organic additives and solvents, instead employing simple metal salts or precondensed oxo-hydroxo metal clusters to produce dense films with ultrasmooth surfaces.2,17–38 In both sol-gel and aqueous solution deposition techniques, water plays an important role in the film formation process. For example, sol-gel metal alkoxide precursors must hydrolyze to metal hydroxides before they condense to form a metal oxide network.39–42 Consequently, water vapor aids in the conversion of sol-gel precursors to corresponding oxide films.5,43,44 Similarly, Zn acetate and acetylacetonate salts react with steam to produce crystalline ZnO.45,46 Researchers are only beginning to examine the general effects of humidity on the formation of oxide films deposited from aqueous metal salts and oxo-hydroxo metal clusters. We recently showed, for example, that water vapor in the spin-coating chamber dramatically affects film thickness, i.e., during the coating process alone.47 Specifically, we found film thickness increases with decreased solvent evaporation rate and increased viscosity, independent of solute identity. This effect is a physical phenomenon based on the properties of water as a solvent. We expect humidity will play a chemical role during thermal processing, where water will affect counterion loss, condensation pathways, and film morphology. For example, water forms azeotropes with both nitric and hydrochloric acids, so the presence of water in the annealing atmosphere may enhance the removal of nitrate and chloride counterions, respectively, via azeotropic evaporation. Indeed, we have observed that exposure of bulk scandium oxo-hydroxo nitrate powders to steam removes nitrate at temperatures well below its decomposition temperature.48 49 In this study, we demonstrate that introducing steam to the annealing ambient significantly reduces residual counterions in zinc oxide, yttrium aluminum oxide, and lanthanum zirconium oxide (LZO) films deposited from aqueous nitrate precursors. We then present a detailed study on the effects of steam annealing on the chemical, physical, and electrical properties of amorphous LZO dielectric films. We find that in addition to reducing residual counterion concentrations, steam annealing enhances resistance to water absorption, prevents void formation in chloride-containing films, and increases the dielectric constant of LZO films in metal-insulator-semiconductor (MIS) devices. Combined, the results from this study demonstrate the pronounced impact of steam in the annealing ambient on the chemical evolution, morphology, and electrical properties of thin films deposited from aqueous precursors. Experimental Precursor Solution Preparation All precursor solutions were prepared by dissolving metal nitrate or metal chloride salts in 18.2 MΩ cm Millipore H2O. Lanthanum zirconium oxide (LZO) precursors were prepared as 0.5, 1.0, 1.5, or 1.8 M (total metal, 1:1 La:Zr) solutions. Solutions were prepared from all NO3 - precursors [La(NO3)3•6H2O (Alfa Aesar, 99.9%) and ZrO(NO3)2•xH2O (Sigma-Aldrich, 99%)] or mixed NO3-/Cl- precursors [La(NO3)3•6H2O (Alfa Aesar, 99.9%) and ZrOCl2•8H2O (Sigma-Aldrich, 99%)]. ZnO precursor solutions were made using Zn(NO3)2•6H2O (Sigma-Aldrich, ≥ 99.0%), and yttrium aluminum oxide (YAO, 3:5 Y:Al) precursor solutions were made using Y(NO3)3•6H2O (Strem, 50 99.9%) and Al(NO3)3•9H2O (Alfa Aesar, 98.0-102.0%). All solutions were filtered through 0.45 μm PTFE syringe filters. Thin Film Preparation Si substrates (2 x 2 cm2) were sonicated in 5% Decon Laboratories Contrad-70 aqueous solution for 5 min, thoroughly rinsed with 18.2 MΩ cm Millipore H2O, and spin- dried using a spin-coater. Substrates were then exposed to a 1 min O2/N2 plasma etch using a Plasma Etch, Inc. PE-50 Benchtop Plasma Cleaner set to maximum power. Substrates were then rinsed with 18.2 MΩ cm Millipore H2O and spin-dried before film deposition. This treatment ensured a hydrophilic substrate surface for good solution wetting.49 Samples prepared for Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) analysis were deposited on lightly doped double-side polished p-type Si substrates. Samples for TPD analysis were deposited on 1 x 1 in2 substrates and physically cleaved into 1 x 1 cm2 samples after annealing. Precursor solutions were filtered through a 0.2 μm PTFE syringe filter onto pretreated Si substrates and spin-cast at 3000 rpm for 30 s. Samples were then immediately transferred to a pre-heated heating stage at 125 °C, held for ~15 s, and annealed under either a “dry” or “humid” annealing atmosphere. Dry-annealed samples were annealed on a custom-built hot plate under ambient laboratory atmosphere (~45% relative humidity at room temperature). Steam-annealed samples were annealed in a tube furnace equipped with a removable heating stage and a water vaporizer (Scheme AC1). Both hot plate and heating stage were equipped with a PID controller for precise control of annealing 51 temperature and ramp rate. All samples were annealed using a 25 °C min-1 ramp rate, unless otherwise noted. Samples were heated at their final annealing temperature for 1 h. Thin Film Characterization Water and residual counterion content was probed via FTIR and TPD analysis. FTIR spectra were collected using a Thermo Fisher Nicolet 6700 spectrometer. Background subtraction was performed using the spectra collected from bare Si substrates with the same thermal history as each sample. TPD analysis was conducted using a TPD Workstation (Hiden Analytical) with a quadrupole mass analyzer (3F PIC, Hiden Analytical). Samples were heated from room temperature to 550 °C (30 °C min-1) under ultrahigh vacuum (base pressure < 5 x 10-9 Torr). Electron-impact ionization (EI) mass spectra were acquired with a 70 eV ionization energy and 20 µA emission current. Selected mass-to-charge (m/z) ratios (m/z 30, 32, and 35 for NO, O2, and Cl desorption, respectively) for each sample were monitored in multiple ion detection (MID) mode with a dwell time of 200 ms and a settle time of 50 ms. Atomic force microscopy (AFM) studies were done using a Dimension ICON AFM (Bruker, U.S.A.) in tapping mode with standard tapping mode probes (42 N m-1, 320 kHz). Images were collected at a scan rate of 1 Hz with 512 lines and 512 measurements per line. NanoScope Analysis 1.50 software was used to calculate root-mean-square roughness (Rrms) and to correct for sample tilt in each image by applying a first order flattering. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 (Thermo Scientific) with an Al Kα monochromated source (150 W, 20 eV pass energy, 52 500 μm spot size). Samples were grounded to the metallic sample holder with a metal clip and charge-neutralized using an electron flood source. The binding energy scale was calibrated using the C-H adventitious C 1s peak at 284.8 eV. Spectra were analyzed using Thermo Scientific Avantage 5.94 software with a linear background. A detailed description of the peak fitting analysis can be found in Appendix C. Cross-sectional scanning electron microscopy (SEM) using a FEI Helios Dual Beam FIB was conducted on samples coated with thermally evaporated Al to prevent charging during imaging (5.0 keV accelerating voltage, 86 pA current).50 Grazing incidence X-ray diffraction (GIXRD) was performed using a Rigaku SmartLab diffractometer with a Cu Kα radiation source (40 kV, 44 mA). Device Fabrication and Testing MIS devices were fabricated from two-coat LZO (NO3 -) films (~100 nm) deposited on degenerately doped n-type Si substrates (<0.005 Ω cm). Al top contacts (0.013 cm2, 100 nm thick) were thermally evaporated onto the stacked LZO films through a shadow mask. Electrical contact to the Si substrate was achieved by scratching through the film to the substrate with a scribe and applying an In/Ga eutectic. Dielectric constants and loss tangents were calculated from impedance measurements taken with an Agilent 4284A Precision LCR meter (1 kHz, 500 mV oscillation amplitude). Three separate samples per annealing temperature were fabricated and tested to evaluate batch-to-batch variations. Current density-electric field (J-E) characterization was done using a Keithley 2400 SourceMeter (0.2 V steps, 0.2 s delay). All device testing was conducted in a dark environment under ambient atmosphere. 53 Results and Discussion Impact of Steam Annealing on Bulk Chemistry FTIR spectra (Figure 5.1) demonstrate steam annealing at 200 °C significantly reduces residual nitrate in LZO films, as evidenced primarily by a decrease in the intensities of the 1280 and 1460 cm-1 nitrate bands.51 This is true for LZO films derived from NO3 - (Figure 5.1a) and NO3 -/Cl- precursors (Figure 5.1b). Additionally, bands at Figure 5.1. FTIR spectra of LZO films deposited from 1.8 M solutions derived from (a) NO3 - precursors or (b) NO3 -/Cl- precursors. All films were annealed under humid (blue) or dry (red) conditions at 200 °C. 1000 and ~800 cm-1 (indicative of NO3 - associated with hydrated metal species)51,52 diminish under humid conditions. To determine whether this effect is specific to LZO films, we examined the effect of steam annealing on zinc oxide and yttrium aluminum oxide films spin-cast from aqueous NO3 - precursors. FTIR studies reveal steam annealing also dramatically reduces residual NO3 - content in these films (Figure AC1). We chose 54 these materials to compare with the LZO system because the metal components have very different hydrolysis and thermal decomposition profiles, and the impact of steam annealing on these disparate systems suggests steam annealing is a general method for removing nitrate from aqueous-derived films at low processing temperatures. FTIR is unable to provide information on Cl- content in films deposited from NO3 -/Cl- precursors. TPD allows mass spectral analysis of gases desorbed from a sample as it is heated and is sensitive enough to probe small volume samples, such as thin films. In the present case, we use TPD to measure residual nitrogen- and chlorine-containing species that evolve during the heating of a previously annealed film. Figure 5.2a shows a clear signal at m/z = 35 (Cl) from the 200 °C dry-annealed film. In contrast, the film annealed under steam desorbs no Cl, indicating that steam annealing at 200 °C effectively removes Cl to below the detection limit of TPD. Figure 5.2b shows desorption of NO from dry- and steam-annealed films, and Figure 5.2c shows O2 desorption in the same experiment. We associate these signals with thermally induced NO3 - decomposition because the NO and O2 signals overlap in the two spectra.53,54 The significant signal reductions between 300 and 400 °C for the humid- annealed films demonstrate the effectiveness of steam for reducing NO3 - content, and is consistent with trends observed by FTIR (see Figure 5.1). We propose the two distinct mass loss events from ~250 to 450 °C and ~450 to 500 °C in the NO trace of the dry- annealed film indicate NO3 - exists in at least two distinct chemical environments. The absence of NO and O2 signals up to 400 °C for the steam-annealed film indicates steam annealing at 200 °C removes loosely bound NO3 -. Additionally, reduced signals at higher temperatures indicate strongly bound NO3 - partially desorbs during steam annealing. 55 Figure 5.2. TPD data for dry- (red) and humid-annealed (blue) LZO films deposited from a 1.5 M solution (NO3 -/Cl- precursors) and annealed at 200 °C prior to TPD experiments: (a) m/z 35, (c) m/z 30, and (d) m/z 32. In general, condensation (Eq. 5.1) of aqueous metal salt precursors requires the initial decomposition or chemical removal of counterions (Eqs. 5.2, 5.3) to produce hydroxide. 56 Eq. 5.1. M-OH + HO-M ↔ M-O-M + H2O Eq. 5.2. M-NO3 + H2O ↔ M-OH + HNO3 Eq. 5.3. M-Cl+ H2O ↔ M-OH + HCl These equations are general for oxide formation from metal salts, but different metals will undergo hydrolysis and condensation at different rates. Nevertheless, counterion removal effectively controls formation of M-O-M linkages. The FTIR and TPD data indicate that steam annealing more effectively removes counterions from films prepared from aqueous metal salt precursors. We postulate a combination of factors controls and assists counterion removal under these conditions. First, Eqs. 5.2 and 5.3 shift to the left, as H2O(g) evolves from the films during dry anneals. This shift drives counterions to the inner coordination sphere of the metal cation. These strongly bound counterions decompose and evolve at high temperatures. In contrast, Eqs. 5.2 and 5.3 shift to the right under steam annealing, which generates counterions loosely bound in matrices of hydrogen bonds that are ostensibly easier to remove. Indeed, a recent report suggests increased inner-sphere hydration of metal cations reduces film processing temperatures.25 Second, counterions may evaporate as HNO3/H2O and HCl/H2O azeotropes (68% HNO3/32% H2O and 20% HCl/80% H2O, which boil at 120 and 110 °C, respectively). Azeotropic evaporation shifts the equilibria of Eqs. 5.2 and 5.3 to form metal hydroxides that may then condense via Eq. 5.1. A vapor pressure of water above the film surface promotes azeotrope formation and provides an alternative to thermal decomposition, allowing low-temperature removal of counterions.48 57 Impact of Steam Annealing on Water Absorption Many solution-processed films absorb water readily after thermal processing,55 but the literature rarely comments on this water resorption. Lanthanum-containing films are particularly prone to water absorption.56–58 We observe that 200 °C dry-annealed LZO films become visibly hazy 15 to 60 min after removal from a heated hot plate (Figure AC2). In contrast, we observe no changes for films annealed under steam at the same temperature, and they retain their visual appearance for weeks in air at room temperature. AFM studies indicate that the visible changes in the dry-annealed films are associated with an increased surface roughness from 0.2 to 3.3 nm (Figure AC3). Because of these effects, we made the measurements described above immediately after the annealing step. To determine the cause of the instability observed for dry-annealed films, we compared FTIR spectra of dry- and humid-annealed films taken immediately after annealing at 200 °C and again after a 1 h exposure to air. Figure 5.3 shows that air exposure does not significantly affect the humid-annealed film; notably, the OH band near 3500 cm-1 changes little. In contrast, the OH band of the dry-annealed film increases dramatically after air exposure, indicating significant water absorption. Differences in surface chemistry may account for the enhanced resistance of steam- annealed films to water absorption. Indeed, a dense capping layer can act as an effective diffusion barrier to water.55,59 We examined the surface chemistry of LZO films (NO3 -/Cl- precursors) annealed under humid and dry conditions with XPS. XPS spectra of films annealed at 200 °C (Figure AC5, Tables AC1 and AC2) show that dry-annealed films contain significantly more N from residual NO3 - (consistent with FTIR and TPD). However, both dry- and steam-annealed films retain considerable nitrate at this annealing 58 temperature, complicating peak fitting of the O 1s spectra. Additionally, the hygroscopic nature of dry-annealed films results in sample-to-sample variations. For these reasons, we also studied LZO films annealed under both atmospheres at 500 °C (Figures 5.4 and AC6, Table 5.1 and AC3). Nitrates are completely removed at this annealing temperature, allowing for more reliable O 1s peak fitting and a more detailed analysis of surface chemistry. Additionally, water absorption is not an issue at this annealing temperature, and samples remain amorphous by GIXRD (Figure AC7). Figure 5.3. FTIR spectra of LZO films deposited from a 1.8 M solution (NO3 -/Cl- precursors) and annealed under dry (red) or humid (blue) conditions at 200 °C. Solid lines are data taken immediately after annealing and dashed lines are data taken after 1 h in ambient lab air (~45% relative humidity). See Figure AC4 for similar FTIR stability data for LZO (NO3 -) films. Figure 5.4 shows C 1s, O 1s, and La 3d spectra for LZO films (NO3 -/Cl-) annealed at 500 °C, and Table 5.1 shows the atomic percentages from the C 1s and O 1s spectra peak fitting. The peak fitting indicates LZO films contain carbonate near the film surface. A higher concentration of carbonate exists in dry-annealed films (~4%) compared to humid- 59 annealed films (~1%). Carbonate near the film surface likely arises from CO2 adsorption, which is known to occur for lanthanum oxides.57,60 Comparison of the ratios of O and C attributed to carbonate from peak fitting provides further insight. For the dry-annealed sample, the relevant O:C ratio is ~2.7, which is similar to the expected 3:1 O:C ratio of CO3 2-. In the case of the humid annealed sample, the O:C ratio is ~7.7, significantly higher than the O:C ratio expected for CO3 2-. Lanthanum oxides also absorb H2O from the atmosphere to form lanthanum hydroxide,56 so it is possible that steam-annealed films contain significant OH at the film surface. Because OH has a similar O 1s binding energy range to CO3 2- (530.8 to 532 and 530.5 to 531.5 eV, respectively61), the presence of OH could account for the anomalously high O:C ratio in the humid-annealed sample. The intensity ratio of the multiplet-split La 3d3/2 and La 3d5/2 peaks further supports enhanced OH content in the steam-annealed films; the shape of these peaks more closely resembles pure La(OH)3 than pure La2O3 for the steam-annealed sample. 62 XPS also shows nitrate is completely removed from films annealed under both atmospheres at 500 °C, but that dry-annealed films retain a small amount of Cl (~1%, Figure AC6, Table AC3). This is consistent with XPS for films annealed at 200 °C (Figure AC5, Table AC2), which shows that both Cl and N content of steam-annealed films are significantly reduced relative to dry-annealed films. From the XPS and FTIR data, it is clear that steam annealing diminishes residual counterion concentrations and effectively limits water resorption. There are two potential explanations for these effects. First, many metal nitrate and chloride salts are hygroscopic. Steam-annealed films may be less hygroscopic simply because they contain fewer residual counterions. Second, the enhanced removal of counterions via steam 60 annealing signifies that metal hydrolysis (Eqs. 5.2, 5.3) and subsequent condensation to the oxide (Eq. 5.1) are more complete. Generally, we observe that steam annealing produces ~10% thinner (and presumably denser) films relative to dry annealing. Increased metal oxide condensation and densification would presumably impede water Figure 5.4. XPS spectra of films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 500 °C. The C 1s (a, b) and O 1s (c, d) data were fit using Avantage peak fitting software, and raw data are shown in gray, envelope peak fits are shown in black, and linear backgrounds are shown in red. Individual fit peaks corresponding to specific C or O species are labeled and color-coded. Overlaid La 3d data (e) are color-coded red and blue for films annealed under dry and humid conditions, respectively. 61 absorption. This explanation for the enhanced resistance to water absorption is consistent with previous reports that show a dense oxide capping layer prevents H2O absorption in water-sensitive oxides.55,59,63 Table 5.1. C and O Atomic Percentages from XPS Peak Fitting Shown in Figure 5.4a-d (see Table AC3 for a complete list of atomic percentages). C Species At. % Dry At. % Humid O Species At. % Dry At. % Humid C-H 8.8 7.3 La-O 29.1 28.4 C-O 2.9 2.6 Zr-O 14.9 20.2 C=O 0.8 0.3 CO3 10.4 - O=C-O 0.4 1.1 CO3 + HO - 10.0 CO3 3.9 1.3 C-O + C=O + O=C-O 3.8 3.2 Impact of Steam Annealing on Morphology Further studies reveal a surprising impact of annealing atmosphere on film morphology. The cross-sectional SEM image in Figure 5.5a shows a dry-annealed LZO film (NO3 -/Cl-) contains large voids in the lower portion of the film and a nonporous, uniform surface region. A thin, continuous layer of material at the LZO-substrate interface indicates voids are not caused by dewetting of the precursor solution. In marked contrast, Figure 5.5b shows a humid-annealed LZO film (NO3 -/Cl-) is uniform and homogeneous, containing no voids. Interestingly, NO3 - precursors produce uniform and homogeneous films regardless of annealing atmosphere (Figure AC8). 62 Figure 5.5. SEM images of LZO films deposited from a 1.5 M solution (NO3 -/Cl- precursors) and annealed at 500 °C under (a) dry or (b) humid conditions. See Figure AC8 for SEM images of LZO (NO3 -) films. Counterion loss during film formation requires thermal decomposition and/or diffusion to the film surface via a charge-compensated mechanism. These processes, however, compete with the densification of the oxide network. If the rate of densification exceeds those of counterion diffusion, decomposition, and desorption, then counterions, hydroxide, and water may form trapped bubbles within the films. Bubble collapse during continued annealing may then produce voids. We propose that for chloride ions, the rate of diffusion relative to densification is low under dry annealing, leading to entrapment and void formation. This explanation is consistent with the observation of residual Cl in dry-annealed films by TPD and XPS. In contrast, steam annealing affects the relative rates of diffusion and condensation, enhancing counterion removal and suppressing void formation. To explore this hypothesis further, we conducted an additional SEM study to examine the importance of the relative rates of diffusion and densification in more detail. Specifically, we varied heating rates and the concentration of the precursor solutions (which determines film thickness64) to monitor void formation in dry-annealed (NO3 -/Cl-) films. Figure 5.6a-c shows SEM images of films prepared from different concentrations using a “standard” ramp rate of 25 °C min-1 to 500 °C. The thinnest film (~20 nm, from 63 0.5 M solution) exhibits no voids (Figure 5.6a). Thicker films (~40 and 70 nm, from 1.0 and 1.5 M solutions, respectively) contain voids near the substrate (Figure 5.6b,c). Curiously, each of these thicker films exhibits a dense, void-free surface region of ~20 nm, suggesting Cl species can diffuse from this depth to desorb as the film is forming. Species below this depth become trapped, forming voids. We also examined the effect of ramp rates on film morphology for a fixed film thickness (Figure 5.6d-f). Films annealed at the slowest ramp rate (0.25 °C min-1, Figure 5.6d) form the smallest voids, while films annealed at the fastest ramp rate (125 °C min-1, Figure 5.6f) form the largest voids. Notably, the thickness of the dense, pore-free region Figure 5.6. SEM images of LZO films deposited from (a) 0.5, (b) 1.0, or (c) 1.5 M solutions annealed using a standard (25 °C min–1) ramp, and LZO films deposited from a 1.5 M solution and annealed using (d) slow (0.25 °C min–1), (e) standard, or (f) fast (125 °C min–1) ramps. All films were deposited from NO3 -/Cl- precursors and annealed under dry conditions at 500 °C. 64 at the top of each film increases with slower ramp rates. These observations indicate slow ramp rates allow species to diffuse and desorb more readily and from deeper within the film. Impact of Steam Annealing on Device Properties From the proceeding data and discussion, it is clear that steam annealing reduces residual NO3 - and Cl- content, mitigates water absorption, and improves the morphology of LZO films. To determine the impact of steam annealing on device properties, we incorporated LZO films into MIS capacitors and examined their dielectric properties. Our SEM studies show that LZO films deposited from NO3 -/Cl- precursors contain voids when dry-annealed but not when steam-annealed. Therefore, a comparison of device properties between dry- and steam-annealed LZO (NO3 -/Cl-) films would not be informative, as the dry-annealed films would display undesirably high leakage currents and poor device properties. For this reason, we characterize devices fabricated from LZO (NO3 -) films. Generally, and regardless of annealing atmosphere, dielectric constants increase and loss tangents decrease with increasing annealing temperature (Figure 5.7). We attribute this to increased condensation and densification as annealing temperature increases. Steam annealing at 350 and 500 °C produced films with higher dielectric constants relative to dry-annealed films. We note that the dielectric constant for the humid- annealed 350 °C film is approximately the same as for the 500 °C dry-annealed film. This is consistent with greater degrees of condensation and densification in the humid- annealed films, which is supported by FTIR, TPD, and XPS studies. Additionally, 65 breakdown and leakage current characteristics were similar for films annealed under dry and humid conditions (Figure AC9), suggesting that the enhanced dielectric constants observed for steam-annealed films are not due to water incorporated from processing under water vapor. Notably, 200 °C steam-annealed films produced stable, functional MIS devices, whereas devices prepared from films dry-annealed at the same temperature were nonfunctional. Figure 5.7. (a) Dielectric constants (κ) and loss tangents (loss δ) for MIS devices made from two-coat LZO (NO3 -, 1.0 M) films (~100 nm) annealed at various temperatures under dry (red squares) or humid (blue circles) conditions. Conclusion We report the dramatic effects of steam annealing on metal oxide films deposited from aqueous precursors. Steam annealing significantly reduces residual NO3 - and Cl- content, enhances resistance to water absorption, prevents void formation, and enhances dielectric properties. We postulate steam annealing increases the inner sphere hydration 66 of metal cations (preventing strong counterion binding) and facilitates improved counterion removal via water/acid azeotropic evaporation. Steam annealing may also suppress condensation at low processing temperatures, thereby enhancing the diffusion and desorption of counterions and condensation byproducts from the films. Overall, this study shows that annealing humidity affects the decomposition temperatures and chemical evolution pathways of metal oxides from aqueous solution precursors. Our findings indicate steam annealing offers a promising avenue for producing high-quality device components with enhanced properties relative to ambient thermal processing methods. Bridge Chapter V discussed the impact of steam annealing on the evolution of aqueous precursors into thin films. Specifically, steam in the annealing atmosphere was shown to enhance counterion removal at low temperatures, promoting hydroxide condensation and subsequent densification of the oxide. Film stability with respect to water absorption was improved and dielectric properties of lanthanum zirconium oxide, introduced in Chapter III, were also augmented. The effects of steam annealing were demonstrated for a variety of oxide materials, making it a general method for lowering processing conditions for aqueous-based oxide thin films. Thin films deposited from homogeneous aqueous solutions are generally assumed to be homogeneous. To date, there have been several reports of non-uniform density profiles in aqueous-based films, but it is unclear if these inhomogeneities stem from density or 67 composition gradients in the films. Chapter VI provides a detailed investigation of the nature and origin of these non-uniform density profiles. 68 CHAPTER VI NON-UNIFORM COMPOSITION PROFILES IN INORGANIC THIN FILMS FROM AQUEOUS SOLUTIONS Authorship Statement This chapter was previously published as a full article in ACS Applied Materials and Interfaces in 2016 (volume 8, pages 667-672) and was coauthored by Kurtis C. Fairley, Devin R. Merrill, Keenan N. Woods, Jeffrey Ditto, Can Xu, Richard P. Oleksak, Torgny Gustafsson, Darren W. Johnson, Eric L. Garfunkel, Gregory S. Herman, David C. Johnson, and Catherine J. Page. The manuscript was written by K.C.F., D.R.M., and K.N.W. with contributions from J.D. Editorial assistance was provided by T.G., D.W.J., E.L.G., G.S.H., D.C.J., and C.J.P. Experiments were performed by K.C.F., D.R.M., K.N.W., J.D., C.X., and R.P.O. with data analysis done by K.C.F., D.R.M., and K.N.W. Introduction Inorganic coatings are ubiquitous in modern technology. While the majority of inorganic coatings are made via high-vacuum processes (e.g., sputtering, evaporation, atomic-layer deposition), there has long been an interest in preparing dense, smooth inorganic coatings using a solution route. Perhaps the most widely studied solution route to thin films is the sol-gel method, which generally employs non-aqueous solvents.1 This method has been used to prepare films with varying degrees of porosity and a wide range of pore sizes. Sol-gel-derived monoliths and films can generally be described as porous rigid oxide networks in which the pores are filled with liquid or gas.1,2 More recently, 69 Keszler and coworkers have developed a “prompt inorganic condensation” (PIC) aqueous route to prepare ultrasmooth amorphous inorganic films with a variety of cations by controlling the condensation process.3–10 This method allows the preparation of dense oxide films with near-atomic surface smoothness, presumably due to the surface tension of the water-based solution during spin coating.3,10 Film thicknesses can easily be controlled via the concentration of the solution and the physical parameters used to spin the films. Thin films made via PIC have been incorporated into thin-film transistors using spin coating and low-temperature annealing to obtain devices that meet or exceed the performance of conventional vapor-deposited devices.5,11–15 PIC films prepared from solutions of hafnium oxychloride dissolved in sulfuric acid (HafSOx) with added peroxide have been shown to function as ultrahigh-resolution resists, patternable with electron beam or extreme ultraviolet radiation.16,17 These resists enable patterning with minimum linewidths and edge roughness superior to those obtainable using organic resist materials using conventional lithography techniques. These advantages are expected to become even more important for extreme ultraviolet (EUV) lithography in next generation semiconductor manufacturing.18 For solution-processed films, developing a fundamental understanding of the chemistry occurring in each processing step is critical for improving film morphology and performance.19–23 In the case of inorganic metal oxide films (using sol-gel or PIC), processing steps include: film deposition (via spin coating or dip coating), a “soft bake” to drive off excess solvent, and higher temperature annealing to facilitate counterion removal and condensation. In the sol-gel process, evaporation of organic solvent molecules and simultaneous hydrolysis reactions at relatively low temperatures lead to a 70 stiff inorganic network, and subsequent drying occurs via evaporation from pores.24 By contrast, in the aqueous PIC route, evaporation of solvent (water) involves considerably more energy due to hydrogen bonding and increased solvent/solute interaction strength. The loss of water leads to condensation reactions that directly link the inorganic species as their concentration increases.12 Although fundamentally different chemistries occur in the various methods used to prepare inorganic films, drying and densification models generally assume the resulting films are homogeneous.24 However, there are a few previous studies that show inhomogeneities in sol-gel-derived multilayer films observable by TEM25 or X-ray reflectivity (XRR).26,27 Denser surface “crusts” in single-layer films have also been observed via ellipsometry in sol-gel silica coatings28 and by XRR in PIC aluminum oxide phosphate films.29 The nature of these inhomogeneities, whether due to density variations or compositional inhomogeneity, has not been determined. This prompted us to undertake a systematic study of density and composition gradients in PIC-prepared films using multiple techniques. In this work we present XRR data on a diverse set of films made using PIC with different metal precursor solutions. The three material systems investigated were selected based on their ability to form dense, smooth, amorphous thin films. Films comprised of multiple layers yielded XRR patterns inconsistent with those expected for homogeneous films, suggesting a general phenomenon of density or composition gradients in the individual layers of PIC-derived films. Using HafSOx as a model system, single and multilayer films annealed at different temperatures were examined using XRR, cross- sectional high-angle annular dark-field scanning transmission electron microscopy 71 (HAADF-STEM), and medium energy ion scattering (MEIS). These techniques reveal the evolution of density and compositional inhomogeneity in the films during processing. They suggest a thin, dense surface crust forms during spin coating, presumably because the reactants near the surface dehydrate faster than the interior of the film. This surface crust persists during subsequent low-temperature annealing, but surprisingly does not increase in thickness. Because the performance of photoresists has been shown to be very sensitive to processing conditions,22 controlling the inhomogeneity in HafSOx films may be an important avenue towards improving performance in ultra-high resolution resist applications. More generally, the presence of a dense surface layer may affect the kinetics of film formation, as well as the final properties of metal oxide films derived using PIC. Therefore, understanding and controlling the formation of the surface layer is important for tailoring the evolution and properties of films made using this method. Experimental InGaOx and AlOx Precursor Solution Preparation A 2.00 M total metal concentration (6:7 In:Ga) solution of In(NO3)3•xH2O (Sigma Aldrich, 99.9% In) and Ga(NO3)3•xH2O (Alfa Aesar, 99.999% Ga) and a 1.70 M metal concentration solution of Al(NO3)3•9H2O (Alfa Aesar, 98% Al) were electrochemically treated to reduce nitrate counterion concentrations according to previously reported methods.30,31 Water contents of the indium and gallium nitrate hydrate salts were determined through calcination of the salts to form the oxide and back calculation of the hydrate content (~5 and ~8 H2O for the indium and gallium nitrate hydrate salts, respectively). Both solutions were diluted to 0.25 M (total metal concentration) with 18 72 MΩ cm nanopure water and filtered through 0.45 μm PTFE filters. Hf(OH)4-2x-2y(O2)x(SO4)y•zH2O (HafSOx) Precursor Solution Preparation A 1.00 M stock solution of HfOCl2•8H2O (Alfa Aesar, 98% Hf) was prepared by dissolution and dilution with 18 MΩ cm nanopure water. Solutions for spin coating were prepared by mixing 2.000 N H2SO4(aq) (VWR) and 30 wt % H2O2(aq) (EMD Millipore) followed by the addition of HfOCl2(aq). 32 The final solution was diluted using 18 MΩ cm water to a concentration of 0.105 M sulfuric acid, 0.45 M hydrogen peroxide, and 0.15 M Hf. The four-coat multilayer was synthesized from a solution without peroxide to decrease the solubility of the film, allowing for a lower annealing temperature to prevent subsequent layers from dissolving the previous.17 InGaOx, AlOx, and HafSOx Film Preparation N-type, Sb-doped silicon substrates (0.008-0.02 Ω cm) received surface treatments using a MΔRCH cs-1701 plasma cleaner running on O2 plasma at 30% O2 in N2 using 150 W for 60 s immediately before spin coating. Films were prepared by filtering the solutions through a 0.45 μm filter and spin coating at 3000 rpm for 30 s. The HafSOx thin films were subjected to a 1 min anneal at either 80 °C for a single layer or 180 °C for the multilayer samples. The InGaOx and AlOx multilayer films were annealed at 450 °C for 20 min per coat followed by a final 60 min anneal at 450 °C. All films were annealed on a preheated, calibrated hot plate. 73 Thin Film Characterization XRR patterns were obtained on a Bruker D8 discover (Cu Kα radiation). Sample alignment was checked to ensure that the incident and exit angle were equal and that the sample was in the center of the goniometer. Alignment procedures involved iteratively performing rocking curves and z (height) scans to ensure the aforementioned criteria were met.33 Fits of the XRR data were performed using the Bede REFS software package, which creates a population of solutions based on an initial model and uses a genetic algorithm to minimize residuals.34 Once the best fit was achieved, the models were perturbed to confirm that the model was not a local minimum. Films were initially fit as a single film, to determine the average density and total film thickness. For single coat films, the model was then split in half, with each half being allowed to vary thickness and density independently to improve the fit. A gradient was also added to the fit, as the abruptness of the interface was unknown. Comparison of the difference between the simulated and experimental data (i.e., residuals) over a constant range allows for the quantitative comparison of the fits for a single data set. Comparison of residuals between different data sets requires a more detailed analysis, as the noise inherent in the experimental data contributes differently to the residuals for each data set. Multilayer films were modeled similarly; each coat was split into a two-layer unit, and each layer was allowed to vary independently in thickness and density (all coats assumed to be identical). For scanning transmission electron microscopy (STEM) investigations, a thin cross- section sample was prepared from a section of the film using an in situ lift-out process on a FEI Helios 600 equipped with a Sidewinder ion column and a Quorum cryo stage.35 74 Steps were taken to avoid exposure of the beam-sensitive film while imaging with the SEM and sputtering with the focused ion beam (FIB). Prior to loading the sample in the FIB-SEM, the sample was coated with a 30 nm protecting layer of evaporated carbon. A second 1.5 μm thick protecting layer of FIB-induced carbon was deposited over the area of interest using a 2 keV accelerating voltage (<5 nm stopping range in evaporated carbon). The lift-out and thinning process was carried out using a 500 eV accelerating voltage on the electron beam to minimize the interaction volume of the beam (<9 nm interaction depth). After completing the lift-out, the sample remained 1.5 μm thick to assure no beam interaction had taken place. The sample was then cooled to -170 °C throughout the thinning process. The FIB accelerating voltages used were lowered as the thickness decreased, 30 keV (1.5 to 0.5 μm), 5 keV (500 to 100 nm), and 2 keV (100 to 40 nm). During thinning the sample was monitored using low dose electron beam conditions to reduce risk of exposure and excessive heat (periodic single frames using 500 eV, 90 pA, 500 ns dwell per pixel, 50 um horizontal field width, 1024 x 768 pixels). The thin sample (approximately 40 nm thick) was warmed slowly overnight in the FIB- SEM vacuum chamber to ambient temperature. HAADF-STEM imaging was completed on a FEI Titan at 300 keV (0.245 m camera length, 10 μs dwell, 2048 x 2048 pixels, and 0.6 nA current) at -175°C with a Gatan single-tilt cryo-transfer holder. MEIS, a high resolution version of Rutherford backscattering (RBS), was performed at the Rutgers MEIS facility using 130 keV protons as the incident ion.36 Results and Discussion Four-coat samples of InGaOx, AlOx, and HafSOx were prepared using PIC as 75 described earlier and examined by XRR (Figure 6.1). For all samples, the XRR patterns consist of regular Kiessig fringes with a pattern of varying intensity. Figure 6.2a contains a schematic of two potential structures of these films, one that is homogeneous and one that contains a repeating structure of four layers (coats) where each layer has an identical non-uniform electron density gradient. Figure 6.2b contains the simulated XRR patterns for these two films. Since XRR is very sensitive to variations in electron density, small (1%) differences in electron density can be detected. The simulated XRR pattern for a homogeneous single-coat film shows a characteristic systematic decay in the Kiessig fringe intensity with increasing diffraction angle. The simulated XRR pattern of the film with a repeating structure of four identical layers with non-uniform electron density contains a characteristic modulation of the Kiessig fringe decay, with more intense diffraction maxima corresponding to the thickness of the individual layers resulting from Figure 6.1. XRR patterns of four-coat films of InGaOx, AlOx, and HafSOx without peroxide (top to bottom, respectively). The patterns are offset for clarity. 76 the coherent scattering of the electron density profiles in each layer. Between these more intense maxima are n - 2 smaller maxima, where n is the number of coats, resulting from incomplete destructive interference. All of the multilayer films prepared via PIC have XRR patterns (Figure 6.1) characteristic of films consisting of repeating layers of non- uniform electron density profiles, inconsistent with those expected for homogeneous films. This suggests that this inhomogeneity is a general characteristic of PIC solution- deposited thin films. We elected to study the effects of this phenomenon in more detail for the HafSOx system. Figure 6.2. (a) A schematic representation of two simple structures of a four-coat stack, one in which each coat is homogeneous and one where each coat has the identical inhomogeneous electron density. (b) Simulated XRR data for the two cases shown in panel (a). 77 In Figure 6.3, we model the XRR data of the four-coat HafSOx film assuming that each deposited coat consists of a bilayer: a “bulk” layer and a surface layer. The density and thickness of the bulk and surface layers were allowed to freely vary in order to obtain the best possible fit. It was also assumed that all four coats were identical as a simple, first approximation. This model is not quantitative, as the actual structure is more complex due to the different thermal treatment for each layer. Within these constraints, the best fits were obtained with a thin surface layer (or crust) in each coat with a higher electron density than the underlying bulk layer. Attempts to perturb the models to give a bulk layer of higher density than the surface layer resulted in low-quality fits. Although this model is an oversimplification, it indicates that the films have a higher density surface region within each deposited layer. Figure 6.3. Raw and modeled XRR data of a four-coat multilayer of HafSOx without peroxide annealed at 180 °C for 1 min. 78 A single coat film was also investigated to determine if the crust is inherent in each coat or whether it is induced by the processing conditions used to make the four-coat multilayer. To match the processing of the four-coat multilayer, the film was annealed at 180 °C. XRR data for the single-coat film was modeled both as a homogeneous single- layer film and as a bilayer separated by a gradient. For the latter model, the surface and bulk layer were allowed to freely vary in thickness and electron density (Figure 6.4). Figure 6.4. XRR data for a single-coat HafSOx film without peroxide annealed at 180 °C for 1 min. The data were modeled as a homogeneous single layer and as a bilayer with a thin, dense surface layer and a less dense bulk layer separated by a density gradient. The addition of the surface “crust” improved the agreement between the model and the data, especially in the first three observed minima in the XRR pattern. Between the two models, the two-layer model with a thin, denser crust has a 20% reduction in the residuals as compared to the single-layer model when both models are allowed to search for the minimum using the genetic algorithm and modeling procedure described previously. Perturbing the models from the fits to find better solutions was unsuccessful, indicating the refined fits represent global minima. In XRR, the density 79 values are derived from the critical angle, which is directly related to electron density. In order to obtain density in g cm-3, the composition of the film must be assumed. Therefore, the density values of the layers should be viewed as being approximate, as they depend on the model used. The modeling suggests the best description of the film within the applied modeling constraints is that of a less electron-dense layer topped by a thin, higher electron density crust. These are probably separated by a thin gradient region between the surface and bulk layers, which was included in the model of the single-coat films. This two-layer model with a thin, dense crust therefore qualitatively agrees well with the modeling of the individual coats in the four-coat film, which was previously discussed. Not surprisingly, the simple model assuming four identical bilayers does not quantitatively match the model used for the single-coat films, where the addition of a gradient layer increases the density of the surface layer. In both models, a denser surface crust and less dense bulk layer are required to obtain good fits. Since it has previously been reported that peroxide-containing HafSOx films used for lithography were homogeneous,17 new films containing peroxide were prepared to test whether the crusts observed above also occur when peroxide is added. These peroxide- containing films were also used to examine the effect of annealing temperature. In order to replicate HafSOx films used for patterning, a single coat film containing peroxide was annealed to the soft-bake temperature of 80 °C for XRR studies. As seen in Figure 6.5, not only does the bilayer model give a better fit to the data but also the residuals are reduced by 50% when the model is allowed to create two layers separated by a gradient. This supports the formation of a crust in films containing peroxide, even at the low soft- bake temperatures used for lithography. 80 XRR studies reveal electron density gradients and periodicity (in multilayers), but do not reveal the source of the density gradients (i.e., whether they are due to density variations of a single chemical composition or a chemical gradient). Additional information on the HafSOx films was obtained using HAADF-STEM. Cross-sections of a Figure 6.5. XRR data for a single-coat HafSOx film containing peroxide annealed at 80 °C for 1 min. The data were modeled as a homogeneous single layer and as a bilayer consisting of a thin, dense surface layer and a less dense bulk layer separated by a density gradient. The addition of a surface “crust” improved the agreement between the model and data, especially with respect to the minima in the XRR pattern between 0.8 and 2°. single-coat film containing peroxide were prepared with care to maintain its condition prior to exposure, and a representative cryo-HAADF-STEM image is shown in Figure 6.6a. The image reveals a bright thin band at the top of the sample, indicating an increased density of heavy atoms at the surface. Integration of the average HAADF signal intensity over the highlighted area in Figure 6.6a provides a two-dimensional plot of the heavy atom density profile across the film (Figure 6.6b). This indicates that, in addition to concentration of heavy atoms at the surface of the film, there is also a slight increase of 81 heavy atom density near the bottom of the film. This data provides direct evidence that there is electron density inhomogeneity within the films and supports the two-layer XRR model used earlier. Figure 6.6. (a) HAADF-STEM image of the cryo-FIB cross-section of a single-layer HafSOx film containing peroxide annealed at 80 °C for 1 min and (b) the heavy atom density profile of the film obtained by integrating the intensity of the pixels in the boxed region in panel (a). 82 In order to obtain information about chemical inhomogeneity, MEIS data were collected as a function of annealing temperature and time. MEIS, a low energy, high- resolution version of RBS, is a quantitative technique. The experiment directly measures the number of backscattered ions of each mass. Using calibrated instrumental parameters and well-established cross-sections, these can be converted into areal densities. Further, the ions lose energy as they travel through the film, with the amount of energy loss being a measure of the distance traveled. The width of a peak in an MEIS spectrum is therefore a measure of the thickness of the corresponding layer, and its integrated area yields a direct quantitative areal density (atoms cm-2). The highest energy peak results from protons backscattered from the heaviest atom (i.e., Hf) in the film. The areal density can be converted into a thickness if the bulk density is known (e.g., from XRD), and, conversely, the density can be determined if one has an alternative measure of thickness (e.g., from TEM). Because there is some uncertainty in the water content of the HafSOx films (which depends on processing conditions, film history, temperature, and gas-phase partial pressures during characterization), precise comparisons between different samples is challenging. Nevertheless, the relative density changes from MEIS in the film as a function of depth are quite meaningful. Hafnium segregation to, or densification at, the surface is readily apparent from the raw MEIS data (Figure 6.7). If Hf were distributed uniformly throughout the film, the highest energy peak (corresponding to Hf) would have a “flat” plateau. However, the intensity of the Hf MEIS peaks is greatest at the highest detected energy (at the surface) and is decreased at lower energies (further below the surface). This is consistent with the increased intensity in the HAADF-STEM image at the surface due to an increased 83 Figure 6.7. (a) MEIS data collected on a HafSOx film containing peroxide annealed for 3 min at 80 °C (blue), 5 min at 150 °C (red), and 5 min at 300 °C (black). (b) An expansion of the Hf area, which visually demonstrates the total film thickness decreases with increasing annealing temperature while the average density increases. The peak at 125 keV indicates that all of the films have a surface layer with higher Hf density. concentration of heavy atoms (i.e., Hf). It is also readily apparent in the MEIS spectra that the film thickness decreases (with corresponding increase in density) as the annealing temperature is increased (Figure 6.7b). Qualitatively, the MEIS data is consistent with the results obtained from XRR and STEM investigations. All three techniques indicate the 84 presence of a Hf-enriched denser surface crust in all HafSOx films reported in this work (annealed from 80 to 300 °C, with or without peroxide). Conclusion The data presented suggests that films prepared from aqueous solutions via the PIC process are inhomogeneous in composition and density throughout their thickness. For HafSOx films, a dense region forms at the surface after a low-temperature anneal. This may result from enhanced evaporation of solvent at the surface of the film, increasing concentration and resulting in condensation reactions between the Hf-containing moieties to yield a dense surface layer on the film. Presumably this acts as a diffusion barrier, leading to the lower density observed in the bulk of the film. We postulate that the slow rate of water diffusion through the crust relative to the rate of diffusion within the bulk layer of the film prevents the formation of density gradients in the bulk of the film during subsequent annealing. In the case of HafSOx, the denser surface layer may influence the patterning resolution and contrast obtainable with this system. It may be possible to control the density and/or thickness of the surface crust by adjusting annealing conditions (e.g., temperature, atmosphere, ramp rate) and to correlate changes with patterning metrics. We also show that crust formation is quite general for PIC processed films and that the density inhomogeneities in single coats result in periodic density gradients in multilayer films, which may affect film properties such as electrical conductivity or breakdown voltage. In the case of multi-metal-component oxides deposited from solutions of mixed metal cations, the chemical composition profiles are expected to be more complicated and dependent on a number of factors (e.g., metal solubility and 85 diffusion rate differences). Determining chemical composition gradients at the nanoscale would enable the development of a mechanistic picture of the chemistry occurring during film formation and provide avenues to design chemical inhomogeneities to obtain enhanced properties. The determination of chemical composition gradients at the nanoscale, however, remains a major analytical challenge. Ultimately, understanding and controlling film inhomogeneities will provide an additional tool to modify the physical and chemical properties of films prepared using the PIC approach. Bridge In this Chapter, non-uniform density profiles were linked to composition inhomogeneities in hafnium sulfate oxide thin films. Specifically, greater electron density at the film surface was attributed to Hf-enrichment resulting from increased evaporation at the film surface. Chapter VI focused on non-uniformities in a single-metal component oxide, but complex compositional gradients likely exist in multi-metal oxides, which are often used to achieve particular properties for specific applications. Chapter VII details density and composition gradients in lanthanum zirconium oxide films. The next Chapter also investigates the implications of stacking inhomogeneous single layer films to prepare multilayer films, a method commonly used to control film thickness. 86 CHAPTER VII NON-UNIFORM COMPOSITION PROFILES IN AMORPHOUS MULTI-METAL OXIDE THIN FILMS DEPOSITED FROM AQUEOUS SOLUTION Authorship Statement This chapter was previously published as a full article in ACS Applied Materials and Interfaces in 2017 (volume 9, pages 37476-37483) and was coauthored by Keenan N. Woods, Milana C. Thomas, Gavin Mitchson, Jeffrey Ditto, Can Xu, Donna Kayal, Kathleen C. Frisella, Torgny Gustafsson, Eric Garfunkel, Yves J. Chabal, David C. Johnson, and Catherine J. Page. The manuscript was written by K.N.W. with contributions from M.C.T., G.M., J.D., and C.X. Editorial assistance was provided by T.G., E.G., Y.J.C., D.C.J., and C.J.P. Experiments were performed by K.N.W., M.C.T., G.M., J.D., C.X., D.K., and K.C.F. Introduction The diversity of chemical and physical properties exhibited by metal oxide thin films make them critical components in a number of technological applications, including electronic display,1–4 photovoltaic,5–8 and fuel cell technologies.9–11 For many applications, oxide films with multiple metal components are necessary to achieve desired properties. Solution deposition is an attractive method for producing multi-metal oxides because it enables facile manipulation of metal compositions by controlling precursor stoichiometry,12 and offers the promise of high-throughput processing and large area coverage.13–15 Sol-gel routes are the most common solution deposition methods, but 87 all-inorganic aqueous routes produce high-quality, dense films without the need for organic additives.16–26 In some cases, aqueous solution deposition yields films with device properties comparable to films made using atomic layer deposition.27,28 Despite the inherent differences between organic sol-gel and aqueous deposition routes, density inhomogeneities have been observed in films derived from both solution deposition methods.29–36 Recently, we reported the first in-depth chemical analysis of non-uniform density profiles in aqueous-derived thin films.34 Specifically, Hf enrichment at the surface of hafnium sulfate oxide (HafSOx) films was observed, and the denser, more condensed surface layer was attributed to enhanced evaporation at the film surface. The HafSOx system has a single metal component, but a multi-metal oxide might be expected to have a more complex distribution of metal components. For example, differences in the network forming ability and diffusion rates of metal species may give rise to inhomogeneous metal distributions. We recently reported density inhomogeneities in lanthanum zirconium oxide (LZO) thin films,35,36 which are of interest as high-κ gate dielectrics35,37–43 and as barrier layers for a number of applications.44–50 We demonstrated that single layer LZO films have non- uniform electron densities, consistent with a bulk layer capped by a thin (1-3 nm) surface layer of different density.35 Additionally, preliminary data indicate that these inhomogeneities persist when multiple coats of solution are stacked to make multilayer films,36 a common practice for producing films with a desired thicknesses. From these initial studies, it is not clear if the observed non-uniform density profiles are the result of inhomogeneous metal distributions or simply a consequence of density gradients in the oxide network (with uniform metal ratios throughout). 88 In this study, we detail an in-depth chemical analysis of non-uniform density profiles in amorphous aqueous-deposited LZO films. We show that density variations in single and multilayer LZO films are associated with inhomogeneous La and Zr distributions. Specifically, we find that La is concentrated at the film surface and at the interface between sequentially deposited layers in multilayer LZO films. We attribute the inhomogeneous metal distributions to differences in solubility and network forming ability of the two metal components. We also show that layering artifacts in multilayer films do not negatively impact electrical properties; multilayer films have higher dielectric constants, in agreement with greater condensation (and densification) relative to single layer films. Similar inhomogeneities are likely present in other solution- deposited oxide films with multiple metal components, and determining the chemical origin of these inhomogeneities is a critical first step for understanding their effect on film properties. Experimental Precursor Solution Preparation LZO precursor solutions were prepared by dissolving La(NO3)3 • 6H2O (Alfa Aesar, 99.9%) and ZrO(NO3)2 • xH2O (Sigma-Aldrich, 99%) in 18.2 MΩ cm Millipore H2O. Solutions with various total La and Zr metal concentrations (1:1 ratio) were prepared via stirring and gentle heating, followed by filtration through a 0.45 μm PTFE syringe filter. Precursor solutions are stable for at least six months, with no observed precipitation. 89 Thin Film Preparation Si substrates were prepared with a 5 min sonication in 5% Decon Labs Contrad-70 solution, followed by rinsing with 18.2 MΩ cm H2O, spin-drying, and a 1 min oxygen plasma etch using a PE-50 Benchtop Plasma Cleaner (Plasma Etch, Inc.) set to maximum power. Five drops of LZO precursor solution were deposited through a 0.2 μm PTFE syringe filter onto the substrates, then spun at 3000 rpm for 30 s. Samples were immediately placed onto a hotplate preheated to 125 °C. Single layer films were ramped to 450 °C (25 °C min-1) and annealed for 1 h. Multilayer films were made by the sequential deposition of individual layers that were ramped from 125 °C to 450 °C (25 °C min-1) and annealed for 10 min per layer, with a final anneal at 450 °C for 1 h. An annealing temperature of 450 °C was chosen because Fourier transform infrared spectroscopy (FTIR) indicates that nitrate is fully removed from the films by that temperature (Figure AD1). Precursor solution concentrations of 0.25, 0.33, 0.50, and 1.00 M were used to prepare four, three, two, and one layer samples, respectively, to give the same approximate overall thickness (~40 nm). All samples were annealed in air, unless otherwise noted. Thin Film Characterization Electron density profiles were studied via X-ray reflectivity (XRR) collected using a Bruker D8 Discover diffractometer with Cu Kα radiation. The incident beam was conditioned with large horizontal and vertical slits, and the exit beam was conditioned with a 0.1 mm detector slit. The data were collected from 0-10° 2θ in 0.015° steps at 25 s step-1. Film density, thickness, and roughness were extracted from the XRR data using 90 Bede REFS modeling software.51 A detailed description of the XRR modeling process can be found in the Appendix D. Heavy atom distributions were examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Cross-sectional lamellae were lifted out and thinned using an FEI Helios 600 FIB for HAADF-STEM imaging with an FEI Titan 80-300 TEM (300 keV, 240 mm camera length, 17.8 mrad convergence angle, 0.5 nA current), and image intensity profiles were extracted using Gatan DigitalMicrograph software. For the STEM energy-dispersive X-ray spectroscopy (STEM-EDS) study, a separate lamella from the HAADF-STEM analysis was prepared from a three layer sample according to the same procedure, but using a C (instead of Pt/C) protective cap to avoid elemental interferences from Pt. The cross-sectional STEM-EDS data were collected as spectral images using a JEOL ARM 200 CF STEM (200 keV, 80 mm camera length, 1.0 nA current) equipped with a Thermo Fisher 80 mm2 SDD (0.98 sr solid angle). The data were processed to obtain integrated intensity profiles using Python scripts written in IPython52 and incorporating functions from the HyperSpy multi-dimensional data analysis toolbox.53 STEM-EDS element intensities were quantified using the Cliff- Lorimer method and known film compositions54 as measured using a quantitative thin film technique for electron probe microanalysis (EPMA).55 X-ray photoelectron spectroscopy (XPS) studies were conducted on films annealed to 450 ˚C under either vacuum (~430 mTorr) or ambient air in a stainless steel reaction chamber. The samples were characterized using a PHI 5600 spectrometer equipped with an Al K monochromatic X-ray source and a hemispherical analyzer under ultrahigh 91 vacuum conditions (~10−9 Torr). The composition was determined by using XPS peak area analysis in the MultiPak Software. Medium energy ion scattering (MEIS), a high resolution version of conventional Rutherford Backscattering Spectroscopy (RBS), was conducted using a He+ beam (190 keV). Backscattered ions were detected in a double alignment configuration, which significantly reduces background contributions to achieve monolayer depth resolutions. Quantitative depth profiles were obtained by analyzing the energy spectrum of backscattered ions. The measured spectrum was simulated with multiple 10 Å layers, and the ratio of La, Zr, and O was allowed to vary for each 10 Å layer until a best fit was obtained for the measured data.56 In situ FTIR absorption measurements were performed using a custom-built annealing chamber with a Thermo Nicolet 6700 IR spectrometer equipped with a liquid nitrogen cooled broadband mercury cadmium telluride (MCT-B) detector. All spectra were collected at 74° (Brewster’s angle) with the sample positioned at 45 °C under N2. Films were deposited on double-side polished Si and heated by direct resistive heating. Annealing was performed at successive temperatures for 5 min each under N2. Device Fabrication and Testing Metal-insulator-semiconductor (MIS) devices were fabricated by depositing and annealing single and multilayer LZO films on degenerately doped p-type Si (0.008 to 0.020 Ω cm) and thermally evaporating Al top contacts (0.013 cm2 dots, ~100 nm thick) onto the LZO films through a shadow mask. Electrical contact to the Si was made by physically scratching the film with a scribe and applying an In/Ga eutectic to the exposed 92 Si. Dielectric constants and loss tangents were calculated from impedance measurements made using an Agilent 4284A Precision LCR meter (20-10,000 Hz, 500 mV oscillation amplitude). Batch-to-batch variation was examined by fabricating two separate samples for each single or multilayer film architecture and measuring the impedance of five MIS devices per sample. Impedance measurements were done in a dark environment under ambient atmosphere. Results and Discussion Previously, we have demonstrated density inhomogeneities in single-layer LZO films.35 In a separate HAADF-STEM image analysis development study, preliminary data suggested that the density inhomogeneities observed in single-layer films are preserved in stacked multilayer films.36 To confirm this, XRR was used to probe the electron density profiles of multilayer LZO films of the same approximate thickness (Figure 7.1). Concentration was scaled inversely with the number of coats to give films of approximately the same overall thickness.57 Kiessig fringe patterns for the multilayer films have modulated periodicity, indicating regular variations in electron density perpendicular to the substrate. The observed modulated periodicity in the fringe intensity is related to the number of deposited layers, with n – 1 low-intensity fringes between higher intensity fringes (where n is the number of coats). This is similar to the variations in Kiessig fringe intensity observed in XRR patterns reported for nanolaminate films made from stacked layers of materials with different densities.16,58 93 Figure 7.1. XRR patterns of four, three, two, and one layer LZO films deposited from 0.25, 0.33, 0.50, and 1.00 M precursor solutions from top to bottom, respectively. Because XRR indicates the presence of periodic inhomogeneous electron density in the LZO multilayer films, a homogeneous single layer model will not be appropriate for fitting this data. Indeed, fitting the XRR data with a homogeneous single layer model results in poor fits (Figure AD2, Table AD1). A more complex model is thus required. Incorporating additional fitting parameters often results in improved fits; therefore, additional data from a complementary technique is necessary to determine a physically meaningful model. HAADF-STEM was used to visualize the films and to examine heavy atom distribution in the same single and multilayer LZO films (Figure 7.2). The images show some variation in individual layer and overall film thickness. Because film thickness is linearly related to concentration,57 we attribute variations in film thickness to differences in the spin-coater humidity during film deposition, which was not controlled in this 94 study.59 Porosity is observed in the lower portion of the single layer LZO film, which we attribute to trapped water and nitrate species. We postulate that porosity is not observed in the multilayer films because nitrate and water can more readily diffuse to the film surface during annealing in the thinner individual layers that comprise the multilayer stack. Notably, the HAADF-STEM images of the multilayer films show distinct bright ‘interfaces’ between deposited coats, which are absent in the single layer LZO film. Because HAADF-STEM is sensitive to high-Z atoms in the sample, the brighter regions correspond to higher concentrations of heavy atoms (presumably La in this case). Figure 7.2. HAADF-STEM images of one (a), two (b), three (c), and four (d) layer LZO films deposited from 1.00, 0.50, 0.33, and 0.25 M precursor solutions, respectively. Bright interfaces can be seen in the multilayer films. Using the HAADF-STEM images as a guide, more complex models with interfacial layers between ‘bulk’ layers and a thin capping layer (Figure AD3, Table AD2) were fit to the XRR data in Figure 7.1. Although the fit of the informed models were greatly 95 improved in comparison to the homogeneous single layer models, these more complex models are still likely an oversimplification of the electron density profiles of the films. For example, the abrupt interfaces between the individual layers in the models do not account for density gradients within the films. The “best fit” models are simply one solution (informed by complementary HAADF-STEM data) that provides insight into the amorphous film structure. As noted previously, additional fitting parameters generally improve data fitting, but additional compositional information would be required to justify models with additional fitting parameters. The chemical nature of the observed electron density inhomogeneities were investigated using HAADF-STEM, MEIS, and XPS experiments. Cross-sectional STEM- EDS provides a means for mapping atomic distributions in the films. Signal-to-noise tends to be an issue because sample volumes are small, and extended collection times can change/damage the sample. In our study we averaged signal from an area of the film to improve signal-to-noise, using a statistical analysis previously developed for averaging HAADF-STEM intensities.36 The effects of lamellae sample thickness on STEM-EDS intensities were removed by generating the La:Zr atomic ratio profile (Figure 7.3) from the La Lα and Zr Lα intensity profiles (Figure AD4), assuming that the overall La and Zr intensities integrated from the whole film were proportional to the average film compositions measured using EPMA (Table AD3). The STEM-EDS La:Zr atomic ratio profile shows an abrupt increase in the La:Zr ratio at the surface of each deposited layer, which confirms the abrupt interfaces observed by HAADF-STEM. Furthermore, the La:Zr atomic ratio decreases as a function of distance from the surface of each individual layer. Each layer displays approximately the same change in La:Zr ratio. This is 96 Figure 7.3. La:Zr atomic ratio as a function of distance from the film surface for a three layer film deposited from a 0.33 M precursor solution as determined by STEM-EDS elemental mapping (a distance of zero corresponds to the surface of the film). The dashed red line is a linear fit to the decrease in the La:Zr ratio as a function of distance for the topmost layer (x = 0 to 9 nm). The raw La Lα and Zr Lα intensity profiles can be found in Figure AD4. highlighted by the red dashed lines in Figure 7.3, which represent a linear fit to the gradient in the topmost layer. The gradient in the La:Zr ratio suggests La enrichment at the surface of each deposited coat in the multilayer film. This enrichment is corroborated by MEIS and XPS data, which show enhanced La concentrations at the film surface (Figure AD5, Table AD4). Collectively, these data suggest that the observed bright interfaces in the HAADF-STEM images are the La-rich surfaces of the sequentially deposited LZO coats that comprise the multilayer films. The surface of a single coat LZO film was investigated further using XPS. The C 1s and O 1s spectra reveal that there are both C and O associated with carbonate in films annealed in ambient air, but not in films annealed under vacuum (289 eV in the C 1s region and 531.5 eV in the O 1s region, Figure 7.4). Most of the carbonate in the air- annealed films is present only at the surface, dissipating after only 60 s of sputtering with Ar+ ions at 1 keV (~1 nm). 97 Figure 7.4. (a) C 1s and (b) O 1s XPS spectra of a single-layer LZO film annealed under vacuum and (c) C 1s and (d) O 1s XPS spectra of a LZO film annealed in air for various Ar+ (1 keV) sputtering times. Peaks assigned to metal oxides (529 eV) and adventitious carbon (285 eV) are observed in all films studied. The collective data demonstrate inhomogeneous La and Zr distributions in LZO films, indicating metal atom segregation during film formation. According to glass network theory, small, highly charged ions such as Zr4+ act as network formers to form metal oxide networks, whereas larger ions such as La3+ act as network modifiers.60 We postulate that a rigid Zr oxide networks forms relatively early in the film formation process, leaving the La3+ ions more mobile within the evolving films. This is supported by previous thermogravimetric analysis studies that indicate that significant mass loss occurs for ZrO(NO3)2 hydrate powders at much lower temperatures than for La(NO3)3 hydrate powders.35 As a film is annealed, water is removed from the film surface. This results in a concentration gradient of La3+ ions at the film surface because the rate of evaporation, which concentrates the ions, is apparently faster than the ability of La3+ ions to diffuse away from the surface. In multilayer films, this La-rich surface forms for each 98 individual layer and is ‘buried’ by the sequential deposition of additional layers. The La- rich surface of the final film absorbs CO2 from ambient air to form a thin carbonate- containing surface layer either during61 or after62 annealing. As discussed previously, LZO films are of interest as dielectric components for microelectronic devices. To investigate the impact of the observed inhomogeneities on device performance, we fabricated LZO-based MIS devices and used impedance spectroscopy to characterize their dielectric properties (Figure 7.5). In general, dielectric constants increased with increasing number of layers (Figure 7.5a). Loss tangents were similar for all devices studied, although the single-layer devices displayed higher deviations from ideal capacitance at low frequencies (Figure 7.5b). Figure 7.5. (a) Dielectric constants (κ) and (b) loss tangent (loss δ) as a function of frequency for MIS devices made from four, three, two, and one layer LZO films deposited from 0.25, 0.33, 0.50, and 1.00 M precursor solutions, respectively. Error bars were determined from two separate batches of LZO-based devices. 99 Although multilayer films exhibit enhanced dielectric properties, we are hesitant to attribute this to the increased number of internal interfaces (and inhomogeneities) in these films. The enhanced dielectric properties are likely due to increased condensation and densification of the multilayer films. These films are comprised of multiple thin coats of solution, in which nitrate and water can more readily diffuse to the film surface during the annealing process. In contrast, counterions and water in the single-layer film must diffuse a greater distance to reach the film surface and desorb. In addition, the multilayer films have been annealed cumulatively longer than the single layer film because each individual coat must be “set” with a short anneal. Indeed, XRR modeling indicates that the multilayer films are denser than the single-layer film (Table AD2). Additionally, the HAADF-STEM images and loss tangent data at low frequencies suggest that more mobile species are trapped within the single-layer film. Although we cannot exclude the effects of the internal interfaces, mass transport considerations likely dominate the observed trend in dielectric constants. Regardless of the cause, these results indicate that multilayer films are preferred for optimal device properties. Conclusion The composition of non-uniform density profiles was investigated for amorphous multi-metal component LZO dielectrics using a suite of complementary techniques. We observe an inhomogeneous distribution of La and Zr and detail the composition of a La- rich carbonate-containing surface layer in the LZO films. We postulate that the La-rich surface results from concentrating the La3+ ions at the film surface as water evaporates, and that a similar effect is not observed for Zr4+ ions due to rapid Zr oxide network 100 formation early in the film formation process. This inhomogeneous distribution of metal components is likely a general phenomenon for multi-metal oxide films deposited from solution. Additionally, because many applications require thicker films than can be prepared with single coats, multilayer films are often used to achieve desired film thicknesses. We show that layering artifacts in multilayer films do not negatively impact dielectric properties, and that the more condensed multilayer films have higher dielectric constants relative to single layer films of the same thickness. Ultimately, understanding the chemical origin and consequences of inhomogeneities will be important for optimizing solution-deposited film properties and their use in devices. 101 CHAPTER VIII SUMMARY AND OUTLOOK This work contributes to the advancement of aqueous deposition methods by addressing major knowledge gaps in three areas. First, all-inorganic aqueous routes to high-quality, high-κ metal oxide dielectrics were developed in Chapters III and IV. Chapter IV also demonstrated the tunability of aqueous approaches, which enables careful optimization of device properties for specific functions. Second, Chapter V detailed the remarkable impact of steam annealing on the evolution of aqueous precursors to the corresponding oxides. This general method reduces residual counterion content in films at annealing temperatures ≤ 200 °C via azeotropic evaporation, improves film stability with respect to post-anneal water absorption, and enhances dielectric properties. Third, non-uniform density profiles were linked to composition gradients in single and multi-metal oxides in Chapters VI and VII, respectively. These composition gradients are attributed to enhanced evaporation at the film surface and differences in metal cation network forming ability. Chapter VII also showed that multilayer dielectrics contain less porosity and display higher dielectric constants than single layer films of the same thickness, making them preferred for device applications. To fully realize the potential advantages of solution-phase deposition over traditional vapor-phase methods, further elucidation and control of the chemistry that occurs during film processing is required. Optimization of deposition and processing conditions will also be important, and development of high-throughput technologies, such as slot die 102 coating, are critical for widespread incorporation of solution deposition methods into industrial device component manufacturing. 103 APPENDIX A CHAPTER III SUPPLEMENTARY INFORMATION Table AA1: X-ray reflectivity (XRR) best-fit modeling parameters Capping Layer Bulk Layer Total T (°C) t (nm) D (g cm-3) R (nm) t (nm) D (g cm-3) R (nm) t (nm) D (g cm-3) 300 0.7 3.33 0.3 50.4 4.35 1.2 51.1 4.35 400 2.1 4.29 0.4 37.8 4.75 0.7 39.9 4.71 500 2.2 4.62 0.4 34.4 5.04 0.6 36.7 4.97 600 1.0 4.37 0.4 32.5 5.30 0.9 33.5 5.27 Table AA1. Best fit XRR modeling parameters for single layer LZO films annealed between 300-600 °C. Best fit models used a two layer fit with a bulk layer capped by a thinner, less dense capping layer. Parameters modeled for the capping and bulk layers were thickness (t), density (D), and roughness (R). Total thickness and the total weighted average density are also shown. 104 Figure AA1: Dielectric constant dispersion Figure AA1. a) Dielectric constant (κ) as a function of frequency (f) for LZO MIS devices and b) corresponding loss tangents (loss δ) as a function of f. 105 Figure AA2: Current density-electric field (J-E) hysteresis test Figure AA2. J-E hysteresis test for a MIS device made from a two-coat (~100 nm) LZO film annealed at 500 °C. Voltage was swept from 0 to +30 V, +30 to -30 V, and -30 to 0 V in three successive sweeps. Sweep 1 is shown in blue, sweep 2 is shown in black, and sweep 3 is shown in red (for clarity, only sweeps from 0 to +30 V and from 0 to -30 V are shown). 106 APPENDIX B CHAPTER IV SUPPLEMENTARY INFORMATION Figure AB1: Fourier transform infrared spectroscopy (FTIR) for ZAO films annealed at 300 and 400 °C Figure AB1. FTIR spectra of two-coat ZAO films annealed at (a) 300 °C and (b) 400 °C. 107 Figure AB2: X-ray reflectivity (XRR) spectra and best fit models for ZAO films annealed at 500 °C Figure AB2. Representative XRR spectra for single-coat ZAO films annealed at 500 °C (solid, colored) and overlaid best fit models (dashed, black). Best fit models were generated using a two-layer model consisting of a thin (~1-3 nm) capping layer over an underlying bulk layer of different density, consistent with models employed in previous XRR studies on aqueous-derived metal oxides.1,2 108 Figure AB3: Film thickness and density of ZAO films extracted from XRR best fit models Figure AB3. (a) Film thickness [t] and (b) density [D] of single-coat ZAO films extracted from XRR best fit, two-layer models. Small differences in film thickness between compositions are attributed to small differences in the humidity of the spin- coating chamber during deposition.3 Density values are calculated from a weighted average of the capping and bulk layer densities. 109 Figure AB4: Dielectric constant dispersion Figure AB4. (a) Dielectric constants [κ] as a function of frequency [f] for ZAO-based MIS devices and (b) corresponding loss tangents [loss δ] as a function of f. MIS devices were fabricated from two-coat (~100 nm) ZAO films annealed at 500 °C. Error bars were determined using measurements taken from three separate batches of MIS devices. 110 APPENDIX C CHAPTER V SUPPLEMENTARY INFORMATION Scheme AC1: Steam annealing apparatus Scheme AC1. A cartoon schematic of the steam annealing apparatus, consisting of a tube furnace equipped with a water vaporizer (connected by Al tubing heated at ~70 °C) and a removable sample heating stage. The tube furnace was kept at 125 °C, while the sample stage was ramped up to the desired annealing temperature (rate = 25 °C min-1). The water vapor content was kept constant with a flow rate of ~5.3 mL min-1. 111 Figure AC1: Fourier transform infrared (FTIR) spectra for ZnO, YAO, and LZO (NO3 -) films Figure AC1. FTIR spectra of (a) ZnO, (b) YAO, and (c) LZO films deposited from 1.8 M aqueous nitrate precursors and annealed under dry (red) or humid (blue) conditions at various temperatures. Peak assignments (colored bands) are consistent with previous reports.1–4 112 Figure AC2: Photographs of LZO (NO3 -/Cl-) films before and after water absorption Figure AC2. Photographs of LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 200 °C. Films were photographed immediately after annealing (0 h) and then after 1, 2, and 72 h of exposure to ambient air. Humid-annealed films retained their visual appearance after >72 h of exposure to air, whereas dry-annealed films changed dramatically. Some variability was observed in dry-annealed films, with some films beginning to visibly change after ~15 min of exposure to air and becoming full cloudy and discolored after ~1 h. This variability is attributed to relative humidity differences in the ambient air. 113 Figure AC3: Atomic force microscopy (AFM) images of LZO (NO3 -) films Figure AC3. Representative AFM images (1 X 1 μm2) taken of LZO films deposited from a 1.0 M solution (NO3 - precursors). Films were annealed at (a) 200 °C under dry conditions, (b) 200 °C under humid conditions, (c) 500 °C under dry conditions, and (d) 500 °C under humid conditions. Sample height is indicated by color scales, with image (a) having a different scale than images (b-d). The root-mean-square roughness (Rrms) for each image is also given. 114 Figure AC4: FTIR water absorption stability data for LZO (NO3 -) films Figure AC4. FTIR spectra of LZO films deposited from a 1.8 M solution (NO3 - precursors) and annealed under dry (red) or humid (blue) conditions at 200 °C. Solid lines are data taken immediately after annealing and dashed lines are data taken after 1 h in ambient lab air (~45% relative humidity). 115 X-ray photoelectron (XPS) peak fitting analysis The C 1s and O 1s XPS spectra were analyzed using Thermo Scientific Avantage 5.94 software to determine differences in carbonate and/or hydroxide content at the film surface for the two different annealing methods. The C 1s spectra were fit with five peaks (linear background), four for adventitious carbon species and one for carbonate species: C-H (peak A) at 284.8 eV, C-O (peak B) at A + 1.5(± 0.3) eV, C=O (peak C) at A + 3.0(± 0.3) eV, O=C-O (peak D) at A + 4.0(± 0.3) eV, and CO3 2- (peak E) at A + 4.5(± 0.3) eV, consistent with energy ranges reported by Moulder et al.5 All peaks were constrained to have the same FWHM and Lorentzian/Gaussian (L/G) ratio, both of which were fit by the software. The FWHM and L/G ratios obtained for the 200 °C samples were 1.63 and 1.20 for the humid-annealed sample and 1.67 and 0.39 for the dry- annealed sample. The FWHM and L/G ratios obtained for the 500 °C samples were 1.59 and 0.21 for the humid-annealed sample and 1.75 and 0.19 for the dry-annealed sample. The O 1s spectra was fit with five or four peaks for the 200 and 500 °C samples, respectively. In the case of the 200 °C samples, five peaks were used, without binding energy constraints, starting from low to high binding energy: O2- from La (peak F), O2- from Zr (peak G), O2- from CO3 2- (peak H), O2- from NO3 - (peak I), and O from adventitious carbon-oxygen species (peak J). In the case of the 500 °C samples, four peaks were used, without binding energy constraints, starting from low to high binding energy: O2- from La (peak F), O2- from Zr (peak G), O2- from CO3 2- (peak H), and O from adventitious carbon-oxygen species (peak J). Peak H was constrained to three times the atomic percentage as peak E, and peak J was constrained to the atomic percentage of peaks B + C + D to account for all carbon-oxygen species. For the 200 °C samples and 116 for the 500 °C humid sample, peak H was not constrained since a larger atomic percentage was needed to get a reasonable fit, presumably due to the presence of hydroxide adding to the peak intensity. All O 1s peaks were constrained to the same FWHM and L/G ratio, which were fit by the software. The FWHM and L/G ratios obtained for the 200 °C samples were 1.53 and 6.03 for the humid-annealed sample and 1.59 and 10.26 for the dry-annealed sample. The FWHM and L/G ratios obtained for the 500 °C samples were 1.65 and 0.39 for the humid-annealed sample and 1.52 and 0.72 for the dry-annealed sample. The position of the La-O, Zr-O, CO3 2-, N-O, and C-O species are consistent with previous reports.5–9 The La 3d spectra of the humid- and dry-annealed 500 °C samples were overlaid and auto-scaled (using Avantage) in order to directly compare peak splitting and intensity ratios. To avoid background subtraction issues, quantification of La for all LZO samples was achieved by using only the La 3d5/2 peaks. 117 Figure AC5: XPS C 1s, O 1s, Cl 2s, N 1s, La 3d, and Zr 3d spectra for LZO (NO3 -/Cl-) films annealed at 200 °C Figure AC5. XPS spectra of LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 200 °C: C 1s (a,g), O 1s (b,h), Cl 2s (c, i), N 1s (d, j), La 3d (e, k), and Zr 3d (f, l). The C 1s (a, g) and O 1s (b, h) data were fit using Avantage peak fitting software, and raw data are shown in gray, envelope peak fits are shown in black, and linear backgrounds are shown in red. Individual fit peaks corresponding to specific C or O species are labeled and color-coded. 118 Table AC1: XPS atomic percentages for fitted C 1s and O 1s spectra for LZO (NO3 -/Cl-) films annealed at 200 °C C Species At. % Dry At. % Humid O Species At. % Dry At. % Humid C-H 5.5 6.1 La-O 14.4 14.2 C-O 1.6 1.3 Zr-O 17.2 20.2 C=O 0.6 0.7 CO3 - - O=C-O 0.8 1.4 CO3 + HO 11.0 7.8 CO3 0.8 1.7 N-O 13.6 16.5 C-O + C=O + O=C-O 4.5 3.5 Table AC1. C and O atomic percentages from XPS peak fitting shown in Figure S5 (see Table S2 for a complete list of atomic percentages). 119 Table AC2: XPS atomic percentages for La, Zr, O, C, Cl, and N species for LZO (NO3 -/Cl-) films annealed at 200 °C La Zr O C Cl N At. % Dry 10.3 8.3 60.6 9.4 3.5 7.9 At. % Humid 12.8 6.7 62.2 11.2 0.7 6.4 Table AC2. XPS atomic percentages for LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 200 °C. 120 Figure AC6: XPS Cl 2s, N 1s, and Zr 3d spectra for LZO (NO3 -/Cl-) films annealed at 500 °C Figure AC6. XPS spectra of LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry or humid conditions at 500 °C: Cl 2s (a, b), N 1s (c, d), and Zr 3d (e, f). 121 Table AC3: XPS atomic percentages for La, Zr, O, C, Cl, and N species for LZO (NO3 -/Cl-) films annealed at 500 °C La Zr O C Cl N At. % Dry 12.1 11.8 58.3 16.7 1.1 0 At. % Humid 14.9 10.8 61.7 12.6 0 0 Table AC3. XPS atomic percentages for LZO films deposited from a 1.0 M solution (NO3 -/Cl- precursors) and annealed under dry and humid conditions at 500 °C. 122 Figure AC7: Grazing incidence X-ray diffraction (GIXRD) data for LZO (NO3 -) films Figure AC7. GIXRD spectra for LZO films deposited from a 1.0 M solution (NO3 - precursors) and annealed at 500 °C under dry (red) or humid (blue) conditions. 123 Figure AC8: Scanning electron microscopy (SEM) images of LZO (NO3 -) films Figure AC8. SEM images of LZO films deposited from a 1.5 M solution (NO3 - precursors) and annealed at 500 °C under (a) dry or (b) humid conditions. 124 Figure AC9: Current density-electric field (J-E) data for LZO-based (NO3 -) MIS devices Figure AC9. J-E data for MIS devices fabricated from two-coat LZO (NO3 -, 1.0 M) films (~100 nm) annealed under dry (red traces) or humid (blue traces) conditions at various temperatures. 125 APPENDIX D CHAPTER VII SUPPLEMENTARY INFORMATION Figure AD1: Fourier transform infrared (FTIR) spectra Figure AD1. FTIR spectra of a single layer LZO film deposited from a 1.00 M precursor solution sequentially heated in situ to various annealing temperatures. 126 X-ray reflectivity (XRR) modeling procedure The XRR spectra were fit using Bede REFS software, which generates a solution based on an initial model input and a genetic algorithm to minimize residuals. Two types of model inputs were used: a homogeneous, single layer model and a model with multiple layers informed by HAADF-STEM studies. An approximate total thickness of 40 nm was used as an initial starting thickness for each model. For the more complex models, the thickness of the individual layers was approximated from the HAADF-STEM images. In all cases, the thickness, density, and roughness of each individual layers were allowed to vary independently. After producing the best fit, the models were perturbed to ensure that they had each settled in a global minimum. Goodness of Fit (GOF) values were output by the modeling program and indicate the quality of the model fitting. 127 Figure AD2: XRR homogeneous single layer best fit models Figure AD2. Single layer models fit to XRR data collected from (a) one, (b) two, (c) three, and (d) four coat films deposited from 1.00, 0.50, 0.33, and 0.25 M precursor solutions, respectively. 128 Table AD1: XRR single layer best fit model parameters Single Layer Best Fit Parameters Layer(s) t (nm) D (g cm-3) (g/ R (nm) 1L Film LZO 38.0 5.83 0.3 2L Film LZO 31.1 6.04 0.5 3L Film LZO 39.2 5.89 0.4 4L Film LZO 41.7 5.95 0.4 Table AD1. XRR single layer best fit model parameters [thickness (t), density (D), roughness(R)] for XRR data and models presented in Figure AD2. 129 Figure AD3: XRR inhomogeneous multilayer best fit models Figure AD3. Multilayer models fit to XRR data collected from (a) one, (b) two, (c) three, and (d) four coat films deposited from 1.00, 0.50, 0.33, and 0.25 M precursor solutions, respectively. Starting models were informed by HAADF-STEM images. 130 Table AD2: XRR multilayer best fit model parameters Multilayer Best Fit Parameters Layer(s) t (nm) D (g cm-3) (g/ R (nm) 1L Film LZO 1 1.71 5.79 0.3 LZO 2 36.2 5.36 0.1 2L Film LZO 1 0.9 5.02 0.3 LZO 2 13.1 5.85 0.1 LZO 3 1.1 5.71 0.3 LZO 4 16.1 5.89 0.2 3L Film LZO 1 1.8 5.89 0.4 LZO 2 12.6 5.71 0.1 LZO 3 1.2 6.02 0.2 LZO 4 13.1 5.78 0.3 LZO 5 1.3 6.04 0.2 LZO 6 9.2 5.86 0.1 4L Film LZO 1 1.8 6.03 0.4 LZO 2 9.9 5.63 0.7 LZO 3 1.3 6.00 0.1 LZO 4 9.8 5.62 0.8 LZO 5 1.2 6.00 0.1 LZO 6 10.0 5.69 0.8 LZO 7 1.3 6.04 0.1 LZO 8 6.6 5.91 0.3 Table AD2. XRR multilayer best fit model parameters [thickness (t), density (D), roughness(R)] for XRR data and models presented in Figure AD3. 131 Figure AD4: Energy-dispersive X-ray spectroscopy (EDS) measurements Figure AD4. Cross-sectional STEM-EDS profiles from a three layer LZO film deposited from a 0.33 M precursor solution. Intensity profiles plotted are the O Kα, La Lα, and Zr Lα signals vs. distance from the film surface (where a distance of zero is the film surface). 132 Table AD3: Electron probe microanalysis (EPMA) measurements Precursor Molarity Layers La at% Zr at% N at% O at% La/Zr (La+Zr)/O (La+Zr)/(O+N) 1.00 1 14.8(1) 14.6(1) 5.6(5) 65.1(3) 1.01(1) 0.45(1) 0.42(1) 0.50 2 15.3(2) 15.2(1) 3.8(6) 65.7(4) 1.01(2) 0.46(1) 0.44(1) 0.33 3 15.5(1) 15.1(1) 2.9(4) 66.5(3) 1.03(1) 0.46(1) 0.44(1) 0.25 4 15.9(1) 15.6(1) 2.4(6) 66.2(6) 1.02(1) 0.48(1) 0.46(1) Table AD3. Atomic ratios measured using EPMA with 95% confidence intervals from 5 EPMA measurements (uncertainty indicated in parentheses). 133 Figure AD5: Medium energy ion scattering (MEIS) data and best fit model He ions that scatter from larger masses are detected at higher energies. Therefore, the highest energy peak corresponds to ions backscattered from La in the film, while the lower energy peak corresponds to ions backscattered from Zr. Figure AD5. MEIS experimental data (black) collected from a thin two layer (~8 nm) film overlaid with the best fit model (red) indicates a surface enrichment of La whereas the amount of Zr throughout the film is essentially constant. 134 Table AD4: X-ray photoelectron spectroscopy (XPS) measurements Films were sputtered with 1 keV Ar+ ions for a total of three minutes (~3-4 nm). The composition was determined by using XPS peak area analysis in the MultiPak Software. 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