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Defect and Metal Oxide Control of Schottky Barriers And DEFECT AND METAL OXIDE CONTROL OF SCHOTTKY BARRIERS AND CHARGE TRANSPORT AT ZINC OXIDE INTERFACES DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Geoffrey Michael Reimbold Foster Graduate Program in Physics The Ohio State University 2018 Dissertation Committee: Professor Leonard J. Brillson Adviser Professor Thomas Lemberger Professor Ilya Gruzberg Professor Robert Perry Copyrighted by Geoffrey Michael Reimbold Foster 2018 ABSTRACT In recent years ZnO has received renewed interest due to its exciting semiconductor properties and remarkable ability to grow nanostructures. ZnO is a wide band gap semiconductor, allowing many potential future applications including electronic nanoscale devices, biosensors, blue/UV light emitters, and transparent conductors. There are many potential challenges that keep ZnO from reaching a full device potential. The biggest challenge is what the role native point defects play in the fabrication of high quality Ohmic and Schottky contacts. The following work examines the impact of these native point defects in the formation of Schottky barriers and charge transport at ZnO interfaces. We have used depth-resolved cathodoluminescence spectroscopy and nanoscale surface photovoltage spectroscopy to measure the dependence of native point defect energies and densities on Mg content, band gap, and lattice structure in non-polar, single- phase MgxZn1-xO (0<x<0.56) alloys grown by molecular beam epitaxy (MBE) on r-plane sapphire substrates. Based on this wide range of alloy compositions, we identified multiple deep level emissions due to zinc and oxygen vacancies whose densities exhibit a pronounced minimum at ~45% Mg corresponding to similar a and c parameter minima at ~52%. This minimum also corresponds to a pronounced change in Schottky barriers reported previously. ii DRCLS allows us to probe buried interfaces. Due to a strong Fermi-level mismatch, about 10% of the electrons in a 5-nm-thick highly-Ga-doped ZnO (GZO) layer grown by molecular beam epitaxy at 250 C on an undoped ZnO buffer layer transfer to the ZnO (Debye leakage), causing the measured Hall-effect mobility (H) of the GZO/ZnO combination to remarkably increase from 34 cm2/V-s, in thick GZO, to 64 2 cm /V-s. From previous characterization of the GZO, it is known that ND = [Ga] = 1.04 x 21 20 -3 10 , and NA = [VZn] = 1.03 x 10 cm , where ND, NA, and [VZn] are the donor, 19 acceptor, and Zn-vacancy concentrations, respectively. In the ZnO, ND = 3.04 x 10 , and 18 -3 2 NA = 8.10 x 10 cm . Assuming the interface is abrupt, theory predicts H = 61 cm /V-s, with no adjustable parameters. The assumption of abruptness in [Ga] and [VZn] profiles is confirmed directly with a differential form of depth-resolved cathodoluminescence spectroscopy coupled with X-ray photoelectron spectroscopy. An anneal in Ar at 500 C for 10 min somewhat broadens the profiles but causes no appreciable degradation in H and other electrical properties. Using our ability to probe abrupt buried interfaces, we probed the IrOx/ZnO interface. IrOx and other metal oxides exhibit higher Schottky barriers than their pure metal counterparts, consistent with wider depletion regions and potentially useful for ohmic contacts to p-type semiconductors. DRCLS with I-V and 1/C2-V barrier height and carrier profile measurements showed high zinc vacancy VZn and CuZn defect densities that compensate free carrier densities, increase depletion widths, and form higher effective barriers than Ir/ZnO contacts. Zn-polar versus O-polar ZnO interfaces with IrOx exhibit 40% higher VZn + CuZn interface segregation and lower carrier densities within a iii wider depletion region, accounting for the significantly higher (0.89 vs. 0.67 eV) barrier heights. The depth of VZn density segregation and the Zn-deficient layer thickness measured microscopically both match the depletion width, and applied electric fields comparable to spontaneous polarization fields across similar layers display analogous defect segregation. These results account for the difference in polarity-dependent segregation due to the electric field-driven diffusion of native defects near ZnO interfaces. By establishing the role that defects play in the effective barrier height we attempted to form Ohmic and Schottky contacts to ZnO nanowires. Ohmic and rectifying metal contacts to semiconductor nanowires are integral to electronic device structures and typically require different metals and process techniques to form. A Pt ion beam alone can form Ohmic, Schottky, or blocking contacts to ZnO nanowires with the same metal on the same wire by controlling native point defects at the intimate metal-semiconductor interface. Spatially- resolved cathodoluminescence spectroscopy both laterally and in depth gauges the nature, density, and spatial distribution of specific native point defects inside the nanowires and at their metal interfaces. Combinations of electron and ion beam deposition, annealing, and outer diameter milling of the same pulsed laser deposited nanowire provide either low contact resistivity (2.6 x 10-3 Ω-cm-2) ohmic, Schottky (Φ > 0.35 eV) or blocking junctions with single Pt deposition, depending on the physical nature and spatial distribution of substitutional Cu on Zn sites, zinc vacancy, and oxygen vacancy defects. These results demonstrate the importance of point defects on electrical iv properties of metal contacts to ZnO nanowires and present methods to tailor contact electronic properties of nanowires in general. v ACKNOWLEDGMENTS This work would not be possible if not for the support and mentorship of Dr Leonard J. Brillson. I would like to thank him for his patience advice and guidance throughout my time as a graduate student. I would also like to thank, and in no particular order, Thaddeus Asel, Jon Cox, Brent Noesges, Hantian Gao, and all of the Columbus School for Girls interns who all assisted this work in one way or another. Through our success and failures we have advanced the study of ZnO. Without all of you the quality and speed of this work would have been greatly diminished. Lastly I would like to thank my family for their continual love and support. My wife Melissa Foster, who put up with the strange hours graduate school sometimes requires you while continually being my biggest cheerleader. My mom, Tracy Reimbold, who in primary school always pushed me to greater academic heights and never let me settle with 'just A's'. And my father, Tom Foster, who constantly encouraged me to take chances and just 'go for it' that it is ok to fail as long as you try. vi VITA 2009................................................................Streetsboro High School 2013................................................................B.S. Physics, University of Akron 2015................................................................M.S. Physics, The Ohio State University 2015 to present ..............................................Graduate Research Associate, Department of Physics, The Ohio State University PUBLICATIONS Foster, Geoffrey M., Hantian Gao, Grace Mackessy, Alana M. Hyland, Martin W. Allen, Buguo Wang, David C. Look, and Leonard J. Brillson. 2017. “Impact of Defect Distribution on IrOx/ZnO Interface Doping and Schottky Barriers.” Applied Physics Letters 111 (10). AIP Publishing: 101604. doi:10.1063/1.4989539. Foster, G. M., Faber, G., Yao, Y. F., Yang, C. C., Heller, E. R., Look, D. C., & Brillson, L. J. (2016). Direct measurement of defect and dopant abruptness at high electron mobility ZnO homojunctions. Applied Physics Letters, 109(14). http://doi.org/10.1063/1.4963888. Dordevic, S V, G M Foster, M S Wolf, N Stojilovic, H Lei, C Petrovic, Z Chen, Z Q Li, and L C Tung. (2016). “Fano Q-Reversal in Topological Insulator Bi2Se3.” Journal of vii Physics Condensed Matter 28 (16). https://doi.org/10.1088/0953-8984/28/16/165602. Foster, G. M., Perkins, J., Myer, M., Mehra, S., Chauveau, J. M., Hierro, a., & Brillson, L. J. (2015). Native point defect energies, densities, and electrostatic repulsion across (Mg,Zn)O alloys. Physica Status Solidi (a), 212(7), 1448–1454. http://doi.org/10.1002/pssa.201532285. Perkins, J., G. M. Foster, M. Myer, S. Mehra, J. M. Chauveau, A. Hierro, A. Redondo- Cubero, W. Windl, and L. J. Brillson. (2015). “Impact of Mg Content on Native Point Defects in MgxZn1-xO (0 ≤ X ≤ 0.56).” APL Materials 3 (6):62801. https://doi.org/10.1063/1.4915491. Dordevic, S. V., G. M. Foster, N. Stojilovic, E. A. Evans, Z. G. Chen, Z. Q. Li, M. V. Nikolic, Z. Z. Djuric, S. S. Vujatovic, and P. M. Nikolic. (2014). “Magneto-Optical Effects in 1-xAsx with x=0.01: Comparison with Topo- Logical Insulator Bi1-xSbx with x=0.20.” Physica Status Solidi (B) Basic Research 251 (8):1510–14. https://doi.org/10.1002/pssb.201451091. FIELDS OF STUDY Major Field: Physics viii TABLE OF CONTENTS Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. vi Vita .................................................................................................................................... vii Publications ....................................................................................................................... vii Table of Contents ..............................................................................................................
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