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Quantum Well State of Cubic Inclusions in Hexagonal Silicon Carbide Studied with Ballistic Electron Emission Microscopy Dissertation

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Quantum Well State of Cubic Inclusions in Hexagonal Silicon Carbide Studied with Ballistic Electron Emission Microscopy Dissertation QUANTUM WELL STATE OF CUBIC INCLUSIONS IN HEXAGONAL SILICON CARBIDE STUDIED WITH BALLISTIC ELECTRON EMISSION MICROSCOPY DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Yi Ding, M.S. * * * * * The Ohio State University 2004 Dissertation Committee: Approved by: Professor Jonathan P. Pelz, Adviser Professor Leonard J. Brillson Professor David G. Stroud Adviser Professor Gregory P. Lafyatis Department of Physics ABSTRACT SiC is a polytypic material that may crystallize in many different close-packing sequences with cubic, hexagonal, or rhombohedral Bravais lattices. All SiC polytypes have wide bandgaps ranging from 2.39 eV in cubic SiC to 3.023 – 3.330 eV in common hexagonal polytypes. This, as well as many other properties favorable to electrical applications, makes SiC a very promising material in electronic device fabrication. However, the many lattice stacking sequences may impair the stability of SiC devices. In the hexagonal 4H polytype, it has been found that thin cubic SiC inclusions may be formed due to stacking fault expansion, and it has been proposed that the inclusions may behave as quantum wells because of the lower bandgap of cubic SiC. We performed ultra-high vacuum ballistic electron emission microscopy (BEEM) measurements on n-type 4H-SiC samples containing double-stacking-fault cubic inclusions to characterize the electrical properties of individual inclusions. Thin Pt films are deposited in ultra-high vacuum on the sample surfaces to form Schottky contacts. A Schottky barrier height of ~1.01 eV is observed over the inclusions in a background of normal 4H-SiC barrier height of 1.54 eV, which directly confirms the cubic inclusions support two-dimensional propagating quantum well states, and the 0.53 eV lowering of ii barrier height indicates the two-dimensional conduction band minimum is located at ~0.53 eV below the conduction band minimum of bulk 4H-SiC. We also used BEEM to study the Schottky contact between Pt and p-type 4H-SiC, and observed a second transmission channel in the BEEM spectrum that suggests a split- off valence band at ~0.11 eV below the valence band maximum. We also measured the barrier heights of p-type and n-type Schottky contacts prepared under identical conditions and the results suggest the existence of an interfacial layer. An earlier study of threading dislocations in GaN using BEEM is also described. Although threading dislocations in GaN had been generally thought to be negatively charged prior to our study, our high-resolution BEEM measurements yielded no evidence of significant charge along the dislocations near the interface between Pt films and our molecular beam epitaxy grown GaN sample. iii Dedicated to my wife iv AKNOWLEDGMENTS First, I wish to thank my adviser, Professor Jonathan P. Pelz, for the tremendous help he has provided to me over my seven years of studies at Ohio State and in his lab. I have learned so much from him that cannot be elaborated here. He has a keen insight in physics and has many times come up with great ideas in experimental methods and data analysis involved in my research projects that originally might not have conceivable by me myself. Now, after his training and mentoring, I have better ability of developing new ideas in my scientific research, which will benefit me throughout my future careers. I also want to thank my lab colleague Benjamin Kaczer, Hsung-Jai Im, Jon- Fredrik Nielsen, Eric Heller, David Lee, Brian Gibbons, Cristian Tivarus, and Kibog Park. It has been a really enjoyable experience to work with these nice people, who are always so helpful. I have learned a lot in physics as well as in other areas through discussions with them. In particular, Hsung Jai Im carefully and patiently trained me to use the equipment needed in my research, and Kibog Park worked together with me on my Ph.D. projects and helped me collect data and perform calculations and modeling. I am very grateful to them for the guidance and assistance. Finally, I want to thank my wife Xiaohuang Fang for her love and support. v VITA June 17, 1976 . Born – Shijiazhuang, China 1994 – 1997 . Undergraduate student, Tsinghua University, China 2003 . M. S. Physics, The Ohio State University 1997 – present . Graduate teaching and research associate, The Ohio State University PUBLICATIONS 1. Y. Ding, K.-B. Park, J. P. Pelz, K. C. Palle, M. K. Mikhov, B. J. Skromme, H. Meidia and S. Mahajan, “Quantum well state of self-forming 3C-SiC inclusions in 4H- SiC determined by ballistic electron emission microscopy,” Phys. Rev. B 69, 041305 (2004). 2. Y. Ding K.-B. Park, J. P. Pelz, A. V. Los, and M. S. Mazzola, “Ballistic electron emission microscopy study of p-type 4H-SiC,” Mater. Sci. Forum 457-460, 1077 (2004). 3. Calculated potential profile near charged threading dislocations at metal/ semiconductor interfaces, C. Tivarus, Y. Ding, and J.P. Pelz, J. Appl. Phys. 92, 6010 (2002). 4. Characterization of individual threading dislocations in GaN using Ballistic Electron Emission Microscopy, H.-J. Im, Y. Ding, J. P. Pelz, B. Heying, and J. S. Speck, Phys. Rev. Lett. 87, 106802 (2001). 5. Nanometer-scale test of the Tung model of Schottky-barrier height inhomogeneity, H.-J. Im, Y. Ding, J. P. Pelz, and W. J. Choyke, Phys. Rev. B 64, 075310 (2001). vi FIELDS OF STUDY Major Field: Physics vii TABLE OF CONTENTS Page Abstract . ii Dedication . iv Acknowledgments . v Vita . vi List of Tables . xi List of Figures . xii Chapters: 1. Overview . 1 2. Silicon Carbide . 5 2.1 Polytypism . 6 2.1.1 Tetrahedrally Bonded Structures . 6 2.1.2 Close-Packing Possibilities . 10 2.1.3 SiC Polytypes . 15 2.1.4 Bravais lattice of Close-Packed Structures . 20 2.1.5 Why So Many Polytypes in SiC? . 25 2.2 Properties of SiC Polytypes . 29 2.2.1 Lattice Constant . 30 2.2.2 Bandgap and Band Structure . 32 2.2.3 Spontaneous Polarization . 35 2.3 Inclusions in SiC . 43 2.3.1 Stacking Faults and Partial Dislocations . 43 2.3.2 Cubic Inclusions in 4H-SiC . 49 2.3.3 Experimental Observation of Self-Forming Inclusions . 51 2.3.4 Quantum Well States of the Inclusions . 53 3. Experimental Methods . 56 3.1 Schottky Barrier . 56 viii 3.2 Ballistic Electron Emission Microscopy . 59 3.2.1 Schematics of BEEM . 60 3.2.2 BEEM Spectrum . 62 3.2.3 BEEM Imaging . 63 3.2.4 Bell-Kaiser Model . 65 3.3 Equipment Setup . 68 3.3.1 Sample Preparation . 69 3.3.2 Measurements in the STM/BEEM Chamber . 70 3.3.3 STM Tip Preparation and Modification . 73 3.3.4 Micro Inchworm . 76 3.4 Data Acquisition and Processing . 77 3.4.1 STM/BEEM Software . 78 3.4.2 Raw Data Processing . 80 4. Results . 81 4.1 Double-Stacking-Fault Cubic Inclusions in 4H-SiC . 81 4.1.1 Sample Processing and Characterization by Collaborators . 82 4.1.2 Identifying the Cubic Inclusions Using BEEM . 84 4.1.3 Quantum Well State of the Inclusions . 91 4.1.4 Electron Scattering in the Metal Film . 98 4.1.5 Electron Scattering at the Inclusion/4H-SiC Interface . 100 4.1.6 Geometry of the Inclusion Opening at the M/S Interface . 101 4.1.7 Possible Evidence of a Wider Inclusion and Deeper Well . 104 4.1.8 Second Transmission Channel in Some Inclusion Spectra . 106 4.2 BEEM Study of p-type 4H-SiC . 107 4.2.1 Sample Preparation . 107 4.2.2 Valance Band Structure of 4H-SiC . 110 4.2.3 Interfacial Oxide Layer at the Pt/4H-SiC Interface . 112 4.3 Threading Dislocations in GaN . 118 4.3.1 Sample Preparation and Barrier Height Determination . 119 4.3.2 Identifying the Threading Dislocations . 120 4.3.3 Expected Effect of Significant Dislocation Charge . 122 ix 4.3.4 BEEM Measurements Across the Threading Dislocations . 124 4.3.5 Upper limit of Dislocation Charge Density Near the Interface . 130 4.3.6 Upper Limit of Acceptor Level Depth . 132 5. Conclusions . 135 5.1 Major Findings . 135 5.1 Future Directions . 137 Appendices . 139 A Description of the Tip Locking Algorithm . 139 B Conversion of Binary STM/BEEM Data Into Text File . 142 List of References . 144 x LIST OF TABLES Table Page 2.1 Comparison of calculated interlayer interaction parameters in SiC . 28 2.2 Comparison of calculated and experimentally measured lattice constants of several common SiC polytypes . 31 xi LIST OF FIGURES Figure Page 2.1 Unit cell of an fcc lattice and a diamond/zincblende lattice . 7 2.2 Unit cell of a simple hexagonal lattice, an hcp lattice, and a wurtzite.
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