EPITAXIAL GRAPHENE FILMS ON SIC: GROWTH, CHARACTERIZATION, AND DEVICES A Thesis Presented to The Academic Faculty by Xuebin Li In Partial Ful¯llment of the Requirements for the Degree Doctor of Philosophy in the School of Physics Georgia Institute of Technology August 2008 EPITAXIAL GRAPHENE FILMS ON SIC: GROWTH, CHARACTERIZATION, AND DEVICES Approved by: Professor Walter A. de Heer, Advisor Professor Phillip N. First School of Physics School of Physics Georgia Institute of Technology Georgia Institute of Technology Professor Thomas Orlando Professor Mei-Yin Chou School of Physics School of Physics School of Chemistry and Biochemistry Georgia Institute of Technology Georgia Institute of Technology Professor James D. Meindl Date Approved: April 21, 2008 School of Electrical and Computer Engineering Georgia Institute of Technology To my wife, Qinyi and my daughter, Kathleen iii ACKNOWLEDGEMENTS When I joined the Ph.D. program in School of Physics at Georgia Institute of Technology in Spring 2002, I could not imagine how much I could bene¯t from the Ph.D program training and how much I could achieve in research. Now what I have achieved is far beyond what I could imagine six years ago. After a 10 minutes' talk with Prof. Walter de Heer, I joined his group in August 2003. It turned out to be a wonderful and exciting experience for me to participate in the pioneering work of Epitaxial Graphene from the beginning. Under his supervision, I have many opportunities to expose myself to di®erent research focuses with di®erent technical skills. More than that, what I learn from him is his great passion for science, acute intuition to unclear research issues, and unusual viewpoints to identify and resolve puzzles. I believe that is why he is always one of the great pioneers in several research ¯elds: Carbon Nanotubes, Epitaxial Graphene, and Cluster Physics. I would like to thank him for his continuous ¯nancial support and research instruction during my thesis work. I would also like to thank people I work with in the Lab of Epitaxial Graphene, including Dr. Claire Berger, Dr. Zhimin Song, Dr. Xiaosong Wu, Nate Brown, Mike Sprinkle, and Fan Ming. I give my special acknowledgement to Dr. Claire Berger not only for her tremendous help in my research, but also her care and encouragement every moment. I also bene¯t a lot from uncountable and meaningful discussions with Dr. Xiaosong Wu and Dr. Zhimin Song. Many collaborators have done a lot of surface study on my graphitized SiC samples in the past several years. Their results strongly support my thesis work from many aspects. They are Prof. Phillip N. First, Prof. Edward H. Conrad, Prof. Thomas Orlando, Dr. Tianbo Li, Joanna Hass, Nikhil Sharma, Kristin Shepperd, Lan Sun, Daniel Ugarte. I would like to thank many collaborators in CNRS, Grenoble, France for their signi¯cant work in infrared Landau level spectroscopy, Raman spectroscopy, and other studies on the iv samples we provided. I would like to thank people I collaborated for graphene device fabrication in the Micro- electronic Research Center at Gatech. Without their support, it is hard for me to achieve so much in the thesis work. They include Prof. James Meindl, Dr. Raghu Murali, Farhana Zaman, Devin Brown, and other cleanroom sta®. Finally, I want to give my great appreciation to my parents and my family. For my parents, Guotong Li and Naiqing Wang, their tremendous support and patience for my whole student career makes them the best parents in my world. For my wife, Qinyi Wu, her support and push on the completion of my Ph.D. program makes her so special and outstanding from most of her peers in my eyes. For my sweet daughter, Kathleen, you not only brings the angel's smile to us, but also makes us understand the meaning of a family. I wish this thesis could make all of you proud. v TABLE OF CONTENTS DEDICATION . iii ACKNOWLEDGEMENTS . iv LIST OF TABLES . x LIST OF FIGURES . xi SUMMARY . xvi I INTRODUCTION . 1 II INTRODUCTION TO GRAPHITE . 4 2.1 Crystal Structure of Bulk Graphite . 4 2.2 Band Structure of Bulk Graphite . 6 2.3 Transport Properties of Bulk Graphite . 8 2.4 Summary . 10 III GRAPHENE: THEORY AND POSSIBLE APPLICATIONS . 11 3.1 Band Structure of Graphene . 11 3.1.1 Single-layer Graphene . 11 3.1.2 Few-layer Graphene . 14 3.2 Ways to Obtain Graphene . 15 3.2.1 Mechanical Cleavage . 15 3.2.2 Epitaxial Graphene Growth on Silicon Carbide . 17 3.3 Transport Properties of Graphene . 17 3.3.1 Anomalous Quantum Hall E®ect in Single-layer Graphene . 18 3.3.2 Anomalous Quantum Hall E®ect in Bi-layer Graphene . 21 3.3.3 Minimal Conductivity . 21 3.3.4 High Field Degeneracy Splitting . 22 3.3.5 Suppression of Weak Localization . 23 3.4 Graphene Devices and Possible Applications . 24 3.4.1 Graphene Nanoribbons (GNRs) . 25 3.4.2 2D Graphene Field E®ect Transistor and Band gap: Experimental Review . 28 vi 3.4.3 Graphene Spintronics . 29 3.4.4 Superconductivity . 30 3.4.5 Suspended Graphene . 31 3.4.6 Key Issues for Graphene Applications . 31 3.5 Summary . 33 IV SILICON CARBIDE . 34 4.1 Crystal Structure and Polytype of Silicon Carbide . 34 4.2 Physical and Electronic Property of Silicon Carbide . 38 4.3 Growth of Silicon Carbide . 39 4.3.1 Bulk Growth of Silicon Carbide . 39 4.3.2 Epitaxial Growth of Silicon Carbide . 41 4.4 Surface Flattening of Silicon Carbide . 44 4.5 Summary . 45 V HYDROGEN ETCHING OF SILICON CARBIDE . 46 5.1 Mechanism of Hydrogen Etching . 46 5.2 Experiment Equipment and Procedure . 48 5.2.1 Design of the Hydrogen Etching System . 48 5.2.2 Sample Preclean and Etching Experiment Procedure . 50 5.3 Result and Discussion . 53 5.3.1 On-axis and O®-axis 6H-SiC . 53 5.3.2 On-axis 4H-SiC . 58 5.4 Summary . 62 VI GRAPHITIZATION OF SILICON CARBIDE . 63 6.1 Mechanism of SiC Graphitization . 63 6.1.1 Graphitization in UHV . 63 6.1.2 Graphitization in a High Vacuum Induction Furnace . 66 6.2 Experiment Equipment and Procedure . 68 6.2.1 Design of the Graphitization System . 68 6.2.2 Graphitization Procedure . 68 6.3 Result and Discussion . 68 vii 6.3.1 Graphitization of the Si Face of 6H-SiC . 69 6.3.2 Graphitization of the 4H-SiC . 71 6.3.3 Thickness Measurement of Epitaxial Graphene . 76 6.3.4 Comparison of Epitaxial Graphene Between the C face and the Si face of 4H-SiC . 77 6.3.5 STM on Epitaxial Graphene . 79 6.3.6 Cross-section View of Graphene by HRTEM . 80 6.3.7 X-ray Reflectivity Experiment on Epitaxial Graphene . 81 6.4 Summary . 82 VII FABRICATION OF EPITAXIAL GRAPHENE DEVICES . 84 7.1 Lithography . 84 7.1.1 Photolithography . 84 7.1.2 E-beam Lithography . 87 7.2 Semiconductor Processing Techniques: Deposition and Etching . 90 7.2.1 Film Deposition . 90 7.2.2 Etching . 91 7.3 Fabrication of Field E®ect Transistors (FETs) . 93 7.3.1 Top Gated FETs . 93 7.3.2 Side Gated FETs . 94 7.3.3 Other Epitaxial Graphene Devices . 103 7.4 Conclusion . 103 VIII TRANSPORT PROPERTIES OF EPITAXIAL GRAPHENE . 104 8.1 Landau Level Spectroscopy . 104 8.2 Transport Properties of Epitaxial Graphene . 106 8.2.1 Shubinkov de Haas (SdH) Oscillations and Weak Anti-localization (WAL) . 107 8.2.2 Long Phase Coherence and Quantum Con¯nement . 111 8.3 Electric Field E®ect on Epitaxial Graphene . 113 8.3.1 Top Gating E®ect . 113 8.3.2 Side Gating E®ect . 119 8.4 Summary . 121 viii REFERENCES . 122 VITA ............................................ 141 ix LIST OF TABLES 4.1 Properties of SiC polytypes in comparison with Si and GaAs at room tem- perature. [27, 46, 143, 248] . 38 5.1 The list of SiC wafers. 51 x LIST OF FIGURES 2.1 Crystal lattice structure of graphite (Bernal). 5 2.2 The stacking sequences of three graphite structures. (a) Bernal(ABAB); (b) rhomboheral (ABCABC). 5 2.3 Graphite Brillouin zone with several high symmetry points. Electron and hole Fermi surfaces are located along the edges of HKH and H'K'H' [43]. 7 2.4 Electronic energy bands near the H-K-H axis in the three-dimensional graphite as obtained from the SWMcC band model. The E3 band is doubly degen- erate along the H-K-H axis (See center ¯gure) and is lifted when away from the H-K-H axis (See left-hand and right-hand ¯gures.) [43]. 8 3.1 (a) Graphene hexagonal lattice. a1 and a2 are the lattice vectors. (b) Sketch of the ¯rst Brillouin zone in the reciprocal lattice. 12 3.2 Band structure of graphene. (a) The energy of the conduction band and the valence band as a function of wavevector K . (b) The energy dispersion relation near the Dirac points (K, K'). 13 3.3 Schemes of three types of quantum Hall e®ects [156]. Red line is Hall con- ductivity as a function of Landau level N. Blue (electron) and orange (hole) regions are state densities as a function of Landau level N. g is the degeneracy for each type of QHE. (a) Integer QHE in normal 2D semiconductor systems. (b) Half-integer QHE in single-layer graphene. (c) Anomalous integer QHE in bi-layer graphene. 20 3.4 The edge types of graphene nanoribbons. N is the ribbon width. (a).