High-resolution x-ray analysis of graphene grown on 4H-SiC ( $000\bar 1$ 000
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Benjamin M. Capano Group 4 Development, LLC, West Lafayette, Indiana 47906
Dallas T. Morisette School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907; and Group 4 Development, LLC, West Lafayette, Indiana 47906
Alberto Salleo and Sangwon Lee Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
Michael F. Toney Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, Menlo Park, California 94025 (Received 5 July 2013; accepted 30 September 2013)
This article explores the growth of graphene under low-pressure Ar conditions. Carbon- and silicon-face 4H–SiC samples are subjected to epitaxial graphene growth at 1600 °C in vacuum, in 1 mbar argon, or in 10 mbar of argon. High-resolution x-ray scattering is used to characterize all graphene films. On the C-face, specular scans reveal a bimodal distribution of thicknesses that decrease with increasing Ar pressure. Thin and thick regions are approximately 15 and 46 monolayers in C-face graphene grown at high vacuum, 14 and 42 monolayers thick in graphene grown at 1 mbar, and 12 and 32 monolayers thick in graphene grown at 10 mbar. Azimuthal scans confirm in all cases that graphene layers are epitaxial and display expected crystallographic relationships with the underlying SiC substrate. In-plane azimuthal scans show the rotational disorder increases as pressure increases. Peaks in radial scans are asymmetric, suggesting the grain structure has a bimodal distribution of large and small domains. The sample displaying the lowest average Hall mobility (grown at 1 mbar) has the largest population of small crystallites (coherence length on the order of ;30 nm). Variations in structure and mobility of C-face graphene are attributed to inadequate control of Si sublimation during growth.
I. INTRODUCTION
Graphitization of silicon carbide (SiC) via thermal decomposition in a high-vacuum environment has been studied for about 40 years.1–4 Since the breakthrough research revealing the remarkable transport characteristics of graphene in 2004,5,6 a dramatic increase in the study of SiC thermal decomposition has occurred. Graphene synthesis by thermal decomposition of SiC exploits incongruent sublimation when the material is exposed to high temperature and low pressure. Under these conditions, the vapor in equilibrium with SiC has a Si concentration which is about four orders of magnitude greater than the carbon concentration,7,8 leading to graphitization of the surface. The underlying SiC substrate, having the space group P63mc with a hexagonal lattice, provides excellent symmetry a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.306 J. Mater. Res., Vol. 29, No. 3, Feb 14, 2014
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matching for graphene epitaxy. Graphene grown in this manner is referred to as “epitaxial graphene,”6 and possesses 2D Dirac electron properties rivaling those of ex
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