Communication pubs.acs.org/crystal
Comparison of Epitaxial Graphene Growth on Polar and Nonpolar 6H-SiC Faces: On the Growth of Multilayer Films B. K. Daas,* Sabih U. Omar, S. Shetu, Kevin M. Daniels, S. Ma, T. S. Sudarshan, and M. V. S. Chandrashekhar Department of Electrical and Computer Engineering, University of South Carolina, 301 South Main Street, Columbia, South Carolina 29208, United States ABSTRACT: We present epitaxial graphene (EG) growth on nonpolar 6H-SiC-faces by solid-state decomposition of the SiC substrate in the Knudsen flow regime in vacuum. The material characteristics are compared with those known for EG grown on polar SiC-faces under similar growth conditions. X-ray photoelectron spectroscopy (XPS) measurements indicate that nonpolar faces have thicker layers than polar faces. Among nonpolar faces, the m-plane (11̅00) has thicker layers than the aplane (112̅0). Atomic force microscopy (AFM) shows nanocrystalline graphite features for nonpolar faces, consistent with the small grain size measured by Raman spectroscopy. This is attributed to the lack of a hexagonal template, unlike on the polar Si- and C-faces. These nonpolar face EG films exhibited stress decreasing with increasing growth temperature. These variations are interpreted on the basis of different growth mechanisms on the various faces, as expected from the large differences in surface energy and step dynamics on the various SiC surfaces. Surfaces with smaller grain sizes systematically exhibited thicker layers. Using this observation, we argue that multilayer EG growth, after the nucleation of the first layers, is determined primarily by Si diffusion through grain boundaries and defects, as Si cannot diffuse through a perfect graphene lattice. A greater density of grain boundaries allows more Si to escape during growth, allowing thicker layers of carbon to be grown.
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2220 erg/cm2 on the Si-face and 300 erg/cm2 on the Cface.10,11 This difference in surface free energy is expected to lead to different growth mechanisms, as the growth process tends to minimize the surface free energy.12 Epitaxial graphene (EG) growth by thermal decomposition of SiC at high temperature can be achieved in two different ways: (i) growth at high pressure13−16 and (ii) growth at high vacuum (low pressure) [see. e.g., refs 6, 8, and 17]. In both these techniques, Si sublimes off and the remaining carbon rearranges in a honeycomb 2D pattern, registered to the substrate. It has been reported that good quality EG films on polar SiC substrate have been grown at high pressure.14−16 The key insight is that Si sublimation slows down at high pressure (usually Ar environment), which allows carbon atoms sufficient time to rearrange themselves to form a good quality crystal. The slower Si-out diffusion rate necessitates higher growth temperatures (by ∼300−400 K) than with vacuum, which also leads to significant step bunching in the underlying SiC substrate, leading to surface steps >10 nm in some cases15,18 on the Si-face. The lower temperatures in vacuum growth prevent step-bunching, potentially leading to smoother surfaces, a potential advantage of vacuum growth over high pressure growth.
INTRODUCTION Graphene is a two-dimensional (2D) carbon honeycomb crystal, the basic building block of other sp2 carbon nanomaterials, such as nanographite sheets and carbon nanotubes, which exhibits unusual electronic and optical properties.1−6 The charge carriers are “massless” Dirac fermions resulting in high electron mobility.3,5 Additionally, other important characteristics such as high crystalline quality, high thermal conductivity and stiffness, room temperature quantum hall effect, and ballistic transport properties have generated enormous interest.6 Since its discovery, graphene has been produced using mechanical exfoliation, where graphene layers are peeled off layer by layer using Scotch tape.1,7 This technique can provide single layer graphene with high structural and electronic properties, but it is not suitable for large-scale production due to poor yield, repeatability, lack of process control, and small sample size. As an alternative, large area epitaxial graphene (EG) is grown by thermal decomposition of the polar c-plane of 4H or 6H SiC in ultrahigh vacuum or Ar environment at high temperatures. In this technique, Si sublimes off from the SiC substrate, leaving behind carbon atoms which rearrange themselves into a graphene layer.8 The growth mechanism of EG on SiC is still a current issue of research because it involves high temperatures with three key steps: (i) silicon desorption, (ii) carbon diffusion, and (iii) island nucleation.9 It is known that different surfaces of SiC have very different surface free energies, e.g., © 2012 American Chemical Society
Received: April 3, 2012 Revised: May 18, 2012 Published: June 6, 2012 3379
dx.doi.org/10.1021/cg300456v | Cryst. Growth Des. 2012, 12, 3379−3387
Crystal Growth & Design
Communication
Figure 1. Inside architecture for epitaxial graphene growth in high vacuum. The large amount of graphite foam surrounding the graphite crucible, as well as the large thermal mass of the graphite crucible minimizes the thermal gradients (∼1 °C/mm) and thermal transients in the system. This design is similar to that used in the bulk growth of SiC single crystals.
geometry. Further details are given in the Experimental Details (see Figure 1). We note that our growth conditions were optimized for Siface growth to produce uniform EG bilayers of quality as good or better than those grown using Ar-mediated growth at high pressure, with domains as large as tens of square micrometers.17 However, the growth on the C-face is of poorer quality than with Ar-growth, as the Si-loss rate is much higher in vacuum, due to defects in the EG, as we will show later. Nevertheless, this study is still of importance, as the Si mass transport changes f rom Knudsen transport in vacuum to Boltzmann dif f usion at high pressures. In this paper, we examine the growth of EG in the Knudsen transport regime (eliminating the Si out-diffusion bottleneck at high pressure) and illustrate the critical role that defects and grain boundaries play in mediating the growth of multilayer films. A detailed understanding of the Knudsen transport regime should enable further optimization of the growth even at higher pressures. For the polar c-plane Si-face, EG growth is due to step flow, as has been discussed in detail elsewhere.9 Studies on the C-face indicate there are two growth modes on the lower energy Cface, one from step flow21 and another from nucleation at higher energy defects.12,21 The role of defects on the C-face EG growth has also been discussed by Hicks et al.22 As defects are associated with higher energies than the stable C-face surface (which has a low free energy), the defect sites are more favorable to Si sublimation and EG growth. Both these modes play a significant role and are also likely sensitive to the growth conditions (pressure, temperature, ambient, etc.). As the above references13,21,23 provide detailed insight into nucleation of carbon on these faces, we do not discuss it in detail in this paper. Rather, we focus on how multilayer EG films grow on SiC, assuming that the initial layers have nucleated. If a perfect crystal of graphene is nucleated on SiC, no further growth is expected, as the graphene lattice is so tight that no Si can diffuse through the grown graphene layer to enable subsequent multilayer EG growth. This can be seen geometrically (Figure 2b), as the free space between carbon atoms in graphene (accounting for the electron cloud radius of ∼0.7 Å/C atom) is
Si sublimation in vacuum can also be slowed down or even completely suppressed using a background pressure of silicon vapor, driving the SiC(s) → Si(g) + C(s) equilibrium backward, with typical equilibrium vapor pressures of
