Stacking of adjacent graphene layers grown on C-face SiC

Abstract

Graphene was grown on the C-face of nominally on-axis SiC substrates using high-temperature sublimation with Ar as the buffer inert gas. The results of studies of the morphology, thickness, and electronic structure of these samples using low-energy electron microscopy (LEEM), x-ray photoelectron emission microscopy, photoelectron spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), and low-energy electron diffraction (LEED) are presented. The graphene thickness is determined to vary from 1 or 2 to 6 or 7 monolayers (MLs), depending on the specific growth conditions utilized. The formation of fairly large grains (i.e., crystallographic domains) of graphene exhibiting sharp 1\ifmmode\times\else\texttimes\fi{}1 spots in micro-LEED is revealed. Adjacent grains are found to show different azimuthal orientations. Macro-LEED patterns recorded mimic previously published, strongly modulated, diffraction ring LEED patterns, indicating contribution from several grains of different azimuthal orientations. We collected selected area constant initial energy photoelectron angular distribution patterns that show the same results. When utilizing a small aperture size, one Dirac cone centered on each of the six K-points in the Brillouin zone is clearly resolved. When using a larger aperture, several Dirac cones from differently oriented grains are detected. Our findings thus clearly show the existence of distinct graphene grains with different azimuthal orientations; they do not show adjacent graphene layers are rotationally disordered, as previously reported for C-face graphene. The graphene grain size is shown to be different on the different samples. In some cases, a probing area of 400 nm is needed to detect the grains. On one sample, a probing area of 5 \ensuremath{\mu}m can be used to collect a 1\ifmmode\times\else\texttimes\fi{}1 LEED pattern from a multilayer graphene grain. ARPES is used to determine the position of the Dirac point relative to the Fermi level on two samples that LEEM shows have dominant coverage of 2 and 3 MLs of graphene, respectively. The Dirac point is found to be located within 75 meV of the Fermi level on both samples, which indicates that the electron carrier concentration induced in the second and third graphene layers on the C-face is less than \ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}10${}^{11}$ cm${}^{\ensuremath{-}2}$. Formation of patches of silicate is revealed on some samples, but the graphene formed on such nonhomogenous surfaces can contain fairly large ordered multilayer graphene grains.