Homogeneous large-area graphene layer growth on 6H-SiC(0001) C. Virojanadara, M. Syväjarvi, R. Yakimova, and L. I. Johansson Department of Physics, Chemistry, and Biology, Linköping University, S-581 83 Linköping, Sweden A. A. Zakharov and T. Balasubramanian Maxlab, Lund University, S-22100 Lund, Sweden Received 13 August 2008; revised manuscript received 18 October 2008; published 1 December 2008 Homogeneous large-area graphene monolayers were successfully prepared ex situ on 6H-SiC0001. The samples have been studied systematically and the results are compared with those from a sample cut from the same wafer and prepared by in situ heating. The formation of smaller graphene flakes was found on the in situ prepared sample, which is in line with earlier observations. Distinctly different results are observed from the ex situ graphene layers of different thicknesses, which are proposed as a guideline for determining graphene growth. Recorded C 1s spectra consisted of three components: bulk SiC, graphene G, and interface I, the latter being a 6  3 layer. Extracted intensity ratios of G / I were found to give a good estimate of the thickness of graphene. Differences are also revealed in micro low energy electron diffraction images and electron reflectivity curves. The diffraction patterns were distinctly different from a monolayer thickness up to three layers. At a larger thickness only the graphitelike spot was visible. The electron reflectivity curve showed a nice oscillation behavior with kinetic energy and as a function of the number of graphene layers. The graphene sheets prepared were found to be very inert and the interface between the substrate and the layers was found to be quite abrupt. No free Si could be detected in or on the graphene layers or at the interface. DOI: 10.1103/PhysRevB.78.245403 PACS numbers: 73.20.-r I. INTRODUCTION Silicon carbide SiC based electronics has attracted much attention due to its excellent properties for devices operable under extreme conditions. 1 Different metals have been widely selected and intensively studied to improve the metal- SiC interface and device properties. However, silicide forma- tion at the interface and an instability of the metal-SiC de- vices operating at high temperatures have been observed. Thanks to the SiC composition it is possible to heat the SiC crystal up to elevated temperatures to sublimate the Si atoms and leave a single or few layers of graphene/graphite on top of the substrate. These layers typically have metal character with thickness-dependent properties but also the lateral ex- tent is important. 2 This is crucial for electronic devices since recent studies found that sub-10 nm graphene nanoribbons with smooth edges were obtained and demonstrated to be semiconductors with a band gap inversely proportional to the width. 3 This gives strong motivation to study how to control the thickness and homogeneity of graphene layers to be in- corporated into reduced scale devices. Yet, it is still very difficult to prepare a homogeneous large-area graphene layer, and it is unclear under which preparation conditions and at what stage of the high-temperature treatment single layers develop. Although epitaxial growth of graphene on 6H-SiC and 4H-SiC is actively pursued, achieving large graphene domains with uniform thickness remains a challenge. 4 It is also unknown what actually happens to the Si atoms if some of them remain within the graphene layer or stay on the top of that layer or even remain at the interface. In this work, we present a successful method to prepare a homogeneous large-area graphene layer. Moreover, we study its quality and indicate the characteristics of layers with dif- ferent thickness, which may be used as a guideline for graphene growth. The results presented below are obtained from low energy electron microscope LEEM, photoelec- tron spectroscopy PES, angle-resolved photoelectron spec- troscopy ARPES, low energy electron diffraction LEED, photoemission electron microscope PEEM, and from micro-LEED and micro-PES at specifically defined small ar- eas. II. EXPERIMENT The ex situ samples in this study were produced in a pro- totype of an inductively heated furnace based on the SB generation Epigress heating systems the Swedish company Epigress is a part of the Aixtron Group and production grade n-type 6H-SiC0001 substrates from SiCrystal with chemical mechanical polishing CMP on the Si face, a re- sistivity of 0.06–0.10  cm, a wafer orientation of 0 + 0.25°, and a micropipe density of 100 cm -2 were uti- lized. This system allows the formation of uniform graphene over a large area. Presently up to 2 in. size substrates are possible in the system. The equipment used for the graphene process has been modified from an earlier version which gave graphitic layers, 5 having qualities which compare quite well with those recorded for natural single crystals. The cru- cible was specially designed so that the axial and radial tem- perature gradients were minimized in order to prevent mass transfer from and to the sample. The base pressure in the reactor was 5  10 -6 mbar. The graphene growth was carried out under highly isothermal conditions at a temperature of 2000 °C and at an ambient argon pressure of 1 atm. The in situ sample in this study was prepared by heating the sample resistively to a temperature of 1275 ° C for a few minutes at a base pressure of approximately 10 -10 mbar. The substrate was cut from the same wafer as the ex situ prepared samples and pretreated the same way by RCA cleaning to remove PHYSICAL REVIEW B 78, 245403 2008 1098-0121/2008/7824/2454036 ©2008 The American Physical Society 245403-1