Temperature Dependence of Epitaxial Graphene Formation on SiC(0001)

LUXMI 0 1 SHU NIE 0 1 P.J. FISHER 0 1 R.M. FEENSTRA 0 1 GONG GU 0 1 YUGANG SUN 0 1 0 1.Department of Physics, Carnegie Mellon University , Pittsburgh, PA 15213, USA . 2.Sarnoff Corporation, CN5300, Princeton, NJ 08543 , USA . 3.Center for Nanoscale Materials , Argonne National Laboratory , Argonne, IL 60439, USA. 4. 1 Luxmi, Nie, Fisher, Feenstra, Gu, and Sun The formation of epitaxial graphene on SiC(0001) surfaces is studied using atomic force microscopy, Auger electron spectroscopy, electron diffraction, Raman spectroscopy, and electrical measurements. Starting from hydrogenannealed surfaces, graphene formation by vacuum annealing is observed to begin at about 1150 C, with the overall step-terrace arrangement of the surface being preserved but with significant roughness (pit formation) on the terraces. At higher temperatures near 1250 C, the step morphology changes, with the terraces becoming more compact. At 1350 C and above, the surface morphology changes into relatively large flat terraces separated by step bunches. Features believed to arise from grain boundaries in the graphene are resolved on the terraces, as are fainter features attributed to atoms at the buried graphene/SiC interface. - Graphene (one or more monolayers of carbon) has been intensively studied for the past few years because of its unique electrical behavior. Graphene exists in two main forms: isolated layers formed by exfoliation of graphite,1 and epitaxial layers residing on a suitable lattice-matched substrate.2 The size of the graphene flakes formed by the exfoliation process is relatively small, so many workers have focused on the epitaxial approach for obtaining films suitable for large-scale fabrication of circuits. There are several methods for forming epitaxial graphene, with the most studied to date being the sublimation of silicon from SiC leaving behind excess carbon in the form of graphene.2 Field-effect transistors fabricated on epitaxial graphene/SiC have yielded room-temperature field-effect mobilities of 5000 cm2/V s or more.3,4 In this work, we produce graphene by sublimation of Si from SiC(0001) (i.e., the so-called Si-face of (Received August 21, 2008; accepted September 30, 2008; published online October 21, 2008) SiC), using the well-known procedure of heating the SiC in vacuum. Use of semi-insulating SiC precludes heating by direct current, and a metal film (which would allow electron-beam heating) cannot be deposited on the backside of the wafer since this metal is found to migrate to the front of the wafer during heating.5 Furthermore, poor thermal contact between sample and heater (due to the vacuum environment) and low optical absorption of the SiC (band gap 3.0 eV, depending on polytype) necessitates temperatures as high as 1850 C for the heater itself. To accomplish this heating we have developed a simple arrangement consisting of a graphite strip, with currents as high as 200 A passing through the strip. Prior to the graphene formation, the substrates are hydrogen-etched at 1600 C in order to remove residual polishing damage. The graphite strips are found to be quite robust in this environment, unlike other heater materials that we have tested. We have investigated the formation of graphene using annealing temperatures ranging from 1100 C to 1500 C, and characterized our samples using atomic force microscopy (AFM), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), Raman spectroscopy, and electrical measurements. The evolution of the morphology is studied in particular, revealing motion of step edges, pit formation, and subsequent coarsening on the surface, and features associated with grain boundaries in the graphene as well as structure of the graphene/SiC interface. The graphite strip heater we use is contained in a dedicated ultra-high-vacuum (UHV) chamber with a base pressure of 1 9 10-10 Torr, pumped by a 150 l/s turbo-molecular pump and a hydrogen-getter pump. A graphite plate with a thickness of 1 mm and an area of 100 mm 9 75 mm is cut into a bow-tie shape, with a narrow neck of 20 mm length and 14 mm width. Two thick (dual, 9.5 mm diameter) watercooled copper feedthroughs are used to transmit the current, mounted onto large copper clamps on the two 75-mm ends of the plate. The current is supplied by a transformer capable of supplying up to 210 A at 6.3 V. Gate valves separate the turbo pump from its backing pump as well as the hydrogen-getter pump from the main chamber; these gate valves are closed and the turbo-pump is switched off for the H-etching, and they are open with the turbo-pump switched on for the graphitization. Most of our experiments have been performed on nominally on-axis, semi-insulating 4H-SiC substrates that were purchased from Cree Corp. As received, these substrates had been mechanically polished on both sides and they are epi-ready (i.e., with further polishing and a damage removal step) on the (0001) surface. Samples measuring 10 mm 9 10 mm were cut from the wafers. Hydrogen-etching was performed at 1 atm pressure, using 99.9995% purity hydrogen with a flow rate of 10 lpm and at a temperature of 1550 C for 3 min to eliminate scratches. Temperature is measured with a disappearing filament pyrometer; the pyrometer is directed at the sample, although since the sample is transparent it is mainly the heater strip that is seen. The turbo-pump is restarted a few minutes after the H-etching and the gate valve to the H-getter pump is opened shortly thereafter. The pressure reaches 1 9 10-8 Torr after pumping for about 30 min, and the annealing to form the graphene is then performed. All results refer to the surface of the sample that is facing away from the heater strip. The material used to fabricate the graphite heater strip was obtained from Poco Graphite, and is semiconductor-grade material. No measurable contamination as seen by residual gas analysis is found to be emitted during the graphitization (these measurements were performed only after the first few heating runs with the strip). The strip is found to be quite robust; we have processed >50 samples with it and it shows only a small amount of pitting on the surface as a result. In contrast, we have previously used thin (25 lm) Ta foils for the H-etching and they are found to disintegrate after each H-etching run, presumably due to embrittlement by H uptake. We also attempted the use of SiC heating strips, but they were found to be relatively brittle and cracked after one or two runs. The thickness (number of graphene monolayers) of our graphene films is determined by AES, using 5-kV incident electrons and a VG Scientific Clam 100 hemispherical analyzer. For calibration, we use a spectrum obtained from the SiC(0001) 3 9 3R30 surface as shown in Fig. 1. This surface has a known structure of Si adatoms sitting on top of a SiC bilayer with one adatom for each three SiC unit cells.6 We analyze the intensities of the C KLL line at 272 eV to the Si LMM (...truncated)