Interaction of Nitrogen With 6H-SiC Surfaces
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V. VAN ELSBERGEN, M. ROHLEDER, W. MONCH Laboratorium ffir Festkbrperphysik, Gerhard-Mercator-Universitiit Duisburg, D-47048 Duisburg, Germany ABSTRACT The interaction of nitrogen with 6H-SiC surfaces with different orientation and composition ranging from Si-rich to graphitized was investigated by use of X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). The samples were cleaned and their surface composition was tuned by heating them in a Si flux at different temperatures or by annealing. Nitrogen exposures were performed using a RF plasma source operated at 13.56 MHz. Two different nitridation mechanisms can be distinguished by XPS. While anion exchange occurs at bulk-truncated SiC surfaces diffusion will control the nitrogen uptake of Si-rich samples. After nitridation, UPS spectra of bulk--truncated surfaces display characteristic valence-band structures of Si 3 N 4. The thickness of the nitride layer amounts to approximately 9-12 A. Thicker silicon nitride films on SiC are obtained by rising the sample temperature during nitrogen exposures or by deposition and subsequent nitridation of thin Si overlayers on SiC surfaces. The nitrogen uptake of graphitized samples is comparable to that of bulk-truncated surfaces. Our photoemission spectroscopy results prove that surface graphite is partly removed by N and the UPS spectra point to the formation of silicon nitride. In addition, C-N bonds exist but the exact bonding configuration cannot be resolved. INTRODUCTION Silicon nitride layers were grown on SiC by chemical vapour deposition [1-4]. They are used, for example, in metal-insulator-SiC devices [3,4]. The influence of oxynitrides on Si0 2/SiC interfaces [5], the adsorption of nitrogen on SiC surfaces [6], the interaction of nitrogen with SiC during implantation [7,8] or growth of III-nitrides [9] on SiC substrates have been studied. In this article we report our experimental results on the direct plasma nitridation of SiC surfaces with surface compositions ranging from Si-rich, i.e., Si overlayers grown on a Si-terminated sample, to graphitized. EXPERIMENT The experiments were performed in an ultrahigh vacuum (UHV) system on Si-terminated (0001) and C-terminated (0001) 6H-SiC sinlge-crystal wafers (Cree Research, Inc.). The samples were first treated ex situ in HF and then cleaned in UHV. Si-terminated surfaces were heated in the presence of a Si flux at different temperatures while C-terminated samples were annealed at 1170 K for 10 min. X-ray photoemission spectroscopy (XPS) and low-energy electron diffraction (LEED) showed the resulting surfaces to be clean and wellordered [10]. The nitrogen exposures were performed with a RF plasma source operated at a frequency of 13.56 MHz [11]. In order to prevent nitrogen ions from reaching the samples during nitridation, they were biased by 500 V at room temperature and by 1000 V at higher temperatures. Oxygen and carbon contaminations were removed from the samples after each nitrogen exposure by annealing at 1150 K for 15 min. The nitridation of the SiC 457 Ma
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