MORPHOLOGY, ATOMIC AND ELECTRONIC-STRUCTURE OF 6H-SIC(0001) SURFACES

Recent findings concerning primarily the root 3 x root 3 and 6 root 3 x 6 root 3 reconstructed surfaces of 6H-SiC(0001) are reviewed. First, the morphology of some different types of 6H-SiC crystals is discusse d. The scanning tunneling microscopy (STM) and atomic force microscopy (AFM) results presented show that surfaces with a morphology suitable for surface investigations can be prepared using sublimation- or hydr ogen-etching. Then results obtained concerning the atomic and electron ic structure for the reconstructed surfaces, prepared using an ex situ method for oxide removal and in situ heating! are presented. For the root 3 x root 3 reconstruction, recent STM and photoelectron spectrosc opy (PES) data are discussed in view of available theoretical results. The STM images presented are shown to be consistent with a structural model of Si or C adatoms in threefold symmetric sites. The theoretica l results favor Si adatoms in T-4 sites as the optimal configuration f or this reconstruction. However, the surface shifted components extrac ted in studies of the C Is and Si 2p core levels and the location of a surface state band mapped out in angle resolved experiments cannot be explained using this structural model. At present there is no structu ral model that satisfactorily can explain all experimental findings fo r the root 3 x root 3 reconstruction. A monocrystalline graphite overl ayer on top of bulk-terminated or root 3 x root 3-reconstructed SIC ha s previously been proposed to explain the 6 root 3 x 6 root 3-reconstr uctcd surface. However, STM and PES results are presented that unambig uously show that there is no graphite on the surface when a well devel oped 6 root 3 x 6 root 3 low-energy electron diffraction (LEED) patter n is observed. The STM images recorded during the gradual development of the 6 root 3 x 6 root 3 surface show growing fractions of pseudo-pe riodic 6 x 6 and 5 x 5 reconstructions. These reconstructed regions do minate on the surface, but small root 3 x root 3-reconstructcd regions are still present when a well developed 6 root 3 x 6 root 3 LEED patt ern is observed. It is shown that the 6 root 3 x 6 root 3 LEED pattern can be fully explained by scattering from surfaces with a mixture of 6 x B, 5 x 5 and root 3 x root 3 reconstructions. Due to the complexit y of the STM data, no structural model is proposed for the 6 x 6 and 5 x 5 reconstructions. STM and PES results are presented showing that g raphitization of the surface is obtained only after heating at higher temperatures than that required for observing a well developed 6 root 3 x 6 root 3 LEED pattern. The STM images then show that the graphite appears as a monocrystalline overlayer on top of the 6 x 6 reconstruct ion and not on bulk-terminated or root 3 x root 3-reconstructed SiC(00 01).