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 discussed. 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 hydrogen-etching. Then results obtained concerning the atomic and electronic 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 spectroscopy (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 theoretical results favor Si adatoms in T-4 sites as the optimal configuration for this reconstruction. However, the surface shifted components extracted 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 structural model that satisfactorily can explain all experimental findings for the root 3 x root 3 reconstruction. A monocrystalline graphite overlayer on top of bulk-terminated or root 3 x root 3-reconstructed SIC has previously been proposed to explain the 6 root 3 x 6 root 3-reconstructcd surface. However, STM and PES results are presented that unambiguously show that there is no graphite on the surface when a well developed 6 root 3 x 6 root 3 low-energy electron diffraction (LEED) pattern 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-periodic 6 x 6 and 5 x 5 reconstructions. These reconstructed regions dominate 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 pattern 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 complexity 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 graphitization 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 reconstruction and not on bulk-terminated or root 3 x root 3-reconstructed SiC(0001).