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Free keywords:
silicon compounds; wide band gap semiconductors; hydrogen; X-ray photoelectron spectra; core levels; valence bands; surface reconstruction; bonds (chemical); chemical shift; binding energy; thermally stimulated desorption; dangling bonds
Abstract:
We report on highly resolved core-level and valence-band photoemission spectroscopies of hydrogenated, unreconstructed 6H-SiC(0001) and (000 (1) over bar) using synchrotron radiation. In the C 1s core level spectra of 6H-SiC(000 (1) over bar) a chemically shifted surface component due to C-H bonds is observed at a binding energy (0.47+/-0.02) eV higher than that of the bulk line. The Si 2p core-level spectra of SiC(0001) suggest the presence of a surface component as well but a clear identification is hindered by a large Gaussian width, which is present in all spectra and which is consistent with values found in the literature. The effect of thermal hydrogen desorption was studied. On 6H-SiC(0001) the desorption of hydrogen at 700-750 degreesC is accompanied by a simultaneous transformation to the Si-rich (root3xroot3)R30degrees reconstruction. On 6H-SiC(000 (1) over bar) first signs of hydrogen desorption, i.e., the formation of a dangling bond state in the fundamental band gap of SiC, are seen at temperatures around 670 degreesC while the (1x1) periodicity is conserved. At 950 degreesC a (3x3) reconstruction is formed. The formation of these reconstructions on thermally hydrogenated 6H-SiC(0001) and (000 (1) over bar) is discussed in the light of earlier studies of 6H-SiC{0001} surfaces. It will be shown that by using the hydrogenated surfaces as a starting point it is possible to gain insight into how the (root3xroot3)R30degrees and (3x3) reconstructions are formed on 6H-SiC(0001) and 6H-SiC(000 (1) over bar), respectively. This is due to the fact that only hydrogen-terminated 6H-SiC{0001} surfaces possess a surface carbon to silicon ratio of 1:1.
