Synchrotron x-ray photoelectron spectroscopy study of hydrogen-terminated $6H-\mathrm{SiC}\{0001\}$ surfaces

We report on highly resolved core-level and valence-band photoemission spectroscopies of hydrogenated, unreconstructed $6H-\mathrm{SiC}\mathrm{}\left(0001\right)\mathrm{}$ and $\left(0001\mathrm{}¯\right)$ using synchrotron radiation. In the C $1s$ core level spectra of $6H-\mathrm{SiC}\left(0001\mathrm{}¯\right)$ a chemically shifted surface component due to C-H bonds is observed at a binding energy $(0.47\mathrm{}\pm 0.02)\mathrm{eV}$ 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 $6H-\mathrm{SiC}\mathrm{}\left(0001\right)\mathrm{}$ the desorption of hydrogen at $700–750°\mathrm{C}$ is accompanied by a simultaneous transformation to the Si-rich $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ reconstruction. On $6H-\mathrm{SiC}\left(0001\mathrm{}¯\right)$ 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 $670°\mathrm{C}$ while the $(1\mathrm{}\times 1)$ periodicity is conserved. At $950°\mathrm{C}$ a $(3\mathrm{}\times 3)$ reconstruction is formed. The formation of these reconstructions on thermally hydrogenated $6H-\mathrm{SiC}\mathrm{}\left(0001\right)\mathrm{}$ and $\left(0001\mathrm{}¯\right)$ is discussed in the light of earlier studies of $6H-\mathrm{SiC}\{0001\}$ surfaces. It will be shown that by using the hydrogenated surfaces as a starting point it is possible to gain insight into how the $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ and $(3\mathrm{}\times 3)$ reconstructions are formed on $6H-\mathrm{SiC}\mathrm{}\left(0001\right)\mathrm{}$ and $6H-\mathrm{SiC}\left(0001\mathrm{}¯\right),$ respectively. This is due to the fact that only hydrogen-terminated $6H-\mathrm{SiC}\{0001\}$ surfaces possess a surface carbon to silicon ratio of $1:1.$