Si-rich 6H- and 4H-SiC(0001) 3x3 surface oxidation and initial SiO2/SiC interface formation from 25 to 650°C -: art. no. 165323

We investigate the initial oxidation of the Si-rich 6H-SiC(0001) 3x3 and 4H-SiC(0001) 3x3 surfaces and the subsequent SiO2/4H-SiC and SiO2/6H-SiC initial interface formation by Si 2p, C 1s, and O 1s core-level photoemission spectroscopy using synchrotron radiation. The 3x3 surface reconstruction is found to be highly reactive to oxygen with an initial oxidation rate of about three to four orders of magnitude larger than for silicon surfaces. Furthermore, for both polytypes, direct SiO2/SiC interface formation is achieved already at room temperature and extremely low oxygen exposures. However, the two polytypes have significantly different behaviors with larger amounts of oxide products having higher oxidation states for the 6H-SiC(0001) 3x3 surface, while mixed oxides including carbon species (Si-O-C) are the dominant oxide products for the 4H polytype surface. The oxidation rate is improved at increased surface temperatures. In all cases, the oxygen uptake remains significantly larger for the 6H polytype when compared to the 4H one. The very different behavior of the 6H and 4H polytypes seems to originate, at least in part, from the presence of two domains in the bulk for the 4H polytype (as evidenced by two bulk components in the Si 2p core-level spectrum) which limits the oxygen insertion into the 4H-SiC lattice. Instead the 6H polytype has only one bulk domain with a single Si 2p bulk component. Abrupt SiO2/6H-SiC interfaces could be achieved by thermal oxidation of a predeposited Si overlayer onto the surface leading to have oxide thicknesses ranging from 10 Angstrom up to 80 Angstrom after postoxidation with a transition layer of less than 5 Angstrom. This study shows that, using a "gentle" initial oxidation approach at low temperatures and low oxygen exposures allow high-quality SiO2/SiC interfaces. It also brings interesting insights into the understanding of polytype crucial effect in SiC surface oxidation.