Studies of oxidized hexagonal SiC surfaces and the SiC/SiO2interface using photoemission and synchrotron radiation

Results of recent photoemission studies of oxidation of hexagonal SiC surfaces and SiO 2 /SiC interfaces are reviewed and discussed. These investigations have focused on two main questions thought to have a significant effect on MOS device characteristics: the existence of carbon clusters or carbon-containing by-products and the existence of sub-oxides at the SiO 2 /SiC interface. The presentation is focused on Si-terminated surfaces of hexagonal n-type SiC(0001) crystals since they to date have been considered the most promising for device applications. The results reviewed show that no carbon clusters or carbon-containing by-product can be detected at the interface of in situ or ex situ grown samples with an oxide layer thickness larger than about 10 Å. Since the presence of carbon clusters was suggested in a recent scanning microscopy study it appears that they may exist, possibly depending on the sample preparation method used, but in such low concentrations that they are not detectable using photoemission. The presence of sub-oxides at the SiO 2 /SiC interface has been revealed in recorded Si 2p core level spectra by several groups. The results were not unanimous, however. The number of sub-oxides present and the shifts reported were different. The results of a recent study including also the Si 1s core level and Si KLL Auger transitions are therefore examined. These together with earlier Si 2p data show the presence of only one sub-oxide, assigned to Si 1+ oxidation states, besides the fully developed SiO 2 (Si 4+ ). Possible reasons for the differences obtained earlier are discussed. That the sub-oxide is located at the interface is concluded from the relative intensity variations observed for the different components versus electron emission angle. An oxide thickness dependence of the SiO 2 chemical shift in the core levels and Auger transitions is shown, similar to but smaller in magnitude than the thickness dependence revealed earlier for SiO 2 /Si. On cooled SiC(0001) substrates, adsorption of metastable molecular oxygen is suggested to occur in the initial oxidation stage like on the Si(111)-7 × 7 surface. Oxidation results from the C-terminated surface and some preliminary results for the non-polar and surfaces are included and they show distinct differences both as regards the sub-oxides present and the amount of carbon-containing by-products at the interface in the initial oxidation stage compared to the Si-terminated SiC(0001) surface.