The thesis work is focused on photoemission studies of clean and oxidized 4H-SiC surfaces since 4H-SiC has been considered the most promising for device applications. Oxide layers can be easily grown by standard wet and dry oxidation methods but to obtain good interface properties (i.e. low defect densities) is a major problem for the utilization of SiC. It had been claimed that formation of carbon containing by-products at the interface was a major problem. Therefore the first effort made was to determine if any carbon byproducts could be detected at the interface of SiO₂/SiC(0001) samples prepared by ex-situ dry oxidation. The results obtained showed no carbon by-products at the interface but a weak contribution from graphite like carbon on the surface of the samples. This surface carbon could however shadow the presence of carbon byproducts at the interface. It was estimated that a 0.12 Å thick graphite layer would have been required at the interface in order to be detected in the experiments performed. To avoid the surface carbon contamination oxide layers were then instead prepared by in-situ oxidation. The results obtained from such samples, where the oxide thickness was varied from about 5 to 25 Å, showed no carbon containing by-products at the interface, the surface, or in the oxide layers. The presence of an intermediate oxide located at the interface and interpreted to correspond to si⁺¹ oxidation states was instead revealed. This suboxide was suggested to be a factor that could contribute to the interface defect density. The observation of only one suboxide (Si⁺¹)was, however, in conflict with results reported by other research groups. They claimed the formation of foursuboxides (Si⁺¹, si⁺², Si⁺³ Si⁺⁴) in oxidation studies of SiC(0001), i.e. similar as on elemental Si surfaces.Therefore an additional effort was made to resolve this issue using higher photon energies. This allowed us to study also Si 1s core level and Si KLL Auger spectra on SiO₂/SiC(0001) samples where the oxide thickness was varied from about 5 to 120 Å. These results confirmed that the presence of only one suboxide was possible to detect and that it was located at the interface. It was moreover found that the SiO₂ oxide chemical shift exhibited a dependence on the oxide thickness. The shift was found to be fairly constant for oxides less than ca 10 Å thick, to increase by about 0.5 eV when increasing the oxide thickness to around 25 Å and then to gradually saturate for thicker oxides. In the O 1s core level no changes in the line shape could be observed with oxide thickness or after different oxygen exposures when keeping the substrate at an elevated temperature.

However, slight differences in the O 1s line shape were reveled after small exposures made at room temperature (RT) and at elevated temperature. After exposures at 800°C the O 1s spectrum exhibited only a slightly asymmetric main peak originating from formation of stable oxides. After exposures at RT and even more pronounced at LN temperature the O 1s spectrum exhibited additional structures at lower and higher binding energies and a considerably broader main peak. To model these spectra three additional components had to be used. From their exposure and time-decay dependence they were identified to originate from metastable oxygen.

Experiments using a nitrogen containing gas (NO and N₂O) for the oxidation of SiC at a substratetemperature of 800°C were also performed. The reason was that the use of a nitrogen containing gas had been claimed to improve the interface properties. For NO exposed samples it was found that the amount of suboxide (Si⁺¹) at the interface was reduced and replaced by a Si₃N_4. No significant difference in C 1s spectra recorded after NO and 0₂ exposure could be revealed, however. This suggested that carbon byproducts do not appear to be a main reason for the interface defect density for SiO₂/SiC (0001) samples grown in NO. For oxide samples grown using N₂O no N 1s signal could be detected and only the Si⁺¹ suboxide was observed besides the fully developed oxide, SiO₂. The absence of nitrogen at the interface for these samples was suggested to be due to that the substrate temperature selected was too low to efficiently dissociate N₂O.

Included in the thesis are also photoemission studies of the polar (0001̅ ) and nonpolar (101̅ 0) and (11 2̅ 0)surfaces. For the clean polar (0001̅ ) surface the C1s spectra showed that the surface was carbon rich and composed of at least two carbon layers on the surface. This surface carbon was found to be not completely eliminated after the largest oxygen exposures made (10⁶ L). This was opposite to the observations made for the polar (0001) surface at similar exposures. Only one suboxide, with a different chemical shift suggesting it to originate from Si⁺² oxidation states, was observed in the Si 2p spectra. It was interpreted to originate from Si-O-C bonding due to the excesses surface/interface carbon present on the (0001̅ ) surface. Also the clean nonpolar surfaces were found to be carbon rich (Si depleted) or rearranged in such a way that the carbon atoms move outward and Si atoms move inward. After similarly large oxygen exposures as for the polar surfaces a weak contribution from carbon by-products was found to remain at the interface for the nonpolar surfaces. The Si 2p data recorded showed the presence of two suboxides (Si⁺¹ and Si⁺²) besides Si0₂. This seemed reasonable and consistent with the earlier observations since the ideal (i.e. bulk truncated) nonpolar surfaces are composed of both silicon and carbon atoms. Studies of oxide samples identically prepared on the four different surfaces were carried out. The results showed the oxidation rate to be highest on the (101̅ 0) surface, somewhat lower on the (0001̅ ) and (11 2̅ 0) surfaces, and about a factor of two lower on the (0001) surface.

The SiO₂/SiC interface and surface properties of Si-rich SiC surfaces, prepared by Si deposition, was also investigated. After Si deposition the Si 2p data collected showed three prominent surface shifted peaks for both the polar and nonpolar surfaces indicating Si surface enrichment. The C 1s spectrum then showed only one sharp bulk SiC peak. After smaller oxygen exposures recorded core level spectra showed that the Si adlayers, created by the Si deposition, were not completely oxidized but that the C 1s spectrum was affected. The C 1s peak was broadened and a small asymmetry on the high binding side could be detected. After large oxygen exposures, i.e. when the Si adlayers were fully oxidized, the presence of the same suboxide(s) as for the corresponding surface prepared without Si deposition was(were) detected for all four surfaces.