Chemically etched and directly load-locked SiO2/SiC samples are investigated using a photon energy of 3.0 keV. Si 2p and C 1s spectra recorded at different electron emission angles each show two components originating from SiC, SiO2 and graphite like carbon, respectively. The relative intensity of these are extracted and compared to calculated intensity variations. For the samples investigated, best agreement between experimental and calculated intensity variations is obtained when assuming a graphite like layer on top of the oxide. No graphite like carbon at the SiO2/SiC interface was detected, even on a sample for which the graphite like carbon contribution at the surface corresponds to a layer thickness of only 0.05 angstrom. The energy separation between the oxide and carbide components in the Si 2p spectrum was monitored before and after Ar+ sputtering cycles and before and after in situ heating. The separation increased directly upon sputtering while only in situ heating does not affect it. We suggest that defects induced by the sputtering give rise to the increase, observed in the energy separation.

An in situ oxidation study of v3 × v3 R30° reconstructed 4H-SiC(001) surfaces is reported. An intermediate oxidation state (interpreted to be Si+1) is revealed in core level photoemission spectra recorded from the in situ prepared SiO2/4H-SiC samples. Oxidation was made at a pressure of ~10-3 Torr in flowing oxygen and at substrate temperatures from 600°C to 950°C. The highest oxidation rate was obtained at 800°C when ˜25 Å thick SiO2 layers were prepared. The surface related C 1s components observed on the clean reconstructed 4H-SiC(0001) surfaces were found to disappear after oxidation. No carbon or carbon containing by-product at the interface or in the oxide were possible to observe for the films grown.

The results of a photoemission study of Si- and C-terminated 4H SiC surfaces after different oxygen exposures are presented and discussed. The surfaces were oxidized gradually from 1 to 1.2 x 10(6) L at both room temperature and at 800 degreesC. Recorded Si 2p and C 1s spectra show at both temperatures only two oxidations states, Si1- and Si4- for the Si-terminated surface and Si2+ and Si4+ for the C-terminated surface, For the Si-terminated surface, no carbon containing by-product can be detected at the interface or at the surface after the largest exposure investigated. For the C-terminated surface, oxygen exposures are shown to affect the surface related carbon components quite strongly and the Si2+ oxidation state is interpreted to originate from a mixture of Si-O-C bonding, The surface/interface related carbon decreases dramatically after the largest exposure investigated but is not eliminated as on the Si-terminated surface. For the latter, a clean and well ordered root3 surface is shown to be possible to re-create by in situ heating even after the largest oxygen exposures made. (C) 2002 Elsevier Science B.V. All rights reserved.

Initial oxygen adsorption at different temperatures on the SiC(0 0 0 1)- v3 × v3 R30° surface has been studied using photoemission. Oxygen exposures with the sample at 800 °C results in formation of stable oxides. However, after small exposures (0.1-10 L) with the sample at room temperature or cooled to ˜100 K additional structures appear in the O 1s spectrum that are identified to originate from metastable oxygen. Similar additional structures were recently revealed on Si(111)-7 × 7 and suggested to originate from adsorption of metastable molecular oxygen in an ins-paul configuration. © 2002 Elsevier Science B.V. All rights reserved.

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL2,3L2,3 Auger transitions from SiO2/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered v3 × v3 reconstructed 4H-SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si+1 and Si+4, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra. The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si2O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å. The SiO2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO2 shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined. The relative final state relaxation energy dR(2p) is determined from the modified Auger parameter. This yields a value of dR(2p) = -1.7 eV and implies, for SiO2/SiC, a "true" chemical shift in the Si 2p level of only ˜0.4 eV for oxide layers of up to 10 Å thick. © 2003 Elsevier Science B.V. All rights reserved.

The results of a photoemission study of clean and oxidized nonpolar (10 (1) over bar0) and (11 (2) over bar0) surfaces of 4H-SiC crystals are reported. For the clean surfaces prominent surface shifted components are revealed in the C 1s spectra, although with different relative strengths for the two surfaces. In the Si 2p spectra only one weak surface related feature is observed. These observations suggest carbon enrichment on both surfaces. Valence band spectra show a semiconducting behavior for both surfaces but also distinct differences. The lower lying subband, originating mainly from C 2s states, is for example found to be considerably weaker on the (10 (1) over bar0) than on the (11 (2) over bar0) surface. Observed differences are suggested to originate from differences in surface carbon arrangement and bonding. It is concluded, from the absence of a graphitelike C 1s peak, that the clean surfaces are not graphitized. The effects induced in the core levels and valence bands upon initial oxidation were investigated. The surfaces were oxidized gradually from 1 to 10(6) L while keeping the substrate at a temperature of 800 degreesC. Recorded Si 2p spectra show three oxidation states for both surfaces, and these are interpreted to originate from Si+1, Si+2, and Si+4, respectively. This is quite different compared to earlier results for the polar surfaces where only Si+4 and one suboxide were revealed on each surface. It is concluded that the Si+4 oxide (SiO2) grow as a layer on top of the Si+1 and Si+2 suboxides that are located at the interface. The surface/interface related carbon is found to decrease dramatically, but not to be totally eliminated, after the large oxygen exposures made.