PHOTOELECTRON SPECTROSCOPIC STUDIES OF THE ELECTRONIC-STRUCTURE OF ALPHA-SIC

The electronic structure of single-crystal alpha-SiC (silicon carbide) has been determined with variable photon energy valence band photoelectron spectroscopy. Synchrotron radiation in the range 30-250 eV was used in addition to a conventional Al K-alpha X-ray source to follow the intensity changes of valence band features as a function of the incident photon energy. The dominant atomic orbital contributions have been assigned to valence band features by comparing them to theoretical atomic photoionization cross sections. The results indicate that C 2s based ionizations appear at the highest binding energy, near 15 eV, while Si 3s features are evident near 10 eV. The highest occupied energy levels (from approximately 1-8-eV binding energies) are predominantly C 2p based. Differences between the photon energy dependence of the data and theoretical atomic orbital cross sections show the valence levels to be highly mixed. Comparison of the valence band spectra with a molecular orbital calculation confirms the experimental assignments, but the calculation appears to underestimate the extent of atomic orbital mixing. The valence band spectrum of a hot-pressed polycrystalline SiC sample compares quite well with the single-crystal spectrum. The implications of these results on the chemisorption behavior and reactivity of SiC are discussed.