Interface states atSiO2/6H−SiC(0001)interfaces observed by x-ray photoelectron spectroscopy measurements under bias: Comparison between dry and wet oxidation

Interface states in almost the entire SiC band gap are observable by means of x-ray photoelectron spectroscopy (XPS) measurements under bias, although SiC is a wide-gap semiconductor having 2.9 eV band-gap energy. When a ${\mathrm{SiO}}_2$ layer is formed by wet oxidation at 1000 °C on $6H-\mathrm{SiC}\mathrm{}\left(0001\right)\mathrm{}$ Si-faced surfaces, only a broad interface state peak is observed at ∼2 eV above the SiC valence-band maximum (VBM), while for dry oxidation at the same temperature, an additional sharp interface state peak is caused at 1.8 eV above the VBM. When the wet-oxidation temperature is increased to 1150 °C, this 1.8-eV interface-state peak also appears. The concentration of graphitic carbon at the ${\mathrm{SiO}}_2/\mathrm{SiC}$ interface is found to increase with the heat treatment temperature. The 1.8-eV interface-state peak is tentatively attributed to graphitic carbon with a special structure near the interface. On the other hand, the broad 2-eV interface-state peak is attributed to Si dangling bonds at the interface. Without the 1.8-eV interface-state peak, current-voltage $\mathrm{}(I-V)\mathrm{}$ curves measured under x-ray irradiation deviate only slightly from the ideal $I-V$ curve (∼0.4 V), while with this peak, the deviation becomes much larger (∼0.8 V). XPS measurements under bias show that the $I-V$ curves under x-ray irradiation are determined by the magnitude of band bending in SiC. Therefore, the deviation from the ideal $I-V$ curve is attributed to the accumulation of holes (i.e., minority carriers), generated by x-ray irradiation, at interface states with energies between the SiC and metal Fermi levels, causing a downward SiC band-edge shift and thus resulting in a decrease in the magnitude of band bending in SiC. This result demonstrates that the interface states affect $I-V$ characteristics by a static effect (i.e., interface state charges), not by a dynamical effect (i.e., electron-hole recombination at the interface states).