Dependence of interface states in the Si band gap on oxide atomic density and interfacial roughness

The energy distribution of interface states in the Si band gap present at ultrathin chemical oxide/Si(111) interfaces is obtained from measurements of x-ray photoelectron spectra under bias. The analysis of interface state spectra, together with the results obtained from measurements of Fourier-transform infrared attenuated total reflection spectra, atomic force micrographs, and synchrotron radiation ultraviolet photoelectron spectra, clarifies the relation of the interface states with the oxide atomic density and interfacial roughness. It is found that when the atomic density of the oxide layer is low (e.g., oxide formed in HCl+H₂O₂), an interface state peak appears near the midgap, which peak is attributable to isolated Si dangling bonds at the interface. For a high-atomic density oxide layer (e.g., oxide formed in HNO₃), interface state peaks are present above and below the midgap and are attributable to Si dangling bonds interacting weakly with a Si or oxygen atom in the oxide layer. The interface state density of an atomically smooth interface is much lower than that of an atomically rough interface. The interface is made smoother after oxidation with HCl+H₂O₂ than the initial surface, while it becomes rougher by oxidation with HNO₃ or H₂SO₄+H₂O₂. Consequently, the interface state density for the HCl oxide layer is lower than those for the HNO₃ and H₂SO₄ oxide layers.