Dependence of interface states for ultra-thin SiO₂/Si interfaces on the oxide atomic density determined from FTIR measurements

The atomic density of ultra-thin thermal silicon oxide layers formed on Si(111) and (100) substrate is determined from Fourier transform infrared (FTIR) measurements, which well explains a dependence of the energy distribution of interface states obtained from X-ray photon spectroscopy measurements under bias on the oxidation temperature. In the case of the thermal oxide layers formed below 450°C, observation of the LO and TO phonons shows that the oxide atomic density is low and an interface state peak is present near the midgap, where the peak is attributable to isolated Si dangling bonds at the interface. With an increase in the oxide formation temperature, the atomic density of the oxide layer increases and interface state peaks are observed above and below the midgap, which peaks are attributable to Si dangling bonds interacting weakly with a Si or oxygen atom in the oxide layer. The energy separation between the interface state peaks for the oxide/Si(111) interfaces is much smaller than that for the oxide/Si(100) interfaces. The small energy separation arises from the long distance between a Si dangling bond and the interacting atom in the oxide layer, originated from the interfacial structure.