Interface Properties of Al₂O₃-Ge Structure and Characteristics of Al₂O₃-Ge MOS Transistors

The interface properties of Al₂O₃-Ge structure formed on germanium wafers by DC reactive sputtering are evaluated. The fast interface states are diminished by subjecting the sample to heat treatment in an inert gas flow. The interface state density is estimated to be typically less than 4×10¹¹ state/cm²-eV from the shift of the flat-band voltage with temperature as originally adopted by Gray and Brown. From the measurement of the parallel conductance of the MOS structure, the electron capture cross section of the trap located at the interface is estimated to be about 0.9×10^-17 cm² using the tunnelling model of Preier, and about 1×10^-18 cm² using the surface potential fluctuation model of Nicollian and Goetzberger. With reactively sputtered Al₂O₃ films as a gate insulator, n-channel germanium MOS transistors are made with a high effective mobility. The effective mobility is typically more than 2000 cm²/V-sec and the gate threshold voltage is about 1 volt or less.