Interface properties of oxidized germanium-doped silicon

Czochralski-grown Si-Ge single crystals, with several atomic per cent of Ge, were investigated from the aspects of their oxide interface properties and their suitability for MOS device making. It was found that the oxide formed is almost pure SiO 2 , but the C(V) curve of both n- and p-type MOS capacitors is shifted in the positive direction by an amount dependent on the oxidation rate and the Ge diffusion coefficient in Si, thereby showing the existence of a negative interface charge. The surface state density function was measured using the high-frequency and the quasistatic capacitance methods and was found to have more peaks, located near the middle of the gap, than the normal Si function. The states were mainly of the acceptor type. Their number, established by the variable temperature method or by the length of the flat region (`ledge') of the C(V) curve which forms at liquid nitrogen temperature, was one to two orders of magnitude larger than in Si but could be much reduced by annealing. The capture cross section, measured by the conductance method, was larger than in Si and varied by jumps along the band gap. MOS transistors of the n-channel enhancement type were made and were found to have a relatively low mobility.