Effects of Thick Oxides on Germanium Surface Properties

Oxides grown on germanium by heating in oxygen are shown to inhibit interaction between the germanium and the ambient atmosphere. The decay time of the dc field effect increases with increasing thickness of the oxide formed. This indicates that the states associated with the oxide lie principally on its outer surface. The decay time decreases, however, in the presence of water vapor, iodine, or ammonia in the ambient. This may be explained as being due to an increase in the density of states on the outer surface of the oxide layer. Desiccation lengthens the decay time greatly. If the oxide layers are of the order of 0.25 μ in thickness, the dc field effect displays no measurable decay, and the electrical properties of the underlying germanium are unaffected by a vapor as active as iodine. The surface underneath a thick oxide is shown to be strongly $n$ type and has a low value of surface recombination velocity.