Large-Signal Surface Photovoltage Studies with Germanium

Abstract

Studies of the surface photovoltage of germanium were carried out over a considerably wider range of excess carrier densities than previously reported. Ambient induced inversion and accumulation layer surfaces were studied on $p$- and $n$-type Ge with resistivities ranging from 1 to 15 ohm-cm. The photovoltage was measured with ac methods and the excess carrier density was monitored by changes in the specimen conductance. The observed dependence of the photovoltage on the excess carrier density agreed quite well with theory that considers the surface space charge, but neglects charge changes in fast surface states. Comparison of the observed and theoretical curves is believed to give the surface potential to within about one $\frac{\mathrm{kT}}{e}$ unit for potentials less than about $\frac{8kT}{e}$ units, even if the effect of previously reported fast states is neglected. Excursions of the surface potential over the ambient cycle were found to be about the same as those reported for other types of surface measurements.The large signal photovoltage, in the range of surface potentials covered in the present work, is insensitive to fast states having the range of parameters extant in the literature: sensitivity is largely restricted to unreported parameter values. Since no evidence for fast states was observed in the present experiments, it is concluded that the present results are at least consistent with previously reported fast-state parameter values.