Electronic transport and photoconductivity in phosphorus-doped amorphous germanium

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Abstract
It is shown that the electronic properties of amorphous germanium (a-Ge) prepared by the glow-discharge decomposition of germane can be controlled systematically by substitutional phosphorus doping from the gas phase. Specimens with different doping levels have been investigated by conductivity, thermoelectric power and Hall effect measurements in a temperature range between 160 and 450 K. The dependence of conductivity on doping level is qualitatively similar to that in a-Si, but the range of control is limited in a-Ge by the smaller mobility gap. Also the larger overall density of gap states in this material reduces the doping sensitivity. The above transport measurements and their temperature dependence can be interpreted in a quantitatively consistent manner by a two-path model in which conduction takes place in the extended states and in another path through the localized states. As in a-Si, the photoconductivity of glow-discharge Ge can be appreciably sensitized by phosphorus doping. The μτ product deduced from such experiments on a-Ge and a-Si are compared for different preparation techniques. The data show that irrespective of the presence of hydrogen the method of deposition remains an important factor in determining the density of gap states.