The electrical properties and impurity profiles of ZnSe films on GaAs and of Gallium-diffused ZnSe single crystals

Abstract

The electrical properties and impurity profiles of ZnSe films and gallium-diffused ZnSe single crystals were investigated. ZnSe films 10–30 μm thick were epitaxially grown on p-type GaAs in the vapour phase by using metallic zinc and metallic selenium without intentional doping. Low resistivity films (of the order of several ohm centimetres) were deposited, even in the presence of excess selenium, at temperatures above 650°C. The carrier concentration profile showed an exponential dependence on the distance from the interface. Donor and acceptor concentrations of about 10 18 cm -3 and a compensation ratio close to unity were obtained. An electron probe microanalyser was used to determine the impurity profiles on the cleaved surfaces. Gallium and arsenic atoms were distributed exponentially throughout the ZnSe films with concentrations of the order of 10 20 cm -3. The gallium concentration was always higher than the arsenic concentration. The concentration of the electrically active impurity was about two orders of magnitude less than the gallium and arsenic concentrations. The gallium-diffused ZnSe was prepared in an evacuated quartz ampoule by diffusing gallium from the melt into a melt-grown ZnSe single crystal in order to compare the electrical properties of gallium-diffused and undiffused ZnSe. The resistivity of the gallium-diffused ZnSe was also low. We conclude from these results that the resistivity of ZnSe on GaAs became low because of gallium auto-doping. The gallium concentration profile in the bulk ZnSe was described by a complementary error function in contrast with the exponential distribution obtained for ZnSe films on GaAs. A trial solar cell of dimensions 4 mm x 4 mm had an open-circuit voltage of 0.65–0.75 V and a short-circuit current of 0.9–2.2 mA.