Annealing behavior of Au(Te)/n-GaAs contacts

Abstract

The microstructure and electrical properties of Au/Te/Au/n-GaAs contacts annealed in the temperature range 360–480 °C, either with or without an insulating capping layer, have been investigated by the combined use of Rutherford backscattering spectrometry, x-ray diffraction, transmission electron microscopy, and current–voltage characterization. In addition, a thin-film collector method was applied to measure evaporative losses of As and Te, in view of their high volatility during heat treatment. The results give evidence that the thermally activated contact reaction strongly depends on the application of the capping layer. Contacts annealed without capping layer become ohmic at 420 °C. In the uncapped system, the contact reaction is dominated by the Au–GaAs interaction, as indicated by the appearance of Au7Ga2 phase and large arsenic losses. Moreover, tellurium is shown to partly evaporate during the heat treatment. For the sealed contacts, the losses of both Te and As are limited. Suppression of As vaporization restrains the Au–GaAs reaction, while reduction of Te sublimation activates the Te–GaAs reaction. Ga2Te3 and As2Te3 are the main reaction products, apart from unreacted Au. These reactions, however, do not lead to the formation of ohmic contacts to n-GaAs. Our results do not support the heterojunction model of ohmic contact, but rather testify in favor of the doping model.