Alloying Behavior of Au and Au–Ge on GaAs

Abstract

Although Au–Ge has often been used in making Ohmic contacts to GaAs, the alloying behavior of this system has not been well characterized and understood. In this paper the behavior of Au and Au–Ge layers on GaAs was investigated as a function of processing temperature (400–600°C) and time by backscattering and channeling-effect measurements with 2-MeV 4He ions. Scanning electron microscopy and current-voltage evaluations were also made. Similarities are found in both systems: There is a deeply penetrating component of Au into the GaAs that remains essentially unaltered once the surface Au has been consumed; a disordered region near the surface is found, the amount of which increased with increased process times. No significant amount of Au is found on substitutional or tetrahedral interstitial sites within about 3000 Å of the surface. The presence of the Ge does have a significant effect: Complete alloying occurs at lower temperatures, and Ohmic contacts are formed. Scanning-electron-microcope displays indicate that surface pits are formed; the depth of these increase with increased amounts of Au. Optical micrographs of a sectioned sample indicate a deep Au penetration.