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.

Gold contacts to semiconductor devices

“…The ohmic contacts are deposited and heat treated. The present metallurgy utilizes gold-germanium alloys because it has been found that lower resistance contacts are formed with about twenty atomic percent germanium in the gold (14,19). Although there is no direct evidence, it is believed that the germanium becomes incorporated in the GaAs lattice during heat-treatment and forms ahigh n-type dopant concentration immediately adjacent to the metallic contact interface.…”

“…One method of making gold ohmic contacts to an n-type semiconductor is therefore to heat the sample. For example in studies (19) of ohmic contacts to gallium arsenide, GaAs, which is a III-V compound semiconductor used for high speed devices, heat treatment of gold coated GaAs at temperatures of 400 to 600°C produced a deeply penetrating component of gold in the semiconductor. A disordered region was found near the surface with the amount of disorder increasing with process time and temperature.…”

Electron Device Contact Studies.

“…Some arise from the non-uniform way in which the alloying occurs (19). However, the most serious problems arise from the redistribution of the Au, Ge, and Ga during the alloying (21,(24)(25)(26).…”

“…The usual technique utilizes the alloying of a Au-Ge contact mixture into the GaAs surface (18)(19)(20)(21)(22). The Ge is believed to move into the GaAs surface thereby creating a highly doped region which forms a thin space charge layer through which electrons can tunnel.…”

“…Other researchers (19) nave shown that alloyed Au-Ge contacts have a problem witn localized formation of various Au-Ge-GaAs phases with their differing conduction properties. A conduction pinhole in this case may be regarded as a localized area where rapid incorporation of Ge has occured giving rise to a highly doped point in the GaAs surface.…”