Auger studies on rapid grain boundary diffusion of Ge through Au

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Abstract
A commonly used, Ohmic electrical contact to n-type GaAs involves sequential evaporation of a 20 nm Au–Ge alloy and a 400 nm Au layer. During Auger analysis of such structures at a pressure of 2×10−8 Pa, we have observed diffusion of Ge through the Au layer at room temperature. When the Ge was removed by a short sputter clean with a 3 keV ion beam, the Ge Auger signal immediately started to increase. The intensity of the Au (69 eV) and Ge (1147 eV) transitions was monitored as a function of time and temperature. Temperatures used were low compared to the melting point of Au, 0.03Tm ≤T≤0.22Tm. The saturation values correspond to 0.5 monolayer of Ge on the Au surface. The experiment was repeated a number of times on the same sample. Auger sputter depth profiles showed that the Ge concentration was below the Auger detectability limit in the bulk of the gold films until the Ge–Au alloy was reached. Secondary ion mass spectrometry of similar samples showed a small Ge concentration throughout the Au film. The results are consistent with diffusion of Ge through grain boundaries in the Au film and subsequent surface accumulation. When the samples were exposed to air, the Ge oxidized but the amount of Ge did not increase. A further set of experiments involved exposing the sputter cleaned surface to hydrocarbon partial pressures or air for 2 h. In both experiments the surface accumulation of Ge is greatly reduced. From our results an activation energy for surface accumulation of 0.44 eV was determined.

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