A stack of Ni/Ge/Au metal layers was deposited sequentially to form the n-ohmic contact as part of the fabrication of an InP-based infrared photodetector product. Concave circles with a cone at the center were observed under optical microscopy after a de-ionized water rinse following a metal lift-off process. Auger electron spectroscopy showed that the composition differed between the normal area of the metal surface and the area inside the concave circles. Inside the circles, the Ge on the top Au surface had been lost, and the Ge layer beneath the top Au layer was significantly reduced. The circle size was investigated experimentally and found to increase logarithmically with the exposure time in de-ionized water. A diffusion model based on the results was developed to explain the growth kinetics of the concave circles. Mathematical simulations were performed with different values for the diffusion coefficient for Ge. The simulated result agreed with experimental data when D = 1.6 x 10–12cm2/s, which was about an order of magnitude larger than that reported previously in literature for grain-boundary diffusion of Ge through Au.

Enhanced Diffusion of Germanium at the Au/Ge Interface in the Presence of Water in a Gold/Germanium/Nickel Metal Stack. S.Zhang, S.J.Dixon-Warren, S.R.Das: Journal of Applied Physics, 2004, 95[7], 3521-6