Enhanced diffusion of germanium at the Au/Ge interface in the presence of water in a gold/germanium/nickel metal stack

Abstract

A stack of nickel/germanium/gold metal layers was deposited sequentially to form the n-ohmic contact as part of the fabrication of an indium phosphide-based infrared photodetector product. Concave circles with a cone at the center were observed under optical microscopy after a deionized (DI) 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 germanium on the top gold surface had been lost, and the germanium 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 DI 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 (D) for germanium. The simulated result agrees with experimental data when D=1.6×10−12 cm2/s, which is about one order of magnitude larger than that reported previously in literature for grain-boundary diffusion of Ge through gold.