Investigation of the role of Ni in forming ultra-low resistance Ni-Ge-Au ohmic contacts to n/sup +/ GaAs

Nickel is commonly thought to act as a wetting agent in alloyed Au-Ge ohmic contacts for GaAs-based devices. However work on the optimisation of ohmic contacts for ultra-low resistance has indicated that Ni plays a more complex role. We have investigated a series of Ni-Au-Ge alloys with varying Ni content using cubic central composite experiments and electron microbeam techniques which have allowed us to characterise the alloy microstructure, dopant distribution and contact resistance as a function of Ni content. We conclude that the amount of Ni must be closely matched to the amount of Ge and Au for the formation of ultra-low resistance contacts.