Dependence of melting, roughness and contact resistances on Ge and Ni content in alloyed AuGe/Ni/Au-type electrical contacts to GaAs/AlGaAs multilayer structures

Annealed AuGe/Ni/Au film structures on GaAs/AlGaAs multilayers have been examined for contact resistance, roughness, magnetization and melting as functions of anneal temperature, Ni-layer thickness and three AuGe compositions. Magnetization data indicate that a solid state, solubility-limited dissolution of Ni into AuGe takes place even for low-temperature anneals and that this dissolution is complete when alloying occurs at ∼400 °C. An apparent melting temperature, detected in differential scanning calorimetry, increases with increasing Ni-layer thickness and decreasing Ge content in the AuGe alloy. Electrical contact formation and roughening of the surface occur in the range of melting temperatures of the structure. The eutectic alloy with a Ni-layer thickness of ∼25-30 nm gives the optimum contact resistance. The contact resistance can be traded off for the reduction in roughness by either increasing the Ni-layer thickness or reducing the Ge content, with the latter being the better choice of the two. The temperature dependence (4-300 K) of the contact resistance shows indications of both thermionic and tunneling behaviors. The barrier height for the current conduction increases with the increase of the Ni-layer thickness and a decrease of the Ge content in the AuGe layer, relative to that of the structure with optimum contact resistance.