Nickel dissolution into AuGe in alloyed AuGe/Ni/Au Ohmic contacts on GaAs/AlGaAs multilayer structures

Magnetic properties of alloyed Ohmic contacts of the type AuGe/Ni/Au on GaAs/AlGaAs multilayers with n⁺ cap layer with different AuGe compositions and Ni-layer thicknesses are examined. Magnetization data indicate that the annealed structures are non-magnetic, at room temperature for commonly used anneal temperatures (∼ 400-430 °C) and Ni-layer thicknesses (10-100 nm). The transformation of Ni to non-magnetic phase begins at ∼ 100 °C, well below temperatures at which extensive alloying with the GaAs substrate takes place. The fraction of Ni transformed to non-magnetic phase on annealing appears to scale with AuGe layer thickness, has a quadratic dependence on anneal temperature and is time independent for time scales of minutes. The data indicate that the Ni layer dissolves into the AuGe layer at temperatures well below that at which alloying between AuGe and GaAs substrate takes place. The dissolved Ni concentration is limited by a solubility that increases with anneal temperature and decreases with decreasing Ge content from that of the AuGe eutectic composition.