Electrical and structural investigations of Ag-based Ohmic contacts for InAlAs/InGaAs/InP high electron mobility transistors

Ge/Ag/Ni and AuGe/Ni/Au Ohmic contacts on InAlAs/InGaAs/InP high electron mobility transistors with excellent contact resistance of 0.07 Ω mm were obtained after annealing at 425 and 265°C, respectively. The Ag-based contacts have a large processing window of >130°C. Structural analyses confirm that Ag and Au protrusions created during annealing effectively linked the two-dimensional electron gas layer with the metal contacts to produce excellent Ohmic characteristics. The formation of liquid AuGe eutectic phase in AuGe/Ni/Au at 300°C is believed to cause overannealing. The eutectic temperature of Ag-Ge is -300°C higher leading to a higher optimum annealing temperature and a wider processing window for the Ge/Ag/Ni contacts.