The mechanisms of formation of ohmic contacts to AlGaAs: A microstructural, elemental diffusion and electrical investigation

Interfacial microstructure and phase composition of PtTiGePd ohmic contacts to heavily C-doped AlGaAs were investigated as a function of annealing temperatures. Results of the material analyses were used to explain the specific contact resistances measured for each thermal treatment. Evidence of interdiffusion and compound formation between AlGaAs and Pd was visible in a Ga-rich Pd-Ga-As reaction zone prior to heat treatment. As the annealing temperature was raised from 530 to 600°C, As began to out-diffuse. At 600°C, this As out-diffusion, which is critical to the formation of good p-type ohmic contacts, contributed to the creation and development of the laterally continous two-phase interfacial region, TiAs/Pd12Ga2Ge5, overlying the AlGaAs substrate. The minimum specific contact resistance was also achieved at this temperature. As the annealing temperature was elevated to 650°C, the specific contact resistance degraded in response to intensive chemical diffusion and development of a broad, nonuniform multiphased interfacial region.