Thermally stable ohmic contacts to n-type GaAs. V. Metal-semiconductor field-effect transistors with NiInW ohmic contacts

Abstract

Thermal stability of self-aligned refractory metal-semiconductor field-effect transistors (MESFETs) with the universally used AuNiGe ohmic contact metallurgy or the recently developed NiInW ohmic contact metallurgy has been studied. In these devices WSi0.1 films with length down to 1 μm were used as the gate material, and AlCu was used as the overlayer material on the ohmic contacts, where a very thin Ti layer was deposited prior to the AlCu deposition. The performance of the as-fabricated devices with the NiInW ohmic contacts was as good as those with the AuNiGe ohmic contacts. During subsequent annealing at 400°C, deterioration of the device performance (defined by the decrease in the FET square-law coefficient) was observed after annealing at 400°C for 2 h in the devices with the AuNiGe ohmic contacts. However, excellent stability was observed in the devices with the NiInW ohmic contacts; no deterioration was observed at 400°C for 180 h, 450°C for 18 h, and 500°C for 2 h. The device deterioration with NiInW contacts, which was observed after annealing for longer times, is believed to be due to an increase in the contact resistances caused by In movement away from the metal/GaAs interfaces. Based on this assumption, an activation energy for In diffusion in GaAs was determined from onset times of the device deterioration to be ∼2.0 eV, which is close to the reported value of 1.9 eV.