Ohmic contacts to n-type GaAs made with Pd/Sn and Pd/Sn/Au metallizations

A non-alloyed ohmic contact system comprising of Pd/Sn metallization has been developed for n-GaAs and compared with Pd/Sn/Au metallization. Contacts are annealed at 300-400°C for 30 min in a graphite strip annealer. Surface morphology of the contacts is investigated using surface profilometry measurements and Scanning Electron Microscopy (SEM). Contact resistivities, ρ_c, of the proposed metallizations are measured utilizing conventional Transmission Line Model (TLM) method. A lowest ρ_c of 2.38×10^-5 Ω-cm² is obtained with Pd(40 nm)/Sn(120 nm) contacts on 2×10¹⁸ cm^-3 n-GaAs after annealing at 400°C for 30 min. A Au overlayer improves both electrical and morphological characteristics of the contacts. The Au overlayer also changes the optimal annealing cycles at the lowest ρ_c points. The Pd(30 nm)/Sn(150 nm)/Au(100 nm) contact shows a lowest ρ_c of ~3.89×10^-6 Ω-cm² after annealing at 330°C for 30 min. The Pd(30 nm)/Sn(90 nm) contact is thermally stable at 300°C for at least 400 h. After 400 h annealing, ρ_c values decrease from 1.13×10^-3 to 1.37×10^-5 Ω-cm ²