Effects of Au overlayers on the electrical and morphological characteristics of Pd/Sn Ohmic contacts to n-GaAs

The effects of Au overlayers on the electrical and morphological characteristics of non-alloyed Pd/Sn Ohmic contacts to n-GaAs have been studied. Surface morphology of the contacts is investigated using surface profilometry measurements and scanning electron microscopy. Contact resistivities, rho(c), of the proposed metallizations are measured using the conventional transmission line model method. A lowest rho(c) of similar to 2.07 x 10(-5) Omega cm(2) is obtained with a Pd(50 nm)/Sn(125 nm) contact on 2 x 10(18) cm(-3) n-GaAs after annealing at 400 degrees C for 30 min under a forming gas atmosphere. An overlayer of Au improves both electrical and morphological characteristics of the Pd/Sn contacts. The Au overlayer also changes the optimal annealing cycles at the lowest rho(c) points. A Pd(50 nm)/Sn(125 nm)/Au(40 nm) contact shows a lowest rho(c) of similar to 5.10 x 10(-6) Omega cm(2) after annealing at 330 degrees C for 30 min. The Pd(30 nm)/Sn(150 nm)/Au(100 nm) contact shows a lowest rho(c) of similar to 3.89 x 10(-6) Omega cm(2) after annealing at 330 degrees C for 30 min.