PT TI/GE/PD OHMIC CONTACTS TO GAAS - A STRUCTURAL, CHEMICAL, AND ELECTRICAL INVESTIGATION/

Interface morphology, phase composition, and elemental diffusion of Pt /Ti/Ge/Pd ohmic contacts to both n+- and p+-GaAs have been investigate d as a function of annealing temperature. Structural and chemical resu lts were correlated with specific contact resistances (rho(c)) measure d for each thermal treatment in an attempt to determine how the indiff usion of contact metal and the out-diffusion of Ga and As affect speci fic contact resistance, and how effective Ti is as a diffusion barrier . Annealing at 450-degrees-C yielded the lowest rho(c), approximately 6.4 X 10(-7) OMEGA CM2 . The interface was observed to be smooth and a brupt, and there was little As out-diffusion and Pt indiffusion. Two i nterface phases were detected; a primary phase, PdGe, and a secondary, Ga rich Pd-Ga-As ternary phase. The presence of this ternary phase is believed to be critical to contact formation on n-type GaAs. The Ti a nd Pt layers remained pristine. Annealing at 550-degrees-C resulted in a slightly higher p(c), approximately 2.1 X 10(-6) OMEGA Cm2. There w as significant elemental diffusion within the contact metal and minor diffusion into the GaAs substrate. The interface possessed limited are as of spiking with uniform composition. Annealing at 600-degrees-C pro ved to have a detrimental effect on the rho(c), approximately 10(-4) O MEGA CM2. This electrical degradation was accompanied by strong chemic al intermixing between the contact and substrate, resulting in a conti nuous nonplanar interface with deep multiphase protrusions, and the Ti layer no longer was effective as a diffusion barrier.