OHMIC CONTACT FORMATION IN PALLADIUM-BASED METALLIZATIONS TO N-TYPE INP

Two Pd-based metallizations have been systematically studied, i.e., Au /Ge/Pd and Pd/Ge contacts to n-type InP, in an attempt to better under stand the role of the metallization constituents in forming ohmic cont acts. Ohmic contacts were obtained with minimum specific resistances o f 2.5 x 10(-6) OMEGA-cm2 and 4.2 x 10(-6) OMEGA-cm2 for the Au/Ge/Pd a nd the Pd/Ge contacts, respectively. The annealing regime for ohmic co ntact formation is 300-375-degrees-C for the Au/Ge/Pd/InP system and 3 50-450-degrees-C for the Pd/GeInP system. Palladium, in both cases, re acts with InP to form an amorphous layer and then an epitaxial layer a t low temperatures, providing good metallization adhesion to InP subst rates and improved contact morphology. Ohmic contact formation in both contacts is attributed to Ge doping, based on the solid state reactio n-driven decomposition of an epitaxial layer at the metallization/InP interface, producing a very thin, heavily doped InP layer. Gold appear s to be responsible for the difference in contact resistance in the tw o systems. It is postulated that Au reacts strongly with In to form Au -In compounds, creating additional In site vacancies in the InP surfac e region (relative to the Au-free metallization), thereby enhancing Ge doping of the InP surface and lowering the contact resistance. Both c ontacts degrade and ultimately become Schottky barriers again if over annealed, due to consumption of additional InP, which destroys the hea vily doped InP layer.