THERMAL-STABILITY OF PD GE-BASED OHMIC CONTACTS TO N-TYPE GAAS/

The microstructural changes that occur during high temperature anneali ng (450-600 degrees C) of Pt/Ti/Ge/Pd ohmic contacts to n-type GaAs ha ve been studied using transmission electron microscopy (TEM). The meta l layers were deposited sequentially by electron beam evaporation onto GaAs doped with Si to a level of approximate to 5x10(18) cm(-3). The deposition sequence and metal layer thicknesses were: Pt (50 nm), Ti ( 30 nm), Ge (90 nm) and Pd (45 nm). The contact microstructure remained uniform up to 550 degrees C and consisted of a continuous polycrystal line layer of GePd at the semiconductor surface, along with pockets of epitaxially grown Ge. Annealing at 550 degrees C resulted in the nucl eation of a Ge-deficient phase, Ge8Pd21 at the GePd/GaAs interface, wh ich protruded into the GaAs and deteriorated the uniformity of the con tact. At higher annealing temperatures, the contact degraded rapidly d ue to inward diffusion of Ti and Pt and outward diffusion of As, leadi ng to the formation of several binary and ternary phases. (C) 1998 Ame rican Institute of Physics.