Electrical characterization, metallurgical investigation, and thermal stability studies of (Pd, Ti, Au)-based ohmic contacts

Although Pd/Ti/Pd/Au contacts are similar to their Pt/Ti/Pt/Au counterparts in providing low specific contact resistance, rho(c), the former exhibits long-term thermal stability. Their projected mean times to 50% increase in rho(c)(mu(50)) at 150 degrees C to p(+)-GaAs (greater than or equal to 3.43 x 10(15) h) are higher than those of the latter by over five orders of magnitude. Contacts to p(+)-In0.53Ga0.47As are not as thermally stable, with a much lower albeit respectable mu(50) at 150 degrees C of greater than or equal to 2.25 x 10(5) h. Contacts with an interfacial Pd layer provide rho(c)'s that are at least two times lower than those without, and the presence of an oxide layer (GaxTiyOz) at the Ti/GaAs interface is identified as a possible cause. Pd-Ga-As phases are formed at the Pd/GaAs interface, being As-rich (PdxGayAs) initially and convert to Ga-rich phases (PduGavAs) upon a high temperature anneal and the eventual composition depends on the evaporated interfacial Pd thickness and annealing conditions. This could probably explain the existence of an optimum interfacial Pd layer thickness of 100 Angstrom for achieving the lowest rho(c). The Ga-rich PduGavAs phases formed are inferred to cause the liberation of As atoms from the GaAs lattice, thus enabling them to diffuse out to the Ti and react to form TixAsy phases that bind the As from further out-diffusion. This has in turn led to the accumulation of As at the Pd/Ti interface. (C) 2000 American Institute of Physics. [S0021-8979(00)02705-5].