Inter-diffusion in compositions for transparent contacts

Contact structures using contact materials Au, Ge and Pd on n-type GaAs were metallized, formed and studied. The transfer characteristics, namely contact resistivity (pc), for the structures was assessed using transmission line model. The results showed that the structures under appropriate annealing conditions (period and temperature) exhibit ohmic behaviour with low contact resistivity pc (~10-6 Q cm2). It was found that pc decreases with increasing annealing temperature up to ~180 0C. The pc was also found to decrease with increasing substrate-doping density. The contact formation mechanism for the structures Au/Ge/Pd was also studied by a compositional analysis. The compositional distribution in metallization was investigated by collecting the Rutherford 2.3 MeV 4He++ backscattering spectra for the as-deposited structures and the structures after annealed. The results supported the Au-Ge inter-diffusion mechanism. However, the factors which were crucially responsible for the Au/Ge/Pd transparent-contacts with low contact resistance are the solid-phase regrowth, inter-diffusion between Au and Ge, and the enhancement of the conductivity of the excess Ge due to the incorporation of Au.