Contact Resistivity Dependence on Ge:Ni Ratio in AuNiAuGe Metallization on n-GaAs

Abstract

It is found that the Ni:Ge ratio in AuGeNiAu metallization on n-GaAs determines the optimally low contact resistance condition independent of the Au layer thickness. In two series of experiments the thickness x of the Ni layer was varied in the structure Au/Ni/Ge/Au with layer thicknesses of 600/x/50/100 nm and 600/x/100/200 nm. Optimally low contact resistivity of 6.4×10-6 Ω cm2 and 1.68×10-5 Ω cm2, respectively, were obtained when the Ni:Ge layer thickness ratio reached 1. Secondary-ion mass spectroscopy showed that, at this Ni:Ge ratio of 1, maximum Ge doping on the n-GaAs was obtained. Such a condition is a result of a balance between the catalytic effect of Ni on the decomposition of GaAs by Au for effective doping by Ge, to achieve low contact resistance, and the formation of NiAs, rather than Ni2GeAs, under the excess-Ni condition, which results in an increase of contact resistivity.