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Abstract
The performance of Pd-Ge based ohmic contacts, with and without Ti-Pt or Ti-Pt-Au capping layers, has been investigated. The contacts were deposited by electron beam evaporation, then characterized electrically using a modified transmission line method (TLM) and structurally using both cross-section and plan-view transmission electron microscopy (TEM). Although both capped and non-capped contact structures underwent the same phase transformations during annealing, capped contacts had significantly better contact resistances (a minimum value of 4×10-7 Ω cm^sup 2^ was achieved) - almost three orders of magnitude better. The superior performance is attributed to the capping layers providing protection for the Pd-Ge layers during contact processing, where the metallization was exposed to a CF^sub 4^-O^sub 2^ plasma, oxyen descumming, organic solvents and deionized water. Non-capped contacts exhibited PdGe decomposition and oxidation of exposed Ge. Long-term reliability testing of capped contacts showed virtually no change in contact resistance at 235°C (1350 h) and a sevenfold increase after ageing at 290°C for 370 h. There were no phase changes during ageing; the increase in contact resistance was attributed to interdiffusion between Ge and GaAs.[PUBLICATION ABSTRACT]