PERFORMANCE OF PD-GE BASED OHMIC CONTACTS TO N-TYPE GAAS

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 d eposited by electron beam evaporation, then characterized electrically using a modified transmission line method (TLM) and structurally usin g both cross-section and plan-view transmission electron microscopy (T EM). Although both capped and non-capped contact structures underwent the same phase transformations during annealing, capped contacts had s ignificantly better contact resistances (a minimum value of 4 x 10(-7) Omega cm(2) was achieved)- almost three orders of magnitude better. Th e superior performance is attributed to the capping layers providing p rotection for the Pd-Ge layers during contact processing, where the me tallization was exposed to a CF4-O-2 plasma, oxyen descumming, organic solvents and deionized water. Non-capped contacts exhibited PdGe deco mposition and oxidation of exposed Ge. Long-term reliability testing o f capped contacts showed virtually no change in contact resistance at 235 degrees C (1350 h) and a sevenfold increase after ageing at 290 de grees C for 370 h. There were no phase changes during ageing; the incr ease in contact resistance was attributed to interdiffusion between Ge and GaAs.