FORMATION OF WSI-BASED OHMIC CONTACTS TO N-TYPE GAAS

An outcome of the indirect doping concept conceived recently in NiGe-b ased Ohmic contacts has led to the development of annealed WSi-based O hmic contacts to n-type GaAs for the first time. It was concluded that simultaneous addition of a ''direct doping element'' of Si in WSi2.7 and an ''indirect doping element (M)'' such as Au, Pd, Cu, or Ag, was essential. The M(5 nm)/WSi2.7(20 nm)/W(50 nm) contacts showed Ohmic be havior after annealing with the lowest contact resistances of 0.4 Omeg a mm (similar to 6 x 10(-6) cm(2)). In addition, the WSi-based contact s with a small amount of Au showed good thermal stability at 400 degre es C after contact formation. Microstructural analysis of the WSI cont acts with Au showed formation of beta-AuGa and WSi2 compounds, which i ndicates that the Ohmic behavior would be due to heavy doping of Si at the GaAs surface induced by Ga out-diffusion. The mechanism of Ohmic contact formation of the present contacts agreed very well with that o f the NiGe-based Ohmic contacts. (C) 1997 Elsevier Science S.A.