MORPHOLOGY OF AL-NI-GE OHMIC CONTACTS TO N-GAAS AS A FUNCTION OF CONTACT COMPOSITION

Al-Ni-Ge ohmic contacts on n-GaAs(001) were prepared by sequential vap or deposition and subsequent annealing at 500 degrees C. Structural pr operties were studied as a function of contact composition, by transmi ssion electron microscopy and scanning electron microscopy, in additio n to other techniques. In all cases, Al was deposited as the top layer at a fixed thickness. While all metallizations exhibit a non-spiking interface with the GaAs substrate, it was found that the contact morph ology varies strongly with the Ge:Ni thickness ratio. For a Ge:Ni thic kness ratio of 3:4 or greater, annealing results in the formation of a thick Al3Ni layer adjacent to the GaAs substrate, as well as a non-un iform surface layer, characterized by dendritic Ge precipitates in an Al matrix. By lowering the Ge:Ni thickness ratio to 1:2, the surface m orphology was greatly improved and the contact displays a stable layer ed structure of the type Al3Ni/(Ni-Ge)/GaAs. These results were accoun ted for on the basis of a recently developed Al-Ni-Ge ternary phase di agram.