Investigation of the structural and electrical properties of Al-Ge-Ni contacts to GaAs

The microstructure of Al-Ge-Ni ohmic contacts to both n- and p-type GaAs has been investigated by high resolution transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS). Electrical assessment of these contacts shows that ohmic contacts with low specific contact resistances are formed on both n- and p-type material and that the thickness of Ge deposited and the alloying time have a large influence over the degree of ohmicity observed, particularly in the case of n-type material. TEM had previously shown the contact interface to be extremely flat and uniform in all cases with a continuous single phase polycrystalline layer of Al₃Ni adjacent to the semicon-ductor. SIMS shows that the contact components, especially the Al, diffuse into the un-derlying semiconductor during alloying. The possible reasons for the observed varia-tions in ohmic behaviour as a result of processing are discussed in terms of the microstructure.