Interfacial reactions in the formation of ohmic contacts to wide bandgap semiconductors

The influence of interfacial reactions on the formation of ohmic contacts to GaAs, ZnSe and GaN based semiconductor layers is reviewed. In the case of semiconductors whose Fermi levels are not pinned (Z&e and GaN), disruption of interfacial contamination layers is critical during the interfacial reaction step. In these cases, interfacial phase formation appears to be detrimental to the contact properties. In GaAs where the Fermi level is pinned near midgap, interfacial reactions which consist of dissociation of the GaAs lattice and regrowth of this lattice in the presence of a dopant, are critical to successful formation of the ohmic contact. Dopant incorporation during GaAs regrowth leads to a heavily doped surface layer, charge transport across the interface by thermionic field emission, and ohmic behavior with a low specific contact resistance. The most difficult contacts to form are those to p-type ZnSe and GaN since the top of the valence band is at too low an energy to match with the work function of any known metal or compound. To date no metallization scheme has been identified which will lead to lattice reactions with ZnSe or GaN and yield a p’+ surface layer and subsequent ohmic contact formation.