Comparison and optimisation of different ohmic contact metallisations for InP-HEMT structures with doped and undoped cap-layers

A direct comparison was made between several ohmic contact metallisations using Ni, Ge and Au for InP based HEMT applications. HEMT structures with both doped and undoped GaInAs cap-layers were investigated. Ni-Ge-Au proved to be the most suitable contact for both structures. An optimum nickel thickness of 100 Å Ni followed by 500 Å Ge and 800 Å Au was found. The contact resistances of the optimised Ni-Ge-Au structure were as low as 0.09 Ωmm and relatively insensitive to anneal time and temperature and are therefore most suitable for IC fabrication. For applications where high anneal temperatures are required, Ge-Ni-Au ohmic contacts show the largest process window for the anneal temperature. Furthermore it is reported that the contact resistance is independent of the doping of the thin (50-100 Å) cap-layer. This indicates that it is the bandgap and not the doping of the cap-layer, which determines the contact resistance to the HEMT structure