NiGe-based ohmic contacts to n-type GaAs
Refractory NiGe ohmic contacts which have excellent thermal stability and smooth surface have been developed. To apply these contacts to the future very large scale integration GaAs devices, reduction of the contact resistance (Rc) of the NiGe contacts is mandatory. In the present paper, in order to obtain a guideline for the Rc reduction, the formation mechanism of the NiGe contacts was investigated. The NiGe contacts were found to have two different ohmic contact formation mechanisms. These mechanisms suggested that facilitation of heavy doping at the GaAs surface and/or in the Ge layer was very effective to reduce the Rc values of the NiGe contacts. Experimentally, the Rc reduction was demonstrated by adding a small amount of third elements. Direct doping elements (Sn, Sb, and Te) and indirect doping elements (Pd, Pt, Au, Ag, and Cu) were chosen as the third elements. In additon, the effect of addition of In, which forms alow barrier layer between metal and GaAs, was investigated. The contact resistance of these NiGe-based contacts were close to 0.3 Ω mm, and they provided smooth surface and shallow reaction depth. Finally, the NiGe-based contacts were compared with the conventional AuGeNi contact.