Formation mechanism of InxGa1?xAs ohmic contacts to n-type GaAs prepared by radio frequency sputtering

The formation mechanisms of InAs/Ni/W ohmic contacts to n-type GaAs prepared by radio-frequency (rf) sputtering were studied by measuring contact resistances (R_c) using a transmission line method and by analyzing the interfacial structure mainly by x-ray diffraction and transmission electron microscopy. Current-voltage characteristics of the InAs/Ni/W contacts after annealing at temperatures above 600°C showed “ohmic-like behavior.” In order to obtain the “ohmic” behavior in the contacts, pre-heating at 300°C prior to high temperature annealing was found to be essential. The contacts showed ohmic behavior after annealing at temperatures in the range of 500∼850°C and contact resistance values of as low as ∼0.3Ω-mm were obtained. By analyzing the interfacial structures of these contacts, In_xGa_1−xAs layers with low density of misfit dislocations at the In_xGa_1−xAs and GaAs interface were observed to grow epitaxially on the GaAs substrate upon heating at high temperatures. This intermediate In_xGa_1−xAs layer is believed to divide the high energy barrier at the contact metal and GaAs interface into two low barriers, resulting in reduction of the contact resistance. In addition, Ni was found to play a key role to relax a strain in the In_xGa_1−xAs layer (introduced due to lattice mismatch between the In_xGa_1−xAs and GaAs) by forming an intermediate Ni_xGaAs layer on the GaAs surface prior to formation of the In_xGa_1−xxAs layer.