Vanadium-based Ohmic contacts to n-AlGaN in the entire alloy composition

The authors report on the formation and evaluation of V-based Ohmic contacts to n-AlGaN films in the entire alloy composition. The films were produced by plasma assisted molecular beam epitaxy and doped n-type with Si. The conductivity of the films was determined to vary from 10(3) to 10(-2) (Omega cm)(-1) as the AlN mole fraction increases from 0% to 100%. Ohmic contacts were formed by e-beam evaporation of V(15 nm)/Al(80 nm)/V(20 nm)/Au(100 nm). These contacts were rapid thermal annealed in N-2 for 30 s at various temperatures. The optimum annealing temperature for this contact scheme to n-GaN is about 650 degrees C and increases monotonically to about 1000 degrees C for 95%-100% AlN mole fraction. The specific contact resistivity was found to be about 10(-6) Omega cm(2) for all films up to 70% AlN mole fraction and then increases to 0.1-1 Omega cm(2) for films from 95%-100% AlN mole fraction. These results were accounted for by hypothesizing that vanadium, upon annealing, interacts with the nitride film and forms vanadium nitride, which is consistent with reports that it is a metal with low work function. (c) 2007 American Institute of Physics.