Comparison of electrical and reliability performances of TiB2-, CrB2-, and W2B5-based Ohmic contacts on n-GaN

Three different metal borides (TiB2, CrB2, and W2B5) were examined for the use in Ti/Al/boride/Ti/Au Ohmic contacts on n-type GaN and the reliability compared to the more usual Ti/Al/Ni/Au metal scheme. The minimum specific contact resistance obtained was in the range of 10(-5) Omega cm(2) with CrB2 and W2B5 and approximately an order of magnitude lower with TiB2. In all cases, the minimum contact resistance is achieved after annealing in the range of 700-900 degrees C. The main current transport mechanism in the contacts after this annealing is tunneling as determined by the absence of any significant measurement temperature dependence to the contact resistance. The TiB2 and CrB2 contacts retain smooth morphology even after annealing at 1000 degrees C. Auger electron spectroscopy depth profiling indicated that the formation of an interfacial TiNx layer is likely responsible for the Ohmic nature of the contact after annealing. All three boride-based contacts show lower contact resistance than Ti/Al/Ni/Au after extended aging at 350 degrees C. (c) 2006 American Vacuum Society.