Comparison of electrical and reliability performances of Ti B ₂ -, Cr B ₂ -, and W ₂ B ₅ -based Ohmic contacts on n-GaN

Three different metal borides (Ti B2, Cr B2, and W2 B5) were examined for the use in TiAlborideTiAu Ohmic contacts on n -type GaN and the reliability compared to the more usual TiAlNiAu metal scheme. The minimum specific contact resistance obtained was in the range of 10-5 Σ cm2 with Cr B2 and W2 B5 and approximately an order of magnitude lower with Ti B2. In all cases, the minimum contact resistance is achieved after annealing in the range of 700-900 °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 Ti B2 and Cr B2 contacts retain smooth morphology even after annealing at 1000 °C. Auger electron spectroscopy depth profiling indicated that the formation of an interfacial Ti NX layer is likely responsible for the Ohmic nature of the contact after annealing. All three boride-based contacts show lower contact resistance than TiAlNiAu after extended aging at 350 °C.