W and W/WSi/In_1-xAl_xN ohmic contacts to n-type GaN

Two approaches are investigated in order to obtain good stable ohmic contacts to u-type GaN. In the first one the ohmic contact is enabled by depositing W on an n⁺-GaN region produced by Si implantation. The contacts were RTA treated at 750, 850, 950, and 1050°C for 10 s. The 750, 950 and 1050°C anneals produced good ohmic contacts with contact resistivities ranging from 2.6·10^-6 to 1.1·10^-4 Ωcm², as measured by the transmission line model technique, while the 850°C anneal produced leaky rectifying diodes. In the second approach, a graded In_1-xAl_xN epilayer facilitates the proper band alignment needed for an ohmic contact. In order to test the feasibility of this approach a stack of 500 angstrom W/500 angstrom WSi/3500 angstrom n⁺-In_1-xAl_xN/AlN with x = 0, 0.27 and 0.46 was deposited on sapphire and RTA treated at temperatures up to 700°C. Contact resistivity values of 1·10^-6 to 1·10^-5 Ωcm² were obtained. The contact resistivity increased as the annealing temperature increased.