Ti/Al Based Ohmic Contact to As-Grown N-Polar GaN

Ti/Al based Ohmic contacts to as-grown N-polar GaN are investigated by cross-section transmission electron microscopy and energy dispersive x-ray spectroscopy. Due to the higher oxygen background doping in the N-polar GaN, the Al metal in Ohmic stacks is found to react with background oxygen more easily, resulting in more AlO_x. In addition, the formation of AlO_x is affected by the Al layer thickness greatly. The AlO_x combined with the presence of AlN is detrimental to the Ohmic contacts for N-polar GaN compared with Ga-polar GaN. With the reduction of the Al layer thickness to some extent, less AlO_x and AlN are formed, and lower Ohmic contact resistance is obtained. The lowest contact resistivity ρ of 1.97×10^?6 Ω?cm² is achieved with the Al layer thickness of 80 nm.