The role of Ti/A1 ratio in nanolayered ohmic contacts for GaN/AlGaN HEMTs

The role of the former-Ti/Al ratio in the nanolayered ohmic contacts to GaN/AlGaN HEMT structures has been studied for three ratios: (30/70) wt.%, (50/50) wt.% and (70/30) wt.%. The dependence of the electrical properties and surface morphology on the initial contact composition and annealing conditions has been investigated. Lowest contact resistivity of 4.22times10⁵ Omegacm² has been achieved for Ti/Al (30/70 wt.%) and Ti/Al (50/50 wt.%) contacts to semi-insulating GaN/AlGaN heterostructures. Smoothest contact surface and most narrow contact periphery have been obtained with a Ti/Al (70/30 wt.%) contact indicating that increase of the Ti/Al ratio improves the contact morphology. The contact composition with former-Ti/Al ratio of (50/50) wt.% has been found to fulfill the best both requirements for low resistivity and a smooth surface of the contact.