Role of Al and Ti for ohmic contact formation in AlGaN/GaN heterostructures

A mechanism for ohmic contact formation using Ti/Al based metals on AlGaN/GaN heterostructures has been investigated by measuring temperature dependence of sheet electron density (ns) and mobility (μ). It was found that both ns and μ at room temperature for Ti/Al deposited sample were increased by annealing in vacuum, while not for Al/Ti deposited one. The results, especially increase in μ, cannot be understood by the conventional ohmic formation model, including Ti-N (nitrogen) complex formation or N vacancy formation. As the most probable mechanism for the increase in ns and μ, we have proposed a model, in which tensile strain is induced by the reaction of Ti/Al and AlGaN after annealing.