Ti/Al ohmic contacts on AlGaN/GaN heterostructures with different defect density

In this paper, the electrical and structural properties of Ti/Al Ohmic contacts have been investigated comparing two AlGaN/GaN heterostructures with different dislocation density (4 × 109 cm−2 and 12 × 109 cm−2, respectively). After thermal annealing, the Ti/Al bilayer deposited on the more defective sample became Ohmic at a lower annealing temperature (500 °C) with respect to the contact formed on the better material (800 °C). Moreover a different temperature dependence of the specific contact resistance RC has been observed in the two samples. The electrical behavior has been correlated with a structural and morphological analysis of surfaces and interfaces, attributing a key role to the presence of V-shaped defects on the AlGaN surface. Indeed, nanoscale conductive atomic force microscopy measurements revealed a preferential conduction through these defects, and consistently explained the temperature dependence of RC.