Thermal stability of metallizations on GaN/Al_xGa _1-xN/GaN heterostructures

This study examines two different forms of thermal stability of gate metallizations on GaN/Al_xGa_1-xN/GaN heterostructures: long-term anneals at moderate temperatures (accelerated ageing) and high-temperature rapid thermal anneals (commonly used for ohmic contact formation). The metallizations Re, Pt, Pd, Au, Ni, Ni/Au, Ni/Ga/Ni, Co and Co/Au were selected based on criteria associated with thermodynamic predictions and the metal work function. Long-term (200 h) thermal stability experiments for unpassivated samples annealed at 500°C within a tube furnace revealed very good thermal stability of Pt/Au diodes. In addition, Re and Ni/Ga/Ni Schottky diodes may also provide thermally stable gate metallizations for high-temperature devices. As a result of short-term annealing, all metals tested apart from Au and Re yielded stable diodes when annealed for 60 s up to 400°C. Re contacts showed increases in reverse current densities after annealing at temperatures between 400 and 600°C, with a dramatic return to as-deposited values following an anneal at 800°C for 60 s. Upon longer-term annealing at 800°C for 10 min, dissolution of Ga into the Re film and complete loss of the thin GaN cap on the heterostructure were observed, resulting in the Re metal and underlying Al_xGa_1-xN layer making direct contact.