Reaction of aluminum-on-titanium bilayer with GaN:: Influence of the Al:Ti atomic ratio

Backscattering spectrometry, x-ray diffractometry, and scanning electron microscopy have been used to study metallurgically the evolution of < GaN >/Ti(40 nm)/Al(180 nm) and < GaN >/Ti(80 nm)/Al(150 nm) metal contacts before and after annealing for 30 min in vacuum between 450 and 800 degrees C. A slight reaction of the titanium with the aluminum is first observable after annealing at 450 degrees C. After 550 degrees C titanium completely converts to Al3Ti. Major differences in the evolution of the samples are observed after annealing at 550 and 700 degrees C depending on the atomic ratio between aluminum and titanium. With excess aluminum remaining after the Al3Ti formation, hillocks form at 550 degrees C and the excess Al melts at 700 degrees C, leading to a very strong roughening of the reacted film. Titanium nitride also appears after annealing at 700 degrees C. The roughening of the surface can be avoided by keeping the atomic ratio of aluminum to titanium below 3. In that case, the excess titanium reacts with the Al3Ti and the second phase AlTi appears. No titanium nitride is observed in the latter case, and aluminum nitride neither, in all cases. These results still need explaining.