Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts using transmission electron microscopy

Ohmic contacts to AlGaN/GaN heterostructures which have low contact resistance and good surface morphology are required for the development of commercial high power, high frequency transistors in the GaN system. The development of such contacts would be helped by a better understanding of the effect of microstructure on electrical behavior, which is studied here. Au/Ni/Al/Ti/AlGaN/GaN ohmic contact structures were rapid thermal annealed in argon for 60 s at temperatures in the range 550–900 °C. The variation of contact resistance with anneal temperature was correlated with the phase distribution observed by transmission electron microscopy (TEM). A combination of TEM techniques was required to determine the resulting microstructure, including energy filtered TEM, high resolution electron microscopy and energy dispersive x-ray spectrum imaging. Contacts with the lowest resistance were formed after 700 °C annealing. Very little consumption of the 30 nm AlGaN layer was observed. An unidentified phase containing Al, Ti and Au is present at the interface in the samples with low specific contact resistance. The identification of the observed thin interfacial phases (including TiN and AlN) is discussed, along with the effects of oxidation and possible mechanisms of ohmic contact formation.