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 resista nce and good surface morphology are required for the development of commerc ial high power, high frequency transistors in the GaN system. The developme nt 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 degreesC. The variation of co ntact resistance with anneal temperature was correlated with the phase dist ribution observed by transmission electron microscopy (TEM). A combination of TEM techniques was required to determine the resulting microstructure, i ncluding energy filtered TEM, high resolution electron microscopy and energ y dispersive x-ray spectrum imaging. Contacts with the lowest resistance we re formed after 700 degreesC annealing. Very little consumption of the 30 n m AlGaN layer was observed. An unidentified phase containing Al, Ti and Au is present at the interface in the samples with low specific contact resist ance. The identification of the observed thin interfacial phases (including TiN and AlN) is discussed, along with the effects of oxidation and possibl e mechanisms of ohmic contact formation. (C) 2001 American Institute of Phy sics.