  In this paper, we report the use of lanthanum (La) in S/D contacts of GaN
HEMTs, achieving 0.97 Ohm-mm contact resistance without S/D recess. The HEMTs
show well-behaved electrical characteristics and satisfactory reliability. Our
studies show that La, a CMOS compatible metal, is promising to lower GaN HEMT
S/D contact resistance. La's low work function (3.5 eV) is beneficial for
reducing the barrier between the metals and GaN. The Ohmic contact formation
mechanism involved was shown to be different from conventional Ti/Al films.
Spherical-shaped high-La regions formed near the surface during annealing. La
diffuses into the AlGaN layer, and the overlap of La and Al peaks is
significantly increased compared with that before annealing.
