High temperature stability of Ti/Al/Pd/Au ohmic contact for AlGaN/GaN HEMTs

In this paper, a high temperature stable Ti/Al/Pd/Au for ohmic contacts of AlGaN/GaN high electron mobility transistors is reported. Compared with the conventional TiAlNiAu structure, the metal scheme reported takes advantage of the thermal stability and the high oxygen blocking ability of palladium, which improves the stability of ohmic resistance, facilitating the formation of high quality AlGaN/GaN HEMT ohmic metals. A low Ti/Au/Pd/Au contact resistance of 0.19 Ω mm was obtained by rapid thermal annealing at 930 °C, 50 s. Transmission line model (TLM), transmission electron microscopy, and energy-dispersive x-ray spectroscopy (EDS) were used to characterize the high-temperature stability of the contact. EDS line scanning indicated that the existence of oxides at the metal/GaN interface in Ti/Al/Ni/Au impede metal to diffuse after rapid thermal annealing (RTA). Metal clusters increase the barrier layer height of the ohmic contact and affect the electrical properties of Ti/Al/Ni/Au during the temperature-dependent TLM test. The results show that Ti/Al/Pd/Au formed a uniform Al-Pd-Au metallic phase during RTA, causing a lower barrier layer height and better contact resistance performance at different measurement temperatures.