Temperature and doping-dependent resistivity of Ti/Au/Pd/Au multilayer ohmic contact to <i>n</i>-GaN

Abstract

The temperature (both measurement temperature and annealing temperature) and doping dependence of specific contact resistivity of Ti/Au/Pd/Au multilayer ohmic contact have been studied. The metallization for the contact involves the deposition of the composite metal layer Ti/Au/Pd/Au (200 Å/600 Å/400 Å/500 Å) on n-GaN. When the contacts were annealed at 800 °C for 30 s in air it was observed that the contact resistivity decreases with increasing doping concentration of the n-GaN, but increases with increasing measurement temperature of the contact. Within the framework of the transmission line measurement method, the specific contact resistivity for doping ND=6×1017 cm−3 was ρS=1.0×10−4 Ω cm2, and for doping ND=1020 cm−3 was ρS=2.38×10−9 Ω cm2. The resistivity ρS=1.0×10−4 Ω cm2 for ND=6×1017 cm−3 decreased to ρS≈8×10−7 Ω cm2 after a second annealing. Thus annealing, some times more than once, appears to play an important role in shaping the best value of the resistivity. The physical cause underlying the realization of doping and temperature dependence of the contact involves the diffusion of much of Pd and Au deep into n-GaN leaving Ti on the surface and allowing Ti to react with GaN to form TiN. The work functions of TiN and GaN are close; also TiN has a very narrow band-gap metallic character. Both of these are important for yielding good ohmic contact to n-GaN. Nitrogen vacancies which behave as donor atoms are also formed. They enhance the quality of the contact resistivity.