Cr/Al and Cr/Al/Ni/Au ohmic contacts to n-type GaN

In this work we investigate the performance of Cr/Al and Cr/Al/Ni/Au ohmic contacts on n-type GaN. Annealing of the contacts was achieved by using a low temperature conventional quartz tube furnace in an Ar ambient and a new vacuum annealing technique using a tungsten strip heater. Low specific contact resistivity (rho(c)) metallizations were achieved with furnace annealing at considerably lower temperatures (550-600 degrees C) than those typically required for GaN contacts by halogen lamp rapid thermal annealing (similar to 900 degrees C). Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. For the Cr/Al bilayer on GaN with n doping of 10(18) cm(-3), minimum specific contact resistivities of 1.6x10(-4) Omega cm(2) and 2.3x10(-5) Omega cm(2) were achieved for furnace annealing and vacuum annealing, respectively. Our experiments showed that, when Cr was used as a contact material, the simultaneous presence of Cr and Al was necessary in order to obtain the best possible ohmic properties. Furthermore, the Cr/Al contacts maintained good stability at elevated temperatures. The Cr/Al/Ni/Au system offers lower rho(c) values and even greater temperature stability than the Cr/Al system when annealed in the 800-1200 degrees C temperature range. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer had the effect of increasing the surface roughness after annealing. (C) 2000 American Institute of Physics. [S0021-8979(00)10201-4].