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 l ow temperature conventional quartz tube furnace in an Ar ambient and a new vacuum annealing technique using a tungsten strip heater. Low specific cont act resistivity (rho(c)) metallizations were achieved with furnace annealin g at considerably lower temperatures (550-600 degrees C) than those typical ly required for GaN contacts by halogen lamp rapid thermal annealing (simil ar to 900 degrees C). Vacuum annealing was found to require temperatures si milar to those used in halogen lamp rapid thermal annealing for forming ohm ic contacts on n-type GaN, but with minimal oxidation of the Al surface. Fo r 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. Ou r experiments showed that, when Cr was used as a contact material, the simu ltaneous presence of Cr and Al was necessary in order to obtain the best po ssible ohmic properties. Furthermore, the Cr/Al contacts maintained good st ability 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 microsco py investigations indicated that the introduction of the Ni/Au overlayer ha d the effect of increasing the surface roughness after annealing. (C) 2000 American Institute of Physics. [S0021-8979(00)10201-4].