Low resistance bilayer Nd/Al ohmic contacts on n-type GaN

A bilayer Nd/Al metallization structure has been deposited onto low pressure organometallic vapor phase epitaxy grown n-type GaN(1x10(18) cm(-3)) by electron-beam evaporation. Ohmic metal contacts were patterned photolithographically for standard transmission line measurement, and then thermally annealed at temperatures ranging from 200 to 350 degrees C and from 500 to 650 degrees C using conventional thermal annealing (CTA) and rapid thermal annealing (RTA), respectively. The lowest values for the specify contact resistivity of 9.8x10(-6) Omega-cm(2) and 8x10(-6) Omega-cm(2) were obtained using Nd/Al metallization with CTA of 250 degrees C for 5 min and RTA of 600 degrees C for 30 s. Examination of the surface morphology using atomic force microscopy as a function of annealing temperature revealed that the surface roughness was strongly influenced by conventional thermal annealing, it was smooth in the temperature range from 550 to 650 degrees C for rapid thermal annealing. Auger electron spectroscopy depth profiling was employed to investigate the metallurgy and interdiffusion of contact formation.