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

Abstract

A bilayer Nd/Al metallization structure has been deposited onto low pressure organometallic vapor phase epitaxy grown n-type GaN ( 1 × 10¹⁸ 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°C and from 500 to 650°C using conventional thermal annealing (CTA) and rapid thermal annealing (RTA), respectively. The lowest values for the specify contact resistivity of 9.8 × 10⁻⁶ Ω−cm² and 8 × 10⁻⁶ Ω−cm² were obtained using Nd/Al metallization with CTA of 250°C for 5 min and RTA of 600°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°C for rapid thermal annealing. Auger electron spectroscopy depth profiling was employed to investigate the metallurgy and interdiffusion of contact formation.