High temperature stable WSi{sub x} ohmic contacts on GaN

Abstract

The authors have sputter-deposited 500--1200{angstrom} thick WSi{sub 0.45} metallization onto n{sup +} GaN (n{ge}10{sup 19}cm{sup {minus}3}) doped either during MOCVD growth or by direct Si{sup +} ion implantation (5{times}10{sup 15}cm{sup {minus}2}, 100 keV) activated by RTA at 1,100 C for 30 secs. In the epi samples R{sub C} values of {approximately}10{sup {minus}14}{Omega}cm{sup 2} were obtained, and were stable to {approximately}1000 C. The annealing treatments up to 600 C had little effect on the WSi{sub x}/GaN interface, but the {beta}-W{sub 2}N phase formed between 700--800 C, concomitant with a strong reduction in near-surface crystalline defects in the GaN. Spiking of the metallization down the threading and misfit dislocations was observed at 800 C, extending >5,000{angstrom} in some cases. This can create junction shorting in bipolar or thyristor devices, R{sub C} values of <10{sup {minus}6}{Omega}cm{sup 2} were obtained on the implanted samples for 950 C annealing, with values of after 1050 C anneals. The lower R{sub C} values compared to epi samples appear to be a result of the higher peak doping achieved. The authors observed wide spreads in R{sub C} values over a wafer surface, with the values on 950 C annealed material ranging from 10{sup {minus}7} to 10{sup {minus}4}{Omega}cm{sup 2}. There appear to be highly nonuniform doping regions in the GaN, perhaps associated with the high defect density in heteroepitaxial material, and this may contribute to the variations observed. They believe that near-surface stoichiometry is variable in much of the GaN currently produced due to the relative ease of preferential N{sub 2} loss and the common use of H{sub 2}-containing growth (and cool-down) ambients. Finally, the ohmic contact behavior of WSi{sub x} on abrupt and graded composition In{sub x}Al{sub 1{minus}x}N layers has been studied as a function of growth temperature, InN mole fraction (x = 0.5--1) and post WSi{sub x} deposition annealing treatment