The Effect of Heat Treatment on Ni/Au Ohmic Contacts to p-Type GaN

The effect of heat treatment temperature on the microstructure and specific contact resistance of oxidized Ni(5 nm)/Au(5 nm) contacts to p-type GaN was investigated. The minimum specific contact resistance (ρ_c) obtained was 4 × 10^—6 Ω cm² after heat treating at 500 °C in air for 10 min. The cross-sectional microstructure of heat treated Ni/Au films on p-type GaN was examined with transmission electron microscope (TEM) in conjunction with compositional analyses. The high value of ρc for samples heat treated at lower temperatures (<400 °C) was attributed to the fact that Au islands and crystalline NiO detached from the p-type GaN. When the temperature increased to 500 °C, NiO films and Au islands epitaxially constructed on p-type GaN matrix. The crystalline NiO may play a crucial role in the formation of low-resistance ohmic contact to p-GaN. Increasing the temperature further to 600 °C, large voids adjacent to p-GaN were observed which resulted in the reduction of contacting area deteriorating the contact resistance.