Stable low resistance ohmic contacts to p-GaN

Stable and low-resistance Ohmic contacts are especially important for laser diodes where high current levels are required. Good contacts are especially difficult on p-type GaN which was the motivation for this study. The GaN was epitaxially grown on (0001) sapphire substrates by MOCVD. Resistivity of this layer was 3.5 Ohm-cm and thickness was 2 microns. After conventional cleaning followed by treatment in boiling HNO₃: HCl (1:3), metallization was by thermally evaporating 40 nm Au / 60 nm Ni or 70 nm Au / 55 nm Pd. Heat treatment in O₂ + N₂ at various temperatures followed, with best results at 600 °C or 700 °C, respectively. Best values of the contact resistance were 1.8×10^-4 Ohm-cm² for Pd/Au and 2.65×10^-4 Ohm-cm² for Ni/Au contacts. After repetitive cycling from room temperature to 600 °C, the Ni contacts were very stable and more stable than the Pd contacts. X-ray photoelectron spectroscopy depth profiling showed the Ni contacts to be NiO followed by Au at the interface for the Ni/Au contacts whereas the Pd/Au contacts exhibited a Pd: Au solid solution. Some contacts were quenched in liquid nitrogen following sintering. These contacts were much more uniform under atomic force microscopy examination and gave a 3 times lower contact resistance with the Ni/Au design. Current-voltage-temperature analysis revealed that conduction was predominantly by thermionic field emission.