The Role of an Overlayer in the Formation of Ni-based Transparent Ohmic Contacts to p-GaN

Abstract

The effect of an overlayer in the Ni-based ohmic contacts on contact resistivity, light transmittance and the performance of light emitting diodes (LEDs) has been investigated. The ohmic contact composed of an Au overlayer on Ni resulted in a low contact resistivity of 9×10-4 Ω·cm2 and a high transmittance of 92% after annealing at 500°C in O2 ambient. The LED with a Ni/Au contact showed a threshold voltage and an output power of 4.1 V and 1.7 mW at 20 mA, respectively. In the case of a Pt overlayer on Ni, the minimum contact resistivity was comparable to that of the Ni/Au contacts. However, the maximum transmittance was only 50%, which reduced the output power of the fabricated LED to 1.0 mW. For a Pd overlayer on Ni, the transmittance of 72% was obtained after annealing at 500°C in O2 ambient, while the contacts showed nonohmic behavior. The LED with the Ni/Pd contact resulted in a high threshold voltage of 7.5 V at 20 mA. These results were explained by the differences in the thermodynamic properties between the overlayer and Ni.