Investigation of n-ohmic contact of vertical GaN-based light-emitting diodes on graphite substrate with Ag-In bonding

Vertical light-emitting diodes (VLEDs) were successfully transferred from a GaN-based sapphire substrate to a graphite substrate by using low-temperature and cost-effective Ag-In bonding, followed by the removal of the sapphire substrate using a laser lift-off (LLO) technique. One reason for the high thermal stability of the AgIn bonding compounds is that both the bonding metals and Cr/Au n-ohmic contact metal are capable of surviving annealing temperatures in excess of 600 °C. Therefore, the annealing of n-ohmic contact was performed at temperatures of 400 °C and 500 °C for 1 min in ambient air by using the rapid thermal annealing (RTA) process. The performance of the n-ohmic contact metal in VLEDs on a graphite substrate was investigated in this study. As a result, the final fabricated VLEDs (chip size: 1000 µm×1000 µm) demonstrated excellent performance with an average output power of 538.64 mW and a low operating voltage of 3.21 V at 350 mA, which corresponds to an enhancement of 9.3% in the light output power and a reduction of 1.8% in the forward voltage compared to that without any n-ohmic contact treatment. This points to a high level of thermal stability and cost-effective Ag-In bonding, which is promising for application to VLED fabrication.