Investigation of indium-tin-oxide ohmic contact to p-GaN and its application to high-brightness GaN-based light-emitting diodes

Indium-tin-oxide (ITO) contacts to p-GaN exhibit ohmic characteristics by inserting a 10-nm-thick p-In0.1Ga0.9N layer as an intermediate. The specific contact resistivity of 4.5 × 10−2 Ω cm2 was obtained while annealing ITO/p-GaN contacts at 500 °C. Based on the variation of contact resistivity with respect to temperature, the dominant transport mechanism of ITO/p-GaN structure tended from thermionic field emission to thermionic emission when the post-annealing temperature was raised from 400 °C to 600 °C. From the XPS, XRD and SIMS results, the outdiffusion of gallium atoms and the formation of Ga-O bonds could introduce the gallium vacancies and increase the net concentration of holes, which would benefit the carrier tunneling through the interface. The GaN-based light-emitting diodes (LEDs) with 500 °C-annealed ITO contacts exhibited the forward voltage of 3.43 V and output power of 4.30 mW at a 20-mA-current injection. Although the forward voltage showed a little higher than the conventional LEDs by 0.2 V, the external quantum efficiency and power efficiency were enhanced by about 46% and 36%, respectively. As for the life test, LEDs with 500 °C-annealed ITO contacts presented a similar reliability as the conventional LEDs. Therefore, ITO contacts can make GaN-based LEDs highly bright and reliable in practice.