Reducing forward bias voltage of InGaN/GaN-based light emitting diodes by using (In)GaN contact layer

The electrical properties of GaN-based light emitting diodes (LEDs) fabricated with indium-tin-oxide (ITO) p-contacts were investigated as functions of the thickness of the (In)GaN contact layers and the In content. The LEDs with the GaN contact layers showed lower forward voltages (in the range of 3.48-3.03 V) than the LEDs with ITO-only contacts (4.2 V); the forward voltages of the LEDs decreased with increasing contact layer thickness (from 1-4 nm) and increasing In content. However, the output power linearly decreased with increasing In content, whereas that of the GaN contact layer LEDs became saturated at a thickness of 2 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level for the samples with the contact layers was shifted toward lower binding energies by 0.11-0.22 eV compared with that of the sample without the contact layer. However, the energy shift decreased with increasing In content. Unlike the contact-layer samples, the sample without the contact layer experienced outdiffusion of N atoms. Based on the XPS and atomic force microscopy results, the contact-layer-induced electrical improvement was described and discussed.