Role of alloy fluctuations in InGaN-based LEDs and laser diodes

UV/blue/green InGaN and GaN single-quantum-well structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The external quantum efficiency (EQE) of the UV InGaN LED on ELOG was much higher than that on sapphire only at high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG even at low- and high-current operations due to the lack of localized energy states formed by alloy composition fluctuations. In order to improve the lifetime of laser diode (LD), ELOG had to be used because the operating current density of the LD is much higher than that of LED. A violet InGaN multi-quantum-well/GaN/AlGaN separate-confinement-heterostructure LD with an emission wavelength of 410 nm was grown on ELOG substrate. The LDs with cleaved mirror facets showed an output power as high as 40 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed at an output power of up to 40 mW. The estimated lifetime of the LDs at a constant output power of 30 mW was more than 500 hours under CW operation at an ambient temperature of 60 degreesC.