Temperature-dependent electroluminescence in InGaN/GaN multiple-quantum-well light-emitting diodes

Abstract

We present a comparative study on temperature dependence of electroluminescence (EL) of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with identical structure but different indium contents in the active region. For the ultraviolet (UV) and blue LEDs, the EL intensity decreases dramatically with decreasing temperature after reaching a maximum at 150 K. The peak energy exhibits a large redshift in the range of 20-50 meV with a decrease of temperature from 200 K to 70 K, accompanying the appearance of longitudinal-optical (LO) phonon replicas broadening the low energy side of the EL spectra. This redshift is explained by carrier relaxation into lower energy states, leading to dominant radiative recombination at localized states. In contrast, the peak energy of the green LED exhibits a minimal temperatureinduced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. We attribute the different temperature dependences of the EL to different degrees of the localization effects in the MQW regions of the LEDs. region is not yet well understood. In this work, we compare the current-and temperature-dependent electroluminescence (EL) of UV, blue, and green InGaN/GaN MQW LEDs over a wide temperature range. It appears that the behavior of the lowtemperature EL is a strong function of In mole fraction in the active region. This enables us to identify the role of localization effects in carrier capture and radiative recombination in the InGaN/GaN MQWs. EXPERIMENTALThe LED samples were grown on (0001) sapphire substrates using metal-organic chemical-vapor deposition. The device structures were identical. They consisted of a low-temperature GaN buffer layer, a 2.5-m Si-doped n-GaN layer (n ϳ 5 ϫ 10 18 cm Ϫ3 ), a 10-period, undoped-InGaN/GaN MQW layer, a 0.1-m p-type AlGaN cladding layer, and a 0.3-m p-GaN contact layer (Mg concentration ϳ1 ϫ 10 19 cm Ϫ3 ). The nominal indium-mole fractions in the MQW layers were 0.04, 0.20, and 0.35 for the UV, blue, and green LEDs, respectively. A standard technology for GaN-based LED fabrication was used, i.e., the p-GaN was partially etched to form

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