Optical Studies on Mixed InGaN/GaN Multiple Quantum-well Structures Grown by Metalorganic Chemical Vapor Deposition

In this research, we grow a series of samples by stacking InGaN/GaN quantum wells (QWs) of different compositions in different quantum wells with different indium composition. A series of optical characterization results of the five mixed InGaN/GaN multiple quantum well structures are reported. First, we present systematical material analyses, including electroluminescence, cathodoluminescence, X-ray diffraction, and high-resolution transmission electron microscopy, of the five samples. Then, the PL peak positions and PL integrated intensities show different behaviors of the blue and green emissions in these samples. Furthermore, the Arrhenius plots are fitted to calibrate the activation energy and defect density of these samples. We can generally see that by increasing the number of green-emitting QWs, the defect density and activation energy become higher.
From time-resolved PL (TRPL) measurements, we can generally find that the PL decay time of the green light is longer than that of the blue one. From the results of photon-energy-dependent PL decay time, the longer PL decay times on the low-energy side are partially attributed to the carrier capture from the high-energy localized states. By increasing the temperature, two-component decays, the early-stage decay (faster) time, related to carrier relaxation after they are excited, and the extended decay (slower) decay time, describing the rate of recombination, can be observed. The PL decay times of the green emission as functions of temperature of the five samples show that the decay times generally decrease with temperature. However, some of the decay times increase with increasing temperature. This trend may be attributed to the destruction of excitons and the strong piezoelectric field.