The effect of internal strain on the luminescence properties of an InGaN single quantum well (SQW) was investigated as a function of modification via an underlayer (UL). Single In0.25Ga0.75N QWs (λ = 520 nm) 3 nm thick were grown on various ULs on a sapphire substrate, where the two UL types included (1) a buffer layer onto which an InGaN layer with a very small amount of In was inserted and (2) a buffer layer grown using different carrier gases. The SQWs were then analyzed by temperature-dependent time-resolved photoluminescence, scanning electron microscopy and cathodoluminescence. The experimental results show that the density of non-radiative recombination centers and the level of potential fluctuation in the SQWs decrease with insertion of an In
x
Ga1−x
N UL possessing a quite low but sufficient indium content (x = 0.007). The density of non-radiative recombination centers in the SQW on the H2 carrier-grown UL, however, is large.
The bandgap energy of InGaN alloy materials can be controlled by changing their alloy composition. Since the compositional fluctuation in the InGaN quantum wells (QWs) greatly affects the characteristics of optical devices, it is very important to understand the electronic structures of such fluctuated InGaN-QW systems for the improvement of their device characteristics. Mobility edge (ME), which is the boundary energy between the localized and delocalized states, can be an index to evaluate the potential fluctuation of carriers. Although several experiments have been proposed to evaluate the ME, they give sometimes different results. In this study, we experimentally and theoretically have studied the evaluation method of ME, and have revealed that the commonly-used evaluation method does not necessarily give accurate results. It is suggested that the method using photoluminescence excitation measurement is the best way to estimate the ME in InGaN-QWs
The effects of the potential fluctuation (alloy compositional fluctuation and/or well-width fluctuation) in InGaN quantum wells (QWs) on the characteristics of InGaN-QW lasers, have been theoretically investigated, and it is found that the temperature dependence of the lasing threshold is strongly affected by the degree of fluctuation. Furthermore, we have experimentally measured the temperature dependence of stimulated-emission threshold excitation power density in photo-pump measurements, and have observed the predicted behavior of temperature dependence. It is considered that the temperature dependence of the lasing threshold could be used for the evaluation of the degree of the potential fluctuation in active layers of InGaN QW lasers.
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