Gain spectroscopy on InGaN quantum well diodes

One major question that has emerged with the demonstration of the InGaN QW diode injection laser concerns the relationship between a macroscopic device parameter, such as the threshold current density, and the optical gain viewed at a microscopic level. At present, the optical gain, when normalized to the threshold electron-hole pair density, is about one order of magnitude higher in the InGaN QW laser than in the ZnCdSe QW laser, although the two wide gap alloys are rather similar at the level of the virtual crystal approximation (effective masses, etc.). As is well known, the GaN-based heterostructures house a wide range of crystalline defects which appear to impact the active physical properties and device performance in a surprisingly mild manner. The specific issue that is raised in paper concerns the correlation between compositional disorder in the InGaN QW on atomic scale with that of optical gain and associated e-h pair dynamics. There is plenty of evidence to date that InGaN, at In-composition exceeding a few atomic percent, is not a simple random alloy. For example, linear optical probes (luminescence, absorption, reflectance) exhibit spectral linewidths on the order of 100 meV, well in excess of estimates obtained from models based on random potential fluctuations in the cation composition.