Comparison of the mechanism of optical amplification in InGaN/GaN heterostructures grown by molecular beam epitaxy and MOCVD

We comprehensively studied InGaN/GaN heterostructures grown by molecular be am epitaxy (MBE) and metal-organic vapor deposition epitaxy (MOCVD) using a variety of methods of optical spectroscopy, such as cathodoluminescence mi croscopy (CL), time-integrated and time-resolved photoluminescence (PL). Mi cro-photoluminescence and cathodoluminescence results show the variation in emission wavelength at different scales, and this reflects the degree of c ompositional fluctuations in the samples. We obtain information on the deca y times of the main emission lines using time-resolved photoluminescence sp ectroscopy and models of stretched exponentials, indicating the importance of nanoscale fluctuations for the recombination mechanism. The temperature dependent behavoir of the InGaN emission is explained in terms of a carrier freeze out at local potential fluctuations combined with a thermionic ther malization at elevated temperatures. To correlate the fluctuations in emiss ion wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. From all these results we conclude, t hat localized carriers at a statistical distribution of potential fluctuati ons act as recombination centers and that the degree of fluctuations determ ines the efficiency of optical amplification. The threshold values for lasi ng and the gain values are compared and discussed with respect to the diffe rent growth procedures. From all these Endings we draw conclusions concerni ng the influence of differences in the growth conditions and their impact o n the optical properties.