Spectroscopy of Non-Radiative Recombination Centers in Blue-Light Emitting Semiconductors by Two-Wavelength Excitation Scheme

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1. Analog Measurement System and Excitation Density Dependence :In addition to single-photon-counting, we arranged an analog measurement system by introducing a low-noize lock-in amplifier. The dependence of an above-gap excitation (AGE) density on the PL intensity decrease due to a below-gap excitation (BGE) light was measured for a GaN grown by MOCVD. The result agreed well with our two-states model.A filtered D_2 lamp (4.1 eV) and aNd : YAG laser (1.17 eV) were used as an AGE and BGE light sources.2. Temperature Dependence of the BGE effect:The normalized PL intensity defined as the ratio of PL intensities with and without BGE, I_<A+B>/I_A, was unity above 150 K in case of a donor-acceptor pair (DAP) luminescence of a GaN epitaxial layer. With lowering the temperature below 150K, it decreased steeply down to 0.42 and then became constant below 80K. This is attributed to a thermal emission of electrons from the upper level of the two-states model to the conduction band. A thermal activation energy of 0.1 eV *s calculated by curve fitting, which was consistent with our previous energy determination of the level. No distinct temperature dependence of yellow luminescence is important to analyze nonradiative recombination (NRR) processes.3. Below-Gap States in GaN-based Semiconductors:Based on these results on GaN, InGaN/GaN and GaN/AlGaN multiple quantum well structures, we summarized the spatial and energy distribution, density and capture coefficients of NRR centers detected by our two-wavelength excited PL. They were presented at International Conferences and published in Academic Journals on the list.