Internal quantum efficiency (IQE) of radiative recombination for photo-excited carriers in compound semiconductor materials is usually estimated from temperature dependence of photoluminescence (PL) intensity by assuming that the IQE at cryogenic temperature is unity. III-nitride semiconductors, however, usually have large defect-density, and the assumption is not necessarily valid. In this study, we developed a new method to estimate accurate IQE values by simultaneous PL and photo-acoustic (PA) measurements, and demonstratively evaluated the IQE values for an InGaN quantum-well sample. The results show that the conventional method cannot give accurate IQE values, and that our method is a promising way for accurate estimation of absolute IQE values, which could lead to the accurate estimation of radiative and nonradiative recombination lifetimes in carrier dynamics studies.
We have successfully developed a method for directly forming organic single-crystal thin films at designated locations on a substrate by solution-phase growth. An original micropattern, in which small rectangular regions were connected to a large rectangular region, was designed. The small regions and the large region were used as nucleation control regions (NCRs) and a growth control region (GCR), respectively. The key to success was to vary local supersaturation of a solution droplet by making a large difference in solvent evaporation between a NCR and a GCR. We found that the NCR played a very important role in forming a single nucleus and in investigating the possibility of control of the crystal orientation. By using the developed micropattern and controlling the solvent vapor pressure during growth, we fabricated single-crystal arrays of a stable organic semiconductor, 3,9-bis(4-ethylphenyl)-peri-xanthenoxanthene (C2Ph-PXX).
In order to elongate lifetime of high power pure-blue GaN based laser diodes, reduction of newly created structural
defects at active region, which consists of multiple quantum well structures, is inevitable. We, first, report on detailed
structural analysis of this new type defects and discuss formation mechanism and reduction methodology of these
defects. We, then, fabricated laser diodes with current injection-free structure at front facets, which is confirmed to be
effective for suppression of degradation by catastrophic optical damage. We also discuss degradation mechanism of the
We developed high-power and long-lived AlGaInN-based pure-blue semiconductor lasers emitting in the 440-450 nm
wavelength range. The half lifetime (the time for the output power to degrade to half its initial value in constant current
mode) was estimated to be more than 10000 hours at a power of 0.75 W under continuous-wave operation at 35°C.
Reducing the density of structural defects newly originating from the multiple quantum well active layer and reducing
the operating current density were shown to be important for producing high-performance pure-blue lasers.
We report on various kinds of structural defects frequently observed in nitride-based laser diodes (LDs). First, we discuss threading dislocations in the nitride-based LDs. By investigating structural analysis of epitaxial lateral overgrown (ELO) GaN layers, comparison study between short-lived LDs and long-lived LDs, and degradation analysis, we show that although threading dislocations do not multiple during the device operation, reduction of threading dislocations is primarily important for improving device reliability. Secondly, we investigate the Mg-related structural defects. The other important aspect for extending the device lifetime is optimization of Mg doping. During the course of our study of LDs, inverse pyramidal defects were often found in Mg-doped layers. We analyze the relationship between the pyramidal defects and the atomic concentration of Mg [Mg] and discuss device degradation mechanism in terms of degradation rate and Mg doping. Thirdly, we describe structural defects observed in GaInN multiple quantum wells (MQWs). Apart from previously reported structural defects such as In-rich precipitates and clusters, we found a new type of structural defects in GaInN MQWs with higher In concentrations. These defects consist of planar defects and associated dislocations. These multiple defects can be formed merely by one monolayer In-In bond.
High power GaN-based laser diodes (LDs) are very desirable for various applications such as optical storage systems. We have obtained GaN films of low dislocation density using epitaxial lateral overgrowth technique and the raised- pressure metalorganic chemical vapor deposition technique. Dislocation density of the improved GaN is about 107 cm-2. Optimized GaN-based LDs fabricated on the improved GaN films have operated up to 35 mW without any kink. The lifetime is more than 500 hours with a constant power of 20 mW at 25 degree(s)C under continuous wave conditions. Furthermore, we have introduced buried-ridge laser diode structure in order to control the optical transverse mode. The features of the far field patterns of LDs with AlGaN burying layers indicate their controllability.
We summarize here recent progress in II-VI blue-green semiconductor laser diodes (LDs). ZnMgSSe quaternary alloy, a promising material for the cladding layer, has enabled us to realize a long lifetime exceeding 100h with improvements in the active layer, the electrode, and growth techniques. Studies of degradation have revealed that II-VI LDs degrade not catastrophically, but gradually with enhancement by electron-hole recombination at defect sites.