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.