Shigetaka Tomiya

Characterization of threading dislocations in GaN epitaxial layers

We investigated Si-doped GaN epitaxial layers on a (0001)-sapphire substrate using a HCl vapor-phase etching technique, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Three kinds of distinctive etch pits correspond to three different types of threading dislocations, edge, mixed, and screw types. Photoluminescence intensity increases with the decrease in the number of etch pits corresponding to mixed and screw dislocations. The number of etch pits corresponding to edge dislocations, however, did not change. We concluded, therefore, that threading dislocations having a screw-component burgers vector act as strong nonradiative centers in GaN epitaxial layers, whereas edge dislocations, which are the majority, do not act as nonradiative centers.

Dislocation related issues in the degradation of GaN-based laser diodes

We investigate degraded GaN-based laser diodes (LDs) on epitaxial lateral overgrown GaN layers in terms of dislocations. Almost all of the threading dislocations that appear in the wing regions are a-type dislocations. Their origins are the lateral extension of dislocations from the seed regions that contingently bend upwards to the episurface. Comparing short-lived LDs and long-lived LDs that have almost the same power consumption, we find that the relative levels of dislocation densities in their respective active layers are different. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations are observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. The degradation rate is almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.

Structural study of defects induced during current injection to II–VI blue light emitter

We have carried out structural studies of nonluminescent areas developed by current injection in ZnMgSSe alloy‐based II–VI blue light emitting diodes by electroluminescence topography and transmission electron microscopy. The nonradiative regions, which spread out in the 〈100〉 direction during current injection, consist of a high density of dislocation dipoles and dislocation loops. The source of these defects is the preexisting stacking faults originating at the substrate/epilayer interface. The dipoles themselves are aligned along both of the 〈110〉 directions lying in the {111} plane. Their Burgers vectors were of the type (a/2)〈011〉 inclined at 45° to the (001) junction plane.

Dependence of crystallographic tilt and defect distribution on mask material in epitaxial lateral overgrown GaN layers

We have investigated the dependence of crystallographic tilt and defect distribution on mask material in metalorganic chemical vapor deposition grown GaN layers formed utilizing an epitaxial lateral overgrowth (ELO) technique using x-ray diffraction and transmission electron microscopy. Crystallographic tilt in the ELO GaN layer was suppressed by changing the mask material from electron beam (EB)-evaporated SiO2 to plasma enhanced chemical vapor deposition (PECVD) grown SiO2 and PECVD SiNx. Defect distribution also changes in accordance with mask materials. By depositing a thin PECVD SiNx layer on the PECVD SiO2 mask, the crystalline quality of the ELO layer changes from that used with the SiO2 mask to that used with the SiNx mask. These results suggest that the interface between the ELO GaN layer and the mask has a significant effect on crystallographic tilt and defect distribution.

Organic Single‐Crystal Arrays from Solution‐Phase Growth Using Micropattern with Nucleation Control Region

A method for forming organic single-crystal arrays from solution is demonstrated using an organic semiconductor, 3,9-bis(4-ethylphenyl)-peri-xanthenoxanthene (C(2) Ph-PXX). Supersaturation of C(2) Ph-PXX/tetralin solution is spatially changed by making a large difference in solvent evaporation to generate nuclei at the designated location. The method is simple to implement since it employs only a micropattern and control of the solvent vapor pressure during growth.

Dislocations in GaN‐Based Laser Diodes on Epitaxial Lateral Overgrown GaN Layers

We have investigated dislocations in GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy and cathodoluminescence microscopy and found a correlation between dislocations and device reliability. Dislocation density in the seed regions of ELO GaN layers is of the order of 10 8 cm -2 , while that in the wing regions is less than mid-10 6 cm -2 . The origin of dislocations in the wing regions is the extension of defects in highly defective regions near the GaN layer/substrate interface in the seed regions. The lifetime of LDs has a strong correlation with consumption power. However, some LDs have a shorter lifetime although their consumption power is almost the same. In the LDs with short lifetimes, dislocations lying in the c-plane were formed below the active regions, bent towards the c-axis and threaded upwards to active regions. These newly created dislocations can become detrimental to the device lifetime.

GaN-based blue laser diodes

We report our recent progress on GaN-based high-power laser diodes (LDs), which will be applied as a light source in high-density optical storage systems. We have developed raised-pressure metal-organic chemical vapour deposition (RP-MOCVD), which can reduce the threading-dislocation density in the GaN layer to several times 108 cm-2, and demonstrated continuous-wave (cw) operation of GaN-based LD grown by RP-MOCVD. Furthermore, we found that the epitaxial lateral overgrowth (ELO) technique is useful for further reducing threading-dislocation density to 106 cm-2 and reducing the roughness of the cleaved facet. By using this growth technique and optimizing device parameters, the lifetime of LDs was improved to more than 1000 hours under 30 mW cw operation at 60 °C. Our results proved that reducing both threading-dislocation density and consumption power is a valid approach to realizing a practical GaN-based LD. On the other hand, the practical GaN-based LD was obtained when threading-dislocation density in ELO-GaN was only reduced to 106 cm-2, which is a relatively small reduction as compared with threading-dislocation density in GaAs- and InP-based LDs. We believe that the multiplication of non-radiative centres is very slow in GaN-based LDs, possibly due to the innate character of the GaN-based semiconductor itself.

ZnCdSe/ZnSSe/ZnMgSSe SCH Laser Diode with a GaAs Buffer Layer

Blue-green laser diodes exhibiting continuous-wave operation during hundreds-of-seconds have been fabricated. This structure is a ZnCdSe/ZnSSe/ZnMgSSe separate-confinement heterostructure with low dislocation density of ~105 cm-2 in the n-ZnMgSSe cladding layer. The use of a GaAs buffer layer has lead to the decrease of dislocation density for laser-diodes, with which we have observed CW operation up to 80° C, that showed the feasibility of ZnCdSe/ZnSSe/ZnMgSSe SCH laser-diodes.

Atomic scale characterization of GaInN/GaN multiple quantum wells in V-shaped pits

High-angle annular dark field scanning transmission electron microscopy and atom probe tomography of the V-shaped pits in GaInN/GaN multiple quantum wells have revealed that a quantum well structure exists at the slope region of the V-shaped pits. Their thickness and In concentrations were found to be much lower compared to those of the flat region. This suggests that threading dislocations in the V-shaped pits act as energy barriers for the lateral transport of charge carries and that the pit center may not work properly for vertical transport because of the collapse of well-defined quantum well structures.

Threading Dislocations and Optical Properties of GaN and GaInN

We categorized threading dislocations in GaN and GaInN multiple quantum wells and epitaxially lateral overgrown GaN into three types of line defects (edge, screw and mixed dislocations), and investigated the optical properties. It was confirmed by cathodoluminescence measurements that not only screw and mixed dislocations but also edge dislocations act as non-radiative centers in GaN. Epitaxial lateral overgrowth (ELO) technique can reduce the densities of all line-defects in a several μm wide wing region. Growth steps in the wing region were disturbed by the defects which were left in a seed region, and a complicated structure was formed at the surface of GaN and GaInN layers grown on ELO-GaN at low temperature. We believe that this surface structure formed by high supersaturation is a cause of In compositional spatial fluctuation and phase separation of GaInN alloy.