Satoru Kijima

Kinetic model for degradation of light-emitting diodes

We present a kinetic model for the optical output degradation of light-emitting diodes based on the carrier-recombination enhanced defect motion. Our model leads to analytical solutions and universal curves for the optical output power and the defect density as a function of the normalized aging time with the initial quantum efficiency as the determining parameter. The theoretical results explain very well the time dependence of the II-VI light-emitting diodes under constant current aging condition. The faster aging rate with increasing bias current or temperature is also investigated both experimentally and theoretically, resulting in a very good agreement. Our model provides a quantitative description of the light-emitting diode aging characteristics for compound semiconductors in the presence of electron-hole recombination-enhanced defect generation.

GaN-Based High Power Blue-Violet Laser Diodes

The epitaxial lateral overgrowth (ELO) technique is an important technology for improving the characteristics of GaN-based laser diodes (LDs). The photoluminescence intensities from GaN and GaInN multiple quantum well active layers in the ELO-GaN wing region were found to be higher than those in the seed region. This indicates that the density of dislocations in the wing region could be reduced significantly. This is evidenced by dislocation densities of less than 106 cm-2 as determined from transmission emission microscopy and etching-pit-density measurements. The cleaved facets of LDs on ELO-GaN and sapphire were observed by atomic forced microscopy. Although the roughness of GaN cleaved facets on sapphire were high (Ra>10 nm), the roughness in the ELO-GaN wing region was found to be as smooth as that of GaAs cleaved facet (Ra<1 nm). The characteristics of LDs on ELO-GaN were found to be superior to those on sapphire as a result of smoother facets and lower dislocation densities.

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.

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.

High-Power AlGaInN Laser Diodes with High Kink Level and Low Relative Intensity Noise

High-power AlGaInN laser diodes (LDs) with both high kink level and low relative intensity noise (RIN) are successfully fabricated. A kink level of higher than 100 mW is obtained by using a new ridge structure and by narrowing the ridge width down to 1.5 µm. This structure consists of a thin Si on SiO2 multiple buried layers instead of the conventional thick SiO2 buried layer. The low RIN of these devices is obtained by decreasing the threshold current; under optical feedback with high-frequency modulation, the RIN values are as low as -125 dB/Hz. As the RIN of these devices exhibits a tendency to deteriorate with increasing operating current, the lifetime criterion was redefined as the point at which the operating current has increased by 20%. Even under this stricter lifetime criterion, the estimated lifetime of these LDs exceeds 15,000 h under 30 mW cw operation at 60°C.

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.

Universal curves for optical power degradation of II–VI light‐emitting diodes

We propose a transient recombination‐enhanced defect‐generation model to analyze the degradation of optical output powers of blue‐green II–VI light‐emitting diodes (LEDs). We find an analytical solution and discover a set of universal curves for the time dependence of optical output power, which agree very well with the experimental data for strained CdZnSe quantum‐well structures. Our model shows that the optical power can be non‐exponential in character and its long‐time behavior has a 1/t dependence. This 1/t dependence is also related to the growth of the defect density, which should behave as a t1/2 dependence. The generation of new defects due to electron‐hole recombination at the defect sites is found to be the dominant degradation mechanism for II–VI LEDs.

Optimized ZnSe:N/ZnTe:N contact structure of ZnSe-based II–VI laser diodes

We have established the basic stability of ZnSe:N/ZnTe:N superlattice (SL) contact needed for 1000 h of operation. We have fabricated ZnCdSe/ZnSSe/ZnMgSSe separated confinement heterostructure laser diodes with the ZnTe:N contact layer/ZnSe:N/ZnTe:N SL/ZnSe:N cap layer to achieve a low voltage (<5 V) at 500 A cm−2 constant current for 1000 h (electrode test) by the reduction of the thickness of the ZnTe:N contact layer and by the optimization of the N concentration in the ZnSe:N cap layer. We found that, when the thickness of the ZnTe:N contact layer becomes 4 nm, the reduction of NA–ND in the ZnSe:N cap layer is prevented, and the defect density in the vicinity of the ZnSe:N/ZnTe:N SL is reduced. It is assumed that the reduction of NA–ND in ZnSe:N is caused by the stress in the ZnSe:N cap layer induced by a large lattice mismatch between ZnTe and ZnSe. We confirmed that the reduction of stress in the ZnSe:N layer and the reduction of the structural defect density were achieved by the reduction of the thi...

AlGaInN-based laser diodes

The longer lifetime is desired for high power AlGaInN based violet lasers. We found that lifetime is strongly dependent on both the initial operating consumption power and the dislocation densities in the laser stripe. Pd/Pt/Au as a metal and AlGaN/GaN superlattice as a p-type cladding layer were incorporated to reduce the operating voltage. The optimization of device parameters as well as the stripe width and the RIE etching device depth led to the lower threshold current of 3.4 kA/cm2. We used the Pendeo epitaxy technique to get lower dislocation density approximately 107 cm-2. The LDs with these technologies showed an output power as high as 35 mW under room temperature CW condition without kink. The lifetime is more than 500 hours under CW operation with a constant power of 20 mW at 25 degree(s)C.

Estimation of Device Properties in AlGaInN‐Based Laser Diodes by Time‐Resolved Photoluminescence

We estimated the optical characteristics of AlGaInN-based laser structures by time-resolved and standard photoluminescence. It was clarified that the microscopic distribution of the peak wavelength of GaInN active layers has a strong correlation with the threshold current and the slope efficiency. As an index representing the extent of this distribution, we used the depth in the tail states of the active layer, E 0 , for which we found an excellent correlation with the device properties. From an estimation of the external quantum efficiency as a function of equivalent cavity length, it was found that the sample with smaller E 0 has higher internal quantum efficiency and smaller internal absorption coefficient than those with larger E 0 .