Kazuhiko Itaya

Room Temperature Pulsed Operation of Nitride Based Multi-Quantum-Well Laser Diodes with Cleaved Facets on Conventional C-Face Sapphire Substrates

We demonstrate room temperature pulsed operation of nitride based multi-quantum-well (MQW) laser diodes with cleaved mirror facets grown on a conventional C-face sapphire substrate. Cleavage was performed along the direction of the sapphire substrate, and the resultant facet was analyzed using an atomic force microscope (AFM) and theoretical calculation. A single peak emisson, at a wavelength of 417.5 nm, with a full width at half-maximum of 0.15 nm, was obtained. The threshold current density of the laser was 50 kA/cm2 and a voltage for the threshold current was 20 V.

Short-wavelength InGaAlP visible laser diodes

Results achieved on the operation of short-wavelength InGaAlP visible-light laser diodes are described. It is found that there is a strong correlation between operation temperature and oscillation wavelength. The deterioration of favorable temperature characteristics in the short-wavelength region is attributed to an increase in the leakage current, which originates in a small conduction-band discontinuity inherent in the InGaAlP material system. The introduction of a highly doped p-cladding layer improves these temperature characteristics. Short-wavelength oscillation at 630-nm-band wavelength was achieved for transverse-mode stabilized InGaAlP lasers. >

High‐brightness InGaAlP green light‐emitting diodes

Candela class InGaAlP surface‐emission green light‐emitting diodes (LEDs) have been successfully fabricated. The growth of InGaAlP on an intentionally misoriented substrate improved emission properties, as confirmed by a quantitative estimation of the diffusion length in the active material. A Bragg reflector using InGaAlP was realized for the first time. A new device structure, including this Bragg reflector, drastically improved the light extraction efficiency. An external quantum efficiency of 0.7% at 573 nm green light was obtained, corresponding to a luminous intensity of 2 cd.

Effects of thermal treatment of low-temperature GaN buffer layers on the quality of subsequent GaN layers

The surface morphology and crystalline quality of GaN layers grown by metalorganic chemical vapor deposition on sapphire (0001) substrates were investigated for various thermal treatment conditions of low-temperature (LT)-grown GaN buffer layers. The surface morphology and crystalline quality of subsequently grown high-temperature (HT) GaN layers strongly depended on thermal effects during the temperature ramping process after LT growth of the buffer layers. We have found that the defect density and structure are affected by this temperature ramping process, and that the generation of growth pits is closely related to defects in the HT-GaN layers. High-quality HT-GaN layers with specular surface morphology were obtained with optimum growth and ramping conditions for the LT-GaN buffer layers. Furthermore, the role of thermal treatment during the temperature ramping process was identified, and mechanisms of nucleus formation, HT-growth initiation on the LT-GaN buffer layers, and defect formation are propose...

Highly reliable InGaP/InGaAlP visible light emitting inner stripe lasers with 667 nm lasing wavelength

To obtain highly reliable InGaP/InGaAlP inner stripe (IS) lasers, the relation between the maximum CW operation temperature and other laser characteristics has been determined. The Al composition of the cladding layer, the carrier concentration of the p-cladding layer, and the thicknesses of the active layer and cladding layer have been optimized. It was found that an Al composition of 0.7 was the most suitable for the cladding layer, and the optimized carrier concentration was 4*10/sup 17/ cm/sup -3/. A maximum temperature of 90 degrees C was obtained for a 0.1- mu m active layer thickness and a 0.6- mu m cladding layer thickness. This is the highest value for InGaP/InGaAlP IS lasers. In the case of a 0.06- mu m active layer thickness and a 0.8- mu m cladding layer thickness, a maximum temperature of 75 degrees C was obtained. IS lasers with facet coating have been operating stably for more than 8000 h at 40 degrees C and 3 mW and for more than 4000 h at 50 degrees C and 3 mW. >

Temperature dependence of the threshold current for InGaAlP visible laser diodes

The temperature dependence of the threshold current for InGaAlP visible-light laser diodes was investigated from the standpoint of gain-current characteristics. The dependence of the light output power versus the current characteristic on the cavity length was evaluated for a 40- mu m-wide InGaP-InGaAlP broad-stripe laser in the temperature range between -70 and 90 degrees C. The threshold-current density dependence on the cavity length shows that a linear-gain approximation is suitable for this system. A minimum threshold-current density of 860 A/cm/sup 2/ was achieved at room temperature with a cavity length of 1160 mu m. The internal quantum efficiency decreased in the temperature range higher than -10 degrees C, which affected the excess threshold-current increase and the decrease in the characteristic temperature at this temperature range. >

Characteristics of a distributed Bragg reflector for the visible‐light spectral region using InGaAlP and GaAs: Comparison of transparent‐ and loss‐type structures

Distributed Bragg reflectors (DBRs) for the visible‐light spectral region constructed by InAlP/InGaAlP pairs and InAlP/GaAs pairs, grown by metalorganic chemical‐vapor deposition, have been investigated with the point of comparing optical and structural properties. In the case of the InAlP/InGaAlP stacked type, a reflectivity above 80% has been realized; however, the bandwidth decreased by shortening the peak wavelength. The reflection spectrum of the InAlP/GaAs DBR showed a wide‐band characteristic and provided a relatively high reflectivity to the green region in spite of the absorption loss of the GaAs layers. These results were in good agreement with theoretical calculations considering the absorption loss and refractive index of individual stacked layers. These DBRs have also been confirmed to have good uniformity and periodicity through cross‐sectional transmission electron microscopy and x‐ray rocking curve analysis. Good electrical conductivity through these DBRs has been obtained for the n‐type s...

New window-structure InGaAlP visible light laser diodes by self-selective Zn diffusion-induced disordering

The behavior of Zn diffusion in InGaAlP double heterostructures during a second metalorganic chemical vapor deposition (MOCVD) growth was investigated, and it found that the behavior of Zn diffusion strongly depends on the presence of an n-GaAs capping layer. A process for InGaAlP window-structure InGaAlP laser diodes was designed using this unique diffusion behavior and disordering of a natural superlattice. A window structure realizing a high power operation was successfully fabricated. A maximum output power above 80 mW was obtained for continuous wave (CW) operation, and 400 mW for pulsed operation. >

Long-term reliability tests for InGaAlP visible laser diodes

Long-term reliability tests of over 10,000 hours have been carried out for 670-nm-wavelength InGaAlP visible laser diodes at elevated ambient temperatures beyond 50°C. Lifetime characteristics of inner stripe structure lasers were investigated from the viewpoints of structural parameter and aging conditions. At least two degradation modes were observed in the InGaAlP laser. One of them was related to facet oxidation and reduced by facet coating. The other degradation mode strongly depended on the operation current density. Mean times to failure of 24,900 hours for facet-coated thin-active-layer (0.06-µm) devices and 17,200 hours for as-cleaved-facet thick-active-layer (0.1-µm) devices were estimated at 50°C. The activation energy of the lifetime was over 1.7 eV in the temperature range between 50°C and 60°C.

Hybrid-Type InGaAlP/GaAs Distributed Bragg Reflectors for InGaAlP Light-Emitting Diodes

Hybrid-type distributed Bragg reflectors (DBRs) with InAlP/GaAs and InAlP/InGaAlP multilayers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated. The structure of the DBRs was designed using a theoretical calculations considering the absorption loss and refractive index of each stacked layer. The wide-band, high-reflectivity characteristics were also experimentally confirmed. Good electrical conductivity through InGaAlP light-emitting diodes (LEDs) with the hybrid-type DBRs was obtained in spite of the many interface needed for the multiple layers of the DBRs. A luminous intensity of 0.8 cd was obtained at 565.7 nm (nearly pure green light).