Takahiro Suyama

Reduction in Threshold Current Density of Quantum Well Lasers Grown by Molecular Beam Epitaxy on 0.5° Misoriented (111)B Substrates

GaAs/AlGaAs graded-index, separate confinement heterostructure quantum-well lasers with an extremely low threshold current density (Jth) were grown by molecular beam epitaxy on 0.5° misoriented (111)B substrates. The Jth of these lasers was reduced by 20 A/cm2 compared with that of (100)-oriented devices, which results from the quantization along the direction. The minimum Jth of 158 A/cm2 was achieved for a 490 µm long device.

Multi-Coloring of Thin-Film Electroluminescent Device

Green, orange-red, orange, yellow, blue and white light emitting thin-film electroluminescent (EL) devices have been developed by using various rare-earth fluoride luminescence centers. A series of technical data on the relation between device performances and film deposition conditions are presented. The device performances, particularly, brightness and efficiency, much depend on ZnS active layer thickness and the concentration of luminescence centers, while the substrate temperature is not so effectual to obtain high brightness. The brightness of more than 100 foot-Lambert (fL) has been obtained for the devices emitting green (TbF3, ErF3 and HoF3), orange-red (SmF3), yellow (DyF3) and white (PrF3) under 5 kHz sinusoidal excitation. Especially, more than 600 fL is obtained by the TbF3 doped device. Typical efficiencies are 0.15 lm/W for the device doped with TbF3 and 0.03–0.05 lm/W for the devices doped with SmF3, DyF3, PrF3, ErF3 and HoF3.

New type of thin‐film electroluminescent device having a multilayer structure

A new kind of multilayered thin‐film electroluminescent (EL) device is proposed. The device consists of multiple alternate layers in which the functions of carrier acceleration and light emission are separated. One big advantage of the proposed structure is that the light emitting phosphor can be selected independently of the carrier accelerating material, thus permitting a variety of color emissions. Therefore, one can optimize the device performance by selecting combinations of materials and cell structure design parameters. For example, a device employing ZnS and Y2O3:Eu thin films as the carrier accelerator and light emitter, respectively, emits a red color, and a brightness level of 40‐ft lambert has been obtained under sinusoidal voltage excitation of 5 kHz. This value is several times higher than that reported for EL in powdered layers of Y2O3:Eu.

Enhancement of Heavy-Hole-Related Excitonic Optical Transitions in (111)-Oriented Quantum Wells

Photoluminescence excitation spectra have been measured at low temperatures on high quality GaAs/Al0.3Ga0.7As and GaAs/AlAs multiple quantum wells grown by molecular beam epitaxy along the [111] and [100] crystallographic axes. Comparisons of these spectra have shown clear evidence of the enhancement of the heavy-hole-related excitonic optical transitions relative to the light-hole-related transitions in (111)-oriented quantum wells in comparison with (100)-oriented quantum wells.

High Reliability in AlGaAs Laser Diodes Prepared by Molecular Beam Epitaxy on 0.5°-Misoriented (111)B Substrates

Slip lines in the molecular beam epitaxial wafer bonded with In during the growth have been eliminated by using (111)-oriented GaAs substrates instead of the usual (100)-oriented ones. AlGaAs double-heterostructure and quantum well lasers were grown on (100)- and 0.5°-misoriented (111)B substrates. The yield of reliable lasers is much higher for (111)-oriented devices than that for (100)-oriented ones, possibly due to the elimination of local defects such as slip lines.

Enhancement of the Capture Rate of Carriers in (111)-Oriented GaAs/AlGaAs Quantum Well Structures

A detailed analysis on the dependence of the photoluminescence spectrum on the excitation density in (111)- and (100)-oriented single-quantum-well (SQW) structures is presented. The results show that the capture rate of photo-excited carriers in the barrier region by the SQW and the radiative recombination rate in the SQW are enhanced in (111)-oriented SQW structures compared to those in (100)-oriented ones.

High Power Semiconductor Lasers

The high power characteristics of broad stripe gain-guiding quantum well lasers are reported. Graded-index separate confinement heterostructure (GRIN-SCH) -type quantum well lasers are grown on 0.5°misoriented (111) B GaAs substrates by molecular beam epitaxy. The reduction in the optical power density on the facet and in the driving current density is effective for increasing the maximum output power. The maximum output power of 3.7 W has been attained by small optical confinement in the graded-index waveguide region and by using a long cavity of 750μm.