野村 一郎

Novel MgZnCdSe layers with various Mg composition and a high optical property ZnCdSe/MgZnCdSe quantum well (QW) structure were grown on (100) InP substrates by molecular beam epitaxy. For Cl-doped Mg-0.25(Zn0.5Cd0.5)(0.75)Se layers, the electron density of 4 x 10(17) cm(-3) was obtained. In addition, refractive indices of the MgZnCdSe layers were evaluated. Using the refractive indices, ZnCdSe/MgZnCdSe strained single quantum well (SSQW) lasers are theoretically investigated for the first time. As a result, it is predicted that the lasing wavelength is controlled from 550 to 640 nm by changing the strain amount of the ZnCdSe SSQW active layer from -1.5% (tensile) to 1.5% (compressive), and that threshold current density values are from 0.2 to 0.4 kA/cm(2).

Cubic GaInN/GaN/AlGaN quantum well (QW) lasers are theoretically analyzed using the dipole moment which is estimated by fitting the theoretical light absorption coefficients to the experimental values. The threshold current density of 1.0kA/cm(2) is calculated for the single QW laser with the lasing wavelength of 415nm.

GaInP-AlGaInP compressively strained multiple quantum-wire layers were fabricated by the irt situ strain induced lateral layer ordering process, during gas source molecular beam epitaxial (GS-MBE) growth, The effect of compositional modulation was described in terms of PE spectra, and TEM images for GaInP-AlGaInP MQWR lasers with 18 period (GaP)(1.5)-(InP)(1.5) SPBS active layers. Based on transmission electron microscopy (TEM) images, the size of quantum-wire width was estimated, and the size fluctuation of quantum wires were discussed, Quantum-wire effect was discussed in terms of anisotropic lasing characteristics and EL polarization, which were reflected by an anisotropic oscillation strength in quantum wires and the comparison with GaInP-AlGaInP compressively strained quantum-film lasers was examined in terms of threshold current density, The condition under which quantum wires were formed by strained induced lateral layer ordering process was discussed in terms of anisotropic behaviors of lasing characteristics, such as threshold current density and lasing wavelength for GaInP-AlGaInP MQWR lasers with (GaP)m/(InP)m SPBS active layers, The lowest obtained J(th) value was 278 A/cm(2) under the room temperature (r.t,) pulsed condition, The first CW operation of GaInP-AlGaInP quantum-wire laser was described, Threshold current was 294 A/cm(2) and CW operation up to 70 degrees C was obtained.

Annealing effects on hreshold current density (Jth) Of GaInP strained single quantum well (SSQW) lasers were systematically investigated. After the annealing, remarkable reduction of the Jth of 630-650nm lasers was observed, accompanied by increase of p carrier density of the p-cladding layers. Especially, the Jth Of 633-nm lasers was reduced from 850 A/cm(2) to 555 A/cm(2). The annealing effect on the Jth reduction can be theoretically explained by suppressing electron leakage currents, especially drift leakage current over p-side heterobarriers.

GaInP strained single quantum well (SSQW) lasers in the 630-710 nm range grown by gas source molecular beam epitaxy (GSMBE) using a novel shutter control method were systematically investigated. A good controllability in strain amount as well as good in-plane compositional homogeneity over 4-cm-long distance of the laser wafers were confirmed. For +1.1% compressive SSQW lasers (708 nm in wavelength), a very low threshold current density (J(th)) Of 175 A/cm(2) was obtained. These facts suggest that the novel shutter control method is very effective for fabricating GaInP SSQW lasers. Furthermore, the post-annealing effect on lasing characteristics of GaInP SSQW lasers was investigated. The increased p carrier density of Be-doped AlInP layers was obtained with the remarkable reduction in J(th) value of 633 nm tensile SSQW lasers from 850 A/cm(2) to 555 A/cm(2)

Gas source molecular beam epitaxy (GSMBE) is one of the most promising technologies for growing AlGaInP which is an important compound system for realizing high performance 550-700 nm range visible light emitting devices. In this paper we review the investigations carried out for GSMBE-grown AlGaInP crystals and visible lasers, involving several results for AlGaInP systems grown by metal-organic chemical vapour deposition. First of all, we investigated growth conditions of high optical-quality GaInP by GSMBE, followed by the fabrication of 600 nm range GaInP/AlInP double-heterostructure lasers. To improve the lasing performances, various techniques described in this paper have been investigated. (GaInP)2.5/(AlInP)2.5 short-period superlattices were utilized to suppress the non-radiative recombination at the interface between the active and AlInP cladding layers. These superlattices were also used as the superlattice barriers in multiple-quantum-well active layers. The effect of misoriented GaAs substrates from the (100) surface toward the [011] direction on material properties and lasing performances was systematically investigated. It was clarified that the spontaneous crystal ordering was suppressed and the Al-related non-radiative recombination was reduced. Also, this substrate misorientation effect was effective in enhancing the electrical activity of Be in AlInP cladding layers with a resultant drastic reduction in threshold current density. The introduction of novel multiple-quantum-barrier structure into lasers was also investigated, for the first time; this was very effective in reducing the carrier overflow from the active layer. As a result, a drastic improvement in lasing performance of 660 nm lasers was obtained. Finally, a new shutter control method to form a strained quantum well layer without growth interruption is presented.

GaInP/AlInP quasi-quaternary (QQ) compounds grown by gas-source molecular-beam epitaxy were systematically investigated. Quasi-quaternary properties were obtained with GaInP/AlInP short-period superlattices when the thickness of the AlInP layers was less than 3 ML. Equivalent band-gap values of QQ compounds were defined in terms of the layer thickness ratios of the GaInP and AlInP layers. Using GaInP/AlInP quasi-quaternary compounds, 607-640-nm wavelength QQ lasers with 20-nm QQ active layers were fabricated. Threshold current densities as low as 1.5 kA/cm2 were attained for QQ lasers emitting at 635 nm without the presence of strain or quantum-well effects.

A novel shutter control method for growing GaInP strained layers without growth interruption by gas source molecular beam epitaxy (GSMBE) is presented. By using the method, strained single quantum well (SSQW) GaInP/AlInP visible lasers were fabricated by GSMBE, for the first time. The threshold current density of the laser was 329 A/cm2 for the compressive strained case (700 nm in wavelength) and 1.7 kA/cm2 for the tensile strained case (634 nm).

Multiquantum barriers (MQBs) were introduced into 660 nm GaInP/AlInP lasers with superlattice confinement (SLC) layers, resulting in drastic improvements in lasing performance. Lowest threshold current densities (840 A/cm2) and highest room-temperature values for T0 (167 K) ever reported for 660-680 nm range lasers with bulk active layers were achieved. High-temperature characteristics of the threshold current densities were measured in order to investigate the enhanced carrier confinement effect of MQBs and to estimate the excess nonradiative recombination current component. From the temperature dependence on the excess current density, the activation energies E0 of nonradiative processes were estimated to be 0.45 eV for MQB-SLC lasers, and 0.26 eV for conventional SLC lasers without MQB. The increase of E0 demonstrates the enhanced heterobarrier effect by MQBs.