Tatsuhiko Niina

Buried Heterostructure GaAs/GaAlAs Distributed Bragg Reflector Surface Emitting Laser with Very Low Threshold (5.2 mA) under Room Temperature CW Conditions

We would like to report on the performance of buried heterostructure (BH) GaAs/Ga0.65Al0.35As surface emitting (SE) lasers with p-type Ga0.9Al0.1As/Ga0.4Al0.6As and SiO2/TiO2 multilayer Bragg reflectors (MBR) under room temperature CW conditins. The buried heterostructure was formed by a selective LPE growth technique with a Ga0.55Al0.45As mask instead of the conventional SiO2 mask. The GaAlAs mask was very useful for forming uniformly small active regions of about 5 µm in diameter. Under room temperature CW conditions, the threshold current was 5.2 mA. The external differential quantum efficiency was about 16% and more than 0.6 mW of output power was obtained. The lasing wavelength was about 897 nm.

Growth of undoped, high purity, high resistivity ZnSe layers by molecular beam epitaxy

The growth of undoped, high purity ZnSe layers by molecular beam epitaxy with extremely high purity Se source materials refined by sublimation is described. The resistivity of these layers is greater than 104 Ω cm. In low‐temperature photoluminescence spectra, only free‐exciton emission line becomes dominant and all the bound exciton emission lines are extinguished, which is characteristic of very high purity ZnSe. Detailed behavior of electrical and optical properties of normally undoped layers is remarkably dependent on the purification cycles of Se materials used as source materials. The results indicate that the high resistivity of undoped, high purity ZnSe layers is due to a reduction of residual donor impurities.

Ga-Doped ZnSe Grown by Molecular Beam Epitaxy for Blue Light Emitting Diodes

The MBE-grown ZnSe layer on GaAs (100) with an optimum doping of Ga is found to exhibit an extremely low resistivity and a strong blue near-band-gap photoluminescence (PL) emission (~4610 A) without any deep broad emissions at room temperature. At lower Ga-cell temperatures, the carrier concentration and blue PL emission intensity increases significantly owing to an increase in substitutional Ga atoms acting as donors. At higher Ga-cell temperatures, an excess incorporation of Ga results in a reduction of carrier concentration, a decrease in blue PL emission, and a marked increase in deep broad emission (~5900 A).

The preparation of conductive ZnS films by using MBE with a single effusion source

We have successfully grown ZnS films on GaAs and GaP substrates by using an MBE technique with a single effusion source of ZnS compounds. The epitaxial ZnS films grown on the (001) face of the substrates have a cubic structure free from twins and exhibit n-type conductivity with a maximum carrier concentration of ≈ 2x1014 cm-3 and a maximum Hall electron mobility of ≈ 90 cm2/Vh.s. This was realized under the optimum growth temperature of ≈ 240°C. Their luminescence spectra show that two kinds of deep level luminescence, divided into a high energy blue broad emission band and a low energy red broad emission band, are dominant. Their major origin may be attributed to the Cu impurity which is incorporated during growth by evaporation of the impurities contained in the source materials. On the other hand, epitaxial ZnS films grown on the(111) face of the substrates contain {111} twins 180° rotated about the axis normal to the grown layer surface so that they exhibit a resistivity greater than 104 Ω cm.

Growth and characterization of MBE-grown ZnTe:P

Abstract Heteroepitaxial P-doped ZnTe films with precisely controlled carrier concentrations ranging from 1.2 × 10 16 to 4 × 10 17 cm -3 through the regulation of P-beam pressure were successfully grown on GaAs by means of MBE. The crystallinity of the P-doped films and the activation efficiency of incorporated P-impurities were strongly affected by beam pressure ratios of Te/Zn beams (BPRs). In the Te rich case, where two-dimensional growth was dominant, a high quality film was grown and an efficient carrier activation from shallow P-acceptors took place together with dominant excitonic PL emission. In the Zn rich case, where three-dimensional growth occured, the quality of the grown film was degraded, so that an uncontrollable carrier concentration was confirmed with the existence of a strong deep PL emission. However, from SIMS analysis, the concentration of P-impurities incorporated in both films was controllable by P-beam pressure; BPRs were not as well-defined a parameter for governing the concentration of P-impurities as the P-beam pressure.

GaAs buried heterostructure vertical cavity top-surface emitting lasers

Room-temperature continuous wave (CW) operation was achieved using GaAs buried heterostructure vertical cavity top-surface emitting lasers with both GaAlAs/AlAs and SiO/sub 2//TiO/sub 2/ distributed Bragg reflectors (DBRs). One-step organometallic vapor phase epitaxy (OMVPE) and two-step liquid phase epitaxy (LPE) growth techniques has been developed. In order to improve the reflectivity of both DBRs, the Bragg wavelength was designed to correspond with a longer mode than the lasing mode under pulsed conditions, and a sufficiently flat planar surface was formed by LPE growth. The threshold current was 17.4 mA, and an output power of up to 0.84 mW was obtained. The lasing wavelength was about 911 nm. A 5*6 common voltage array was used as a trial structure for a two-dimensional array consisting of the buried heterostructure top-surface emitting laser diodes. >

Fabrication of SiC Blue LEDs Using Off-Oriented Substrates

SiC blue LEDs were fabricated by the dipping technique, using 6H-SiC substrates whose (000)C face varied toward the [110] or [100] direction. The surface morphology of the epitaxial layers on the off-oriented substrates was a striped pattern perpendicular to the off-axis direction. By using off-oriented substrates, the degradation of SiC blue LEDs can be greatly reduced. High-efficiency blue LEDs (12 mcd at 20 mA) were fabricated by increasing the amount of Al dopant in the n-type layer.

Single Crystal Growth of 6H-SiC by a Vacuum Sublimation Method, and Blue LEDs

Undoped inch-sized n-type single crystal ingots of 6H-SiC have been successfully fabricated, and degradation (emission color shift from blue to greenish-blue) of SiC blue LEDs prepared using off-angle substrates is effectively prevented.

Suppression of Cu diffusion from a bulk ZnSe substrate to a homoepitaxial layer by Se‐beam irradiation as a pregrowth treatment

Cu diffusion of homoepitaxial ZnSe was investigated through low‐temperature photoluminescence (PL) spectra and secondary ion mass spectroscopy. Though Cu in ZnSe normally diffuses easily, we found that Cu in a ZnSe substrate did not diffuse into the homoepitaxial layer when the substrate was heated under Se‐beam irradiation prior to growth. Cu‐related emissions, such as Ideep1 and Cu‐green, disappeared from the PL spectrum of the homoepitaxial layer grown on the Se‐beam irradiated substrate. This suppressed Cu diffusion can be explained by site transformation of interstitial Cu atoms in the ZnSe substrate into Cu atoms occupying the Zn lattice site. This treatment is very useful for improving the purity of homoepitaxial ZnSe layers.

Fundamental transverse mode 100-mW semiconductor laser with high reliability

A 100 mW high-power AlGaAs laser with the current-blocking regions near the facets and a long cavity length has been developed. The length and the channel width of the current-blocking region, and cavity length were examined in order to obtain a high-output power. As a result, a stable fundamental transverse mode operation is obtained up to 180 mW, and maximum output power is 230 mW under CW operation. Stable operation under 100 mW of output power was confirmed for more than 2000 hours at 60 C.