Nomura Ichirou

Abstract In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (μ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15–20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumn μ-LEDs with an φ12 μm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm⁻². The process of nanocolumn μ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window of φ5 μm.

Conduction band discontinuity (ΔEc) of MgSe/ZnCdSe heterojunctions were evaluated using n–i–n diodes consisting of an undoped i-MgSe layer sandwiched by n-doped ZnCdSe layers. The n–i–n diodes were fabricated on InP substrates by molecular beam epitaxy. Injection current density versus applied voltage (J–V) characteristics of the n–i–n diodes were measured at 77 K and room temperature. In addition, the theoretical J–V characteristics of the n–i–n diode were calculated while varying ΔEc. By fitting the theoretical data to the experimental data, ΔEc was estimated to be 1.2 eV from the result at 77 K. This value is similar to the ΔEc estimated from the literature.

The application of indium tin oxide (ITO) as the p-cladding layer of II-VI compound semiconductor laser diodes (LDs) on InP substrates was investigated. The waveguide analysis of the LD structures revealed that the optical confinement effect around the active layer was obviously improved by changing the p-cladding layer from the conventional MgSe/BeZnTe superlattice to ITO. For example, the estimated optical confinement factors were 0.15 and 0.27 for the conventional and ITO LD structure, respectively, when the emission wavelength was 580 nm. In addition, we investigated optimum LD structures, considering the optical and carrier confinements at the active layer. In experiments, light emitting devices with an ITO layer were fabricated on InP substrates via molecular beam epitaxy and radio-frequency (RF) magnetron sputtering. Yellow emissions at 582 nm were observed by current injections at room temperature. These results indicate that ITO is a promising p-cladding layer material for II-VI LDs on InP substrates. (C) 2016 WILEY-VCH Verlag GmbH & Co.

II-VI-compound-semiconductor laser diode (LD) structures on InP substrates were investigated using device simulations and waveguide analysis. Our simulations showed that electron injection from the n-cladding into the active layer is hindered by the n-side barrier layer between the n-cladding and active layer. Consequently, holes are not injected into the active layer but instead leak to the n-side layers. It was shown that carrier injection efficiency can be improved by removing the n-barrier. On the contrary, no large differences were observed between the optical confinement factors of the LD structures with and without the n-barrier layer. In ex-periments, we have fabricated the LD structures with and without the n-barrier layer on InP substrates using molecular beam epitaxy. The turn-on voltage of the device without the n-barrier was smaller than that for the device with the n-barrier by about 5 V. Spontaneous orange emissions around 603 nm were observed for the devices without the n-barrier. In contrast, no emission was observed for the devices with the n-barrier. These results prove that the carrier injection into the active layer is enhanced by the removal of the n-barrier, leading to improved the device performances. (C) 2016 WILEY-VCH Verlag GmbH & Co.

N-doped p-type ZnTe and ZnSeTe contact layers were investigated to evaluate which is more suitable for use in II-VI compound semiconductor optical devices on InP substrates. Contact resistances (R-c) between the contact layers and several electrode materials (Pd/Pt/Au, Pd/Au, and Au) were measured by the circular transmission line model (c-TLM) method using p-n diode samples grown on InP substrates by molecular beam epitaxy (MBE). The lowest R-c (6.5 x 10(-5) Omega cm(2)) was obtained in the case of the ZuTe contact and Pd/Pt/Au electrode combination, which proves that the combination is suitable for obtaining low R-c. Yellow light-emitting diode devices with a ZnTe and ZnSeTe p-contact layer were fabricated by MBE to investigate the effect of different contact layers. The devices were characterized under direct current injections at room temperature. Yellow emission at around 600 urn was observed for each device. Higher emission intensity and lower slope resistance were obtained for the device with the ZuTe contact layer and Pd/Pt/Au electrode compared with other devices. These device performances are ascribed to the low R-c of the ZnTe contact and Pd/Pt/Au electrode combination. (C) 2015 Elsevier B.V. All rights reserved.

The photoluminescence (PL) characteristics of ZnCdSe/BeZnTe type-II superlattices (SLs) with various thickness combinations of ZnCdSe and BeZnTe layers were investigated at room temperature. High PL peak intensities that were comparable to those of other direct-bandgap materials, such as BeZnSeTe, were obtained. The PL peak wavelength was widely controlled from 508 to 739 nm by changing the layer thickness combination (ZnCdSe/BeZnTe) from 1 monolayer (ML)/5 ML to 6 ML/5 ML. The peak wavelengths were evaluated by comparison with theoretical values. Three-primary-color luminescence with PL peaks of 506, 530, and 617 nm was demonstrated for the SL sample with three types of monolithically stacked SLs with different layer thickness combinations (i.e., 1 ML/5 ML, 2 ML/5 ML, and 5 ML/5 ML). (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnTe and ZnSeTe p-contact layers were investigated for use in BeZnSeTe/MgZnCdSe optical devices on InP sub-strates. The contact resistances (Rc) of Au electrodes to N-doped p-ZnTe and p-ZnSeTe contact lay-ers were evaluated using the circular transmission line model (c-TLM) method. A lower Rc (3.1x10(-3) Omega cm(2)) was obtained for the ZnSeTe p-contact sample than for the ZnTe sample (6.3x10(-3) Omega cm2). BeZnSeTe/MgZnCdSe yellow-light-emitting devices with a ZnTe or ZnSeTe p-contact layer grown on InP substrates were characterized. A comparison of the current-voltage (I-V) characteristics of these devices showed that the turn-on voltage of the ZnSeTe contact device was about 4 V lower than that of the ZnTe device. From the aging characteristics of these devices under direct-current injection at room temperature, it was shown that the lifetime of the ZnSeTe contact device was 30 times longer than that of the ZnTe contact device. These results show that a ZnSeTe contact layer is more suitable for BeZnSeTe/MgZnCdSe optical devices on InP substrates than a ZnTe contact layer.

Indium tin oxide (ITO) transparent electrodes formed on II-VI compound semiconductor optical devices on InP substrates were characterized. Two kinds of ZnCdSe/BeZnTe p-n diode devices with a different p-contact layer (i.e., p-ZnTe and p-ZnSeTe layers) were prepared to investigate more suitable p-contact material for ITO electrodes on the II-VI devices. Thin ITO layers were deposited on the devices by magnetron sputtering. From injection current density versus applied voltage (J-V) characteristics, the turn-on voltage of the ZnTe contact device was found to be lower than that of the ZnSeTe contact device by about 2 V. The J-V characteristic of the ZnTe device was comparable to that of the device with Au electrodes on the p-contact instead of ITO. Using ITO electrodes, ZnCdSe/BeZnTe erlattice photovoltaic devices with ZnTe and ZnSeTe p-contact layers were fabricated. The devices were characterized under 1.5 AM illumination at room temperature. The J-V characteristics showed that the device performances of the ZnTe contact device were almost erior to those of the ZnSeTe device. Open-circuit voltage, short-circuit current density, fill factor, and maximum conversion efficiency of the ZnTe device were 0.90 V, 4.7 A/cm(2), 0.35, and 1.5%, respectively

We proposed BeZnTe/ZnSeTe superlattice quasi-quaternaries (SLQQs) as an active layer material replaced with BeZnSeTe quaternaries for yellow/green light emitting devices. BeZnTe/ZnSeTe SLQQs with various layer thickness combinations of BeZnTe and ZnSeTe were grown on InP substrates by a molecular beam epitaxy (MBE). In photoluminescence (PL) measurements at room temperature, high PL intensities were obtained for the SLQQs compared with BeZnSeTe. The peak wavelength was controlled from 589 to 480 nm by changing the layer thickness combination (BeZnTe/ZnSeTe) from 1 monolayer (ML)/20 ML to 4 ML/2 ML Applying the SLQQ for the active layer, LEDs were fabricated on n-type InP substrates. Single-peak yellow electroluminescence at 584 nm was observed with the full wave half maximum (FWHM) of 105 meV. (c) 2013 Elsevier B.V. All rights reserved.

The photopumped lasing characteristics of double heterostructures with a BeZnSeTe active layer grown on InP substrates were systematically investigated. Green-to-yellow lasing emissions from 538 to 570nm were observed at room temperature (RT). The threshold excitation power density (P-th) was approximately 30 kW/cm(2). From the temperature dependence of Pth, stable lasing emissions were obtained up to 353 K. The characteristic temperatures of Pth were 106 to 140 K above RT. The relationship between the threshold gain (G(th)) and the threshold carrier density (N-th) was estimated from the cavity length dependence of Pth and by waveguide analysis. Using the relationship between G(th) and N-th, the threshold current densities (J(th)) of electrically pumped BeZnSeTe laser diode structures were calculated to be less than 1.3 kA/cm(2). Jth decreases as the lasing wavelength increases from 538 to 570 nm. The above results demonstrate that BeZnSeTe is a promising active-layer material for high-performance green-to-yellow LDs. (C) 2011 The Japan Society of Applied Physics

BeZnSeTe/(MgSe/BeZnTe) double heterostructures were fabricated on InP substrates by molecular beam epitaxy and their optical exciting laser properties were evaluated. Yellow-green lasing at 564 nm at room temperature was achieved for the first time. The threshold excitation power density was 149 kW/cm(2) when the cavity length was 2 mm. The threshold power density linearly decreased with a decrease in the inverse of cavity length, which agreed with the theoretical expectation. The potential of BeZnSeTe for the use as an active layer for yellow-green laser diodes has been proved. (C) 2009 Elsevier B.V. All rights reserved.

Double heterostructures (DHs) consisting of BeZnSeTe active and MgSe/BeZnTe superlattice cladding layers were fabricated on InP substrates by molecular beam epitaxy. By photoexciting the DHs, green lasing emissions at 548 nm were obtained at room temperature. The threshold excitation power density was 70 kW/cm(2), from which we estimated the threshold carrier density and threshold current density assuming a green laser diode (LD) with a BeZnSeTe active layer to be (1.4-4.6)x10(18) cm(-3) and 0.22-0.73 kA/cm(2), respectively. The experiment proved the applicability of BeZnSeTe as an active layer of green LDs.

MgSe/BeZnSeTe II-VI compound superlattices (SLs) were grown on InP substrates by molecular beam epitaxy (MBE). In the growth, Zn molecular beam irradiation was performed at each hetero-interface between MgSe and BeZnSeTe layers to suppress in-plane compositional fluctuation of the BeZnSeTe layers and to enhance steepness of the interfaces. The Zn irradiation effect was systematically investigated changing the Zn irradiation time. As a result, it was found that structural and optical properties of the SLs strongly depended on the Zn irradiation time. Explicit high-order X-ray diffraction (XRD) satellite peaks appeared by introducing the Zn irradiation, which means the interface steepness was improved. In photoluminescence (PL) measurements at room temperature (RT), single peak emissions around 520 run were observed. PL intensities at 15 K and RT strongly depended on the Zn irradiation time, and they were maximized when the Zn irradiation time was 4 s. The supply amount of the Zn irradiation for 4 s corresponds to about two-monolayer growth. From the dependencies of the PL intensity, it was shown that appropriate Zn irradiation was effective for both increase in the radiative recombination rate and decrease in the non-radiative recombination rate. (c) 2006 Elsevier B.V. All rights reserved.

ZnCdTe/MgZnSeTe laser diodes (LDs) were fabricated on ZnTe substrates by molecular beam epitaxy. The LDs were characterized under single short pulse current injections at low temperature. Yellow-green single-mode lasing emissions at 564 nm were successfully obtained, for the first time. The threshold current density (J(th)) at 100 K was 2.8 kA/cm(2). The full width at half maximum value of the lasing emission peak was 1.1 meV (0.29 nm). From the temperature dependency of the J(th) it was confirmed that the lasing operations were obtained up to 170 K. The characteristic temperature value was estimated to be 228 K. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Aging characteristics of BeZnSeTe yellow light emitting diodes (LEDs) fabricated on InP substrates by molecular beam epitaxy were investigated under direct current injections at room temperature. It was shown that the decay speed of the light output during the aging was slower than that of conventional ZnCdSe/MgZnSSe LEDs. A long lifetime more than 5000 h and a half lifetime of 5180 h were obtained at a current density of 130 A/cm(2). The half lifetimes of the BeZnSeTe LEDs were about three orders of magnitude greater than that of the ZnCdSe/MgZnSSe LEDs. These results proved high reliability of the BeZnSeTe LED. Investigating the aging characteristics of the applied voltage, the injection current, and the emission spectra showed that the output decay was caused by degradation of the active layer of the LEDs. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A novel p-type doping method for MgZnCdSe on InP substrates was proposed. In this method, highly p-type-doped thin layers, such as N-doped ZnTe, are inserted at proper intervals into MgZnCdSe layers. The inserted thin layer emits holes and the p-type coduction was achieved. ZnCdSe/ZnTe superlattices (SLs), where thin p-type-doped ZnTe layers were periodically inserted, were grown on InP substrates by molecuar beam epitaxy. When the ratio of ZnTe layer thickness to one insertion period was 0.17, the net acceptor cocentration (N A-ND) value reached 8.0×1017 cm -3. This value was more than one order of magnitude greater than that of a simple N-doped ZnCdSe layer (3.5×1016 cm-3). To obtain wide bandgap p-type materials, MgSe/ZnCdSe/ZnTe SLs were fabricated. NA-ND values from 4.7×1017 to 8.0×1017 cm-3 were observed for samples with bandgap energies from 2.33 to 2.04 eV. © 2006 WILEY-VCH Verlag GmbH &amp Co. KGaA.

Current injection lasing operation of ZnTe based yellow-green light semiconductor laser diodes (LDs) with ZnCdTe active layer and MgZnSeTe cladding layers have been achieved, for the first time. The lasing characteristics were evaluated with a single pulse current injection mode at low temperature. The lowest threshold current density of the LD was 2.7 kA/cm2 at 100 K. The maximum lasing temperature was 175 K and the observed lasing wavelength ranged from 564 to 576 nm. The green to yellow light region is a missing wavelength of semiconductor laser diode. The ZnTe based II-VI compound semiconductor is one of a most attractive material system to realize the green to yellow light LDs. Several researches have been reported aiming to ZnTe based light emitter [1]-[4]. Figure 1 shows a schematic diagram of ZnCdTe/MgZnSeTe LDs. The LD structures were grown on p-type ZnTe substrates by molecular beam epitaxy as a sequence of nitrogen-doped low temperature grown p-type ZnTe:N buffer, p-ZnTe:N, p-MgZnSeTe:N cladding layer, p-ZnTe/MgZnSeTe:N superlattice (SL) optical confinment layer (OCL), undoped ZnTe, undoped ZnCdTe active layer, undoped ZnTe, n-ZnTe/MgZnSeTe:Cl SL OCL, n-MgZnSeTe:Cl cladding, n-ZnTe:Cl, n-CdSe/ZnTe:Cl MQW layer, and n-CdSe:Cl contact layer. Precise growth conditions of the ZnTe based device structures are described in ref. [4]. After the growth, Au was deposited by resistive heat evaporation as p- and n-type electrodes. Here, the n-type electrode was formed into 16μm-wide contact stripe geometry by photolithograph technique. The wafer was cleaved into several hundreds-μm-long LD chips. The LDs were mounted in a 15 K-cryostat and evaluated under single pulse operation mode with a pulse width of 300-500 ns at 100-175 K. The light output was detected by a photo-multiplier through a fiber optical system or by a CCD array detector with an intensifier through a 30 cm spectrometer. Figure 2 shows a typical current-voltage characteristic of the ZnCdTe/MgZnSeTe LD at 100 K. The clear rectification characteristic was observed. Due to the low operation temperature, the turn-on voltage was relatively high to be about 14 V. Typical light output versus injection current (I-L) characteristic are shown in Fig. 3. The non-liner increase of light output due to lasing was observed for TE mode light with a threshold current of about 500mA. The large TE/TM ratio is typical for conventional semiconductor laser diodes. Figure 4 shows lasing and spontaneous emission spectra of the ZnCdTe/MgZnSeTe LD at 100 K under various injection current of the single pulse operation. A remarkable narrowing of emission spectrum above the threshold was confirmed. The lasing wavelength was 563.9 nm and the full width at half maximum (FWHM) was narrower than 0.29 nm. The FWHM was limited by a resolution of detection system. The dependence of threshold current on temperature is shown in Fig. 5. The lowest threshold current of 265 mA was obtained for a 612 μm-long LD (Fig. 5) at 100 K. This value corresponds to the threshold current density of 2.7 kA/cm2. With increasing temperature, the threshold current also increased. In this experiment, the maximum lasing temperature was 175 K. The characteristic temperature of the threshold current (T0) was evaluated to be 181 K. We also measured a temperature dependency of the lasing wavelength (dλL/dT) to be 0.144 nm/K as shown in Fig. 6. © 2005 IEEE.

Aging characteristics of BeZnSeTe II-VI yellow light emitting diodes (LEDs) fabricated on InP substrates by molecuar beam epitaxy were investigated under higher current injections from 130 to 900 A/cm(2) at room temperature. Light output decays during the aging are approximated using the decay model based on the carrier-recombination enhanced defect motion. Half lifetimes estimated from the approximations are compared with the literature data of ZnCdSe/MgZnSSe LEDs, which clearly indicates quite long lifetimes of the BeZnSeTe LEDs. In fact a long lifetime more than 2500 h was obtained at the injection current density of 130 A/cm(2) with no gradual decay except for the slight initial decay. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Bandgap energy and refractive indices of ZnTe-based II-VI compounds such as ZnCdTe, ZnCdSeTe, and MgZnSeTe were systematically investigated. The refractive index data were approximated by the modified single effective oscillator method. Especially the approximation for MgZnSeTe made it possible to estimate the refractive index values for any Mg composition in any wavelength of the transmission region. Using the refractive index data, waveguide analysis and design of ZnCdTe/MgZnSeTe laser structures were performed to obtain confinement factors more than 4.5%. ZnCdTe/MgZnSeTe light emitting diodes were fabricated on ZnTe substrates observing yellow light emissions around 583 nm. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A novel BeZnSeTe quaternary has been proposed for active layers of II-VI visible light emitting devices on InP substrates. BeZnSeTe, which lattice parameters were matched to InP, were grown on InP substrates by molecular beam epitaxy (MBE). The Be composition was changed from 0.11 to 0.35. The photoluminescence (PL) spectra of these samples at 15 K showed single peak emissions with wavelengths from 487 to 506 mn without any deep level emission. From the reflectivity and PL spectra at 15 K, the Gamma-Gamma direct and Gamma-X indirect bandgaps of BeZnSeTe were evaluated, and it is shown that the crossover point from direct to indirect is around Be composition of 0.3. Applying the BeZnSeTe for the active layer, visible light emitting diodes (LEDs) were fabricated on InP substrates by MBE, resulting in yellow light single peak emissions around 575 mn at room temperature. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have fabricated yellow-green light emitting diodes (LEDs) and laser diodes (LDs) by using II-VI compounds grown on an InP substrate. A separate confinement hetero-structure was fabricated by means of Cl-doped MgSe/ZnCdSe SuperLattices(SLs) for n-side layers, a ZnCdSe Single Quantum Well (SQW) for an active region, and N-doped MgSe/BeZnTe SLs for p-side layers. Lasing emission around 560 nm could be observed at a current density over 2.5 kA/cm(2) under the pulsed current injection at 77 K. In order to obtain a lasing operation at room temperature, it is necessary to decrease the resistance of the LD and to suppress the current leakage at the interface between the active region and the p-side region. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Middle-range visible light emitting diodes (LEDs) were fabricated using MgZnCdSe, BeZnTe, and BeZnSeTe II-VI materials grown on InP substrates by molecular beam epitaxy. By changing the elemental compositions of BeZnSeTe active layers, yellow-green-to-orange emissions were obtained at 542, 575, and 594 nm in wavelength. A long lifetime CW operation of the 575nm yellow LED for more than 3000 hours has been achieved. The injection current density was 130 A/cm2. This result shows higher reliability of the BeZnSeTe LEDs against the precedent ZnCdSe/MgZnSSe-based II-VI LEDs on GaAs substrates. © 2004 IEEE.

Aging characteristics of BeZnSeTe II-VI yellow light emitting diodes (LEDs) fabricated on InP substrates by molecuar beam epitaxy were investigated under higher current injections from 130 to 900 A/cm2 at room temperature. Light output decays during the aging are approximated using the decay model based on the carrier-recombination enhanced defect motion. Half lifetimes estimated from the approximations are compared with the literature data of ZnCdSe/MgZnSSe LEDs, which clearly indicates quite long lifetimes of the BeZnSeTe LEDs. In fact a long lifetime more than 2500 h was obtained at the injection current density of 130 A/cm2 with no gradual decay except for the slight initial decay. © 2004 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

II-VI compound materials containing beryllium-chalcogenides such as BeZnTe, ZnCdSe/BeZnTe, and MgSe/BeZnTe superlattices grown on InP substrates have been investigated for yellow-green emitters employing molecular beam epitaxy. Photoluminescence peaks of ZnCdSe/BeZnTe superlattices were widely controlled from 740 to 507 nm by changing the layer thickness combination. Applying ZnCdSe/BeZnTe and MgSe/BeZnTe superlattices, visible light emitting diodes (LEDs) were fabricated emitting at the wavelengths from 554 (yellow-green) to 644 nm (red) at room temperature. For yellow (575nm) LEDs, a long lifetime more than 3500 hours was demonstrated. Laser diodes consisting of a ZnCdSe active, a MgSe/ZnCdSe superlattice n-cladding, and a MgSe/BeZnTe superlattice p-cladding layers successfully operated with yellow-green lasing emissions around 560 nm at 77 K. © 2004 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

Yellow-green (560 nm) II-VI laser diodes on InP substrates were successfully operated under the pulsed current injection at 77 K. A separate confinement heterostructure was formed by employing MgSe/BeZnTe:N superlattices (SL) as p-cladding layers and MgSe/ZnCdSe:Cl SL as n-cladding layers. The threshold current density was about 2.5 kA/cm(2). (C) 2002 American Institute of Physics.

MgZnCdSe/BeZnTe II-VI light emitting diodes (LEDs) with ZnCdSe/BeZnTe type-II superlattice (SL) active layers have been fabricated by molecular beam epitaxy. Yellow-green emissions around 575 nm were observed from electroluminescence measurements. We have also investigated the degradation behavior of the LEDs. In the simple aging test, the lifetime was drastically improved by optimizing the condition of an InGaAs buffer layer. Under the constant current condition at about 18 A/cm(2) at room temperature, a long life operation over 1500 h was achieved.

MgSeTe/ZnTe superlattices were proposed as carrier-confinement layers for high-performance yellow-green light emitting diodes (LEDs) on ZnTe substrates for the first time. ZnCdTe/ZnTe LEDs consisting of ZnCdTe quantum well active and ZnTe cladding layers were fabricated by molecular beam epitaxy incorporating the MgSeTe/ZnTe superlattices between the active and the cladding layers. The LEDs were characterized by electro-luminescence (EL) measurements at room temperature. As a result, a narrow yellow emission at 578 nm with the FWHM value of 25.0 meV was observed in the EL measurements. Comparing LEDs with and without the superlattices, no influence on emission spectra was observed even by incorporating the superlattices. On the other hand, the external quantum efficiency of the LEDs with the superlattice was drastically improved by 5.5 times. This result indicated that the superlattice confinement layers were very effective to increase the emission efficiency of the LEDs.

Novel I I-VI compound materials such as MgZnCdSe, BeZnCdSe, BeZnTe, and related superlattices grown on InP substrates have been investigated for yellow-green emitters employing molecular beam epitaxy. MgZnCdSe was grown in the Mg composition range of 0similar to0.6 to clarify the compositional dependency of the bandgap and refractive index. MgSe-ZnCdSe and MgSe-ZnSeTe short-period superlattices were investigated; the superlattices behaved as quasi-quaternaries (QQs), so that their bandgap energies were controlled by the layer thickness combination of superlattices. For realizing strong lattice hardness, Be-contained II-VI compounds, such as BeZnCdSe and BeZnTe bulk crystals, and MgSe-BeZnCdSe, ZnCdSe-BeZnTe, and MgSe-BeZnTe short-period superlattices were investigated. The superlattices also behaved with QQ properties, by use of which multilayered heterostructures could be grown without growth interruption. Applying the superlattices, visible LEDs were fabricated emitting at the wavelengths from 554 (yellow-green) to 644 nm (red) at room temperature. For yellow (575 nm) LEDs, a long lifetime more than 3500 h was demonstrated even for defect densities as high as 10(5) cm(-2). The BeZnTe buffers were effective in suppresing the defect density to less than 7 x 10(3) cm(-3). Finally, MgZnCdSe-based II-VI LDs were successfully operated with yellow-green lasing emissions around 560 run at 77 K.

The ZnCdSe-based laser diodes (LD) were fabricated on S-doped oriented InP substrates demonstrating the successful lasing operation. The emission spectra of the LD under various current injection levels was presented. Below the threshold, the full width at half maximum of the spontaneous emission was 200 nm and emission peak was at 560 nm.

Refractive indices of BeZnTe, ZnCdSe/BeZnTe superlattices (SLs) and MgSe/BeZnTe SLs grown on InP substrates were measured for the first time. The refractive index of BeZnTe with a wide bandgap (E-g) of 3.12 eV was higher than that of ZnCdSe with a narrower E-g of 2.1 eV. This tendency in BeZnTe is unexpected, because for other materials, wider bandgaps may generally give lower indices. The refractive indices of the ZnCdSe/BeZnTe and MgSe/BeZnTe SLs were controlled in the wide range by changing the layer thickness combination. Waveguide analysis is performed for laser structures consisting of ZnCdSe/BeZnTe SL active layer and MgSe/BeZnTe SL p-cladding layer. As a result, optical confinement factors around 5.9% are estimated at a wavelength of 564 nm.

We report the fabrication and characterization of ZnCdSe/BeZnTe superlattices (SLs) visible LEDs which were grown on InP substrates by molecular beam epitaxy (MBE). Applying the MgSe/BeZnTe SL for a p-cladding layer of the LED structure, the carrier confining effect was enhanced. The evaluation of the LEDs under pulsed current injections at room temperature (RT) resulted in observation of single peak emissions in the visible range, i.e. at 640 and 554 nm, corresponding to the different layer thickness combinations of the ZnCdSe/BeZnTe SLs active layers. A simple aging test of the LED was performed under the injection current density of 28 A/cm(2). In spite of the higher dislocation density of the LED, in mid 10(5) cm(-2), a relatively longer lifetime over 130 h was confirmed.

The extended defect density of ZnCdSe on InP substrates was successfully reduced by introducing BeZnTe buffer layers. A very low etch pit density (EPD) around 7 x 10(3) cm(-3) was obtained for ZnCdSe layers grown on BeZnTe buffer layers. In the photoluminescence (PL) measurements at 15 K for the ZnCdSe sample, sharp emission spectra without any deep-level emissions were observed around 579 nm.

ZnCdTe/ZnTe pn-junction LEDs, which consisted of a 10 nm ZnCdTe active layer, and p- and n-type ZnTe cladding layers doped with N and Cl, respectively, were grown on ZnTe substrates by molecular beam epitaxy. The current injection into the ZnCdTe active layer provided yellow emission (587 nm) of a single sharp spectrum at room temperature. The full width at half maximum (FWHM) was 25.9 meV (the corresponding FWHM in wavelength, 7.4 nm). In the devices, n-type CdSe contact layers were introduced for a low oh-mic contact.

ZnCdSe/BeZnTe II-VI compound superlattices (SLs) were grown on InP substrates by molecular beam epitaxy for the first time. In the X-ray diffraction studies, definite satellite Peaks were observed for each sample, which indicates that fine periodic SL structures were obtained. For SL samples with several layer thickness combinations, wide-range visible emissions from 740 to 507 nm were observed during photoluminescence (PL) studies at 15 K. Comparison of PL emission properties at 15 K for a ZnCdSe/BeZnTe SL and ZnCdSe bulk samples showed the superior emission intensity of the SL sample, A double hetero-structure (DH) consisting of ZnCdSe/BeZnTe SLs and BeZnTe cladding layers was fabricated. In the PL spectrum of the DH sample at 15 K, sharp emission peaks at 517 and 604 nm were observed. Visible light emitting diodes were fabricated by applying ZnCdSe/BeZnTe SLs as the active layer, and were evaluated under pulsed current injections at room temperature. A single-peak yellow-green emission around 562 nm was obtained.

Novel ZnCdSe/BeZnTe type-II superlattices (SLs) are very attractive For active layers of new visible light emitting devices using II-VI compounds on InP substrates. It is expected that the transition energy of the SL can be controlled in a wide visible range from 761 to 397 nut by only changing layer-thickness combinations. In this study, the ZnCdSe/BeZnTe SLs grown on InP substrates by molecular beam epitaxy with the shutter Control method for the first time. In the X-ray diffraction measurements, sharp satellite peaks from the SL structures could be observed, which indicates that high crystal qualities and steep interfaces between ZnCdSe and BeZnTe layers were obtained by using the shutter control method. For the SL samples with several layer-thickness combinations, wide-range visible emissions from 740 to 507 nut could be observed in the Pl measurements at 15 K. The PL peaks are comparable to theoretical transition energies calculated for ZnCdSe/BeZnTe SL structures. A double hetero (DH) structure consisting of ZnCdSe/BeZnTe SLs and BeZnTe cladding layers was fabricated. In the PL spectrum of the DH sample at 15 K. sharp peak emissions at 517 and 614 nm could be observed.

BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice (SL) and BeZnCdSe quantum well (QW) structures were grown on InP substrates by molecular beam epitaxy for the first time. As for a lattice-matched Be-0.08(Zn0.30Cd0.70)(0.92)Se sample, a single-peak photoluminescence (PL) spectrum was observed at the temperature range from 15K to room temperature. The PL peak wavelength and the full-width at half-maximum (FWHM) value at 15 K were 572 nm and 8.8 meV, respectively. PL measurements of MgSe/BeZnCdSe SL samples were observed single-peak emissions in the green-to-blue color range from 525 to 470 nm at 15 K with changing MgSe layer-thickness ratio from 0.20 to 0.60. As for the QW sample consisting of four BeZnCdSe QWs with a different well width, PL peaks corresponding to each QW were observed in the wavelength range from 459 to 561 nm at 15 E(. By fitting calculated transition energies in the QWs to the PL peak energies. the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated to be DeltaE(c) : DeltaE(v) = 9 : 1. (C) 2001 Elsevier Science B.V. All rights reserved.

BeZnTe ternary alloys were grown on InP substrates by molecular beam epitaxy. The bandgap energies of BeZnTe were investigated as a function of Be composition by absorption coefficient measurements. As a result, it was shown that the bowing parameter was as small as 0.1 eV, and the wide bandgap energy of 3.12 eV was given for BeZnTe lattice-matching to InP, with a Be composition of 0.48. The p-type doping of BeZnTe was performed using a RF-radical nitrogen source. From Hall effect measurements of the samples, high hole concentrations in the range of 10(18) to 10(19) cm(-3) were obtained. Using N-doped BeZnTe as a p-cladding layer, MgZnCdSe/BeZnTe Light emitting diodes (LEDs) were fabricated on InP substrates. The active and the n-cladding layers were ZnCdSe and MgSe/ZnCdSe superlattices, respectively. The LEDs were evaluated under pulsed current injections at room temperature. Single-peak orange light emissions around 605 nm were observed under low applied voltages of about 3 V.

BeZnTe II-VI compound ternary alloys with various Be composition (x(Be)) were grown on InP substrates by molecular beam epitaxy. From absorption coefficient measurements at room temperature. the bandgap energies (E-g) were evaluated as a function of x(Be). The bandgap energy monotonically increases from 2.24 to 3.29 eV when increasing x(Be) from 0 to 0.62 By fitting a quadratic equation in terms of x(Be) to the E-g values, E-g = 2.24(1 - x(Be)) + 4.1x(Be) + 0.1x(Be)(x(Be) - 1) is obtained, and tho bowing parameter is seen to be as small as 0.1eV. Thus, the E-g value of BeZnTe, when the lattice constant is matched to InP, is estimated to be 3.1eV. p-type doped BeZnTe alloys were grown using an RF-radical nitrogen source. A high hole concentration of 4.8 x 10(18) cm(-3) was obtained for a Be0.4Zn0.6Te sample with a wide bandgap of 2.97eV. These results suggest that BeZnTe is very promising for p-cladding layer materials in short wavelength II-VI laser diodes. (C) 2000 Elsevier Science B.V. All rights reserved.

Novel MgSe/ZnSeTe II-VI compound superlattice quasi-quaternaries (SL-QQs) were grown on InP substrates by molecular beam epitaxy with p-type doping using a RF-radical nitrogen source. The SL-QQs with various equivalent Mg compositions (x(Mg)) were prepared by changing the MgSe layer composition in the superlattice. Thf photoluminescence peak energy at 15 K increased from 2.11 to 2.68 eV with increasing x(Mg) from 0 to 0.76. A hole concentration over 2 x 10(18) cm(-3) was obtained for x(Mg) of 0.14, and the doping level monotonically decreased with increasing x(Mg). By changing the doping process, the doping property was improved, and a hole concentration of about 3 x 10(17) cm(-3) was realized even for x(Mg) of 0.4. ZnCdSe/MgZnCdSe(Te) light emitting diodes were fabricated on InP substrates using MgSe/ZnSeTe SL-QQs as p-side cladding layers. Yellow light emissions around 577 nm were observed under a pulsed current injection at 77 K. (C) 1999 Elsevier Science B.V. All rights reserved.

Superlattice quasi-quaternaries (SL-QQs), consisting of MgSe/ZnSeTe short-period superlattices, were grown on InP substrates by molecular beam epitaxy with nitrogen p-doping. By changing the Mg composition (i.e., the layer thickness ratio of MgSe in the superlattice) from 0 to 0.76, the photoluminescence peak energy at 15 K increased from 2.11 to 7.68 eV. The N-doping of MgSe/ZnSeTe SL-QQs was performed by two processes, that is by the modulation-doping into ZnSeTe and by the all-doping. For the modulation-doping, a hole concentration of over 1 x 10(18) cm(-3) was obtained for Mg composition of less than 0.14, with decreased monotonically with increasing Mg composition. The hole concentration increased by changing the doping process from the modulation-doping to the ail-doping. Far example, the hole concentration increased from 1 x 10(15) to 3 x 10(17) cm(-3) for Mg composition of 0.4. ZnCdSe/MgZnCdSe(Te) light emitting diodes (LEDs) were fabricated using MgSe/ZnSeTe SL-QQs as p-cladding layers having yellow emissions around 577 nm at 77 K.

Selective removal of the InP substrates of MgZnCdSe LI-VI compound samples by chemical etching using HCl:H3PO4 solutions was investigated for the first time. Using a selective removal technology, the absorption coefficients of the MgZnCdSe layers were estimated from the optical transmission measurements at room temperature. Furthermore, the bandgap energies of MgZnCdSe were obtained as a function of the Mg composition by fitting the theoretical absorption coefficients to the experimental values. In addition, for ZnCdSe/MgZnCdSe multiple quantum well (MQW) samples, absorption coefficient variations due to the quantum confined Stark effect (QCSE) were observed near the absorption edge of the MQW by applying an electric field.

Novel MgSe/ZnCdSe superlattice quasi-quaternaries (SL-QQs) lattice-matched to (1 0 0) InP substrates were grown by molecular beam epitaxy for the first time. From the X-ray diffraction measurements of the SL-QQs, it was made sure that the designed superlattice periods were actually obtained. While, the equivalent Mg composition of the SL-QQs was controlled from 0.12 to 0.60 by changing the MgSe layer thickness ratio to the superlattice period. Optical properties of the SL-QQs were investigated by photoluminescence (PL) and reflectance measurements at 15 K. As a result, the PL peak energies of the SL-QQs monotonically increased from 2.24 to 2.91 eV with increasing the Mg composition from 0.12 to 0.60 like MgZnCdSe bulk compounds. (C) 1998 Elsevier Science B.V. All rights reserved.

ZnCdSe/MgZnCdSe multiple quantum well (MQW) devices were fabricated on InP substrates by molecular beam epitaxy, and quantum-confined Stark effect (QCSE) in the devices were investigated by measurements of reflectance variations at the top surface and absorption coefficient variations in the MQW region with changing applied voltage at room temperature. As a result, remarkable reflectance variations were observed near 587 nm wavelength, accompanied by red shifts of the transition wavelength with increasing applied voltage. A maximum reflectance variation of 5.2% was obtained under the applied voltage of -7.5 V (the electric field of 1.9 x 10(5) V/cm). From the electromagnetic analysis of the device structure. the refractive index variation of 2.4% per well in the MQW was theoretically estimated for the maximum reflectance variation. (C) 1998 Elsevier Science B.V. All rights reserved.

Refractive indices of Mg-x(Zn0.48Cd0.52)(1-x)Se compounds grown on InP substrates were systematically investigated as a function of Mg composition (x). The refractive indices with various Mg compositions were estimated from the reflectance measurements. By approximating the refractive indices by the modified single effective oscillator method, the direct band gap energy E-Gamma = 2.03 + 1.45x, the dispersion energy E-d = 24.5 - 15.2x, and the oscillator energy E-0 = 5.13 - 1.03x were obtained. MgZnCdSe multilayer distributed Bragg reflectors (DBRs) designed by using the refractive indices obtained in this study were fabricated by a molecular beam epitaxy. As a result, high reflectance values over 98% at 595 nm were experimentally obtained for the 30 pairs Mg-0.1(Zn0.48Cd0.52)(0.9)Se/Mg-0.6(Zn0.48Cd0.52)(0.4)Se DBR, and the reflectance spectrum agreed with the theoretical values. Furthermore, good agreements of the experimental and the theoretical maximum reflectance of the DBRs as a function of the layer pair number are obtained. From theoretical investigations of 500-600 nm wavelength range DBRs, reflectance values of 99.9% are calculated for the layer pair numbers from 30 to 40. (C) 1997 American Institute of Physics.

Cubic GaInN/GaN/AlGaN quantum well (QW) lasers are theoretically analyzed. For the gain calculation, the dipole moment of cubic GaN which is extrapolated from those of other several III-V compounds is corrected by fitting the theoretical light absorption coefficients to the experimental values. Using the corrected dipole moment, the threshold current density value of 1.0 kA/cm(2) is theoretically calculated in the case of the single QW laser with the lasing wavelength of 415 nm. For the 10 mu m wide stripe single QW lasers, the threshold currents (I-th) are calculated as a function of the cavity length. The I-th drastically decreases with decreasing the cavity length because the corrected dipole moment gives the higher differential gain. The I-th values below 20 mA are theoretically given for a short cavity length of 100 mu m. (C) 1997 Elsevier Science Ltd.

A mechanism for self-organization of GalnP strained quantum wires in (GaP)(m)/(lnP)(m) short-period binary superlattice (SPBS) is discussed. To elucidate the self-organization mechanism, GalnP/AllnP compressively strained multi-quantum-wire (CS-MQWR) lasers were fabricated, changing the superlattice monolayer number m in (GaP)(m)/ (InP)(m) SPBS active layers. The self-organization occurred for m > 1.2, determined from transmission electron microscopy images and from the anisotropic TM/TE polarization ratio in electroluminescence, i.e. an anisotropic dipole moment. The mechanism by which quantum wire axes were selected to the [01(1) over bar] direction is discussed in terms of the anisotropy in adatom diffusion between [011] and [01(1) over bar] directions. To confirm this, (GaP)(1.2)/(lnP)(1.2) SPBS layers were grown on GaAs (100) substrates misoriented towards the [011] direction, on which the [011] adatom diffusion is suppressed. Enhanced quantum wires self-organization by substrate misorientation was observed, showing that anisotropic diffusion played an important role. The mechanism modelling of the lateral compositional modulation is discussed considering the initial growth of films largely mismatched to bottom crystals. The lateral compositional modulation is supposed to be related to GaP wire-like nuclei induced by large strain energy in the first GaP layer growth in (GaP)(m)/(lnP)(m) SPBSs. GalnP/AllnP CS-MQWR lasers with low J(th) values of 257 A cm(-2) were obtained at m = 1.5.

Wide-gap II-VI MgZnCdSe quaternary compounds were grown on InP substrates by molecular beam epitaxy, for the first time. Changing the Mg composition (x = 0 to 0.63), various Mg-x(ZnyCd1-y)(1-x)Se lattice-matched to InP were grown. Mirror-like surface morphologies and streaky reflection high energy electron diffraction patterns of MgZnCdSe were obtained. With increased Mg compositions, the band-edge emissions wavelength in photoluminescence spectra was shifted from 572 nm (2.17 eV) to 398 nm (3.12 eV) at 15K. Furthermore, the absolute PL peak intensity increased drastically with increased band-edge emission, being accompanied by a relative decrement in the deep level emission intensities were also observed.