Yasuyuki Endoh

Structural and Photoluminescence Characterization of CdS/GaAs Films and CdS-ZnS Strained-Layer Superlattices Grown by Low-Pressure MOCVD Method

Single-crystalline CdS films have been epitaxially grown on (100) and (111)A and B GaAs substrates at temperatures between 200 and 350°C by a low-pressure metalorganic chemical-vapor-deposition (MOCVD) method using (CH3)2Cd and H2S gases. The (100)-oriented film exhibits perfectly cubic modification, but contains dislocations and stacking faults. Moreover, the films on (111)A and B substrates show pure hexagonal growth. In particular, the photoluminescence (PL) spectrum at 4.2 K of the film grown on the (111)A substrate is characteristic of a hexagonal wurtzite crystal and is dominated by the presence of the strong I2 (neutral-donor bound exciton) line at 2.547 eV and I1 (neutral-acceptor bound exciton) line at 2.534 eV, and a donor-acceptor pair (IE) band at 2.447 eV. CdS-ZnS strained-layer superlattice has for the first time been fabricated on GaAs substrate by MOCVD. The structural and 4.2 K PL properties are also presented.

Properties of the Excitonic Emission at Room Temperature in Cd0.3Zn0.7S/ZnS Strained-Layer Superlattices

Cd0.3Zn0.7S/ZnS multiple-quantum-well strained-layer superlattices with a range of well widths can produce excitonic emissions around 3.4 eV at room temperature (RT), due to the quantum confinement of excitons in the CdZnS wells. The emission spectra of a 41 A Cd0.3Zn0.7S well were extensively studied under high optical excitation. We found a non-linear dependence of the spontaneous RT exciton-emission intensity on the excitation intensity for high excitations. With increasing excitation density, the emission line moves to slightly the lower-photon energy and in addition its linewidth is broadened. A shoulder on higher-energy side of the excitonic spectrum appears above about 100 kW/cm2, which is related to an optical transition associated with a higher excited-level state of the electrons confined in the conduction-band potential well.

Relaxation due to multiple longitudinal‐optical‐phonon emission and dissociation of exciton in Cd0.3Zn0.7S/ZnS strained‐layer superlattices

The optical properties of Cd0.3Zn0.7S/ZnS strained‐layer superlattices grown on (100) GaAs substrates by low‐pressure metalorganic chemical vapor deposition, were investigated by means of photoluminescence excitation spectroscopy and through the effect of an electric field on exciton emission. A multiple longitudinal optical‐phonon emission process, related to the relaxation of excitons, has been observed for the first time in the excitation spectra. The localization of excitons has been found to be the dominant cause of the linewidth broadening. The effects on an axial electric field on exciton emission intensity and peak position have revealed that the dissociation of the localized excitons on the low‐energy side of the spectrum principally takes place due to impact ionization.

Active-Nitrogen-Doped P-Type ZnSe Grown by Gas-Source Molecular Beam Epitaxy for Blue-Light-Emitting Devices

Active-nitrogen-doped p-type ZnSe epitaxial layers were grown by gas-source molecular beam epitaxy using H2Se. Electrical properties of the N-doped ZnSe layers changed drastically depending on the growth temperature and the VI/II ratio. With decreasing growth temperature, the net acceptor concentration increased. At the growth temperature of 300°C, the net acceptor concentration was as high as 1.02×1018 cm-3. Using this technique, p-n junction diodes with an active layer of ZnCdSe/ZnSe multiple quantum wells were fabricated. These diodes emitted clearly visible blue light (477 nm) at room temperature.

Gas Source Molecular Beam Epitaxial Growth of ZnSe Using Metal Zn and H2Se

ZnSe films were grown on GaAs substrates by gas source molecular beam epitaxy using metal Zn and H2Se as sources. H2Se was thermally cracked in a high pressure gas cell. The effects of growth temperature, VI/II ratio and H2Se cracking temperature on ZnSe growth were investigated. Under the condition where the cracked- H2Se flux controlled the growth rate, ZnSe films with smooth surface morphology were obtained at growth temperatures between 250 and 410° C. With increasing cracked- H2Se flux and/or H2Se cracking temperature, facets emerged on the growing surface and surface morphology was degraded.

Optical Properties of CdZnS-ZnS Strained-Layer Superiattices

The Cd 0.3 Zn 0.7 S-ZnS strained-layer superlattices has for the first time been fabricated on (100)GaAs substrates by a low-pressure MOCVD method. Exciton luminescence properties were investigated by the photoluminescence spectroscopies which show that the quantum confinement effect of excitons occurs in the CdZnS alloy layer. Temperature dependence of the exciton properties reveals a dominant scattering process which originates from exciton-phonon interaction(Γ 10 =68meV). Nevertheless, the large exciton binding energy of about 166meV makes it possible to produce the exciton peak at room temperature. The effect of external electric field on the exciton intensity and its peak position is found and is tentatively interpreted in terms of a quantum confined Stark effect.

Electrical characterization of Au/p-ZnSe structure

Au electrodes were deposited on p-ZnSe layers grown by molecular beam epitaxy (MBE) and gas source (GS) MBE. The influences of the chemical pretreatment before electrode formation and the following heat treatment on electrical characteristics were investigated. All of the samples showed nonohmic current flow. In MBE-grown samples before heat treatment, pretreatment lowers the voltage of current flow rise from 6 V to 4–5 V. On the other hand, GSMBE-grown samples with and without pretreatment showed current flow rise at 4–5 V. The increase of the voltage of current flow rise was observed after heat treatment, irrespective of pretreatment and the growth method. This is related to the difference in the as-grown surface between MBE and GSMBE. The resistivity of the p-ZnSe layer increased slightly after heat treatment below the growth temperature.

Blue Light Emitting Laser Diodes Based on ZnSe/ZnCdSe Structure Grown by Gas Source Molecular Beam Epitaxy

Diode structure with a (ZnSe/ZnCdSe)5 multiple quantum-well active layer sandwiched between n- and p-ZnSe layers was grown on a (100) GaAs substrate by gas source molecular beam epitaxy using H2Se. Stimulated emission from the diode structure was observed at a wavelength of 486 nm under a pulsed current injection at 77 K. The threshold current density was 1.16 kA/cm2.

Excitonic properties of CdxZn1−xS⧸ZnS strained-layer quantum wells

Abstract We review the fundamental optical properties of excitons in the multiple-quantum wells of CdxZn1−xS⧸ZnS (x = 0.3) strained-layer superlattices with a ternary alloy as the well. Excitonic properties in the strained-layer quantum well (SLQW) with a range of well widths were investigated by means of temperature and DC electric field dependence of photoluminescence (PL) and photocurrent, and PL excitation spectra. We report some evidence of excitonic transitions in the photocurrent spectra (PCS) at 14 K and at room temperature. In electric field dependent PCS, a blue shift in the peak energy and photocurrent enhancement with increasing applied field were observed. These results are tentatively explained by assuming that the Wannier-Stark localization takes place in the SLQW under reverse bias condition. It is demonstrated that multiple-LO-phonon emissions observed in the PL excitation spectra are related to a relaxation process of the hot excitons. We suggest the possibility of producing a photomodulator on the basis of exciton absorption in ultraviolet region of the spectrum.

II–VI widegap superlattices

Abstract We review our recent results of the excitonic properties in ZnSeZnS and Cd x Zn 1−x SZnS strained-layer superlattices (SLSs). The most important physical insights in the II–VI widegap superlattices are to understand the relationship between the optical properties of quasi-two-dimensional exciton and strain because the well layer frequently receives biaxial compression or tension. The strain thus causes the significant shifts of the bandgap and splitting of the valence band. Semi-quantative calculations lead to an expectation that ZnSeZnS SLS always exhibits a type I band lineup within 100 A thicknesses of the ZnSe well at a constant ZnS barrier width of several tens angstrom. This is in good agreement with the experimental results of exciton absorption and its luminescence excitation spectra. The Cd 0.3 Zn 0.7 SZnS SLSs with a range of well widths can produce intense excitonic emissions around 3.4 eV at room temperature due to the quantum confinement of excitons in the ternary CdZnS well. In order to elucidate localisation and relaxation processes of excitons, we have for the first time reported a multiple-LO-phonon emission process in the excitation spectra. The electric-field studies suggest that the concomitant decrease in intensity and the energy downshift of the exciton line may originate from the quantum confined Stark effect.