Tsunemasa Taguchi

Ultraviolet stimulated emission and optical gain spectra in CdxZn1-xS-ZnS strained-layer superlattices

Stimulated emission at 374.9 nm has been observed in an optically pumped Cd0.22Zn0.78S‐ZnS strained‐layer superlattice at room temperature. Using a pump‐and‐probe technique with nanosecond excitation pulses, optical gain has also been observed at the tail part of the n=1 heavy‐hole exciton absorption spectrum in the presence of the clear absorption peak of the exciton. Our experimental results suggest that the stimulated emission originates from excitonic gain.

Excitonic and edge emissions in MOCVD-grown ZnS films and ZnSe-ZnS superlattices☆

Abstract Excitonic lines and edge-emission bands in metalorganic chemical vapour deposition (MOCVD) heteroepitaxially-grown ZnS films on (100) GaAs substrates have been extensively investigated at 4.2 K using He-Cd (325 nm) and Xe-Cl excimer (308 nm) lasers. Distinct edge-emission bands were observed; the LO-phonon-replicated 3.641 and 3.666 eV bands are ascribed to a donor-acceptor pair and free-to-bound acceptor transition, respectively, from time-resolved spectral measurements. The Na acceptor level is tentatively assigned to be about 170 meV above the valence band. ZnSe-ZnS and ZnSe-ZnSSe superlattices, which were for the first time prepared by low-pressure MOCVD, have been characterized by the well-width dependence of the excitonic lines and by optical absorption. We observed heavy-hole and light-hole excitons in the absorption spectra of the ZnSe-ZnS superlattices.

A new 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs

Abstract We have studied the temperature dependence of emission intensity and peak energy, photoluminescence excitation and time-resolved spectra (TRS) of the LO-phonon replicated 1.47 eV band in CdTe films grown on (100) GaAs substrates by a low-pressure metalorganic chemical vapour deposition (MOCVD) method. The TRS studies indicate that there is no observable energy shift toward lower photon energy after time delay. The excitation spectrum represents an exciton resonant peak at 1.598 eV, suggesting that the contribution of free excitons to the 1.47 eV band is predominant. This evidence is also revealed by the thermal quenching of the emission intensity which shows that a thermal liberation of excitons from the localized center takes place. We therefore propose that the observed 1.47 eV band looks like the Y line series, attributable to recombination of excitons at an extended defect, which has been already observed in ZnSe.

Growth of high-purity ZnSe by sublimation THM and the characteristics of the Y and Z deep-level emission lines

Abstract The sublimation traveling-heater method (THM) has been employed to produce good-quality ZnSe single crystals using chemical-vapor-desposition grown polycrystalline ZnSe as a suitable feed material. The crystals that were grown at a temperature around 900°C and a growth rate of 1 mm/day exhibited excellent excitonic-emission characteristics and contained a relatively low density of dislocations (below 10 4 cm -2 ). The 2.60 eV ( Y ) and 2.44 eV ( Z ) photoluminescence lines with unusual spectral properties have been characterized by means of the effect of heat treatment and temperature dependence. These luminescences become very much smaller in intensity or vanish completely in sublimation THM-grown crystals. A correlation between the presence of the Y and Z lines and dislocation densities is confirmed, and tentatively suggests that both lines arise from recombination of excitons.

Effects of strain and temperature on excitonic emissions in ZnSeZnS strained-layer superlattices grown by low-pressure MOCVD

Abstract The dominant excitonic-emission line in low-pressure metalorganic chemical-vapour deposition (MOCVD)-grown ZnSeZnS strained-layer superlattices (SLS) has been extensively characterized by the temperature dependence of its linewidth, peak energy and emission intensity. The emission line of the ZnSe well shows a band tail on its low energy side. It is tentatively suggested that the n = 1 heavy-hole free and light-hole free excitons, which are separated from each other by about 200 meV, were clearly observed in the absorption spectra at 4.2 K. Taking into account the variations of both the band offsets and the bandgap of the ZnSe well and the ZnS barrier layers as a function of strain, the Kronig-Penney analysis has been carried out in order to interpret quantatively the energy shifts of the n = 1 heavy-hole free exciton absorption peak to the high-energy side with decreasing ZnSe well width.

Fabrication and photoluminescence properties of ZnTe and CdZnTe films by low pressure metalorganic chemical-vapour deposition

Abstract Unintentionally-doped heteroepitaxial ZnTe and CdZnTe films have successfully been grown on (100)GaAs substrates at about 350°C using a low-pressure metalorganic chemical-vapour deposition (MOCVD) method by cracking (CH 3 ) 2 Te. Photoluminescence spectrum at 4.2 K of ZnTe layers is dominated by a neutral-acceptor-bound-exciton line (I l a′ ) at 2.366 eV and the characteristic oxygen-bound exciton band around 1.9 eV. The free-exciton line from the epitaxial layer is greatly shifted toward lower photon energy with respect to that of bulk ZnTe. From the thickness dependence of both the emission peak and linewidth of the I l a′ line, it is evident that the epitaxial film is strained. We tentatively suggest that tensile stress induced by the different thermal expansion coefficients between ZnTe and GaAs strongly affects the optical properties in the films. In a mixed crystalline Cd 0.33 Zn 0.67 Te film, a dominant emission band appears at 2.035 eV at 4.2 K and is ascribed to a donor-acceptor pair luminescence as indicated from the excitation intensity dependence of its peak position.

Interface properties and the effect of strain of ZnSe/ZnS strained-layer superlattices

Abstract The ZnSe/ZnS alternately layered system is one of the fundamental examples of the wide-gap II–VI compound strained-layer superlaticee (SLS) used to understand the basic physics associated with optical, electronic and structural properties. Interface properties of ZnSe/ZnS SLSs have been extensively investigated by means MeV transmission electron microscopy (TEM), ion Rutherford backscattering (RBS)/channeling, X-ray diffraction (XRD), X-ray photoelectron emission spectra (XPS) and cross-sectional TEM measurements. It is shown that the ZnSe/ZnS SLS can be grown coherently up to a layer thickness of about 100 A and to exhibit good crystalline quality. The experimental X-ray diffraction patterns are analyzed in terms of the kinematic approximation from which the strains being induced in each layer can be quantatively evaluated; the ZnSe well is certainly under compressive strain, while the ZnS barrier is under tensile strain. Band offsets in the conduction and valence bands in the strained layers are experimentally determined to be about 0.25 and 0.73 eV by XPS, respectively, and are theoretically considered as a function of the well width thickness using the ‘model solid theory’. The channeling in the SLS shows much higher dechanneling yields than along the growth direction. This may be derived from the small angle change in the inclined crystal directions at each interface which result in significant dechanneling, and can be discussed in terms of the bond relaxation model or beam steering effects. The periodic oscillatory structure of the superlattices in RBS/channeling spectra has been observed in this SLS system. The interface properties can be understood in terms of the excitonic linewidth broadening which is influenced by either the interface roughness or by well width fluctuation. Photoluminescence and absorption spectroscopies of the ZnSe/ZnS and ZnSe/ZnSSe SLS are used to characterise the interface, and to clarify the energy separation of the degenerate hole bands due to a compressive stran in the ZnSe quantum well. The effects of ion irradiation and thermal stability during annealing in SLS are briefly introduced with a view to providing the fundamental physical properties of the creation of lattice defects and impurity diffusion. A Fibonacci SLS sequence with a quasi-periodic superlattice system demonstrates the possibility of new functional devices.

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.

Gamma and electron radiation effects in CdTe

Abstract In order to reveal radiation-induced characteristics of CdTe detectors, we present the irradiation effects between bulk and detector properties under radiation levels of 10 5 R/h gamma-rays. Drift mobilities both for holes and electrons are remarkably reduced after irradiation and then charge transport is dominated by appearances of an electron-trap level at E c − 0.5 eV and a hole-trap level at E v + 0.14 eV. The fwhm energy resolution acquired at 278 K with a detector produ from undoped THM crystals obtained under a controlled Cd over-pressure is approximately 8 keV for 59.5 keV gamma rays from 241 Am and 12 keV for 122 keV gamma rays from 57 Co. The effect due to successive irradiations of 60 Co gamma-rays markedly degraded the output pulse-height spectra where ultimately the 59.5 keV and 122 keV photopeaks could not be discerned. Following annealing of about 600 K, the pulse-height spectrum recovers almost to the preirradiation value. This agrees with the recovery stage of the electron-trap level at E c − 0.5 eV.

Growth of high-quality cubic ZnS crystals and their application to MIS blue light-emitting diodes

Cubic ZnS crystals, some 1.5 cm 3 in size, have been successfully grown at about 850 ° C using a conventional iodine-transport method. The quality of the crystal obtained can be significantly improved by prebaking the ZnS powder in H 2 S gas prior to growth. Low-resistive n-type crystals, with a room temperature (RT) resistivity as low as 3 Ω cm, have been characterized by their excitonic and impurity-related photoluminescence emission spectra at 4.2 K. A correlation between the emission properties and surface morphology was found with the various oriented homoepitaxial ZnS layers grown by low-pressure metalorganic chemical vapour deposition (MOCVD). Using the high-quality n-ZnS crystals doped with I donors, an MIS-structured blue LED has been fabricated and yielded an external quantum efficiency as high as 0.05% at RT when a ZnS insulating (I) layer with a thickness of 300 A was deposited on the (110)-oriented ZnS substrate.