ZnSe ZnTe Research Articles

Ultrafast exciton dynamics in Type II ZnTe–ZnSe colloidal quantum dots

Ultrafast transient absorption spectroscopy is used to investigate the exciton dynamics of Type II ZnTe-ZnSe core-shell colloidal quantum dots. Surface-trapping is shown to occur within a few picosecond for hot electrons and with a few 10s of picoseconds for electrons cooled to the band-edge, and is the dominant process in the decay of the band-edge bleach for well-stirred samples pumped at moderate powers. The surface-trapped electrons produce a broad photo-induced absorption that spectrally overlaps with the band-edge, distorting and partially cancelling out the bleach feature. At high pump powers and for unstirred samples, these surface-trapped electrons can survive sufficiently long within the pumped volume to accumulate under repeated excitation of the sample, resulting in the formation of an additional exciton decay channel.

Electric Field Analysis of the ITO-ZnSe-CdTe Photo Sensor Cell

In this report, the ITO-ZnSe-CdTe cell and the ITO-ZnSe-CdTe-ZnTe cell as a photo sensor were analyzed comparatively with the use of the electric field analysis from spectral sensitivity data. From analysis, it is presumed that there is the depletion layer from the junction surface to 0.194μm in zone of CdTe layer at the ITO-ZnSe-CdTe cell. On the other hand, the ITO-ZnSe-CdTe-ZnTe cell has the depletion layer in whole region of CdTe layer. It is presumed that both ZnSe layer and ZnTe layer form the depletion layer of CdTe layer in the ITO-ZnSe-CdTe-ZnTe cell. The CdTe layer, which has 0.8μm in thickness, absorbs all of visible wavelengths, and both of two cells have CdTe layer 0.8μm in thickness. Luminous sensitivity of the ITO-ZnSe-CdTe cell is much less than luminous sensitivity of the ITO-ZnSe-CdTe-ZnTe cell. It is presumed that differences of the luminous sensitivity between those two cells arise from differences of the electric field distribution. From this analysis, it is presumed that the structure of ZnSe-CdTe-ZnTe makes depletion layer in whole region of CdTe layer.

Light up-conversion mechanism of ZnSe–ZnTe superlattices

The specially designed ZnSe–ZnTe superlattice (SL), type-II SL, has the light up-conversion effect that SL emits a photoluminescence (PL) peak under the irradiation of the excitation light with the lower energy than the PL peak. The excitation intensity dependence of the energy of up-converted PL peak obeys the characteristic of type-II SL. The PL peak intensity has the super-linear dependence on the excitation intensity. The five kinds of samples in this work showed these phenomena. The model to explain these situations has been proposed.

Highly sensitive ultraviolet PIN photodiodes of ZnSSe n+–i–p structure/p+‐GaAs substrate grown by MBE

AbstractHighly sensitive and stable ultraviolet PIN photodiodes of ZnSSe junction structure on p+‐GaAs substrates are developed by molecular beam epitaxy (MBE) growth. The short‐wavelength photodiodes are grown on p‐type (100)GaAs by MBE utilizing an optimized “hetero‐interface super‐lattice (SLE) buffer” consisting of a ZnTe–ZnSe multiple quantum well, by which a large energy barrier (>1 eV) for hole ohmic conduction is overcome. ZnSSe n+–i–p photodiodes with a thin interface SLE are grown in a complete lattice‐matched condition (S = 6%). High sensitivities of 0.30 A/W (external quantum efficiency: η = 83.6%) in the blue (450 nm), 0.24 A/W (η = 75.8%) in the blue–violet (400 nm: for BlueRay and HD‐DVD systems) and 0.12 A/W (η = 50.9%) in the ultraviolet (300 nm) regions are obtained with an extremely low dark leakage current (∼sub pA/mm2) under high reverse bias (20 V) at 300 K. Because of the low dark current, a very low noise equivalent power value below 10–14 W $ \sqrt {{\rm Hz/cm}^{-2}} $ (450 nm) is proved in the blue–violet region. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Zn 3N 2 compensated ZnTe films and ZnTe–ZnSe superlattices grown by hot wall epitaxy

Nitrogen-doped (N-doped) p-type ZnTe films and ZnTe–ZnSe superlattices (SLs) were prepared on GaAs (1 0 0) substrates by hot wall epitaxy (HWE) using NH 3 gas and Zn 3N 2 as a codoping source. We have investigated the compensation effect of Zn 3N 2 and optimized the growth conditions. Then the nitrogen-doped ZnTe films and ZnTe–ZnSe superlattices of high crystal quality with a high hole concentration were obtained.

Excitation intensity dependence of up-converted emission in ZnSe–ZnTe superlattice grown by MBE

The photoluminescence (PL) spectra of the type II ZnSe–ZnTe superlattices with dual-subband structures have two emission peaks. The PL peak at the higher energy side appears by the excitation even with the light, with the middle energy between the PL peaks. The emission energy of the up-converted light shows dependence similar to the case of direct excitation (high-energy excitation) on the excitation intensity. The emission energy increases with increase in the excitation intensity. The up-converted and the direct excited emissions are due to the same state in the superlattice.

A Zn(II) ion-selective electrode based on chalcogenide As2Se3–Sb2Se3–ZnSe and GeSe2–ZnSe–ZnTe glasses

Zn(II) ion-selective electrodes with chalcogenide glassy As2Se3–Sb2Se3–ZnSe and GeSe2–ZnSe–ZnTe membranes were developed. Basic analytical characteristics such as stability, linearity, slope of the electrode function, limits of detection, effect of pH on the electrode potential and response time were studied with varying glass compositions. A structural mechanism for explanation of the dependencies obtained is suggested.

The role of oxygen in formation of radiative recombination centers in ZnSe1−xTex crystals

The present study was aimed at revealing the influence of oxygen on formation of alloy ZnSe–ZnTe as well as on development of luminescence centers under thermal treatment of scintillators based on isovalently doped ZnSe crystals. The presence of oxygen in ZnSe(Te) crystals as ZnO phase has been shown to hinder formation of (V′ZnTexSe)′ type luminescence centers responsible for luminescence band peaked at 635–640 nm, which is the principal emission band of this scintillator. In ZnSe(Te) crystals doped by mechanically stimulated ion implantation with active forms of oxygen (O′, Ox, O′2, etc.), luminescence quenching effects are not observed. However, the luminescence band in these crystals is blueshifted towards 600–610 nm. Strong luminescence band peaked at λmax=590–610 nm is observed in Te-free ZnSe crystals doped with oxygen in active forms. The experimental results are interpreted by formation of centers (Zni⋅Oxi)⋅ or (Zni⋅OxSe)⋅. Having larger charge carrier capture cross-section than (Zn⋅iV′ZnTexSe)x, the oxygen-containing centers determine kinetics of luminescence in ZnSe1−xTex crystals (x<0.001) treated by active forms of oxygen during mechanical activation of the initial raw material for crystal growth.

Up-conversion effect of ZnSe–ZnTe superlattices with modulated periodicity

A light up-conversion effect has been observed in the ZnSe–ZnTe superlattices with a modulated periodicity. A photoluminescence (PL) peak at 2.1 eV is observed in the samples even under excitation by an He–Ne laser (633 nm, 1.96 eV). The samples, which we grow on InP by molecular beam epitaxy, consist of a combination of two kinds of superlattices. The PL spectrum, measured by using an He–Cd (325 nm, 3.81 eV) or Ar + (488 nm, 2.54 eV) laser as the excitation source, indicates two peaks due to the respective superlattices at 1.9 and 2.1 eV. The intensity of the up-converted peak has a super-linear dependence on the excitation intensity. The mechanism of the effect has been explained by a dual-subband model.

Distribution of tellurium in melt-grown ZnSe(Te) crystals

The properties of semiconductor scintillation crystals are largely determined by the presence and distribution of the activator dopant. Influence of growth technological parameters (overheating Δ T, inert gas pressure P, crystallization rate V k) upon distribution of tellurium dopant was studied for ZnSe(Te) crystals grown by Bridgman technique in compression furnaces. Values of the “apparent” distribution coefficient of tellurium ( k Te) over crystal length were shown to be dependent on Δ T and the related value of charge carry-over (Δ m/ m 0). It was established that at Δ m/ m 0=12%, K Te=1; at Δ m/ m 0<12%, K Te<1; at Δ m/ m 0>12%, K>1. This relationship is explained by assuming a competition between two processes affecting the distribution coefficient of tellurium: movement to the tail part in accordance with the phase diagram of the ZnSe–ZnTe pseudobinary system and thermal desorption of ZnTe due to its high volatility. Conditions required for preparation of ZnSe(Te) crystals of highly uniform composition are determined.

Stretched-exponential decay of the luminescence in ZnSeZnTe superlattices

The photoluminescence decay of fractional-layer superlattices of ZnSeZnTe has been measured to investigate the origin of “slow-bands” (K. Suzuki et al. [4]). The decay of this band is found to be well described by a stretched exponential function. Distributions of effective life-time of excitons at several wavelength positions of the bands are derived from the stretched exponential parameters with which the decay is fitted. From the distributions, the radiative life-time of localized excitons in the slow bands are estimated to be 10–20 ns.

Time-Resolved Photoluminescence Measurements on ZnSe-ZnTe Superlattices

Time-resolved photoluminescence of ZnSe–ZnTe superlattices with different Te concentrations has been measured in the picosecond time region. The samples show typical S1 (blue band) and S2 (green band) emissions in the time-integrated spectra. In the time-resolved spectra, both samples exhibit an asymmetrically broadened shallow component at the high-energy side of the luminescence band, indicating the presence of shallow localized excitons due to long-range potential fluctuations.

Phase matched second harmonie generation using a χ(2) modulated [formula omitted] optical waveguide

Second harmonic generation is investigated in a ZnTe ZnSe waveguide structure grown by metalorganic vapour phase epitaxy on (001)GaAs substrate. A focused Ga +-ion beam is used to generate a first order periodic modulation of the nonlinear susceptibility χ (2) within the waveguide. Using a tuneable ps-KTP optical parametric oscillator a clearly enhanced SHG signal is observed at a fundamental wavelength of λ F = 1164 nm.

Emission mechanism of blue and green bands in ultrathin ZnSeZnTe superlattices

The electron-phonon interaction strength (Huang-Rhys parameter S) of ultrathin ZnSeZnTe superlattices has been estimated. The values of S are 3.5 and 7.4 for S1 blue and S2 green bands caused by the Te isoelectronic traps. Using these values, we have constructed configuration coordinate diagrams to explain absorption and emission mechanisms in the ZnSeZnTe superlattices.

Absorption Spectra of Type I and Type II Strained-Layer Superlattices

Absorption spectra which originate from direct valence-band-to-conduction-band transitions in strained-layer superlattices (SLSs) are studied theoretically. The electronic structures are calculated by using the Luttinger-Kohn Hamiltonian in the Kronig-Penney model. The absorption coefficients are evaluated by taking the mixing of heavy- and light-hole states into account. As a result of calculation, firstly, origins of fine structures in the absorption spectra observed at a room temperature for type I ZnSe(22 A)–ZnS(23 A) SLS are clarified satisfactorily. Secondly, novel properties of type II ZnTe–ZnSe are predicted, i.e. it is revealed that the absorption intensity is one order of magnitude less in type II SLS than in type I SLS, and that in type II SLS the strength for any transitions to the higher conduction and from the higher valence subbands is greatly enhanced, in sharp contrast to the type I SLS.

Investigations of the thermodynamic properties of Te-saturated ZnSeZnTe solid solutions

The activity of ZnTe in Te-saturated ZnSeZnTe solid solutions was measured in the temperature range 710–845 K by an electrochemical technique using LiClKCl + 5 wt.% ZnCl2 as the molten salt electrolyte. The activity of ZnSe has been calculated by Gibbs-Duhem integration using the α function. From the e.m.f. values of the cell measured at different temperatures, the partial, integral and excess molar thermodynamic quantities have been calculated. All the excess partial molar thermodynamic quantities at infinite dilution have been accurately estimated by Chiotti's method. The composition dependence of the α function confirms that the system does not follow a regular solution model. The results have been analysed in terms of Darken's stability and excess stability parameters. Detailed analyses of the results confirm that the system is completely miscible in the solid state and consists of a single-phase field throughout the entire range of composition.

Electronic Structures and Absorption Spectra of II-VI Compound Strained-Layer Superlattices

Electronic structures and absorption spectra for direct valence-band-to-conduction-band transitions in ZnSe–ZnS and ZnTe–ZnSe strained-layer superlattices, which have recently attracted interest for an application to optoelectronic devices, are studied theoretically. The calculations of the electronic states near the band edges are carried out by using the Kronig-Penney model in the Luttinger-Kohn Hamiltonian. The calculated absorption spectra are decomposed into each transition component, examined in connection with the mixing of heavy- and light-hole states in detail, and compared with the experimental results. Overall features of the absorption spectra observed recently by Shen et al . are explained reasonably well. For ZnSe–ZnS, it is predicted that a drastic change in the absorption spectrum may occur by changing the incident-photon polarization, and for ZnTe–ZnSe a similar property will be observed, if samples with a thicker barrier layer are studied.

Electronic structure of short-period ZnSe/ZnTe superlattices grown by MOVPE at 300ºC

ABSTRACTThe ZnSe-ZnTe combination is a potential candidate for the realisation of visible light-emitting devices. The lattice mismatch between bulk ZnSe and bulk ZnTe is important (∼ 8%). Therefore, their hetero-structures are strained and high quality superlattices will only be grown if having small periods. This prescription can be fulfilled in the case of metal organic vapour phase epitaxy (MOVPE) growth by combining triethylamine dimethyl zinc adduct with di-isopropyl telluride as precursors for the growth of the ZnTe layers. The growth of high quality ZnTe can then be performed at a temperature of 300ºC , close to the best MOVPE-growth temperature for ZnSe (280ºC). Lowering the growth temperature of ZnTe to this value, we could thus obtain sharp interfaces. This work reports on ZnSe-ZnTe superlattices grown on ZnSe and ZnTe buffers deposited on GaAs substrates. We demonstrate that the stokes-shift between the reflectance and photoluminescence features ( ∼ 40 meV ) measured when the thickness of ZnSe layers does not exceed 20 Å, drastically increases for layer thicknesses beyond this critical value. This, we interpret in terms of the onset of plastic relaxation which favours tellurium diffusion in the ZnSe slices. Then photoluminescence spectra broaden ( contributions of trapped-excitons dominate), and observation of free excitons in reflectance become impossible. We have studied in detail the optical properties of the superlattices and compared our findings with the predictions of a multiband envelope function calculation. We show that both zone centre excitons as well as excitons associated with the miniband dispersions (saddle-point excitons) are observed in these superlattices.

Phonon Folding in ZnSe–ZnTe Strained Layer Superlattices

Folding of longitudinal acoustic (LA) mode phonons in ZnSe–ZnTe strained layer superlattices, grown on the (001) GaAs without buffer layers by molecular beam epitaxy (MBE), was measured as a function of period and thickness of the layers by Raman scattenng. The dispersion relation between observed LA frequencies and wave number of the superlattices was analyzed by fitting the sound velocity as a parameter in the Rytov model. Two analytical solutions were obtained from the model. It is supported by consideration of calculation and experimental results that the sound velocity of the ZnSe and ZnTe layers in the superlattices is smaller and larger than their bulk values respectively, and this implies elongation of the ZnSe layers and compression of the ZnTe layers.

The study of Raman spectra of II–VI wide gap ZnSeZnTe strained layer superlattices

Vacuum heat treatment is an increasingly popular thermal processing technique which is now widely established practice in the aerospace, automotive and machine tool industries.This paper is concerned with the development of a pressure quenching system for vacuum furnaces which provides both the optimum metallurgical properties and minimal distortion of components during hardening.First introduced by Consarc in the mid-1980s the Multiflow system utilises multiple gas flow patterns to achieve uniform quenching conditions and hence minimise the distortion of component during hardening. This is in contrast to vacuum furnaces employing a conventional, single gas flow pattern which results in a less uniform quench.Since 1989 the ongoing development programme at Consarc Engineering Ltd, has resulted in further improvements to the Multiflow system. The latest generation of furnaces incorporates an improved design of heat exchanger which has significantly increased the cooling rate of the furnace. The improved heat exchanger is the by-product of extensive in-house testing and state-of-the-art computer aided design techniques. The latest multiflow furnaces therefore offer the capability to harden awider range of materials while maintaining the uniformity of the quench and the dimensional stability of components.