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The Zeeman splitting and degree of circular polarization (DCP) of photoluminescence (PL) from type-II submonolayer ZnTe/ZnSe quantum dots (QDs) have been investigated in magnetic fields up to 18 T. To explain the observed relative intensities and energy positions of the σ+ and the σ− PL, a non-Boltzmann distribution for holes with ultra-long spin-flip time, confined to submonolayer QDs, is proposed. The g-factor of electrons, located in the ZnSe barriers, was obtained from fitting the temperature dependence of the DCP, and its value is in excellent agreement with that of bulk ZnSe. The g-factor of type-II excitons was extracted by analyzing the Zeeman splitting, from which the g-factor of holes confined within submonolayer ZnTe QDs was found to be ∼2.65 ± 0.40. This value is considerably larger than that in bulk ZnTe. Tight-binding calculations using an sp3s* model were employed to understand the origin of such an increase. The results of the simulation match the experiment and show that the enhancement of the hole g-factor is mostly caused by a reduced orbital contribution to Zeeman splitting arising from the submonolayer thickness of these QDs. [ABSTRACT FROM AUTHOR]

Linearly polarized photoluminescence is observed for type-II ZnTe/ZnSe submonolayer quantum dots (QDs). The comparison of spectral dependence of the degree of linear polarization (DLP) among four samples indicates that the optical anisotropy is mostly related to the elongation of ZnTe QDs. Numerical calculations based on the occupation probabilities of holes in px and py orbitals are performed to estimate the lateral aspect ratio of the QDs, and it is shown that it varies between 1.1 and 1.4. The value of anisotropic exchange splitting for bright excitonic states is found to be ~200 μeV from the measurement of the degree of circular polarization as a function of the magnetic field. The results also show that heavy-light hole mixing ratio is about 0.16. [ABSTRACT FROM AUTHOR]

Sub-monolayer quantities of Mg are introduced in multilayer stacked ZnMgTe quantum dots (QDs) embedded in ZnSe barriers in order to reduce the hole confinement energy by controlling the bandgaps and band-offsets of ZnTe/ZnSe system having type-II band alignment. The photoluminescence (PL) emission from such ZnMgTe/ZnSe QD structure is found to be a broad band centered at 2.35 eV. The higher energy side of the PL band shows a larger blue-shift with increasing excitation intensity and a faster life-time decay due to a greater contribution of the emission from the smaller size dots and the isoelectronic bound excitons. It is found that the characteristic decay time of the PL evolves along the band with a value of 129 ns at 2.18 eV to 19 ns at 2.53 eV. The temperature dependent PL emission is controlled by two thermally activated processes: ionization of electrons away from QD state to the barrier (EA1 ∼ 3 meV) by breaking the type-II excitons and thermal escape of the holes from the ground state to the barrier (EA2 ∼ 114-116 meV). We propose a modified band diagram and energy levels for this ZnMgTe/ZnSe multilayer QD system by determining the composition of Mg inside the QDs and solving the 1-D Schrodinger's equation and show that Mg incorporation lowers the hole activation energy via modification of the valence band offset without changing the barrier significantly. [ABSTRACT FROM AUTHOR]

We report the structural properties and spatial ordering of multilayer ZnMgTe quantum dots (QDs) embedded in ZnSe, where sub-monolayer quantities of Mg were introduced periodically during growth in order to reduce the valence band offset of ZnTe QDs. The periodicity, period dispersion, individual layer thickness, and the composition of the multilayer structures were determined by comparing the experimental high resolution x-ray diffraction (HRXRD) spectra to simulated ones for the allowed (004) and quasi-forbidden (002) reflections in combination with transmission electron microscopy (TEM) results. Secondary ion mass spectroscopy (SIMS) profiles confirmed the incorporation of Mg inside the QD layers, and the HRXRD analysis revealed that there is approximately 32% Mg in the ZnMgTe QDs. The presence of Mg contributes to higher scattering intensity of the HRXRD, leading to the observation of higher order superlattice peaks in both the (004) and (002) reflections. The distribution of scattered intensity in the reciprocal space map (RSM) shows that the diffuse scattered intensity is elongated along the qx axis, indicating a vertical correlation of the dots, which is found to be less defined for the sample with larger periodicity. The diffuse scattered intensity is also found to be weakly correlated along the qz direction indicating a weak lateral correlation of the dots. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and photoreflectance (PR) were used to characterize ZnxCd1-xSe/MgSe multiple quantum well (MQW) structures grown on InP substrates by molecular beam epitaxy for mid-infrared (IR) device applications. The PL spectra yielded information of the fundamental excitonic recombination and ZnxCd1-xSe cap/spacer band edge emission of the samples. The PR spectra revealed multitude of possible interband transitions in MQW structures. The ground state transitions were assigned by comparing with the PL emission signals taken from the same structures. A comprehensive analysis of the PR spectra led to the identification of various interband transitions. The intersubband transitions were then estimated and found to be in a good agreement with the previous report of Fourier-transform IR absorption measurements [Li et al., Appl. Phys. Lett. 92, 261104 (2008)]. The results demonstrate the potential of using PL and PR as nondestructive optical techniques for characterization of ZnxCd1-xSe/MgSe MQWs for mid-IR device applications. [ABSTRACT FROM AUTHOR]

Bias voltages applied to Zn0.24Cd0.76Se quantum well light emitting diodes (QW-LEDs) affect both the intensity and wavelength of room temperature cathodoluminescence (CL). These effects have been studied experimentally and theoretically to advance understanding of the CL and optoelectronic behavior of these devices. QW CL intensity and photon energy are increased by forward bias, and they are decreased by reverse bias, with an exponential dependence of CL intensity on bias voltage from -1 to +1 V and little dependence from 1.5 to 2.5 V. The p-n junction current and electroluminescence increase rapidly for forward bias greater than 2.34 V, the calculated built-in potential. The bias dependence of QW CL intensity is little affected when electron beam currents change by ∼300 times, from 0.1 to 29 nA with 10 kV beam voltage and ∼1 μm2 irradiated area. The QW CL intensity increases sublinearly with beam current. Small hysteresis effects are seen in bias-dependent CL intensity for low beam currents. The effects of bias voltage on CL intensity and photon energy have been modeled, including bias dependence of carrier transport, QW energy levels, wave functions, overlap integrals, internal electric fields, exciton ionization, and rates of carrier capture in and escape from the QW. For the QW-LED and experimental conditions used in this study, the bias dependence of CL intensity at room temperature results mainly from electric field dependence of exciton ionization and of electron and hole captures in the QW, and the bias dependence of CL photon energy results from field-dependent shifts in QW energy levels of electrons and holes. [ABSTRACT FROM AUTHOR]

Carrier recombination through radiative and nonradiative processes in lattice-matched n-Zn0.5Cd0.5Se/Zn0.21Cd0.19Mg0.6Se multi-quantum-wells (MQWs) was investigated by temperature-dependent time-resolved photoluminescence (PL) spectroscopy. The n-Zn0.5Cd0.5Se/Zn0.21Cd0.19Mg0.6Se MQW samples with different well widths were grown on InP substrates by molecular beam epitaxy. The PL decay times and the PL intensities were measured as functions of temperature. For a doping level of 1×1018 cm-3, the dominant mechanism of the radiative process was found to be free carrier recombination while excitonic recombination was absent due to the effect of strong carrier screening. The nonradiative mechanism was determined to be hole capture through multiphonon emission (MPE). The expressions of the nonradiative MPE recombination lifetime, the PL decay time, and the PL intensity were deduced as functions of temperature and were used to fit the measured temperature dependence of the PL decay times and the PL intensities. The MPE activation energies and relative defect densities for the samples with different well widths were obtained. A simple method is suggested to investigate the interfacial defects of quantum wells. [ABSTRACT FROM AUTHOR]

The Verleur and Barker-like multiphonon description proposed for the refined understanding of the two-mode transverse optical (TO) and longitudinal optical (LO) Raman responses from the stiff/short Be–Se bond in the percolation range (0.19<=Be<=0.81) of the random BeSe–ZnSe mixed crystal [O. Pagès et al., Phys. Rev. B 70, 155319 (2004)] is extended to the similar Be–Te bond in the percolation range of the random BeTe–ZnTe mixed crystal. Moreover this description is shown to remain valid just outside the percolation range, in spite of appearances. We deduce that it is not activated by the close intermixing of the stiff and soft continua at the mesoscopic scale but relies directly on the contrast in the bond properties at the microscopic scale. The whole study is supported by extensive ZnBeTe Raman data, and full contour modeling of the TO and LO Raman line shapes by using our phenomenological 1-bond→2-mode percolation model. Care is taken that our ZnBe(Se,Te) epitaxial layers undergo large substrate-induced residual biaxial strains at the two edges of the percolation range. This generates significant phonon shifts, which have to be taken into account. [ABSTRACT FROM AUTHOR]

We here report results of high resolution x-ray diffraction, x-ray reflectivity (XRR), as well as optical absorption and reflection measurements on ZnSe samples grown by molecular beam epitaxy, with insertion of planar (δ-) regions of both N as an acceptor dopant and Te as a “co-dopant” to facilitate a p-type doping. We note that to enhance the surface diffusion of Te, migration enhanced epitaxy was adopted in the growth of the “δ-layers;” i.e., Te is deposited in the absence of Zn flux. Structural parameters were extracted by simulating the experimental x-ray diffraction curves using a dynamical model. The results show that only the “δ-layers” (with submonolayer thickness) are rich in ZnTe, while the nominally undoped “spacers” have only a low Te concentration. Moreover, the morphology of the surface and interfaces are studied by XRR. Furthermore, the optical absorption and reflection results show that our samples largely preserve the optical properties of the host material (ZnSe). We note that our results, in particular those on the Te concentration, explain the observed good p-type doping of such samples. [ABSTRACT FROM AUTHOR]

Electron bombardment enhancement of luminescence from ZnCdMgSe quantum well (QW) light emitting diodes has been studied by time-resolved and bias- and temperature-dependent cathodoluminescence (CL) spectroscopy and mapping. Effects of electron bombardment are seen as bright regions in CL images. The enhancement of CL intensity decays with time after electron bombardment for tens and hundreds of hours, and the rate of decay is faster for higher temperatures in the range studied, 20–200 °C. We propose that the enhancement of QW CL intensity results from buildup of internal fields due to persistent charge trapping at pre-existing or electron bombardment created defects. A simple trapping-detrapping model involving two types of traps with different activation energies reproduces most aspects of the observed time and temperature dependence of decay of the luminescence enhancement. [ABSTRACT FROM AUTHOR]

The mechanisms controlling the growth rate and composition of epitaxial CdTe and Cd1-xZnxTe films were studied. The films were grown by isothermal closed space configuration technique. A GaAs(100) substrate was exposed sequentially to the elemental sources (Zn, Te, and Cd) in isothermal conditions. While growth of ZnTe followed an atomic layer epitaxy (ALE) regime [self-regulated at one monolayer (ML) per cycle]; the CdTe films revealed different growth rates in dependence of the growth parameters (exposure and purge times). Combination of short purge times and larger Cd exposure times led to not self-regulated growth regime for CdTe. This is ascribed to large Cd coverages that were dependent on Cd exposure times (following a Brunauer-Emmett and Teller-type adsorption). However, for longer purge times and/or short Cd exposure times, an ALE self-regulated regime was achieved with 2 ML/cycle. In this sense, the self-regulation of the growth is limited by desorption, instead of absorption, as in the traditional growth technique. Cd atoms substitution by Zn atoms and subsequent evaporation of surface Cd atoms during Zn exposure has been proved. The influence of these facts on the growth and composition of the alloy is discussed. [ABSTRACT FROM AUTHOR]

We report the optical properties of BeCdSe/ZnCdMgSe single quantum well (QW) structures that consist of closely lattice matched ZnCdMgSe barrier layers and a strained BeCdSe QW layer (Δa/a=1.95%) grown on InP substrates. Emission from the red to the green regions of the visible spectrum was obtained from the structures with the QW thickness varying from 95 to 12 Å. Efficient QW emission, dominated by an exciton recombination behavior, was observed. From the Arrhenius plot of the integrated emission intensity as a function of temperature, an activation energy of 61 meV was obtained for a BeCdSe QW structure with a 48 Å thick QW layer. Parameters that describe the temperature dependence of the near band edge emission energy and the broadening of the excitonic emission were evaluated. Our results indicate that the BeCdSe-based QW structures are attractive for application as red light emitters. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High crystalline quality ZnBeSe epilayers were grown nearly lattice matched to GaAs (001) substrates by molecular beam epitaxy with a Be-Zn co-irradiation. A (12) reflection high energy electron diffraction pattern was observed after the Be-Zn co-irradiation of the GaAs (24) surface. A high p-type doping level of 1.510cm was achieved for (N+Te) triple-delta doping ( doping) of ZnBeSe epilayers, whereby three adjacent layers of N and Te were deposited in each doping cycle. X-ray diffraction measurements reveal that (N+Te) -doped ZnBeSe samples with a Te concentration of about 0.5% remain of very high crystalline quality with an X-ray rocking curve linewidth of 51 arcsec. Low temperature photoluminescence measurements show some emission peaks related to Te clusters and/or Ten≥3 clusters. c 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Distributed Bragg reflectors (DBRs) based on ZnCdSe and ZnCdMgSe quarter wave layers have been grown on InP (001) substrates by molecular beam epitaxy. Their reflectivity maxima were controlled by the individual thicknesses of constituent layers, and were adjusted in the range of 615-500 nm, covering the red, green, and blue-green regions of the visible spectrum. The crystal quality of the structures was assessed by double crystal x-ray diffraction measurements. Reflectivity as high as 95% was achieved for the DBR structures that consisted of 16 periods. Theoretical calculations, based on the transfer matrix method, predict that a reflectivity above 99% can be achieved by increasing the number of periods to between 20 and 26. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Temperature-dependent time-resolved photoluminescence spectroscopy is used to investigate the carrier recombination through radiative and nonradiative processes in lattice-matched ZnCdSe/ZnCdMgSe multi-quantum-wells (MQWs). The carrier recombination processes found in these samples are attributed to the interplay between the free carrier recombination and hole capture through multiphonon emission.

Investigates the factors determining the interface quality in II-VI and III-V compound semiconductors such as zinc selenide and gallium arsenide (GaAs). Issue on polar heteroepitaxy between compound semiconductors; Polar heteroepitaxial configuration of GaAs surface; GaAs growth on zinc selenide.

Studies the electrical characterization of gallium planar-doped zinc selenide grown by molecular-beam epitaxy. Applications of II-VI compound semiconductor; Details on the experimental procedure; Discussion on the results of the study.