Magnetic field effect on transitions between direct and indirect excitons in diluted magnetic semiconductor double quantum wells

Transitions between direct and indirect excitons with change of magnetic field in double quantum well heterostructure Cd_1−xMg_xTe/Cd_1−yMg_yTe/Cd_1−xMg_xTe/Cd_1−zMn_zTe/Cd_1−xMg_xTe in external magnetic field are studied. The structure contains diluted magnetic semiconductor (Cd,Mn)Te layer that forms magnetic quantum well with the depth depending on the magnetic field intensity. Above some magnetic field the indirect exciton becomes the lowest excited state of the system. The indirect exciton lifetime exceeds by several orders of magnitude of the direct exciton one. The range of quantum well widths for which the indirect exciton is the exciton lowest state was estimated for the proposed system.     

Far-infrared dielectric functions and phonon spectra of alloys determined by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to determine the complex dielectric function of a series of ternary Be_xZn_1−xTe thin films grown by molecular beam epitaxy. The II–VI semiconductor alloys were grown on InP substrates that had an InGaAs buffer layer. After the growth, X-ray diffraction experiments were performed in order to determine the alloy concentration. A standard inversion technique was used to obtain the dielectric functions from the measured ellipsometric spectra, obtained between 2000 nm (5000 cm⁻¹) and 40,000 nm (250 cm⁻¹). By modelling the dielectric function as a collection of oscillators, representing longitudinal and transverse optical phonons of the Be_xZn_1−xTe lattice, we were able to recover the phonon spectra for this alloy system. It is argued that the additional phonon modes that are obtained from ellipsometry are best understood from the recently-proposed percolation model.     

Optically pumped lead chalcogenide infrared emitters on silicon substrates

Two low cost-infrared sources emitting above 4 μm wavelength are described: (i) Double heterostructure or quantum well EuSe/PbSe/Pb_1−xEu_xSe edge emitting lasers on silicon substrates show peak powers up to 200 mW and differential quantum efficiencies up to 20%. They operate up to 250 K when pumped with 870 nm laser diodes (with peak powers of 5.5 W). (ii) A “wavelength transformer”, a EuSe/PbSe/Pb_1−xEu_xSe active resonant cavity with epitaxial bottom and top mirror on a Si(1 1 1) substrate transforms the incoming 870 nm pump radiation into e.g. 4.2 μm wavelength. The device operates at room temperature, and the width and value of the emission wavelength can be tuned by design.     

Electronic properties of intersubband transition in (CdS/ZnSe)/BeTe quantum wells

In view of the fact that the bandwidth required in optical fiber communication systems will exceed 100 Gb s^-1, ultrafast optical switching and modulation devices with high efficiency must be developed. Given that intersubband transitions (ISBT) in quantum wells (QWs) are one of the important ultrafast phenomena, a numerical study of intersubband transition (ISBT) properties in (CdS/ZnSe)/BeTe QWs is considered. The structure modeled consists of a few monolayers of CdS embedded in a ZnSe/BeTe QW. A self-consistent analysis is made to achieve the desired properties and device applications. Variation of CdS well thickness leads to tailoring of the band alignment, achieving optical transitions in the wavelength range of 1.33–1.55 μm wavelengths for applications in optical fiber transmission. To analyze the optical behavior of the heterostructure under investigation, we have calculated the CdS well thickness-dependant oscillator strengths and electron emission energy of the intersubband transition between the two first states in the well. An attempt to explain our results will be presented.     

Dynamical nonlinear refraction in PbTe/Pb1−xSnxTe MQWs

An analysis of nonlinear refraction due to the dynamical free carrier filling of minibands in PbTe/Pb_1−xSn_xTe (X = 0.2) MQEs on (111) BaF₂ substrates is presented. The results are compared with those for bulk Pb_1−xSn_xTe layers. The abrupt substantial increase of n(I) in MQWs is connected with crossing of the minibands in oblique valleys of Pb_1−xSn_xTe wells by the Fermi quasi-level.     

Dispersive optical constants of glassy AsSe1−xTex system

The compositional dependence of the optical constants, the refractive index n, and the absorption index k, of the AsSe_1−xTe_x thin films with 0<x<1.0 were determined in the spectral range of 400–2500 nm. The maximum value of the refractive index n, is shifted toward the long wavelength by increasing the Te content in the examined system. The values of the forbidden energy gap of the system have been determined and were correlated with the type and the amount of chemical bonds formed by the increasing Te content in the AsSe_1−xTe_x glassy system. The value of the dispersion energy E_d exhibits low value at the composition containing the same atomic percent of Se and Te.     

Atomic and electronic structure formation at the metal-Cd1−xMnxTe interface

Atomic and electronic structure modification of a metal-Cd_1−xMn_xTe interface is achieved using selective etching of the Cd_1−xMn_xTe surface (x=0, 0.34) and Cd adsorption. It is revealed that Te, TeO₂, Mn₃O₄, and CdTeO₃ are formed at the Cd_1−xMn_xTe surface etched in Br₂ solution. Te and Cd_1−xMn_xTe produce TeCd_1−xMn_xTe heterojunctions, the salient features of which are nearly symmetric nonlinear I-V characteristics. At the Cd_1−xMn_xTe surface with adsorbed Cd, CdTe might form, resulting in a CdTe-Cd_1−xMn_xTe heterojunction. The metal-CdTe-Cd_1−xMn_xTe microstructure is characterized by a nonlinear dependence of current on voltage and rectifying behaviour. The results obtained give deep insight into electronic processes in metal-Cd_1−xMn_xTe microstructures.     

Far-infrared spectra of Pb1−xMnxTe alloys

Far-infrared reflectivity spectra of Pb_1−xMn_xTe (0.0001x0.1) single crystals were measured in the 10–250 cm⁻¹ range at room temperature. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed an intermediate one–two mode behavior.     

Einstein relation in n-i-p-i and microstructures of nonlinear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestions for an experimental determination

We investigate the Einstein relation for the diffusivity-mobility ratio (DMR) for n-i-p-i and the microstructures of nonlinear optical compounds on the basis of a newly formulated electron dispersion law. The corresponding results for III-V, ternary and quaternary materials form a special case of our generalized analysis. The respective DMRs for II-VI, IV-VI and stressed materials have been studied. It has been found that taking CdGeAs₂, Cd₃As₂, InAs, InSb, Hg_1−xCd_xTe, In_1−xGa_xAs_yP_1−y lattices matched to InP, CdS, PbTe, PbSnTe and Pb_1−xSn_xSe and stressed InSb as examples that the DMR increases with increasing electron concentration in various manners with different numerical magnitudes which reflect the different signatures of the n-i-p-i systems and the corresponding microstructures. We have suggested an experimental method of determining the DMR in this case and the present simplified analysis is in agreement with the suggested relationship. In addition, our results find three applications in the field of quantum effect devices.     

Intersubband resonant enhancement of the nonlinear optical properties in asymmetric (CdS/ZnSe) based quantum wells

In this work, the linear and nonlinear optical properties are studied theoretically in asymmetric (CdS/ZnSe/BeTe)/(ZnSe/BeTe) quantum wells. The electronic states are calculated using the envelope wave function approximation and the intersubband transition energies are studied as a function of CdS and ZnSe well thicknesses as well as doping concentration. The optimum parameters carrying out the transition energy 0.8 eV (1.55 μm wavelength) are given. Results are presented for the linear, the third order nonlinear optical absorption and the refractive index changes in the studied heterostructure. Results show that the changes in the linear and the third order nonlinear optical absorption as well as refractive index change are as important as the temperature is high, the nonlinear terms must be taken into consideration especially near the resonance.     

Structural and electronic properties of GaAsBi

The structural and electronic properties of the GaAs_1−xBi_x ternary alloy are investigated by means of two first principles and full potential methods, the linear augmented plane waves (FPLAPW) method and a recent version of the full potential linear muffin-tin orbitals method (FPLMTO) which enables an accurate treatment of the interstitial regions. In particular, we have found that the maximal GaBi mole fraction x for which GaBi_xAs_1−x remains a semiconductor is probably around x=0.5. The electronic properties of (GaAs)_m/(GaBi)_n quantum well superlattices (SLs) have also been calculated and it is found that such SLs are semiconductors when m is larger or equal to n.     

Optical properties and ferromagnetism of ternary Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te nanocrystals

Cd_1−x Mn _x Te (x = 0, 0.1, 0.2) nanocrystals have been synthesized by mechanical alloying (MA) Cd, Mn, and Se elemental powders. XRD patterns and HRTEM images confirmed the formation of cubic Cd_1−x Mn _x Te nanocrystals. All the diffraction peaks from elemental Cd, Mn, and Te powders disappeared completely in those XRD patterns of as-milled Cd_1−x Mn _x Te nanocrystals for more than 20 h. When the MA process was carried out for 40 h, typical zinc blende structure diffraction mode was exhibited in the XRD pattern. Subsequently, capping the surface of as-milled Cd_1−x Mn _x Te nanocrystals with long chain trioctylphosphine/trioctylphosphine oxide/nitric acid (TOP/TOPO/NA) molecules has achieved colorful dispersion solution, which shows similar optical properties to those CdTe nanocrystals prepared by wet chemical process. The grain size is within the range of 2–8 nm for the capped Cd_1−x Mn _x Te nanocrystals being ball milled for 40 h. The PL excitation peak red shifts to longer wavelength side with increasing Mn concentration. Pure CdTe nanocrystals show ferromagnetism behavior at room temperature, the saturation magnetization value and magnetic hysteresis loop increase with the content of substituting Mn ions within the Cd_1−x Mn _x Te nanocrystals.     

Properties of optical phonons in Zn1−x−yMgyBexSe quaternary mixed crystal

Long wavelength optical lattice vibration and dielectric constants of the quaternary mixed crystal Zn_1−x−yMg_yBe_xSe are investigated based on the pseudo-unit-cell mode and Born-Huang procedure. It is found that this material shows a three-mode behavior and the oscillator strength of each mode is mainly controlled by only one component. The theoretical results also show that the linear interpretation method for dielectric constants is reliable. The vibrational frequencies and the oscillator strengths of the ternary mixed crystals Be_xZn_1−xSe, Be_xMg_1−xSe and Mg_xZn_1−xSe are also calculated as special cases of the quaternary mixed crystal for comparing with experiments. The calculation shows agreement with the experimental results.     

Modeling of electrical and optical characteristics of near room-temperature CdS/ZnSe based NIR photodetectors

Results of modeled photodetector characteristics in (CdS/ZnSe)/BeTe multi-well diode with p–i–n polarity are reported. The dark current density (J–V) characteristics, the temperature dependence of zero-bias resistance area product (R₀A), the dynamic resistance as well as bias dependent dynamic resistance (R_d) and have been analyzed to investigate the mechanisms limiting the electrical performance of the modeled photodetectors. The quantum efficiency, the responsivity and the detectivity have been also studied as function of the operating wavelength. The suitability of the modeled photodetector is demonstrated by its feasibility of achieving good device performance near room temperature operating at 1.55 μm wavelength required for photodetection in optical communication. Quantum efficiency of ∼95%, responsivity ∼0.6 A/W and D^* ∼ 5.7 × 10¹⁰ cm Hz^1/2/W have been achieved at 300 K in X BeTe conduction band minimum.     

Scanning tunneling optical spectroscopy in mercury cadmium telluride and related compounds

We consider II–VI narrow gap semiconducting alloys: mercury cadmium telluride, Hg_(1−x)Cd_(x)Te (MCT), mercury zinc telluride, Hg_(1−x)Zn_(x)Te (MZT), and mercury zinc selenide, Hg_(1−x)Zn_(x)Se (MZS). MCT is emphasized for actual calculations, but a table of values needed in all calculations is presented. These materials are of interest because of their application to infrared detectors and related devices, and because they are candidates for low gravity crystal growth to improve uniformity. We present new calculations of the scanning tunneling optical spectroscopy (STOS) current from which the local energy gap, a function of x, and hence the stoichiometry (x) can be determined as a function of position with presumably high spatial resolution. The low temperature tunneling current (vs. photon frequency) has a sharper onset at the band gap than the low temperature optical absorption. This sharp onset originates from the rapid increase in the integrated transmission probabilities and is greatly enhanced by large diffusion lengths. Thus, STOS should be a competitive technique, compared to optical absorption, for determining the local stoichiometry, a property that is important for characterizing crystals.     

Optical properties of Cd1−xZnxTe thin films fabricated through sputtering of compound semiconductors

Cd_1−xZn_xTe thin film fabrication is necessary for its photovoltaic and imaging applications in large scale. Thermally annealed and thereby interdiffused r.f. sputtered multilayers comprising of CdTe and ZnTe have been utilized here for the fabrication of Cd_1−xZn_xTe thin films. Photoluminescence and change of resistance of the multilayer under illumination were studied using different annealing temperatures and varying number of repetitions. It was found that three number of repetitions annealed at 300 °C exhibited the best results.     

High-resolution X-ray diffraction analysis of SnTe/Sn1−xEuxTe superlattices grown on (1 1 1) BaF2 substrates

A set of SnTe/Sn_1−xEu_xTe superlattice (SL) samples with increasing nominal Eu content x up to 0.28 was successfully grown on (1 1 1) BaF₂ substrates by molecular beam epitaxy. A complete structural characterization was performed by triple-axis X-ray diffractometry and reciprocal space mapping. The X-ray results showed that, despite the phase separation that normally occurs for unstrained Sn_1−xEu_xTe layers with x0.02, an SL stack with homogeneous individual layers can be formed for SL samples with a nominal Eu content up to 0.16. No SL satellite peak structure could be identified for samples with x values higher than 0.24. The structural parameters of the individual layers that compose the SL were determined using a best-fit simulation procedure which compared the calculated X-ray spectra to the measured (2 2 2) ω/2Θ scans. The strain information used in the simulation was obtained from the reciprocal space maps measured around the (2 2 4) lattice point.     

Development of epitaxial silicon lattice-matched insulators: silicon heterostructures for quantum confinement

Epitaxial films of the wide-bandgap II–VI beryllium chalcogenide semiconductors, BeTe, BeSe, and BeSeTe were grown on arsenic-terminated silicon substrates by MBE. Silicon was also epitaxially regrown on Be-chalcogenide films. Initial structural characterization revealed the desired smooth two-dimensional nature of the layer growth. The composition of BeSeTe ternary films was governed by the Be/Se flux ratio during deposition rather than by the Se/Te flux ratio. The variation in Be/Se flux ratio or in the sticking coefficients due to temperature gradients led to radial compositional inhomogeneity. Current versus temperature measurements of the Be-chalcogenide films at elevated temperatures analyzed assuming thermionic emission over the heterojunction barrier, showed conduction band offsets of 1.2 eV for the BeSe_0.41Te_0.59/As/Si and 1.3 eV for the BeSe/As/Si heterostructures. At room temperature, current density through BeSe/Si and BeSe_0.41Te_0.59/Si films was mid-10 ^{− 9}A cm ^{− 2}at 0.1 MV cm ^{− 1}, similar to previously reported values for ZnS/Si, while BeTe/Si films had orders of magnitude higher current density, possibly due to interfacial recombination.     

Spin polarization dependent optical transition rates observed in the integer quantum Hall region

We report on a field-dependent photoluminescence (PL) emission rate for the transitions between band states in modulation-doped CdTe/Cd_1−xMg_xTe single quantum wells in the integer quantum Hall region. The recombination time observed for the magneto-PL spectra varies in concomitance with the integer quantum Hall plateaus. Furthermore, different PL decay times were observed for the two circular polarizations, i.e. for the transitions between the Zeeman split subbands of the Landau levels. We analyzed the data in comparison with the experimentally determined spin polarization of the conduction electrons and the Zeeman splitting of the valence band. Furthermore, we discuss the relevance of the spin polarization of the conduction electrons, the electron–hole exchange interaction and the spin-flip processes of the hole states for the PL decay time.     

Study of one-mode,two-mode and three-mode phonon behavior in mixed zinc blende alloys

In this paper, we put forth a microscopic interpretation of the characteristics of vibrational spectra of zinc blende semiconductor alloys arising due to either difference in masses or contrast in bond lengths. Previous Raman and infrared experiments have helped in identifying two mode vibrational behaviors in mixed systems of Zn_1−xMg_xSe and Zn_1−xMg_xTe in contrast to the one-mode behavior in Zn_1−xMg_xS. Our lattice dynamics computations have elucidated that in addition to the mass of the anion, bond length anomalies, energy separation between the two sets of optical modes, and the magnitudes of the scattering cross section play an important role in the observance of one mode behavior in the S system, two mode behavior for the whole of the composition range in the Se and Te systems, and an additional Be–Te like vibrational doublet in case of Zn_1−xBe_xTe. Our calculations incorporate the treatment of disorder through a supercell approach. The calculated lattice constants for different concentrations, the bimodal bond length distribution, as well as the phonon frequencies at the Brillouin zone centre are in good agreement with the available experimental data.