Se Barriers Research Articles

PbSnSe/PbSrSe quantum well materials for thermophotovoltaic devices

Multiple quantum well (MQW) materials composed of Pb0.81Sn0.19Se wells and Pb0.80Sr0.20Se barriers with intersubband energy gaps of 343 meV and 450 meV were modeled for thermophotovoltaic (TPV) device performance. The effect of L-valley degeneracy removal in these (111)-oriented IV-VI semiconductor quantum wells was evaluated. Degeneracy splitting reduces the effective densities of states in both the valence and conduction bands. Thermally generated intrinsic charge carrier concentrations are smaller by a factor of three as compared to bulk materials with the same bandgap energies. A current-matched dual junction TPV cell made from these MQW materials in a generator with a 1215°C radiator is predicted to have a power density of 2.34 W/cm2, 49% better than the power density generated by a cell made from bulk materials with the same bandgap energies.

Epitaxial growth and photoluminescence excitation spectroscopy of CdSe quantum dots in (Zn,Cd)Se barrier

Design, epitaxial growth, and resonant spectroscopy of CdSe Quantum Dots (QDs) embedded in an innovative (Zn,Cd)Se barrier are presented. The (Zn,Cd)Se barrier enables shifting of QDs energy emission down to 1.87eV, that is below the energy of Mn2+ ions internal transition (2.1eV). This opens a perspective for implementation of epitaxial CdSe QDs doped with several Mn ions as, e.g., the light sources in high quantum yield magnetooptical devices. Polarization resolved Photoluminescence Excitation measurements of individual QDs reveal sharp (Γ<150μeV) maxima and transfer of optical polarization to QD confining charged exciton state with efficiency attaining 26%. The QD doping with single Mn2+ ions is achieved.

Growth and characterization of mid-infrared microdisk lasers operating in continuous-wave mode up to 2 °C

Growth and characterization of lead salt microdisk lasers with near-room-temperature continuous-wave (cw) emission at mid-infrared wavelengths around 4.3μm is reported. The laser structures were grown by molecular beam epitaxy on BaF2 substrates and consist of a PbSe single quantum well embedded in Pb0.94Sr0.06Se barrier layers. Under optical excitation, cw lasing is obtained up to temperatures of 275K, i.e., 2°C. With increasing pump power, the narrow laser line shifts to shorter wavelengths, from which an internal temperature of about 20°C is estimated. We also find a good agreement between the observed laser lines and the calculated optical microdisk resonator modes.

Zero field spin splitting in CdSe quantum dots

AbstractWe report the fabrication and characterization of self assembled CdSe quantum dots embedded in a dilute magnetic (Zn,Mn,Be)Se barrier. Transport experiments indicate a finite spin splitting at zero magnetic field although the materials are intrinsically paramagnetic. The result can be explained by the formation of a single magnetic polaron by ferromagnetic interaction between the surrounding Mn ions mediated by the carriers inside the dot. The effect prevails up to relatively high temperatures which we ascribe to a feedback mechanism induced by the tunneling current, previously predicted theoretically. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoluminescence of CdSe quantum dots with Zn0.38Cd0.23Mg0.39Se barriers under hydrostatic pressure

AbstractWe have investigated the pressure dependence of the room‐temperature photoluminescence (PL) of self‐assembled CdSe quantum dots (QDs) with quaternary ZnCdMgSe barriers. Four samples with different CdSe deposition times, i.e. different average dot size, were studied. Samples exhibit a very distinct emission energy covering the visible spectrum from the red to the blue. We found, however, a similar pressure coefficient of around 47 meV/GPa for the PL emission of all samples. This value is close to that of bulk CdSe, which gives evidence that dot and quaternary barrier material have a similar compressibility. In contrast, the PL linewidth displays a clear trend with respect to the emission energy, the PL line being broader for the smaller dots emitting in the blue. This is explained by taking into account the huge band‐gap offset of ≈1 eV between dot and barrier and considering quantum confinement effects within a very simplistic infinitely deep square‐well model. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Size-dependent strain effects in self-assembled CdSe quantum dots with Zn0.38Cd0.23Mg0.39Se barriers

From resonant Raman scattering measurements under hydrostatic pressure the authors were able to identify both the strain-shifted longitudinal optical phonon of CdSe quantum dots with different average size embedded in a quaternary ZnCdMgSe material and a Mg-local dot mode due to interdiffusion from the barrier. They show that the large tunability of the light emission, which covers most of the visible spectrum, is not only due to a different degree of confinement but also to a size-dependent built-in strain. The frequency of the Mg mode allowed them also to roughly estimate the amount of Mg interdiffusion into the quantum dots.

Magnesium effects on CdSe self-assembled quantum dot formation on Zn xCd yMg 1−x−ySe layers

Optical and morphological studies are used to investigate the effects of chemical composition and, in particular, the magnesium content of the Zn x Cd y Mg 1− x − y Se barrier layers on the size, density and uniformity of CdSe self-assembled quantum dots (QDs). A reduction of the uncapped QD size, as well as a blue shift of the capped QD photoluminescence peak position by increasing Mg concentration in the Zn x Cd y Mg 1− x − y Se barrier has been demonstrated by changing the Mg cell temperature during growth. In addition, a more uniform and more densely packed QD layer has been observed with an increase of the MgSe fraction in the Zn x Cd y Mg 1− x − y Se barrier layer using three-dimensional topographic atomic force microscopy images of the surface of uncapped QDs. Results point to Mg as a chemical factor that induces QD formation, either by increasing the density of atomic steps or/and by changing the energy of the Zn x Cd y Mg 1− x − y Se surface.

Remanent Zero Field Spin Splitting of Self-Assembled Quantum Dots in a Paramagnetic Host

We report on the observation of a finite spin splitting at zero magnetic field in resonant tunneling experiments on CdSe self-assembled quantum dots in a (Zn,Be,Mn)Se barrier. This is remarkable since bulk II-VI dilute magnetic semiconductors are paramagnets. Our experiment may be viewed as tunneling through a single magnetic polaron, where the carriers contained inside the dot act to mediate an effective ferromagnetic interaction between Mn ions in their vicinity. The effect is observable up to relatively high temperatures, which we tentatively ascribe to a feedback mechanism with the electrical current, previously predicted theoretically.

CdSe self-assembled quantum dots grown on ZnMnSe diluted magnetic semiconductors with different Mn concentration

We have investigated magneto-optical properties of a series of CdSe self-assembled quantum dots (QDs) grown on the Zn 1− x Mn x Se diluted magnetic semiconductors (DMSs). The QDs are formed by depositing 2.5 ML CdSe layer on 1 μm thick Zn 1− x Mn x Se buffer layer prepared by molecular beam epitaxy (MBE). The Mn concentration x in the Zn 1− x Mn x Se buffer was varied from 0.05 to 0.25. The photoluminescence (PL) peaks from CdSe QDs and from Zn 1− x Mn x Se barrier are clearly observed at zero magnetic field. The PL position of CdSe QDs appears to be systematically shifted toward low energy with increasing Mn concentration in the Zn 1− x Mn x Se buffer layer. The PL intensity ratio of CdSe QDs with respect to the Zn 1− x Mn x Se barrier also monotonically increases with Mn concentration x. This systematic behavior observed in the series of CdSe/Zn 1− x Mn x Se QD systems indicates that the CdSe QD formation strongly depends on the Mn concentration of the Zn 1− x Mn x Se buffer layer. The observed large Zeeman shift of the QD system in the presence of magnetic field was discussed in terms of the wavefunction overlap of carriers with Mn ions in the barrier.

Structural characterization of ZnSe/ZnMgSe MQWs grown on (1 0 0)GaAs by low pressure MOVPE

A detailed structural characterization of ZnSe/ZnMgSe multiple quantum wells (MQWs) grown on GaAs by low pressure metalorganic vapour phase epitaxy is presented. ZnSe/Zn 0.83Mg 0.17Se MQWs having between 6 and 12 periods were deposited at 330°C and 304 mbar reactor pressure on (1 0 0)GaAs after a 4.2 nm ZnSe buffer layer. The MQWs had nominal 4.4 nm thick ZnSe wells and 5.3 nm thick Zn 0.83Mg 0.17Se barriers. The MQW structural properties were investigated by high-resolution X-ray diffraction (HRXRD) and X-ray specular reflectivity (XSR) measurements. Besides the MQWs-substrate mismatch, simulation of the HRXRD and XSR patterns allowed to determine the MQW period, individual layer thickness and barrier composition. Between 8 and 10 periods the MQW structure begins to relax, its critical thickness on GaAs being between 92 and 113 nm. Furthermore, HRXRD showed broader zeroth and first-order satellite peaks with increasing MQW periods, a result ascribed to strain fluctuations induced by either inhomogeneous Mg incorporation in the ZnSe lattice and/or interface roughening. Comparison of experimental and simulated XSR patterns allowed to determine the rms roughness at each multilayer interface, which linearly increases along the growth direction due to a cumulative intrinsic roughening.

Binding energy of charged excitons in ZnSe-based quantum wells

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively ${(X}^{\ensuremath{-}})$ and positively ${(X}^{+})$ charged excitons are measured as functions of quantum well width, and free carrier density and in external magnetic fields up to 47 T. The binding energy of ${X}^{\ensuremath{-}}$ shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well width from 190 to $29 \AA{}.$ The binding energies of ${X}^{+}$ are about 25% smaller than the ${X}^{\ensuremath{-}}$ binding energy in the same structures. The magnetic field behavior of ${X}^{\ensuremath{-}}$ and ${X}^{+}$ binding energies differ qualitatively. With growing magnetic field strength, ${X}^{\ensuremath{-}}$ increases its binding energy by 35--150%, while for ${X}^{+}$ it decreases by 25%. Zeeman spin splittings and oscillator strengths of excitons and trions are measured and discussed.

Fabrication of wurtzite quantum-well structures of CdSe/ZnCdSe by molecular beam epitaxy

We successfully fabricated wurtzite quantum-well structures with Zn 0.4Cd 0.6Se barrier layers and a four monolayer CdSe well layer on cubic ZnSe/GaAs(1 1 1)B substrates by molecular beam epitaxy. Although the cubic CdSe well layer grew on the cubic ZnSe/GaAs(1 1 1)B substrate, the wurtzite CdSe well layer grew on the wurtzite-Zn 0.4Cd 0.6Se/cubic-ZnSe/GaAs(1 1 1)B structure. The CdSe well layer was compressed in the (0 0 0 1) plane and dilated to the [0 0 0 1] direction judging from the X-ray diffraction measurement. Weak orange photoluminescence was observed in this quantum-well structure.

Magneto-optical properties ofZn0.95Mn0.05Se/Zn0.76Be0.08Mg0.16Sequantum wells andZn0.91Mn0.09Se/Zn0.972Be0.028Sespin superlattices

We have performed magnetoluminescence and magnetoreflectance studies on novel semimagnetic semiconductor heterostructures based on $(\mathrm{ZnMn})\mathrm{Se}$/$(\mathrm{ZnBeMg})\mathrm{Se}$. The successful fabrication and various optical properties of that system are demonstrated by ${\mathrm{Zn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Se}$/'' ''${\mathrm{Zn}}_{0.76}{\mathrm{Be}}_{0.08}{\mathrm{Mg}}_{0.16}\mathrm{Se}$ single quantum wells. The exciton spin splitting was used to determine the valence-band offset for $(\mathrm{ZnMn})\mathrm{Se}$/$(\mathrm{ZnBeMg})\mathrm{Se}$ to $0.22$ of the total band-gap discontinuity. Weakly confining multiple quantum wells made of ${\mathrm{Zn}}_{0.91}{\mathrm{Mn}}_{0.09}\mathrm{Se}/{\mathrm{Zn}}_{0.972}{\mathrm{Be}}_{0.028}\mathrm{Se}$ demonstrate a change of the band alignment from type I to type II for one of the exciton spin components in external magnetic fields which results in the formation of a spin superlattice. This spin superlattice formation manifests itself in an asymmetric Zeeman splitting of a spatially direct exciton caused by a spin-dependent change of the exciton binding energy. A pronounced broadening of the exciton in reflectance and photoluminescence excitation spectra was observed when scattering into the (ZnBe)Se barriers becomes possible.

Magnetic tailoring of the nature of the fundamental optical transition in a ZnSe/(Zn,Mn)Se heterostructure.

We report on optical measurements performed on two 53 and 106 \AA{} wide ZnSe quantum wells separated by a 350-\AA{}-thick ${\mathrm{Zn}}_{0.73}$${\mathrm{Mn}}_{0.27}$Se barrier. The measurements were done by means of cw photoluminescence, cw photoluminescence excitation, and time-resolved photoluminescence spectroscopies at low temperature and under a magnetic field up to 5.5 T. They allow us to determine the strain state of the different layers and the valence-band offset between the ternary and binary alloys. Moreover magnetic tailoring of the nature of the fundamental optical transition from a type-I light-hole exciton to a type-II heavy-hole exciton is evidenced. Calculations taking into account the strain, Zeeman, and excitonic effects are presented and support the experimental findings.

Steady state and time-resolved magneto-luminescence in ZnSe/(Zn,Mn)Se Heterostructures

We report on optical measurements performed on two 53Å and 106Å wide ZnSe quantum wells separated by a 350Å thick Zn 0.73Mn 0.27Se barrier. The measurements were performed by means of cw photoluminescence up to 20T, cw photoluminescence excitation and time-resolved photoluminescence spectroscopies up to 9T, at low temperature (4.2K). The fundamental optical transition changes its nature from a type-I light hole excition to a type-II heavy hole exciton as a function of applied magnetic field. Calculations taking into account the strain, Zeeman, and excitonic effects support the experimental findings and allows us to specify the value of the relative valence band offset.

Spin superlattice formation in ZnSe-based diluted magnetic semiconductor heterostructures

We have observed spin–polarized confinement of both electrons and holes in ZnSe-based single and multiple quantum well structures. The samples were grown by molecular beam epitaxy with 100 Å Zn0.99Fe0.01Se barriers and 100–200 Å ZnSe wells. In these systems, the band offsets are very small, so that the large spin–splitting of the diluted magnetic semiconductor levels dominates the alignment of both the conduction and valence bands, permitting magnetic field-tunable confinement. Magnetoreflectivity measurements show a pronounced asymmetry of the heavy hole spin–splitting, a signature of spin-dependent quantum confinement. The relative phase and near-unity intensity ratio of the ‘‘spin–up’’ and ‘‘spin–down’’ excitons confirm localization of spin–up carriers to the ZnSe wells and spin–down carriers to the ZnFeSe layers, producing a field-tunable ‘‘spin–superlattice.’’ These results will be compared to those obtained for higher magnetic ion concentrations in the barriers.

Spin separation in diluted magnetic semiconductor quantum well systems (invited)

Heterostructures containing diluted magnetic semiconductor (DMS) layers offer the possibility of magnetically tuning the heterojunction band alignment due to the extraordinarily large spin-splitting of the DMS bands (large effective g-factor). This field-dependent band alignment has significant consequences for spin-dependent carrier confinement as evidenced in magnetooptic or magneto-transport experiments. We have examined two wide-gap DMS quantum well systems in which the band alignment is dominated by the DMS spin-splitting rather than by the more commonly observed effects of differences in bandgap, natural band offset, and strain. Quantum well structures with (Zn,Fe)Se or (Zn,Mn)Se barriers and ZnSe wells have been grown to investigate magnetically tuned, spin-dependent quantum confinement. In these systems, the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. However, the hole confinement is continuously tunable by an external magnetic field applied normal to the layer plane, since the valence band spin-splitting is approximately an order of magnitude larger at modest fields (1 T) than the zero field VB offset. This produces a field-induced spin dependent type I/type II band alignment and a consequent spatial spin separation of the holes. The quantum well structure exhibits a type I ‘‘straddling’’ alignment for the mj=+3/2 level, with the spin ‘‘up’’ holes localized in the ZnSe wells, and a type II ‘‘staggered’’ alignment for the mj=−3/2 level in which the spin ‘‘down’’ holes are localized in the barriers. The observed exciton splittings, intensities and temperature dependence are consistent with this model. In the Zn0.91Mn0.09Se/ZnSe system, a second bound state is observed in the conduction band in the σ− polarization in magnetoreflectivity measurements. A quantitative fit to the data yields a value of approximately 1 meV for the zero field valence band offset. These systems represent the first DMS-based heterostructures in which such field-dependent confinement has been observed.

Spin separation and type I/type II behavior in ZnSe/(Zn,Fe)Se quantum wells (abstract)

Quantum well structures with 100-Å Zn0.9Fe0.1Se barriers and 100–200-Å ZnSe wells have been grown by molecular-beam epitaxy. The magnetic field splitting of the heavy-hole exciton was studied by magnetoreflectivity at 4.2 K and B≤8 T, and is observed to be strikingly asymmetric, in contrast to the behavior observed in thick single epilayers. This behavior may be quantitatively understood through a model which incorporates strain effects, quantum confinement in the conduction and valence bands, and the strong spin exchange interaction in the (Zn,Fe)Se layers. In this system the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. The valence-band (VB) lineup is thus determined primarily by the field-induced spin splitting of the heavy-hole level, which at modest fields (1 T) is an order of magnitude larger than the zero-field VB offset. This produces a field-induced spin-dependent band alignment, with the quantum well structure exhibiting a type I ‘‘straddling’’ alignment for the mj=+ (3)/(2) level, and a type II ‘‘staggered’’ alignment for the mj=− (3)/(2) level. This leads to a spatial spin separation of the holes, with the mj=+ (3)/(2) spin-‘‘up’’ holes localized in the wells, and the mj=− (3)/(2) spin-‘‘down’’ holes localized in the barriers. The observed exciton intensities and splittings are both consistent with such a model.

Photosensitive tunnel junctions and photosensitive granular films with Te or Se barriers

Abstract SnTePb and SnSePb junctions are found to exhibit a photosensitive behavior. A preliminary experiment is performed to prepare a photosensitive granular film in which the interparticle coupling can be controlled by light exposure.