Te Barriers Research Articles

Universal magnetic proximity effect in ferromagnet-semiconductor quantum well hybrid structures.

Hybrid ferromagnet-semiconductor systems possess new outstanding properties, which emerge when bringing magnetic and semiconductor materials into contact. In such structures, the long-range magnetic proximity effect couples the spin systems of the ferromagnet and semiconductor on distances exceeding the carrier wave function overlap. The effect is due to the effective p-d exchange interaction of acceptor-bound holes in the quantum well with d-electrons of the ferromagnet. This indirect interaction is established via the phononic Stark effect mediated by the chiral phonons. Here, we demonstrate that the long-range magnetic proximity effect is universal and observed in hybrid structures with diverse magnetic components and potential barriers of various thicknesses and compositions. We study hybrid structures consisting of a semimetal (magnetite Fe3O4) or dielectric (spinel NiFe2O4) ferromagnet and a CdTe quantum well separated by a nonmagnetic (Cd,Mg)Te barrier. The proximity effect is manifested in the circular polarization of the photoluminescence corresponding to the recombination of photoexcited electrons with holes bound to shallow acceptors in the quantum well induced by magnetite or spinel itself, in contrast to interface ferromagnet in case of metal-based hybrid systems. A nontrivial dynamics of the proximity effect is observed in the studied structures due to recombination-induced dynamic polarization of electrons in the quantum well. It enables the determination of the exchange constant Δexch ≈ 70μeV in a magnetite-based structure. The universal origin of the long-range exchange interaction along with the possibility of its electrical control offers prospects for the development of low-voltage spintronic devices compatible with existing solid-state electronics.

Light and heavy excitons in strained CdTe/CdZnTe quantum wells

The spectra of photoluminescence and polarized reflection under normal and oblique incidence of light from structures with quantum wells with symmetric Cd0.9Zn0.1Te/CdTe/Cd0.9Zn0.1Te and asymmetric Cd0.9Zn0.1Te/CdTe/Cd0.4Mg0.6Te barriers have been studied. Due to the mechanical stresses caused by the mismatch of the crystal lattices of the wells and barriers, the energy of light holes in quantum wells was higher than in barriers, i.e., the band structure for them was of type II. However, in the reflection spectra, the lines of heavy and light excitons had comparable intensities. In structures with symmetric barriers, exciton resonances, which do not appear in the photoluminescence spectra, were found in the reflection spectra. A detailed calculation of the energy levels and reflection spectra has been carried out Keywords: quantum well, reflection, excitons, strain.

Легкие и тяжелые экситоны в напряженных квантовых ямах CdTe/CdZnTe

The spectra of photoluminescence and polarized reflection under normal and oblique incidence of light from structures with quantum wells with symmetric Cd0.9Zn0.1Te/CdTe/Cd0.9Zn0.1Te and asymmetric Cd0.9Zn0.1Te/CdTe/Cd0.4Mg0.6Te barriers have been studied. Due to the mechanical stresses caused by the mismatch of the crystal lattices of the wells and barriers, the energy of light holes in quantum wells was higher than in barriers, i. e., the zone structure for them was of type II. However, in the reflection spectra, the lines of heavy and light excitons had comparable intensities. In structures with symmetric barriers, exciton resonances, which do not appear in the photoluminescence spectra, were found in the reflection spectra. A detailed calculation of the energy levels and reflection spectra has been carried out.

Проявление анизотропии интерфейсов в квантовых ямах CdTe/CdZnTe

The spectra of polarized reflection from a structure with a single quantum well and asymmetric Cd 0.9 Zn 0.1 Te/CdTe/Cd 0.4 Mg 0.6 Te barriers are studied in this work. The Stokes parameters of reflected light were measured. In the structure with asymmetric barriers under study in the region of exciton resonances, the phenomenon of light birefringence caused by a reduced symmetry of the interfaces compared to the bulk was found.

Bilayer graphene/HgCdTe heterojunction based novel GBn infrared detectors

This paper presents different configurations of bilayer graphene (BLG)/mercury cadmium telluride (Hg1–xCdxTe) based GBn detectors operating in the mid- and long-wave infrared (IR) regime. One type of configuration consists of a contact graphene layer (G) of p+-BLG and a barrier layer (B) of p–-Hg1–xCdx = 0.41Te deposited on the n+-Hg1–xCdxTe absorber layer (n) and is named as p+-p–-n+ detector. In another configuration, the n+-BLG contact layer and n–-Hg1–xCdx = 0.41Te barrier layer are deposited on n+-Hg1–xCdxTe absorber layer and is called a n+-n–-n+ unipolar detector. The Silvaco Atlas TCAD software is used for the optoelectronic characterizations which closely matches the results obtained through analytical modeling. The proposed GBn detectors demonstrate about 106 times improvement in photocurrent density and self-powered mode operation. The proposed detectors demonstrate rapid photoswitching time <0.1 ps. The external quantum efficiency (QEext) of 26.06, 11.68, 54.08, and 50.53% are found for p+-p–-n+ mid-wave, p+-p–-n+ long-wave, n+-n–-n+ mid-wave, and n+-n–-n+ long-wave IR detectors, respectively, at −0.5 V and 77 K. Such detection performances are ascribed to the huge built-in electric field at contact/barrier heterojunction, and large barrier height in conduction band resulting in enhanced photogenerated carriers which contributes to the net photocurrent. Further, it is shown that the multiplication effect of carriers in BLG and high electric field across the interface results in the QEext >100% at near room temperature.

Two-Dimensional Antiferroelectric Tunnel Junction.

Ferroelectric tunnel junctions (FTJs), which consist of two metal electrodes separated by a thin ferroelectric barrier, have recently aroused significant interest for technological applications as nanoscale resistive switching devices. So far, most existing FTJs have been based on perovskite-oxide barrier layers. The recent discovery of the two-dimensional (2D) van der Waals ferroelectric materials opens a new route to realize tunnel junctions with new functionalities and nm-scale dimensions. Because of the weak coupling between the atomic layers in these materials, the relative dipole alignment between them can be controlled by applied voltage. This allows transitions between ferroelectric and antiferroelectric orderings, resulting in significant changes of the electronic structure. Here, we propose to realize 2D antiferroelectric tunnel junctions (AFTJs), which exploit this new functionality, based on bilayer In_{2}X_{3} (X=S, Se, Te) barriers and different 2D electrodes. Using first-principles density functional theory calculations, we demonstrate that the In_{2}X_{3} bilayers exhibit stable ferroelectric and antiferroelectric states separated by sizable energy barriers, thus supporting a nonvolatile switching between these states. Using quantum-mechanical modeling of the electronic transport, we explore in-plane and out-of-plane tunneling across the In_{2}S_{3} van der Waals bilayers, and predict giant tunneling electroresistance effects and multiple nonvolatile resistance states driven by ferroelectric-antiferroelectric order transitions. Our proposal opens a new route to realize nanoscale memory devices with ultrahigh storage density using 2D AFTJs.

Electron g-factor in coupled quantum wells CdTe and CdMnTe

In tunnel-coupled quantum wells (QWs) CdTe (20 nm wide) and CdMnTe (8 nm wide) separated by Cd0.88Mg0.12Te barrier with a thickness of LB = 5, 7, 9, and 11 monolayers (MLs), the dependence of electron g-factor on the barrier thickness and temperature is investi-gated by means of the pump-probe Kerr rotation technique. The renormalization of the electron g-factor occurs due to the s–d exchange interaction of electrons with manganese ions in a magnetic QW. The most change of the electron g-factor value Δg = 0.25 is registered at the least barrier width of 5 MLs and temperature T = 5 K. In this case, the penetration of the electron wave function from the nonmagnetic QW into the magnetic one is estimated to be 0.6%.

Low voltage control of exchange coupling in a ferromagnet-semiconductor quantum well hybrid structure

Voltage control of ferromagnetism on the nanometer scale is highly appealing for the development of novel electronic devices with low power consumption, high operation speed, reliable reversibility and compatibility with semiconductor technology. Hybrid structures based on the assembly of ferromagnetic and semiconducting building blocks are expected to show magnetic order as a ferromagnet and to be electrically tunable as a semiconductor. Here, we demonstrate the electrical control of the exchange coupling in a hybrid consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well, separated by a thin non-magnetic (Cd,Mg)Te barrier. The electric field controls the phononic ac Stark effect—the indirect exchange mechanism that is mediated by elliptically polarized phonons emitted from the ferromagnet. The effective magnetic field of the exchange interaction reaches up to 2.5 Tesla and can be turned on and off by application of 1V bias across the heterostructure.

Ultralow Interface Recombination Velocity (∼1 cm/s) at CdTe/MgxCd ${}_{1\hbox{-}}$xTe Heterointerface

The interface recombination velocity (IRV) at the interfaces in CdTe/Mg <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> Cd <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}_{1\hbox{-}}$</tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> Te double heterostructures (DHs) is studied using time-resolved photoluminescence. It is found that both thermionic emission and tunneling effects can cause photogenerated carrier loss over or through the Mg <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x </i> Cd <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}_{1\hbox{-}}$</tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x </i> Te barriers, either due to the low barrier potential or the thin barrier thickness. Thus, carrier lifetime measurements reveal only an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">effective</i> IRV. The thermionic emission induced interface recombination can be distinguished by conducting temperature-dependent carrier lifetime measurements, and the tunneling induced IRV can be quantified by comparing samples with different barrier thicknesses. When both thermionic emission and tunneling effects are suppressed or even eliminated, the actual IRV (due to the recombination at the DH interface trap states) is measured to be ∼1 cm/s, with a very long carrier lifetime of 3.6 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> s achieved in the DHs.

Antireflective Photonic Structure for Coherent Nonlinear Spectroscopy of Single Magnetic Quantum Dots

This work presents epitaxial growth and optical spectroscopy of CdTe quantum dots (QDs) in (Cd,Zn,Mg)Te barriers placed on top of the (Cd,Zn,Mg)Te distributed Bragg reflector. The photonic mode formed in our half-cavity structure permits enhancement of the local excitation intensity and extraction efficiency of the QD emission, while suppressing the reflectance within the spectral range covering the QD transitions. This allows us to perform coherent, nonlinear, resonant spectroscopy of individual QDs. The coherence dynamics of a charged exciton is measured via four-wave mixing, with the estimated dephasing time T2 = (210 ± 40) ps. The same structure contains QDs doped with single Mn2+ ions, as detected in photoluminescence spectra. Our work therefore paves the way toward investigating and controlling coherence of individual excitons coupled, via s,p-d exchange interaction, with individual spins of magnetic dopants.

Electrical and Optical Properties of n-Type Indium-Doped CdTe/Mg0.46Cd0.54Te Double Heterostructures

CdTe/Mg0.46Cd0.54Te double heterostructures with n-type In doping concentrations, varied from 1 × 1016 to 7 × 1018 cm−3, have been grown on InSb substrates using molecular beam epitaxy. Secondary ion mass spectroscopy measurements show strong diffusion of In from the InSb substrate to the CdTe buffer layer, while the In concentration is constant in the CdTe layer between the two Mg 0.46Cd0.54Te barrier layers. Capacitance–voltage measurements show that the dopants are 100% ionized for the doping concentration range from 1 × 1016 to 1 × 1018 cm −3. The carrier lifetime decreases with increasing doping concentration (from 0.73 μs for an unintentionally doped sample to 0.74 ns for a 1 × 1018 cm−3 doped sample) due to the decrease of both radiative and nonradiative lifetimes. Decent carrier lifetimes are achieved (∼100 ns) between 1 × 1016 and 1 × 1017 cm−3 doping levels, which is beneficial for developing n-type monocrystalline CdTe solar cells, photodetectors, and other optoelectronic devices. The strongest photoluminescence intensity is observed when the doping concentration is 1 × 1017 cm −3, which corresponds to the highest internal quantum efficiency.

Long-range p–d exchange interaction in a ferromagnet–semiconductor hybrid structure

Hybrid structures synthesized from different materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wave function overlap and is therefore short-ranged, so that it may be compromised across the interface. Here we study a hybrid structure consisting of a ferromagnetic Co-layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wave function overlap of quantum well holes and magnetic Co atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 10 nm. We suggest that the resulting spin polarization of the holes is induced by an effective p-d exchange that is mediated by elliptically polarized phonons.

Absence of nonlocal resistance in microstructures of PbTe quantum wells

AbstractWe report on experiments allowing to set an upper limit on the magnitude of the spin Hall effect and the conductance by edge channels in quantum wells of PbTe embedded between PbEuTe barriers. We reexamine previous data obtained for epitaxial microstructures of n‐type PbSe and PbTe, in which pronounced nonlocal effects and reproducible magnetoresistance oscillations were found. Here we show that these effects are brought about by a quasi‐periodic network of threading dislocations adjacent to the BaF2 substrate, which give rise to a p‐type interfacial layer and an associated parasitic parallel conductance. We then present results of transport measurements on microstructures of modulation doped PbTe/(Pb,Eu)Te:Bi heterostructures for which the influence of parasitic parallel conductance is minimized, and for which quantum Hall transport had been observed, on similar samples, previously. These structures are of H‐shaped geometry and they are patterned of 12 nm thick strained PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. The structures have different lateral sizes corresponding to both diffusive and ballistic electron transport in non‐equivalent L valleys. For these structures no nonlocal resistance is detected confirming that PbTe is a trivial insulator. The magnitude of spin Hall angle γ is estimated to be smaller than 0.02 for PbTe/PbEuTe microstructures in the diffusive regime.

Exciton–Mn exchange interactions as a function of translational wavevector in Cd1‐xMnxTe quantum wells

AbstractThe exchange interaction between excitons and manganese ions have been studied in wide quantum wells of Cd1‐x Mnx Te with Cd1‐y Mgy Te barriers. The widths L of the wells are sufficiently large for the excitons to be described by the centre of mass approximation, in which the translational wavevector is quantised according to Kz = Nπ/L. Excitonic transitions with different values of the quantisation index N can be resolved in the optical spectra. The giant Zeeman splittings induced by a magnetic field then enable the separate determination of the exchange energy for each value of the wavevector. We find that the quantity N0 (α – β), which describes the exchange splitting, is constant to within 3% over the range of wavevectors up to 4 × 106 cm–1 (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Luminescence of CdMnTe/CdMgTe structures with periodically arranged narrow-gap inclusions

The study is concerned with the excitonic and intracenter emission spectra of planar structures with periodically arranged Cd0.9Mn0.1Te layers (with a nominal thickness corresponding to one monolayer) separated by Cd0.5Mg0.5Te barriers with different thicknesses. The complex structure of the spectra and their variations in the temperature range from 5 to 80 K are indicative of the inhomogeneity of the Cd0.9Mn0.1Te layers, the important role of vertical diffusion, and the correlation between irregularities in the direction of growth at a small spacing between the narrow-gap Cd0.9Mn0.1Te layers. A substantial factor controlling the spectrum and quantum efficiency of excitonic emission is the relation between the exciton radius and the period of the structure.

Wave-vector dependence of magnetic properties of excitons in ZnTe

The magnetic properties of heavy-hole excitons in wide quantum wells of ZnTe with Znx Mg1−x Te barriers have been studied with photoluminescence and reflectivity measurements. The exciton magnetic moments (as characterized by the g values) and the diamagnetic shifts of the exciton transitions are found to depend strongly on the wave-vector component Kz associated with translational motion of the exciton normal to the plane of the quantum well. The case of ZnTe differs from examples of this behavior previously reported for GaS, CdTe, and ZnSe since the ZnTe is under tensile biaxial strain, so that the heavy-hole exciton states lie higher in energy than the corresponding states of the light-hole excitons. The dependence of the magnetic properties on Kz is nevertheless still in excellent agreement with the predictions of a model proposed by Smith et al. [Phys. Rev. B 78, 085204 (2008)], in which mixing of the heavy-hole 1S exciton state with light-hole nP states is found to be responsible for motion-induced changes in the internal structure of the exciton.

Contact superconductivity in In–PbTe junctions

We report on electron transport studies on superconductor/semiconductor hybrid structures of indium and n-type PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. In/PbTe contacts form by spontaneous alloying, which occurs already at room temperature. The alloyed phase penetrates deeply into the semiconductor and forms metallic contacts even in the presence of depletion layers at the semiconductor’s surface. Although the detailed structure of this phase is unknown, we observe that it exhibits a superconducting transition at temperatures below 7 K. This causes such substantial reduction in the contact resistances that they even become comparable to those predicted for ideal superconductor-normal conductor contacts. Our findings suggest that the new superconducting phase is result of Josephson coupling arising between tiny superconducting precipitates randomly distributed in the interface region. In analogy to the granular superconducting systems, the coupling would lead to the occurrence of the global superconductivity only when the Coulomb charging energies of the precipitates are sufficiently small. This condition is exceptionally well fulfilled in PbTe because of a huge static dielectric constant of this material, ε&amp;gt;1000.

10.1007/s11451-008-2021-0

The luminescence spectra of Cd0.75Mn0.25Te/Cd1 − y Mg y Te superlattices with narrow-band-gap Cd0.75Mn0.25Te nanolayers having nominal thicknesses of 0.5, 1.5, and 3.0 monolayers (MLs) exhibit exciton emission bands of Cd1 − y Mg y Te barriers and nanolayers, as well as intracenter luminescence of Mn2+ ions. For all samples, the luminescence spectra of Cd0.75Mn0.25Te nanolayers consist of two bands. From analyzing the dependences of the luminescence intensity on the nanolayer thickness and temperature, these bands are assigned to excitons localized at two-and zero-dimensional potentials. The intracenter emission of Mn2+ in 3.0-ML-thick Cd0.75Mn0.25Te clearly reveals excitation migration.

Modification of the surface state and doping of CdTe and CdZnTe crystals by pulsed laser irradiation

The photoelectric and electrical properties of high-resistivity p-like CdTe and Cd 0.96Zn 0.04Te single crystals and barrier structures on their base before and after laser irradiation in different conditions are studied. Irradiation of samples with nanosecond ruby laser pulses was carried out in two different ways. In the first case, the Cd(Zn)Te crystals were subjected to laser action directly from the surface and irradiation within a certain range of intensities resulted in a decrease in the surface recombination rate and increase in the photoconductivity signal. The surface region with a wider bandgap in CdZnTe crystals was formed. In the second case, the samples were irradiated from the side pre-coated with a relatively thick In dopant film and it caused rectification in the I–V characteristics as a result of laser-induced doping of the thin Cd(Zn)Te surface region and formation of a built-in p-n junction. The application of the fabricated M- p- n structured In/Cd(Zn)Te/Au diodes for X-ray and γ-ray detectors is discussed.

Linear polarization of the photoluminescence of quantum wells subject to in-plane magnetic fields

The degree and orientation of the magnetic-field induced linear polarization of the photoluminescence from a wide range of heterostructures containing (001) (Cd,Mn)Te quantum wells between (Cd,Mn,Mg)Te barriers has been studied as a function of detection photon energy and of the strength and direction of a magnetic field applied in the plane of the quantum well. Three field-induced contributions to the linear polarization of the photoluminescence are observed which differ in their dependence on the angle, $\ensuremath{\varphi}$, between the magnetic field and the [110] direction, being, respectively, independent of $\ensuremath{\varphi}$, varying as $\mathrm{cos}(2\ensuremath{\varphi})$, and as $\mathrm{cos}(4\ensuremath{\varphi})$. A theoretical description of each of these contributions in terms of an in-plane deformation acting on the valence band states is presented and verified by comparison with the experimental data. In our model, we account for the possibility that the in-plane deformations are distributed in both magnitude and direction. We conclude that it is possible to account for the magnetic-field induced linear polarization of the photoluminescence via in-plane deformations and without invoking terms in the valence band spin Hamiltonian which are cubic in $J$. The models developed in the present paper apply in full measure to nonmagnetic quantum wells as well as ensembles of disklike quantum dots with shape and/or strain anisotropy.