CdTe Quantum Wells Research Articles

Nuclear spin relaxation mediated by donor-bound and free electrons in wide CdTe quantum wells

Nuclear spin relaxation mediated by donor-bound and free electrons in wide CdTe quantum wells

Molecular Beam Epitaxy Growth of Cadmium Telluride Structures on Hexagonal Boron Nitride.

We investigate the feasibility of the epitaxial growth of a three-dimensional semiconductor on a two-dimensional substrate. In particular, we report for the first time the molecular beam epitaxy growth of cadmium telluride (CdTe) quantum wells on hexagonal boron nitride (hBN). The presence of the quantum wells is confirmed by photoluminescence measurements conducted at helium temperatures. Growth of the quantum wells on two-dimensional, almost perfectly flat hBN appears to be very different from growth on bulk substrates; in particular, it requires 70-100 °C lower temperatures.

Spectroscopic evidence of tunnel coupling between CdTe quantum wells in the CdTe/ZnTe heterostructures

The photoluminescence (PL) spectra of CdTe/ZnTe double quantum wells (QWs) are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The QWs were grown in atomic-layer epitaxy and separated by ZnTe spacers with the thicknesses dsp =40−160 ML. The dependences of the relative intensity of shallow QW1 and deep QW2 PL bands (I1 and I2 , respectively) on the pump intensity (J) when excited by the lasers with different radiation wavelengths are investigated. It is found that in the sample with dsp=40 ML, the ratio Y(J)=I1/I2 depends on J and the shape of the Y(J) dependency changes with the excitation wavelength. In the samples with dsp > 70 ML Y(J) also changes with the excitation intensity J, but the shape of this dependence is the same for various excitation wavelengths. It is concluded that the energy relaxation in these samples is influenced not only by the tunneling of charge carriers from QW1 to QW2, but also by carrier relaxation at the nonradiative centers, for which the recombination rate is different for shallow and deep QWs.

Hot exciton relaxation in coupled ultra-thin CdTe/ZnTe quantum well structures

The photoluminescence (PL) and PL excitation (PLE) spectra of CdTe/ZnTe asymmetric double quantum well (QW) structures are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The samples differ in the thickness of the ZnTe spacer between CdTe QWs which is 45, 65 and 75 ML. It has been found that at above-barrier excitation the PL from a shallow QW at sufficiently weak excitation intensities is determined by recombination of hot excitons. It is shown that under these conditions, when PL is excited by lasers with different wavelengths, the ratio of the PL intensities from shallow and deep QWs decreases exponentially with an increase of the initial kinetic energy of hot excitons. It is found that energy relaxation of hot excitons with LO phonon emission determine the shape of the PLE spectrum of shallow QW in the range of exciton kinetic energies up to more than 20 LO phonons above ZnTe bandgap. We have shown that the results obtained are well described by the model of charge and energy transfer between QWs.

Longitudinal spin relaxation time of donor-bound electrons in a CdTe quantum well

We study the magnetic-field dependence of the longitudinal spin relaxation time ${T}_{1}$ of donor-bound electrons placed in the middle of an 8-nm CdTe quantum well with different doping concentrations in the range from $1\ifmmode\times\else\texttimes\fi{}{10}^{9}$ to $2.9\ifmmode\times\else\texttimes\fi{}{10}^{11}$ $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ and at low temperature. We use an extended photoinduced Faraday rotation technique, which expands the usual domain of the measured decays from tens of ns to $\ensuremath{\mu}\mathrm{s}$. As in high-purity bulk semiconductors, a maximum relaxation time of around ${T}_{1}\ensuremath{\sim}10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ is observed for a residually doped sample at low magnetic field of $B=0.08\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. For higher doping concentrations, the magnetic-field dependence of ${T}_{1}$ shows a nonmonotonic behavior: first a rapid increase, followed by a plateau or a decrease of ${T}_{1}$. The fast increase of ${T}_{1}$ at low magnetic fields is explained by the inhibition of the mechanisms identified at zero field---hyperfine and anisotropic exchange interactions---while the behavior at high magnetic field can be succesfully explained by a mechanism proposed by Lyubinskiy and associated to electron hops [I. S. Lyubinskiy, JETP Lett. 88, 814 (2008)]. A good agreement between experiment and theory is found for samples below the metal-insulator transition, when Dresselhaus terms of spin-orbit coupling are considered to be the dominant ones in the Hamiltonian describing the system.

All-electrically tunable networks of Majorana bound states

Second-order topological superconductors (SOTSs) host localized Majorana fermions and provide a new platform for topological quantum computation. We propose a remarkable and feasible way to realize networks based on SOTSs which allow to nucleate and braid Majorana bound states (MBSs) in an all-electrical manner without fine-tuning. The proposed setups are scalable in a straightforward way and can accommodate any even number of MBSs. Moreover, the MBSs in the networks allow defining qubits whose states can be initialized and read out by measuring Josephson currents flowing between SOTS islands. Our proposal can be implemented in monolayers of $\text{FeTe}{}_{1-x}\text{Se}_{x}$, monolayers of 1T'-WTe$_2$, and inverted Hg(Cd)Te quantum wells in proximity to conventional superconductors.

High-resolution resonance spin-flip Raman spectroscopy of pairs of manganese ions in a CdTe quantum well

We report the observation of tens of minor lines of the combinational spin-flip Raman scattering in a CdTe:Mn quantum well by means of the high-resolution optical spectroscopy. Classification of this manifold leads to four characteristic values of energy, that correspond to four different types of pair clusters of Mn ions: the nearest, second, third etc. neighbors. All the four energies show up in a single experiment with a very high precision, providing experimental grounds for a deeper understanding of the d-d exchange interactions in a diluted magnetic semiconductor and demonstrating the capacity of the employed method. The major (nearest-neighbor) exchange constant J_1 = 6.15 K was found to consent with its previously reported value. Other detected characteristic energies are as follows: J_{(2)} = 1.80 K, J_{(3)} = 1.39 K, J_{(4)} = 0.81 K.

Photoluminescence of CdTe quantum wells doped with cobalt and iron

Cobalt and iron dopants in semiconductors are killers of excitonic photoluminescence. In particular, photoluminescence of quantum wells doped with Co and Fe poses a challenge and has not been reported so far. Here, we show molecular beam epitaxy growth and photoluminescence studies of (Cd,Co)Te and (Cd,Fe)Te quantum wells in (Cd,Mg)Te. We compare the results to well-known structures: CdTe and (Cd,Mn)Te quantum wells, and we conclude that 0.2% of Fe reduces photoluminescence intensity by 3 orders of magnitude, and similar concentration of Co reduces PL intensity by about 5 orders of magnitude. We also present the influence of the dopant's concentration on emission efficiency in external magnetic field. Structures with low doping levels require weaker fields to reduce the quenching effect.

Short range proximity effect induced by exchange interaction in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells

The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by ${\mathrm{Mn}}^{2+}$ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron $g$ factor by the exchange interaction with the ${\mathrm{Mn}}^{2+}$ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the ${\mathrm{Cd}}_{0.88}{\mathrm{Mg}}_{0.12}\mathrm{Te}$ barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations.

Temperature dependence of the spin relaxation time of donor-bound electrons immersed in a CdTe quantum well

The behavior of the spin relaxation time of electrons bound to donors immersed in the middle of a CdTe quantum well was measured in the range of temperatures 10--80 K, by using picosecond pump-probe Kerr rotation. Different doping concentrations spanning from isolated donors up to a concentration beyond the metal-insulator transition were considered at a fixed magnetic field; at very low temperature, in insulating regime all electrons are bound to donors but for one high concentration we had to consider that a fraction of electrons is in conduction states. By increasing the temperature, the number of conduction electrons increases. The experimental temperature dependences were explained by invoking spin exchange between electron spins localized on donors and the spin of electrons promoted to conduction states. A good agreement between experiment and theory was found and allowed us to conclude that, while the spin of localized electrons undergoes the effect of both hyperfine and anisotropic exchange interactions, the D'yakonov-Perel' mechanism governs the spin relaxation of the conduction electrons for the whole range of the studied doping concentrations. Moreover, we identified the scattering mechanisms possibly undergone by the conduction electrons at low temperature.

Spin relaxation time of donor-bound electrons in a CdTe quantum well

Low-temperature spin relaxation time of electrons localized on iodine donors placed in the middle of a 8-nm CdTe quantum well is studied by using the photoinduced Faraday rotation technique, for donor concentrations in the range from $1\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ to $3.6\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. A maximum relaxation time of the order of 20 ns is found at a doping concentration near the metal-insulator transition. Adapting the theoretical description developed for bulk GaAs [R. I. Dzhioev et al., Phys. Rev. B 66, 245204 (2002)] to a quantum well, we have fitted the experimental results taking as a parameter the spin-orbit constant in CdTe. A satisfactory fit is obtained with ${\ensuremath{\alpha}}_{so}=0.079\ifmmode\pm\else\textpm\fi{}0.011,$ which is also very close to the calculated theoretical value. A comparison of the experimental spin relaxation times measured previously in bulk CdTe to those predicted by the theory is done. Finally, we compare the predicted spin relaxation times in bulk CdTe to those predicted in one of the most studied bulk semiconductor materials: GaAs, and in (less studied) ZnSe.

Some radiation effects in II-VI quantum dots

The effect of electron irradiation (E=0.6-1.8 MeV) on the optical characteristics (photoluminescence, PL) of CdTe/ZnTe structures with quantum dots, QDs, was investigated in the temperature range from 4.2 to 250 K. The data on the influence of irradiation on the temperature dependence of PL intensity, energy position and PL line width, W, from QDs were obtained. The narrowing of PL band and the blue shift of the QD peak position are explained by quenching of the low energy component connected with larger QDs. A slight decrease in PL intensity for both QDs and the buffer ZnTe layer as well as a sharp drop in PL intensity for 1 ML CdTe quantum wells, QWs, and fragments accompanied by a change in the activation energy of the PL quenching is explained by the radiation defects localized near the QD interface. The obtained results show that under e-beam irradiation QDs are more stable than QWs, which is in agreement with our previous investigations.

Persistent spin helix manipulation by optical doping of a CdTe quantum well

Time-resolved Kerr-rotation microscopy explores the influence of optical doping on the persistent spin helix in a [001]-grown CdTe quantum well at cryogenic temperatures. Electron spin diffusion dynamics reveal a momentum-dependent effective magnetic field providing SU(2) spin-rotation symmetry, consistent with kinetic theory. The Dresselhaus and Rashba spin-orbit coupling parameters are extracted independently from rotating the spin helix with external magnetic fields applied parallel and perpendicular to the effective magnetic field. Most importantly, a non-uniform spatiotemporal precession pattern is observed. The kinetic theory framework of spin diffusion allows for modeling of this finding by incorporating the photocarrier density into the Rashba ($\alpha$) and the Dresselhaus ($\beta_3$) parameters. Corresponding calculations are further validated by an excitation-density dependent measurement. This work shows universality of the persistent spin helix by its observation in a II-VI compound and the ability to fine-tune it by optical doping.

Magnetospatial dispersion of semiconductor quantum wells

Polarization conversion of light reflected from quantum wells governed by both magnetic field and light propagation direction is observed. We demonstrate that the polarization conversion is caused by the magnetospatial dispersion in quantum wells which manifests itself in the reflection coefficient contribution bilinear in the in-plane components of the magnetic field and the light wave vector. The magnetospatial dispersion is shown to arise due to structure inversion asymmetry of the quantum wells. The effect is resonantly enhanced in the vicinity of the heavy-hole exciton. We show that microscopically the magnetospatial dispersion is caused by the mixing of heavy- and light-hole states in the quantum well due to both orbital effect of the magnetic field and the in-plane hole motion. The degree of the structure inversion asymmetry is determined for GaAs/AlGaAs and CdTe quantum wells.

Excitons in tunnel coupled CdTe and (Cd,Mn)Te quantum wells

The photoluminescence (PL) from structures containing Cd0.95Mn0.05Te and CdTe quantum wells (QWs) separated by a narrow (1.94 nm) barrier was studied. The PL lines of comparable intensities from several possible exciton states were observed simultaneously at energy distances substantially exceeding kT. This means that the energy transfer in the studied systems is slower than the radiative recombination of the confined excitons. For the CdTe QW width of about 8.7–9 nm, indirect excitons with the electron and heavy hole chiefly localized in the CdTe and Cd1−xMnxTe QWs, respectively, were detected in the magnetic field. These indirect excitons have PL energy of about 10–20 meV above the PL line of the direct excitons in the CdTe QW. The observation of the PL from the indirect excitons which are not the lowest excitations in the structure is a distinctive feature of the system.Photoluminescence intensity dependence on the energy and the magnetic field.

Raman scattering in a two-dimensional Fermi liquid with spin-orbit coupling

We present a microscopic theory of Raman scattering by a two-dimensional Fermi liquid (FL) with Rashba and Dresselhaus types of spin-orbit coupling, and subject to an in-plane magnetic field (B). In the long-wavelength limit, the Raman spectrum probes the collective modes of such a FL: the chiral spin waves. The characteristic features of these modes are a linear-in-q term in the dispersion and the dependence of the mode frequency on the directions of both q and B. All of these features have been observed in recent Raman experiments on CdTe quantum wells.

Electrostatic control of quantum Hall ferromagnetic transition: A step toward reconfigurable network of helical channels

Ferromagnetic transitions between quantum Hall states with different polarization at a fixed filling factor can be studied by varying the ratio of cyclotron and Zeeman energies in tilted magnetic field experiments. However, an ability to locally control such transitions at a fixed magnetic field would open a range of attractive applications, e.g. formation of a reconfigurable network of one-dimensional helical domain walls in a two-dimensional plane. Coupled to a superconductor, such domain walls can support non-Abelian excitation. In this article we report development of heterostructures where quantum Hall ferromagnetic (QHFm) transition can be controlled locally by electrostatic gating. A high mobility two-dimensional electron gas is formed in CdTe quantum wells with engineered placement of paramagnetic Mn impurities. Gate-induced electrostatic field shifts electron wavefunction in the growth direction and changes overlap between electrons in the quantum well and d-shell electrons on Mn, thus controlling the s-d exchange interaction and the field of the QHFm transition. The demonstrated shift of the QHFm transition at a filling factor $\nu=2$ is large enough to allow full control of spin polarization at a fixed magnetic field.

Conductance oscillations in quantum point contacts of InAs/GaSb heterostructures

We study quantum point contacts in two-dimensional topological insulators by means of quantum transport simulations for InAs/GaSb heterostructures and HgTe/(Hg,Cd)Te quantum wells. In InAs/GaSb, the density of edge states shows an oscillatory decay as a function of the distance to the edge. This is in contrast to the behavior of the edge states in HgTe quantum wells, which decay into the bulk in a simple exponential manner. The difference between the two materials is brought about by spatial separation of electrons and holes in InAs/GaSb, which affects the magnitudes of the parameters describing the particle-hole asymmetry and the strength of intersubband coupling within the Bernevig-Hughes-Zhang model. We show that the character of the wave function decay impacts directly the dependence of the point contact conductance on the constriction width and the Fermi energy, which can be verified experimentally and serve to determine accurately the values of relevant parameters. In the case of InAs/GaSb heterostructures the conductance magnitude oscillates as a function of the constriction width following the oscillations of the edge state penetration, whereas in HgTe/(Hg,Cd)Te quantum wells a single switching from transmitting to reflecting contact is predicted.

Nanosecond spin coherence of excitons bound to acceptors in a CdTe quantum well

We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times.

Turnover of Exciton Spin States in CdTe/Cd0.88Mn0.12Te Quantum Wells

Photoinduced exciton spin states in diluted magnetic semiconductor CdTe/Cd0.88Mn0.12Te quantum wells (QWs) were investigated by the time-resolved Kerr rotation (TRKR) technique at various temperatures and applied magnetic fields. Two oscillation components were observed in the TRKR signals. One component is ascribed to the spin precession of a localized electron, which was not affected by the Mn ions in the CdMnTe barrier layers. Its frequency increased linearly with the applied magnetic field. The other component was strongly dependent on the temperature, and its frequency showed a minimum at around 20 K in the 8.4 nm QW. This result was well explained by the turnover of exciton spin states in the CdTe QW, which is determined by the competition between the exciton Zeeman effect and an exciton–Mn exchange interaction. The temperature dependence of the exciton spin relaxation is also discussed.