Holes In Quantum Wells Research Articles

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.

Фононный спектр LED-гетероструктуры InGaN/GaN с квантовыми ямами

Measurements of the phonon spectrum of the LED heterostructure based on the In0.12Ga0.88N/GaN barrier showed the presence in it of four peaks of phonon radiation with energies of 0.193, 0.207, 0.353, and 0.356 eV. From a comparison of the results of the calculation of the energy spectrum of the electron and hole quantum wells with the obtained experimental data, it was assumed that these peaks can be interpreted as the energies of phonons generated during the capture of electrons from the barrier layer to the second level of dimensional quantization, as well as during the relaxation of electrons from the second level to the radiation level and trapping holes to the upper level of the quantum well.

Large optical gain from the 2D-transition metal dichalcogenides of MoS2/WSe2 quantum wells

In this work, the optical gain in the new class of 2D-materials is calculated for MoS2/WSe2 single quantum well (QW). For this purpose, numerical solution of the k.p equation at (K+, K−) points of the Brillouin zone is done in order to obtain the energy levels and their corresponding wave functions of electrons and holes in QW. Then, we discussed the band gap dependence on temperature of MoS2 and WSe2 materials. Further, we examined the temperature and the QW width effects on the optical and the maximum of gain. We found that MoS2/WSe2 QW can satisfy the lasing requirements and can give interesting results about the performance of laser emitting devices. The results are compared to the available data reported in the literature.

Composite fermion states around the two-dimensional hole Landau level filling factor 32 in tilted magnetic fields

Transport measurements under tilted magnetic fields were performed on a series of C-doped (001) GaAs/AlGaAs two-dimensional hole quantum wells. Due to a large $g$ factor, Zeeman energy is large and comparable to the cyclotron energy in these samples. On the other hand, it was found that the in-plane component ${g}_{\ensuremath{\parallel}}$ is small, and the effect of a tilted magnetic field is mainly to increase the effective mass of the holes. We investigate the spin transition of composite fermion states around Landau level (LL) filling factor 3/2. We found that the $\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}4/3$ state encounters a partial- to full-spin-polarization transition, conforming to the same pattern as that of electron samples. In addition, a high-resistance phase emerges at $\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}3/2$ under very high tilt angles. We interpret both of these phenomena as a consequence of LL crossing under a purely perpendicular magnetic field.

Evidence of exchange interaction of localized carriers and transition metals in diluted II‐VI nanostructures: ODMR study

AbstractOptically detected magnetic resonance study of (CdMn)Te/(CdMg)Te quantum wells allowed to reveal the formation of exchange‐coupled complexes consisting of Mn ions and localized holes in quantum wells with excess hole concentration and the directional electron tunneling towards wider wells in multiple quantum well structures. The existence of a distribution of Mn‐hole complexes that differ in a number of Mn ions interacting with a localized hole is justified.In colloidal cobalt doped ZnO nanocrystals, several nm in diameter, the interaction between the magnetic ions and the shallow donor electron in the confined system of ZnO quantum dots has been revealed. Direct evidence of interaction of Co ions with the interstitial Li shallow donor in the ZnO nanocrystal core and hyperfine coupling with 1H in the quantum dot shell have been demonstrated. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

W-dispersion particles in repulsive potentials: Quasibound states and their lifetime

We consider fundamental features which emerge in the mechanics of quasiparticles with nonmonotonic (as a function of ${p}^{2}$) dispersion law. Quasiparticles of this kind abound in modern physics, with examples ranging from holes in quantum wells to edge magnetic states in quantum wires to photons in atomic vapors to polaritons in photonic crystals and in trapped-atom lattices. The motion of such a particle in repulsive potentials gives rise to a number of counterintuitive phenomena, which carry a promise of unusual optical manifestations. A classical particle can be trapped by repulsive potentials, and the likelihood of this trapping may increase with the value of the angular momentum. Further, in contrast to the usual quantum-mechanical notion, the particle always has a quasibound state in a two-dimensional, central-force repulsive potential, while it may have no bound states in a one-dimensional analog of this potential. The binding energy of these states and their inherent decay rate are determined by a complex interplay of the parameters of the potential, the particle dispersion law, and the value of the angular momentum. We construct the energy spectrum of quasibound states in a repulsive Coulomb potential, estimate their lifetime, and predict their optical manifestations as inverted hydrogen spectral-line series.

Inhomogeneous injection in III-nitride light emitters with deep multiple quantum wells

Excessive depth of optically active quantum wells (QWs) and related increase in QW population capacity is one of the main causes of inhomogeneous carrier injection and unequal QW populations in multiple-quantum-well III-nitride light emitters operating in the visible range. In turn, uneven distribution of injected carriers across the device's active region creates imbalance between confined electron and hole QW populations and supports large residual QW charges, especially in marginally located n-side and p-side QWs. QW charges are strongly non-equilibrium as determined by dynamic balance between carrier capture and recombination rates, with the later being progressively faster in excessively deep QWs.

Type-I interband cascade lasers near 3.2 μm

Interband cascade (IC) lasers have been demonstrated based on type-I InGaAsSb/AlAsSb quantum well (QW) active regions. These type-I IC lasers are composed of 6-cascade stages and InAs/AlSb superlattice cladding layers. In contrast to the use of quinary AlGaInAsSb barriers for active region in previous type-I QW lasers, the type-I QW active region in each stage is sandwiched by digitally graded multiple InAs/AlSb QW electron injector and GaSb/AlSb QW hole injector. The fabricated type-I IC lasers were able to operate in continuous wave and pulsed modes at temperatures up to 306 and 365 K, respectively. The threshold current densities of broad-area lasers were around 300 A/cm2 at 300 K with a lasing wavelength near 3.2 μm. The implications and prospects of these initial results are discussed.

Magnetic field spectral crossings of Luttinger holes in quantum wells

We develop an analytic approach to two-dimensional (2D) holes in a magnetic field that allows us to gain insight into physics of measuring the parameters of holes, such as cyclotron resonance, Shubnikov-de Haas effect and spin resonance. We derive hole energies, cyclotron masses, and the $g$ factors in the semiclassical regime analytically, as well as analyze numerical results outside the semiclassical range of parameters, qualitatively explaining the experimentally observed magnetic field dependence of the cyclotron mass. In the semiclassical regime with large Landau level indices, and for size quantization energy much bigger than the cyclotron energy, the cyclotron mass coincides with the in-plane effective mass, calculated in the absence of a magnetic field. The hole $g$ factor in a magnetic field perpendicular to the 2D plane is defined not only by the constant of direct coupling of the angular momentum of the holes to the magnetic field, but also by the Luttinger constants defining the effective masses of holes. We find that the $g$ factor for quasi-2D holes with heavy mass in the [001] growth direction in GaAs quantum well is $g=4.05$ in the semiclasssical regime. Outside the semiclassical range of parameters, holes behave as a species completely different from electrons. Spectra for size- and magnetic-field-quantized holes are nonequidistant, not fanlike, and exhibit multiple crossings, including crossing in the ground level. We calculate the effect of Dresselhaus terms, which transform some of the crossings into anticrossings, and the effects of the anisotropy of the Luttinger Hamiltonian on the 2D hole spectra. Dresselhaus terms of different symmetries are taken into account, and a regularization procedure is developed for the ${k}_{z}^{3}$ Dresselhaus terms. Control of the nonequidistant levels and crossing structure by the magnetic field can be used to control the Landau level mixing in hole systems, and thereby control hole-hole interactions in the magnetic field.

High-temperature interfacial electroluminescence in type-II broken-gap heterostructures based on InSb quantum dashes in n-InAs matrix

Room-temperature electroluminescence is observed for the first time in type-II heterostructures based on InSb quantum dashes embedded in a narrow-gap n-InAs matrix. The heterostructures exhibit positive luminescence at wavelengths in the range 3–4 μm. This is due to the interfacial radiative transitions of electrons from self-consistent quantum wells on the side of InAs matrix layers across the broken-gap type-II InSb/InAs heterointerface to quantum-well hole levels in the InSb quantum dashes, situated in the energy gap of the matrix near the InAs conduction-band bottom.

Exporting superconductivity across the gap: Proximity effect for semiconductor valence-band states due to contact with a simple-metal superconductor

The proximity effect refers to the phenomenon whereby superconducting properties are induced in a normal conductor that is in contact with an intrinsically superconducting material. In particular, the combination of nano-structured semiconductors with bulk superconductors is of interest because these systems can host unconventional electronic excitations such as Majorana fermions when the semiconductor's charge carriers are subject to a large spin-orbit coupling. The latter requirement generally favors the use of hole-doped semiconductors. On the other hand, basic symmetry considerations imply that states from typical simple-metal superconductors will predominantly couple to a semiconductor's conduction-band states and, therefore, in the first instance generate a proximity effect for band electrons rather than holes. In this article, we show how the superconducting correlations in the conduction band are transferred also to hole states in the valence band by virtue of inter-band coupling. A general theory of the superconducting proximity effect for bulk and low-dimensional hole systems is presented. The interplay of inter-band coupling and quantum confinement is found to result in unusual wave-vector dependencies of the induced superconducting gap parameters. One particularly appealing consequence is the density tunability of the proximity effect in hole quantum wells and nanowires, which creates new possibilities for manipulating the transition to nontrivial topological phases in these systems.

3.3–3.4-$\mu{\rm m}$ Diode Lasers Based on Triple-Layer GaInAsSb Quantum Wells

Diode lasers based on triple-layer GaInAsSb quantum wells (QWs) operating in continuous wave regime at room temperature were designed and fabricated. The fine adjustment of the device operating wavelength was achieved by independent control of the effective electron and hole QW widths. The modified W-QW design approach allowed using 50% In composition in 10-12-nm-wide QW to achieve laser operation above 3.2 μm. The diode lasers emitting at 3.3 and 3.4 μm demonstrated 50 mW per 50- μm-wide stripe and 30 mW per 100- μm-wide stripe at 17 °C, respectively.

Universal density scaling of disorder-limited low-temperature conductivity in high-mobility two-dimensional systems

We theoretically consider the carrier density dependence of low-temperature electrical conductivity in high-quality and low-disorder two-dimensional (2D) `metallic' electronic systems such as 2D GaAs electron or hole quantum wells or gated graphene. Taking into account resistive scattering by Coulomb disorder arising from quenched random charged impurities in the environment, we show that the 2D conductivity \sigma(n) varies as \sigma ~ n^{\beta(n)} as a function of the 2D carrier density n where the exponent \beta(n) is a smooth, but non-monotonic, function of density with possible nontrivial extrema. In particular, the density scaling exponent \beta(n) depends qualitatively on whether the Coulomb disorder arises primarily from remote or background charged impurities or short-range disorder, and can, in principle, be used to characterize the nature of the dominant background disorder. A specific important prediction of the theory is that for resistive scattering by remote charged impurities, the exponent \beta can reach a value as large as 2.7 for k_F d ~ 1, where k_F ~\sqrt{n} is the 2D Fermi wave vector and d is the separation of the remote impurities from the 2D layer. Such an exponent \beta (>5/2) is surprising because unscreened Coulomb scattering by remote impurities gives a limiting theoretical scaling exponent of \beta = 5/2, and naively one would expect \beta(n) \le 5/2 for all densities since unscreened Coulomb scattering should nominally be the situation bounding the resistive scattering from above. We find numerically and show theoretically that the maximum value of \alpha (\beta), the mobility (conductivity) exponent, for 2D semiconductor quantum wells is around 1.7 (2.7) for all values of d (and for both electrons and holes) with the maximum \alpha occurring around k_F d ~ 1. We discuss experimental scenarios for the verification of our theory.

High speed photodetectors based on a two-dimensional electron/hole gas heterostructure

We report on high-speed metal-semiconductor-metal (MSM) resonant cavity enhanced photodetectors based on Schottky-contacted (Al,In)GaAs heterostructures containing both electron and hole quantum wells. Interdigitated detectors were fabricated and characterized with and without an underlying Distributed Bragg Reflector (DBR). All detectors had very low dark currents and high linear responsivities. The fastest measured temporal response with a 16 ps full-width at half-maximum and a 29 ps fall time was demonstrated on a device with 1 μm gap between electrodes and an underlying DBR. Single quantum well detectors have previously demonstrated increased responsivity and speed but were limited by a slow decaying tail in the high speed photoresponse, attributed to the long collection path of minority carriers. The use of an electron and hole well, separated by a 110 nm absorption region as well as an underlying DBR, eliminates the slow tail by providing an enhanced collection path for both optically generated electrons and holes. Here, we present the fabricated device structure along with the DC and high speed photoresponse under varying incident powers. We briefly compare these results to those of the previous single well devices and attribute improvements in the time response tail to enhanced diffusion created by the presence of the separated dual well structure.

Hole spin relaxation inp-type (111) GaAs quantum wells

Hole spin relaxation in $p$-type (111) GaAs quantum wells is investigated in the case with only the lowest hole subband, which is heavy-hole like in (111) GaAs/AlAs and light-hole like in (111) GaAs/InP quantum wells, being relevant. The subband L\"{o}wdin perturbation method is applied to obtain the effective Hamiltonian including the Dresselhaus and Rashba spin-orbit couplings. Under a proper gate voltage, the total in-plane effective magnetic field in (111) GaAs/AlAs quantum wells can be strongly suppressed in the whole momentum space, while the one in (111) GaAs/InP quantum wells can be suppressed only on a special momentum circle. The hole spin relaxation due to the D'yakonov-Perel' and Elliott-Yafet mechanisms is calculated by means of the fully microscopic kinetic spin Bloch equation approach with all the relevant scatterings explicitly included. For (111) GaAs/AlAs quantum wells, extremely long heavy-hole spin relaxation time (upto hundreds of nanoseconds) is predicted. In addition, we predict a pronounced peak in the gate-voltage dependence of the heavy-hole spin relaxation time due to the D'yakonov-Perel' mechanism. This peak origins from the suppression of the unique inhomogeneous broadening in (111) GaAs/AlAs quantum wells. Moreover, the Elliott-Yafet mechanism influences the spin relaxation only around the peak area due to the small spin mixing between the heavy and light holes in quantum wells with small well width. We also show the anisotropy of the spin relaxation. In (111) GaAs/InP quantum wells, a mild peak, similar to the case for electrons in (111) GaAs quantum wells, is also predicted in the gate-voltage dependence of the light-hole spin relaxation time. The contribution of the Elliott-Yafet mechanism is always negligible in this case.

Pecularities of Hall effect in GaAs/δ〈Mn〉/GaAs/InxGa1−xAs/ GaAs (x ≈ 0.2) heterostructures with high Mn content

Transport properties of GaAs/{\delta}<Mn>/GaAs/In\timesGa1-\timesAs/GaAs structures containing InxGa1-xAs (\times {\approx} 0.2) quantum well (QW) and Mn delta layer (DL) with relatively high, about one Mn monolayer (ML) content, are studied. In these structures DL is separated from QW by GaAs spacer with the thickness ds = 2-5 nm. All structures possess a dielectric character of conductivity and demonstrate a maximum in the resistance temperature dependence Rxx(T) at the temperature {\approx} 46K which is usually associated with the Curie temperature Tc of ferromagnetic (FM) transition in DL. However, it is found that the Hall effect concentration of holes pH in QW does not decrease below TC as one ordinary expects in similar systems. On the contrary, the dependence pH(T) experiences a minimum at T = 80-100 K depending on the spacer thickness, then increases at low temperatures more strongly than ds is smaller and reaches a giant value pH = (1-2)\cdot10^13 cm^(-2). Obtained results are interpreted in the terms of magnetic proximity effect of DL on QW, leading to induce spin polarization of the holes in QW. Strong structural and magnetic disorder in DL and QW, leading to the phase segregation in them is taken into consideration. The high pH value is explained as a result of compensation of the positive sign normal Hall effect component by the negative sign anomalous Hall effect component.

Coherent States in Double Quantum Well Systems

Coherent states, where a macroscopic system of many quantum particles exhibits properties of a single quantum particle, have been a topic of intense investigations in physics. The formation of such states is usually accompanied by and experimentally indicated by dissipationless transport of mass, charge, or spin. Excitonic condensation occurs when an electron-hole system develops spontaneous coherence between the electron and hole bands. This effect, predicting the coherent state of electron-hole pairs, has led to a deeper theoretical understanding of condensates with metastable particles. Over the past two decades, this field has seen tremendous experimental developments, starting with the development of spatially separated electron and hole quantumwells, which increased the exciton lifetime by orders of magnitude. The increased exciton lifetime, in turn, has led to a detailed exploration of electrical transport, drag, and photoluminescence properties in such double quantum well systems. These systems have been traditionally fabricated using semiconductor heterojunctions; however, recently, graphene and topological insulators have emerged as promising new candidates as well. The goal of this special issue is to review the recent theoretical and experimental developments on coherent states in such systems. With the rapid progress in fabrication techniques, it is possible to study coherent states with multiple broken symmetries. Thus, this issue starts with the state of experimental efforts on excitonic condensation in semiconductor quantum wells that are doped or optically pumped. It is followed by theoretical analyses of excitonic condensation in bilayer quantum Hall systems. The last paper extends the discussion on coherent states in graphene double layers.

10.1007/s11453-008-3012-7

A theory describing the optical orientation and Hanle effect for holes in quantum wells or quantum dots based on cubic semiconductors is developed. It is demonstrated that the presence of internal or external strain in quantum-confinement heterostructures leads to the dependence of the Hanle effect on the orientation of the magnetic field with respect to the heterostructure growth axis.

Shubnikov-de Haas oscillations of the conductivity of a two-dimensional gas in quantum wells based on germanium and silicon. Determination of the effective mass and g factor

The Shubnikov-de Haas oscillations of the conductivity of a two-dimensional gas of holes in quantum wells consisting of pure germanium and silicon with low germanium content (13%) are analyzed to determine the effective masses and the g factor in these regions. The magnetic-field dependences of the resistivity ρxx obtained at temperatures from 33mKto4K in magnetic fields up to 11T are used for the analysis.