The excition tunneling in ZnCdSe/ZnSe asymmetric double quantum well

Photoluminescence spectra of asymmetric double-quantum-well structure are studied in this paper. We show the excitation power dependence of exciton tunneling. Due to the different tunneling time of electrons and holes, space-charge effect is observed.     

Evidence of thermally activated transfer of excited carriers between CdSe/ZnSe quantum dots

Temperature dependent photoluminescence and cathodoluminescence of selfassembled CdSe/ZnSe quantum dots grown by metalorganic vapor phase deposition were investigated. We found an unusual large red shift and a narrowing of the photoluminescence peak with temperature increases. Cathodoluminescence studies of a small number of quantum dots showed that the broad peak observed in the photoluminescence spectra is, in fact, made up of a series of narrower peaks, coming from quantum dots of different sizes. While the intensity of luminescence from small dots drops monotonously with temperature rises, that from the large dots displays a peculiar behavior. It actually increases within the temperature range of 140–170 K, the same range in which the photoluminescence peak shows narrowing. The simultaneous increase of luminescence from some quantum dots and decrease from others are believed to be responsible for the red shift and narrowing of the observed photoluminescence peak. A simple analytically solvable rate equation model was used to understand the spectral data. We suggest that the unusual behaviors observed can be understood as resulting from a transfer of thermally activated carriers from small to large quantum dots.     

CdSe/ZnSe量子点的合成与荧光特性

采用低温成核生长与一步法相结合的方式合成了CdSe/ZnSe核壳结构量子点,并通过吸收光谱、荧光光谱、X射线衍射等分析手段证明了ZnSe壳层包覆成功.对加入空穴传输材料后CdSe/ZnSe量子点的荧光变化情况进行了深入的研究.稳态光谱结果表明.空穴传输材料对量子点发光有较强的淬灭作用;时间分辨光谱结果显示,随着空穴传输材料分子浓度的增加,量子点的荧光寿命明显缩短,其荧光淬灭过程可以解释为静态淬灭和动态淬灭过程.静态淬灭来源于量子点表面与空穴传输材料间的相互作用;而动态淬灭则来源于量子点到空穴传输材料的空穴转移过程.因此,量子点的壳层结构及空穴传输材料的种类对量子点的荧光淬灭起关键作用.     

Growth and characterization of digital alloy quantum wells of CdSe/ZnSe

We report a study of digital alloy quantum wells of CdSe/ZnSe grown by migration enhanced epitaxy. The quantum well regions consist of various numbers of periods of one monolayer of CdSe and three monolayers of ZnSe, and the barriers are ZnSe. It will be shown that the optical properties of such quantum wells are greatly affected by the structural quality of the digital alloy. Both structural and optical properties will be discussed. Such digital alloy quantum wells are shown to have excellent room temperature optical characteristics.     

Photon-assisted tunneling in coupled double quantum wells

Using different techniques to individually contact two closely spaced electron gases, we study the tunneling characteristics between weakly coupled GaAs quantum wells, with and without resonant far-infrared excitation. We find that for barriers as thick as 300 Å, the alignment between the subbands in the wells can be observed as an increased tunneling conductivity. To study photon-assisted tunneling in our samples, we use the cyclotron resonance as a strong, tunable electronic excitation in the far-infrared. When the Landau-level spacing [hstrok]ω_c corresponds to the laser energy [hstrok]ω_L, the carriers are effectively pumped to higher Landau levels, which leads to a reduced resistance across the tunnel barrier. This photo-conductive signal is “doubly resonant” in that it is at maximum when ω_c coincides with ω_L, and at the same time the subbands of the two wells are aligned.     

Relaxation of excitons in semimagnetic asymmetric double quantum wells

The steady-state circular-polarized photoluminescence in semimagnetic asymmetric double quantum wells based on Cd(Mn,Mg)Te is studied thoroughly in relation to the polarization of intrawell nonresonance photoexcitation in magnetic fields Bup to 9 T. In low fields B, in which the exciton in the magnetic well is higher in energy than the exciton in the nonmagnetic well, the complete interwell relaxation of excitons is observed. In fields higher than B _c = 3–6 T, at which the exciton level in the magnetic well crosses the field-independent exciton level in the nonmagnetic well, the magnetic-field-induced red shift of the exciton in the magnetic well is accompanied by the establishment of a nonequilibrium distribution of excitons. This suggests that spin relaxation plays an important part in the interwell separation of excitons in the spin-dependent potential of the heterostructure. The efficiency of spin relaxation is controlled by mixing of valence band states in the nonmagnetic well and by splitting of heavy and light holes Δ _hh-lh . Different modes of interwell tunneling are observed in different field regions separated by the field B _c ^* > B _c corresponding to the crossing of the localized excitons in the nonmagnetic well and free excitons in the magnetic well. Possible mechanisms of interwell tunnel relaxation are discussed.     

Light-scattering determination of electron tunneling gaps in double quantum wells

Resonant inelastic light-scattering techniques have been used to measure directly the single-particle tunneling gap (Δ_SAS) in Al_xGa_1−xAs/GaAs double quantum wells. We have observed a systematic decrease in Δ_SAS with increasing height of the barrier in agreement with the Δ_SAS calculated self-consistently within a Hartree approximation.     

Germanium quantum dots in an unstrained GaAs/ZnSe/Ge/ZnSe heterosystem

Arrays of Ge quantum dots in unstrained GaAs/ZnSe/Ge heterostructures were obtained by molecular-beam epitaxy for the first time. Their spatial parameters are examined by scanning tunneling microscopy, and their electronic structure is studied by Raman spectroscopy.     

Optical properties of CdSe quantum dots grown on ZnSe and ZnBeSe by molecular beam epitaxy

We report detailed studies of the optical properties of CdSe quantum dots (QDs) grown on ZnSe and ZnBeSe by molecular-beam epitaxy (MBE). We performed steady-state and time-resolved photoluminescence (PL) measurements and observe that nonradiative processes dominate at room temperature (RT) in the CdSe/ZnBeSe QDs structures, though these nonradiative processes do not dominate in the CdSe/ZnSe QDs structures up to RT. We performed secondary ion-mass spectrometry (SIMS) measurement and propose that the oxygen incorporation in the ZnBeSe layers (possibly caused by the reactivity of Be) may contribute to the dominant nonradiative processes at high temperatures in the QDs grown on ZnBeSe.     

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si_0.85Ge_0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.     

Excitons in single and double GaAs/AlGaAs/ZnSe/Zn(Cd)MnSe heterovalent quantum wells

Exciton photoluminescence spectra, photoluminescence excitation spectra, and magnetophotoluminescence spectra of single (GaAs/AlGaAs/ZnMnSe) and double (GaAs/AlGaAs/ZnSe/ZnCdMnSe) heterovalent quantum wells formed by molecular beam epitaxy are studied. It is shown that the exciton absorption spectrum of such quantum wells mainly reproduces the resonant exciton spectrum expected for usual quantum wells with similar parameters, while the radiative exciton recombination have substantial distinctions, in particular the additional localization mechanism determined by defects generated by heterovalent interface exists. The nature of these localization centers is not currently clarified; their presence leads to broadening of photoluminescence lines and to an increase in the Stokes shift between the peaks of luminescence and absorption, as well as determining the variation in the magnetic g factor of bound exciton complexes.     

Intersubband absorption of light in selectively doped asymmetric double tunnel-coupled quantum wells

The spectrum of equilibrium intersubband absorption has been studied in selectively doped asymmetrical double tunnel-coupled quantum wells designed for research into the modulation of IR light in a longitudinal electric field. The comparison of calculated and experimental spectra at different temperatures is carried out. In calculations, the influence of the space charge on the energy spectra of electrons and the difference in the electron effective mass in different subbands are taken into account. The data obtained on the intersubband absorption spectra in equilibrium conditions and under electron excitation by high-power picosecond pulses of light in the mid-IR range allow us to refine the energy spectrum of electrons in the actual structure.     

Raman spectra of CdTe/ZnTe self-assembled quantum dots

In this paper, we present Raman scattered spectra and results of photoluminescence (PL) measurements of CdTe/ZnTe self-assembled quantum dots (SAQD). The PL spectrum displayed two main emission peaks, both connected with the existing of QDs. One presents direct deexcitation to ground state and the other is optical phonon (ω=204.2 cm^?1)-assisted deexcitation. The registered multiphonon (MP) emission process depends on temperature. At low temperature, one-phonon spectra shows line at 200.4 and 210.3 cm^?1 (ZnTe LO mode confined by degenerate superlattices), 217 cm^?1 (TA+LO in ZnTe) and 386 cm^?1 (LO+TO(Γ) in ZnTe).     

New system of self-assembled GaSb/GaP quantum dots

The atomic structure and energy spectrum of self-assembled GaSb/GaP quantum dots are discussed. It is shown that the quantum dots consist mainly of fully relaxed GaSb and have type-I band alignment with the ground electron state at the indirect valley of the GaSb conduction band.     

Temperature dependence of excitonic transition in ZnSe/ZnCdSe quantum wells

In recent years ,the interest in the quantum hetero-structures composed of theⅡ-Ⅵwide gap materials haveincreased dueto potential device applications ,suchasthehigh-brightness blue/green light emitting diodes(LEDs) andlaser diodes (LDs)[1].Sincethe applicationof semiconductor materials to optoelectronic devices re-quires a precise design of the optical properties at theoperation temperature, it is i mportant to have theknowledge of the temperature effects on the opticalproperties of the mat…     

Production of quantum dots by selective interdiffusion in CdTe/CdMgTe quantum wells

Individual quantum dots are produced by selective interdiffusion between the barriers and the quantum well layer in a CdTe/CdMgTe heterostructure. The heterostructure, with a SiO₂ mask preliminarily deposited onto the surface, was subjected to short-term annealing for 1 min at the temperature 410°C. The mask contained open apertures with diameter up to 140 nm. The annealing induces diffusion of Mg atoms into the depth of the quantum well. Diffusion is substantially enhanced under the mask. The induced lateral potential, with minimums in the regions of apertures of the mask, stimulates efficient localization of charge carriers that form quasi-zero-dimensional excitons. The study of radiative recombination suggests complete spatial confinement of the excitons. The confinement manifests itself in the observation of a substantially narrowed line of excitonic transitions, as well as in the observation of biexcitons and excited states at high levels of photoexcitation. The characteristic energies of interlevel splitting and the biexciton binding energy show that charge carriers are under the condition of weak confinement in the quantum dots.     

Calculation of the size-quantization levels in strained ZnCdSe/ZnSe quantum wells

The ZnSe and CdSe parameters required to calculate levels in ZnCdSe/ZnSe quantum wells are determined by fitting to published data. The model is shown to be adequate for the example of structures with a collection of quantum wells whose thickness and composition were determined by independent methods. Fiz. Tekh. Poluprovodn. 31, 939–943 (August 1997)     

Magnetospectroscopy of double HgTe/CdHgTe quantum wells

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.     

Interaction of CdSe/ZnS quantum dots: Among themselves and with matrices

Photoluminescence (PL) of CdSe/ZnS quantum dots (QDs) deposited on Si, fused silica, Au film, shows red-shift; and blue-shift whenever two peaks are present, particularly on silica nanospheres. The red-shift with increasing density of QDs is attributed to interaction between QDs with PL emerging from the lower bonding state, and the second peak is attributed to molecular complexes on the surface of QD interacting with its surrounding matrices. The second peak is too weak to be detected on Si wafer with native oxide. The couplings between QD/QD and QDs/silica spheres via the molecular complexes are explained with a simple model. We also demonstrate that the band-gap of photonic crystals consisting of silica spheres can be stabilized by dehydration, annealing at high temperatures up to 1000 °C.     

Persistent photoconductivity in InAs/AlSb heterostructures with double quantum wells

Effects of persistent photoconductivity in InAs/AlSb heterostructures with the cap GaSb layer and two-dimensional electron gas in the InAs double quantum wells at T = 4.2 K are studied. From Fourier analysis of the Shubnikov-de Haas oscillations, electron concentrations in each quantum well are determined at various wavelengths of illumination and pronounced asymmetry of the structure caused by the built-in electric field is demonstrated explicitly. The self-consistent calculations of the energy profile of the double quantum well are performed and the concentrations of ionized donors on both sides of the well are determined, which provided concretization of the previously suggested mechanism of bipolar persistent photoconductivity in such structures.