An (AlGaAs/GaAs/AlGaAs)<Subscript>60</Subscript> resonant Bragg structure based on the second quantum-confinement level of heavy-hole excitons in quantum wells

Photoluminescence and optical-reflection spectra of a periodic structure consisting of 60 tunneling-isolated GaAs quantum wells separated by AlGaAs barriers are studied. The structure is designed so that, for a certain angle of incidence (∼23°), the Bragg resonance condition is satisfied for light with a photon energy equal to the energy of heavy-hole excitons at the second quantum-confinement level in the wells. It is established experimentally that, under the conditions of double exciton-polariton and Bragg resonance, a superradiant optical mode is formed. Dependences of Bragg and exciton-polariton reflection on the angle of incidence and polarization of light and the temperature are investigated.     

ZnMgSe/ZnCdSe多量子阱的光学性质

通过光致发光 (PL)和拉曼 (Raman)光谱研究了分子束外延 (MBE)生长的 Zn Mg Se/ Zn Cd Se多量子阱的光学性质。在 80 K到 3 0 0 K温度范围内 ,观测到了 PL光谱中来自量子阱的自由激子发光 ,通过发光强度与温度的变化关系 ,计算了激子束缚能。结果表明在 Zn Mg Se/ Zn Cd Se多量子阱 (MQWs)势垒层中 ,Mg的引进增强了量子阱的限制效应 ,导致激子具有较好的二维特性。在室温下的 Raman光谱中观测到了多级纵光学声子(LO)和横光学声子 (TO)的限制模 ,表明多层结构具有较高的质量     

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.     

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.     

Modulation optical spectroscopy of excitons in structures with GaAs multiple quantum wells separated by tunneling-nontransparent barriers

Contactless optical electroreflectance measurements at different temperatures are used to study exciton states in a structure involving a periodic system of 36 GaAs quantum wells separated by tunneling-nontransparent AlGaAs barriers with thickness 104 nm. In the structure, the width of 32 of the quantum wells is 15 nm, while the width of the remaining four quantum wells, numbered 5, 14, 23, and 32, is 20 nm. The periodicity of the structure corresponds to the Bragg interference condition at the excitonic frequency in quantum wells at the angle of incidence of light ～43°. From the quantitative analysis of the shape of the contactless electroreflectance line, the parameters of the exciton ground states and excited states are determined for both types of quantum wells. It is established that, for the system of four 20-nm-wide quantum wells separated by a distance of 830 nm, the size-quantization energy in the ground state is 8.4 ± 0.1 meV, and the parameter of broadening of the excitonic peak is 1.8 ± 0.1 meV at 17 K and increases with temperature up to 2.0 ± 0.1 meV at 80 K. For the system of 32 wells with the width 15 nm, the quantum confinement energy in the ground state is 14.9 ± 0.1 meV, and the parameter of broadening of the excitonic peak is 2.2 ± 0.1 and 2.6 ± 0.1 meV at 17 and 80 K, respectively. The possible causes of radiative and nonradiative broadening of exciton states in the systems are discussed.     

Energy characteristics of excitons in structures based on InGaN alloys

Samples containing ultrathin InGaN layers that emit radiation in the spectral range from the ultraviolet to yellow region are studied. The samples are grown by metal-organic vapor-phase epitaxy. The Urbach energy, the localization energy of excitons, and the activation energy of charge carriers are determined to characterize radiative and nonradiative processes in the quantum dots and barriers of the structures. It is shown that these energy parameters are linearly dependent on the photon energy in the range from 3.05 to 2.12 eV. It is established that temperature variations in the emission intensity are due to the increase in the number of charge carriers thermally activated from the quantum wells into barriers as well as due to the enhancement of scattering of free excitons at defects.     

Excitonic spectrum of the ZnO/ZnMgO quantum wells

Excitonic spectrum of the wurtzite ZnO/Zn_{1 ? x} Mg _x O quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schrödinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.     

非对称耦合双阱P—I—N结构的光伏谱研究

首次报道了GaAs/Al_(0.35)Ga_(0.65)As/GaAs(50(?)/40(?)/100(?))非对称耦合双阱P-I-N结构的室温光伏谱,清楚地观察到双阱的轻、重空穴激子峰,这些激子峰的能量位置与光荧光和光电流谱得到的数据吻合得很好,而且光伏谱谱形非常类似于光电流谱,呈现清晰的台阶状结构。此外,还观察到轻、重空穴激子跃迁对激发光偏振态的依赖,并讨论了产生光伏谱的机制。     

In_（0．2）Ga_（0．8）As／GaAs应变多量子阱结构的光调制反射和热调制反射谱的研究

对Ｉｎ（０．２）Ｇａ（０．８）Ａｓ／ＧａＡｓ应变多量子阱在７７Ｋ下的光调制反射谱（ＰＲ）和热调制反射谱（ＴＲ）进行了实验研究．对ＰＲ结果的线形拟合指认了应变多量子阱中子能级的跃迁，并与理论计算结果作了比较．实验对比确认ＰＲ中１１Ｈ、１３Ｈ等跃迁结构为非耦合态、具有电场调制机构的一阶微商性质．而１１Ｌ、３１Ｈ、２２Ｈ等跃迁结构为阶间耦合态，对这些隧穿耦合的低场调制产生三阶微商特性．     

Heat induced nanoforms of zinc oxide quantum dots and their characterization

In our studies we observed heat induced phase transitions of Zinc oxide quantum dots at 60, 200, 360 and 400°C, where all the transitions were irreversible except the transition at 60°C which wasa reversible one. The phase transition at 60°C indicated a heat induced conformational change which was supported here by studying polarizing micrographs of ZnO quantum dots thin film. The X-ray diffraction studies of the sample fired at different temperatures as indicated by the thermal analysis were performed in order to understand the changes occurred due to transitions. The study also indicated a new and simple approach to develop ZnO nanorods by just thermal decomposition of the ZnO quantum dots firing in furnace at 200°C with 2h soaking. In order to have a proper insight of the structural changes we performed scanning electron microscopy. Optical characterization was done by UV-Vis and fluorescence spectrophotometer.     

Optical properties of germanium monolayers on silicon

Photoluminescence and Raman spectra of thin germanium layers grown on silicon at a low temperature (250°C) have been studied. In structures of this kind, in contrast to those grown at high temperatures, luminescence from quantum wells is observed at germanium layer thicknesses exceeding ～9 monolayers (ML). With the development of misfit dislocations, the luminescence lines of quantum wells are shifted to higher energies and transverse optical (TO) phonons involved in the luminescence are confined to a quasi-2D germanium layer. Introduction of an additional relaxed Si_0.95Ge_0.05 layer into the multilayer Ge/Si structure leads to a substantial rise in the intensity and narrowing of the luminescence line associated with quantum dots (to 24 meV), which points to their significant ordering.     

Optical Properties of AlGaAs/GaAs Resonant Bragg Structure at the Second Quantum State

Photoluminescence, optical reflectance and electro-reflectance spectroscopies were employed to study an AlGaAs/GaAs multiple-quantum-well based resonant Bragg structure, which was designed to match optical Bragg resonance with the exciton-polariton resonance at the second quantum state in the GaAs quantum wells. The structure with 60 periods of AlGaAs/GaAs quantum wells was grown on a semi-insulating substrate by molecular beam epitaxy. Broad and enhanced optical and electro-reflectance features were observed when the Bragg resonance was tuned to the second quantum state of the GaAs quantum well excitons manifesting an enhancement of the light-matter interaction under double-resonance conditions. By applying an alternating electric field, we revealed electro-reflectance features related to the x(e2-hh2) and x(e2-hh1) excitons. The excitonic transition x(e2-hh1), which is prohibited at zero electric field, was allowed by a DC bias due to brake of symmetry and increased overlap of the electron and hole wave functions caused by electric field.     

Photoluminescence study of AlGaAs/GaAs/AlGaAs double quantum wells separated by a thin AlAs layer

Photoluminescence (PL) spectra of Al_0.21Ga_0.79As/GaAs/Al_0.21Ga_0.79As double quantum wells (DQWs) separated by a thin AlAs barrier have been studied in the temperature range 77–300 K. The well width was varied from 65 to 175 Å, and the thickness of the AlAs barrier was 5, 10, or 20 Å. In the case of a sufficiently thin (5, 10 Å) AlAs barrier, the energy spectrum of QW states is considerably modified by coupling between the QWs. This effect shifts the main spectral peak of PL, and specific features associated with the splitting of the ground state into symmetric and asymmetric states are observed in the spectra at higher temperatures. The DQW structure with a 20-Å-thick AlAs barrier is a system of two uncoupled asymmetric Al_0.21Ga_0.79As/GaAs/AlAs QWs. The energy levels in double coupled QWs were calculated as functions of the well width and AlAs barrier thickness, and good correlation with the experimentally observed energies of optical transitions was obtained.     

Polarization of the optical emission of polaron excitons in anisotropic quantum dots

A theory of large polarons in ellipsoidal quantum dots is developed. The optical spectrum of polaron excitons and its dependence on the degree of anisotropy of a quantum dot are analyzed. It is shown that the polaron ground state exhibits specific anisotropic polarization of the medium. The symmetry of the wave function of the ground state depends on the band structure of the material and on the shape of the quantum dot. The conditions under which strong polarization of the zero-phonon emission line produced by interband optical transitions occurs are determined. The possible polarization of this line is determined for various relationships between the polaron energy and the energy of the exchange interaction.     

Exciton states and photoluminescence of silicon and germanium nanocrystals in an Al2O3 matrix

This paper deals with the theoretical and experimental study of radiative processes in zero-dimensional Si and Ge nanostructures consisting of a system of Si or Ge nanocrystals embedded in an Al₂O₃ matrix. The Al₂O₃ films containing Si or Ge quantum dots were produced by pulsed laser-assisted deposition. The timeresolved photoluminescence spectra of the films were recorded in the energy range from 1.4 to 3.2 eV in the range of photoluminescence relaxation times between 50 ns and 20 μs. The exciton binding energy and the energy of radiative excitonic transitions are calculated, taking into account the finite barrier height and the polarization of heterointerfaces. In addition, the excitonic photoluminescence spectra are calculated, taking into account the effect of quantum mesoscopic fluctuations and the possible nonmonotonically varying dependence of the radiative zero-phonon lifetime of excitons on the dimensions of the quantum dots. The observed agreement between the calculated and recorded photoluminescence spectra confirms the excitonic nature of photoluminescence and provides a means for the determination of the model parameters of photoluminescence in the nanostructures.     

Excitonic photoluminescence of silicon quantum-well structures

The exciton binding energy and the energies of radiative excitonic transitions in the separate SiO_x-Si-SiO_x quantum wells are calculated in the effective-mass approximation with the quadratic dispersion relation. Along with the real finite offsets of the bands in such quantum well structures, the effect of dielectric enhancement of the exciton binding energy due polarization of the heterointerfaces is taken into account. In addition, the dependence of the zero-phonon radiative excitonic recombination time on the width of the SiO_x-Si-SiO_x quantum well is calculated. This dependence exhibits unsteady (oscillating) behavior, which is caused by the indirect band gap of the silicon material. It is shown that the theoretically calculated energies of the radiative excitonic transitions in the SiO₂-Si-SiO₂ quantum wells match the experimentally determined energies for the quantum wells whose widths are larger than 1.5 nm. Good agreement between the theoretically calculated and experimental spectral dependences of photoluminescence in the SiO₂-Si-SiO₂ quantum wells is attained.     

Modeling of optical gain in InGaN-AlGaN andInxGa1-xN-InyGa1-yNquantum-well lasers

This paper presents computations of the optical gain in In_x Ga_1-xN-In_yGa_1-yN and InGaN-AlGaN quantum-well lasers involving the contributions of excitons as well as free carriers transitions. The behavior of optical gain in GaN based quantum wells due to excitonic transitions is quite similar to that of ZnCdSe-ZnSSe system, as the magnitude of the exciton binding energies (~30 meV) is comparable. The model compares the exciton emission energy with the experimental data reported on In_0.22Ga_0.78N-In_0.06Ga_0.94N multiple quantum wells as well as in GaN layers (cubic grown on 3C SiC), including the effect of strain induced band gap changes. The optical gain is also computed as a function of the injection current density for the InGaN-AlGaN multiple quantum-well lasers. The model evaluates the feasibility of obtaining GaN based blue and ultraviolet lasers. It is shown that the excitonic transitions reduce the threshold current density which is adversely affected by the presence of dislocations and other defects     

Experimental Study of Spontaneous Emission in Bragg Multiple- Quantum-Well Structures with InAs Single-Layer Quantum Wells

The time-resolved photoluminescence of a Bragg structure formed by InAs single-layer quantum wells in a GaAs matrix is investigated experimentally. The comparison of photoluminescence spectra recorded from the edge and the surface of the sample indicates that Bragg ordering of the quantum wells leads to substantial modification of the spectra, in particular, to the appearance of additional modes. The spectrum recorded at the edge of the sample features a single line corresponding to the exciton ground state. In the spectrum recorded at the surface, an additional line whose frequency and propagation angle correspond to the Bragg condition for quantum wells, appears at high excitation levels. The calculation of the modal Purcell factor explains the fact that spontaneous emission is enhanced only for specific propagation angles and frequencies, rather than for all angles and frequencies satisfying the Bragg condition.     

Terahertz emission and photoconductivity in n-type GaAs/AlGaAs quantum wells: the role of resonant impurity states

Terahertz emission spectra in a longitudinal electric field and lateral photoconductivity spectra under terahertz illumination have been studied in structures with GaAs/AlGaAs quantum wells (QWs). It is shown that the spectra contain features associated with electron transitions involving resonant impurity states related to the second quantum-well subband. Calculations of the energy spectrum of impurity states and matrix elements of optical transitions made by taking into account various positions of the impurity relative to the QW center confirm the assumptions made.     

Type-II Ge/Si quantum dots

The electronic structure of spatially indirect excitons, multiparticle excitonic complexes, and negative photoconductivity in arrays of Ge/Si type-II quantum dots (QDs) are considered. A comparison is made with the well-known results for type-II III-V and II-VI QD heterostructures. The following fundamental physical phenomena are observed in the structures under study: an increase in the exciton binding energy in QDs as compared with that for free excitons in homogeneous bulk materials, a blue shift in the excitonic transitions during the generation of multiparticle complexes (charged excitons, biexcitons), and the capture of equilibrium carriers to localized states induced by the electric ield of charged QDs.