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。     

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.     

Optical spectroscopy of a resonant Bragg structure with InGaN/GaN quantum wells

The optical reflectance and transmittance spectra of a periodic InGaN/GaN semiconductor heterostructure with 60 quantum wells are studied at room temperature. The period of the structure was chosen such that, at some angles of incidence of light, the energy of a photon resonantly reflected from the Bragg structure coincides with the excitation energy for quantum-well excitons in quantum wells. The parameters of these excitons are determined by fitting the spectra measured at two different angles of incidence, 30° and 60°. We take into account the resonant exciton transitions in quantum wells as well as the transitions into the continuous spectrum. The radiative decay parameter is determined to be (0.20 ± 0.02) meV.     

A study of the excitonic characteristics in heterostructures with quantum wells and corrugated surface

The excitonic characteristics of In_xGaAs_1−x -GaAs heterostructures in quantum wells in the presence of a corrugated surface has been investigated. The corrugated surface gives a substantial (up to 20%) polarization of the excitonic spectra even when the exciting light is incident in the direction of the normal. The exciton binding energies are calculated on the basis of data on the phonon repetitions of the photoluminescence spectra and the results agree well with the theoretical calculations. The parameters of the surface microrelief of the experimental layered structures are estimated on the basis of the degree of polarization of the photoluminescence and the magnitudes of the reflection and transmission. Fiz. Tekh. Poluprovodn. 31, 875–879 (July 1997)     

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.     

Size effect of semiconductor quantum wells in excitonic spin generation under drift

The excitonic spin polarization in dependence of the size of semiconductor quantum well (QW) was investigated by observing the two different circular polarizations of photoluminescence (PL). From the measurements of PL in QWs, it was found that there is a difference between the two different polarization conditions, which is caused by spin-dependent phase-space filling. The PL spin polarization was estimated from the signals of the left and right circularly polarized PL and was found to depend on the size of the wells as well as on the strength of the bias field. The effects of the size of the well and applied electric field on the excitonic PL spin polarization were studied.     

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.     

Excitonic dynamics of a quantum dot coupled to a laser-driven semiconductor microcavity

Within the density matrix formalism, we report on the quantum control of the excitonic coherences in quantum dots coupled to a single mode field resonant semiconductor cavity. We use an external classical laser field to drive the dynamical response of the excitonic states. Dissipation mechanisms associated with the cavity field and the excitonic states are explicitly included in the model. Our numerical simulations of the excitonic dynamics are in good agreement with recent experimental reports. Furthermore, we compute and show how to tailor such a dynamics in the presence of the laser field by means of controlling the detuning between the laser and the cavity field frequencies. The results are analyzed with a view to implementing quantum control of local qubit operations.     

Molecular orbital topology and optical properties of galliumarsenide clusters

The oscillator strength is expressed in terms of a density matrix formulation for optical absorption and is calculated for a GaAs cluster by using the self-consistent-field Xα-scattered wave cluster molecular orbital method. The discrete value of oscillator strength due to optical excitation defined by the selection rule leads to a model of the fundamental nature of the excitonic absorption behavior in GaAs bulk material and quantum well structure. Calculated results of band-to-band transition and exciton states are found to agree well with absorption spectra published in literature. The implication of the excitonic states for the nonlinear-optical behavior observed in GaAs quantum well structures is discussed. The presence of excitonic states may cause a `saturation' phenomenon which will lead to the optical nonlinearity     

Resonance reflection of light by a periodic system of excitons in GaAs/AlGaAs quantum wells

The optical reflection spectra of periodic structures with two quantum wells in the unit cell are investigated. The dependences of the features of Bragg reflection and exciton-polariton reflection on the angle of incidence and polarization of light and the sample temperature are studied. Analysis of the experimental data shows that a 60-period structure acts as a distributed Bragg reflector with a peak reflectivity exceeding 90% and a spectral band width as large as 16 meV.     

Optical investigation of piezoelectric field effects on excitonic properties in (111)B-grown (In, Ga)As/GaAs quantum wells

The excitonic properties in two (111)B-grown In_0·15Ga_0·85As/GaAs multiple quantum well p-i-n diodes are investigated by thermally-detected optical absorption and electroreflectance rneasurements. The lineshape of the electroreflectance spectra is analysed by means of a multilayer model enabling the energies and the oscillator strengths of excitonic transitions to be deduced. The excitonic characteristic values are calculated by using a variational method. The determination of the variation of the binding energy with in-well field leads to an accurate value of the piezoelectric field in the InGaAs layers. The theoretical oscillator strengths are compared to those obtained from electroreflectance for different excitonic transitions.     

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.     

Tunable photodetectors based on strain compensated GaInAsSb/AlAsSbmultiple quantum wells grown by molecular beam epitaxy

GaInAsSb/AlGaAsSb strain compensated multiple quantum well (MQW) p-i-n structures grown by molecular beam epitaxy on GaSb substrates have been successfully fabricated into tunable photodiodes, which showed a large photoresponse peak shift up to 30 meV (80 nm) at 77 K under a reverse bias of 10 V due to quantum confined Stark effect (QCSE). The QCSE persists up to room temperature and the values of the excitonic absorption peak shifts agree well with the calculated results. Based on the observed QCSE, an electrically tunable resonant cavity enhanced photodetector with strain compensated MQW structure is proposed and modeled     

Spin-dependent tunneling recombination in heterostructures with a magnetic layer

We propose a mechanism for the generation of spin polarization in semiconductor heterostructures with a quantum well and a magnetic impurity layer spatially separated from it. The spin polarization of carriers in a quantum well originates from spin-dependent tunneling recombination at impurity states in the magnetic layer, which is accompanied by a fast linear increase in the degree of circular polarization of photoluminescence from the quantum well. Two situations are theoretically considered. In the first case, resonant tunneling to the spin-split sublevels of the impurity center occurs and spin polarization is caused by different populations of resonance levels in the quantum well for opposite spin projections. In the second, nonresonant case, the spin-split impurity level lies above the occupied states of electrons in the quantum well and plays the role of an intermediate state in the two-stage coherent spin-dependent recombination of an electron from the quantum well and a hole in the impurity layer. The developed theory allows us to explain both qualitatively and quantitatively the kinetics of photoexcited electrons in experiments with photoluminescence with time resolution in Mn-doped InGaAs heterostructures.     

太赫兹场作用下半导体超晶格的动力学过程及光吸收谱研究

基于激子基，采用密度矩阵理论研究了太赫兹场作用下半导体超晶格的子带间动力学过程及光吸收谱。在太赫兹场的驱动下，激子作布洛赫振荡。子带间极化的缓慢变化依赖于太赫兹频率，随着太赫兹频率的增加，子带间极化向下振荡，极化强度降低。以 和 两种超晶格为例进行研究，它们的光吸收谱出现了卫星峰结构，这是由于太赫兹场与万尼尔斯塔克阶梯激子作用的非线性效应产生的。但是就 与 超晶格相比而言，我们研究发现，n<0的激子态与n=0的激子态耦合作用较强使得光吸收谱吻合性较好，n＝0时的激子态吸收光谱出现红移，n>0的激子态光吸收谱中出现的边带效应不是很明显。     

Effects of dispersion and birefringence on the performance ofquantum well mode converters

The device characteristics of GaAs multiple quantum well (MQW) polarization modulators, designed for operation at wavelengths of 865 and 870 nm, respectively, are investigated and shown to depend strongly on the dispersive and anisotropic optical properties of the quantum well medium. Results indicate that the observed decrease in spectral bandwidth and conversion efficiency at wavelengths detuned from the excitonic bandgap by ~250 Å can be accounted for, theoretically, if electroabsorptive loss terms are included in the coupled-mode analysis of polarization conversion in MQW waveguides. Device design considerations and applications of MQW polarization modulators to integrated optic filtering, wavelength division multiplexing/demultiplexing, and the frequency tuning of semiconductor lasers are presented     

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO₂ is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO₂ QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO₂ QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO₂ QDs.     

V-grooved quantum wires as prototypes of 1D-systems: Single particle properties and correlation effects

We present a combined theoretical and experimental study of the electronic and excitonic properties of GaAs-based V-grooved quantum wires as well as their dependence on external magnetic fields. We propose a numerical approach which allows us to compute electron and hole states, as well as their mutual correlation effects for realistic wire geometries. Our predictions for the optical spectra and their dependence on magnetic field are compared with several spectroscopical observations on samples grown by MBE on holographic patterns. The agreement is excellent, thus demonstrating the one-dimensional nature of the ground state exciton and the excitonic nature of the first-subband transitions in the presence of a magnetic field.     

Electroabsorption in narrow coupled double quantum wells: Coulombiccoupling effects

The effects of Coulombic coupling between different subband pairs on the electroabsorption spectra of narrow coupled double quantum wells (QWs) have been studied. It is shown via detailed comparison between electroabsorption spectra calculated using a full excitonic Green's function method, a decoupled excitonic Green's function method, a variational method, and experimental data that inclusion of the Coulombic coupling between different subband pairs is required for correct prediction of electroabsorption in the narrow well system. It is also shown that, due to Coulombic coupling, it is necessary to include unbound QW states, above the QW edge, in the simulation of electroabsorption and electrorefraction. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation of QW electroabsorption spectra, in coupled QW structures containing narrow 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.