FIRST-PRINCIPLE SELF-CONSISTENT PSEUDOPOTENTIAL CALCULATION OF THE ELECTRONIC STRUCTURES OF SHORT-PERIOD (GaAs)m(AlAs)n SUPERLATT1CES

With the local density approximation, the band structares of the short-period (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated by using the first-principle self-consistent pseudopotential method. The results show that the (GaAs)₁(AlAs)₁ superlattice is an indirect semiconductor, and the lowest conduction band state is at point R in the Brillouin zone; the (GaAs)₂(AlAs)₁ superlattice is a direct semiconductor and the lowest conduction band state is at point Γ. The squared matrix elements of transition are calculated. The pressure coefficients of energy gaps of the (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated and compared with those obtained by hydrostatic pressure experiments.     

量子限制受主的光致发光研究

对一系列δ掺杂浅受主铍（Be）原子的GaAs/AlAs多量子阱和均匀掺杂Be受主的GaAs体材料中Be原子的能级间跃迁进行了光致发光（PL）研究.实验中所用的样品是通过分子束外延技术生长的均匀掺杂Be受主的GaAs外延单层样品和一系列GaAs/AlAs多量子阱样品,并在每量子阱中央进行了Be原子的δ掺杂,量子阱宽度为30 到200 ?.在4.2 K温度下测量了上述系列样品的光致发光谱,清楚地观察到了束缚激子的受主从基态1s_3/2（Γ₆）到第一激发态 关键词： 量子限制受主 光致发光 多量子阱 δ掺杂     

GaAs/AIAs superlattices under pressure: steady-state and subpicosecond time-resolved measurements

Abstract

Low-temperature photoluminescence measurements under hydrostatic pressure were performed on [100]-, [311]-and [111]-grown GaAs/AlAs superlattices. The indirect optical transitions for all three growth directions were identified by their characteristic pressure dependences as originating from the X point of the AlAs conduction band. Subpicosecond-time-resolved measurements on a GaAs/AIAs superlattice show a decrease of electron transfer times from the GaAs layer into the A1As layer with pressure from 400 fs to 5Ofs and an intensity dependence of the pressure-induced crossover from type I to type II.     

Quantum-Statistics Behavior of the Exciton-Biexciton System In GaAs/AlAs Type-II Superlattices

We have investigated the quantum-statistics behavior of the exciton-biexciton system from the photoluminescence properties in (GaAs) m /(AlAs) m type-II superlattices with m = 12 and 13 monolayers, where the lowest-energy type-II exciton consists of the n = 1 X electron of AlAs and n = 1 o heavy hole of GaAs. The long exciton lifetime of the order of w s due to the indirect transition nature enables us to obtain precisely the density relation between the exciton and biexciton from the line-shape analysis of time-resolved photoluminescence spectra. In a relatively low exciton-density region, the biexciton density obeys a well-known square law. At an exciton density around 1 2 10 10 cm m 2 , the biexciton density suddenly increases with a threshold-like nature. This behavior, which is realized at a bath temperature up to 8 K under an excitation power of the order of 100 mW/cm 2 , results from the characteristics of Bose-Einstein statistics of the exciton-biexciton system.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)     

Photoconductivity of GaAsAlAs multiple quantum wells: Temperature dependent optical transitions

Employing temperature dependent photoconductivity, photoluminescence and photoreflectivity measurements, we have analyzed a GaAsAlAs multiple quantum well. The above optical techniques clearly resolve the fundamental inter-subband transitions, including heavy hole-light hole splittings. At T < 60K an anomalously high photoconductivity was discovered below the direct inter-subband transitions and is attributed to the presence of interface states. For T ≥ 100K the fundamental indirect Λ - J transition was discovered and associated with L0 (L) - phonon absorption. The energetic positions of the direct and the indirect gaps are in agreement with previous band structure calculations for GaAs/AlAs superlattices.     

Energy relaxation and transport of indirect excitons in AlAs/GaAs coupled quantum wells in magnetic field

The evolution of indirect exciton luminescence in AlAs/GaAs coupled quantum wells after excitation by pulsed laser radiation has been studied in strong magnetic fields (B⩽12 T) at low temperatures (T⩾1.3 K), both in the normal regime and under conditions of anomalously fast exciton transport, which is an indication of the onset of exciton superfluidity. The energy relaxation rate of indirect excitons measured in the range of relaxation times between several and several hundreds of nanoseconds is found to be controlled by the properties of the exciton transport, specifically, this parameter increases with the coefficient of excitonic diffusion. This behavior is qualitatively explained in terms of migration of excitons between local minima of the random potential in the plane of the quantum well. Zh. éksp. Teor. Fiz. 114, 1115–1120 (September 1998)     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Exchange interactions of excitons localized at opposite interfaces in type-II GaAs/AlAs superlattices: Optically detected magnetic resonance and level anticrossing

Two types of excitons, localized at opposite interfaces and characterized by different magnitudes of the exchange interactions at the same radiation energies, are simultaneously in type-II GaAs/AlAs superlattices. It is shown that the additional long-wavelength luminescence line in superlattices grown with growth interruptions after the GaAs layers is due to the recombination of an exciton localized at an inverted interface in regions where the quantum-well width is increased by one monolayer. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 701–706 (25 November 1996)     

Magneto-optical studies of GaAs/AlAs quantum wells

We present a magnetoreflectivity study of three GaAs/AlAs multiple quantum wells with widths 75, 100 and 150 Å. At T = 5K, the reflectivity spectra exhibit features associated with the excitons as well as interband Landau transitions. The slopes of these transitions imply that the electrons are confined in the GaAs layers. In addition, experimental values for the exciton binding energies are determined from the zero field intercepts of the Landau transitions.     

High-Density Effects on the Exciton Systems and Their Dynamical Behavior in Al x Ga 1− x As/AlAs Quantum Wires

In Al x Ga 1 m x As/AlAs quantum wire (QWR) structures, the lower lying indirect exciton (IE) photoluminescence (PL) peak shows remarkable blue-shift under intense light-excitation contrary to the higher lying direct exciton (DE) PL band with very small blue-shift, although the two kinds of exciton states consist of the common hole state. In time-resolved PL spectra in the type-II QWR of x = 0.4, the DE PL band appears at an earlier stage without peak-shift and the excitons relax to the IE state making the IE PL peak dominant with time evolution. The blue-shift of the IE peak in a quasi equilibrium after the relaxation seriously depends on the excitation density. The origin of the blue-shift is explained in terms of many-body effects including band-bending effect due to the electric field induced by spatially separated electrons and holes in the QWR structures.     

二维共价有机骨架中的直接激子和间接激子

本文利用基于GW方法和Bethe-Salpeter方程的第一性原理计算,研究了两种二维共价有机骨架材料(COF)的激发态性质. 单层COF是直接带隙材料,而体相COF呈现间接带隙. 根据直接激子计算的体相COF的光学带隙和吸收光谱与实验一致,而由位于导带底的光生电子和位于价带顶的空穴形成的间接激子能量的理论计算值远低于实验荧光光谱的测量值. 研究表明,可以排除间接带隙COF材料的发光由声子主导的可能性. 研究认为体相COF的发光可能源于缺陷处直接激子的复合. 体相COF的AA堆叠结构导致其带隙是间接的. 如果将堆叠方式由AA变成AB,体相COF将转变成直接带隙材料,它的发光效率可能会增强.     

Luminescence of excitons in slightly asymmetric double quantum wells

We report the first studies of exciton luminescence spectra from asymmetric double quantum wells (DQWs) of very similar width. The DQWs were of GaAs/AlGaAs and the differences in widths of the coupled wells were one or two monolayers. The coupled direct and indirect exciton states anticross with a resonance splitting of 1.33 meV. An additional luminescence line appearing at low temperatures is identified as a localized indirect exciton. Fiz. Tverd. Tela (St. Petersburg) 39, 735–739 (April 1997)     

Raman scattering from confined LO phonons and dispersion relation in GaAs/AlAs superlattices

The Raman scattering from both GaAs and AlAs confined LO phonons for several GaAs/AlAs superlattices is presented. The GaAs confined LO phonons were observed for two narrow GaAs layer samples at room temperature. The frequencies of observed phonons fit fairly well with the theoretical dispersion curves.     

Spectroscopic studies of real space indirect symmetric GaAs/AlAs short period superlattices

The band structure and the optical transitions of symmetric ultra-short period (GaAs) _m /(AlAs) _n superlattices (n=m is the number of monolayers) have been studied using different spectroscopic techniques, namely photoacoustic spectroscopy, reflectivity and luminescence, photoluminescence excitation and high excitation intensity time-resolved luminescence. Direct observation of the transition from type I to type II energy band alignment is reported for superlattices whose configuration consists of more than 12 monolayers (i.e. m=n>12). The radiative recombination processes associated with the real space indirect transitions have been investigated as a function of the density of photogenerated electron-hole pairs, revealing an unusual density-dependent behaviour. A tentative interpretation in terms of a condensed electron-hole state at the indirect gap is given, which accounts for the long decay time of the type II luminescence at high excitation rate.     

Staggered excitonic resonances of Stark-ladder transitions in an asymmetric double-well GaAs/AlAs superlattice

Excitonic effects on Stark-ladder transitions have been investigated experimentally and theoretically in a novel asymmetric double-well superlattice consisting of wide and narrow GaAs quantum wells separated by a constant AlAs barrier. In this superlattice strong electron resonance can occur under the applied electric field between the wide and narrow wells. It is found that due to existence of the two different heavy-hole localized states two types of excitonic resonances which are staggered in field are observed in the low-temperature photocurrent spectra. This field difference in the staggered exciton resonances is rigorously explained by variational calculations of the changes in the direct and indirect exciton binding energies with the field.     

Fine structure splitting and biexciton binding energy in single self-assembled InAs/AlAs quantum dots

We report on photoluminescence investigations of individual InAs quantum dots embedded in an AlAs matrix which emit in the visible region, in contrast to the more traditional InAs/GaAs system. Biexciton binding energies, considerably larger than for InAs/GaAs dots, up to 9 meV are observed. The biexciton binding energy decreases with decreasing dot size, reflecting a possible crossover to an antibinding regime. Exciton and biexciton emission consists of linearly cross polarized doublets due to a large fine structure splitting up to 0.3 meV of the bright exciton state. With increasing exciton transition energy the fine structure splitting decreases down to zero at about 1.63 eV. Differences with InAs/GaAs QDs may be attributed to major dot shape anisotropy and/or larger confinement due to higher AlAs barriers.     

The role of the electron-hole coulomb interaction in quantum confined stark effect

The luminescence peak energy and tunneling lifetime of an exciton in a semiconductor quantum well with a small valence band offset in the presence of a perpendicular electric field is calculated by generalizing the variational approach of quantum confined Stark effect normally used for systems of GaAs/AlGaAs quantum wells. At a finite electric field, the electron-hole Coulomb interaction provides additional confinement to each of the carriers and significantly enhances the Stark shift and the exciton lifetime against field ionization. Numerical results are presented for ZnSe/Zn_1−xMn_xSe heterostructures studies in recent experiments.     

Deuterium in In-based quantum wells

The photoluminescence (PL) of high quality InGaAs/GaAs typically shows one strong intrinsic band, due to the heavy-bole free-exciton (HHFE) recombination. After sample irradiation with deuterium, two bands appear at energies below that of HHFE: a deeper band, D, due to radiative recombination at a deuterium-related site, and a shallower band, (D,X), attributed to an exciton bound to the same state. A maximum of the binding energy has been observed as a function of the well width for both states. As the indium molar fraction x increases, the strength of the D band decreases, together with its binding energy, until the band becomes no more detectable at x=0.37. The (D,X) band cannot be resolved for x ≥ 0.19. Deuterium diffusion has no beneficial effect on the PL intensity of InGaAs/GaAs quantum wells. Different is the case of InAs/GaAs quantum wells having well width below 2 monolayers. Although no strain relaxation occurs in the samples, the PL of the virgin samples is typical of structures with a high number of defects. After deuterium diffusion, the PL intensity increases by one to three orders of magnitude, depending whether non-radiative centers or thermal escape of carriers from the well rule the PL efficiency.     

Collective behavior of interwell excitons in GaAs/AlGaAs double quantum wells

Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell exciton in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circularly polarized light, the luminescence line of interwell excitons exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon is observed at temperatures lower than the critical point and can be interpreted in terms of the collective behavior of interwell excitons.