Observation of Internal Transitions of Confined Excitons in GaAs/AlGaAs Quantum Wells (original) (raw)
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Physica B: Condensed Matter, 1998
Optically detected resonance (ODR) spectroscopy has been used to study electron and hole cyclotron resonance (CR) and various internal excitonic transitions in one sample in magnetic fields up to 15 T. The consequences of the cylindrical symmetry of the Hamiltonian for this system are observed directly. The energy difference between electron and hole CR equals the energy difference between any pair of 1sPnp ! internal excitonic transitions (IET). The two principal hole CR transitions were identified from comparison with theoretical calculations. In addition to the nearly degenerate 1sP2p > IET(s), two 1sP2p \ IETs resulting from the two distinct heavy-hole magnetoexcitons were observed. The capability of observing electron and hole CR as well as several IETs in a single sample is unique to the ODR technique and demonstrates its potential for elucidating the electronic states of semiconductor nanostructures.
2001
Spin-singlet and spin-triplet internal transitions of quasi-two-dimensional, negatively charged magneto-excitons (X −) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum-wells by optically detected resonance (ODR) spectroscopy. In the dilute electron limit, due to magnetic translational invariance, the ODR spectra are dominated by bound-to-continuum bands in contrast to the superficially similar negativelycharged-donor system D − , which exhibits strictly bound-to-bound transitions. With increasing excess electron density in the wells in the magnetic field region corresponding to Landau level filling factors ν < 2 the X −-like transitions
Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells
Solid State Communications, 2003
Many body effects contribute significantly to the energy states of electron-hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron-hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor , 2, and similar effects are found in CdTe QWs. For filling factor .2 no internal transitions are observed. These measurements demonstrate the strong effects of electron-electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.
Internal transitions of confined magnetoexcitons in GaAs-(Ga,Al)As quantum wells
Physical Review B, 2000
Internal transitions of confined magnetoexcitons in GaAs-͑Ga,Al͒As quantum wells have been theoretically studied under magnetic fields applied along the growth direction. Results are obtained within the effectivemass approximation and by using a variational procedure. Calculations are performed for transitions from 1s-like to 2p-, 3p-, and 4p-like magnetoexciton states as functions of the applied magnetic field, and for several well widths. Theoretical results for the far-infrared intraexcitonic transition energies are then compared with recent experimental measurements using optically detected resonance techniques.
The excitonic states have been investigated in In 0.045 Ga 0.955 As/GaAs heterostructures consisting of i quantum wells (iϭ1,2,3,4) with 7.5 nm well thickness. For a 2.5 nm barrier thickness between the wells, the electronic states are strongly coupled. Because of the coupling, the heavy-hole exciton nshh of each single quantum well is split into i 2 states. The states can be characterized according to their symmetry under a combination of the reflections of the single particles at the quantum-well plane. The energy order of the symmetric and antisymmetric states as a function of quantum-well number is investigated in detail, and compares well to the theoretical calculation. These coupled quantum-well structures exhibit somewhat three-dimensional character based on the study of their exciton binding energies and wave functions. Highly resolved photoluminescence excitation spectra are presented, measured in magnetic fields up to 13 T using circularly polarized light. Strong mixing between light-and heavy-hole excitons causes optical transitions into high-angular-momentum exciton states and strong anticrossing effects. An anticrossing between the 3dhh 11 and hh 21 exciton is observed. Also, the light-hole exciton is found to possess ⌫ 7g and ⌫ 6g symmetries.
Experimental and theoretical study of excitonic transition energies in GaAs/Al Gat "As quantum wells
Physical Review B, 1994
High-quality molecular-beam-epitaxy-grown decoupled GaAs-(Al, Ga)As multiple quantum wells (MQW) of various well thickness (2.7 Ls 11.9 nm; x=0.3) and different barrier compositions (0.12 x 1; L~= 11 nm) have been studied by x-ray diffraction (XRD), photoluminescence excitation, and emission (PL). The temperature dependence of the MQW properties has also been studied. The well width and barrier composition of the MQW were obtained by XRD and PL, respectively. 2s-excitonic features and the free electron-hole sublevel transitions can be resolved. We compare the 1s-2s energy difference and the ground-state binding energies of the heavy-and light-hole excitons with a recent accurate theory of exciton binding energies, taking the structure parameters from an independent determination. Experimental and theoretical values of the heavy-and light-hole exciton binding energies are found to agree within 1 meV. The theoretically predicted and experimentally observed excitonic transition energies associated with the lowest (n =1) electron, heavy-hole, and light-hole sublevels agree well, if the theoretical approach includes the split-off valence band. Interpolation formulas for the heavy-and light-hole ground-state exciton binding energies and for the n = 1 electron, heavy-, and light-hole sublevel energies are given.
Intra-magnetoexciton Transitions in Semiconductor Quantum Wells
2002
Highly sensitive optically detected resonance experiments have shown that magnetoexcitons in GaAs-(Ga,AI)As semiconductor quantum wells have discrete internal energy levels, with transition energies found in the far-infrared (terahertz) region. Here we are concerned with a theoretical study of the terahertz transitions of light-hole and heavy-hole confined magnetoexcitons in GaAs-(Ga,A1)As quantum wells, under a magnetic field applied in the growth direction of the semiconductor heterostructure. The various magnetoexciton states are obtained in the effective-mass approximation by expanding the corresponding exciton-envelope wave functions in terms of appropriate Gaussian functions. The electron and hole cyclotron resonances and intra-magnetoexciton transitions are theoretically studied by exciting the allowed electron, hole and internal magnetoexcitonic transitions with far-infrared radiation. Theoretical results are obtained for both the intra-magnetoexciton transition energies and oscillator strengths associated with excitations from Is -like to 2s, 2p±, and 3p± -like magnetoexciton states, and from 2pto 2s -like exciton states. Present results are in overall agreement with available optically detected resonance measurements and clarifies a number of queries in previous theoretical work.
Semiconductors, 2007
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.
Interwell excitons in GaAs/AlGaAs double quantum wells and their collective properties
Journal of Experimental and Theoretical Physics, 2000
Luminescence spectra of interwell excitons in GaAs/AlGaAs double quantum wells with electricfield-tilted bands ( n -i -n ) structures were studied. In these structures the electron and the hole in the interwell exciton are spatially separated between neighboring quantum wells by a narrow AlAs barrier. Under resonant excitation by circularly polarized light the luminescence line of the interwell excitons exhibited appreciable narrowing as their concentration increased and the degree of circular polarization of the photoluminescence increased substantially. Under resonant excitation by linearly polarized light the alignment of the interwell excitons increased as a threshold process with increasing optical pumping. By analyzing time-resolved spectra and the kinetics of the photoluminescence intensity under pulsed excitation it was established that under these conditions the rate of radiative recombination increases substantially. The observed effect occurs at below-critical temperatures and is interpreted in terms of the collective behavior of the interwell excitons. Studies of the luminescence spectra in a magnetic field showed that the collective exciton phase is dielectric and in this phase the interwell excitons retain their individual properties.
Optical Properties of Excitons in Semiconductor Quantum Wells
Japanese Journal of Applied Physics, 1995
Oscillator strength of excitons in various quantum wells is systematically investigated analyzing Fourier transform reflectance spectra. The temperature dependence of the oscillator strength of the zero-phonon line transition is well described by a modified Debye-Waller expression with an averaged phonon mode. This allows us to investigate the exciton-phonon interaction. The oscillator strength in GaAs/AlGaAs quantum wells increases as the well width is reduced in agreement with the existing theory, while in InGaAs/GaAs and GaAs/GaAsP quantum wells takes a maximum at a certain well width, showing the variation from two- to three-dimensional characters. The oscillator strength in InGaAs/GaAs and GaAs/GaAsP quantum wells is differently dependent on the alloy composition variation.