Properties Of Semiconductor Quantum Wells Research Articles

All-optical control of excitons in semiconductor quantum wells

Applying the Floquet theory, we developed the method to control excitonic properties of semiconductor quantum wells (QWs) by a high-frequency electromagnetic field. It is demonstrated, particularly, that the field induces the blue shift of exciton emission from the QWs and narrows width of the corresponding spectral line. As a consequence, the field strongly modifies optical properties of the QWs and, therefore, can be used to tune characteristics of the optoelectronic devices based on them.

Intense laser field effect on the nonlinear optical properties of triple quantum wells consisting of parabolic and inverse-parabolic quantum wells

The nonlinear optical rectification, the second harmonic generation coefficient, and the third harmonic generation coefficient in parabolic-inverse parabolic-parabolic quantum wells (PIPPQWs) and inverse parabolic-parabolic-inverse parabolic quantum wells (IPPIPQWs) are calculated varying the intense laser field (ILF) parameter (). The modifications of the dipole moment matrix elements and the energy levels are depending on the potential shape. The results show that the ILF intensity exerts an active influence on the profile, height, and width of the confinement potential of both PIPPQW and IPPIPQW. The potential profile of IPPIPQW has been affected otherwise than PIPPQW for different ILF intensities. The nonlinear optical rectification, the second harmonic generation, and the third harmonic generation coefficients of PIPPQW and IPPIPQW could be altered in the energy range and the size of the resonance peak by rising the ILF intensity. By changing the parameter, it is conceivable to organize red or blue shift at the resonance peak positions of the nonlinear optical rectification, the second harmonic generation, and the third harmonic generation coefficients. The shift of the nonlinear optical rectification coefficient occurs when the difference between the ground and the second energy levels changes. According to the parameters used here, while for PIPPQW the spectrum of the resonance peak of the nonlinear optical rectification displays a blue shift with increasing ILF, this spectrum displays a red shift for IPPIPQW. The consequences can be valued in investigating new ways of changing the optical and electronic properties of semiconductor quantum wells.

Fano resonances in optical spectra of semiconductor quantum wells dressed by circularly polarized light.

The optical properties of semiconductor quantum wells irradiated by a strong circularly polarized electromagnetic field are studied theoretically. Since the field can induce the composite electron-light states bound at repulsive scatterers, it drastically modifies all optical characteristics of the system. Particularly, it is demonstrated that the quantum interference of the direct interband optical transitions and the transitions through the light-induced intermediate states leads to the Fano resonances in the optical spectra, which can be detected in the state-of-the-art measurements.

Quantitative characterization of the interface roughness of (GaIn)As quantum wells by high resolution STEM

The physical properties of semiconductor quantum wells (QW), like (GaIn)As/GaAs, are significantly influenced by the interface morphology. In the present work, high angle annular dark field imaging in (scanning) transmission electron microscopy ((S)TEM), in combination with contrast simulation, is used to address this question at atomic resolution. The (GaIn)As QWs were grown with metal organic vapor phase epitaxy on GaAs (001) substrates under different, precisely controlled conditions. In order to be able to compare different samples, a carefully applied method to gain reliable results from high resolution STEM micrographs was used. The thickness gradient of the TEM samples, caused by sample preparation, was compensated by the intensity of group V atomic columns, where no alloying takes place. After that, the In concentration map was plotted for the investigated regions based on the intensity of the group III atomic columns. The composition maps show that the Indium distribution across the quantum well is not homogeneous. The growth temperature of the QW can greatly influence the composition fluctuation and the interface morphology, with higher growth temperature resulting in larger composition fluctuations in the QWs and slightly wider interfaces, i.e. larger In-segregation. Growth interruptions are shown to significantly homogenize the elemental depth profile especially along the (GaIn)As/GaAs interface and hence have a positive effect on interface smoothness.

Nonlinear optical properties of semiconductor quantum wells under intense terahertz radiation

The nonlinear optical properties of semiconductor quantum wells driven by intense in-plane terahertz electric fields are investigated theoretically by employing the extended semiconductor Bloch equations. The dynamical Franz-Keldysh effect of the optical absorption near the band edge is analyzed with Coulomb correlation among the carriers included. The in-plane terahertz field induces a variety of behavior in the absorption spectra, including terahertz replicas of the (dark) 2p exciton and terahertz sidebands of the 1s exciton. The dependence of these interesting features on the intensity, frequency, and phase of the terahertz field is explored in detail.

Influence of surface segregation on the optical properties of semiconductor quantum wells

We studied the influence of surface segregation on optical properties of semiconductor quantum wells. This effect leads to significant deviations of composition profiles from expected rectangular profiles. The model of Muraki is used to simulate composition profiles of InGaAs∕GaAs quantum wells for different segregation efficiencies from which we derive potentials for electrons and holes. To compute eigenenergies the Schrödinger equation is numerically solved. The transition energies are calculated from the energy differences of electrons and holes as a function of segregation efficiency. We find that the optical properties are influenced for segregation efficiencies larger than 0.7.

Enhanced light-matter interaction in semiconductor heterostructures embedded in one-dimensional photonic crystals

The optical properties of semiconductor quantum wells embedded in one-dimensional photonic crystal structures are analyzed by a self-consistent solution of Maxwell's equations and a microscopic many-body theory of the material excitations. For a field mode spectrally below the photonic band edge it is shown that the optical absorption and gain are enhanced, exceeding by more than 1 order of magnitude the values of a homogeneous medium. For the photonic crystal structure inside a microcavity the gain increases superlinearly with the number of wells and for more than five wells exceeds the gain of a corresponding vertical-cavity surface-emitting laser.

Effects of an intense laser field radiation on the optical properties of semiconductor quantum wells

Laser-dressed quantum well potentials for both conduction and valence band are derived in the frame of non-perturbative theory and variational approach. A significant blue shift is obtained for the interband recombination energy and quantum well fundamental absorption edge, when an intense, non-resonant laser field radiation, is applied. Strong enhancement of the blue shift occurs in the higher lying electron–heavy-hole recombination processes. Reduction of the absorption coefficient due to the laser-dressing potential is obtained as well. The blue shift and the reduction of absorption coefficient are found to be both dependent upon the well thickness, thus allowing direct investigation of the lasing frequency of GaAs–Ga 1− x Al x As quantum well lasers, by applying an intense, non-resonant, laser field radiation. A novel approach to build GaAs–Ga 1− x Al x As lasers, by tailoring the semiconductor parameters and the external long-wavelength laser field radiation, is theoretically predicted.

Is the spatially averaged spectrum equal to the global spectrum?

Near-field optical spectroscopy and microscopy give access to excitations that cannot be revealed in the far zone. In order to investigate these remarkable differences, we present a theoretical analysis of the local optical properties of semiconductor quantum wells including the effects of disorder arising from interface fluctuations. The far-field absorption spectrum is compared with spatially averaged absorption spectra calculated at different spatial resolutions. We find that summing up local optical spectra does not reproduce the global spectrum in contrast to findings at diffraction-limited resolutions.

Spatially resolved spectra in semiconductor quantum structures: Spatially averaged spectra compared to far-field spectra

The optical spectra of homogeneous surface systems can display remarkable differences in the near and far zones. The spectral changes occur due to the loss of evanescent modes in the far zone. These changes clearly show that near-field optical spectroscopy, besides resolving nanometric structures give also access to excitations that cannot be revealed in the far zone. Are these spectral changes detectable in real systems affected by disorder and imperfections? We address this issue by presenting a theoretical analysis of the local optical properties of semiconductor quantum wells including the effects of interface fluctuations. In particular we compare the far-field absorption spectrum with spatially averaged absorption spectra calculaded at different spatial resolutions. We find that summing up local optical spectra does not reproduce the global spectrum in contrast to findings at diffraction-limited resolutions.

Nonlinear optics of normal-mode-coupling semiconductor microcavities

The authors review the nonlinear optical properties of semiconductor quantum wells that are grown inside high-Q Bragg-mirror microcavities. Light-matter coupling in this system is particularly pronounced, leading in the linear regime to a polaritonic mixing of the excitonic quantum well resonance and the single longitudinal cavity mode. The resulting normal-mode splitting of the optical resonance is observed in reflection, transmission, and luminescence experiments. In the nonlinear regime the strong light-matter coupling influences the excitation-dependent bleaching of the normal-mode resonances for nonresonant excitation, leads to transient saturation and normal-mode oscillations for resonant pulsed excitation and is responsible for the density-dependent signatures in the luminescence characteristics. These and many more experimental observations are summarized and explained in this review using a microscopic theory for the Coulomb interacting electron-hole system in the quantum well that is nonperturbatively coupled to the cavity light field.

Optical properties of microcavity polaritons

This work contains a theoretical analysis of the optical properties of semiconductor quantum wells embedded in planar Fabry-Perot microcavities. In particular, the properties of the system in correspondence to the excitonic transition are studied by means of the polariton formalism. The polariton states in microcavities are derived and the polar-iton dispersion is presented. Particular emphasis is put on the existence of two well distinct regimes depending on the exciton and cavity parameters: strong coupling and weak coupling regime. The main experimental results are reviewed and compared with the prediction of the theory. After the polariton states have been characterized, the optical response of the system is discussed, with particular attention to the photoluminescence measurements. The polariton formation and relaxation through phonon scattering and the effect of the exciton inhomogeneous broadening are considered and, finally, a phe-nomenological model for the polariton photoluminescence spectra is presented.

Microscopic theory for the influence of Coulomb correlations in the light-emission properties of semiconductor quantum wells

A nonequilibrium Green's-function $T$-matrix approach is presented for the consistent computation of semiconductor quantum-well optical spectra including strong Coulomb correlations in the coupled photon and carrier system. Numerical solutions of the Bethe-Salpeter-type equations yield good agreement with recent absorption and/or gain and emission experiments.

Electronic and Optical Properties of Semiconductor Quantum Wells and Superlattices

Electronic and Optical Properties of Semiconductor Quantum Wells and Superlattices

Successes and failures of the k

The k\ensuremath{\cdot}p method combined with the envelope-function approximation is the tool most commonly used to predict electronic properties of semiconductor quantum wells and superlattices. We test this approach by comparing band energies, dispersion, and wave functions for GaAs/AlAs superlattices and quantum wells as computed directly from a pseudopotential band structure and using eight-band k\ensuremath{\cdot}p. To assure equivalent inputs, all parameters needed for the k\ensuremath{\cdot}p treatment are extracted from calculated bulk GaAs and AlAs pseudopotential band structures. Except for large exchange splittings in the in-plane dispersion for thin superlattices, present in pseudopotential calculations but absent from the k\ensuremath{\cdot}p results, we find generally good agreement for heterostructure hole bands within \ensuremath{\sim}200 meV of the GaAs valence-band maximum. There are systematic errors in band energies and dispersion for deeper hole bands (all other than hh1 and lh1) and significant qualitative and quantitative errors for the conduction bands. Errors for heterostructure conduction states which are derived from the zinc-blende \ensuremath{\Gamma} point diminish as length scales increase beyond \ensuremath{\sim}20 ML, while significant errors persist for L- and X-derived states.For bulk GaAs and AlAs, eight-band k\ensuremath{\cdot}p bands agree well with pseudopotential results very near the zinc-blende \ensuremath{\Gamma} point (where k\ensuremath{\cdot}p parameters are fit) but the first GaAs X point conduction band is \ensuremath{\simeq}26 eV too high with respect to the pseudopotential result. We show that this inadequate description of the bulk band dispersion is the principal source of k\ensuremath{\cdot}p errors in these heterostructures. A wave-function projection analysis shows that k\ensuremath{\cdot}p errors for heterostructures simply reflect corresponding errors for the bulk constituents, weighted by the amount that such bulk states participate in heterostructure states. \textcopyright{} 1996 The American Physical Society.

Raman studies of interface properties in semiconductor quantum wells and superlattices

Interface roughness and layer thickness fluctuations in superlattices and quantum wells cause a partial breakdown of crystal-momentum conservation in Raman scattering for wave vectors both along (qz) and perpendicular (q||) to the growth direction. In the acoustic-phonon regime this leads to a continuous emission spectrum which reflects features of the foldedphonon density of states. Dispersion gaps at centre and edge of the mini-Brillouin zone as well as internal gaps at anticrossings of longitudinal and transverse phonon dispersions appear as pronounced peaks and dips superimposed on the emission background. We analyse the strength of these features in terms of an in-plane scattering potential and obtain lateral length scales of interface growth islands. From the comparison of crystalmomentum conserving folded-phonon scattering with the continuous emission signal we determine homogeneous and inhomogeneous broadenings of the electronic structure.

Optical Characterization of Quantum Well Structures

A survey of the most common and useful interband optical techniques for characterizing semiconductor quantum well structures is presented. These are divided into three sections; ground state spectroscopy which deals with the transition between the lowest energy state of the conduction band well and the highest energy state of the valence band well; density-of-states spectroscopy which probes higher energy transitions between excited states in the conduction and valence band wells; and other spectroscopies which entails more sophisticated experiments which don’t fit conveniently into the first two categories. Examples are given of each in order to demonstrate the power of optical spectroscopy to elucidate the properties of semiconductor quantum wells.

Versatile approach to the excitonic properties in semiconductor quantum wells

A new approach is described for the calculation of binding energies and oscillator strengths of confined excitons as a function of the quantum well width. The method is well adapted to III–V semiconductors, it provides a direct interpretation of the effective dimensionality of the confined system and shows evidence of greater flexibility than previous work. Results are presented for the GaAs-GaAlAs and InGaAs-InGaAlAs systems.

Excitonic properties in semiconductor quantum wells: numerical calculations and scaling behavior

An approach is described for the calculation of binding energies of confined excitons in III-V semiconductors as functions of the quantum-well width. The method, which avoids the prescription of trial functions, provides direct interpretations of the dimensionality of the confined systems and shows evidence of greater flexibility than has been previously reported. Numerical results are provided concerning four technologically relevant material systems. With the method presented, the evolution of the exciton oscillator strength, as an explicit function of the well width, has been obtained. The analysis of the results suggests the formulation of scaling laws yielding the exciton properties as functions of the well width for all the materials considered. >

Optical properties of quasi-zero-dimensional magneto-excitons

We discuss the evolution of optical properties of semiconductor quantum wells, as the quasi-two-dimensional electronic states are further confined into quasi-zero dimensions by a perpendicular magnetic field. We show that confinement in all three directions strongly modifies both linear and nonlinear optical response. In particular, quasi-zero-dimensionality makes an ensemble of magneto-excitons a unique many-body system, distinct from higher-dimensional excitons and the one-component Coulomb system in the fractional quantum Hall regime or Wigner crystal.