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Dynamics of vertical transport in all-binary GaAs/AlAs short-period superlattices (SPSs) have been investigated as a function of temperature by steady-state and time-resolved photoluminescence (PL) experiments. When two different, enlarged quantum wells are embedded in the SPSs, photoexcited carriers compete to be trapped into various localized exciton states and a variety of exciton radiative recombination dynamics appears accompanied by phonon heating. Redistribution of carrier population in the different wells by phonon scattering is significantly influenced by vertical miniband transport in the SPSs. Three different transport regimes are pointed out: (i) tunnelling transport affected by irreversible carrier cooling into deep localized states, (ii) interplay of carrier relaxation and phonon-assisted activation mediated by an efficient miniband Bloch transport, and (iii) thermalization processes. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the framework of effective mass envelope function theory, the electronic structures of GaAs/Al x Ga1-x As quantum double rings (QDRs) are studied. Our model can be used to calculate the electronic structures of quantum wells, wires, dots, and the single ring. In calculations, the effects due to the different effective masses of electrons and holes in GaAs and Al x Ga1-x As and the valence band mixing are considered. The energy levels of electrons and holes are calculated for different shapes of QDRs. The calculated results are useful in designing and fabricating the interrelated photoelectric devices. The single electron states presented here are useful for the study of the electron correlations and the effects of magnetic fields in QDRs.

The cyclotron spin-flip excitations of a two-dimensional electron system have been investigated by inelastic light scattering (ILS). At small electron filling factors, νnot, vert, similar0.1, cyclotron spin-singlet and spin-triplet excitations have been observed, which are the magnetoplasmon and the three possible spin-flip modes that differ by their spin orientations along the axis of the quantizing magnetic field. At larger electron filling factors, ν>0.2, the electron system is fully spin-polarized. The cyclotron spin-wave (CSW) mode reenters in the ILS spectra at an electron filling factor ν>1. The CSW energy is insensitive to both the experimentally accessible in-plane momenta and the electron concentration, whereas its ILS efficiency is strongly influenced by the spin polarization of the electron system.

In this article we review our recent experimental and theoretical investigations of electronic excitations in GaAs–AlGaAs quantum dots. In the experimental structures we have observed collective spin- (SDE) and charge-density excitations (CDE), and, under conditions of extreme resonance, excitations which can be attributed to nearly unrenormalized single-particle-like transitions (SPE). We compare our experimental results to a theory based on the time-dependent local-density approximation (TDLDA) concerning (i) parity selection rules and (ii) conditions when SPE occur in the spectra. This shows that only a theory which takes into account the resonance enhancements mediated by the valence band states can qualitatively reproduce the experimental findings. To get a deeper understanding of the character of the SPE, we present an intriguing comparison of Kohn–Sham calculations with results of an exact treatment by numerical diagonalization. We demonstrate that even for six electrons per quantum dot, SPE can be expected.

We have grown GaN films and GaN-AlGaN quantum wells (QWs) on homoepitaxial substrates, by molecular beam epitaxy using ammonia. Both the GaN film and the QW are found to have superior excitonic recombination properties which are extremely promising for the development of indium free ultra-violet lasers based on nitrides. (C) 2001 Published by Elsevier Science Ltd.

The dependence of the exciton binding energy on the structural parameters in T-shaped quantum wires is studied numerically, including nonparabolicity and dielectric mismatch. The symmetric structure with pure AlAs barriers was found to give the highest exciton binding energy. Optimized asymmetric T-structures show a binding energy that is nearly constant in a large range of parameters. A large single-particle confinement does not necessarily lead to a large exciton binding energy. A weak correlation is observed between the maximum of the binding energy and the localization of the uncorrelated electron wave function.

We report on the optical properties of m-plane GaInN/GaN quantum wells (QWs). We found that the emission energy of GaInN QWs grown on m-plane SiC is significantly lower than on non-polar bulk GaN, which we attribute to the high density of stacking faults. Temperature and power dependent photoluminescence reveals that the GaInN QWs on SiC have almost as large internal quantum efficiencies as on bulk GaN despite the much higher defect density. Our results indicate that quantum-wire-like features formed by stacking faults intersecting the quantum wells provide a highly efficient light emission completely dominating the optical properties of the structures.

A detailed discussion of the optical properties of Al-rich Al1−x In x N alloy films is presented. The (0 0 0 1)-oriented layers with In contents between x = 0.143 and x = 0.242 were grown by metal-organic vapour phase epitaxy on thick GaN buffers. Sapphire or Si(1 1 1) served as the substrate. High-resolution x-ray diffraction revealed pseudomorphic growth of the nearly lattice-matched alloys; the data analysis yielded the composition as well as the in-plain strain. The complex dielectric function (DF) between 1 and 10 eV was determined from spectroscopic ellipsometry measurements. The sharp onset of the imaginary part of the DF defines the direct absorption edge, while clearly visible features in the high-photon energy range of the DF, attributed to critical points (CPs) of the band structure, indicate promising crystalline quality of the AlInN layers. It is demonstrated that the experimental data can be well reproduced by an analytical DF model. The extracted characteristic transition energies are used to determine the bowing parameters for all CPs of the band structure. In particular, strain and the high exciton binding energies for the Al-rich alloys are taken into account in order to assess the splitting between the valence band with symmetry and the conduction band at the centre of the Brillouin zone. Finally, the compositional dependence of the high-frequency dielectric constants is reported.

The luminescence emission of InGaN/GaN multi-quantum wells (MQW) is affected by several factors, i.e. the composition, the QW thickness, the piezoelectric field and the high density of threading dislocations. In the case of heterostructures containing several QWs piled up, one has to consider the homogeneity of each QW and the thickness of the barrier layers between them, which have influence on the strain. We present herein a Cathodoluminescence (CL) study of a series of InGaN QW structures, paying special emphasis to the problem of the lateral distribution of In, and how it influences the emission properties of the QWs. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate theoretically Raman spectra of few-electron quantum dots. Spectra obtained by an exact many-body treatment and by a time-dependent local-density approximation are compared. We show that single-particle-like excitations can be expected for systems with only six electrons. The energies of these excitations are close to the Kohn-Sham level spacings.

We extend to any temperature, the sophisticated calculation of the evolution of the 2 K photoluminescence energy of InN proposed by Arnaudov et al. [Phys. Rev. B 69, 115216 (2004)], in view of determining the residual doping of thin films. From the detailed line shape modeling, we extract the full width at half maximum of the photoluminescence line which, in the first order, varies like n0.51 at low temperature. This allows us to propose a handy tool for rapid residual doping evaluation. Last, temperature and inhomogeneous broadening effects are analyzed. Ignoring the latter is shown to lead to an overestimation of the residual doping.

We describe semiconductor cavities based on strained ${\mathrm{Ga}}_{0.85}{\mathrm{In}}_{0.15}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ multilayers with permanent built-in piezoelectric fields that confine acoustic phonons in the THz range. The possible role of piezoelectric fields on the phonon lifetimes and on Raman scattering is discussed. Phonon mirrors and cavities grown along [001] (non-piezoelectric) and along [311] (piezoelectric) are studied and compared using high-resolution Raman spectra and photoelastic model calculations. The high quality of the grown [001]([311]) structures is demonstrated by the observation of up to 7(5) orders of folded acoustic phonons and the excellent agreement with theory. We observe a large broadening of the acoustic phonon peaks in the piezoelectric [311] structures upon carrier injection with near-gap excitation. Such acoustic phonon lifetime reduction evidences a strongly modified electron-acoustic phonon interaction in these structures.

International audience; We have performed transmittance and piezomodulated transmittance measurements of as-grown GaAsN layers on GaAs substrates. The absorption shows energy an splitting of the ground state transition and a simultaneous increase of the splitting with the increase of the N content. This indicates the presence of a strain, which lifts the light- and heavy-hole valence band degeneracy. Surprisingly the piezomodulated transmittance shows that the heavy-hole exciton is the ground state! This implies that the GaAsN layers have a lattice parameter larger than that of GaAs and are under compression. The origin of the lattice parameter increase is the incorporation of N atoms on interstitial sites.

A weak Fano-type structure one LO phonon energy above the fundamental exciton in bulk GaAs is investigated via highly sensitive femtosecond transmission spectroscopy. It is identified as the response of an exciton-LO phonon quasiparticle. The low-density dephasing time of this excitation is directly determined to be 300 fs. Temperature and carrier density dependent measurements elucidate the properties of this resonance which is inaccessible in linear spectroscopy.

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 μ eV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.

Voltage-controlled lateral superlattices of various periods are employed to demonstrate the trapping of photogenerated excitons in quasi-one-dimensional regions and to store light in form of ionised excitons in the quantum well of a semiconductor heterostructure. The superlattices are induced by applying spatially alternating external voltages via interdigitated metal gates. Exciton localisation arises from a periodical modulation of the strength of the quantum confined Stark effect in the plane of the quantum well. At large superlattice potential amplitudes the excitons are ionised due to the strong lateral electric fields. The thus spatially separated electrons and holes can be stored efficiently in the structure. Resetting the potential amplitude to zero induces their radiative recombination after very long storage times.

The in-plane optical anisotropy (IPOA) in InAs/GaSb superlattices has been studied by reflectance difference spectroscopy (RDS) at different temperatures ranging from 80 to 300 K. We introduce alternate GaAs- and InSb-like interfaces (IFs), which cause the symmetry reduced from D 2d to C 2v . IPOA has been observed in the (001) plane along [110] and [1 1 ¯ 0] axes. RDS measurement results show strong anisotropy resonance near critical point (CP) energies of InAs and GaSb. The energy positions show red shift and RDS intensity decreases with the increasing temperature. For the superlattice sample with the thicker InSb-like IFs, energy positions show red shift, and the spectra exhibit stronger IPOA. The excitonic effect is clearly observed by RDS at low temperatures. It demonstrates that biaxial strain results in the shift of the CP energies and IPOA is enhanced by the further localization of the carriers in InSb-like IFs. PACS 78.67.Pt, 78.66.Fd, 78.40.Fy