Search

We investigate the properties of a X - trion formed in a polarized nitride heterostructure from an exciton interacting with a two dimensional electron gas (2DEG). We evidence here dramatic changes induced in the excitonic features by moderate to large densities of carriers in such quantum structures. Band filling effects and dielectric constant screening are accounted for in our model which also provides us with the ability to determine the spin-flip exchange energy separating the singlet from the triplet states of such a trion as a function of the 2DEG density.

We have found experimentally that the exciton oscillator strength decreases dramatically with increase of the QW width in a GaN/Al0.07Ga0.93N system. The collapse of the oscillator strength is a manifestation of the polarisation field effect, as confirmed by our variational calculation. We find that only excitons in very thin quantum wells have an oscillator strength exceeding that of the exciton in bulk GaN.

We show that resonant Rayleigh scattering measurements combined with conventional reflectivity measurements can easily allow the determination of the most relevant parameters of nitride-based quantum well excitons. In particular, comparing the Fourier-transformed reflection spectra with calculated time-resolved reflectivities, we have found the exciton oscillator strength to decrease dramatically with increase of the QW width in the GaN/Al 0.07 Ga 0.93 N system. The collapse of the oscillator strength is a manifestation of the polarization field effect, as confirmed by our variational calculation. We find that only excitons in very thin quantum wells have an oscillator strength exceeding that of the exciton in bulk GaN.

We propose the first tentative calculation of exciton binding energies and oscillator strengths in GaAs 1-x N x -GaAs quantum wells with nitrogen contents below x = 0.05. Our model is based on the envelope function approximation and permits us to determine the excitonic properties of GaAs-GaAsN single quantum wells with a variational and self-consistent process for marginal type I or type II potentials due to the very small valence band offset for this combination. We conclude that this is a type I system.

We theoretically study the evolution of exciton binding energies in GaN/AIGaN quantum structures that are photo-pumped. The exciton binding energies are obtained by a variational approach. Dealing with strong intensities of injected carriers in the wells, we solve self-consistently Schrodinger and Poisson equations. Quantum exclusion principle due to the filling of the reciprocal space is included in the model via the proper choice of the exciton trial wavefunction. The excitation intensity leading to the bleaching of the electron-hole interaction is shown to depend strongly on the well width and on the lattice temperature.

The self-consistent procedure of solving both the Schrodinger and Poisson equations for electron and hole wave functions has been combined with the variational calculation of exciton states in strained GaN/AlxGa1−xN quantum wells. The procedure accounted properly for the free-carrier effects on the excitonic wave function, namely, the exciton bleaching and the quantum exclusion effects. It also allowed us to quantify the dependence of the exciton energy and of the oscillator strength on the optical pumping density. The calculation revealed an interesting interplay between the screening of the polarization fields, which leads to the increase of the electron-hole overlap, and the screening of the electron-hole interaction, which affects the exciton Bohr radius. The peculiar nonmonotonic behavior of the exciton binding energy as a function of the density of the electron-hole plasma results from these combined effects.

MBE-grown samples containing several GaN-AlxGa1-xN quantum wells of varying widths are studied by picosecond time-resolved photoluminescence. Extremely strong built-in electric fields are present in such systems, due to piezoelectric and pyroelectric effects. Our results first show that we change the field In the quantum wells by changing the thickness of the separation barriers, as predicted by theory. We calculate the dependence of exciton radiative lifetimes on barrier thickness and compare these predictions with experimental data. We find that the carrier lifetimes are controlled by efficient nonradiative inter-well transfers, rather than by radiative recombination alone. However, the observed barrier-thickness dependence of decay times is inconsistent with a mere tunnel effect. Extrinsic processes, such as scattering by impurities ol composition fluctuations in the barriers are tentatively invoked to explain our results.

Picosecond time-resolved photoluminescence is used to investigate the recombination dynamics of excitons in samples which ail contain four GaN-AlxGa1-xN quantum wells of respective widths of 4, 8, 12 and 16 molecular monolayers, grown by molecular beam epitaxy. The compositions and thicknesses of the barriers have been varied, in order to change the electric fields induced by piezo- and pyro-electric effects. The dependences of experimental decay times with barrier characteristics indicate the presence of efficient inter-well carrier escaping. Calculations of electronic tunneling times versus barrier width show that the present carrier escaping is enhanced by some additional process, such as scattering by impurities or composition fluctuations in the barriers.

We calculate the original properties of excitons in GaN–AlGaN quantum wells by a variational approach in the envelope function formalism. The separation of electrons and holes by huge internal electric fields induces an enhanced dependence of exciton binding energy and oscillator strength on the well width. We demonstrate the necessity to perform excitonic calculations for obtaining, in particular, reliable values of oscillator strengths (thus radiative lifetimes), which are extremely sensitive to the well width.

We combined the self-consistent procedure of solving the Schrodinger and Poisson equations for electron and hole wave functions with the variational calculation of exciton states in strained GaN/AlGaN quantum wells. The procedure accounted properly for the free-carrier effects on the excitonic wave function, namely, bleaching and quantum exclusion effects and allowed to quantify the dependence of the exciton energy and oscillator strength on the optical pumping density. The calculation revealed an interesting interplay between the screening of the polarisation fields which leads to the increase of the electron–hole overlap and the screening of the electron–hole interaction which affects the exciton Bohr radius. Peculiar non-monotonic behaviour of the exciton binding energy as function of the density of electron–hole plasma results from these effects.

Microcalorimetric measurements of small absorption coefficients have been performed on thin GaN–AlGaN quantum wells grown by Reactive Molecular Beam Epitaxy on Al 2 O 3 substrates. In addition to strong absorption at the energy of the GaN buffer and at the energy of the thick AlGaN barrier layers, we could also readily detect transitions associated to the quantum well. These measurements which pave the way to a precise determination of the gap mismatch between the well and the barrier layers are combined with self consistent excitonic and envelope function calculations in the context of a model for the band line-ups which includes the piezoelectric effect.

We analyze the low-temperature photoluminescence decay times, for a series of MBE-grown samples embedding GaN-AlGaN quantum wells. We investigate a variety of configurations in terms of well widths, barrier widths and overall strain states. We find that not only the wells but also the barriers are submitted to large built-in electric fields. In the case of narrow barriers (5 um), these fields favor the nonradiative escape of carriers from narrow wells into wider wells. When all wells have the same width, the field in such narrow barriers allow us to observe the recombination of long-lived “inter-well” excitons at energies close to those of the short-lived “intra-well” excitons. Our results also prove that the energies and the dynamics of excitonic recombinations depend on the parameters of the heterostructures in a complicated way, due to the interplay of piezoelectric and spontaneous polarizations.

We suggest a new mechanism of Bose condensation of excitons in a quantum well with semimagnetic barriers. Attraction of excitons is due to their exchange interaction with semimagnetic environment. The long exciton lifetime needed for formation of a condensed phase is provided due to exchange-induced displacement of the hole into one of the barriers leading to formation of a long-living spatially indirect state. The resulted condensed state is a collective exciton magnetic polaron.

We report here on the study of the theoretical optical properties of Ga based nitrides systems. The dipolar matrix elements and hole effective masses are presented as a function of the reciprocal wavevector for strained bulk GaN. We compute then the A, B and C exciton binding energies for strained GaN/Al0.2Ga0.8N quantum structures and present the calculated optical density for a 50 A wide well.

International audience; We report here on the study of the optical properties of ZnCdSe-ZnSe quantum structures elaborated by metal-organic chemical vapour deposition on GaAs substrates. The band structure of these samples is experimentally investigated by means of photoluminescence and photoreflectance spectroscopy. This makes clear the relationship between strain, carrier confinement and excitonic effects. We have computed for the first time heavy- and light-hole excitons associated with ground and excited states sub-bands, in the context of a self-consistent two-parameter trial function.

We report on the growth of ZnS-ZnSe superlattices by MOVPE. These superlattices were characterized by optical spectroscopy. We have calculated the exciton binding energy in ZnSe-ZnS quantum wells in the context of the variational approach using models of varying degree of sophistication.

We address the formalism adapted to calculate the exciton binding energy in type I and type II quantum wells with one marginal potential, using a two parameter trial function and the correction of marginal potential by the self-induced exciton polarisation field. The calculation can be applied to an ecclectical series of semiconductor heterostructures. As an illustration of this versatility, we first give the results we obtain for the type II light-hole exciton in (Ga, InAs)GaAs, with a marginal light-hole potential. Next we address the ZnSeZnS case (marginal conduction potential) and type I heavy-hole and light-hole excitons. The third application we present here concerns the type I light-hole exciton in (Zn, Cd)SeZnSe. The improvement of the calculation due to utilisation of the two-parameter trial function is discussed.

We present a detailed examination of the optical properties and electronic structure taken from photoreflectance and photoluminescence data collected on a series of short-period ZnS-ZnSe superlattices grown by low pressure metalorganic vapor phase epitaxy. We studied the band offset problem and calculated the exciton binding energy using several variational models. The temperature dependence of the photoluminescence properties of these superlattices was analyzed in the context of a model which includes the influence of the interfacial disorder.

We show theoretically that the longitudinal–transverse splitting of exciton–polaritons in semiconductor microcavities induces precession of polariton pseudospins. This precession manifests itself in pronounced oscillations of the polarization degree of time-resolved photoluminescence of microcavities. This effect combined with spin-dependent stimulated scattering of exciton–polaritons is responsible for the unusual polarization dynamics of emission of microcavities, recently reported [M. D. Martin et al., Phys. Rev. Lett 89, 077402 (2002)].

We have performed a self consistent calculation for both the heavy-hole and light-hole excitons in (Ga, In)As-GaAs quantum wells. The self-consistent light-hole wave function has been found to peak either in the alloy layer or in the GaAs layer, depending both on the indium content and thickness of the (Ga, In)As layer. We have calculated the average light-hole and electron positions and have found that both type I and type II excitons can be obtained from a type II band to band profile with a marginal light-hole potential in agreement with spin orientation measurements.

We have performed a variational calculation of the heavy-hole exciton for (Ga,In)As-GaAs strained-layer quantum wells embedded in p-i-n structures. The calculation has been made for sample growth along both the (001) and (111) directions. We show that the influence of the piezoelectric field may sometimes lead to strong orientation-dependent properties. In particular, when piezoelectric fields are present, the radiative lifetimes of heavy-hole excitons are strongly dependent on the thickness of the (Ga,In)As layer

Exciton properties have been calculated for polarized GaN/AlGaN quantum well (QW) as a function of the injection intensity for various temperatures. A self-consistent process is performed to solve Schrodinger and Poisson equations, and a trial function that takes into account quantum exclusion principle in the filling of the reciprocal space is chosen for the exciton. When carriers are injected in the quantum structure, three different regimes are evidenced before the complete bleaching of the electron–hole interaction at large excitation intensities. We show that, for a given structure, the critical injection intensity of carriers at which this exciton starts to dissociate due to kinetic effects increases with the temperature of the lattice.

We describe the internal structure of acceptor-bound excitons in wurtzite semiconductors. Our approach consists in first constructing, in the context of angular momentum algebra, the wave functions of the two-hole system that fulfill Paulis exclusions principle. Second, we construct the acceptor-bound exciton states by adding the electron states in a similar manner that two-hole states are constructed. We discuss the optical selection rules for the acceptor-bound exciton recombination. Finally, we compare our theory with experimental data for CdS and GaN. In the specific case of CdS for which much experimental information is available, we demonstrate that, compared with cubic semiconductors, the sign of the short-range hole-exchange interaction is reversed and more than one order of magnitude larger. The whole set of data is interpreted in the context of a large value of the short-range hole-exchange interaction Xi(0)=3.4 +/- 0.2 meV. This value dictates the splitting between the ground-state line I-1 and the other transitions. The values we find for the electron-hole spin-exchange interaction and of the crystal-field splitting of the two-hole state are, respectively, -0.4 +/- 0.1 and 0.2 +/- 0.1 meV. In the case of GaN, the experimental data for the acceptor-bound excitons in the case of Mg and Zn acceptors, show more than one bound-exciton line. We discuss a possible assignment of these states. Original Publication:Bernard Gil, Pierre Bigenwald, Plamen Paskov and Bo Monemar, Internal structure of acceptor-bound excitons in wide-band-gap wurtzite semiconductors, 2010, PHYSICAL REVIEW B, (81), 8, 085211.http://dx.doi.org/10.1103/PhysRevB.81.085211Copyright: American Physical Societyhttp://www.aps.org/

We calculate the fine structure splitting of the near band edge donor-bound excitons in major cubic semiconductors using an approach inspired by an earlier one that consists in replacing the Morse potential by a Kratzer one, in order to account for the repulsion between the donor and the hole. A regular trend is observed when plotting the computed results in terms of donor binding energies for all these semiconductors. Second, we extend the method to wurtzite semiconductors, namely CdS, GaN, and ZnO. The previously reported trend is found again, but enriched with the strong anisotropy of the dispersion relations in the valence band of these semiconductors. We end up in addressing a quantitative interpretation of the fine structure splitting of the donor bound exciton complex which includes the jj coupling between the valence band Bloch and the envelope nonrigid rotator hole states. © 2007 The American Physical Society.

We present the theory of spin relaxation of exciton polaritons treated as a gas of weakly interacting bosons. The model is based on the spin density matrix approach in the Born-Markov approximation. In its framework we have described the spin and energy relaxation of exciton-polaritons in semiconductor microcavities accounting for polariton-polariton and polariton-acoustic phonon scattering. We include various mechanisms of spin relaxation linked with the fine structure of the polaritons. The kinetic equation for time-dependent intensity and polarization of light emitted by the microcavities both at resonant and nonresonant pumping is obtained. The parametric amplification regime, in which the resonant polariton-polariton scattering plays the major role, is specifically analyzed. We show that the polarization plane of the emitted light can rotate as a function of the polarization degree of the pumping light which is a manifestation of the ``spin-optronic'' effect of self-induced Larmor precession of polariton pseudospins.

Photoluminescence of GaN/AlGaN quantum well structures was studied under high intensity of excitation. The blue shift of photoluminescence peak energy was observed when excitation intensity increased. The blue shift was most prominent for the wide wells, becoming smaller with decreasing the well width. We attribute the observed effect to the screening of the built-in electric field by photoexcited carriers, which leads to reducing of the initial red shift caused by the quantum confined Stark effect. The variation of exciton binding energy with carrier concentration also makes the contribution to the blue shift. The theoretical calculations of the blue shift were performed and compared with experimental data.

Exciton reflectivity from GaN/AlxGa1-xN quantum wells (QWs) shows broad peaks that are difficult to analyze within a conventional single-free-exciton model. We have applied a new formalism that allows us to separate numerically radiative and inhomogeneous broadenings of an exciton resonance comparing the Fourier-transformed reflection spectra with calculated time-resolved reflectivities. We have found the exciton oscillator strength to decrease dramatically with the increase of the QW width in GaN/Al0.07Ga0.93N system. The collapse of the oscillator strength is a manifestation of the polarization field effect, as confirmed by our variational calculation. We find that only excitons in very thin quantum wells have an oscillator strength exceeding that of the exciton in bulk GaN.

We model the variation of the exciton binding energy and of the oscillator strength versus temperature in strained GaN/AlxGa1−xN quantum wells by using a self-consistent variational procedure. In addition, this method is extended to the case of high photoinjection conditions. We thus can properly account for the effect of a dense electron-hole plasma on the excitonic wave function, and we can quantitatively address the exciton bleaching phenomenon via quantum exclusion effects. A surprising behavior has been found: the robustness of the exciton to screening by the dense plasma increases with increasing temperature. In other words, the pumping intensity necessary to tear apart electrons and holes increases with increasing temperature. This is quantitatively interpreted in terms of the quantum exclusion effect as a straightforward result of the Pauli principle and of the fundamental prescriptions of quantum mechanics. The limitations imposed by this effect on the excitonic wave functions are relaxed with increasing temperature.

Very strong coefficients for spontaneous and piezoelectric polarizations have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry. Such polarizations influence significantly the mechanisms of radiative emissions in quantum-confinement heterostructures based on these materials. The photoluminescence decay time of excitons is used as a probe of internal electric fields in GaN-(Ga, Al)N quantum wells in various configurations of strain, well widths, and barrier widths. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN wells but also on the nonradiative escape of carriers through Ga1 - xAlyN barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the held in the wells is not a simple function of the strain of these layers via the only piezoelectric effect, but rather the result of the interplay of spontaneous and piezoelectric polarizations in both well and barrier materials. [S0163-1829(99)02923-9].

The 2-K recombination dynamics of coupled GaN-AlGaN multiple quantum wells reveals a composite nature in terms of the joint contributions of short-lived direct excitons and long-lived oblique ones. The possibility to observe these oblique excitons, which produce a unusual blueshift of the photoluminescence at long decay times is found to be in straightforward correlation with the presence of internal electric fields together with the existence of a slight disorder at the monolayer scale. [S0163-1829(99)09815-X].

We perform a careful study of the combined influence of the quantum well width and light polarization on the optical properties of strained GaN–AlGaN quantum wells oriented along the [0 0 0 1] direction. This is made in the context of a six-band envelope function approach together with utilization of the most recent experimental values of crystal field splitting, spin–orbit interaction and deformation potentials. We calculate the holes confinement, exciton binding energies and absorption spectra. We show that under σ polarization conditions, for chosen quantum wells, the only noticeable features can be attributed to enhanced A and B excitons; the C exciton process cannot be stimulated due to inter-valence band mixings.

This paper addresses the growth of short period ZnS-ZnSe superlattices by low pressure Metal-Organic Vapor Phase Epitaxy. We have correlated the photoluminescence line shape to the interface roughness within the context of a phenomenological interface disorder. Using additional reflectivity experiments we could develop envelope function calculation and find the band offset. Finally we have calculated the exciton binding energy in ZnSe-ZnS quantum wells in the context a the variational approach using models of varying sophistication.

We present a detailed study of the optical properties of short-period ZnS-ZnSe strained-layer superlattices. These superlattices have been grown by metalorganic vapor-phase epitaxy. We show that the photoluminescence exhibits a low-energy tail due to localization of the exciton to interfacial potential fluctuations. A detailed analysis of the electronic structure has been performed using the envelope-function approach to obtain the valence-band and conduction-band envelope functions and band lineups. This was completed by a self-consistent calculation of the exciton binding energies. In this calculation the marginal conduction-band offset deduced from the standard envelope-function calculation is corrected by the electrostatic deformation produced by the presence of a localized hole wave function.

We study the optical properties of metalorganic vapour phase epitaxy (MOVPE) grown (Ga,In)As-GaAs single quantum wells as a function of the growth parameters. Our objective is to study the properties of the type II light-hole excitons with light-hole wave function delocalized in the thick GaAs layers and electrons confined in the ternary alloy. However, marked structures (like for type I excitons) are observed in the reflectivity spectra. This we interpret in the context of a novel approach of the exciton problem via a self-consistent calculation of the electron-hole interaction.

We compare the binding energy of interacting electron and hole pairs in double quantum wells with and without internal piezo-electric fields. We show that the exciton binding is less sensitive to the piezo electric field than the oscillator strength. This allows many body-effects and bandgap renormalization to be easily produced in strained-layer quantum wells with internal built-in piezo-electric fields, under photo excitation. Our observation was made at low temperature by comparing the behaviour of Ga 0.92 In 0.08 gAs-GaAs strained layer single and double quantum wells grown along the (001) and (111) directions when the densities of photoinjected carriers are tuned over several decades. Comparison between experimental data and the results of a Hartree calculation including the space charge effects reveals that manybody interactions are efficiently photo-induced in the (111)-grown samples. Moreover, tunnelling of the two first excited heavy-hole levels can be stimulated for moderate carrier densities making such structure promissive for realising self electrooptic effect device (SEED) modulators

We propose a mechanism of electronic control of the polarization of the light emitted by polariton lasers. An electric field applied along the growth axis of the microcavity splits the ground polariton state into [110] and [1–1¯0] polarized components. We perform kinetic simulations which show that above a pumping threshold, a condensate of exciton polaritons is formed at the lowest-energy state. In this regime, the emission of the polariton laser is either [110] or [1–1¯0] linearly polarized depending on the direction of the applied field.

We examine the influence of strain fields in ZnO epilayers. We show that a reversal of the nature of the fundamental valence band can be observed similarily to what was reported in GaN epilayers.

A mechanism for exciton Bose condensation is suggested. We show that the interaction of localized exciton magnetic polarons (EMP's) can lead to the formation of a free collective exciton magnetic polaron (CEMP) able to move as a whole particle. Critical conditions for this transition and characteristics of the CEMP are found. We interpret the recent data on giant magnetic polaron mobility in ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{M}\mathrm{n}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ quantum wells [G. A. Balchin, C. D. Poweleit, and L. M. Smith, in Proceedings of the 23rd International Conference on the Physics of Semiconductors, edited by M. Scheffler and R. Zimmermann (World Scientific, Singapore 1996), p. 2055] as a manifestation of the free CEMP formation due to clusterization of long-living indirect EMP's.