Search

We calculate spontaneous and piezoelectric polarization fields in wurtzite crystals via the utilization of crystallography considerations, and elasticity theory. As an application of it, we derive equations appropriate to GaN–Ga 1− x In x N quantum wells, heterostructures that have impact in realizing blue light emitting solid state devices, grown on ( h k l ) planes. We show that some specific crystal orientations lead to a cancelation of the Quantum Confined Stark Effect in such heterostructures, whilst other orientations lead to a reduction of the density of elastic energy stored in the Ga 1− x In x N layers. This imposes a compromise for device designers, if it is to improve the light emission efficiency of the related devices.

We analyse the optical properties of GaN homoepitaxies grown on semipolar ( 11 2 n )- and ( 10 1 p )- orientated GaN substrates. We find the optical anisotropy of the GaN films to be strictly ruled by the angle between the growth plane and the 〈0001〉 direction of the crystal. This anisotropy offers a wide range of applications in the domain of surface emitting devices like VCELs and cavity polariton lasers based on GaN but also for similar structures relying on ZnO-related materials.

We have grown (11-22)-oriented GaN/Al0.5Ga0.5N quantum wells (QWs) using molecular beam epitaxy on GaN (11-22)-oriented templates grown by metal-organic vapor phase epitaxy on m-plane oriented sapphire substrates. The performance of epitaxial growth of GaN/Al0.5Ga0.5N heterostructures on the semi-polar orientation (11-22) in terms of surface roughness and structural properties, i.e., strain relaxation mechanisms is discussed. In addition, high resolution transmission electron microscopy reveals very smooth QW interfaces. The photoluminescence of such samples are strictly originating from radiative recombination of free excitons for temperatures above 100 K. At high temperature, the population of localized excitons, moderately trapped (5 meV) at low temperature, is negligible.

We derive a kinetic equation for relaxation of polaritons in a microcavity in the strong coupling regime. We show that, not making the usual Born approximation between ground and excited states, second order coherence builds up in the ground state once enough polaritons accumulate here. This comes from particles conservation, without polariton–polariton interactions.

Cavity exciton–polaritons are considered to be two-dimensional weakly interacting true bosons. We analyse their thermodynamic properties and show that they can exhibit local condensation or Kosterlitz–Thouless phase transition towards superfluidity, so that polariton lasing can be achieved. The dynamical evolution of the condensate in a non-resonantly pumped cavity is described by a quantum kinetic formalism. The distribution function of polaritons is described by a semi-classical Boltzmann equation. A master equation for the ground-state density matrix is derived in the framework of the Born–Markov approximation. The dynamics of the ground-state population and its coherence are deduced.

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.

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.

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.

Theoretical studies of the excitonic properties for different designs of MgS-ZnSe structures are presented. Excitonic binding energies are calculated for single quantum wells. For MgS-ZnSe superlattices, the energy of electron-hole transitions are evaluated and compared with the experimental photoluminescence spectra.

We have investigated theoretically the optical properties of strained (001) ZnSe-(Zn, Cd)Se single quantum wells with a low cadmium content (~10%). Due to the band offset and strain energy, the electron-to-light-hole transitions are marginally type I. We compute the dispersion relations and joint density of states for a typical structure away from Λ and show that the degeneracy can occur between lh 1 and hh 3 sub-bands in a large area of the Brillouin zone. This can cancel the selection rules for interband processes.

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.

The successful growth of MgS epitaxial layers with the MOVPE technique is reported. The samples were grown on GaAs substrates in a classical horizontal reactor, using bis(methylcyclopentadienyl) magnesium and H 2 S as precursors. The zincblende structure of MgS layers is evidenced by careful X-ray diffraction analysis. The lattice constant is found to be about 5.66 A. The influence of the growth temperature on the morphology and quality of the layers is studied in detail.

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.

The properties of single and double (Ga,ln)As-GaAs strained-layer quantum wells embedded in (pin) diodes are studied. These properties are found to be orientation-dependent, mainly due to the existence of a strong internal piezoelectric field in the (Ga,ln)As layers when the growth axis is polar. We first calculate how large the influences of the (pin) and piezoelectric field are to produce carrier tunnelling out of the active part of the heterostructure. This enables us to compute the carrier’s lifetime in the heterostructures and the corresponding resonance widths. Next, we compare the binding energies of interacting electron and hole pairs in double quantum wells with or without internal piezo electric fields. We show that the exciton binding energy is less sensitive to the piezoelectric field than the oscillator strength. Under photo excitation, many body-effects and bandgap renormalization can be easily produced in strained-layer quantum wells with internal built-in piezo-electric fields. We illuminated at low temperature single and double Ga 0.92 ln 0.08 As-GaAs strained layer quantum wells grown either along the (001) or (111) direction, and tuned over several decades the densities of photo-injected carriers. The comparison between experimental data and the results of a Hartree calculation including the space charge effects reveals that many body interactions are efficiently photo-induced in the (111)-grown samples. Moreover, we show that the tunnelling of the two lowest-lying heavy-hole levels can be stimulated for moderate carrier densities making such structures promissive in order to realise self electrooptic effect device (SEED) modulators.

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 show that manybody-effects and bandgap renormalization can 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 As-GaAs strained layer single and double quantum wells grown along the (001) and (111) directions when the densities of photoinjected carriers is 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, in the case of double quantum wells, additional transitions appear in the photoluminescence spectra, due to the tunnelling of the two first excited heavy-hole levels for moderate densities

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 evaluate the properties of nitride heterostructures on semipolar substrates using two indicators: the cancellation of the Quantum Confined Stark Effect and the volume density of elastic energy stored in the strained layers of GaN-GaInN heterostructures lattice-matched to (hkl)-oriented GaN semipolar substrates. These two parameters indicate that the growth on specific orientations is a plus when compared to growth on (0001) polar substrates. We will show that, unfortunately, one cannot simultaneously minimize the stored elastic energy and cancel the Quantum Confined Stark Effect, but it is possible to significantly reduce both of them. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

We present the quantum theory of momentum and spin relaxation of exciton-polaritons in microcavities. We show that giant longitudinal-transverse splitting of the polaritons mixes their spin states, which results in beats between right- and left-circularly polarized photoluminescence of microcavities, as was recently experimentally observed [M. D. Martin et al., Phys. Rev. Lett. 89, 077402 (2002)]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.

Polariton lasers [Phys. Rev. A 53, 4250 (1996), Phys. Rev. A 62, 063809 (2000)] are microcavities–based devices which favour the formation of a Bose condensate of polaritons in a single mode, the ground state, with properties known to be efficient for condensation up to room temperature. The spontaneous disintegration of the condensate results in emission of light expected to display all features of a laser light. We apply a well known formalism of quantum optics suitable to deal with fields exhibiting coherence. We do not delve much in numerical computations and concentrate here on a linear approximation, although we conclude with general remarks linking coherence to nonlinearity.

GaN/AlGaN quantum wells (QWs) of dominant N polarity with inversion domains (IDs), grown by molecular-beam epitaxy, have been studied. Two-band photoluminescence (PL), with the lower-energy band an ...

We motivate the relevance of simultaneous spin-flip processes between two polariton condensates distinguished by differing spin values (′1). We find that associated coherence parameters have nontrivial dynamics, displaying oscillations with a frequency depending on the interaction strength. Interestingly, no beats of coherence between the two states take place, with both order parameters decreasing and increasing in phase. We show how intensity of light emitted by the system in any linear polarization exhibit oscillations directly linked to oscillations of the order parameters, thus allowing for experimental test of the predicted effect.

Graded index separate confinement heterostructures (GRIN-SCH) for microgun blue-green laser were grown by MOVPE. A first study was carried out using selenium hydride, dimethyl cadmium and two dimethylzine adducts as Se, Cd and Zn precursor respectively. This combination leads to intense prereactions and poor layer morphology. To limit this problem we have then used the tetrahydrothiophene:dimethylcadmium adduct. We have first investigated the quality of thick ZnCdSe layers grown using this adduct. GRIN-SCH structures grown using the two cadmium metalorganics were studied using photoluminescence and optical pumping experiments. Stimulated emission was observed for the two kind of samples.

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.