Pierre Bigenwald

Confined Excitons in GaN–AlGaN Quantum Wells

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.

Strain-Fields Effects and Reversal of the Nature of the Fundamental Valence Band of ZnO Epilayers.

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.

Polariton laser: thermodynamics and quantum kinetic theory

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.

Variational treatment of the exciton binding energy problem in low-dimensional systems with one marginal potential

Abstract 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.

A variational calculation of light-hole envelope functions and exciton binding energies in (Ga, In)As-GaAs quantum wells

Abstract 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.

The calculation of semipolar orientations for wurtzitic semiconductor heterostructures: application to nitrides and oxides

We investigate how the template crystal orientation indices (hk.??) can influence the intensity of density of elastic energy and polarization fields in wurtzite nitrides and oxides semiconductor strained layer heterostructures. We propose analytical relations between the angle ?, defined as the direction of the sixfold axis of unstrained material and the direction normal to the growth plane, and (i) the value of the total polarization and (ii) the density of elastic energy stored in the strained layer. We find that quasi-cancellation of quantum confined stark effect (QCSE) can be generally obtained by carefully selecting the (hk.??) set. This situation does not lead to minimal strain density of elastic energy, but the increase of this parameter may be moderate compared to the minimum value. In the case of materials submitted to biaxial tension, we observe that the total density of elastic energy stored shows no minimum except when ? = 0.

Dynamics of Excitons in GaN–AlGaN MQWs with Varying Depths, Thicknesses and Barrier Widths

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.

Electronic control of the polarization of light emitted by polariton lasers

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.

Coherence dynamics in microcavities and polariton lasers

We study theoretically the second-order coherence g(2)(0) of light emitted by polariton lasers, i.e., devices based on stimulated relaxation and condensation of exciton–polaritons in microcavities. We solve kinetic equations for the polaritons in different approximations and show that (i) the coherence introduced into the polariton condensate by an external source can be conserved by the system over a macroscopically long time, and (ii) if the total number of polaritons is fixed by the excitation conditions, the correlations between the populations of the ground and excited polariton states can also result in the spontaneous buildup of second-order coherence in the polariton condensate. Both results are obtained neglecting polariton–polariton interactions in the condensate.

Dual contribution to the stokes shift in InGaN-GaN Quantum wells

By comparing photoluminescence and photo reflectance spectra taken on a series of InGaN-GaN quantum wells grown under identical conditions except the growth time of the InGaN layers, we could monitor the Quantum Confined Stark Effect (QCSE) without changing the nanotexture of the alloy layers. Our results indicate that, for quantum wells which radiate in the red, the contribution of the QCSE superimposes on the intrinsic localization phenomena of the carriers in the InGaN alloy. and is larger by one order of magnitude. Interpretation of data for samples that emit from the blue to the red can provide only partial conclusions if both localization effects and QCSE are not taken into consideration.