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We present a methodology to quantify polarization and electron affinity changes at interfaces by combining scanning tunneling spectroscopy, off-axis electron holography in transmission electron microscopy (TEM), and self-consistent calculations of the electrostatic potential and electron phase change. We use a precisely known grown-in doping structure to calibrate the surface potential of the TEM lamella and thereby achieve a quantitative analysis of electron phase changes measured by off-axis electron holography. Using this calibration, we deduce quantitatively polarization and electron affinity changes for ${\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}/\mathrm{GaN}$ and ${\mathrm{In}}_{\text{0.05}}{\mathrm{Ga}}_{\text{0.95}}\mathrm{N}/{\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}$ interfaces. The latter interface reveals, as expected, biaxial relaxation as well as polarization and electron affinity changes. However, at the ${\mathrm{Al}}_{\text{0.06}}{\mathrm{Ga}}_{\text{0.94}}\mathrm{N}/\mathrm{GaN}$ interface anomalous lattice relaxations and vanishing polarization and electron affinity changes occur, whose underlying physical origin is anticipated to be total energy minimization by the minimization of Coulomb interactions between the polarization-induced interface charges.

International audience; Polariton lasers are mostly based on planar cavities. Here, we focus on an alternative configuration with slab waveguide modes strongly coupled to excitons confined in GaN/AlGaN quantum wells. We study experimentally and theoretically polariton relaxation at temperatures ranging from 4 to 200 K. We observe a good robustness of the lower polariton population peak energy position against temperature changes due to a balance between the shift of the exciton energy and the change in the normal mode splitting, a promising feature for future applications such as lasers and amplifiers where a small temperature drift in the emission wavelength is a desired asset. Finally, at T = 4 K we observe the signature of polariton nonlinearities occurring in the continuous wave regime that are assigned to an optical parametric oscillation process.

The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.

We present a comprehensive study of the anisotropic optical properties of nonpolar GaN/AlGaN multiple quantum wells intentionally designed to act as an active region of a planar microcavity operating in the strong-coupling regime. The strain induced by the underlying AlGaN-based Bragg reflector leads to a redistribution of exciton oscillator strength as revealed by photoluminescence and reflectivity measurements. Complementary k . p calculations show an excellent agreement with experiments and emphasize the opportunity to tune the nature of the light-matter coupling in a microcavity by means of strain engineering. Finally, the validity of the developed model is proven by angle-resolved photoluminescence studies carried out on the complete microcavity structure. The recorded eigenmode spectra reveal the coexistence of the weak- and the strong-coupling regime along the two orthogonal polarization planes.

By performing pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in a wurtzite GaN/AlGaN quantum well (QW)grown by metal organic vapor phase epitaxy. The circular degree of polarization of the differential reflectivity signal reaches 55-60 % for the heavy-hole exciton when it is resonantly excited before decaying in a few picoseconds. This is an indication that, even if excitons are confined in the growth direction, their propagation in the well plane makes them very sensitive to scattering processes which are responsible for exciton dephasing and, therefore, for efficient spin relaxation. Above 80 K the thermal escape of excitons toward the AlGaN barrier increases the spin-relaxation rate. However, when compared to bulk GaN epilayers, where no spin polarization could be detected above 50 K, the optical orientation is preserved in those QWs up to about 100 K.

A detailed experimental investigation of the phonon-assisted emission in a high-quality c-plane GaN epilayer is presented up to 200 K. By performing photoluminescence and reflectivity measurements, we find important etaloning effects in the phonon-replica spectra, which have to be corrected before addressing the lineshape analysis. Direct experimental evidence for free exciton thermalization is found for the whole temperature range investigated. A close comparison with existing models for phonon replicas originating from a thermalized free exciton distribution shows that the simplified and commonly adopted description of the exciton-phonon interaction with a single excitonic band leads to a large discrepancy with experimental data. Only the consideration of the complex nature of the excitonic band in GaN, including A and B exciton contributions, allows accounting for the temperature dependence of the peak energy, intensity, and lineshape of the phonon replicas.

The pressure-induced changes in the electronic band structures of In-containing nitride alloys, InxGa1-xN and InxAl1-xN are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, dEg/dp, exhibit very large bowing with x, and calculations with simulation of clustered distributions of the In atoms over the cation sites show a strong enhancement of this effect. This relates well to the experimental data obtained from pressure dependent photoluminescence measurements for InxGa1-xN and InxAl1-xN layers, performed in this work, and combined with existing data for InxGa1-xN layers. We discuss possible explanations of the anomalously large magnitude of the dEg/dp bowing in these nitride alloys.

The signature of the strong-coupling regime is unambiguously evidenced in a GaN-based microcavity (MC) above the polariton lasing threshold P-thr at room temperature through the observation of the upper polariton branch. The MC system exhibits a renormalization of the polariton dispersion curve, namely, a reduced normal-mode splitting compared to the low-density regime. Next the dependence of P-thr as a function of exciton-photon detuning is investigated in the 4-340 K temperature range, which allows accessing the polariton lasing phase diagram. The observation of polariton lasing over such a broad range of temperatures reveals a clear transition from a kinetic to a thermodynamic regime with increasing temperature.

We report on the experimental achievement of narrow ultraviolet excitonic and biexcitonic emission from GaN/AlGaN quantum wells. The very large biexciton binding energy (up to 12 meV) inhibits thermalization between the two species allowing to study the recombination kinetics of the biexciton gas. By performing resonant and nonresonant time resolved measurements, we elucidate the interplay between biexciton formation and exciton acoustic-phonon mediated relaxation within localized states.

An AlInN/AlGaN microcavity (MC) containing a GaN/Al0.2Ga0.8N multiple quantum well (MQW) structure is investigated through room temperature (RT) photoluminescence and reflectivity experiments. A vacuum Rabi splitting as large as 50 meV at RT is reported for this MC structure, the highest value reported so far for a semiconductor MC containing QWs. This is shown to result from a geometry combining a state of the art nitride-based MC with a MQW system where the built-in electric field has a reduced impact on the oscillator strength of optical transitions. The contribution of Bragg modes seen in optical spectra is well accounted for by transfer matrix simulations. In addition, the sensitivity of the present system to the tuning between the various optical components of the microcavity (bottom and top Bragg reflectors and active cavity region) to maximize the strong-coupling regime is also shown through simulations. Prospects regarding the nonlinear polariton emission from such a structure indicate that this type of MCs could potentially sustain ultrafast polariton parametric amplification up to 440 K, thanks to an increased exciton binding energy. More generally, it is predicted that, owing to a large exciton saturation density in excess of 1x10(12) cm(-2) per QW, such a MC structure would be suitable for the observation of nonlinear effects associated with cavity polaritons (polariton lasing and polariton Bose-Einstein condensates) at RT and above.

GaN/AlGaN and InGaN/GaN quantum wells (QWs) are investigated as the active region for room-temperature strong exciton-photon coupling in high-quality AlInN/(Al)GaN microcavities (MCs). Angular resolved photoluminescence (PL) measurements performed on an AlInN/AlGaN MC with GaN/AlGaN QWs reveal cavity polariton dispersion curves. A vacuum-field Rabi splitting of 30 meV is observed, from which an exciton oscillator strength of 4.8x10(13) cm(-2) per QW is deduced, a value ten times larger than for InGaAs QWs. In contrast, the PL spectra of an InGaN/GaN QW MC do not exhibit such an anticrossing behavior as should be expected from the strong-coupling regime (SCR). By modeling cavity absorption and PL spectra at the resonance, the conditions, in terms of inhomogeneous excitonic broadening, for the observation of the SCR in these nitride MCs are determined.

We report strong exciton-photon coupling at room temperature in a hybrid high quality bulk 3 lambda/2 GaN cavity with a bottom lattice-matched AlInN/AlGaN distributed Bragg reflector through angle-resolved polarized photoluminescence (PL). Coupling of the optically active free excitons (X-A, X-B, and X-C) to the cavity mode is demonstrated, with their contribution to the PL spectra varying with polarization. Under TE polarization, exciton oscillator strengths for X-A and X-B are about one order of magnitude larger than in bulk GaAs. Photoluminescence exhibits a strong bottleneck effect despite its thermal lineshape.

Angular-dependent emission spectra are investigated in a strongly coupled InGaAs-GaAs-AlAs-based semiconductor microcavity as a function of excitation intensity and of detuning between the uncoupled exciton and photon modes. Under conditions of nonresonant excitation, it is shown that the onset of stimulated emission always occurs in the weak coupling regime. Angular-dependent studies show that the transition to weak coupling occurs when the linewidth of high k excitons becomes of the order of the normal mode splitting of the exciton-polariton coupled modes. We conclude that, under nonresonant excitation, ‘‘polariton lasing,’’ where stimulated polariton scattering followed by photon emission occurs in the strong coupling regime, is only likely to be achieved in systems with larger exciton binding energy than in GaAs-based structures or possibly also in GaAs microcavities containing excess concentrations of free electrons. The experimental results below threshold are found to be in good agreement with numerical solution of Boltzmann kinetic equations for the photoexcited polariton distribution.

The pronounced optical in-plane anisotropy of a suitably designed nonpolar GaN-based microcavity, mainly inherited from the valence-band complexity of wurtzite semiconductors, allows the coexistence of two different light-matter coupling regimes along orthogonal polarization planes. When increasing the excitation power density, a transition to a nonlinear coherent emission is observed under nonresonant optical pumping for both polarization directions at carrier densities remaining well below the exciton-saturation level. Their differing nature is experimentally revealed by means of polarization-resolved Fourier space and near-field imaging. While one polarization direction exhibits all specificities of the strong light-matter coupling regime and features polariton lasing at high injection, the other one lacks a straightforward interpretation due to its complex coupling regime. Possible mechanisms are addressed and evaluated.

The main emission characteristics of electrically driven polariton lasers based on planar GaN microcavities with embedded InGaN quantum wells are studied theoretically. The polariton emission dependence on pump current density is first modeled using a set of semiclassical Boltzmann equations for the exciton polaritons that are coupled to the rate equation describing the electron-hole plasma population. Two experimentally relevant pumping geometries are considered, namely the direct injection of electrons and holes into the strongly coupled microcavity region and intracavity optical pumping via an embedded light-emitting diode. The theoretical framework allows the determination of the minimum threshold current density J(thr),(min) as a function of lattice temperature and exciton-cavity photon detuning for the two pumping schemes. A J(thr, min) value of 5 and 6 A cm(-2) is derived for the direct injection scheme and for the intracavity optical pumping one, respectively, at room temperature at the optimum detuning. Then an approximate quasianalytical model is introduced to derive solutions for both the steady-state and high-speed current modulation. This analysis makes it possible to show that the exciton population, which acts as a reservoir for the stimulated relaxation process, gets clamped once the condensation threshold is crossed, a behavior analogous to what happens in conventional laser diodes with the carrier density above threshold. Finally, the modulation transfer function is calculated for both pumping geometries and the corresponding cutoff frequency is determined.