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International audience; In contrast to GaN-based structures grown along [0001], nonpolar growth allows for the realization of thick quantum wells (QWs) with an optimal overlap between electron and hole wavefunctions. However, nonpolar GaN layers grown on non-lattice-matched substrates exhibit high densities of defects. Then, even at low temperature (T), the carrier recombination is mainly nonradiative.Here, we present the dynamics of charge carriers in AlGaN/GaN QWs of various widths and Al-contents grown on the a-facet of GaN crystals. The QW dislocation density and the exciton diffusion length at 300K are 2.105 cm-2 and 100 nm, respectively. Therefore, dislocations play a minor role in the carrier dynamics at 300K. For all samples, we observe an increasing QW PL lifetime with T, indicating the absence of nonradiative phenomena in the low-T range. Purely radiative recombination is observed up to 240K for a 7 nm thick QW. This suggests the possibility of realizing UV emitters with narrow emission and high internal quantum efficiency at 300K.The observed decrease in QW PL lifetime in the high-T range arises from the thermal escape of carriers from the QW to the AlGaN barriers and their subsequent nonradiative recombination. The efficiency of nonradiative recombination at 300K is thus reduced by limiting the thermal escape of carriers from the QW. This is obtained by growing thick QWs rather than increasing the barrier Al-content, which is important from point of view of defect generation.

International audience; Contrary to GaN-based structures grown along [0001], nonpolar heterostructures allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions. However, the use of lattice-mismatched substrates induces high densities of stacking faults and dislocations. Then, even at low temperature (T), the exciton decay is dominated by the capture on defects [1]. We investigate here the dynamics of excitons in AlGaN/GaN QWs deposited on the a-facet of GaN crystals. We first determine by cathodoluminescence QW dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations do not affect QW excitons at room-temperature [2]. We then study the time-resolved photoluminescence (PL) of excitons in the 10-320 K range for QW samples with various widths and barrier Al-contents. For all samples, the effective lifetime of QW excitons increases with T, indicating the absence of nonradiative phenomena in the low-T range. We observe purely radiative recombination up to 240 K for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [3]. This observation therefore demonstrates the possibility of achieving UV emitters with a rather narrow emission line and a good radiative efficiency at 300 K. In the high-T range, a drop in the QW PL lifetime is accompanied by an increase in the barrier PL lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the equilibrium between excitons and free carriers in the QWs and the barriers, we show that the nonradiative recombination of charge carriers in the AlGaN barriers is the mechanism limiting the PL lifetime of excitons in the QWs [3]. The nonradiative recombinations at 300 K are suppressed by reducing the thermal escape of carriers from the QW. This is achieved by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010). [2] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [3] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).

International audience; Currently, a growing interest is paid to the study of nonpolar nitride-based heterostructures, as they allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions [1]. The growth of wide QWs is indeed a key issue to produce high-power nitride-based optoelectronic devices, as they allow reducing the carrier density in the QW, lessening the efficiency of Auger-like mechanisms. However, non-lattice matched foreign substrates are generally used to grow nonpolar GaN, inducing strain in the heteroepitaxial layers. Even when processing techniques such as epitaxial lateral overgrowth are used, strain relaxation through the generation of dislocations or basal stacking faults leads to a drastic reduction of exciton lifetime [2]. In this work, we therefore investigate the dynamics of excitons in single (Al,Ga)N/GaN QWs deposited directly on the a-facet of GaN crystals. We first extract by cathodoluminescence experiments dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations should not play any significant role in the recombination of excitons at room-temperature [3]. We then study by time-resolved photoluminescence the dynamics of excitons in the 10-320 K range for QW samples with various width and barrier Al-content. We first show that, for all samples, the effective lifetime of QW excitons increases with temperature, evidencing the absence of nonradiative phenomena in the low-temperature range. The largest temperature range of purely radiative recombination (up to 240 K) has been observed for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [4]. This observation therefore evidences the possibility of achieving nonpolar room-temperature UV emitters combining a rather narrow emission line with a good radiative efficiency at 300 K. In the high temperature range, a drop in the QW photoluminescence lifetime is always accompanied by an increase in the barrier emission lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the thermodynamic equilibrium between excitons and free carriers in the QWs and the (Al,Ga)N barriers, we demonstrate that at high temperatures, the nonradiative recombination of charge carriers in the (Al,Ga)N barriers is the mechanism limiting the photoluminescence lifetime of excitons confined in the QWs [4]. We finally propose to tackle the thermal escape of carriers by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Waltereit et al., Nature 406, 865 (2000). [2] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010); T. J. Badcock et al., Appl. Phys. Lett. 93, 101901 (2008). [3] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [4] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).

Best Paper Award.; International audience; Since their prediction in 1996, non-equilibrium polariton condensates have attracted a lot of interest, as they should allow for the realization of ultralow threshold coherent light-emitters [1]. However, the steep dispersion and the short radiative lifetime of lower polaritons (LP) within the light cone hinder their relaxation toward the center of the Brillouin zone, limiting the polariton lasing threshold. Experimental studies carried out on GaAs and CdTe-based microcavities however revealed that one available channel to bypass efficiently the relaxation bottleneck was the relaxation of LPs from the excitonic reservoir directly to the ground state of the LP branch (LPB) through the emission of one LO-phonon [2]. Here, we study by non-resonant angle- and time-resolved photoluminescence experiments the LP relaxation dynamics in a GaN-based multi-quantum well microcavity, for which polariton condensation has been reported between 10 and 340 K [3]. Regarding the strong Fröhlich interaction in GaN, we particularly investigate the role of LO-phonon on the overall LP relaxation mechanism. We first show that LPs efficiently bind into cavity biexcitons and we demonstrate that the latter complexes are in thermal equilibrium with the excitonic reservoir. We then not only demonstrate the existence of a LO-phonon enhanced relaxation channel from the excitonic reservoir toward the ground state, but also the direct feeding of the LPB by the radiative dissociation of cavity biexcitons mediated by one LO-phonon. In other words, when the energy of the bottom of the LPB corresponds to that of the first LO-phonon replica of the cavity biexciton, we observe an enhanced scattering of polaritons toward the k// = 0 state, as well as a decrease in the condensation threshold. This peculiar observation indicates that the LO-phonon assisted dissociation of excitonic molecules constitutes an additional and efficient relaxation channel driving the condensation threshold of polaritons [4]. A. Imamoglŭ et al., Phys. Rev. A 53, 4250 (1996). see for instance F. Boeuf et al., Phys. Rev. B 62, R2279 (2000). J. Levrat et al., Phys. Rev. B 81, 125305 (2010). P. Corfdir et al., submitted to Phys. Rev. B (2012).

International audience; Picosecond and femtosecond spectroscopy allow for a detailed study of carrier dynamics in nanosctructured materials [1]. In such experiments, a laser pulse usually excites several nanostructures at once. However, spectroscopic information may also be acquired using pulses from an electron beam in a modern scanning electron microscope (SEM), exploiting cathodoluminescence (CL) where electrons are promoted from the conduction band to the valence band upon impingement of the high energy electron beam onto a semiconductor. This approach offers several advantages over the usual optical spectroscopy . The multimode imaging capabilities of the SEM enable the correlation of optical properties (via CL) with surface morphology (secondary electron mode) at the nanometer scale [2] and the large energy of the electrons allows the excitation of wide -bandgap materials. Here, we present results obtained with an original time-resolved cathodoluminescence (TRCL) setup [3]. This setup uses ultrafast UV laser pulses to create short photoelectron pulses. The light pulses from an ultrafast UV laser illuminate a metal photocathode from which the electrons are extracted and accelerated inside the high voltage column of the microscope and focused on the sample surface. The collected CL signal is dispersed in a spectrometer and analyzed with an ultrafast STREAK camera to obtain high time resolution. Our current setup reaches combined space and time resolutions of 50 nm and 10 ps, respectively. Measurements can be carried out at temperatures between 25 K and 300 K. We will describe the TRCL setup in detail and will also present results obtained on a-plane GaN and a-plane (Al,Ga)N/GaN quantum wells (QW) [4]. We first study an epitaxial lateral overgrown (ELO) a-plane GaN grown by hydride vapor phase epitaxy on r-plane sapphire that has been studied at 27 K. Large densities of basal stacking faults (BSFs) are usually observed in a-plane GaN. These extended defects can be seen as a type-II QWs and give rise to a broad and intense emission at 3.42 eV (50 meV below the emission energy of the D°X of wurtzite GaN [6]). We evidence that exciton localization and recombination processes are strongly dependent on the local BSF density. In low-BSF-density zones, we show that the diffusion of free excitons towards BSFs is donor assisted. On the other hand, zones with BSF bundles present a totally inhibited D°X emission. The change in BSF-bound exciton luminescence decay time is explained through direct relation to the local BSF density.As a next step, we proceeded to grow a (Al,Ga)N/GaN single QW by molecular beam epitaxy on the GaN sample studied above. Interest has been very high in a-plane GaN since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QWs) [5]. Built-in electric fields are indeed absent in a-plane GaN (non-polar GaN), which allows for the growth of wide QWs without decreasing the radiative recombination probability of electrons and holes. However, even when processing techniques such as epitaxial lateral overgrowth (ELO) are used, non-polar GaN grown on sapphire presents high densities of extended defects. While dislocations are considered as non-radiative recombination centers, basal plane stacking faults (BSFs) are optically active and give rise to an emission centered 50 meV below the excitonic bandgap of GaN [7]. We present a low-temperature TRCL study of exciton dynamics as a function of the local BSF density in a-plane (Al,Ga)N/GaN single QWs grown by molecular beam epitaxy on an ELO-GaN template. First, CL experiments demonstrate the existence of nearly BSF-free regions as well as the existence of regions with BSF bundles. This indicates that the BSF distribution of the underlying a-plane GaN template [8] is reproduced in the QW. We confirm the results obtained by Badcock et al., who demonstrated that the intersection of BSFs with the QW leads to the formation of quantum wires (QWR) [9]. We then study the local relaxation-recombination dynamics of excitons in both QW and QWRs. In particular, we show that the dynamics of QW excitons is dominated by their capture by the BSFs. The QW CL decay time therefore exhibits a strong spatial dependency, explaining the large range of values reported so far for exciton radiative lifetimes in non-polar (Al,Ga)N/GaN QWs [4]. We finally demonstrate that below 60 K, QWR excitons exhibit a zero-dimensional behavior, which we relate to their binding on localization centers such as QWR-width fluctuations. [1] Shah, J. Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, Ch. 8 (Springer, Berlin, 1999).[2] Reimer, L. Scanning Electron Microscopy, Ch. 1 (Springer, Berlin, 1998).[3] M. Merano et al. , Nature 438, 479 (2005).[4] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010). [5] P. Waltereit et al., Nature 406, 865 (2000).[6] P. Corfdir et al., J. Appl. Phys. 105,043102(2009). [7] G. Salviati et al., Phys. Stat. Sol. (a) 171, 325 (1999). [8] P. Corfdir et al.,Appl. Phys. Lett. 94, 201115 (2009). [9] T. J. Badcock et al., Appl. Phys. Lett. 93, 101901 (2008).

International audience

International audience; Contrary to c-plane heterostructures, the absence of built-in electric fields in non-polar nitride-based heterostructures allows for the growth of thick quantum wells (QWs) without any decrease in the overlap between electron and hole wave functions [1]. However, lattice mismatched substrates are generally used to grow non-polar GaN, inducing strain in the heteroepitaxial layers that relax through the generation of non-radiative recombination centers. Here, we investigate by time-resolved photoluminescence the temperature (T) dependence of charge carriers recombination mechanisms in non-polar (Al,Ga)N/GaN quantum wells (QW) grown by molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al-content on the dynamics of excitons in the 10 - 320 K range.

International audience; Microcavity polaritons attract nowadays a lot of interest from the scientific community as they allow for the realization of low-threshold coherent light emitters that operate up to 300 K. It is usually admitted that, at low-temperature and negative cavity detunings, the mechanism limiting the polariton lasing threshold is the relaxation of polaritons towards the bottom of the lower-polariton branch, which is mediated by scattering with acoustic phonons. However, it has been theoretically predicted that in multi quantum well (QW) microcavities, dark exciton and biexciton states should play an important role in the overall relaxation mechanisms of cavity polaritons. Here, we address this issue through a time-resolved photoluminescence (TR-PL) study of the relaxation dynamics at 10 K of microcavity polaritons in a 3!-cavity containing 67 Al0.2Ga0.8N/GaN QWs. The vacuum Rabi splitting of our microcavity is 60 meV and cavity detunings (!) ranging between -100 and 0 meV are investigated. At 10 K, the microcavity PL at small angle is dominated by a transition at 3.63 eV attributed to lower polaritons with a small in-plane wave vector. Two additional non-dispersive lines at 3.673 and 3.693 eV are also observed. From excitation/power-dependent experiments, we attribute them to the recombination of cavity biexcitons and localized dark excitons, respectively. We show by TR-PL that the lower polariton PL at zero-angle exhibits at all times the same PL decay as biexcitons. From this dynamical behavior, together with the !-dependence of the biexciton emission energy, we can describe the interplay between polaritons and biexcitons. In particular, we demonstrate that in nitride-based multi QW microcavities, the mechanism limiting the relaxation of polaritons towards the center of the Brillouin zone is the radiative dissociation of cavity biexcitons into lower polaritons with high in-plane wave vector.

International audience

Microcavity polaritons attract nowadays a lot of interest from the scientific community as they allow for the realization of low-threshold coherent light emitters that operate up to 300 K. It is usually admitted that, at low-temperature and negative cavity detunings, the mechanism limiting the polariton lasing threshold is the relaxation of polaritons towards the bottom of the lower-polariton branch, which is mediated by scattering with acoustic phonons. However, it has been theoretically predicted that in multi quantum well (QW) microcavities, dark exciton and biexciton states should play an important role in the overall relaxation mechanisms of cavity polaritons. Here, we address this issue through a time-resolved photoluminescence (TR-PL) study of the relaxation dynamics at 10 K of microcavity polaritons in a 3!-cavity containing 67 Al0.2Ga0.8N/GaN QWs. The vacuum Rabi splitting of our microcavity is 60 meV and cavity detunings (!) ranging between -100 and 0 meV are investigated. At 10 K, the microcavity PL at small angle is dominated by a transition at 3.63 eV attributed to lower polaritons with a small in-plane wave vector. Two additional non-dispersive lines at 3.673 and 3.693 eV are also observed. From excitation/power-dependent experiments, we attribute them to the recombination of cavity biexcitons and localized dark excitons, respectively. We show by TR-PL that the lower polariton PL at zero-angle exhibits at all times the same PL decay as biexcitons. From this dynamical behavior, together with the !-dependence of the biexciton emission energy, we can describe the interplay between polaritons and biexcitons. In particular, we demonstrate that in nitride-based multi QW microcavities, the mechanism limiting the relaxation of polaritons towards the center of the Brillouin zone is the radiative dissociation of cavity biexcitons into lower polaritons with high in-plane wave vector.

International audience

International audience; In order to achieve efficient doping of GaN layers, both incorporation and activation ratio of dopants must be carefully controlled. In thick polar GaN layers, n-doping is usually obtained by the incorporation of silicon or oxygen atoms. Silicon binding energy in bulk GaN is 30.4 meV, in good agreement with the 28.8 meV calculated in the effective mass approximation. However, impurity ground states in non-flat potential band profiles are strongly modified, as it is the case in heterostructures or in presence of electric fields. In this work, we have thus calculated in the effective potential formalism the energy of an electron confined on a shallow donor nucleus located in any given conduction band-profile. We study particularly the influence on donor binding energy of basal stacking fault (BSFs) in non-polar GaN and of surface in GaN nanocolumns, and then discuss some of their consequences on the electrical and the optical properties of these two systems. We first show that in non-polar GaN, a donor in the vicinity of a BSF can exhibit a binding energy increased up to a factor of 1.8 compared to the 30 meV expected for stacking fault free GaN. Donor activation ratio is thus decreased at room temperature, in agreement with the incomplete donors ionization reported by M. McLaurin et al., J. Appl. Phys. 100, 063707 (2006). We then discuss the consequences of D°-BSF complexes on the emission properties of non-polar GaN. In particular, we propose the statistical distribution of donors in the vicinity of the BSFs planes as origin of both the broad emission from BSF-excitons and intra-BSF exciton localization. In the case nanocolumns, we find that donor binding energy goes from 28.8 meV at the core of the structure to 7.2 meV at its surface. We show that such variation in binding energy is responsible of the broad D°X emission reported for strain-free GaN nanocolumn with high surface-to-volume ratio. We finally suggest that the alteration of D°X wave functions in the vicinity of the nanocolumn surface is responsible of the strong emission at 3.449 eV typically observed in these nanostructures.

International audience; Exciton relaxation and recombination mechanisms in a-plane GaN probed by time-resolved cathodoluminescence.

International audience; Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QW) a lot of work has been carried out on non-polar nitride-based heterostructures. The current interest in non-polar GaN is indeed motivated by the absence of built-in electric fields, allowing the growth of wide QWs without any decrease in the overlap between electron and hole wave functions. However, the use of non lattice-matched substrates induces strain that relaxes through the generation of dislocations or stacking faults. The dynamics of QW excitons is then dominated by their trapping on these extended defects. In this work, we have adopted a different approach consisting in the homoepitaxial growth of a 4 nm-thick non-polar Al0.06Ga0.94N/GaN single QW. We first show that the resulting heterostructure is free of stacking fault and presents a low dislocation-density that we estimate to 2.105 cm-2. From 10 to 150 K, the QW effective decay time increases from 110 to 350 ps, evidencing the absence of non-radiative recombination channel in this temperature range. The temperature dependence of exciton decay time is instead ascribed to the combined delocalization of excitons bound to interface roughness and to the thermal distribution of free excitons in k-space. From these results, we deduce an oscillator strength of 2.1013 / cm2 for QW free excitons, in agreement with values reported for thin polar (Al,Ga)N/GaN MQWs where the influence of electric fields is negligible.

International audience; Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QWs) a lot of work has been carried out on non-polar nitride-based heterostructures. The current interest in non-polar GaN is indeed motivated by the absence of built-in electric fields, allowing the growth of wide QWs without any decrease in the overlap between electron and hole wave functions. However, even when using processing techniques such as epitaxial lateral overgrowth (ELO), non-polar GaN grown on sapphire presents high densities of extended defects. While dislocations are considered as non-radiative recombination centers, basal plane stacking faults (BSFs) are optically active and give rise to an emission centered at 3.42 eV. We present here a low-temperature time-resolved (TR) cathodoluminescence (CL) study of exciton dynamics as a function of the local BSF density in a-plane (Al,Ga)N/GaN single QWs grown by molecular beam epitaxy on a ELO-GaN template. Our TR-CL set-up has spatial and temporal resolutions of 50 nm and 50 ps, respectively, and experiments can be carried out between 25 and 300 K. We first demonstrate through CL experiments the existence of regions nearly free of any BSF as well as of regions with bundles of BSFs, indicating that the BSF distribution in the QW plane reproduces well the one of the underlying a-plane GaN template. We confirm the results obtained by Badcock et al., who demonstrated that the intersection of BSFs with the QW leads to the formation of QWRs, and we then study the local relaxation-recombination dynamics of excitons in both QW and QWRs. We show, in particular, that the dynamics of QW excitons is dominated by their capture by the BSFs and therefore exhibits a strong position dependency, explaining the large range of values reported so far for exciton radiative lifetimes in non-polar (Al,Ga)N/GaN QWs. We finally demonstrate that below 60 K, QWR excitons exhibit a zero-dimensional behavior, which we relate to their binding on localization centers such as QW-width fluctuations.

International audience; Low-temperature picosecond time-resolved cathodoluminescence study of localization in a-plane GaN.

International audience; Optical properties of V-pits in GaN through temperature-dependent picosecond Time-Resolved Cathodolminescence (pTRCL)

International audience; Time-resolved cathodoluminescence at 27 K has been performed on a-plane GaN grown by epitaxial lateral overgrowth. We detail the relaxation and recombination mechanisms of excitons [free or bound to neutral donors, or bound to I1-type basal stacking faults (BSFs)] in relation to the local density in BSFs. We describe the slow exciton capture rate on isolated BSFs by a diffusion model involving donors via a hopping process. Where BSFs are organized into bundles, we relate the shorter rise time to intra-BSF localization processes and the multiexponential decay to the type-II band alignment of BSFs in wurtzite GaN.

International audience; Picosecond time-resolved cathodoluminescence to study GaN based materials

oral

poster

Microcavity comprising two reflectors, at least one semiconductor layer separating said reflectors and a semiconductor quantum well wherein at least one of said reflectors and of said at least one semiconductor layer comprises a structure which is adjusted to localize a polariton in said microcavity.

Optical parametric oscillator including a semiconductor microcavity being configured to spatially localize polaritons of at least three quantized polariton energy levels to effect an optical parametric oscillation.