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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).

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; 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

International audience

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; 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.

poster

We study wurtzite GaN/AIN quantum dots (QDs) by time-resolved photoluminescence. The properties of nitride based nano-objects are significantly affected by strong built-in electric fields existing in this crystalline phase. These fields induce a spatial separation of electrons and holes in the quantum dot, a Stark shift of the transition energy, and they slow their radiative recombination down to tens of microseconds of decay time for the largest quantum dots. Such long decay times are responsible for the screening of internal fields by using moderate excitation intensities, thus modifying drastically both the energy and the rate of photon emission. Consequently it is difficult to perform spectroscopic studies of these QDs in the regime where a single electron-hole (e-h) pair is present at the most in each dot. Using a pulsed laser excitation, we study the time-resolved dynamics of carrier recombinations, waiting for complete recombination of all electron-hole pairs in the dots. We clearly exhibit the conditions where no screening of the internal electric field occurs. We especially obtain the transition energy and the mono-exponential decay, i.e the radiative recombination time, of the last e-h pair present in the dot, for a wide range of QD sizes. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0 +/- 0.5 MV/cm.

International audience; Wurtzite GaN∕AlN quantum dots (QDs) are studied by time-resolved photoluminescence. Careful measurements allow us to reach the regime of a single electron-hole pair per dot, evidenced by a monoexponential decay of the luminescence and a stop of the time-dependent shift of the emission energy. The transition energy and the radiative lifetime of the QD ground state are measured in the absence of any electric field screening effect, for a wide range of QD heights. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0±0.5MV∕cm.

The aim of this work was to study and fabricate monolithic white light emitting diodes (LEDs) grown by molecular beam epitaxy (MBE) using NH3 as nitrogen source. The method proposed at the lab consist in inserting in the active zone of the LED some (Ga,In)N/GaN quantum wells emitting in the blue and yellow range. Even if the In content of the InxGa1-xN alloy is limited at 20-25%, the presence of an internal electric field permits to obtain wave lengths in all the visible spectrum. Unfortunately, this electric field decreases the oscillator strength of the quantum well for well width of more than 2 nm. The radiative efficiency of quantum wells emitting in the yellow is thus low compared to a quantum well emitting in the blue. Therefore, the yellow emission is a problem and we have had to try to counterbalance the electric field effects in order to increase the quantum efficiency of thick wells. In an other way, the power of MBE LEDs is lower than the EPVOM ones. One of the differences between the two growth techniques is the growth of p-type GaN. Different parameters have been tested in order to determine the right growth conditions to obtain good optoelectronic quality of p-type GaN layer. To finish, these different refinings will be checked by elaborating LEDs and in particular white monolithic LEDs with large spectra.; Ce travail concerne la croissance par épitaxie sous jets moléculaires avec NH3 comme source d'azote de diodes électroluminescentes (DELs) blanches monolithiques. La méthode proposée au laboratoire consiste à insérer dans la zone active de la DEL des puits quantiques (Ga,In)N émettant dans le bleu et le jaune. Bien que la concentration en In de l'alliage InxGa1-xN soit limitée à 20-25%, la présence d'un champ électrique interne dans les hétérostructures nitrures permet d'obtenir des longueurs d'onde balayant tout le spectre visible. Malheureusement, ce champ électrique diminue aussi la force d'oscillateur de puits quantiques de largeur de plus de 2nm. Ainsi, l'efficacité radiative de puits quantiques émettant dans le jaune est faible comparé à un puits quantique émettant dans le bleu. L'émission dans le jaune pose donc problème et il nous a fallu tenté de contrebalancer les effets du champ électrique afin d'accroître le rendement quantique des puits larges. D'autre part, la puissance des DELs EJM est faible par rapport à celles épitaxiées en EPVOM. Une des grandes différences entre ces deux techniques est la croissance du GaN de type p. Différents paramètres ont été testés afin de déterminer les conditions de croissance adéquates à l'épitaxie du GaN de type p de bonne qualité optoélectronique. Finalement, ces différents affinages ont été vérifiés par l'élaboration de DELs et en particulier d'une DEL blanche monolithique à large spectre.

International audience

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via 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. We first show that the effective lifetime of QW excitons s increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing s depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs.