Elastic strain engineering is an important way to reversibly tune the properties of micro/nanoscale semiconductors for promising applications in the emerging advanced nanotechnologies, such as the strain enhanced high-mobility transistors, nanogenerators, etc. However, direct observation and control of energetic carrier dynamics with precise strain gradient fields remains a challenge. Here, observation of temperature-dependent exciton funnel dynamics is reported in a precisely controlled strain gradient field by time-resolved photoluminescence. An efficient exciton hopping process is revealed at picosecond time scale as the donor-bound excitons in ZnO microwires funnel along the strain gradient, which strongly depends on the temperature. Combined experiments and simulations unravel that, in addition to the exciton funnel by the strain gradient, both the excitation efficiency and emission lifetime of the excitons increase gradually from the compressive side to the tensile side due to the role of local strain regulation. The results give a clear physical picture of the energetic carrier dynamics in the strain gradient field in semiconductors, which provides a promising paradigm for the design of high-performance optoelectronic devices.

The exciton transport is studied in high quality ZnO microwires using time resolved cathodoluminescence. Owing to the available picosecond temporal and nanometer spatial resolution, a direct estimation of the exciton average speed has been measured. When raising the temperature, a strong decrease of the effective exciton mobility (hopping speed of donor-bound excitons) has been observed in the absence of any remarkable change in the effective lifetime of excitons. Additionally, the exciton hopping speed was observed to be independent of the strain gradient value, revealing the hopping nature of exciton movement. These experimental results are in good agreement with the behavior predicted for impurity-bound excitons in our previously published theoretical model based on Monte-Carlo simulations, suggesting the hopping process as the main transport mechanism of impurity-bound excitons at low temperatures.

Optimizing the electronic structures and carrier dynamics in semiconductors at atomic scale is an essential issue for innovative device applications. Besides the traditional chemical doping and the use of homo/heterostructures, elastic strain has been proposed as a promising possibility. Here, we report on the direct observation of the dynamics of exciton transport in a ZnO microwire under pure elastic bending deformation, by using cathodoluminescence with high temporal, spatial, and energy resolutions. We demonstrate that excitons can be effectively drifted by the strain gradient in inhomogeneous strain fields. Our observations are well reproduced by a drift-diffusion model taking into account the strain gradient and allow us to deduce an exciton mobility of 1400 ± 100 cm(2)/(eV s) in the ZnO wire. These results propose a way to tune the exciton dynamics in semiconductors and imply the possible role of strain gradient in optoelectronic and sensing nano/microdevices.

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton.

We report on the mechanism of hopping for bound excitons under an energy gradient. By means of a Monte-Carlo simulation, we show that this mechanism explains the movement of bound excitons observed experimentally. We show that the speed of the excitons decreases quickly with temperature. Thanks to an effective medium approximation, we deduce an analytical model to estimate the average speed at T = 0 K. Finally, we compare our simulations results to the speed observed in bent ZnO wires and find a good agreement between theory and experiments.

This article presents the dynamics of excitons ina-plane (Al,Ga)N/GaN single quantum wells of various thicknesses grown on bulk GaN substrates. For all quantum well samples, recombination is observed to be predominantly radiative in the low-temperature range. At higher temperatures, the escape of charge carriers from the quantum well to the (Al,Ga)N barriers is accompanied by a reduction in internal quantum efficiency. Based on the temperature-dependence of time-resolved photoluminescence experiments, we also show how the local disorder affects the exciton radiative lifetime at low temperature and the exciton non-radiative lifetime at high temperature.

The nanoindentation fracture behavior of gallium arsenide (GaAs) is examined from two perspectives in two parent papers. In the first paper (part I), we address the morphology of the crack field induced by different types of indenters by means of in situ nanoindentation inside a scanning electron microscope (SEM) and of cleavage cross-sectioning techniques. In the present paper (part II), we investigate the early stage of crack nucleation under wedge nanoindentation through cathodoluminescence and transmission electron microscopy. We find that the apex angle of the wedge indenter influences the dislocation microstructure and, as a consequence, the mechanism of crack nucleation under nanoindentation. The formation of microtwins depends on both the orientation of the indenter with respect to the orientation of the GaAs crystal and on the apex angle of the indenter. For dicing applications of GaAs wafers, it is desirable to have an opening angle of the indenter smaller than 70° to facilitate the formation of precursor cracks.

Despite tremendous progress, the optoelectronic properties of GaN and related compounds still pose a rich field for scientific research. The aim of this session is to bring together leading experts on materials, characterization, and theory in order to discuss the physics of group-III nitride-based heterostructures and various approaches to realize future optoelectronic devices. Open questions are in particular the socalled *green gap* describing lower efficiency in GaN based green light emitters as opposed to their blue and ultraviolet counterparts, polarization of the crystal structure, as well as high-In containing alloys for light emitters in the entire visible range. (Organizers: Bernd Witzigmann, University of Kassel and Frank Bertram, University of Magdeburg)

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).

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 Frohlich 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).

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).

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.

The design of cost-effective standards for the quality of nano-objects is currently a key issue toward their massive use for optoelectronic applications. The observation by photoluminescence of narrow excitonic and biexcitonic emission lines in semiconductor nanowires is usually accepted as evidence for high structural quality. Here, we perform time-resolved cathodoluminescence experiments on isolated ZnO nanobelts grown by chemical vapor deposition. We observe narrow emission lines at low temperature, together with a clear biexciton line. Still, drastic alterations in both the CL intensity and lifetime are observed locally along the nano-object. We attribute these to non-radiative recombinations at edge dislocations, closing basal plane stacking faults, inhomogeneously distributed along the NB length. This leads us to the conclusion that the observation of narrow excitonic and biexcitonic emission lines is far from sufficient to grade the quality of a nano-object.

We present a low-temperature time-resolved cathodoluminescence study on (In, Ga)N/GaN quantum wells grown on the (11-22) facets of non-coalesced ELO-GaN. Taking advantage of the quantum confined Stark effect, such structures have been proposed as promising monolithic white light emitters. Here, we show that in order to achieve solar-like white light emission from this kind of structure, one has to pay attention on the respective evolutions of radiative and non-radiative QW exciton decay times along the (11-22) facets. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim