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Crystallographic point defects (PDs) can dramatically decrease the efficiency of optoelectronic semiconductor devices, many of which are based on quantum well (QW) heterostructures. However, spatially resolving individual non-radiative PDs buried in such QWs has so far not been demonstrated. Here, using high-resolution cathodoluminescence (CL) and a specific sample design, we spatially resolve, image, and analyse non-radiative PDs in InGaN/GaN QWs. We identify two different types of PD by their contrasting behaviour with temperature, and measure their densities from $10^{14}$ cm$^{-3}$ to as high as $10^{16}$ cm$^{-3}$. Our CL images clearly illustrate the interplay between PDs and carrier dynamics in the well: increasing PD concentration severely limits carrier diffusion lengths, while a higher carrier density suppresses the non-radiative behaviour of PDs. The results in this study are readily interpreted directly from CL images, and represent a significant advancement in nanoscale PD analysis., Main text: 8 pages, 6 figures. Supplementary: 11 pages, 8 figures

Ultrafast nonlinear photonics enables a host of applications in advanced on-chip spectroscopy and information processing. These rely on a strong intensity dependent (nonlinear) refractive index capable of modulating optical pulses on sub-picosecond timescales and on length scales suitable for integrated photonics. Currently there is no platform that can provide this for the UV spectral range where broadband spectra generated by nonlinear modulation can pave the way to new on-chip ultrafast (bio-) chemical spectroscopy devices. We demonstrate the giant nonlinearity of UV hybrid light-matter states (exciton-polaritons) up to room temperature in an AlInGaN waveguide. We experimentally measure ultrafast nonlinear spectral broadening of UV pulses in a compact 100 μm long device and deduce a nonlinearity 1000 times that in common UV nonlinear materials and comparable to non-UV polariton devices. Our demonstration promises to underpin a new generation of integrated UV nonlinear light sources for advanced spectroscopy and measurement., Nonlinearity enhancement in different materials is relevant for many scientific applications. Here the authors demonstrate pulse modulation in the UV regime due to polariton-based nonlinearity in an AlInGaN waveguide structure, including at room temperature.

International audience; c-plane GaN/AlN quantum dots (QDs) are promising zero-dimensional quantum nanostructures that exhibit single photon emission properties up to room temperature and even above. In this context, it is of prime interest to gain a deeper insight into the recombination dynamics of photogenerated electron-hole pairs captured by such dots. Hence, in this work, we study the time-resolved photoluminescence (PL) properties in the low injection regime and at cryogenic temperatures of c-plane GaN/AlN QD ensembles emitting above the bulk GaN bandgap in order to properly understand the nature of the recombination channels behind the observed non-exponential decay time profiles. Such decays reveal the existence of a relaxation channel competing with the radiative recombination one. It is thus observed that for the former process the dynamics is independent of the dot height, which is attributed to a reversible nonradiative transfer that could be mediated by a spin-flip process to a dark-level state. The radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, which is well accounted for by the built-in electric field inherent to quantum nanostructures grown along the c-axis and the variations in the lateral confinement at play in such QDs. Those conclusions are drawn from the analysis of the time-evolution of the PL spectra by means of a simple analytical model that enables to exclude any screening of the built-in electric field.

Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. Recent studies in this field have triggered a vivid debate on how to prove and interpret lasing in the high-β regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-β lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power-dependent interplay between zero-dimensional and two-dimensional gain contributions., Here the authors present temperature dependent studies of GaN nanobeam lasers grown on a silicon substrate and demonstrate high-βlasing at room temperature. Comprehensive optical and quantum-optical characterization, complemented by microscopic modeling, of the nanolasers allow identification of lasing behavior.

Since the early 1960s, alloys are commonly grouped into two classes that feature either bound states in the band gap (I) or additional, nondiscrete band states (II). Consequently, one can observe either excitons bound to isoelectronic impurities or the typical band edge emission of a semiconductor that shifts and broadens with rising isoelectronic doping concentration. Microscopic parameters for class I alloys can directly be extracted from photoluminescence (PL) spectra, whereas any conclusions drawn for class II alloys usually remain limited to macroscopic assertions. Nonetheless, we present a spectroscopic study on exciton localization in a mixed-crystal alloy (class II) that allows us to access microscopic alloy parameters. In order to illustrate our approach, we study bulk InxGa1-xN epilayers at the onset of alloying (0

Blue light-emitting diodes based on III-nitride semiconductors are nowadays widely used for solid-state lighting. They exhibit impressive figures of merit like an internal quantum efficiency close to 100%. This value is intriguing when considering the high dislocation density running throughout the InGaN/GaN quantum well (QW) active region. This striking feature is currently ascribed to carrier localization occurring in the InGaN alloy, which hinders their diffusion toward dislocations. However, it was recently reported that another source of defects, disconnected from dislocations, dramatically decreases the radiative efficiency of InGaN/GaN QWs. Those defects, present at the surface, are usually trapped in an InGaN underlayer (UL), which is grown before the QW active region. To get insight into the trapping mechanism, we varied the UL thickness, In content, and materials system (InGaN or InAlN) and studied the photoluminescence decay time at 300 K of a single InGaN/GaN QW. Our data demonstrate that defects are incorporated proportionally to the indium content in the UL. In addition, we show that those defects are created during the high-temperature growth of GaN and that they segregate at the surface even at low-temperature. Eventually, we propose an intrinsic origin for these surface defects. (C) 2018 Author(s).

We demonstrate optically-pumped waveguides grown on freestanding GaN substrate featuring AlInN claddings and GaN/Al0.1Ga0.9N multiple quantum wells exhibiting narrow bandwidth (3.8 nm) optical gain around 370 nm. Due to the high refractive index contrast between the cladding layers and the active region, the confinement factor is as high as 48% and net modal gain values in excess of 80 cm−1 are measured. The results agree well with self-consistent calculations accounting for built-in electric field effects and high carrier density related phenomena. These results open interesting perspectives for the realization of more efficient near-UV lasers and optical amplifiers.

III-nitride surface states are expected to impact high surface-to-volume ratio devices, such as nano- and micro-wire light-emitting diodes, transistors, and photonic integrated circuits. In this work, reversible photoinduced oxygen desorption from III-nitride microdisk resonator surfaces is shown to increase optical attenuation of whispering gallery modes by 100 cm−1 at λ = 450 nm. Comparison of photoinduced oxygen desorption in unintentionally and n+-doped microdisks suggests that the spectral changes originate from the unpinning of the surface Fermi level, likely taking place at etched nonpolar III-nitride sidewalls. An oxygen-rich surface prepared by thermal annealing results in a broadband Q improvement to state-of-the-art values exceeding 1 × 104 at 2.6 eV. Such findings emphasize the importance of optically active surface states and their passivation for future nanoscale III-nitride optoelectronic and photonic devices.

Thin-wall tubes composed of nitride semiconductors (III-N compounds) based on GaN/InAlN multiple quantum wells (MQWs) are fabricated by metalorganic vapor-phase epitaxy in a simple and full III-N approach. The synthesis of such MQW-tubes is based on the growth of N-polar c-axis vertical GaN wires surrounded by a coreshell MQW heterostructure followed by in situ selective etching using controlled H-2/NH3 annealing at 1010 degrees C to remove the inner GaN wire part. After this process, well-defined MQW-based tubes having nonpolar m-plane orientation exhibit UV light near 330 nm up to room temperature, consistent with the emission of GaN/InAlN MQWs. Partially etched tubes reveal a quantum-dotlike signature originating from nanosized GaN residuals present inside the tubes. The possibility to fabricate in a simple way thin-wall III-N tubes composed of an embedded MQW-based active region offering controllable optical emission properties constitutes an important step forward to develop new nitride devices such as emitters, detectors or sensors based on tubelike nanostructures.

The ternary semiconductor alloy Al0.25Ga0.75N has been analyzed by means of correlated photoluminescence spectroscopy and atom probe tomography (APT). We find that the composition measured by APT is strongly dependent on the surface electric field, leading to erroneous measurements of the alloy composition at high field, due to the different evaporation behaviors of Al and Ga atoms. After showing how a biased measurement of the alloy content leads to inaccurate predictions on the optical properties of the material, we develop a correction procedure which yields consistent transition and localization energies for the alloy photoluminescence. (C) 2016 AIP Publishing LLC.

Thanks to its high refractive index contrast, band gap, and polarization mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an attractive solution for applications such as distributed Bragg reflectors, ultraviolet light-emitting diodes, or high electron mobility transistors. In order to study the structural degradation mechanism of InAlN layers with increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN substrates with a low density of threading dislocations, for In compositions of 13.5% (layers under tensile strain), and 19.7% (layers under compressive strain). In both cases, a surface morphology with hillocks is initially observed, followed by the appearance of V-defects. We propose that those hillocks arise due to kinetic roughening, and that V-defects subsequently appear beyond a critical hillock size. It is seen that the critical thickness for the appearance of V-defects increases together with the surface diffusion length either by increasing the temperature or the In flux because of a surfactant effect. In thick InAlN layers, a better (worse) In incorporation occurring on the concave (convex) shape surfaces of the V-defects is observed leading to a top phase-separated InAlN layer lying on the initial homogeneous InAlN layer after V-defects coalescence. It is suggested that similar mechanisms could be responsible for the degradation of thick InGaN layers. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790424]

We demonstrate polariton lasing in a bulk ZnO planar microcavity under non-resonant optical pumping at a small negative detuning (delta similar to -1/6 the 130 meV vacuum Rabi splitting) and a temperature of 120 K. The strong coupling regime is maintained at lasing threshold since the coherent nonlinear emission from the lower polariton branch occurs at zero in-plane wavevector well below the uncoupled cavity mode. The contribution of multiple localized polariton modes above threshold and the non-thermal polariton statistics show that the system is in a far-from-equilibrium regime, likely related to the moderate photon lifetime and in-plane photonic disorder in the cavity. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3650268]

The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN/GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at similar to 420 nm is achieved under pulsed excitation at room temperature for a peak power density of 400 kW/cm(2). The lasing emission linewidth is down to 0.033 nm.

Intersubband (lSB) optical absorption in different nitride-based heterostructures grown by metal-organic chemical vapour deposition (MOCVD) is reported. The role of indium in AlInN/GaN multi-quantum wells (MQWs) is investigated. At high concentration (15%) AlInN is quasi lattice-matched to GaN and no cracks appear in the structure. At very low indium concentration (similar to 2%) the material quality is improved without decreasing the ISB transition wavelength with respect to the case of indium-free structures. Different mechanisms of strain relaxation in pure and 2% indium-doped AlN/GaN MQW structures are also investigated. ISB transition wavelengths of 2 urn for AlN/GaN MQWs, and 3 mu n for AlInN/GaN MQWs with 15% of In are achieved. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report the observation of midinfrared intersubband (ISB) absorption in nearly lattice-matched AlInNGaN multiple-quantum-wells. A clear absorption peak is observed around 3 μm involving transitions from the conduction band ground state to the first excited state. In addition to ISB absorption, photoluminescence experiments were carried out on lattice- matched AlInNGaN single quantum wells in order to determine the spontaneous polarization discontinuity between GaN and Al0.82 In0.18 N compounds. The experimental value is in good agreement with theoretical predictions. Our results demonstrate that the AlInNGaN system is very promising to achieve crack-free and low dislocation density structures dedicated to intersubband devices in the 2-4 μm wavelength range.

The strong coupling of excitons and photons inside a semiconductor microcavity forms a trap in k-space for the coupled polaritons. This trap produces new behaviour from the strong parametric scattering of polaritons on the walls of the trap, from condensation of polaritons at the bottom of the trap and from the bottleneck in relaxation at the neck of the trap. We predict the operation of polariton lasers pumped non-resonantly. either electrically or optically, based on efficient scattering of polaritons into the trap. (C) 2002 Elsevier Science B.V. All rights reserved.

We report on a practical method for developing InGaN-based edge emitting laser diodes of cavity length down to 45 μm. Samples consisting of one uncoated cleaved facet and one etched facet coated with a high-reflectivity (HR) dielectric distributed Bragg reflector (DBR) exhibit lasing in the continuous wave (cw) regime for cavity lengths down to 250 μm and lasing under pulsed injection for lengths as short as 100 μm. For samples having a second HR dielectric DBR, we could demonstrate cw lasing for a cavity length as short as 45 μm with a threshold current below 10 mA being reported for a 75 μm long device. Through a systematic study of the threshold current (I th) and the slope efficiency (η s ) as a function of cavity length, it is proposed that the parameters underpinning the evolution of I th and η s with decreasing cavity length and their overall degradation in the short cavity regime are free carrier absorption, Auger processes and the decrease in the recombination losses due to nonuniform carrier distribution across the multiple quantum well active region.