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The ideal single-photon source displaying high brightness and purity, emission on-demand, mature integration, practical communication wavelength (i.e., in the telecom range), and operating at room temperature does not exist yet. In 2018, a new single-photon source was discovered in gallium nitride (GaN) showing high potential thanks to its telecom wavelength emission, record-high brightness, good purity, and operation at room temperature. Despite all these assets, its coupling to photonic structures has not been achieved so far. In this article, we make a first step in this direction. First, we analyze whether stacking faults are indeed a necessary condition for obtaining such emitters in GaN layers. Then, we discuss the challenges associated to a low spatial density and to a spectrally wide distribution of emitters, which necessitate their location to be determined beforehand and the photonic structure resonance to be tuned to their emission wavelength. The design and fabrication of bullseye antennas are thoroughly described. Finally, we fabricate such bullseyes around telecom emitters and demonstrate that the embedded emitters are able to sustain the necessary clean-room process and still operate as single-photon emitters after the fabrication steps, with room-temperature purities up to 99% combined with repetition rates in the order of hundreds of kHz. The findings in this work demonstrate that telecom single-photon emitters in GaN operating at room temperature are well adapted for single-photon applications where brightness and purity are the required figures of merit, but highlight the numerous difficulties that still need to be overcome before they can be exploited in actual quantum photonic applications.

Controlling light properties with diffractive planar elements requires full-polarization channels and accurate reconstruction of optical signal for real applications. Here, we present a general method that enables wavefront shaping with arbitrary output polarization by encoding both phase and polarization information into pixelated metasurfaces. We apply this concept to convert an input plane wave with linear polarization to a holographic image with arbitrary spatial output polarization. A vectorial ptychography technique is introduced for mapping the Jones matrix to monitor the reconstructed metasurface output field and to compute the full polarization properties of the vectorial far field patterns, confirming that pixelated interfaces can deflect vectorial images to desired directions for accurate targeting and wavefront shaping. Multiplexing pixelated deflectors that address different polarizations have been integrated into a shared aperture to display several arbitrary polarized images, leading to promising new applications in vector beam generation, full color display and augmented/virtual reality imaging., Controlling light with planar elements requires full polarization channels and reconstruction of optical signals. Here, the authors have demonstrated a general method relying on pixelated metasurfaces that enables wavefront shaping with arbitrary output polarization, allowing full utilization of polarization channels.

Intensity and polarization are two fundamental components of light. Independent control of them is of tremendous interest in many applications. In this paper, we propose a general vectorial encryption method, which enables arbitrary far-field light distribution with the local polarization, including orientations and ellipticities, decoupling intensity from polarization across a broad bandwidth using geometric phase metasurfaces. By revamping the well-known iterative Fourier transform algorithm, we propose “à la carte” design of far-field intensity and polarization distribution with vectorial Fourier metasurfaces. A series of non-conventional vectorial field distribution, mimicking cylindrical vector beams in the sense that they share the same intensity profile but with different polarization distribution and a speckled phase distribution, is demonstrated. Vectorial Fourier optical metasurfaces may enable important applications in the area of complex light beam generation, secure optical data storage, steganography and optical communications., Though multiplexing meta-holograms remains an attractive approach for realizing optical encoding, existing methods encode information based on the intensity of the holographic images. Here, the authors report vectorial metasurfaces that decouple and encode intensity and polarization information.

A general method of designing polarization and angular nondispersive metasurface with unlimited bandwidth is provided., Any arbitrary state of polarization of light beam can be decomposed into a linear superposition of two orthogonal oscillations, each of which has a specific amplitude of the electric field. The dispersive nature of diffractive and refractive optical components generally affects these amplitude responses over a small wavelength range, tumbling the light polarization properties. Although recent works suggest the realization of broadband nanophotonic interfaces that can mitigate frequency dispersion, their usage for arbitrary polarization control remains elusively chromatic. Here, we present a general method to address broadband full-polarization properties of diffracted fields using an original superposition of circular polarization beams transmitted through metasurfaces. The polarization-maintaining metasurfaces are applied for complex broadband wavefront shaping, including beam deflectors and white-light holograms. Eliminating chromatic dispersion and dispersive polarization response of conventional diffractive elements lead to broadband polarization-maintaining devices of interest for applications in polarization imaging, broadband-polarimetry, augmented/virtual reality imaging, full color display, etc.

International audience; We report a general method for full-polarization generation based on pixelated metasurface. By encoding the holographic phase profile into such pixels, vectorial holograms are constructed for the application of multidirectional display and cylindrical vector beam (CVB).

Correlation between off-axis electron holography and atom probe tomography (APT) provides morphological, chemical and electrical information about Mg doping (p-type) in gallium nitride (GaN) layers that have been grown at different temperatures at a nanometric scale. APT allows access to the three-dimensional distribution of atoms and their chemical nature. In particular, this technique allows visualisation of the Mg-rich clusters observed in p-doped GaN layers grown by metal-organic chemical vapour deposition. As the layer growth temperature increases, the cluster density decreases but their size indicted by the number of atoms increases. Moreover, APT reveals that threading dislocations are decorated with Mg atoms. Off-axis electron holography provides complementary information about the electrical activity of the Mg doping. As only a small fraction of dopant atoms are ionised at room temperature, this fraction is increased by annealing the specimen to 400 °C in situ in a transmission electron microscope (TEM). A strong reduction of the dopant electrical activity is observed for increases in the layer growth temperature. The correlation of APT with TEM-based techniques was shown to be a unique approach in order to investigate how the growth temperature affects both the chemical distribution and electrical activity of Mg dopant atoms.

Securing optical information to avoid counterfeiting and manipulation by unauthorized persons and agencies requires innovation and enhancement of security beyond basic intensity encryption. In this paper, we present a new method for polarization-dependent optical encryption that relies on extremely high-resolution near-field phase encoding at metasurfaces, down to the diffraction limit. Unlike previous intensity or color printing methods, which are detectable by the human eye, analog phase decoding requires specific decryption setup to achieve a higher security level. In this work, quadriwave lateral shearing interferometry is used as a phase decryption method, decrypting binary quick response (QR) phase codes and thus forming phase-contrast images, with phase values as low as 15°. Combining near-field phase imaging and far-field holographic imaging under orthogonal polarization illumination, we enhanced the security level for potential applications in the area of biometric recognition, secure ID cards, secure optical data storage, steganography, and communications.

The last two decades have shown an increasing need for GaN-based laser diodes (LDs), which are currently only grown on bulk GaN substrates, which remain to date very expensive and/or only available in small sizes. The ever growing laser market will expand in the coming years, thanks to the development of automotive laser lighting, high-speed Li-Fi optical data transmission, LiDAR sensing for autonomous vehicles and smart cities, head-up displays, and AR/VR systems, in addition to biomedical and further industrial applications. These emerging technologies demand for mass-production of GaN-based lasers to be produced on large-size, low-cost, and industrially compatible substrates. To address this issue, we demonstrate the first electrically injected semipolar 440 nm LD on high-quality and low-defect-density (11-22) GaN templates grown on scalable and low-cost sapphire substrates. The LDs exhibit a threshold current density of 17 kA/cm2, a single facet output power of more than 200 mW at 2 A with a slope efficiency of 0.85 W/A, and a TE polarization having a ratio of 97.6%. These results enable the advancement of ultra-low-cost LDs while benefiting from the inherent advantages of semipolar GaN properties.

Using strain-reducing partially relaxed AlInN buffer layers, we observe reduced nonradiative recombination in semipolar green-emitting GaInN/GaN quantum wells. Since strain is a key issue for the formation of defects that act as nonradiative recombination centers, we aim to reduce the lattice mismatch between GaInN and GaN by introducing an AlInN buffer layer that can be grown lattice-matched along one of the in-plane directions of GaN, even in the semipolar ( 11 2 ¯ 2 ) orientation. With the increasing thickness, the buffer layer shows partial relaxation in one direction and thereby provides a growth template with reduced lattice mismatch for the subsequent GaInN quantum wells. Time-resolved photoluminescence measurements show reduced nonradiative recombination for the structures with a strain-reducing buffer layer.Using strain-reducing partially relaxed AlInN buffer layers, we observe reduced nonradiative recombination in semipolar green-emitting GaInN/GaN quantum wells. Since strain is a key issue for the formation of defects that act as nonradiative recombination centers, we aim to reduce the lattice mismatch between GaInN and GaN by introducing an AlInN buffer layer that can be grown lattice-matched along one of the in-plane directions of GaN, even in the semipolar ( 11 2 ¯ 2 ) orientation. With the increasing thickness, the buffer layer shows partial relaxation in one direction and thereby provides a growth template with reduced lattice mismatch for the subsequent GaInN quantum wells. Time-resolved photoluminescence measurements show reduced nonradiative recombination for the structures with a strain-reducing buffer layer.

International audience; Nanopendeo-epitaxy of gallium nitride (GaN) is considered in this study as a way of producing freestanding GaN with reduced strain and threading dislocation density (TDD) for optoelectronic applications. The novelty of this work lies in the use of silicon on insulator (SOI) substrates patterned into nano-pillars down to the buried oxide (BOX). We actually want to benefit from the creeping properties of SiO2 at the growth temperature of GaN for strain relaxation and grain-boundary dislocations reduction. In this paper, we report on the fabrication of 40×40 µm 2 and 300×300 µm 2 freestanding GaN platelets, up to 10 µm-thick, spontaneously separated from the initial pillars. Structural and optical characterizations show that the platelets are crack-free and almost fully relaxed, with a TDD of ∼ 4×10 8 /cm 2. We underline the different benefits of this approach, but most importantly, we believe that it will be the founding-brick for transferable GaN-based devices.

International audience; We investigated the electrical and optical performances of semipolar (11-22) InGaN green µLEDs with a size ranging from 20 × 20 µm 2 to 100 × 100 µm 2 , grown on a low defect density and large area (11-22) GaN template on patterned sapphire substrate. Atom probe tomography (APT) gave insights on quantum wells (QWs) thickness and indium composition and indicated that no indium clusters were observed in the QWs. The µLEDs showed a small wavelength blueshift of 5 nm, as the current density increased from 5 to 90 A/cm 2 and exhibited a size-independent EQE of 2% by sidewall passivation using atomiclayer deposition, followed by an extremely low leakage current of ~0.1 nA at −5 V. Moreover, optical polarization behavior with a polarization ratio of 40% was observed. This work demonstrated long-wavelength µLEDs fabricated on semipolar GaN grown on foreign substrate, which are applicable for a variety of display applications at a low cost.

In this work, we fabricated five different sizes of 520 nm semipolar micro-light-emitting-diodes ( $\mu \text{LEDs}$ ), ranging from 100×100 to $20\times 20\mu \mathrm{m}^{2}$ , on (11–22) GaN/sapphire template. Based upon the results from atom probe tomography (APT), no InGaN cluster was observed in the active region. For the current-voltage characteristics, the voltages were 3.2 V and 3.0 V at 60 A/cm2 for the 100×100 and $20\times 20\ \mu \mathrm{m}^{2}\ \mu \text{LEDs}$ , respectively. In terms of the optical aspect, the $20\times 20\mu \mathrm{m}^{2}\mu \text{LED}$ demonstrated a trivial blue-shift of 9 nm as the inject current density increases from 5 A/cm2 to 100 A/cm2. Moreover, for the external quantum efficiency (EQE) performances, $\mu \text{LEDs}$ in all sizes yielded 6-8% efficiency droop between the peak EQE current density and at 100 A/cm2, and the peak EQE was 2% for all $\mu \text{LED}$ sizes. Although further improvements on growth and materials are required for better $\mu \text{LED}$ performances, this demonstration revealed the feasibility to produce long-wavelength semipolar InGaN $\mu \text{LEDs}$ with high efficiency on large-area sapphire substrates.

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International audience; In this work, we investigate the impact of the quantum confined Stark effect and of the carrier localization on the internal quantum efficiency of polarized single or multiple InxGa1-xN/GaN quantum well(s), and semi-polar (11e22) multiple InxGa1-xN/InyGa1-yN quantum well. We find that increasing the influence of the quantum confined Stark effect at constant indium content with increasing the well-width induces a reduction of the internal quantum efficiency onsets for a decreasing value of the photoexcitation density. Similar but no so dramatic trend is reported when increasing the indium content and thus when increasing the localization of carriers to localized fluctuations of the chemical composition of the alloy. In addition, a change of the electric field internal to active layers (quantified by using time-resolved photoluminescence spectroscopy) realized by growing samples along a semi-polar orientation leads to experimental observation of a substantial enhancement of the threshold of photoexcitation density at which onsets the reduction of the internal quantum efficiency. A correlation is found through several orders of magni- tude between the photoexcitation density PT for the onset of the collapse of IQE and the values of the photoluminescence radiative decay time trad. A scaling law is found in the investigated samples: PT ~ trad-n with n 1⁄4 3/2 ± 0.15 which evidences that quantum confined Stark effect is the main origin for the efficiency droop in nitride light-emitting diodes based on InxGa1-xN active layers.

We demonstrate efficient semipolar (11-22) 550 nm yellow/green InGaN light-emitting diodes (LEDs) with In0.03Ga0.97N barriers on low defect density (11-22) GaN/patterned sapphire templates. The In0.03Ga0.97N barriers were clearly identified, and no InGaN clusters were observed by atom probe tomography measurements. The semipolar (11-22) 550 nm InGaN LEDs (0.1 mm2 size) show an output power of 2.4 mW at 100 mA and a peak external quantum efficiency of 1.3% with a low efficiency drop. In addition, the LEDs exhibit a small blue-shift of only 11 nm as injection current increases from 5 to 100 mA. These results suggest the potential to produce high efficiency semipolar InGaN LEDs with long emission wavelength on large-area sapphire substrates with economical feasibility.

International audience; The variation of the internal quantum efficiency (IQE) of single InGaN quantum well structures emitting from blue to red is studied as a function of the excitation power density and the temperature. By changing the well width, the indium content, and adding a strain compensation AlGaN layer, we could tune the intrinsic radiative recombination rate by changing the quantum confined Stark effect, and we could modify the carrier localization. Strong quantum confined Stark effect and carrier localization induce an increase in the carrier density and then favor Auger non-radiative recombination in the high excitation range. In such high excitation conditions with efficient Auger recombination, the variation of the IQE with the photo-excitation density P is ruled by a universal power law independent of the design: IQE = IQEMAX – a log10P with a close to 1/3. The temperature dependences of the different recombination mechanisms are determined. At low temperature, both quantum confined Stark effect and carrier localization trigger electron-electron repulsions and therefore the onset of the Auger effect. The increase in the value of coefficient C with changing temperature reveals indirect Auger recombination that relates to the interactions of the carriers with other phonons than the longitudinal optical one.

Despite an already existing abundant literature dedicated to report of lot of experimental investigations towards the understanting of the mechanisms that rule the limitation of intense light emission in nitride-based heterostructures, there are still some issues that are not fully elucidated. This is probably related to the lack of investigations away from the blue and aquamarine light regions. In this communication we cover the 480 nm to 620 nm range by using a series of samples with different designs: single and multiple GaInN-GaN quantum wells. This paper is limited to heterostructures grown along the polar orientation. By changing the well width, and the indium content we could tune in the one hand the Quantum Confined Stark Effect that is to say the intrinsic radiative recombination rate while we could tune the crystalline quality by using strain-compensated GaInN-GaN-AlGaN designs. Finally, by changing our optical pump density we modified the intrinsic non radiative Auger. Given an emission wavelength, we find that the photoexcitation density P for the onset of substantial Auger effect to increase with the number of wells. Using an ABC-type modelling we find a clear 3/2 power law correlation between parameters B and C. This behavior is discussed in terms of electron-hole coulomb interaction and electron-electron repulsion in photo-excited samples. In condition of efficient Auger recombination, the variation of the internal quantum efficiency with photoexcitation density is ruled by a universal power law independent of the design: IQE = IQE0 – a log10 P with a =16 %cm2/Watt.

International audience; We comparatively study the onset of photo-induced non-radiative intrinsic Auger recombination processes for red, yellow and green light emitting InGaNsingle bondGaN hetero-structures grown along the polar orientation. We find a dramatic reduction of the photo excitation densities triggering the domination of Auger effect with increasing emission wavelength; that is to say in concert with the enhancement of the internal electric field in the structure. In long wavelength emitters, the internal electric field is stronger, and hence reducing the impact of the internal electric field is more critical.

We report on the selective area growth of semipolar (11-22) GaN epilayers on wet etched r-plane patterned sapphire substrates (PSS) by metal organic chemical vapor deposition. Using a three-step growth method, planar (11-22) GaN epilayers on 2 in. wafers with significant optical and structural quality improvements have been obtained. The filtering of basal stacking faults and dislocations was achieved by overlapping adjacent crystals and forming voids between them. These voids act as a barrier to defect propagation which results in reduced defect density at the surface of the epilayer. Cathodoluminescence measurements at 80 K revealed a dislocation density of 5.1×107 cm−2 and a basal stacking fault density below 30 cm−1. Moreover, photoluminescence and X-ray diffraction measurements attested a material quality similar to conventional GaN on c-plane sapphire. Such large scale semipolar GaN templates are opening the way for efficient semipolar devices grown heteroepitaxially.

(Al,Ga)N-based quantum dots (QDs) grown on Al 0.5 Ga 0.5 N by molecular beam epitaxy have been studied as the active region for the fabrication of ultra-violet (UV) light emitting diodes (LEDs). In the first part, using both “polar” (0001) and “semipolar” (112¯2) surface orientations, the structural and optical properties of different QD structures are investigated and compared. In particular, their propensity to get an emission in the UV range is analyzed in correlation with the influence of the internal electric field on their optical properties. In a second part, (0001) and (112¯2)-oriented LEDs using GaN/Al 0.5 Ga 0.5 N QD as active regions have been fabricated. Their main current-voltage characteristics and electroluminescence properties are discussed, with a focus on the LED emission wavelength range reached for both surface orientations: it is shown that a large part of the UV-A region can be covered, with longer wavelengths-from 415 to 360 nm-for the “polar” LEDs, and shorter ones-from 345 to 325 nm-for the “semipolar” LEDs. In addition, the influence of the internal electric field on the QD-LEDs working operation is shown.

The Auger effect and its impact on the internal quantum efficiency (IQE) of yellow light emitters based on silicon-doped InGaN–AlGaN–GaN quantum wells are investigated by power dependence measurement and using an ABC model. Photoluminescence intensity recorded as a function of excitation power density follows a linear dependence up to a threshold P T that depends on the design of the sample. Above this threshold, the variation of the intensity becomes sublinear, which is characteristic of the onset of Auger recombination processes. After extracting the evolution of IQE with pump power from the experimental data, we use a modified ABC modeling that includes the residual n-type doping to estimate the contribution of different recombination channels. We find that the Auger effect dominates in the high-excitation regime. In addition, we find that intercalating an AlGaN-strain-compensating layer reduces not only the coefficient of nonradiative recombination rates but also reduces the onset of Auger recombination.

CVD. The bare chip device shows a strong UV emission at 380 nm, with a maximum output power of 1.6 mW at 140 mA, and a junction temperature of 120 °C. The saturation of the external quantum efficiency (EQE) occurs at 0.5 kA/cm2 (∼80 mA), a much higher value than the 10 A/cm2 commonly reported in quantum wells (QWs) based nitride LEDs. This behavior can be accounted for by a temperature dependent non-radiative recombination rate which increases at high injection currents. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We determine the internal quantum efficiency of strain-balanced AlGaN-InGaN-GaN hetero-structures designed for yellow-amber light emission, by using a recent model based on the kinetics of the photoluminescence decay initiated by Yoshiya Iwata, Ryan G. Banal, Shuhei Ichikawa, Mitsuru Funato, and Yoichi Kawakami, Journal of Applied Physics 117, 075701 (2015). Our results indicate that low temperature internal quantum efficiencies sit in the 50 % range and we measure that adding an AlGaN layer increases the internal quantum efficiency from 50% up to 57 % with respect to the GaN-InGaN case. More dramatic, it almost doubles from 2.5 % up to 4.3 % at room temperature.

We have grown (11-22)-oriented GaN/Al0.5Ga0.5N quantum wells (QWs) using molecular beam epitaxy on GaN (11-22)-oriented templates grown by metal-organic vapor phase epitaxy on m-plane oriented sapphire substrates. The performance of epitaxial growth of GaN/Al0.5Ga0.5N heterostructures on the semi-polar orientation (11-22) in terms of surface roughness and structural properties, i.e., strain relaxation mechanisms is discussed. In addition, high resolution transmission electron microscopy reveals very smooth QW interfaces. The photoluminescence of such samples are strictly originating from radiative recombination of free excitons for temperatures above 100 K. At high temperature, the population of localized excitons, moderately trapped (5 meV) at low temperature, is negligible.

GaN-Al0.5NGa0.5N quantum dots deposited on (11-22) planes have been grown by combining Molecular Beam Epitaxy and Metal Organic Vapour Phase Epitaxy. This combination is interesting for realization of ultraviolet operation light emitting diodes, lasers andsingle photon sources,… (1,3) The growth of dots was achieved by MBE using ammonia as nitrogen precursor and growth interruption in ammonia less conditions to trigger corrugation of GaN and dot formation (4). The (11-22) GaN oriented peudosubstrate was realized by MOVPE starting from a M-plane oriented sapphire substrate. The orientation of the growth plane dictates in-plane anisotropies which are effectively found leading to a transition from isolated dots to nanochains - oriented along the direction as evidenced from Atomic Force Microscopy features or optical properties: polarization rates and temperature dependent measurements of the radiative recombination process for instance(5). We here restrict to small size isolated quantum dots and present innovative optical properties among which are micro-photoluminescence data versus pump power, polarization of the emitted photons at different temperatures. We also analyse the photoluminescence decay times and model our finding in the context of the effective mass approximation. The crystal field splitting is measured in Al 0.5NGa 0.5N lattice-matched to (11-2) oriented GaN by polarized microphotoluminecence under high photo- injection conditions.

We present an investigation of optical properties of yellow light emitting (Ga,In) N-based devices grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane sapphire with different designs: well width, indium composition up to 23 percent in the well layer. Using time-resolved photoluminescence measurements collected in range of 8-300K, temperature – dependent photoluminescence decays are determined, they exhibit similar behaviors for all samples. These quantum devices always display a two-mode exponential decay with a long decay time and a short one in a ratio about four to five. The photoluminescence intensities measured from low temperature to room temperature give large values of activation temperature that indicate the increasing of the non-radiative recombination rate when the temperature increases. The average decay times are found by a procedure using fitting sigmoidal functions. These decay times increase exponentially with the indium concentration and the well width due to influence of quantum confined Stark effect on these devices. Finally, in order to estimate the performances of our samples, we plotted the decay times obtained versus product of the indium content and the well width together with others published decay times.

High quality GaN-based light-emitting diodes (LEDs) can be achieved on alumina-rich spinel (MgAl6O10), a new substrate material with better thermal and lattice match to GaN than sapphire. As alumina-rich spinel is chemically very close to sapphire (Al2O3), the GaN metal-organic chemical vapour deposition (MOCVD) conditions developed for c-plane sapphire have been successfully replicated on it. In this article, we report on the electroluminescence (EL) properties of GaN-based LEDs grown by MOCVD simultaneously on Al2O3-(0001) and MgAl6O10-(111). Both devices exhibit a linear increase of the light output power versus current density, with a higher EL-intensity and a narrower EL-linewidth for the LEDs grown on alumina-rich spinel. However, due to the lower thermal conductivity of spinel, these LEDs start suffering from device heating at lower current densities than on sapphire. Irreversible damage occurs for DC current densities of ∼0.9 kA/cm2 and at an estimated junction temperature of ∼325 °C. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The current transport properties of an n -doped, lattice matched AlInN layer with multiple, periodic GaN insertions are reported. Samples with three and seven insertions were grown and mesa structures were etched through the AlInN structures to different depths to determine the voltage contribution from each heterointerface. The quality of the AlInN surface improved with the GaN insertions and with reducing the thickness of the individual AlInN layers from 80 nm to 55 nm. Simulations suggest that the large conduction band discontinuity can lead to high turn on voltages which can be reduced by high levels of doping. Current-voltage measurements through the full structure show a diode characteristic which is temperature activated. A quasi-linear current-voltage characteristic is measured when the current is driven only through the uppermost GaN interlayer with 80 nm thick AlInN layers, indicating current leakage through defects in these layers (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Commercially available inorganic white light emitting diodes (LEDs) are essentially based on the combination of a blue InGaN based LED chip covered by a long wavelength emitting (yellow, red) phosphor. We propose to avoid this step of phosphor deposition by taking advantage of the fact that yellow to red emission can be achieved using InGaN alloys. By stacking an InGaN/GaN multiple quantum well (QW) emitting in the yellow, acting as a light converter, and a short wavelength blue-violet pump LED grown on top, white light emission can be obtained. Furthermore, if we extend the emission spectrum of the light converter into the red, a warm white light color is demonstrated when a pump LED is grown on top. However, the high In content InGaN QWs of the light converter have a low thermal stability and the QW efficiency tends to degrade during the growth of the pump LED. Three different solutions are explored to avoid the thermal degradation of the light converter. The monolithic LED structures were grown by molecular beam epitaxy (MBE), by a combination of both MBE and metal-organic chemical vapor phase epitaxy (MOCVD), or by a low temperature full-MOCVD process. The best results are obtained using a complete MOCVD growth process. The structure and the MOCVD growth conditions are specifically adapted in order to avoid the thermal degradation of the large In composition InGaN QWs emitting at long wavelength during the growth of the subsequent layers.

Al, Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al, Ga) N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al, Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented "polar" and (11 (2) over bar 2) oriented "semipolar" materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al, Ga) N nanostructure-based LED demonstrators are presented.

The aim of this work is to provide an overview on the recent advances in the selective area growth (SAG) of ( In ) GaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs. The first three sections deal with the basic growth mechanisms of GaN SAG and the emission control in the entire ultraviolet to infrared range, including approaches for white light emission, using InGaN disks and thick segments on axial nanocolumns. SAG of axial nanostructures is developed on both GaN /sapphire templates and GaN -buffered Si (111). As an alternative to axial nanocolumns, section 4 reports on the growth and characterization of InGaN / GaN core-shell structures on an ordered array of top-down patterned GaN microrods. Finally, section 5 reports on the SAG of GaN , with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the templates is strongly reduced as indicated by a dramatic improvement of the optical properties. In the case of SAG on non-polar (11-22) templates, the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of these nanostructures.

Semipolar GaN crystal stripes larger than 100 mu m with dislocation densities below 5 x 10(6) cm(-2) are achieved using a low cost fabrication process. An original sapphire patterning procedure is proposed, enabling selective growth of semipolar oriented GaN stripes while confining the defects to specific areas. Radiative and non-radiative crystalline defects are investigated by cathodoluminescence and can be correlated to the development of crystal microstructure during the growth process. A dislocation reduction mechanism, supported by transmission electron microscopy, is proposed. This method represents a step forward toward low-cost quasi-bulk semipolar GaN epitaxial platforms with an excellent structural quality which will allow for even more efficient III-nitride based devices. Published by AIP Publishing.

Thanks to the use of two successive selective growths by metal organic chemical vapor deposition reactor, high quality semipolar (11-22) GaN with a homogenous defect repartition over the surface was achieved. The procedure starts with a first selective growth on a patterned sapphire substrate, leading to continuous stripes of three dimensional (3D) GaN crystals of low defect density. Then, a second selective growth step is achieved by depositing a SiNx nano-mask and a low temperature GaN nano-layer on the top of the GaN stripes. Hereby, we demonstrate an original way to obtain a homoepitaxial selective growth on 3D GaN crystals by taking advantage of the different crystallographic planes available. Basal stacking faults (BSFs) are generated during this second selective growth but could be eliminated by using a three-step growth method in which elongated voids are created above the defective area. For a fully coalesced sample grown using the 2 step method, dislocation density of 1.2 × 108 cm−2 and BSFs density of 154 cm−1 with a homogenous distribution have been measured by cathodoluminescence at 80 K. Consequently the material quality of this coalesced semipolar layer is comparable to the one of polar GaN on c-plane sapphire.

The difference of growth temperatures between InGaN quantum wells and GaN barriers has detrimental effects on the properties of the wells. Different capping processes of InGaN quantum well with a thin AlGaN layer have been investigated to prevent these effects. Both structural and optical properties of the samples, grown on c-plane sapphire substrates by metalorganic vapor phase epitaxy, were studied through transmission electron microscopy (TEM), x-ray diffraction and room temperature photoluminescence. The average quantum well thickness and its indium composition were determined by digital processing of lattice fringes in cross-sectional TEM images. From the analysis of the well thickness distribution, it is shown that AlGaN as a capping layer helps to compensate an unwanted undulation at the upper InGaN QW-barrier interface. Moreover, when deposited at the same temperature as InGaN, the AlGaN layer is effective in avoiding or reducing the evaporation and/or diffusion of indium from InGaN wells, which results in the thinning of the well. It therefore helps to extend the emission wavelength up to 540 nm with a reduced degradation of the room temperature photoluminescence efficiency.

Monolithic InGaN-based light-emitting diodes (LEDs) using a light converter fully grown by metal organic vapor phase epitaxy are demonstrated. The light converter, consisting of 10–40 InGaN/GaN quantum wells, is grown first, followed by a violet pump LED. The structure and growth conditions of the pump LED are specifically adapted to avoid thermal degradation of the light converter. Electroluminescence analysis shows that part of the pump light is absorbed by the light converter and reemitted at longer wavelength. Depending on the emission wavelength of the light converter, different LED colors are achieved. In particular, for red-emitting light converters, a color temperature of 2100 K corresponding to a tint between warm white and candle light is demonstrated.

We present a comparative study of semipolar (1122) and polar (0001) (In,Ga)N/GaN light emitting diodes grown simultaneously under equal conditions by metalorganic vapor deposition. These structures are grown on 2-in. GaN templates with two different (1122) and (0001) orientations. The templates were formerly obtained by MOCVD, on (1010) m-plane and (0001) c-plane sapphire substrates. The semipolar quantum wells exhibit a 30% higher indium content compared to the polar quantum wells. This was confirmed by complementary X-ray diffraction and energy dispersive X-ray spectroscopy measurements on thick (~60 nm) InGaN layers grown on (1122) and (0001) GaN templates. The results of electroluminescence and photocurrent show that the emission of semipolar QWs is strongly red-shifted with respect to the band edge. This large Stokes' shift is attributed to carrier localization at potential minima induced by indium composition fluctuations. A strong blueshift under forward DC injection is observed for the semipolar (Ga,In)N diodes. Since a reduced piezoelectric field is expected in the semipolar diodes, the observed blueshift is mainly due to carrier filling of the localized states.

We report on the growth and characterization of green InGaN light-emitting diodes (LEDs) grown on Si (111) substrates using metalorganic vapor phase epitaxy. A single InGaN quantum well active layer has been used. The optical qualities of InGaN on Si(111) and the p–doping efficiency of GaN are discussed. The turn-on voltage of the LED is 6.8 V and the operating voltage is 10.7 V at 20 mA. Electroluminescence of the LEDs starts at a forward bias of 3.5 V. The electroluminescence peaks at 508 nm, with a full-width at half maximum of 52 nm. An optical output power of 6 µW (in ∼ 8π/5 sr) was achieved for an applied current of 20 mA.