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Planar UV-C light emitting diodes still suffer from low efficiency, mainly due to substrate crystalline quality, p doped conductivity and extraction efficiency. One possible way to overcome partly these issues is to realize the whole UV structure on AlGaN pyramids by selective area growth in order to benefit from the advantages of such structures, i.e. the dislocation filtering and the semi polar planes. We present here a detailed study about the epitaxy of AlGaN nano-sized pyramids by metal organic vapor phase epitaxy on patterned templates presenting different holes apertures and pitches as 1.5 μm and 4 μm or 100 nm and 250 nm respectively. While increasing the Al content, their height decreases while the thickness of the deposition on the mask increases whatever the design of the mask. Those changes of the pyramid shapes and deposition are directly linked to the properties of Al adatoms, i.e. low Al diffusion length. Using the conventional growth mode for the epitaxy of those pyramids did not permit the incorporation of Al from the base of the pyramids to their truncated apex. Its presence was concentrated on the edges and top of the pyramids. On the contrary, a pulsed growth mode, coupled with a strongly reduced pitch, allowed an incorporation of Al since the base of the nanopyramid, and a decrease of the deposition height on the mask. These results can be explained by the desorption of Ga species, due to the presence of H2 in the reactor chamber during the step without the metal precursors, leading to a higher Al/Ga ratio. It is even enhanced inside the holes by the reduced pitch.

Micro-displays with pixel pitch smaller than 10 µm are required for augmented reality. The three primary colors can then be achieved with the same material: the InxGa1-xN alloy. However, when strained on GaN, its InN mole fraction x is limited to 25%. By using a full InGaN structure grown on a relaxed InGaN pseudo-substrate, such as the InGaNOS substrate from Soitec, the In incorporation rate is enhanced. An internal quantum efficiency higher than 10% at 640 nm an In content higher than 25% in InxGa1-xN/InyGa1-yN quantum wells will be shown. 10x10 µm² red micro-light emitting diodes will be also presented.

InGaN-based devices have attracted a lot of attention, thanks to versatile optoelectronic applications. We show that the growth of these layers in an AIXTRON Close Coupled Showerhead is complicated due to the presence of gallium pollution in the chamber after the growth of GaN buffer layers. This pollution reacts with the TMIn precursor in the growth chamber to give thicker InGaN layers with reduced indium incorporation. We overcame this problem by limiting the presence of the metallic gallium in the growth chamber, resulting in more stable and predictable growth of InGaN layers.

International audience; The InGaN pseudo-substrate, namely InGaNOS (InGaN On Sapphire), is used to enhance the In incorporation rate in In$_y$Ga$_{1-y}$N/In$_x$Ga$_{1-x}$N multiple quantum wells (MQWs) to get red emission for micro-display applications. However, the starting material for the InGaNOS fabrication is a nonoptimized In0.08Ga0.92N layer grown on GaN on sapphire substrate which exhibits V shaped defects (V pits). Such V pits remain afterwards in the final InGaNOS substrate. We demonstrate here that In$_x$Ga$_{1-x}$N/GaN superlattice has the potential to cover or fill the native V pits while maintaining a pseudomorphic growth. A combination of a thin GaN interlayer and an InGaN layer in a slight tensile strain state for each pair of the superlattice is necessary to achieve this goal. In addition, it is shown that the presence of GaN interlayers improves the material quality and the surface roughness. (002) X-ray diffraction rocking curve linewidth reduces to 780 arcsec compared to 3000 arcsec for the substrate. Finally, In$_y$Ga$_{1-y}$N/In$_x$Ga$_{1-x}$N multiple quantum wells grown on In$_x$Ga$_{1-x}$N/GaN superlattice buffer layer on InGaNOS 3.205Ȧ substrate shows a central emission wavelength, measured by photoluminescence, of 624 nm at 290K with an optical internal quantum efficiency value of 6.5%.

International audience; The distribution and electrical activity of p-type doping (Mg) in gallium nitride (GaN) grown by metal organic chemical vapor deposition was investigated by correlating atom probe tomography (APT) and off-axis electron holography. APT results revealed that high Mg concentrations promote the formation of Mg-rich clusters. This is associated with the formation of pyramidal inversion domains (PIDs). The direct measurement of the doping concentration outside the clusters provided by APT suggests a saturation in the p-type electrical activity for Mg concentrations above 7 x 10 19 cm-3. Maps of the electrostatic potential provided by off-axis electron holography confirm that the highest carrier concentration was achieved in the regions with the highest dopant concentration of 2 x 10 20 cm-3 , despite the presence of a high density of Mg-rich clusters revealed by APT. The correlation of these techniques suggests that PIDs are not the major cause of the reduction in electrostatic potential.

International audience; In this work is studied the growth of InGaN on epitaxial graphene by molecular beam epitaxy. The nucleation of the alloy follows a three-dimensional (3D) growth mode, in the explored temperature range of 515-765°C, leading to the formation of dendrite-like islands. Careful Raman scattering experiments show that the graphene underneath is not degraded by the InGaN growth. Moreover, lateral displacement of the nuclei during an atomic force microscopy scan demonstrate weak bonding interactions between InGaN and graphene. Finally, a longer growth time of the alloy gives rise to a compact thin film in partial epitaxial relationship with the SiC underneath the graphene.

The origin of the nanoscale emission inhomogeneities of red emitting InGaN/InGaN quantum wells (QWs) grown directly on a GaN template and on an InGaN on sapphire (InGaNOS) substrate is investigated. InGaNOS is a partly relaxed InGaN pseudo-substrate fabricated by Soitec. As the latter approach provides an interesting optical internal quantum efficiency of 6.5% at 624 nm at 290 K, a deeper study, at the microstructure level, was conducted. The emission inhomogeneities on InGaNOS were highlighted by cathodoluminescence wavelength mappings where three areas were chosen: one emitting at a shorter wavelength, i.e., 588 nm, and two at a longer wavelength, i.e., 607 and 611 nm. Specimens from these zones were extracted by focused ion beam milling to perform cross-sectional characterization techniques. High-angle annular dark field scanning transmission electron microscopy images demonstrated that, while red emitting areas present homogeneous QWs, shorter wavelength areas exhibit non-uniform QWs, in terms of thickness and In composition. Complementary deformation mappings in the growth direction obtained by geometrical phase analysis show that longer emission wavelengths are originating from homogeneous QWs with an InN mole fraction evaluated at 39.0 ± 1.5%. This result demonstrates the possibility of achieving red emission with a coherent (In,Ga)N alloy when using an adapted substrate. A comparison of identical QWs grown on a GaN template is also given.

International audience; Atom Probe Tomography (APT) has emerged as a reliable chemical analysis technique as it provides access to the chemical composition of the analyzed material together with the 3D distribution of the atoms in a tip-shaped specimen with hundred-nanometer diameter. Moreover, combining APT with EDX enables first to overcome some artefacts inherent to APT to get quantitative data but also permits to investigate nanostructures from nano-to micro-scale. We have successfully applied these techniques for studying both n-type (Ge) and p-type (Mg) doping of AlGaN layers.

Micro displays targeting virtual and augmented reality applications require pixel size in the range of $10\times 10\mu \mathrm{m}$ or below. Having InGaN based LEDs to emit directly over the full red-green-blue (RGB) spectrum would enable a cost effective transfer and integration. Green and red InGaN based LEDs initially suffer from low internal quantum efficiencies (IQE) and external quantum efficiencies (EQE) as compared to blue LEDs. One solution could be to reduce the strain in the overall structure thanks to an InGaN pseudo-substrate. The Smart Cut™technology enables the layer transfer and strain relaxation of patterned InGaN films. Pattern size can extend from $10\mu \mathrm{m}$ up to 1mm. This process is today available on 100mm and is being transferred to 150mm wafers. According to the In content of the donor InGaN layer, the a (in-plane) lattice parameter can be tuned up to 3.219A. Full InGaN LED containing InGaN/InGaN multiple quantum wells structures are grown by metalorganic vapor phase epitaxy (MOVPE) on this engineered substrate with a lattice parameter of 3.205A. Long wavelengths have been reached, up to 630nm with FWHM of 48nm at 0.8 A/cm2, thanks to the reduced strain in the overall structure which enables a higher In incorporation during the growth.

Micro light emitting diodes have been grown by metal organic vapor phase epitaxy on standard GaN and partly relaxed InGaNOS substrates with the purpose of incorporating higher concentrations of indium for identical growth conditions. Green emission has been demonstrated at wavelengths of 500 nm for the GaN template and 525 and 549 nm for the InGaNOS substrates, respectively. The structure, deformation, indium concentration and piezoelectric potentials have been measured with nm-scale spatial resolution in the same specimens by transmission electron microscopy. We show by off-axis electron holography that the piezoelectric potential and information about the indium concentration from the mean inner potential are obtained simultaneously. By separating the components using a model, we show that for higher concentrations of indium in the quantum wells (QWs) grown on InGaNOS substrates, the piezoelectric potentials are reduced. The measurements of the indium concentrations by electron holography have been verified by combining energy dispersive x-ray spectrometry, x-ray diffraction and from the tensile deformation made by precession electron diffraction. A discussion of the limitations of these advanced aberration-corrected transmission electron microscopy techniques when applied to nm-scale QW structures is given.

We show that gallium pollution in an AIXTRON Close Coupled Showerhead (CCS) Metal-Organic Vapor Phase Epitaxy (MOVPE) reactor can strongly affect the growth process stability of InGaN layers. By comparing growth with and without gallium pollution from the showerhead, we find a decrease in the indium incorporation and an increase in layer thickness of up to three times when the pollution is present. This phenomenon varies with the quantity of gallium on the showerhead, with increased gallium due to thicker GaN growth causing a stronger effect. Higher TMIn flows also give a more pronounced effect. These data support our previous hypothesis of TMIn reacting with metallic gallium present on the showerhead surface to release TMGa, which is then incorporated as Ga into the grown layers. This study of the effect of gallium contamination on InGaN layers in CCS MOVPE reactors leads us to suggest that the growth in CCS reactors should be performed free from gallium pollution to give more stable and comprehensible results.

InGaN based LEDs are known to be very efficient in the blue range. However, although InGaN can theoretically cover all visible range, quantum efficiency drops when emission wavelength emission is increased due to quantum confined Stark effect. Furthermore, indium incorporation is hindered by compressive strain induced by lattice mismatch between InGaN and GaN. To tackle the lattice mismatch problem, a full InGaN structure on a relaxed InGaN substrate is proposed. The structure consisting of five InxGa1-xN / InyGa1-yN multi quantum wells on top of an InyGa1-yN buffer layer is grown by MOVPE on an InGaNOs substrate from Soitec company. Three InGaNOs substrates of lattice parameters of 3.190, 3.200 and 3.205 Angstroms were co-loaded in order to compare their ability to incorporate indium for the same growth conditions. For reference, a sample grown on GaN template will allow us to compare the wavelength red-shift resulting of the use of InGaNOs template. The samples were characterized by photoluminescence at room temperature using 375 nm and 405 nm laser diodes. It is shown that long wavelengths can be reached thanks to the use of InGaNOs substrates. For same active region growth conditions as reference sample, a red-shift up to 65 nm (from 445 to 510 nm) is observed, demonstrating InGaNOs potential for easier In incorporation. Using different growth conditions, wavelengths up to 600 nm have been reached. First internal quantum efficiency measurements demonstrate a good quality material. InGaNOs seems promising for emission in the “green gap”and beyond.

The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.

International audience; The impact of a relaxed InGaN pseudosubstrate on indium incorporation in a full InGaN heterostructure was investigated. Three types of InGaN pseudosubstrates were tested with different a lattice parameters ranging from 3.190 to 3.205 angstrom, that is to say, greater than that of a GaN template on sapphire. Samples were loaded together in the growth chamber in order to apply exactly the same growth conditions. The effect of the photoluminescence (PL) emission redshift was observed on InyGa1-yN buffer layers and also on InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs). It was found that these pseudosubstrates have the ability to improve the indium incorporation rate, with an increasing effect as the a lattice parameter increases. A strong PL emission redshift was observed in InxGa1-xN/InyGa1-yN MQWs as a function of the increasing a lattice parameter of the InGaN pseudosubstrate, compared to a reference grown on a GaN template. It has been shown that green and amber emissions can be easily reached. A redshift of up to 42 nm was detected between various InGaN pseudosubstrate samples and up to 62 nm compared to a conventional structure emerged from a GaN buffer on the sapphire substrate. The average QW width less than 3 nm indicates a higher In content. The reduced compressive strain originating from the relaxed InGaN substrate allows the reduction in the compositional pulling effect and consequently enables an enhanced In incorporation rate.