abstract: The goal of this research was to reduce dislocations and strain in high indium content bulk InGaN to improve quality for optical devices. In an attempt to achieve this goal, InGaN pillars were grown with compositions that matched the composition of the bulk InGaN grown on top. Pillar height and density were optimized to facilitate coalescence on top of the pillars. It was expected that dislocations within the pillars would bend to side facets, thereby reducing the dislocation density in the bulk overgrowth, however this was not observed. It was also expected that pillars would be completely relaxed at the interface with the substrate. It was shown that pillars are mostly relaxed, but not completely. Mechanisms are proposed to explain why threading dislocations did not bend and how complete relaxation may have been achieved by mechanisms outside of interfacial misfit dislocation formation. Phase separation was not observed by TEM but may be related to the limitations of the sample or measurements. High indium observed at facets and stacking faults could be related to the extra photoluminescence peaks measured. This research focused on the InGaN pillars and first stages of coalescence on top of the pillars, saving bulk growth and device optimization for future research. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

Presented in this thesis are extensive theoretical investigations into the causes and effects of carrier localization in InGaN/GaN quantum wells. The results of the calculations agree well with experimental data, where it is available, and provide additional insights into the mechanisms that lead to some of the experimentally observed effects of localization. Firstly, the wave functions of the electrons and holes in InGaN/GaN quantum wells have been calculated by numerical solution of the effective-mass Schrödinger equation. In our calculations we have assumed a random distribution of indium atoms, as suggested by the results of atom probe tomography: this allows us to find the contributions to the carriers' potential energy that arise from band gap fluctuations, the deformation potential and the spontaneous and piezoelectric fields. We show that the fluctuations in alloy composition can be sufficient to localize the carriers; our results are in good agreement with the results of experiment and more detailed ab-initio calculations, but we also obtain information about the distribution of localized states which those methods cannot yet provide. We find that the holes are localized on a short scale in randomly-occurring regions of high indium content, whereas the electrons are localized on a longer length scale. We consider the effect of well width fluctuations and find that these contribute to electron localization, but not to hole localization. We also simulate the low-temperature photoluminescence spectrum and find good agreement with experiment for the energy, width and shape of the photoluminescence peak. Secondly, we have used first-order time-dependent perturbation theory to study the diffusion of the carriers between their localized states at non-zero temperatures. The rates for scattering via the interaction with acoustic phonons are calculated using the carrier wave functions, and the resulting master equation for the distribution of the carriers is solved by a Monte Carlo method. We find that, even towards room temperature, the carriers are localized to a small number of states, and that their diffusion lengths are proportional to a combination of the density of localized states and the localization length. The experimentally-observed `S-shape' of the photoluminescence peak energy as a function of temperature is reproduced in our results and is explained by the thermal redistribution of holes among the localized states. A reduction of the depth of this S-shape is found as the excitation power is increased, as has been observed experimentally, and which we attribute to the saturation of the localized states.

Since the AlGaInN alloy has a continuous direct bandgap from about 0.7 eV (InN) to 6.2 eV (AlN), nitride-based materials can cover most of the electromag netic spectrum from near-infrared to ultraviolet. Based on this feature, nitride based light-emitting diode (LED) devices have been widely used. With the first commercialization of blue LED devices in 1993, the LED industry became more and more important in the field of lighting. As a typical light-emitting material, LED devices are not only determined by their own components, but also their luminous efficiency is one of the focuses of attention. Generally speaking, the standard for measuring the luminous efficiency of LED devices is the ratio of the number of injected carriers to the number of emitted photons, that is, the external quantum efficiency (EQE). In order to obtain a higher EQE, it can be improved from three aspects, namely internal quantum efficiency (IQE), light extraction ef ficiency (LEE) and injection efficiency (IE). However, since LED devices are often grown by vapor phase epitaxy, the epitaxial growth substrate often absorbs the light emitted by the LED device, thereby reducing the EQE of the entire device and affecting the luminous efficiency. Especially as the light-emitting wavelength of LEDs becomes longer and longer, the EQE of LED devices tends to drop from more than 80% to 4% or even lower (the decline of red LEDs will be more signif icant). At the same time, as the size of LED devices decreases, the proportion of damage caused by the mesa etching process and the surface recombination area of devices (such as Micro LED devices) increases accordingly, and EQE will also show a clear downward trend. Therefore, in addition to further improving EQE through internal quantum efficiency, increasing LEE as much as possible through structural changes is also a key point to improve EQE. In our study, based on our group’s own grown red LEDs, we successfully transferred structured vertical InGaN red LEDs from Si(111) substrates to new substrates, achieving further improvements in LEE. At the same time, it also provides options for applying this technology to LED devices and micro-LED devices of various wavelengths in the future. The LED device with the vertical structure has a low turn-on work ing voltage and a small series resistance. The whole process adopts dry etching technology, which makes the process more precise and reliable. Compared to standard LED devices, the operating voltage and series resistance of LEDs are changed from 30Ω to less than 10Ω respectively, and the LEE is improved by 70%, which is mainly attributed to the removal of the light-collecting substrate and the use of metal reflective layers to improve light extraction efficiency. Furthermore, although the process is an improvement over LEE, this structure-based process improvement can be used for LEDs of various wavelengths as well as micro-LEDs in the future. This typical substrate transfer technique can transfer very thin (3 micron) LED structures from one substrate to another without damaging the device itself, thus providing a way to realize flexible substrates in the near future.

L’alliage de Nitrure de Gallium et d’Indium (InGaN) est devenu au cours des dernières années un semi-conducteur important pour la réalisation de composants optoélectroniques, du fait de sa bande interdite modulable en fonction de la composition d’indium, entre 0.7 eV à 3.4 eV. Ceci permet l’absorption d’une grande partie du spectre solaire et fait de l’alliage InGaN un excellent candidat pour la réalisation de cellules solaires multijonctions à très haut rendement. Ce travail de thèse a permis une investigation approfondie sur les performances de différentes structures de cellules solaire à base d’InGaN. Il s’inscrit dans le cadre d’un projet visant à associer des méthodes d’optimisation mathématique multivariée à une démarche rigoureuse de simulation s’appuyant autant que possible sur des modèles et résultats expérimentaux. Il s’agit d’une nouvelle approche qui permet d’étudier les performances des cellules solaires en optimisant simultanément plusieurs paramètres (physiques et géométriques) de la cellule solaire. Nous avons étudié pour cette thèse, différentes structures de cellules solaires à simple jonction, notamment de nouvelles structures sans couche P et avons fait également l’étude d’une structure complexe à double jonction. Ces études nous ont permis d’évaluer les performances optimales que pourraient avoir les cellules à base d’InGaN et seront importantes pour la conception et l’élaboration future de cellules solaires InGaN à haut rendement / In recent years, Gallium Indium Nitride (InGaN) alloy has become a semiconductor of choice for the realization of optoelectronic devices, because of its wide spectral coverage, with a bandgap that can be modulated, by changing the indium composition, between 0.7 eV and 3.4 eV. This allows the absorption of a large part of the solar spectrumand makes the InGaN alloy an excellent candidate for the realization of high efficiency multi-junction solar cells. This thesis work led to a further investigation into the performance of different InGaN-based solar cell structures. It is part of a project aiming to associate mathematical optimization methods with a rigorous simulation process based as much as possible on models and experimental results. This is a new optimization approach that optimizes the performance of solar cells by simultaneously optimizing several parameters (physical and geometrical) of the solar cell. We have studied for this thesis, different structures of single junction solar cells, including new structures without P layer and we have also studied a complex structure with double junction. These studies allowed us to evaluate the optimal performance that InGaN-based solar cells can achieve for their design and future development

Graded InGaN buffers were employed to relax the strain arising from the lattice and thermal mismatch in GaN/InGaN epilayers grown on sapphire. An enhanced strain relaxation was observed in GaN grown on a stack of five InGaN layers, each 200 nm thick with the In content increased in each layer, and with an intermediate thin GaN layer, 10 nm thick inserted between the InGaN layers, as compared to the conventional two-step growth of GaN epilayer on sapphire. The function of the intermediate layer is to progressively relax the strain and to annihilate the dislocations that build up in the InGaN layer. If the InGaN layers were graded too rapidly, more dislocations will be generated. This increases the probability of the dislocations getting entangled and thereby impeding the motion of the dislocations to relax the strain in the InGaN layer. The optimum growth conditions of the intermediate layer play a major role in promoting the suppression and filling of the V-pits in the GaN cap layer, and were empirically found to be a thin 10 nm GaN grown at 750 0°C and annealed at 1000 0°C. / Singapore-MIT Alliance (SMA)

Graded InGaN buffers are employed to relax the strain arising from the lattice and thermal mismatches between GaN/InGaN epilayers grown on sapphire. The formation of V-pits in linearly graded InGaN/GaN bulk epilayers is illustrated. The V-pits were sampled using Atomic Force Microscopy and Scanning Electron Microscopy to examine their variation from the theoretical geometry shape. We discovered that the size of the V-pit opening in linearly graded InGaN, with and without GaN cap layer, has a Gaussian distribution. As such, we deduce that the V-pits are produced at different rates, as the growth of the InGaN layer progresses. In Stage I, the V-pits form at a slow rate at the beginning and then accelerate in Stage II when a critical thickness is reached before decelerating in Stage III after arriving at a mean size. It is possible to fill the V-pits by growing a GaN cap layer. It turns out that the filling of the V-pits is more effective at lower growth temperature of the GaN cap layer and the size of the V-pits opening, which is continued in to GaN cap layer, is not dependent on the GaN cap layer thickness. Furthermore, graded InGaN/GaN layers display better strain relaxation as compared to conventionally grown bulk GaN. By employing a specially design configuration, the V-pits can be eliminated from the InGaN epilayer. / Singapore-MIT Alliance (SMA)

Ce travail traite de la croissance épitaxiale et de la caractérisation d’hétérostructures àbase de multi-puits quantiques (MPQ) pour des applications dans le photovoltaïque. Leséchantillons ont été obtenus par la technique d’épitaxie en phase vapeur aux organométalliques(EPVOM) sur des substrats de saphir (0001). La caractérisation structurale etoptique est réalisée par diffraction de rayons X, microscopie électronique en transmission,spectroscopie de photoluminescence et de transmission. Pour étudier la présence de l’effetphotovoltaïque et pour estimer la performance électrique des échantillons, les MPQ ont étéintégrés dans la géométrie de cellules solaires en utilisant de la photolithographie, desattaques réactives ioniques assistées par plasma inductif et des métallisations pour contacterles parties dopées n et p.Nous avons étudié l’influence de différents designs des régions actives InGaN/GaN surles propriétés optiques et électriques des échantillons, c’est-à-dire le nombre de puitsquantiques InGaN, les épaisseurs des puits et des barrières et la composition en indium dansles puits. Deux mécanismes principaux doivent être pris en compte pour une optimisationefficace de composants photovoltaïques: l’absorption des photons et la collections desporteurs. Nous avons montré qu’une augmentation du nombre de MPQ, de leur épaisseur etde la composition d’In améliorait l’absorption, mais causait aussi des pertes dans l’efficacitéde collection du fait de l’augmentation de l’épaisseur de la couche active (champ électriqueplus faible), de la difficulté des porteurs pour s’échapper de puits plus profonds et derelaxation des contraintes (création de défauts structuraux). La décroissance de l’épaisseur desbarrières peut résoudre les deux premiers points, mais le problème de la relaxation de lacontrainte reste entier. Pour notre meilleur design, nous obtenons une efficacité de conversionde 2 % pour des couches 15×In0.18Ga0.82N/GaN qui ont une réponse spectrale qui s’étendjusqu’à 465 nm. / In this work we report on epitaxial growth and characterization of InGaN/GaN multiquantumwells (MQWs) heterostructures for application in photovoltaic devices. The sampleswere grown by metal-organic vapor phase epitaxy (MOVPE) on (0001) sapphire substrate.The structural and optical characterization is performed by X-ray diffraction, transmissionelectron microscopy, photoluminescence spectroscopy and transmission measurements. Toinvestigate the presence of photovoltaic effect and estimate the electrical performance of thesamples, they were processed into solar cells by means of the photolithography, inductivelycoupled plasma reactive-ion etching and metallization to manage n and p contacts.We studied the influence of different InGaN/GaN active region designs on thestructural, optical and electrical properties of the samples, i.e. number of InGaN quantumwells, QW and quantum barrier thicknesses and indium composition in the wells. Two mainmechanisms have to be taken into account for an efficient optimization of photovoltaicdevices: photon absorption and carrier collection. We showed that an increase of the MQWsnumber, their thickness and the In-content allows absorption improvement, but causes lossesin the carrier collection efficiency due to: the increase of the active region thickness (lowerelectric field), the difficulty of the carrier to escape from deeper QWs and the strain relaxation(structural defect creation). The decrease of the barrier thickness can solve the first two issues,but the problem with strain relaxation remains. In the best design, we report the value of2.00% of conversion efficiency for 15×In0.18Ga0.82N/GaN samples with spectral responseextending to 465 nm.

The thesis is aimed at gaining new knowledge on carrier localization and recombination in InGaN epilayers and structures by using photoluminescence spectroscopy with sub-micrometer spatial resolution. Optical characterization is combined with the structural analysis to provide a deeper insight into peculiarities of InGaN luminescence. Studies of InGaN epitaxial layers showed the relaxed layers to contain nanocolumn-like structures that additionally contribute to inhomogeneous photoluminescence distribution in InGaN layers. The feasibility of suppressing the defect-related emission in InGaN epilayers by laser annealing is demonstrated. The influence of unintentional annealing at elevated temperatures during fabrication of InGaN structures is revealed. A novel interpretation for negative correlation between photoluminescence intensity and band peak wavelength in high-indium-content InGaN multiple quantum wells is suggested. The enhancement of emission efficiency in InGaN quantum wells due to coupling of the optical dipole with localized surface plasmons in silver nanoparticles is investigated and the influence of potential fluctuations on the coupling with localized surface plasmons is revealed. / Disertacija yra skirta krūvininkų lokalizacijos ir rekombinacijos tyrimams InGaN epitaksiniuose sluoksniuose ir dariniuose panaudojant fotoliuminescencinę spektroskopiją su submikrometrine erdvine skyra. Optinius bandinių tyrimus papildo struktūrinė analizė. Darbe parodyta, jog relaksavusiuose InGaN sluoksniuose egzistuoja į nanokolonas panašūs dariniai, kurie papildomai prisideda prie netolygaus fotoliuminescencijos pasiskirstymo InGaN epitaksiniuose sluoksniuose. Pademonstruota galimybė nuslopinti priemaišinės kilmės liuminescenciją InGaN sluoksnyje jį iškaitinus lazerio spinduliuote. Atskleistas netyčinis InGaN darinių aktyviojo sluoksnio iškaitinimas, vykstantis formuojant p tipo sluoksnius, kuris keičia InGaN kvantinių darinių optines sąvybes. Išnagrinėta neigiama koreliacja tarp fotoliuminescencijos intensyvumo ir juostos viršūnės bangos ilgio InGaN kvantiniuose dariniuose ir pasiūlytas naujas šios ypatybės interpretavimo modelis. Lokaliai stebėtas liuminescencijos intensyvumo padidėjimas dėl sąveikos su lokalizuotais paviršiniais plazmonais, indukuotais sidabro nanodalelėse. Parodytas potencialo fliuktuacijų daromas poveikis rezonansinei sąveikai tarp optinių dipolių kvantinėse duobėse ir lokalizuotų paviršinių plazmonų.

Disertacija yra skirta krūvininkų lokalizacijos ir rekombinacijos tyrimams InGaN epitaksiniuose sluoksniuose ir dariniuose panaudojant fotoliuminescencinę spektroskopiją su submikrometrine erdvine skyra. Optinius bandinių tyrimus papildo struktūrinė analizė. Darbe parodyta, jog relaksavusiuose InGaN sluoksniuose egzistuoja į nanokolonas panašūs dariniai, kurie papildomai prisideda prie netolygaus fotoliuminescencijos pasiskirstymo InGaN epitaksiniuose sluoksniuose. Pademonstruota galimybė nuslopinti priemaišinės kilmės liuminescenciją InGaN sluoksnyje jį iškaitinus lazerio spinduliuote. Atskleistas netyčinis InGaN darinių aktyviojo sluoksnio iškaitinimas, vykstantis formuojant p tipo sluoksnius, kuris keičia InGaN kvantinių darinių optines sąvybes. Išnagrinėta neigiama koreliacja tarp fotoliuminescencijos intensyvumo ir juostos viršūnės bangos ilgio InGaN kvantiniuose dariniuose ir pasiūlytas naujas šios ypatybės interpretavimo modelis. Lokaliai stebėtas liuminescencijos intensyvumo padidėjimas dėl sąveikos su lokalizuotais paviršiniais plazmonais, indukuotais sidabro nanodalelėse. Parodytas potencialo fliuktuacijų daromas poveikis rezonansinei sąveikai tarp optinių dipolių kvantinėse duobėse ir lokalizuotų paviršinių plazmonų. / The thesis is aimed at gaining new knowledge on carrier localization and recombination in InGaN epilayers and structures by using photoluminescence spectroscopy with sub-micrometer spatial resolution. Optical characterization is combined with the structural analysis to provide a deeper insight into peculiarities of InGaN luminescence. Studies of InGaN epitaxial layers showed the relaxed layers to contain nanocolumn-like structures that additionally contribute to inhomogeneous photoluminescence distribution in InGaN layers. The feasibility of suppressing the defect-related emission in InGaN epilayers by laser annealing is demonstrated. The influence of unintentional annealing at elevated temperatures during fabrication of InGaN structures is revealed. A novel interpretation for negative correlation between photoluminescence intensity and band peak wavelength in high-indium-content InGaN multiple quantum wells is suggested. The enhancement of emission efficiency in InGaN quantum wells due to coupling of the optical dipole with localized surface plasmons in silver nanoparticles is investigated and the influence of potential fluctuations on the coupling with localized surface plasmons is revealed.

Ce travail traite de la croissance épitaxiale et de la caractérisation d’hétérostructures àbase de multi-puits quantiques (MPQ) pour des applications dans le photovoltaïque. Leséchantillons ont été obtenus par la technique d’épitaxie en phase vapeur aux organométalliques(EPVOM) sur des substrats de saphir (0001). La caractérisation structurale etoptique est réalisée par diffraction de rayons X, microscopie électronique en transmission,spectroscopie de photoluminescence et de transmission. Pour étudier la présence de l’effetphotovoltaïque et pour estimer la performance électrique des échantillons, les MPQ ont étéintégrés dans la géométrie de cellules solaires en utilisant de la photolithographie, desattaques réactives ioniques assistées par plasma inductif et des métallisations pour contacterles parties dopées n et p.Nous avons étudié l’influence de différents designs des régions actives InGaN/GaN surles propriétés optiques et électriques des échantillons, c’est-à-dire le nombre de puitsquantiques InGaN, les épaisseurs des puits et des barrières et la composition en indium dansles puits. Deux mécanismes principaux doivent être pris en compte pour une optimisationefficace de composants photovoltaïques: l’absorption des photons et la collections desporteurs. Nous avons montré qu’une augmentation du nombre de MPQ, de leur épaisseur etde la composition d’In améliorait l’absorption, mais causait aussi des pertes dans l’efficacitéde collection du fait de l’augmentation de l’épaisseur de la couche active (champ électriqueplus faible), de la difficulté des porteurs pour s’échapper de puits plus profonds et derelaxation des contraintes (création de défauts structuraux). La décroissance de l’épaisseur desbarrières peut résoudre les deux premiers points, mais le problème de la relaxation de lacontrainte reste entier. Pour notre meilleur design, nous obtenons une efficacité de conversionde 2 % pour des couches 15×In0.18Ga0.82N/GaN qui ont une réponse spectrale qui s’étendjusqu’à 465 nm. / In this work we report on epitaxial growth and characterization of InGaN/GaN multiquantumwells (MQWs) heterostructures for application in photovoltaic devices. The sampleswere grown by metal-organic vapor phase epitaxy (MOVPE) on (0001) sapphire substrate.The structural and optical characterization is performed by X-ray diffraction, transmissionelectron microscopy, photoluminescence spectroscopy and transmission measurements. Toinvestigate the presence of photovoltaic effect and estimate the electrical performance of thesamples, they were processed into solar cells by means of the photolithography, inductivelycoupled plasma reactive-ion etching and metallization to manage n and p contacts.We studied the influence of different InGaN/GaN active region designs on thestructural, optical and electrical properties of the samples, i.e. number of InGaN quantumwells, QW and quantum barrier thicknesses and indium composition in the wells. Two mainmechanisms have to be taken into account for an efficient optimization of photovoltaicdevices: photon absorption and carrier collection. We showed that an increase of the MQWsnumber, their thickness and the In-content allows absorption improvement, but causes lossesin the carrier collection efficiency due to: the increase of the active region thickness (lowerelectric field), the difficulty of the carrier to escape from deeper QWs and the strain relaxation(structural defect creation). The decrease of the barrier thickness can solve the first two issues,but the problem with strain relaxation remains. In the best design, we report the value of2.00% of conversion efficiency for 15×In0.18Ga0.82N/GaN samples with spectral responseextending to 465 nm.