Real structure of heteroepitaxial GaN and AlGaN layers is studied by diffuse x-ray scattering. A new developed method based on Monte Carlo simulation enabling to determine densities of threading dislocations in c-plane GaN and stacking faults in a-plane GaN is presented. The results of Monte Carlo simulations are compared with ones obtained by use of other conventional techniques. The advantages and limitations of the new method are discussed in detail. The methods accuracy is estimated as about 15%. We have shown that our method is a reliable tool for threading dislocations and stacking faults densities determination.

abstract: Group III-nitride semiconductors have wide application in optoelectronic devices. Spontaneous and piezoelectric polarization effects have been found to be critical for electric and optical properties of group III-nitrides. In this dissertation, firstly, the crystal orientation dependence of the polarization is calculated and in-plane polarization is revealed. The in-plane polarization is sensitive to the lateral characteristic dimension determined by the microstructure. Specific semi-polar plane growth is suggested for reducing quantum-confined Stark effect. The macroscopic electrostatic field from the polarization discontinuity in the heterostructures is discussed, b ased on that, the band diagram of InGaN/GaN quantum well/barrier and AlGaN/GaN heterojunction is obtained from the self-consistent solution of Schrodinger and Poisson equations. New device design such as triangular quantum well with the quenched polarization field is proposed. Electron holography in the transmission electron microscopy is used to examine the electrostatic potential under polarization effects. The measured potential energy profiles of heterostructure are compared with the band simulation, and evidences of two-dimensional hole gas (2DHG) in a wurtzite AlGaN/ AlN/ GaN superlattice, as well as quasi two-dimensional electron gas (2DEG) in a zinc-blende AlGaN/GaN are found. The large polarization discontinuity of AlN/GaN is the main source of the 2DHG of wurtzite nitrides, while the impurity introduced during the growth of AlGaN layer provides the donor states that to a great extent balance the free electrons in zinc-blende nitrides. It is also found that the quasi-2DEG concentration in zinc-blende AlGaN/GaN is about one order of magnitude lower than the wurtzite AlGaN/GaN, due to the absence of polarization. Finally, the InAlN/GaN lattice-matched epitaxy, which ideally has a zero piezoelectric polarization and strong spontaneous polarization, is experimentally studied. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1-xN layer. The V-grooves coalesce at 200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. The structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves. / Dissertation/Thesis / Ph.D. Physics 2012

Ce travail de thèse porte sur l’étude des propriétés optiques et électroniques des puits quantiques de GaN / AlGaN grâce à des techniques classiques de réflectivité résolue en angle et de photoluminescence, ainsi qu’avec la technique de photoluminescence résolue temporellement. Les expériences de photoluminescence en régime continu ont permis d’estimer les énergies des transitions excitoniques qui sont également accessibles en réflectivité. Ces techniques ont ainsi permis de mettre en évidence l’effet Stark dans les puits quantiques GaN / AlGaN. L’effet Stark sur les énergies de transition est cohérent avec la théorie des fonctions enveloppes. Les spectres de réflectivité permettent d’accéder à la force d’oscillateur des excitons grâce à leur modélisation par le formalisme des matrices de transfert, prenant en compte les phénomènes d’élargissement homogène et inhomogènes des transitions optiques. Enfin, les mesures de photoluminescence résolue en temps en fonction de la température, ont également permis d’extraire la force d’oscillateur qui est inversement proportionnelle au temps de recombinaison radiative. Cette étude a également permis de mettre en évidence l’effet Stark responsable de la diminution de la force d’oscillateur en fonction de l’épaisseur du puits quantique mais aussi en fonction de la composition d’aluminium. L’augmentation de l’épaisseur du puits entraîne une diminution du recouvrement des fonctions d’onde, et une augmentation de la composition d’aluminium intensifie le champ électrique et diminue également le recouvrement des fonctions d’onde. / This work deals with the study of optical and electronic properties of GaN / AlGaN quantum wells, by classical techniques of spectroscopy including angle resolved reflectivity or photoluminescence, but also by time resolved photoluminescence. The continuous wave photoluminescence experiments allowed estimating the energies of the excitonic transitions, which are also available through reflectivity. These techniques highlighted the Stark effect in GaN / AlGaN quantum wells. The influence of the Stark effect on the energies of the excitonic transitions is well reproduced by envelop functions theory. Reflectivity spectra give access to the oscillator strength via their fitting by transfer matrix formalism, taking in account both homogeneous and inhomogeneous broadenings of the optical transitions. Finally, time resolved photoluminescence measurements as a function of temperature were performed to extract the oscillator strength, which is proportional to the inverse of the radiative recombination time. This technique also highlighted the Stark effect which is responsible of the vanishing of the oscillator strength with the thickness of the well and the aluminium composition. Increasing of the quantum well’s thickness induces decreasing of wave functions overlap, as well as an increasing of the aluminium composition which intensifies the electric field and splits the wave functions.

Neste trabalho estudamos as propriedades físicas das impurezas de elementos de terra rara (TR) nos cristais de GaN, nas estruturas cristalinas zincblenda e wurtzita, e de ZnO. Para tal, consideramos as impurezas de Eu, Gd, Tb, Dy, Ho, Er e Tm substitucionais no sítio do cátion (Ga ou Zn), pois esta posição é mais estável do que no sítio do ânion (N ou O). As investigações foram efetuadas através de simulações computacionais baseadas em métodos de primeiros princípios dentro do formalismo da teoria do funcional da densidade e utilizando o método FP-LAPW (Full Potential Linear Augmented Plane Waves), implementado no código computacional WIEN2k, dentro do esquema de supercélula, com relaxações atômicas tratadas de modo apropriado. No tratamento dos estados de valência 3d (Ga ou Zn) e 4f dos átomos de TR foi introduzida uma correção local de Hubbard, para levar em conta efeitos de alta correlação eletrônica. Inicialmente, estudamos as propriedades eletrônicas e estruturais dos elementos de TR em suas fases metálicas. Estes resultados serviram para validar a metodologia de obtenção dos valores dos parâmetros U de Hubbard de modo autoconsistente, pois para este grupo de materiais existem resultados experimentais. Comparando a localização e o desdobramento entre os estados ocupados e desocupados relacionados aos orbitais 4f, em relação ao nível de Fermi dos sistemas, obteve-se uma ótima concordância. Nossos resultados para os sistemas dopados, utilizando a correção do potencial U de Hubbard, mostram que a suas descrições estão adequadas. Somente com a introdução da correção é possível localizar corretamente, em relação ao topo da banda de valência, os estados ocupados e desocupados relacionados aos orbitais 4f. Estes resultados estão de acordo com esquemas propostos na literatura, mostrando que o estado de oxidação das impurezas de TR é trivalente e que, em geral, em ambos sistemas cristalinos, não há a introdução de níveis de energia na região do gap dos materiais. Nosso estudo da correção do potencial U de Hubbard, obtido autoconsistemente, na teoria do funcional da densidade, mostra que seus valores não são universais, dependendo do estado de carga e do ambiente em que o átomo está inserido. Mais ainda, mostra que o procedimento adotado é totalmente apropriado para descrever a correlação eletrônica dos elétrons 4f. / In this work we studied the physical properties of rare earth (RE) impurities in GaN, in the zincblend and the wurtzite crystal structures, and in ZnO. Eu, Gd, Tb, Dy, Ho, Er and Tm substitutional impurities in the cation site (Ga or Zn) were considered, since this position is more stable than anion site (N or O). The investigations were carried out by computational simulations using ab initio methods, based on the density functional theory and the FP-LAPW (Full Potential Linear Augmented Plane Waves) method, as implemented in the WIEN2k code, considering the supercell approach and atomic relaxations. The 3d-Ga or 3d-Zn and the 4f-RE valence states were treated with the introduction of on-site Hubbard correction, in order to correctly describe the strongly correlated electrons. First, the electronic and structural properties of RE metallic systems were investigated and the results were compared with available experimental data, showing a good agreement. Those results helped to identify the appropriate procedure to compute the Hubbard U potential. This procedure should also provide a reliable description about the electronic properties of RE elements as impurities in semiconductors. Then, we have computed the properties of substitutional RE impurities in the cation site using the same methodology and procedures. The Hubbard U potential was necessary to correctly describe the position of the 4f occupied and unoccupied states, related to the valence band top. These results are in agreement with proposed electronic properties of RE doped GaN and ZnO found in the literature. The RE impurities oxidation states are in general trivalent in both crystal systems and introduce no energy levels in the gap region. Our Hubbard U potential correction, obtained in a self-consistent way, depends on the considered element and the neighborhood, and it is not an universal parameter. Moreover, our investigation shows that the adopted procedure is totally appropriate to describe the electronic correlation of the 4f electrons.

Im Rahmen der Arbeit wurde eine neuartige Variante der Hochtemperatur-Gasphasenepitaxie (HTVPE) für die Herstellung von GaN entwickelt, die eine hohe Flexibilität und bessere Kontrolle des Züchtungsprozesses ermöglicht. Für die Realisierung des Konzeptes wurde eine Züchtungsanlage für die HTVPE entworfen und aufgebaut. Des Weiteren wurde ein numerisches Modell des Wärme- und Stofftransports entwickelt und für die Untersuchungen der Transportphänomene im HTVPE-Reaktor sowie für die Weiterentwicklung des Züchtungsreaktors verwendet. Die systematischen Züchtungsexperimente zeigten eine gute Übereinstimmung mit den Simulationsergebnissen und lieferten ein besseres Verständnis der HTVPE und ihres Anwendungspotentials. Die versbesserte Prozesskontrolle ermöglichte die erstmalige Anwendung der Niedertemperatur-Nukleation für die heteroepitaktische Abscheidung von GaN auf Saphir mit der HTVPE. Weiterhin wurden Wachstumsraten über 80 µm/h erreicht und das Potential der HTVPE für die Herstellung von GaN-Volumenschichten demonstriert.

Un des avantages majeurs des nanofils (NFs) semi-conducteurs est la possibilité d'intégrer ces nano-matériaux sur divers substrats. Cette perspective est particulièrement intéressante pour les nitrures d'éléments III qui manquent d'un substrat idéal. Nous avons étudié l'utilisation de nouveaux supports pour la croissance de NFs de GaN en épitaxie par jets moléculaires assistée par plasma. Nous avons exploré trois approches avec une caractéristique commune : le support de base est un substrat amorphe bas-coût. Pour deux d'entre elles, une fine couche d'un matériau cristallin est déposée sur ce support pour promouvoir la croissance épitaxiale des NFs. Dans la première approche, nous avons formé un film mince de Si poly-cristallin par "cristallisation induite par l'aluminium (AIC-Si)". Les conditions ont été optimisées pour obtenir une forte texture de fibre orientée [111] du film de Si qui nous a permis de faire croitre des NFs de GaN verticaux. La même idée a été mise en ¿uvre avec le graphène transféré sur SiOx. Nous avons montré pour la première fois dans la littérature que les NFs de GaN adoptent une orientation basale bien définie par rapport au graphène. La troisième approche consiste à faire croitre des NFs directement sur les substrats amorphes. Nous avons utilisé la silice thermique et la silice fondue. Nous avons examiné le temps de latence avant la formation des premiers germes et obtenu des NFs de GaN de bonne verticalité sur les deux types de silice. Sur la base de nos observations, nous concluons que la croissance épitaxiale de NFs de GaN sur graphène est particulièrement prometteuse pour le développement de dispositifs flexibles. / A major advantage of semiconductor nanowires (NWs) is the possibility to integrate these nano-materials on various substrates. This perspective is particularly attractive for III-nitrides, for which there is a lack of an ideal substrate. We examined the use of novel templates for growing GaN NWs by plasma assisted molecular beam epitaxy. We explored three approaches with a common feature: the base support is a cost-efficient amorphous substrate and a thin crystalline material is deposited on the support to promote epitaxial growth of GaN NWs.In the first approach, we formed polycrystalline Si thin films on amorphous support by a process called aluminum-induced crystallization (AIC-Si). The conditions of this process were optimized to get a strong [111] fiber-texture of the Si film which enabled us to grow vertically oriented GaN NWs. The same idea was implemented with graphene as an ultimately thin crystalline material transferred on SiOx. We illustrated for the first time in literature that GaN NWs and the graphene layer have a single relative in-plane orientation. We propose a plausible epitaxial relationship and demonstrate that the number of graphene layers has a strong impact on GaN nucleation. Proof-of-concept for selective area growth of NWs is provided for these two approaches. As a simple approach, the possibility of growing NWs directly on amorphous substrates was explored. We use thermal silica and fused silica. Self-induced GaN NWs were formed with a good verticality on both substrates. Based on our observations, we conclude that the epitaxial growth of GaN NWs on graphene looks particularly promising for the development of flexible devices.

No description available.

Les matériaux III-N sont présents dans la vie quotidienne pour des applications optoélectroniques (diodes électroluminescentes, lasers). Les propriétés remarquables du GaN (grand gap, grand champ de claquage, champ de polarisation élevé, vitesse de saturation des électrons importante…) en font un candidat de choix pour des applications en électronique de puissance à basse fréquence, mais aussi à haute fréquence, par exemple en gamme d'ondes millimétriques. L’enjeu de ce travail de thèse consiste à augmenter la fréquence de travail des transistors tout en maintenant une puissance élevée. Pour cela, des hétérostructures HEMTs (High Electron Mobility Transistors) sont développées et les épaisseurs de cap et de barrière doivent être réduites, bien que ceci soit au détriment de la puissance délivrée. Une étude sera donc menée sur l’influence des épaisseurs de cap et de barrière ainsi que le type de barrière (AlGaN, AlN et InAlN) de manière à isoler les hétérostructures offrant le meilleur compromis en termes de fréquence et de puissance. De plus, les moyens mis en œuvre pour augmenter la fréquence de travail entrainent une dégradation du confinement des électrons du canal. De manière à limiter cet effet, une back-barrière est insérée sous le canal. Ceci fera l’objet d’une deuxième étude. Enfin, une étude de la passivation de surface des transistors sera menée. La combinaison des ces trois études permettra d’identifier la structure optimale pour délivrer le plus de puissance à haute fréquence (ici à 40 GHz). / Nitride based materials are present in everyday life for optoelectronic applications (light emitting diodes, lasers). GaN remarkable properties (like large energy band gap, high breakdown electric field, high polarization field, high electronic saturation velocity…) make it a promising candidate for low frequency power electronic applications, but also for high frequency like microwaves range for example. The aim of this work is to increase the transistors working frequency by keeping a high power. To do this, high electron mobility transistor heterostructures are developed, and cap and barrier thicknesses have to be reduced, although it is detrimental for a high power. A first study deals with the influence of cap and barrier thicknesses as well as the type of barrier (AlGaN, AlN and InAlN), in order to isolate heterostructures offering the best compromise in terms of power and frequency. Moreover, the means implemented to increase the working frequency lead to electron channel confinement degradation. In order limit this effect, a back-barrier is added underneath the channel. It will be the subject of the second study. Finally, a transistor surface passivation study will be led. The combination of those three parts will allow identifying the optimum structure to deliver the highest power at high frequency (here at 40 GHz).

III-nitridy běžně krystalizují v hexagonální (wurtzitové) struktuře, zatímco kubická (sfaleritová) struktura je metastabilní a má pouze mírně vyšší energii. Jejich fyzikální vlastnosti jsou silně ovlivněny přítomností rozsáhlých defektů, které jsou v těchto dvou strukturách od sebe odlišné. U wurtzitových nitridů se jedná primárně o vláknové dislokace. Některé vláknové dislokace tvoří hluboké energetické stavy v zakázaném pásu, kterými ovlivňují elektrické a optoelektronické vlastnosti těchto materiálů. Oproti tomu, kubické nitridy obsahují množství vrstevných chyb, které představují lokální transformace do stabilnější wurtzitové struktury. Cílem této práce je charakterizovat rozsáhlé defekty v obou krystalových strukturách pomocí elektronové mikroskopie, mikroskopie atomárních sil a rentgenové difrakce. Prokázali jsme, že vzorky GaN/AlN a AlN s orientací (0001) rostlé na substrátu Si (111) pomocí epitaxe z organokovových sloučenin obsahují velkou hustotu vláknových dislokací. Nejčastější jsou dislokace s Burgersovým vektorem s komponentou ve směru a wurtzitové struktury, následované dislokacemi s Burgersovým vektorem s komponentou ve směru a+c, zatímco dislokace s Burgersovým vektorem s c komponentou jsou relativně vzácné. Pravděpodobný původ vláknových dislokací je diskutován v souvislosti s různými mechanismy růstu těchto vrstev. Prizmatické vrstevné chyby byly nalezeny v tenkých nukleačních vrstvách AlN, ale v tlustších vrstvách již nebyly přítomny. Na rozhraní AlN / Si byla nalezena amorfní vrstva složená ze SiNx a částečně taky z AlN. Navrhujeme, že by tato amorfní vrstva mohla hrát významnou roli při relaxaci misfitového napětí. Analýza elektrické aktivity rozsáhlých defektů v AlN byla provedena pomocí měření proudu indukovaného elektronovým svazkem. Zjistili jsme, že vláknové dislokace způsobují slabý pokles indukovaného proudu. Díky jejich vysoké hustotě a rovnoměrnému rozložení však mají větší vliv na elektrické vlastnosti, než mají V-defekty a jejich shluky. Topografické a krystalografické defekty byly studovány na nežíhaných a žíhaných nukleačních vrstvách kubického GaN deponovaných na 3C-SiC (001) / Si (001) substrátu. Velikost ostrůvků na nežíhaných vzorcích se zvyšuje s tloušťkou nukleační vrstvy a po žíhání se dále zvětšuje. Po žíhání se snižuje pokrytí substrátu u nejtenčích nukleačních vrstev v důsledku difúze a desorpce (nebo leptání atmosférou reaktoru). Vrstevné chyby nalezené ve vrstvách GaN, poblíž rozhraní se SiC, byly většinou identifikovány jako intrinsické a byly ohraničené Shockleyho parciálními dislokacemi. Jejich původ byl diskutován, jako i vliv parciálních dislokací na relaxaci misfitového napětí. Díky velkému množství vrstevných chyb byly podrobněji studovány jejich interakce. Na základě našich zjištění jsme vyvinuli teoretický model popisující anihilaci vrstevných chyb v kubických vrstvách GaN. Tento model dokáže předpovědět pokles hustoty vrstevných chyb se zvyšující se tloušťkou vrstvy.

This master’s thesis deals with the design and realization of a functional sample power inverter for an electric compressor, which is used in hybrid cars. The electric compressor powered by the inverter is E-compressor by Garrett Advancing Motion. An inverter will be using modern High Electron Mobility Transistors which are based on gallium nitride (GaN). The purpose of this thesis is to find if GaN transistors can be used in E-boosting application.