Dentre os materiais de estrutura perovskita, o titanato de estrôncio, SrTiO3, é um dos mais conhecidos e estudado por apresentar interessantes propriedades como a ferroeletricidade, fotoluminescência, entre outras. Além disso, tem sido reportado na literatura que a adição de diferentes dopantes ao composto SrTiO3 modifica suas propriedades dando origem, por exemplo, a materiais para serem aplicados como sensores de gás ou na fotocatálise. Diferentes trabalhos encontrados na literatura descrevem a síntese do composto SrTiO3 e suas soluções sólidas através do método hidrotermal. Entretanto, poucos trabalhos tiveram como foco o estudo do precursor de Ti e do íon dopante nas propriedades estruturais, morfológicas, ópticas e elétricas destes materiais. Desta forma, este trabalho de tese teve como objetivo o estudo da influência da composição e de alguns parâmetros de síntese sobre as propriedades estruturais, morfológicas e ópticas do composto SrTiO3 preparado através do método hidrotermal assistido por microondas HAM. Inicialmente, foi realizada a síntese do composto SrTiO3 a 140oC por 10 minutos, onde se verificou a possibilidade de controlar a morfologia e a propriedade fotoluminescente das amostras através do tipo de precursor de Ti utilizado. Em uma segunda etapa do trabalho, o composto SrTiO3 foi sintetizado por tempos variando de 10 a 640 minutos. A caracterização destas amostras mostrou a formação da fase cúbica do SrTiO3 exibindo morfologia na forma de cubos, formados pelo processo de auto-organização, que tornam-se mais bem definidos com o aumento do tempo de síntese. Medidas de espectroscopia XANES e EXAFS na borda K do Ti mostraram que as amostras de SrTiO3 sintetizadas pelo método HAM apresentam um alto grau de desordem na estrutura local, caracterizada pela coexistência de unidades do tipo TiO5 e TiO6. Além disso, observou-se que o aumento do tempo de síntese leva a uma redução da intensidade fotoluminescente que foi atribuído à redução na concentração de vacâncias de oxigênio. Medidas de DRX das amostras do sistema SrTi1-xFexO3, ao nosso conhecimento pela primeira vez sintetizadas com sucesso através do método HAM, mostraram a formação da fase cúbica quando x &#8804 0,4. Medidas do espectro XANES e EXAFS revelaram a substituição Ti por íons Fe+2 e Fe+3, levando a formação de vacâncias de oxigênio as quais contribuem para a diminuição da energia do band-gap das amostras de 3,2 para 2,8 eV, bem como na total supressão da intensidade fotoluminescente das amostras. A análise por microscopia eletrônica de varredura FE-MEV mostrou a existência de partículas na forma de cubos cuja cinética de formação é influenciada pela variação da quantidade de ferro. Imagens de microscopia eletrônica de transmissão (MET) confirmaram a natureza mesocristalina destas partículas cujo mecanismo de crescimento ocorre por coalescência orientada, originada pelo processo de auto-organização. As amostras do sistema SrTi1-xFexO3 na forma de filmes finos foram avaliadas como sensores O3, NO2, NH3 e CO. As análises indicaram que os filmes depositados pela técnica de evaporação de feixe de elétrons apresentam um grande potencial para serem aplicados como sensor de gás ozônio, exibindo uma boa sensibilidade e seletividade comparada a outros tipos de materiais sensores. / Strontium titanate, SrTiO3, it is one of the most known and intensively studied perovskite compounds due its interesting properties such as ferroelectricity, photoluminescence, etc. According to the literature, the dopant addition into SrTiO3 network can create materials with desirable functions, for example, gas sensing and photocatalytic activity. Despite some authors reported the synthesis of pure or doped SrTiO3 by hydrothermal method, few studies have been devoted to investigate the effects of the Ti precursor and type of dopant ion on the structural, morphological and electrical properties of SrTiO3 compound. In this work, we investigated the influence of synthesis parameters and concentration iron on the structural, morphological and optical properties of SrTiO3 prepared by microwave-assisted hydrothermal method (MAH). Firstly, it was observed that the appropriate choice of the Ti precursor allowed the control of morphological and photoluminescence properties of SrTiO3 compound synthesized at 140oC for 10 minutes by MAH method. Next, SrTiO3 was synthesized during different treatment times varying from 10 to 640 minutes. X-ray diffraction (XRD) measurements indicate a SrTiO3 cubic perovskite structure and FE-SEM images revealed that the samples exhibit a cube-like shape formed by an assembly process, becoming well defined as a function of MAH treatment time. Ti-K edge XANES and EXAFS measurements indicated a large local structural distortion, revealed by the presence of TiO6 and TiO5 units. Moreover, we observed a reduction of the photoluminescence intensity as a function of treatment time probably due to decreasing of the oxygen vacancy concentration. To the best of our knowledge, this is the first time that the SrTi1-xFexO3 solid solution was synthesized by MAH method. XRD analyses indicated a cubic perovskite structure when x &#8804 0.4. XANES and EXAFS measurements revealed that iron ions present a mixed Fe+2/Fe+3 oxidation state and occupy preferentially the Ti4+-site. A UV-visible spectrum shows that the addition of iron reduces the value of optical gap of the 3.2 eV to 2.8 eV and consequently suppresses the photoluminescence intensity. An analysis of FE-SEM and HRTEM images point out that, independently of iron content, the nanoparticles have a cube-like morphology and are formed by a self-assembly of small primary nanocrystals. In addition, SrTi1-xFexO3 thin films were investigated as gas sensor towards O3, NO2, NH3 and CO gases The results indicate that the films deposited by electron beam deposition method exhibits a good response as ozone sensor compared to others gas sensors materials.

Made available in DSpace on 2015-05-14T13:21:38Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 8317996 bytes, checksum: 64db9066ac027260f6795886de515804 (MD5) Previous issue date: 2013-10-07 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Strontium stannate (SrSnO3) and titanate (SrTiO3) are perovskite type oxides that have orthorhombic (Pbnm) and cubic (Pm3m) structures, respectively. These materials have received much attention due to their interesting physical and chemical characteristics, leading to a variety of technological applications. In this sense, these two materials were combined to each other in order to obtain powders and thin films of a solid solution, SrSn1-xTixO3. In relation to the powders, this solid solution presented successive phase transitions ranging from orthorhombic and tetragonal structures to a cubic one with increasing of Ti4+ amount in the composition. These transitions were observed by XRD Rietveld refinement of the samples and confirmed by Raman spectroscopy. The different crystalline structures of the compositions within the solid solution led to different photoluminescent properties in the visible spectrum varying the range of emission, moving from a lower energy region to a higher one with increasing of Ti4+ in the structure (from orthorhombic (SrSnO3), tetragonal (SrSn0.75Ti0.25O3) to cubic (SrSn0.50Ti0.50O3 ; SrSn0.25Ti0.75O3 and SrTiO3). These emissions were probably favored by specific defects created inside the band gap of these materials. On the other hand, the thin films of this system showed different growth orientations that are associated to the crystalline nature of the substrates, the composition of the thin films and the deposition method (Chemical Solution Deposition - CSD - and Pulsed Laser Deposition - PLD). The films deposited on silica substrate were polycrystalline (random growth of the crystallites), whereas the films deposited on sapphire-R (Al2O3-012) were also polycrystalline, with a preferred orientation (h00) for the films deposited by PLD (textured growth), except SrTiO3 which was rotated 45° epitaxially in the sapphire plane. On the contrary, all of the films deposited on LAO (LaAlO3-100) had an (h00) epitaxial growth. In addition to that, the morphological characteristics and photocatalytic properties were strongly influenced also by the same parameters described above. Regarding the photocatalytic efficiency of the films, those obtained by CSD were more efficient than the ones obtained by PLD. Moreover, the films with Sn4+ richer compositions were the most active in the photodegradation of the azo dye Remazol yellow gold, reaching a maximum efficiency with the polycrystalline SrSnO3 thin film obtained by CSD whose degradation and decolorization percentage were 55 and 90 %, respectively. The type of orientation of the SrSnO3 films showed also a strong influence on the photodegradation of the dye. The polycrystalline films with a random orientation obtained on silica were more efficient than the textured films and these ones were more than the epitaxial films / O estanato (SrSnO3) e o titanato de estrôncio (SrTiO3) são óxidos do tipo perovskita que apresentam estrutura ortorrômbica (Pbnm) e cúbica (Pm3m), respectivamente. Estes materiais têm recebido bastante atenção nos últimos anos devido às suas características físicas e químicas interessantes, levando a diferentes tipos de aplicações tecnológicas. Portanto, estes dois materiais foram combinados com intuito de obter uma solução sólida, SrSn1-xTixO3, na forma de pós e filmes finos. Em relação aos pós, esta solução sólida apresentou transições de fase sucessivas com o aumento da quantidade de Ti4+ no sistema, passando de uma estrutura ortorrômbica à tetragonal levando à cúbica. Estas transições foram observadas através de difração de raios-X e refinamento Rietveld, e confirmadas por espectroscopia vibracional Raman. Os diferentes tipos de estrutura cristalina das composições da solução sólida levaram a diferentes propriedades fotoluminescentes no espectro visível, variando a região de emissão de mais baixa à mais alta energia com o aumento de Ti4+ na estrutura, ou seja, passando da estrutura ortorrômbica (SrSnO3), tetragonal (SrSn0,75Ti0,25O3) até à cúbica (SrSn0,50Ti0,50O3, SrSn0,25Ti0,75O3 e SrTiO3). Estas emissões foram provavelmente favorecidas pelos tipos de defeitos específicos criados dentro do band gap destes materiais. Por outro lado, os filmes finos deste sistema apresentaram diferentes tipos de crescimento os quais são associados à natureza cristalina dos substratos, composição dos filmes e com o método de deposição utilizado (método de deposição química em solução - CSD - e método de deposição por laser pulsado PLD). Os filmes depositados sobre sílica e safira-R (Al2O3-012) apresentaram como policristalinos (crescimento aleatorio dos cristalitos), enquanto os filmes depositados por PLD apresentaram orientação preferencial (h00) (crescimento texturizado), com exceção do filme de SrTiO3 que é epitaxial rotacionado em 45º no plano da safira-R. Todos os filmes depositados através dos dois métodos de deposição sobre LAO (LaAlO3-100) apresentaram um crescimento epitaxial (h00). Além disso, as características morfológicas e as propriedades fotocatalíticas também apresentaram forte influencia dos mesmos parâmetros acima citados. Em relação às propriedades fotocatalíticas, os filmes obtidos pelo método CSD apresentaram maior eficiência que os obtidos por PLD e os filmes com composições mais ricas em Sn4+ foram os mais ativos na fotodegradação do corante azo Remazol amarelo ouro, chegando a uma eficiência fotocatalítica máxima com o filme policristalino de SrSnO3 obtido por CSD com percentagem de degradação e de descoloração de 55 e 90 %, respectivamente. A orientação dos filmes de SrSnO3 também mostrou uma forte influencia na fotodegradação do corante, sendo os filmes policristalinos com orientação aleatória mais eficiente que o texturizado e estes por sua vez mais eficientes que os epitaxiais

Dentre os materiais de estrutura perovskita, o titanato de estrôncio, SrTiO3, é um dos mais conhecidos e estudado por apresentar interessantes propriedades como a ferroeletricidade, fotoluminescência, entre outras. Além disso, tem sido reportado na literatura que a adição de diferentes dopantes ao composto SrTiO3 modifica suas propriedades dando origem, por exemplo, a materiais para serem aplicados como sensores de gás ou na fotocatálise. Diferentes trabalhos encontrados na literatura descrevem a síntese do composto SrTiO3 e suas soluções sólidas através do método hidrotermal. Entretanto, poucos trabalhos tiveram como foco o estudo do precursor de Ti e do íon dopante nas propriedades estruturais, morfológicas, ópticas e elétricas destes materiais. Desta forma, este trabalho de tese teve como objetivo o estudo da influência da composição e de alguns parâmetros de síntese sobre as propriedades estruturais, morfológicas e ópticas do composto SrTiO3 preparado através do método hidrotermal assistido por microondas HAM. Inicialmente, foi realizada a síntese do composto SrTiO3 a 140oC por 10 minutos, onde se verificou a possibilidade de controlar a morfologia e a propriedade fotoluminescente das amostras através do tipo de precursor de Ti utilizado. Em uma segunda etapa do trabalho, o composto SrTiO3 foi sintetizado por tempos variando de 10 a 640 minutos. A caracterização destas amostras mostrou a formação da fase cúbica do SrTiO3 exibindo morfologia na forma de cubos, formados pelo processo de auto-organização, que tornam-se mais bem definidos com o aumento do tempo de síntese. Medidas de espectroscopia XANES e EXAFS na borda K do Ti mostraram que as amostras de SrTiO3 sintetizadas pelo método HAM apresentam um alto grau de desordem na estrutura local, caracterizada pela coexistência de unidades do tipo TiO5 e TiO6. Além disso, observou-se que o aumento do tempo de síntese leva a uma redução da intensidade fotoluminescente que foi atribuído à redução na concentração de vacâncias de oxigênio. Medidas de DRX das amostras do sistema SrTi1-xFexO3, ao nosso conhecimento pela primeira vez sintetizadas com sucesso através do método HAM, mostraram a formação da fase cúbica quando x &#8804 0,4. Medidas do espectro XANES e EXAFS revelaram a substituição Ti por íons Fe+2 e Fe+3, levando a formação de vacâncias de oxigênio as quais contribuem para a diminuição da energia do band-gap das amostras de 3,2 para 2,8 eV, bem como na total supressão da intensidade fotoluminescente das amostras. A análise por microscopia eletrônica de varredura FE-MEV mostrou a existência de partículas na forma de cubos cuja cinética de formação é influenciada pela variação da quantidade de ferro. Imagens de microscopia eletrônica de transmissão (MET) confirmaram a natureza mesocristalina destas partículas cujo mecanismo de crescimento ocorre por coalescência orientada, originada pelo processo de auto-organização. As amostras do sistema SrTi1-xFexO3 na forma de filmes finos foram avaliadas como sensores O3, NO2, NH3 e CO. As análises indicaram que os filmes depositados pela técnica de evaporação de feixe de elétrons apresentam um grande potencial para serem aplicados como sensor de gás ozônio, exibindo uma boa sensibilidade e seletividade comparada a outros tipos de materiais sensores. / Strontium titanate, SrTiO3, it is one of the most known and intensively studied perovskite compounds due its interesting properties such as ferroelectricity, photoluminescence, etc. According to the literature, the dopant addition into SrTiO3 network can create materials with desirable functions, for example, gas sensing and photocatalytic activity. Despite some authors reported the synthesis of pure or doped SrTiO3 by hydrothermal method, few studies have been devoted to investigate the effects of the Ti precursor and type of dopant ion on the structural, morphological and electrical properties of SrTiO3 compound. In this work, we investigated the influence of synthesis parameters and concentration iron on the structural, morphological and optical properties of SrTiO3 prepared by microwave-assisted hydrothermal method (MAH). Firstly, it was observed that the appropriate choice of the Ti precursor allowed the control of morphological and photoluminescence properties of SrTiO3 compound synthesized at 140oC for 10 minutes by MAH method. Next, SrTiO3 was synthesized during different treatment times varying from 10 to 640 minutes. X-ray diffraction (XRD) measurements indicate a SrTiO3 cubic perovskite structure and FE-SEM images revealed that the samples exhibit a cube-like shape formed by an assembly process, becoming well defined as a function of MAH treatment time. Ti-K edge XANES and EXAFS measurements indicated a large local structural distortion, revealed by the presence of TiO6 and TiO5 units. Moreover, we observed a reduction of the photoluminescence intensity as a function of treatment time probably due to decreasing of the oxygen vacancy concentration. To the best of our knowledge, this is the first time that the SrTi1-xFexO3 solid solution was synthesized by MAH method. XRD analyses indicated a cubic perovskite structure when x &#8804 0.4. XANES and EXAFS measurements revealed that iron ions present a mixed Fe+2/Fe+3 oxidation state and occupy preferentially the Ti4+-site. A UV-visible spectrum shows that the addition of iron reduces the value of optical gap of the 3.2 eV to 2.8 eV and consequently suppresses the photoluminescence intensity. An analysis of FE-SEM and HRTEM images point out that, independently of iron content, the nanoparticles have a cube-like morphology and are formed by a self-assembly of small primary nanocrystals. In addition, SrTi1-xFexO3 thin films were investigated as gas sensor towards O3, NO2, NH3 and CO gases The results indicate that the films deposited by electron beam deposition method exhibits a good response as ozone sensor compared to others gas sensors materials.

Dans un contexte où les limites intrinsèques des matériaux classiques de l’industrie CMOS sont en passe d’être atteintes du fait de la forte miniaturisation des composants, le développement de la microélectronique requiert la définition de nouvelles solutions pour combiner sur un même substrat (le silicium) des matériaux différents aux propriétés physiques variées. Ceci devrait permettre d’intégrer sur silicium des fonctionnalités nouvelles. Parmi les matériaux d’intérêt, les oxydes fonctionnels de la famille des pérovskites offrent une large gamme de propriétés et attirent donc une attention particulière. D’autre part, la recherche se porte aussi sur les semi-conducteurs de la classe III-V et le Ge pour leurs propriétés optiques ou de transport de charges. Cependant, la grande hétérogénéité chimique et cristallographique entre ces matériaux rend leur association sur silicium par voie épitaxiale particulièrement délicate. Dans ce contexte, ce travail de thèse consiste en une étude approfondie de l’interface Ge sur SrTiO3et des mécanismes à l’origine des modes d’accommodation et de croissance du semi-conducteur sur le substrat pérovskite. Les échantillons, fabriqués par épitaxie par jets moléculaires, ont été étudiés par caractérisations in situ, au synchrotron, diffraction de rayons X en incidence rasante et spectroscopie de photoémission. Des images de microscopie électronique en transmission sont venues compléter cette étude. La combinaison de ces résultats a permis de comprendre et de décrire deux aspects spécifiques des systèmes III-V et Ge sur SrTiO3. Le mode de croissance Volmer-Weber et la compétition entre les orientations cristallines(001) et (111) du Ge sont décrits dans une première partie. La relation d’épitaxie de Ge/SrTiO3est identifiée et l’influence des énergies d’adhésion et de surface libre du semi-conducteur sur sa croissance est élucidée. Dans une deuxième partie, le mode d’accommodation du Ge est plus spécifiquement étudié. La mise en place d’un réseau de dislocations d’interface est observée expérimentalement et analysée à l’aide d’un modèle numérique. Ce travail de thèse a permis de discuter de l’interface d’un système épitaxié très hétérogène et il ouvre des perspectives intéressantes, liées aux spécificités de l’accommodation aux interfaces semi-conducteurs/oxydes, pour l’intégration monolithique de Ge et de III-V sur des substrats d’oxydes/Si. / With the recent developments of the microelectronic industry, the intrinsic limits of the classical CMOS materials are being reached because of the strong miniaturization. Thus, the microelectronic industry is waiting for new solutions for combining, on the same substrate (silicon), different materials with various physical properties in the framework of integrating new functionalities on silicon. Research is now focusing on perovskite oxides because of the very wide range of properties they are offering (electronic, magnetic, etc.), but also on III-V semiconductors for the development of integrated photonic devices and on Ge for its electronic transport properties. However, combining these materials is challenging due to their strong chemical and crystallographic heterogeneity. Thus, this thesis focuses on the Ge/SrTiO3 system. The accommodation mode and growth mechanism have been studied by in situ, synchrotron-based, characterization methods like grazing incidence X-ray scattering and X-ray photoemission spectroscopy. The samples were prepared by molecular beam epitaxy. Transmission electron microscopy images complemented the study. The combination of these results have allowed for highlighting two specificities of the III-V or Ge/SrTiO3epitaxial systems. In a first chapter, the Volmer-Weber growth mode and a competition between (001)and (111)-oriented Ge islands is described. Epitaxial relationship between Ge and SrTiO3, chemical bonds at the interface and influence of adhesion and surface energies on the growth mode are described. In a second part, the specific accommodation mode of the Ge/SrTiO3 interface is studied. The development of a misfit dislocation network during the growth is experimentally observed and analyzed on the basis of a numerical model of the interface. This work provides state of the art understanding of the interface of weakly bonded epitaxial systems and opens interesting perspectives, especially related to the accommodation mode of semiconductors/oxides interfaces, for the monolithic integration of III-V or Ge on oxides/Si substrates.

Les semiconducteurs III-V ayant des propriétés électroniques et optiques très intéressantes, leur intégration sur Si permettrait la combinaison de fonctionnalités variées sur la même puce, une solution potentielle aux obstacles affrontés par les composants CMOS. Les travaux pionniers de McKee et al ont démontré que le SrTiO3 (STO) peut être directement épitaxié sur Si par EJM (Epitaxie par Jets Moléculaires). Plus tard, une équipe de Motorola a montré qu’il était possible d’épitaxier des couches minces de GaAs sur des templates de STO/Si, ouvrant une voie nouvelle pour l’intégration monolithique de III-V sur Si. Sur cette base, l’INL a entrepris l’étude de la croissance de semiconducteurs III-V sur STO. Il a notamment été montré que la faible adhésion caractéristique de ces systèmes favorisait un mode d’accommodation spécifique du désaccord paramétrique par la formation d’un réseau de dislocations confinées à l’interface entre les deux matériaux sans défauts traversant liés à une relaxation plastique, ce qui ouvre des perspectives intéressantes pour l’intégration monolithique de III-V sur Si. Dans ce contexte, lors de cette thèse, Nous nous sommes d’abord focalisé sur l’optimisation de la croissance des templates de STO/Si. Nous avons en particulier montré qu’une couche de STO relaxée et riche en oxygène favorisait la reprise de croissance de l’InP. Nous avons ensuite étudié de manière systématique la croissance d’InP sur STO. La faible adhésion caractéristique de ce système conduit à la formation d’îlots aux premiers stades de la croissance, ainsi qu’à l’observation d’une compétition entre plusieurs orientations cristallines de l’InP. Nous avons fixé des conditions de croissance et de préparation de la surface de STO permettant d’obtenir des îlots purement orientés (001). Nous avons ensuite optimisés l’étape de coalescence de ces îlots pour former des couches 2D d’InP intégrées sur STO/Si. Une étude structurale et optique complète de ces hétérostructures, nous a permis d’analyser le potentiel de notre approche et pointer certaines limitations des templates de STO/Si. Sur cette base, nous avons enfin initié l’étude de templates alternatifs pour la croissance d’InP, en effectuant quelques études préliminaires de l’épitaxie d’InP sur substrats de LaAlO3. / Due to their electrical and optical properties, the integration of III-V semiconductors on Si would open the path to the combination of a various functionalities on the same chip, a potential solution to the challenges faced by CMOS components. The pionner studies by McKee and al have shown that SrTiO3 (STO) could be directly epitaxied on Si by MBE (Molecular Beam Epitaxy). Few years later, a Motorola team has shown that it is possible to epitaxy thin GaAs layers on STO/Si templates, hence opening a new path for III-V monolithic integration on Si. Based on this, the INL has undertaken the study of III-V semiconductors growth on STO. In particular, it has been shown that the weak adhesion specific to these systems favors a preferential accommodation mode of the lattice mismatch by breaking interfacial bonds rather than by plastic relaxation of an initially compressed layer. Hence, it is possible in spite of a strong lattice mismatch to grow III-V semiconductors without threading defects related to a plastic relaxation mechanism, which opens interesting perspectives for IIIV monolithic integration on Si. In this context, during this thesis, we have focalised in the beginning on optimising the growth of the STO/Si templates. In particular, we have shown that a relaxed and oxygen-rich STO layer favors undertaking InP growth. Next, we have studied systematically the InP growth on STO. The weak adhesion specific to this system leads to islands formation at the early stages of growth, as well as the observation of a competition between different crystalline orientations of the InP islands. We have worked out STO growth conditions and surface preparation strategies that allow obtaining purely (001) oriented InP islands. We have next optimised the islands coalescence step in order to form 2D InP layers on STO/Si. Based on a complete structural and optical study of these heterostructures, we have been able to analyse our approach’s potential and to point out cetain limitations of the STO/Si templates. On this basis, we have finally initiated the study of alternative templates for InP growth, by undergoing some preliminary studies of InP epitaxy on LaAlO3 substrates.

La diversification des fonctionnalités intégrées dans les systèmes micro-optoélectroniques est l'un point clé du développement de ces filières. Combiner sur une même puce des matériaux ayant des propriétés différentes doit permettre de faire émerger de nouveaux concepts de composants basés sur de nouveaux effets physiques ou sur la combinaison des propriétés physiques des matériaux intégrés. Parmi les matériaux d'intérêt, les semi-conducteurs III-V présentent des propriétés optiques exceptionnelles et sont couramment utilisés pour réaliser des composants photoniques. Les oxydes fonctionnels, quant à eux, offrent une grande variété de propriétés physiques qui en font des matériaux très prometteurs pour de nombreuses applications. Dans ce contexte, l'objectif global de cette thèse est de démontrer la possibilité d'intégrer des oxydes fonctionnels cristallins sur des hétérostructures à base de GaAs par épitaxie, et de montrer que de telles structures peuvent présenter des propriétés nouvelles pour la photonique. Plus précisément, nous avons focalisé nos efforts sur l'intégration de couches minces de PZT sur des structures à puits quantiques InGaAs/GaAs via des couches tampons de SrTiO3 (STO). Nous avons étudié et développé la croissance par épitaxie par jets moléculaires (MBE) des templates de STO sur GaAs. La forte hétérogénéité entre ces deux types de matériaux nécessite d'avoir recours à des stratégies d'ingénierie d'interface spécifiques et à un excellent contrôle des paramètres de croissance. Nous avons mis en évidence les effets bénéfiques sur la qualité structurale du STO d'une préparation de la surface de GaAs au Ti. Pour ces études, nous avons utilisé la spectroscopie de photoélectrons (XPS, in-situ ou en collaboration avec la ligne TEMPO du synchrotron SOLEIL) et microscopie électronique en transmission (TEM, en collaboration avec le LPN). Ces expériences nous ont permis de sonder structure et chimie de l'interface semi-conducteur/oxyde. Nous avons également étudié les mécanismes de croissance et de cristallisation du STO sur GaAs, en mettant notamment en œuvre des expériences d'XPS in-situ au synchrotron SOLEIL. La compréhension de ces mécanismes spécifiques nous a permis d'adapter les conditions de croissance du STO et d'obtenir des couches tampons d'excellente qualité. Nous avons étudié la croissance de couches minces de PZT sur des structures à puits quantique d'In- GaAs/GaAs via des templates de STO. Nous avons tout d'abord montré que les procédés standards de croissance de PZT (sol-gel ou ablation laser (collaboration avec l'IEF)) conduisaient à de fortes dégradations des puits quantiques du fait des réactions chimiques entre l'oxyde et le matériau III-V. Nous avons étudié les mécanismes de ces dégradations et mis en évidence une forte affinité chimique entre l'As, le Pb et le Sr. Pour pallier cette difficulté, nous avons modifié le procédé de croissance du PZT ainsi que l'hétérostructure III-V (enfouissement du puits, ajout d'AlAs ...). Ces actions combinées nous ont permis de réaliser des couches minces de PZT ferroélectriques sur des structures à puits quantiques d'InGaAs/GaAs. Nous avons ensuite défini un design d'émetteur accordable basé sur une hétérostructure PZT/GaAs/InGaAs. De tels émetteurs ont été réalisés en collaboration avec l'IEF) et mesurés leurs propriétés mécaniques et optiques en effectuant des expériences sous champ. Enfin, nous avons effectué un certain nombre d'études préliminaires visant à démontrer la possibilité d'intégrer des hétérostructures à base de GaAs sur des substrats de Si recouverts de couches tampons de STO. Nous avons pour cela envisagé et étudié la possibilité d'utiliser des composés Zintl-Klemm d'interface susceptibles de minimiser l'énergie d'interface entre le GaAs et le STO. / Diversification of the materials and functionalities integrated on silicon is an important issue for further progression in the field of micro-optoelectronics. The monolithic heterogeneous integration of new materials on silicon, and more generally the combination on the same wafer of materials having different physical properties is a key challenge. Amongst the materials of interest, III-V semiconductors are the object of specific attention because their optoelectronic and transport properties are superior to those of silicon. Similarly, the so-called functional oxides have interesting physical properties (ferroelectricity, ferromagnetism, piezoelectricity, etc.) making them suitable for various applications (NVM, energy harvesters, MEMS . . . ). In this context, the goal of this thesis is to demonstrate the possible integration of crystalline functional oxides on GaAs-based heterostructures using epitaxy and that such structures show new properties for photonic. More precisely, we focused on integration PZT thin film on InGaAs/GaAs quantum wells structures thanks to SrTiO3 (STO) buffer layer. We first studied and developed the growth of STO on GaAs templates using molecular beam epitaxy (MBE). Because of the strong heterogeneity between the two materials, specific interface engineering strategies are required. We highlight the benefit of a Ti-based GaAs surface treatment on the structural quality of STO. For these studies we used photoelectrons spectroscopy (XPS, in-situ and collaboration with TEMPO beam line of SOLEIL synchrotron) and transmission electron spectroscopy (TEM, collaboration with LPN/C2N). Those experiments allowed us to probe both structural and chemical aspects of the semiconductor/ oxide interface. We also studied the growth mechanism of STO on GaAs through in-situ XPS experiments at SOLEIL. Thanks to the understanding of those specifics mechanisms, we could accommodate the growth conditions to obtain good quality STO buffer layers. Then we studied the growth of thin film PZT on InGaAs/GaAs quantum well structures by means of STO templates. We first showed that standard growth process (sol-gel and pulsed laser deposition at IEF/C2N) lead to strong deterioration of quantum well due to chemical reactions between the oxide and the III-V material. We studied the mechanisms involved in this deterioration and highlight the strong chemical affinity between As, Pb and Sr. To palliate this difficulty, the growth process of PZT has been modify and an AlAs “sacrificial” layers has been added in order to limit the oxygen difiusion into the substrate. Thanks to these two solutions, it has been possible to realize a PZT ferroelectric thin film on an InGaAs/GaAs quantum well heterostructure. A tunable source based on such heterostructure has been designed. In this device, the strain induced in the ferroelectric PZT by an electric field is transmitted to the substrate and the quantum well modifying its emitted wavelength. We simulated this device in order to optimize its dimensions. Then we realized this device (collaboration with IEF/C2N) and measured its mechanical and optical properties under an electric field. We also performed preliminary studies in order to demonstrate the possible integration of GaAs-based heterostructures on Si substrates in by the means of STO buffer layer. We considered the use of Zintl- Klemm compounds to minimize the interface energy between GaAs and STO allowing 2D growth of the semiconductor on the oxide.

La mise en évidence d'une conductivité métallique à l'interface entre deux oxydes isolants, le SrTiO3 et le LaAlO3, a ouvert un champ nouveau pour l'électronique tout oxyde (A. Ohtomo & H. Y. Hwang, Nature 427, 2004). Au-delà du fort potentiel applicatif de cette découverte, par exemple pour l'électronique de faible puissance, de nombreuses questions restent posées sur les propriétés de ces interfaces et les différents moyens de les contrôler. L'apparition de la conductivité à l'interface LaAlO3 / SrTiO3 est attribuée à la survenue d'une reconstruction électronique au-dessus d'une épaisseur critique de 4 cellules unités (u.c.) de LaAlO3, visant à compenser la discontinuité de charge à cette interface entre matériaux polaire (LaAlO3) et non polaire (SrTiO3). En plus de cet effet, qui requiert une interface d'excellente qualité, divers paramètres sont susceptibles d'affecter les propriétés de cette interface et un effort de recherche très important porte sur le contrôle et l'amélioration de ces propriétés par des moyens tels que le changement de la nature du substrat, l'application d'un champ de contraintes ou l'introduction de légères modifications chimiques dans la couche ou à l'interface. Ce travail de thèse participe à cet effort en explorant les propriétés de transport et la structure fine des interfaces LaAlO3 / SrTiO3 dopées avec des atomes de métaux de transition. Il est centré sur la fabrication et l'étude d'interfaces LaAlO3 / SrTiO3 dopées avec des atomes d'iridium ou de cobalt. La croissance de nanostructures de haute qualité, typiquement LaAlO3 (5 u.c.) / SrTiO3 dopé (1 u.c.) / SrTiO3 (substrat), a été effectuée dans un bâti de dépôt laser pulsé (PLD) équipé d'un RHEED in situ. L'effet des dopants Ir ou Co sur les propriétés de transport et sur la structure électronique et atomique de l'interface LaAlO3 / SrTiO3 a été analysé en fonction du niveau de dopage grâce à une combinaison de différentes méthodes expérimentales et de calculs DFT. En particulier, des investigations par des techniques avancées comme la diffraction de photoélectrons sur synchrotron (XPD) et la microscopie électronique de résolution atomique en mode STEM-HAADF ont été mises en œuvre pour sonder la structure locale à l'interface et quantifier le niveau de déformation dans la couche LaAlO3. La structure électronique a été étudiée par spectroscopie de perte d'énergie électronique dans un STEM. Les résultats montrent qu'il est possible d'induire des changements dans les propriétés des interfaces LaAlO3 / SrTiO3 en dopant la surface de SrTiO3 avec un dopant et une concentration appropriés. En particulier, les effets du niveau de dopage sur la déformation élastique et les distorsions locales dans les couches LaAlO3 et ses conséquences sur les propriétés mesurées sont décrits. / The demonstration in 2004 of a metallic conductivity at the interface between two insulating oxides, SrTiO3 and LaAlO3, opened a new field for all-oxide electronics (A. Ohtomo & H. Y. Hwang, Nature 427, 2004). Beyond the important applicative potential of this discovery for multifunctional and low power electronics, many questions remain about the properties of such interfaces and the different ways to control them. The occurrence of a conductive behaviour of the LaAlO3 / SrTiO3 interface is attributed to the advent of an electronic reconstruction above a critical thickness of 4 unit cells (u.c.) of LaAlO3, in order to compensate the charge discontinuity at the interface between a polar (LaAlO3) and a non-polar (SrTiO3) materials. In addition to this effect, which implies an interface of excellent quality, various parameters are likely to affect the properties of this interface, and a major research effort aims to control and improve these properties by ways such as the change in the nature of the substrate, the application of a stress field or the introduction of slight chemical modifications in the layer or at the interface. This thesis participates to this effort by exploring the transport and structural behaviour of LaAlO3 / SrTiO3 interfaces doped with transition metal atoms. It focuses on the fabrication and study of LaAlO3 / SrTiO3 interfaces doped with iridium or cobalt atoms. High quality nanostructures, typically LaAlO3 (5 u.c.) / doped-SrTiO3 (1 u.c.) / SrTiO3 (substrate), were grown by pulsed laser deposition (PLD) equipped with in-situ RHEED. The effect of Ir or Co dopants on both the transport properties and the electronic and atomic structure of the LaAlO3 / SrTiO3 interface was analysed as a function of the doping level thanks to a combination of different experimental methods and DFT calculations. In particular, advanced investigations by hard X-ray photoelectron diffraction on a synchrotron facility and by atomically resolved high angle annular dark field- scanning transmission electron microscopy (HAADF-STEM) were implemented to probe the local structure at the interface and quantify the level of strain in the LaAlO3. The electronic structure was investigated by electron energy loss spectroscopy in a STEM. Our results show that it is possible to induce changes in the properties of the LaAlO3 / SrTiO3 interfaces by doping the SrTiO3 surface with suitable dopant and concentration. In particular, effects of the doping level on the elastic deformation and the local distortions in the LaAlO3 layers and its consequences on the measured properties are described.

No description available.

Orientador: Prof. Dr. Gustavo Martini Dalpian / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, 2015. / Em 2004, foi descoberto que a interface formada entre as superf'ýcies (001) do SrTiO3 e LaAlO3 apresenta um g'as de el'etrons bidimensional (2DEG), apesar de o bulk desses materiais ser isolante. Em 2011, foi reportado um 2DEG similar na superf'ýcie (001) do SrTiO3. Apesar de haver muitos trabalhos acerca desse assunto, n¿ao h'a um consenso sobre a origem do 2DEG nesses sistemas. Inclu'ýda nesse contexto, esta tese reporta os resultados de c'alculos de primeiros princ'ýpios baseados na Teoria do Funcional da Densidade (DFT) da superf'ýcie (001) dos filmes finos de SrTiO3 e LaAlO3 e da interface (001) formada entre esses dois 'oxidos. Filmes finos de LaAlO3 e SrTiO3 tendem a ter vac¿ancias com menores energias de forma¸c¿ao da superf'ýcie, e quando com vac¿ancias apresentaram comportamento condutor. Filmes finos de SrTiO3 sem defeitos demonstraram comportamento isolante. A vac¿ancia de oxig¿enio na superf'ýcie com termina¸c¿ao TiO2 'e a menos custosa energeticamente, e nesse caso 'e detectado um 2DEG da superf'ýcie do SrTiO3. O mesmo ocorre em filmes finos de LaAlO3, com vac¿ancias de oxig¿enio e termina¸c¿ao AlO2 e vac¿ancias de La e termina¸c¿ao LaO, podendo haver forma¸c¿ao espont¿anea nesses casos. Em ambos os casos, as simula¸c¿oes sugerem que 'e formado um 2DEG na superf'ýcie. As simula¸c¿oes da interface mostraram que para a hete2 roestrutura sem defeitos 'e necess'aria a deposi¸c¿ao de quatro ou mais camadas de LaAlO3 sobre o substrato de SrTiO3 para que ocorra o comportamento met'alico. Vac¿ancias de oxig¿enio se formam preferencialmente na interface quando a espessura do LaAlO3 'e de tr¿es camadas ou menos. Para quatro ou mais camadas de LaAlO3, as vac¿ancias de oxig¿enio tendem a se localizar na superf'ýcie. Para todas as simula¸c¿oes da interface com defeitos, as heteroestruturas apresentaram comportamento met'alico, por'em n¿ao foi detectado um 2DEG na interface, j'a que as cargas n¿ao se apresentaram confinadas na regi¿ao da interface. Vac¿ancias justificam o 2DEG na superf'ýcie (001) de filmes finos de LaAlO3 e SrTiO3, por'em n¿ao explicam o 2DEG na interface entre os mesmos. / In 2004 it was discovered that the interface between the (001) SrTiO3 and LaAlO3 surfaces presents a two dimensional electron gas (2DEG), although the bulk of these materials are insulators. In 2011 it was reported a similar 2DEG at (001) SrTiO3 surface. Despite many studies on this subject, there is no consensus about the origin of the 2DEG in these systems. Included in this context, this thesis reports results of first principles calculations based on Density Functional Theory (DFT) about (001) SrTiO3 and LaAlO3 thin films surfaces and the (001) interface formed between these two oxides. For LaAlO3 and SrTiO3 thin films, vacancies tend to have lower formation energies at the surface. Non-defective SrTiO3 thin films have demonstrated an insulator behavior. The oxygen vacancy at the TiO2 terminated surface is the most stable and in this case is observed a 2DEG at the SrTiO3 surface. The same occurs for LaAlO3 thin films with oxygen vacancies and for the AlO2 termination and with La vacancies with LaO termination. These vacancies may be formed spontaneously. In both cases, the simulations suggest a 2DEG at the surface. The interface simulations showed that in the non-defective heterostructure it is necessary the deposition of four or more layers of LaAlO3 over SrTiO3 substrate to occur a metallic behavior. Oxygen 4 vacancies are preferably formed at the interface when the LaAlO3 thickness is three layers or less. For four LaAlO3 layers or more, the oxygen vacancies tend to be located at the surface. For all simulations of the defective interface, the heterostructures showed metallic behavior. However it was not detected a 2DEG at the interface, owing the fact that the charges are not confined at the interface region. Vacancies can justify the 2DEG at the SrTiO3 and LaAlO3 thin films surfaces, however does not explain the 2DEG at the interface between them.

La question cruciale dans l'exploration du stockage ultime à haute densité est l'anisotropie magnéto-cristalline (MCA) qui provient du couplage spin-orbite. Utilisant à la fois la méthode des liaisons fortes et les calculs « premiers principes », nous calculons la MCA de nanocristaux de fer et de cobalt qui peuvent être obtenus par croissance épitaxiale sur un substrat SrTiO3 avec un contrôle remarquable de leur taille, forme et structure. Afin de définir une décomposition locale appropriée de la MCA, nous avons implémenté le « Théorème de Force » à l'aide d'une formulation grand-canonique dans le code QUANTUM ESPRESSO ainsi que dans notre modèle de liaisons fortes. Il est intéressant de noter que pour les deux éléments, la MCA totale de nanocristaux isolés est largement dominée par les facettes (001) dont il résulte un comportement opposé: une anisotropie « hors-plan » pour les nanocristaux (contenant plusieurs centaines d'atomes) de fer et « dans le plan » pour ceux de cobalt. Nous avons également mis en évidence un fort renforcement de la MCA pour les petits clusters (contenant quelques atomes seulement) déposés sur un substrat SrTiO3. En conséquence, nous prévoyons que les nanocristaux de fer (même de très petite taille) devraient être magnétiquement plus stables et sont donc de bons candidats potentiels pour le stockage magnétique. Enfin, notre analyse MCA résolu en orbitales s'applique également à d'autres système et permet, par exemple, de prédire le comportement de la MCA de films minces magnétiques après déposition de matériaux organiques comme le graphène ou de molécules tel C60. / The crucial issue in exploring ultimate density data storage is magneto-crystalline anisotropy (MCA) which originates from spin-orbit coupling. Using both tight-binding and first-principles methods, we report the MCA of Fe and Co nanocrystals that can be grown epitaxially on SrTiO3 with a remarkable control of their size, shape and structure. In order to define the proper local decomposition of MCA, we implemented the “Force Theorem” within the grand-canonical formulation in QUANTUM ESPRESSO as well as in our tight-binding model. Interestingly, for both elements, the total MCA of free nanocrystals is largely dominated by (001) facets resulting in the opposite behavior: out-of-plane and in-plane magnetization direction is favored in Fe and Co nanocrystals (containing up to several hundred atoms), respectively. We also find a strong enhancement of MCA for small clusters (containing only several atoms) upon their deposition on a SrTiO3 substrate. As a consequence, we predict that the Fe nanocrystals (even rather small) should be magnetically stable and are thus good potential candidates for magnetic storage devices. Finally, our rather general orbital-resolved analysis of MCA applies also to other systems and allows, for example, predicting the MCA behavior of magnetic thin films upon covering by various organic materials such as graphene or C60 molecule.