To accelerate environmentally friendly thin film photovoltaic (PV) technologies, copper-based chalcogenides are attractive as absorber materials. Chalcopyrite copper indium gallium selenide (CIGS ≡ CuIn1–xGaxSe2) is today a commercially important PV material, and it is also in many aspects a very interesting material from a scientific point of view. Copper zinc tin sulfide selenide (CZTSSe ≡ Cu2ZnSn(S1–xSex)4) is considered as an emerging alternative thin film absorber material. Ternary Cu2SnS3 (CTS) is a potential absorber material, thus its related alloys Cu2Sn1–xGexS3 (CTGS) and Cu2Sn1–xSixS3 (CTSS) are attractive due to the tunable band gap energies. CuSb(Se1–xTex)2 and CuBi(S1–xSex)2 can be potential as ultra-thin (≤ 100 nm) film absorber materials in the future. In the thesis, analyses of these Cu-based chalcogenides are based on first-principles calculations performed by means of the projector augmented wave method and the full-potential linearized augmented plane wave formalisms within the density functional theory as implemented in the VASP and WIEN2k program packages, respectively. The electronic and optical properties of CIGS (x = 0, 0.5, and 1) are studied, where the lowest conduction band (CB) and the three uppermost valence bands (VBs) are parameterized and analyzed in detail. The parameterization demonstrates that the corresponding energy dispersions of the topmost VBs are strongly anisotropic and non-parabolic even very close to the Γ-point. Moreover, the density-of-states and constant energy surfaces are calculated utilizing the parameterization, and the Fermi energy level and the carrier concentration are modeled for p-type CIGS. We conclude that the parameterization is more accurate than the commonly used parabolic approximation. The calculated dielectric function of CuIn0.5Ga0.5Se2 is also compared with measured dielectric function of CuIn0.7Ga0.3Se2 collaborating with experimentalists. We found that the overall shapes of the calculated and measured dielectric function spectra are in good agreement. The transitions in the Brillouin zone edge from the topmost and the second topmost VBs to the lowest CB are responsible for the main absorption peaks. However, also the energetically lower VBs contribute significantly to the high absorption coefficient. CTS and its related alloys are explored and investigated. For a perfectly crystalline CTS, reported experimental double absorption onset in dielectric function is for the first time confirmed by our calculations. We also found that the band gap energies of CTGS and CTSS vary almost linearly with composition over the entire range of x. Moreover, those alloys have comparable absorption coefficients with CZTSSe. Cu2XSnS4 (X = Be, Mg, Ca, Mn, Fe, Ni, and Zn) are also studied, revealing rather similar crystalline, electronic, and optical properties. Despite difficulties to avoid high concentration of anti-site pairs disordering in all compounds, the concentration is reduced in Cu2BeSnS4 partly due to larger relaxation effects. CuSb(Se1–xTex)2 and CuBi(S1–xSex)2 are suggested as alternative ultra-thin film absorber materials. Their maximum efficiencies considering the Auger effect are ~25% even when the thicknesses of the materials are between 50 and 300 nm. / <p>QC 20170523</p>

A investigação das propriedades ópticas e sua relação com as propriedades magnéticas dos semicondutores é de grande interesse para a comunidade científica, em virtude da enorme demanda por novas tecnologias e funcionalidades que podem surgir dessas pesquisas. Os calcógenos de európio são semicondutores intrinsecamente magnéticos, transparentes na região visível do espectro eletromagnético e integráveis em matrizes de silício e nitreto de gálio, sendo assim fortes candidatos a aplicações tecnológicas envolvendo magnetismo e óptica. Neste trabalho são investigados os espectros de absorção e fotoluminescência dos calcógenos de európio, com base no modelo 4f -> 5d(\'t IND. 2g\') da transição óptica de dipolo elétrico, o qual mostrou-se totalmente adequado para a descrição da absorção óptica nos calcógenos de európio em função do campo magnético aplicado, explicando a presença de linhas de absorção estreitas e dicróicas nos espectros em campo alto e a forma larga dos espectros de absorção em campo nulo. Nos espectros de fotoluminescência do EuTe, entretanto, foram detectados estados eletrônicos não contemplados pelo modelo 4f -> 5d(\'t IND. 2g\') , em especial uma banda de emissão denominada \'MX IND. 0\', acoplada a modos vibracionais da rede. Uma linha zero-fônon correspondente a uma transição que não produz fônons pôde ser bem definida e a partir do deslocamento dessa linha em função do campo magnético foi possível detectar inequivocamente a formação de polarons magnéticos no EuTe pela primeira vez; o raio polarônico foi estimado como R = 3.6a, onde a é o parâmetro de rede e a energia de ligação desse polaron foi estimada em \'E IND. p\' = 45 meV, um resultado que foi confirmado através de medidas do deslocamento da linha zero-fônon em função da temperatura. Adicionalmente procurou-se identificar o estado eletrônico associado à emissão \'MX IND. 0\': a partir de medidas da intensidade da fotoluminescência em função da potência de excitação foi possível sugerir que a emissão \'MX IND. 0\' provem de estados eletrônicos ligados a defeitos da rede e foi possível estimar a concentração desses defeitos como menor que 0.1 ppm. / Investigation of optical properties and their relation to magnetic properties of semiconductors is of great interest to scientific community, due to the large demand for new technologies and features that can arise from these studies. Europium chalcogenides are intrinsically magnetic semiconductors, transparent in the visible region of electromagnetic spectrum and integrable into silicon and gallium nitride matrices, beeing strong candidates for technological applications involving magnetism and optics. The present study investigates absorption and photoluminescence spectra of europium chalcogenides, based on the 4f -> 5d(t2g) model of the electric dipole optical transition, which proved to be entirely appropriate to describe polarized optical absorption in europium chalcogenides as a function of magnetic field, explaining the presence of narrow dichroic lines at high fields and the broad shape of the zero-field absorption spectrum. However, in photoluminescence spectra of EuTe, electronic states not covered by the 4f -> 5d(t2g) model were detected, in particular an emission band labeled MX0, which is coupled to vibrational modes of the lattice. A transition without production of phonons, corresponding to a zero-phonon line, could be well resolved and from the displacement of the zero-phonon line as a function of magnetic field the formation of magnetic polarons in EuTe could be detected unambiguously for the first time. The polaronic radius is estimated as R = 3:6a, where a is the lattice parameter, and the polaron binding energy is estimated as Ep = 45 meV, a result that was confirmed by measurements of zero-phonon line displacement as a function of temperature. Additionally, we sought to identify the electronic state associated with MX0 emission: from measurements of the photoluminescence intensity as a function of excitation power, it was possible to suggest that MX0 emission comes from an electronic state coupled to lattice defects of low concentration, which we estimate to be of less than 0.1 ppm.

Recentemente foi descoberto que diversos calcogenetos de metais de transição podem ter o estado charge density waves (CDW) suprimido a partir de pressão hidrostática e dopagens, e, por conseguinte, o estado supercondutor emerge. Nesse contexto, este trabalho apresenta um estudo sistemático de propriedades físicas de amostras poli e monocristalinas de dois compostos do sistema Nb-Te, NbTe2 e NbTe4. Com relação ao composto NbTe2, os resultados aqui apresentados demonstram que esse composto é mais um exemplo de material que exibe ambos os estados a pressão atmosférica e sem dopagens. No que tange as propriedades do NbTe4, este trabalho demonstra que amostras deficientes em telúrio tem a anomalia na curva de resistividade elétrica relacionada a formação do estado CDW amplificada e, a deficiência em telúrio é também capaz de fazer emergir supercondutividade em 5.5 K. Este trabalho também sugere algumas mudanças no diagrama de equilíbrio de fases Nb-Te publicado na base de dados da sociedade americana de metalurgia (ASM). As fases Nb5Te4 e Nb3Te4 foram identificadas como sendo fases de altas temperaturas que são formadas a partir de reações eutetóides. Além do mais, nossos resultados demonstram que a região entre as fases NbTe2 e NbTe4 consiste, na verdade, de uma região bifásica. Durante a realização deste trabalho, outro composto foi investigado, o NiTe2. Nesse composto, nossos resultados demonstram que a intercalação de Ti faz emergir um estado supercondutor em 4.5 K e cuja temperatura de transição parece insensível a pressão hidrostática. Cálculos de estrutura de bandas sugerem fortemente que o composto NiTe2 intercalado com Ti pode ser mais um exemplo de supercondutor com aspectos topológicos em sua superfície de Fermi. / Recently was demonstrated that it is possible to suppress the charge density waves (CDW) ground states while, simultaneously, a superconductor state emerges in several transition metal chalcogenides (TMC), by means of hydrostatic pressure or chemical doping. Within this context, this work presents a systematic study on physical properties of two Nb chalcogenides, NbTe2 and NbTe4. Our results demonstrate that NbTe2 is another example of a TMC which exhibit both stabilities at atmospheric pressure and without doping. Regarding the physical properties of NbTe4, we have demonstrated that Te deficiency increases significantly the anomaly in the electrical resistivity as function of temperature behavior related with the CDW formation. At the same time, Te deficiency can also cause a SC state to emerge at 5.5 K. This work also presents a review of the binary phase diagram, Nb-Te, and some changes are proposed. Nb5Te4 and Nb3Te4 were identified as high temperature phases originated from eutectoid reactions. Furthermore, our results demonstrated that the region between the phases NbTe2 and NbTe4 are, in fact, a two-phase region, differently from what is proposed in the actual version of the phase diagram. Also, during this work, another chalcogenide was investigated, NiTe2. Our results demonstrate that Ti can be intercalted between the Van der Waals gaps of the structure and consequently a superconductor state emerges at 4.5 K. The critical temperature is found to be insensitive to hydrostatic pressure. Band structure strongly suggests that NiTe2 could be another example of a superconductor with topological aspects in its Fermi surface.

Recentemente foi descoberto que diversos calcogenetos de metais de transição podem ter o estado charge density waves (CDW) suprimido a partir de pressão hidrostática e dopagens, e, por conseguinte, o estado supercondutor emerge. Nesse contexto, este trabalho apresenta um estudo sistemático de propriedades físicas de amostras poli e monocristalinas de dois compostos do sistema Nb-Te, NbTe2 e NbTe4. Com relação ao composto NbTe2, os resultados aqui apresentados demonstram que esse composto é mais um exemplo de material que exibe ambos os estados a pressão atmosférica e sem dopagens. No que tange as propriedades do NbTe4, este trabalho demonstra que amostras deficientes em telúrio tem a anomalia na curva de resistividade elétrica relacionada a formação do estado CDW amplificada e, a deficiência em telúrio é também capaz de fazer emergir supercondutividade em 5.5 K. Este trabalho também sugere algumas mudanças no diagrama de equilíbrio de fases Nb-Te publicado na base de dados da sociedade americana de metalurgia (ASM). As fases Nb5Te4 e Nb3Te4 foram identificadas como sendo fases de altas temperaturas que são formadas a partir de reações eutetóides. Além do mais, nossos resultados demonstram que a região entre as fases NbTe2 e NbTe4 consiste, na verdade, de uma região bifásica. Durante a realização deste trabalho, outro composto foi investigado, o NiTe2. Nesse composto, nossos resultados demonstram que a intercalação de Ti faz emergir um estado supercondutor em 4.5 K e cuja temperatura de transição parece insensível a pressão hidrostática. Cálculos de estrutura de bandas sugerem fortemente que o composto NiTe2 intercalado com Ti pode ser mais um exemplo de supercondutor com aspectos topológicos em sua superfície de Fermi. / Recently was demonstrated that it is possible to suppress the charge density waves (CDW) ground states while, simultaneously, a superconductor state emerges in several transition metal chalcogenides (TMC), by means of hydrostatic pressure or chemical doping. Within this context, this work presents a systematic study on physical properties of two Nb chalcogenides, NbTe2 and NbTe4. Our results demonstrate that NbTe2 is another example of a TMC which exhibit both stabilities at atmospheric pressure and without doping. Regarding the physical properties of NbTe4, we have demonstrated that Te deficiency increases significantly the anomaly in the electrical resistivity as function of temperature behavior related with the CDW formation. At the same time, Te deficiency can also cause a SC state to emerge at 5.5 K. This work also presents a review of the binary phase diagram, Nb-Te, and some changes are proposed. Nb5Te4 and Nb3Te4 were identified as high temperature phases originated from eutectoid reactions. Furthermore, our results demonstrated that the region between the phases NbTe2 and NbTe4 are, in fact, a two-phase region, differently from what is proposed in the actual version of the phase diagram. Also, during this work, another chalcogenide was investigated, NiTe2. Our results demonstrate that Ti can be intercalted between the Van der Waals gaps of the structure and consequently a superconductor state emerges at 4.5 K. The critical temperature is found to be insensitive to hydrostatic pressure. Band structure strongly suggests that NiTe2 could be another example of a superconductor with topological aspects in its Fermi surface.

Thin films of GeTe₂ were deposited on glass substrates using RF magnetron sputtering with various hydrogen flow rates in the growth chamber. Transmission data of deposited films were taken and used to determine optical constants: refractive index (n), extinction coefficient (κ), and absorption coefficient (α)) and the energies: E₀₄, E₀₃, Tauc band gap E[subscript]Tauc and Urbach energy E[subscript]U. An increase of these energies was observed with increasing hydrogen flow rate. This increase is interpreted on the basis of the density of state model proposed by Mott and Davis. An increase of network disorder due to the inclusion of hydrogen into the GeTe₂ thin films was determined from the B[superscript]1/2 parameter, Urbach energy and full width at half maximum of Raman vibrational modes. The crystallization process induced by thermal annealing on GeTe₂ was studied. X-ray diffraction measurements were performed and the results suggest that crystallization of GeTe₂ occurs via a phase separation into Te and GeTe crystalline phases. This observation is in agreement with a previous report. The crystallization temperature increases with the addition of hydrogen. This increase is explained in terms of dangling bonds. A large change (approximately 60 percent decrease) of the optical transmission occurs after the phase change from amorphous to crystalline. This decrease is interpreted as a result of the observed phase separation.

Pour que l'énergie photovoltaïque devienne compétitive, les coûts de production doivent être baissés et l'efficacité des cellules augmentée. Les cellules solaires à base de nanocristaux semi-conducteurs constituent une approche prometteuse pour remplir ces objectifs combinant une mise en œuvre par voie liquide avec la possibilité d'ajuster précisément la largeur de bande interdite et les niveaux électroniques. Aujourd'hui, les rendements de conversion des cellules constituées de nanocristaux de sulfure de plomb approchent les 7%. Seulement, à cause des normes européennes destinées à l'affranchissement du plomb du fait de ses risques pour la santé et l'environnement, de nouveaux matériaux doivent être trouvés. Cette thèse concerne la synthèse de nouveaux types de nanocristaux semi-conducteurs et leur application dans des cellules solaires. La synthèse des nanocristaux de CuInSe2 et de SnS de taille et de forme contrôlées a été effectuée, notamment par des voies de synthèses reproductibles dont le passage à grande échelle est facilement possible. Une analyse approfondie de la structure des nanocristaux de SnS par spectroscopie Mössbauer a montré que ces nanocristaux avaient une forte tendance à s'oxyder, ce qui limite leur utilisation dans des dispositifs électroniques après exposition à l'air. La constitution de couches minces continues ayant de bonnes propriétés électriques a été effectuée par le dépôt contrôlé de nanocristaux ainsi que l'échange de leurs ligands de surface. En particulier, un nouveau type de ligand inorganique a été utilisé qui a montré une augmentation de la conductivité des films multiplié par quatre ordres de grandeurs par rapport aux ligands initiaux. Enfin, la préparation de cellules solaires basées sur ces couches minces de nanocristaux a montré des résultats encourageants et notamment un clair effet photovoltaïque lorsque le dépôt est effectué sous atmosphère inerte.

Les mémoires non-volatiles sont devenues récemment un moteur clé de la croissance du secteur des semiconducteurs, et constituent un pivot pour les nouvelles applications et les nouveaux concepts dans le domaine des technologies de l'information et de la communication (TIC). Afin de surmonter les limites en termes de miniaturisation, de consommation électrique et de complexité de fabrication des mémoires non-volatiles à grille flottante (FLASH), l'industrie des semiconducteurs évalue actuellement des solutions alternatives. Parmi celles-ci, les mémoires résistives à pont conducteur ou CBRAM (Conductive Bridge Random Access Memory), qui reposent sur la commutation de résistance d'un électrolyte par migration et oxydo/réduction d'ions métalliques, semblent être des plus prometteuses. L'attractivité de cette technologie innovante vient d'une part de la simplicité de sa structure à deux terminaux et d'autre part de ses performances électriques très prometteuses en termes de consommation électrique et vitesse d'écriture/effacement. De surcroît la CBRAM is une technology mémoire qui s'intègre facilement dans le back end of line (BEOL) du procédé CMOS standard. Dans cette thèse, nous étudions les performances électriques et la fiabilité de deux technologies CBRAM, utilisant des chalcogénures (GeS2) ou un oxyde métallique pour l'électrolyte. Tout d'abord nous nous concentrons sur les CBRAM à base de GeS2, ou l'effet du dopage de l'électrolyte avec de l'argent (Ag) ou de l'antimoine (Sb) est étudié à la lumière d'une analyse des caractérisations électriques. Les mécanismes physiques gouvernant la cinétique de commutation et la stabilité thermique sont aussi discutés sur la base de mesures électrique, d'un modèle empirique et des résultats de calculs ab initio. L'influence des différentes conditions de set/reset est étudiée sur une CBRAM à base d'oxyde métallique. Grâce à cette analyse, les conditions permettant de maximiser la fenêtre mémoire, améliorer l'endurance et minimiser la variabilité sont déterminées. / Non-volatile memory technology has recently become the key driver for growth in the semiconductor business, and an enabler for new applications and concepts in the field of information and communication technologies (ICT). In order to overcome the limitations in terms of scalability, power consumption and fabrication complexity of Flash memory, semiconductor industry is currently assessing alternative solutions. Among them, Conductive Bridge Memories (CBRAM) rely on the resistance switching of a solid electrolyte induced by the migration and redox reactions of metallic ions. This technology is appealing due to its simple two-terminal structure, and its promising performances in terms of low power consumption, program/erase speed. Furthermore, the CBRAM is a memory technology that can be easily integrated with standard CMOS technology in the back end of line (BEOL). In this work we study the electrical performances and reliability of two different CBRAM technologies, specifically using chalcogenides (GeS2) and metal oxide as electrolyte. We first focus on GeS2-based CBRAM, where the effect of doping with Ag and Sb of GeS2 electrolyte is extensively investigated through electrical characterization analysis. The physical mechanisms governing the switching kinetics and the thermal stability are also addressed by means of electrical measurements, empirical model and 1st principle calculations. The influence of the different set/reset programming conditions is studied on a metal oxide based CBRAM technology. Based on this analysis, the programming conditions able to maximize the memory window, improve the endurance and minimize the variability are determined.

La crise de l’énergie ainsi que les problèmes écologiques sont considérés comme les défis les plus importants de demain. Cependant, les sources d’énergies renouvelables et respectueuses de l’environnement ne sont pas suffisamment développées, ce qui entraîne une contribution faible à la production d’énergie. Les cellules solaires font partie des sources d’énergies renouvelables les plus attractives et prometteuses. Cependant, les panneaux solaires existants ont toujours un facteur de qualité négatif, c’est-à-dire que leur production et leur entretien demandent plus d’énergie qu’ils ne sont capables de produire pendant leur cycle de fonctionnement. Malgré la réduction exponentielle du prix des panneaux solaires, leur efficacité de conversion n’est pas suffisante. Les succès récents dans la science des matériaux ont beaucoup contribué à son amélioration, néanmoins des études sur les nouveaux matériaux photovoltaïques sont nécessaires. Les travaux de recherche présentés dans ce manuscrit concernent les vitrocéramiques de chalcogénures dans le système GeSe2-Sb2Se3-CuI. De nombreuses compositions chimiques différentes ont été préparées et analysées du point de vue structural et électrique dans le cadre de ce système. Il a été démontré que la phase Sb2Se3, responsable des propriétés photoélectriques dans les vitrocéramiques, possède un grand potentiel pour les applications photovoltaïques grâce à ses propriétés optiques bien adaptées. L’influence des modifications de la composition des cristaux de Sb2Se3 sur la structure et les propriétés électriques a été étudiée. La flexibilité du type de conductivité et la résistivité des cristaux massifs de Sb2Se3 ont été montrées. Enfin, les couches minces à base de vitrocéramiques et cristaux massifs étudiés ont été préparées par la pulvérisation cathodique RF magnétron. Leur structure et les propriétés électriques sont également décrites dans cette thèse. Une cellule solaire complètement fonctionnelle, basée sur Sb2Se3 dopé iode et sur la vitrocéramique de 40GeSe2-40Sb2Se3-20CuI, a été préparée et caractérisée. / Energy crisis and global ecological problems are considered as important challenges of nearest future. Eco-friendly and renewable sources of energy are still severely undeveloped and contribute weakly to the energy production. One of the most attractive and promising domains of renewable energy is a solar light harvesting. However, existing solar panels still possess negative quality factor, i.e. their fabrication and maintenance require more energy that they are capable to produce during their life cycle. Despite exponential reduction of the price, solar cells are not efficient enough in terms of light to energy conversion. Recent breakthroughs in material science contributed a lot to the increase of efficiency, however further investigation of novel materials are needed. Here, chalcogenide glassceramics of GeSe2-Sb2Se3-CuI system were studied in details. Within this system various chemical compositions were prepared and analyzed for their structure and photoelectric properties. We found that Sb2Se3 phase, responsible for the appearance of photoelectric effect in glass-ceramics, have a great potential for light harvesting due to its suitable optical properties. In the present work we demonstrated the influence of various dopants on photoelectric properties of Sb2Se3 crystals. We showed a possibility of conductivity type and resistivity tuning of bulk Sb2Se3 crystals in a wide range of values. Thin film devices based on studied bulk compositions were prepared by RF sputtering and characterized structurally and electrically as well. We demonstrated a fully functional thin film solar device based on iodine doped Sb2Se3 and 40GeSe2-40Sb2Se3-20CuI glass-ceramic.

A investigação das propriedades ópticas e sua relação com as propriedades magnéticas dos semicondutores é de grande interesse para a comunidade científica, em virtude da enorme demanda por novas tecnologias e funcionalidades que podem surgir dessas pesquisas. Os calcógenos de európio são semicondutores intrinsecamente magnéticos, transparentes na região visível do espectro eletromagnético e integráveis em matrizes de silício e nitreto de gálio, sendo assim fortes candidatos a aplicações tecnológicas envolvendo magnetismo e óptica. Neste trabalho são investigados os espectros de absorção e fotoluminescência dos calcógenos de európio, com base no modelo 4f -> 5d(\'t IND. 2g\') da transição óptica de dipolo elétrico, o qual mostrou-se totalmente adequado para a descrição da absorção óptica nos calcógenos de európio em função do campo magnético aplicado, explicando a presença de linhas de absorção estreitas e dicróicas nos espectros em campo alto e a forma larga dos espectros de absorção em campo nulo. Nos espectros de fotoluminescência do EuTe, entretanto, foram detectados estados eletrônicos não contemplados pelo modelo 4f -> 5d(\'t IND. 2g\') , em especial uma banda de emissão denominada \'MX IND. 0\', acoplada a modos vibracionais da rede. Uma linha zero-fônon correspondente a uma transição que não produz fônons pôde ser bem definida e a partir do deslocamento dessa linha em função do campo magnético foi possível detectar inequivocamente a formação de polarons magnéticos no EuTe pela primeira vez; o raio polarônico foi estimado como R = 3.6a, onde a é o parâmetro de rede e a energia de ligação desse polaron foi estimada em \'E IND. p\' = 45 meV, um resultado que foi confirmado através de medidas do deslocamento da linha zero-fônon em função da temperatura. Adicionalmente procurou-se identificar o estado eletrônico associado à emissão \'MX IND. 0\': a partir de medidas da intensidade da fotoluminescência em função da potência de excitação foi possível sugerir que a emissão \'MX IND. 0\' provem de estados eletrônicos ligados a defeitos da rede e foi possível estimar a concentração desses defeitos como menor que 0.1 ppm. / Investigation of optical properties and their relation to magnetic properties of semiconductors is of great interest to scientific community, due to the large demand for new technologies and features that can arise from these studies. Europium chalcogenides are intrinsically magnetic semiconductors, transparent in the visible region of electromagnetic spectrum and integrable into silicon and gallium nitride matrices, beeing strong candidates for technological applications involving magnetism and optics. The present study investigates absorption and photoluminescence spectra of europium chalcogenides, based on the 4f -> 5d(t2g) model of the electric dipole optical transition, which proved to be entirely appropriate to describe polarized optical absorption in europium chalcogenides as a function of magnetic field, explaining the presence of narrow dichroic lines at high fields and the broad shape of the zero-field absorption spectrum. However, in photoluminescence spectra of EuTe, electronic states not covered by the 4f -> 5d(t2g) model were detected, in particular an emission band labeled MX0, which is coupled to vibrational modes of the lattice. A transition without production of phonons, corresponding to a zero-phonon line, could be well resolved and from the displacement of the zero-phonon line as a function of magnetic field the formation of magnetic polarons in EuTe could be detected unambiguously for the first time. The polaronic radius is estimated as R = 3:6a, where a is the lattice parameter, and the polaron binding energy is estimated as Ep = 45 meV, a result that was confirmed by measurements of zero-phonon line displacement as a function of temperature. Additionally, we sought to identify the electronic state associated with MX0 emission: from measurements of the photoluminescence intensity as a function of excitation power, it was possible to suggest that MX0 emission comes from an electronic state coupled to lattice defects of low concentration, which we estimate to be of less than 0.1 ppm.

Cette thèse porte sur l'étude des chalcogénures de bore sous conditions extrêmes. Après un bref examen de la littérature (Chapitre I) sur le bore et ses composés sous haute pression (HP), la partie expérimentale (Chapitre II) donne une description des appareils HP utilisés dans ce travail de thèse. Les techniques d'analyses employées sont décrites ainsi que les grands principes des calculs théoriques ab initio utilisés. La partie suivante est consacrée à β-B2O3 et r-BS. Le chapitre III présente les résultats de diffraction X in situ à HP et les mesures de phonons (Raman et IR) de β-B2O3 à température ambiante. Les données expérimentales ont été examinées et complétées par des calculs ab initio. Avec le jeu de données obtenues, les descriptions complètes des modifications sous pression de la structure de β-B2O3 et du comportement des phonons en compression à température ambiante ont été faites. Le chapitre IV présente les résultats de diffraction X in situ à HP et de spectroscopie vibrationnelle sur r-BS à température ambiante. En combinaison avec les calculs théoriques, la structure et la nature des phonons de r-BS sur une large plage de pression à température ambiante sont décrites. En outre, la formation d'une nouvelle phase HP métastable de BS est présentée et sa structure probable ainsi que son équation d’état sont discutés. Le chapitre V concerne la première étude in situ de diffraction X sous HP-HT du système B-Se. Selon les résultats de diffraction X et des mesures Raman effectuées sur les échantillons récupérés, un nouveau composé a été synthétisé. Ce composé est métastable à l’ambiante. Sa composition chimique et sa structure probable sont discutées. / This thesis deals with the study of the boron chalcogenides under extreme conditions. After a short review of boron and boron compounds under extreme conditions (Chapter I), the experimental part (Chapter II) provides a brief description of the high-pressure devices used in this PhD work. The employed analytical techniques are described as well as the main principles of the performed ab initio theoretical calculations. The following part is devoted to our experimental and theoretical studies of β-B2O3 and r-BS. Chapter III presents the results of in situ high pressure XRD and phonon measurements (Raman and IR) of β-B2O3 at room temperature. The experimental data were considered and completed with results of ab initio calculations. Based on the total obtained dataset the complete description of β-B2O3 structure change and phonon behavior under compression at room temperature are detailled. Chapter IV contains the results of in situ high pressure XRD and vibrational spectroscopy studies on r-BS at room temperature. In combination with results of theoretical calculations the structure and phonon nature of r-BS in a wide pressure range at ambient temperature are described. Also, the formation of a new metastable high-pressure phase of BS is described and its probable structure and EoS are discussed. The last part (Chapter V) concerns the primary in situ XRD HP-HT studies in B-Se system. Based on the results of XRD and Raman measurements of the recovered samples, a new compound was synthesized. This compound was found to be metastable at ambient conditions. Its probable chemical composition and structure are discussed.