Pour étudier l'influence de la gravité sur la croissance par T.H.M. du semi-conducteur CdTe dopé indium (programme EURECA), nous avons effectué une analyse comparative d'expérience spatiales et terrestres, complétée par une simulation numérique de l'hydrodynamique de la phase fluide utilisant un code 3D. Notre deuxième objectif a été de prépérer des monocristaux de CdTe dopé vanadium par la technique Bridgman présentant de bonnes propriérés photoréfractives. Pour une meilleure compréhension du mécanisme de l'effet photoréfractif, les matériaux ont été caractérisés par R.P.E., D.C.M. et P.I.C.T.S. afin de déterminer le degré de valence de V et la signature des niveaux vanadium dans la bande interdite. Des mesures optiques (absorption, photoconductivité...) et d'effet photoréfractif ont été effectuées. Enfin, pour évaluer la pertinence spatiale de cette croissance, nous avons étudié par simulation numérique l'influence de la gravité sur la configuration hydrodynamique de la phase fluide.

Ces travaux de doctorat proposent d'apporter une contribution à l'identification et à la compréhension des phénomènes limitant les performances de détecteurs de rayon X à base de CdTe:Cl développés pour des applications en radiographie. En effet, des inhomogénéités spatiales non-stables dans le temps sont observées dans la réponse de ces capteurs. Les défauts des cristaux utilisés pour la détection ont été caractérisés. Notamment, les dislocations révélées par attaque chimique et par topographie X présentent des arrangements en mur à la surface des échantillons, ces défauts sont majoritairement traversant dans toute l'épaisseur du cristal. Il a ensuite été montré que ces murs de dislocations sont responsables des inhomogénéités de photo-courant sous irradiation par des rayons X et de courant de fuite d'un détecteur à base de CdTe:Cl. De plus, les niveaux pièges dans le gap du CdTe ont été investigués par des méthodes de spectroscopie optique à basse température : les images de cathodoluminescence mettent en évidence le caractère non-radiatif des murs de dislocations, mais ne montrent pas l'apparition de la luminescence Y au niveau de ces défauts, normalement attribuée aux dislocations dans la littérature. Enfin, l'influence des murs de dislocations sur les propriétés de transport des porteurs de charge a été étudiée par la méthode " Ion Beam Induced Current " (IBIC) montrant qu'ils entraînent une diminution de la valeur du produit mobilité-temps de vie des électrons et des trous

L'amélioration des performances des détecteurs (au niveau de la résolution en énergie et de la résolution spatiale) pour la radiographie X médicale par l'utilisation d'un semi-conducteur montre l'intérêt de remplacer les détecteurs à base de cristaux scintillateurs (majorité des systèmes commerciaux) par un détecteur à base de semi-conducteur. Avec une perspective de respect environnemental, ces travaux portent sur le développement d'une nouvelle génération de détecteurs à base de semi-conducteur, différent du CdTe, pour l'imagerie médicale de rayons X fonctionnant en mode comptage, ce qui permet la réduction de la dose de rayonnement envoyée sur le patient. Deux axes de recherches en découlent, avec le choix d'un nouveau matériau semi-conducteur, le GaAs semi-isolant et d'une nouvelle géométrie de détection, la géométrie 3D.Ces travaux ont consisté à évaluer expérimentalement le semi-conducteur, afin de choisir un matériau (fournisseur, croissance) et une électrode métallique qui ont la capacité de compter des photons. Puis, la structure de détection, au travers de caractérisations des procédés technologiques nécessaires pour la réalisation de la géométrie 3D (usinage des électrodes, dépôt des électrodes, connexion à un circuit électronique) et de la validation du concept avec un dispositif de test, a été également étudiée. Enfin, les premiers résultats d'un détecteur 3D à base de GaAs semi-isolant, montrant la concrétisation de ces objectifs, sont proposés.

The work presented in this thesis focuses on the investigation and improvement of the window stack of layers for thin film CdTe solar cells fabricated in the Center for Renewable Energy Systems Technology (CREST) laboratories. In particular the aim was to change the standard structure including TCO, high resistive transparent (HRT)layer and CdS which is limited by the low transparency of the CdS layer, to a better performing one. The first result chapter of the thesis describes the study of ZnO HRT layers. ZnO thin films were deposited by radio frequency (RF) magnetron sputtering with different structural, optical and electrical properties which were characterized by X-ray diffraction, electron microscopy, spectrophotometry, Hall Effect method and 4-point probe. ZnO films were then incorporated in CdTe solar cells with the structure: FTO/ZnO/CdS/CdTe/Au back contact and the performance of these devices were compared with the film properties to single out trends and identify optimal film characteristics. By varying the deposition pressure of ZnO films, it was possible to increase their transparency and significantly increase their resistivity. While better transparency positively affected the solar cell current density output and efficiency, the resistivity of ZnO films did not show any clear impact on device efficiency. By increasing the deposition temperature the ZnO film grain size was increased. Increased FF was observed in devices incorporating ZnO layers with bigger grains, although this gain was partially counterbalanced by the Voc degradation, leading to a limited efficiency improvement. Finally the addition of oxygen had the main effect of increasing the resistivity of ZnO films, similarly to what happened with the increase of the sputtering pressure. In this case however, an improvement of FF, Jsc and efficiency was observed, especially at an O2/Ar ratio of 1%. By simulating the solar cells behavior with SCAPS-1D, it was found that these performance change can be explained by the variation of interface properties, precisely the amount of interface defects, rather than by bulk properties. The study presented in the second result chapter focuses on magnesium-doped zinc oxide (MZO) and the variation of its energy band structure. MZO was initially used as the HRT layer within a solar cell structure: FTO/MZO/CdS/CdTe/Au back contact. Sputtering MZO films with a target containing MgO 11 weight% and ZnO 89 weight% allowed for and increased band gap from 3.3 eV of intrinsic ZnO to 3.65 eV for MZO deposited at room temperature. Increasing the superstrate deposition temperature allowed for a further band gap increase up to 3.95 eV at 400 °C due mainly to an conduction band minimum upward shift. It was highlighted the importance to create a positive conduction band offset with the MZO layer conduction band slightly above the CdS conduction band, with an optimum found in this case to be 0.3 eV (efficiency 10.6 %). By creating a positive conduction band offset all the performance parameters (Voc, FF, Jsc, efficiency) significantly increased. One of the reasons for this improvement was found to be a diminished interface recombination due to a more ideal MZO/CdS band alignment. In the second part of this investigation the MZO was used as a replacement for the CdS in a simplified structure: FTO/MZO/CdTe/Au back contact. The concepts used to optimise the performance of these devices also involved tuning the conduction band alignment between MZO/CdTe and efficiencies of 12.5 % were achieved with a at conduction band offset. The efficiency increase was achieved mainly thanks to a better transparency of the MZO layer and a higher Jsc output, compared to devices using a CdS buffer layer. The MZO buffers have been tested in combination with different TCOs. Results are presented in the third result chapter and showed that AZO is a good alternative to FTO working effectively in combination with MZO. AZO/MZO efficiency thin film CdTe solar cells (12.6%, compared to 12.5% with FTO). It was found that increasing the IR transparency of the TCOs leads to a potentially higher Jsc. Achieving a better transparency was obtained by using TCOs with high mobility and lower carrier concentration (AZO and ITiO) and also by using a boro-aluminosilicate glass with low iron content. ITiO yielded the best opto-electrical properties among all the TCO materials. Devices incorporating ITiO however, showed lower performance then those using FTO and AZO. ITO/MZO windows also yielded poor performance. In addition, the ITO films deposited had a high carrier concentration leading to a high NIR absorption by plasma resonance and resulted not ideal for application in thin film CdTe PV.

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / CdTe semiconductor nanoparticles have unique optical properties, which can be tunable sizes, due to the quantum confinement effect. In the present work, stable CdTe nanocrystals were grown in the mesoporous environment of thiol-functionalized MCM-41 mesoporous silica from an approach employing 3-mercaptopropionic acid, an efficient capping agent. The obtained materials were characterized by infrared spectroscopy, N2 dessortion/adsorption, UV-Vis emission and absorption spectroscopy, transmission electron microscopy (TEM) and electrochemical characterization; X-ray diffraction (XRD) and thermogravimetric analyses were used to MCM-41 silica. The emission properties of CdTe were found to be dependent on the proportion of mesoporous matrix used in the synthesis, yielding CdTe nanocrystals emitting below the wavelength range usually obtained from aqueous colloidal synthesis. The detection of Cu2+ ions using modified carbon paste electrodes by anodic stripping voltammetry was possible only with samples of CdTe in situ, evidencing the role of the semiconductor as active phase. The most active sample obtained here was exactly that containing the lowest CdTe loading, showing it is possible to reduce the dosage of Cd-containing nanocrystals while improving their properties, which is crucial to concretize applications of CdTe nanocrystals. / Nanopartículas semicondutoras de CdTe possuem propriedades ópticas únicas ajustáveis, que são dependentes de seus tamanhos, devido ao efeito do confinamento quântico. No presente trabalho, nanopartículas estáveis de CdTe foram sintetizadas confinadas em um ambiente mesoporoso de uma sílica MCM-41 funcionalizada com grupos tióis, através do método de funcionalização com o uso do ácido 3-mercaptopropiônico. Os materiais obtidos foram caracterizados por espectroscopia vibracional na região do infravermelho, adsorção/dessorção de nitrogênio, espectroscopias eletrônicas de absorção UV-Vis e emissão, microscopia eletrônica de transmissão (MET) e caracterização eletroquímica; já as caracterizações por difração de raios X (DRX) e análise termogravimétrica foram usadas para sílica MCM-41. As propriedades de emissão do CdTe mostraram relação de dependência da proporção da matriz mesoporosa usada nas sínteses, resultando em emissão abaixo da faixa de comprimento de onda usualmente obtido para CdTe em meio coloidal aquoso. A detecção de íons Cu2+ através de eletrodos de pasta de carbono modificados foi realizada por voltametria de redissolução anódica, a qual apresentou sinal somente para as amostras de CdTe in situ, o que evidenciou o papel do semicondutor como fase ativa do eletrodo. A amostra mais ativa foi aquela que continha a menor proporção CdTe, evidenciando que é possível reduzir a dosagem de Cd2+ nos nanocristais enquanto se aperfeiçoam suas propriedades, o que é crucial do ponto de vista das aplicações de nanocristais de CdTe.

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L'imagerie gamma est une technique qui permet la localisation spatiale de sources radioactives. Les différentes applications de cette technique couvrent les phases de démantèlement des installations nucléaires ou de gestion des déchets nucléaires, mais aussi la radioprotection ou la sécurité intérieure. L'utilisation de caméras gamma permet de réduire la dose reçue par les opérateurs, et, par conséquent, de respecter le principe ALARA. Il existe deux techniques d’imagerie permettant la localisation de radioéléments émetteurs gamma : l’imagerie à masque codé et l’imagerie Compton. L’imagerie à masque codé utilise la modulation spatiale du flux de photons gamma incidents par collimateur multi-trous placé entre la source et le détecteur. Elle présente l’avantage d’être extrêmement performante pour des émetteurs gamma « basses énergies », aussi bien en matière de sensibilité, qu’en matière de résolution angulaire. L'imagerie Compton, quant à elle, repose sur l’utilisation de la mécanique de diffusion Compton. L'énergie déposée pendant le processus de diffusion déterminera l'angle de diffusion, et les positions des interactions détermineront la direction des rayons gamma entrants. La position de la source radioactive peut ainsi être limitée à un cône. Si plusieurs cônes sont utilisés, alors la position où le plus grand nombre de cônes se chevauchent correspond à la position de la source radioactive. Une des limitations de cette technique concerne la localisation des émetteurs gamma « basses énergies », pour lesquels la résolution angulaire est fortement dégradée allant jusqu’à l’impossibilité complète de trouver la position. L’objectif de ces travaux est de développer un prototype d’imageur hybride associant les techniques d’imagerie à masque codé et d’imagerie Compton, afin de tirer profit des avantages de chacun des types d’imagerie. Les différents travaux menés, autour du détecteur pixellisé Timepix3, mais aussi en matière de développement d’algorithmes mathématiques, ont permis de proposer deux prototypes d’imageurs hybrides. Les résultats obtenus à l’issue de ces travaux de recherche ont permis de valider expérimentalement les performances d’un des prototypes d’imageurs et d’illustrer l’intérêt d’un système hybride. / Gamma imaging is a technique that allows the spatial localization of radioactive sources. The various applications of this technique cover decommissioning phases of nuclear facilities, nuclear waste management applications, but also radiation protection or Homeland Security. Using gamma camera reduces the dose received by operators and consequently contributes to the respect of the ALARA principle. There are two imaging techniques for the localization of gamma ray emitters: coded aperture imaging and Compton imaging. Coded aperture imaging relies on the spatial modulation of the incident gamma-ray flux by a multi-hole collimator placed between the detector and the radioactive source. It has the advantage of being extremely efficient for « low energy » gamma-ray emitters in terms of sensitivity and angular resolution. On the other hand, Compton imaging is based of the Compton scattering kinematic. The energy deposited during the scattering process will determine the scattering angle, and the positions of the interactions will determine the direction of the incoming gamma-ray. The position of the radioactive source can thus be limited to a cone. If several cones are used, then, the position where the greatest number of cones overlap corresponds to the position of the radioactive source. One limitations of this technique concerns the location of « low energy » gamma-ray emitters, for which the angular resolution is strongly degraded until it is completely not localizable. The objective of this work is to develop a prototype of hybrid imager that combines coded aperture and Compton imaging techniques in order to take advantage of each type of imaging. The different studies carried out, around the Timepix3 pixel detector, but also in the development of mathematical algorithms, have led to propose two prototypes of hybrid imager. The results obtained from this research work made it possible to validate experimentally the performance of one of the imager prototypes, and to illustrate the interest of a hybrid system.

Title: Photo-Hall effect spectroscopy and laser-induced transient currents in CdTe-based semiconductor radiation detectors Author: Artem Musiienko Department / Institute: Institute of Physics, Faculty of Mathematics and Physics, Charles University Supervisor of the doctoral thesis: Prof. RNDr. Roman Grill, CSc, Institute of Physics, Faculty of Mathematics and Physics, Charles University Abstract: Cadmium Telluride, Cadmium Zinc Telluride, and Cadmium Manganese Telluride are important semiconductors with applications in radiation detection, solar cells, and electro-optic modulators. Their electrical and optical properties are principally controlled by defects forming energy levels within the bandgap. Such defects create recombination and trapping centers capturing photo- created carriers and depreciating the performance of the detector. Simultaneously, the changed occupancy of levels leads to the charging of detector's bulk, which results in the screening of applied bias and the loss of detector's sensitivity. Detailed knowledge of crystal defect structure is thus necessary for the predictable detector work and also for the possibility to reduce the structural defects concentration. This thesis reports on the investigation of deep energy levels in CdTe-based high resistivity and detector-grade materials by...

Title: Point defects in materials for detection of X-ray and gamma radiation Author: Martin Rejhon Department: Institute of Physics of Charles University Supervisor: prof. Ing. Jan Franc, DrSc., Institute of Physics of Charles Uni- versity Abstract: Cadmium telluride and its compounds are suitable materials for pro- duction of X-ray and gamma-ray detectors working at room temperature. How- ever, the detector quality is affected by material imperfections, such as crystal defects and impurities. It results into forming of deep levels which act as re- combination and trapping centers. Then, the accumulated space charge at these deep levels influences electric and spectroscopic properties of the detector. In the end it may result in the polarization effect, when the electric field is localized in vicinity of one contact and detection properties are decreased. This thesis reports a complex study of a detector band structure by various meth- ods with focus on differences between CdTe, CdZnTe, CdTeSe and CdZnTeSe. The electro-optic Pockels effect is used to investigate the influence of the illumi- nation in range 900 − 1800 nm on the inner electric field. The temperature and time evolutions of the electric field after application of bias or switching of the additional light at 940 nm were measured to determine deep levels...

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