Title: Diffusion of native defects and impurities in CdTe/CdZnTe. Author: Lukáš Šedivý Author's e-mail address: luky.sedivy@seznam.cz Department: Institute of Physics of Charles University Supervisor: Doc. Ing. Eduard Belas, CSc. Supervisor's e-mail address: belas@karlov.mff.cuni.cz Abstract: In this thesis, the influence of structural defects on the electrical and de- tection characteristics of CdTe material was investigated. The performed research fo- cused on the reduction of structural defects in the material by annealing in Cd or Te vapor, while preserving acceptable features for X-ray and γ-ray detection. The mate- rial was characterized by measurement of the electrical resistivity and concentration and mobility of free carriers. Tellurium and cadmium inclusions were studied using infrared microscope. The static and dynamic properties of defect structures at high temperatures and de- fined Cd pressures was investigated, as well, and chemical diffusion coeficients describing the dynamic properties of these defects were experimentally determined. Keywords: monocrystal CdTe, structural defects in semiconductors, annealing in Cd or Te, chemical diffusion coefficient, γ-ray detectors. 1

A study combining infrared (IR) absorption spectroscopy and first-principles theory is presented for a sulfur–oxygen complex in CdSe characterized by IR absorption lines located at 1094, 1107, and 1126 cm-1 (10 K). The properties of the center are compared to a similar species occurring in CdTe that gives rise to two absorption lines at 1097 and 1108 cm-1 (10 K). Temperature- and polarization-sensitive measurements performed on 18O-enriched samples reveal that for both materials the IR absorption lines are due to split ν3 stretch vibrations of a distorted sulfate (SO4) tetrahedron, whereby the local point group of the SO4 complex is reduced to Cs and C3v in hexagonal CdSe and cubic CdTe, respectively. Measurements on the vibrational spectrum of the sulfate species in the spectral range of symmetric stretch (ν1), bend (ν4), and combinationmodes (ν1 þ ν3) are presented. The cation vacancy VCd is discussed as a likely site occupied by SO4 in CdSe.

Novel photovoltaic cells receive considerable attention from researchers as evidenced by high numbers of published articles. Different types of materials are currently being examined in order to reduce the cost and improve the efficiency of solar cells. Essentially, solar cells are constructed by placing layers of light absorber between electron and hole transport materials. Electricity generation by solar cells involves multiple processes. These processes require an understanding of the physical properties of the surfaces and interfaces of the materials. In this thesis, materials for novel photovoltaic cells are studied by X ray photoelectron spectroscopy (XPS), a surface and interface characterisation technique. The materials studied in this thesis are colloidal quantum dots (CQDs) of the core/shell systems CdTe/CdSe and PbS/CdS, and CQDs that have been surface passivated using Cl- (CdTe/Cl) and CdS (CdTe/CdSe/CdS and PbS/CdS). Moreover, CsSnI3, a perovskite material, is also studied in both bulk and thin film form. CQDs can be used as light absorbers in solar cells while CsSnI3 can be employed as the hole transport material. The role of the core shell structure and surface passivation treatment is to improve or maintain charge transport as well as acting as a protective layer to the CQDs. Depth profiling synchrotron radiation XPS is used to determine these structures. In the CdTe/CdSe samples, the elemental ratio between Se (shell) and Te (core) increases with decreasing sampling depth, demonstrating the presence of a CdSe shell located at the surface of the CQDs. The shell thicknesses of the core-shell systems are estimated from XPS and show that the addition of the third thin shell (of CdS) protects the CQD during ligand exchange. Cl- passivation is shown to reduce the energy the valence band maximum and the energy gap of CdTe CQDs. This is associated with the passivation of midgap trap states due to the removal of dangling bonds at the surface of CQDs. Surface passivation is shown to improve the stability of CQDs to air exposure. This is indicated by a significant reduction of the surface oxide species in the passivated PbS/CdS samples. In the unpassivated core-only PbS samples, however, oxidation rapidly occurs which affects the electronic states required for charge transport in solar cells. XPS studies of CsSnI3 show that this material is reactive to air exposure. Surface preparation techniques are performed to remove the contamination layer and reveal the physical properties of the perovskite itself. This is confirmed by the elemental ratios from XPS. The metallic character of CsSnI3 is also observed in the valence band spectra as evidenced by the appearance of the Fermi edge.

Etude expérimentale de la condensation de Bose-Einstein des excitons dans les solides.

In this work we investigated fabrication of semiconductor nanomaterials and evaluated their potential for photo-chemical and photovoltaic applications. We investigated different II-VI semiconductor nanomaterial systems; (i) ZnO oriented nanowire arrays non-epitaxially grown from a substrate; and (ii) colloidal CdE (E=Te,Se,S) quantum structures synthesized by solution-based thermal decomposition of organo-metallic precursors. We have studied the synthesis of vertically aligned ZnO nanowire arrays (NWA), by a wet chemical process on various substrates. We have extended this method wherein nanofibers of poly-L-lactide act as a substrate for the radially oriented growth of ZnO nanowires. By combining the large surface area and the flexibility of the PLLA-ZnO hierarchical nanostructure we have shown the proof-of-principle demonstration of a ‘continuous-flow’ water treatment system to decompose known organic pollutants in water, as well as render common waterborne bacteria non-viable. We have studied synthesis of colloidal quantum dots (QD), and show size, morphology and composition tailored nanocrystals for CdE (E=S, Se, Te) compositions. We have studied the influence of crystal growth habits of the nanocrtsyals on the final morphology. Furthermore we have synthesized core-shell, CdSe-CdS QDs with spherical and tetrahedral morphologies by varying the reaction conditions. We show that these core-shell quantum dots show quasi-type II characteristics, and demonstrate with I-V measurements, the spatial localization of the charge carriers in these hetero-nanocrystals. For this purpose, we developed hybrid materials consisting of the core-shell quantum dots with electron acceptors (ZnO nanowires) and hole acceptors (polymeric P3HT nanofibers). In addition we have also compared the synthesis reaction when carried out with conventional heating and microwave-mediated heating. We find that the reaction is enhanced, and the yield is qualitatively better when using microwave induced heating. / QC 20120525

The research presented has been focused on understanding the surface effects on the optical and electronic properties of some metallic and semiconductor nanomaterials. When the particle sizes are on the nanometer length scale, a large fraction of atoms in the particles are on the surface. The bonding of the surface atoms being unsaturated could cause trapping and introduce defects that interact with the excited electrons. The effect of the surface on the optical and electronic properties of some semiconductor and metallic nanoparticles is investigated. When the size and shape of nanomaterials change, both the electron density of the excited electrons on the surface and the electronic structure change. Therefore, it becomes important to understand how these changes affect the electronic motion in the particles in order to exploit their full potential in a variety of applications. Semiconductor nanoparticles studied include cadmium selenide (CdSe) and cadmium sulfide (CdS). Effect of changing CdSe shape and size on optical and electronic properties has been investigated and the ability for the CdS nanoparticles to show optical gain (stimulated emission) in solution at room temperature is reported. Effect of surface phonon contribution on the exited electron relaxation in copper nanoparticles is investigated. For the particles size smaller than the mean free path of the electrons in the metal, electron-surface phonon coupling becomes an important factor (contribution) for hot electron relaxation. In the thesis presented, it is shown for the first time the size depended electronic relaxation in copper nanoparticles. Fluorescence due to surface plasmon field enhancement is observed for copper nanoparticles to be million times stronger than the fluorescence observed from bulk copper.

La compréhension des propriétés électriques de nano-objets est essentielle pour le développement s des nanotechnologies. La microscopie à force Kelvin (KFM) est une des techniques les plus utiles pour cartographier simultanément la topographie et la différence de potentiel de contact (CPD) à l'échelle nanométrique. Après 20 ans de développement, la KFM est principalement utilisé dans des conditions normales de pression et de température, permettant d'effectuer, de manière simple, de multiples analyses comparatives. Toutefois, sous ultra-vide (UHV), comme la surface est contrôlée et que la sensibilité est meilleure, des mesures plus précises et plus fiables sont réalisables. Dans la première partie, la mesure KFM sous atmosphère ambiante est améliorée en développant la technique simple-passage à la fois en modulation de fréquence (FM) et en modulation d'amplitude (AM). Une électronique externe Nanonis a été adaptée sur les AFMs commerciaux (Dimension 3100 et MultiMode, Bruker). Une étude comparative avec le mode Lift a été réalisée sur des couches de graphène épitaxié sur échantillon de SiC. L'effet de la distance pointe-échantillon sur le contraste et la résolution est décrit ainsi que l'influence des paramètres expérimentaux. Une amélioration significative du contraste et de la résolution est clairement observée sur les résultats obtenus par la technique simple passage en modulation de fréquence, indépendamment de la distance pointe-échantillon. Dans une deuxième partie, la technique KFM est développée sous vide secondaire. Le travail instrumental est réalisé sur un AFM EnviroScope de chez Bruker, qui a été équipé d'une électronique externe Nanonis, permettant de mesurer simultanément la topographie en mode non-contact et la CPD en modulation d'amplitude ou de fréquence. Les résultats montrent que la CPD mesurée est comparable à celle obtenue avec une mesure sous ultravide. Enfin, après avoir posé les bases à la fois expérimentale et théorique de la KFM, cette technique est utilisée pour caractériser les hétérostructures CdTe/CdS en films minces utilisés pour les applications de cellules solaires. Un protocole de préparation d'échantillon sur la tranche a été spécialement développé. L'hétérojonction CdTe/CdS est étudiée sous polarisation à la fois dans l'obscurité et sous éclairement. L'influence de l'épaisseur de la couche de CdS est également étudiée pour comprendre son effet dramatique sur le rendement des cellules solaires.

Thin film cadmium telluride (CdTe) technology is the most important competitor for silicon (Si) based solar cells. Pulsed direct current (DC) magnetron sputtering is a new technique has been developed for thin film CdTe deposition. This technique is industrially scalable and provides uniform coating. It is also possible to deposit thin films at low substrate temperatures. A series of experiments are presented for the optimisation of the cadmium chloride (CdCl2) activation process. Thin film CdTe solar cells require CdCl2 activation process to improve conversion efficiencies. The role of this activation process is to increase the grain size by recrystallisation and to remove stacking faults. Compaan and Bohn [1] used the radio-frequency (RF) sputtering technique for CdTe solar cell deposition and they observed small blisters on CdTe layer surface. They reported that blistering occurred after the CdCl2 treatment during the annealing process. Moreover, void formation was observed in the CdTe layer after the CdCl2 activation process. Voids at the cadmium sulphide (CdS)/CdTe junction caused delamination hence quality of the junction is poor. This issue has been known for more than two decades but the mechanisms of the blister formation have not been understood. One reason may be the stress formation during CdTe solar cells deposition or during the CdCl2 treatment. Therefore, the stress analysis was performed to remove the defects observed after the CdCl2 treatment. This was followed by the rapid thermal annealing to isolate the CdCl2 effect by simply annealing. Small bubbles observed in the CdTe layer which is the first step of the blister formation. Using high resolution transmission electron microscopy (HR-TEM), it has been discovered that argon (Ar) working gas trapped during the deposition process diffuses in the lattice which merge and form the bubbles during the annealing process and grow agglomeration mainly at interfaces and grain boundaries (GBs). Blister and void formation were observed in the CdTe devices after the CdCl2 treatment. Therefore, krypton (Kr), neon (Ne) gases were used as the magnetron working gas during the deposition of CdTe layer. The results presented in this thesis indicated that blister and void formation were still existing with the use of Kr an Ne. Xe, which has a higher atomic mass than Kr, Ne, Ar, Cd and Te, was used as the magnetron working gas and it resulted in surface blister and void free devices.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Inorganic nanomaterials are a class of advanced materials, which have been widespread in both science and nanotechnology. The use of nanomaterials in several areas makes nanoscience an interdisciplinary field. Some challenges, such as the type of synthesis and toxicity control should be explored, mainly because those materials can be controlled at the nanometer level by means of the synthesis variables. In this work, we performed the synthesis of cadmium telluride (CdTe) nanoparticles with the surface capped by a mixture of organic thiol stabilizers such as mercaptopropionic acid (MPA), cysteine (CYS), glutathione (GLU), and biopolymer sodium alginate (ALG). Synthesis were performed by two distinct aqueous methods hydrothermal and reflux and the synthesis parameters such as pH and ratio Cd/Te/stabilizer were fixed based on the literature. The colloidal dispersions obtain present remain stable for at least 3 months (time observed so far). Optical absorption spectra of all mixtures of CdTe - stabilizers presented a red shift with increasing synthesis time, and furthermore, the position of this band was at a wavelength below the bulk CdTe (790 nm). The emission spectra in all cases were intense, with little difference between them. The diameter of the particles, calculated from an empirical equation from the of maximum absorption wavelength, were comparable compared to those obtained by X-ray diffraction (XRD) (Scherrer equation) as well as transmission electron microscopy (TEM). The results from the three techniques agreed very well for CdTe via reflux. However for hydrothermal synthesis it was not possible to determine the diameter of the particles by TEM due to morphology of nanostructured aggregates. The crystal structures as obtained by XRD indicating the incorporation of sulfur atoms in the CdTe structure, in agreement to the literature for this type of synthesis. Infrared spectroscopy confirmed the presence of organic stabilizers on the surface of CdTe. The morfology of samples identified by TEM/HRTEM was different depending on the method of syntesis aggregates for the hydrothermal samples and isolated particles for reflux samples so the mechanisms of nucleation and growth were proposed oriented attachment (OA) and Ostwald ripening (OR) for synthesis CdTe-hydrothermal and reflux, respectively. / Os nanomateriais inorgânicos são uma classe de materiais avançados, que vêm sendo difundidos tanto em ciência como em nanotecnologia. O uso destes materiais está localizado em diversas áreas de atuação, fazendo com que tal classe seja interdisciplinar. Alguns desafios como o tipo de síntese e a toxidade devem ser explorados, principalmente porque estes materiais podem ser controlados a nível nanométrico por meio das variáveis de síntese. Neste trabalho, foi realizada a síntese de nanopartículas de Telureto de Cádmio (CdTe) com a superfície coberta pela mistura de estabilizantes orgânicos tióis tais como, ácido mercaptopropiônico (MPA), cisteína (CYS) e glutationa (GLU) e o biopolímero alginato de sódio (ALG). Esta síntese foi realizada por dois métodos aquosos, via hidrotermal e refluxo, e os parâmetros de síntese como pH e relação Cd/Te/estabilizante foram fixos com base na literatura. As dispersões coloidais obtidas apresentaram-se com estabilidade coloidal por pelo menos 3 meses (tempo observado até o momento). Os espectros eletrônicos de absorção óptica de todas as misturas de CdTe-estabilizantes se apresentaram largos, com deslocamento para o vermelho com o aumento do tempo de síntese, e além disso, a posição desta banda estava em comprimento de onda abaixo do CdTe bulk (790 nm), indicando confinamento quântico. Os espectros de emissão em todos os casos foram intensos e com pouca diferença entre os mesmos. Os diâmetros das partículas foram calculados usando-se uma fórmula empírica, a partir do comprimento de onda máximo de absorção e comparado aos resultados obtidos por difratometria de raios X (XRD) (equação de Scherrer) e por microscopia eletrônica de transmissão (TEM). Os resultados das três técnicas corroboram entre si para as amostras de CdTe via refluxo. Contudo, no caso do hidrotermal, não foi possível determinar os diâmetros por TEM porque a morfologia foi de agregados nanoestruturados. As estruturas cristalinas forma obtidas por XRD, indicando a incorporação de átomos de enxofre na estrutura do CdTe, estando de acordo com a literatura para o tipo de síntese. A espectroscopia de infravermelho confirmou a presença dos estabilizantes orgânicos na superfície do CdTe. A morfologia identificada por TEM/HRTEM das amostras sintetizadas pelas duas sínteses foi diferente agregados para a síntese hidrotermal e partículas isoladas para refluxo sendo que os mecanismos proposto para nucleação e crescimento foram o oriented attachment (OA) e o Ostwald ripening (OR) para CdTe-hidrotermal e o CdTe-refluxo, respectivamente.

Gamma-ray spectroscopy is undoubtedly the most effective tool for understanding the structure of the nucleus. In common with many other problems however, there is more information available that can be readily measured by standard experimental facilities. Therefore, this thesis investigates the potential for the use of a new detector material, CdZnTe, in nuclear physics applications. To evaluate the requirements of detection systems for nuclear physics applications, a y-ray spectroscopy experiment was performed to investigate neutron alignments in 100Mo, 104Ru and 108Pd using deep-inelastic reactions. This showed that a detector capable of detecting low energy (< 100 keV) X- rays without compromising y-ray detection efficiency could have significant benefit. A room temperature CdZnTe semiconductor detector could reasonably form part of a standard escape suppressed spectrometer. However, there is a substantially higher leakage current associated with room temperature semiconductor devices than standard cryogenically cooled semiconductor detectors. CdZnTe suffers from significant charge trapping, and therefore the rise time of the radiation induced pulses forms an important part of the signal analysis from such detectors. These two problems have implications on the design of preamplifier systems for CdZnTe detectors. For this reason, this thesis describes the design of optimised electronic systems for use with room-temperature operated CdZnTe detectors. Here, the focus is on the preamplifier design, and on practical ways of analysing noise performance of the preamplifier. A new preamplifier configuration with digital output has been developed, and a detailed signal-to-noise analysis performed. Such a circuit facilitates simultaneous measurement of both energy and pulse shape information.