Compound semiconductors are the foundation of many electronic and optoelectronic devices. As a result semiconductor epitaxy can be viewed as the first significant step in device engineering. Accurate and reliable characterization methods are needed to measure semiconductor properties including optical, electrical, vibrational and crystal structure. In this thesis, the epitaxy of ZnTe thin films on sapphire substrate by Pulsed Laser Deposition system at different growth temperatures is studied. The texture analysis is inspected by Two Dimensional X-Ray Diffraction. The lattice constant of the films and strain studies are investigated by High Resolution X-Ray Diffraction. UV-Vis spectroscopy is applied to find absorption edge in ZnTe thin film in order to estimate optical bandgap. These common characterization methods reveal the great effect of growth temperature on crystalline and optical properties of ZnTe thin films. In addition, Raman spectroscopy is used for the first time in the Preston's group to examine vibrational modes in ZnTe thin films. This new characterization method, which is the main focus of this thesis, uncovers some new features of ZnTe thin films not accessible through other techniques. In this thesis, optimum experimental conditions, instrumentation and data analysis of Raman observations in thin films are studied in detail. The final results are in good agreement with other characterization methods and they can justify crystalline and optical observations. These results demonstrate that Raman spectroscopy is a non-destructive characterization method applicable to thin film analysis. / Thesis / Master of Applied Science (MASc)

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-08T20:19:50Z No. of bitstreams: 1 geraldohenriquessilva.pdf: 20536600 bytes, checksum: 9b626b89b88c550fa0fff1dcca169621 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-06-26T20:29:26Z (GMT) No. of bitstreams: 1 geraldohenriquessilva.pdf: 20536600 bytes, checksum: 9b626b89b88c550fa0fff1dcca169621 (MD5) / Made available in DSpace on 2017-06-26T20:29:26Z (GMT). No. of bitstreams: 1 geraldohenriquessilva.pdf: 20536600 bytes, checksum: 9b626b89b88c550fa0fff1dcca169621 (MD5) Previous issue date: 2014-02-28 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho estudou-se vidros co-dopados por meio de técnicas espectroscópicas de absorção óptica, espectroscopia Raman, fotoluminescência, fotoluminescência resolvida no tempo, lente térmica e capacidade térmica volumétrica. A classe de vidro estudada é a de vidros fosfatos denominados PZABP (P205 – ZnO – Al203 – BaO – PbO), os quais foram dopados com íons de európio (Eu3+), em diferentes concentrações, e telúrio (Te), visando o surgimento de nanopartículas semicondutoras de Telureto de Zinco (ZnTe). O objetivo desse trabalho é investigar a emissão do Eu3+ e sua interação com as nanopartículas semicondutoras no sentido de melhorar as propriedades ópticas e/ou térmicas do sistema vítreo co-dopado. Com a técnica de absorção óptica foi possível observar a alta transparência desse sistema, com uma janela óptica cobrindo a região desde o ultravioleta (em torno de 300 nm) até ao infravermelho médio (em torno de 3000 nm). Observou-se uma banda de absorbância larga centrada em torno de 540 nm quando o sistema vítreo PZABP foi dopado com Te. Essa banda foi atribuída a nanopartículas de ZnTe com comportamento de material na forma bulk. Com a técnica de fotoluminescência observou-se uma emissão avermelhada intensa atribuída aos íons de Eu3+, sendo a transição 5D0 —> 7F2 (611 nm) a mais intensa. Os parâmetros de Judd-Ofelt foram obtidos a partir dos espectros de emissão e absorção. De posse desses parâmetros, encontrou-se o tempo de vida radiativo do estado 5D0 que, juntamente com o tempo de vida experimental, permitiu a determinação da eficiência quântica. Observou-se que a mesma não sofreu uma alteração significativa em função da concentração dos íons de Eu3+ e com a presença do ZnTe. Com a técnica de lente térmica, na presença de nanopartículas de ZnTe, observou-se que a difusividade térmica (D) permaneceu praticamente constante com o aumento da concentração de Eu3+. / IIn this work we have studied co-doped glasses by means of spectroscopic techniques such as optical absorption, Raman spectroscopy, photoluminescence, time-resolved photoluminescence, thermal lens and volumetric heat capacity. The class of studied glasses was of phosphate glasses called PZABP (P2O5 – ZnO – Al203 – BaO – PbO) doped with different concentrations of europium ions (Eu3+), and tellurium (Te), to nucleation of semiconductor nanoparticles of zinc telluride (ZnTe). The goal of this work is to investigate the emission of the Eu3+ and its interaction with the semiconductor nanoparticles aiming to improve the thermo/optical properties of the glass system. With the optical absorption technique was possible to observe the high transparency of the system, which covers an optical window going from the ultraviolet region (about 300 nm) to mid-infrared (about 3000 nm). Also, was observed a large absorbance band centered around 540 nm when the glass system PZABP was doped with Te. This band was attributed to bulk-like ZnTe nanoparticles. Through the photoluminescence technique was possible to observe an intense reddened emission attributed to Eu 3+ ions, being the 5D0 —>7 F2 transition ( 611 nm) the most intense. The Judd-Ofelt parameters were obtained from the absorption and emission spectra. From these parameters, the radiative lifetime of the state 5D0 was found which together with the experimentally measured lifetime allowed the determination of the quantum efficiency. The latter does not present a significant change when the concentration of Eu3+ ions increases and with the presence of ZnTe. With the thermal lens technique, in the presence of ZnTe nanoparticles, was observed that the thermal diffusivity (D) remained almost constant with the increasing of concentration of Eu3+.

Cette thèse présente l'étude théorique et expérimentale de la génération et de la mise en forme d'impulsions térahertz (THz). Dans un premier temps, nous avons étudié la génération d'impulsions THz par redressement optique d'impulsions laser femtosecondes intenses dans des cristaux semiconducteurs de ZnTe. Nous avons mis en évidence une forte dépendance de l'efficacité de ce processus de génération avec l'intensité de l'impulsion laser génératrice. Ceci se traduit en particulier par un décalage progressif du spectre vers les basses fréquences et par une évolution anormale de l'énergie THz avec l'intensité laser. Ces comportements résultent d'une combinaison de trois phénomènes : la déplétion de l'impulsion laser au cours de sa propagation dans le cristal, l'absorption de l'onde THz par les porteurs libres créés par absorption à deux photons et une modification de la condition d'accord de phase induite par les porteurs chauds. Dans un deuxième temps, nous avons développé une approche analytique permettant de générer des impulsions THz de formes particulièrement intéressantes pour les expériences de spectroscopie cohérente ou de contrôle cohérent. Nous avons notamment généré des paires d'impulsions verrouillées en phase, des trains d'impulsions, ainsi que des impulsions THz accordables de grande finesse spectrale. Cette technique repose sur le redressement optique d'impulsions laser femtosecondes mises en forme à l'aide d'un masque à cristaux liquides placé dans le plan de Fourier d'une ligne à dispersion nulle. Afin de démontrer la validité de notre approche, nous avons également développé un programme de simulation qui donne des résultats en très bon accord avec l'expérience.

Cette thèse présente l’étude théorique et expérimentale de la génération et de la mise en forme d’impulsions térahertz (THz). Dans un premier temps, nous avons étudié la génération d’impulsions THz par redressement optique d’impulsions laser femtosecondes intenses dans des cristaux semiconducteurs de ZnTe. Nous avons mis en évidence une forte dépendance de l'efficacité de ce processus de génération avec l'intensité de l'impulsion laser génératrice. Ceci se traduit en particulier par un décalage progressif du spectre vers les basses fréquences et par une évolution anormale de l'énergie THz avec l'intensité laser. Ces comportements résultent d'une combinaison de trois phénomènes : la déplétion de l’impulsion laser au cours de sa propagation dans le cristal, l’absorption de l'onde THz par les porteurs libres créés par absorption à deux photons et une modification de la condition d’accord de phase. Dans un deuxième temps, nous avons développé une approche analytique permettant de générer des impulsions THz de formes particulièrement intéressantes pour les expériences de spectroscopie cohérente ou de contrôle cohérent. Nous avons notamment généré des paires d'impulsions verrouillées en phase, des trains d'impulsions, ainsi que des impulsions THz accordables de grande finesse spectrale. Cette technique repose sur le redressement optique d’impulsions laser femtosecondes mises en forme à l’aide d’un masque à cristaux liquides placé dans le plan de Fourier d’une ligne à dispersion nulle. Afin de démontrer la validité de notre approche, nous avons également développé un programme de simulation qui donne des résultats en très bon accord avec l'expérience. / This thesis deals with the theoretical and experimental study of the generation and shaping of terahertz (THz) pulses. At first, we have studied the generation of THz pulses by optical rectification of intense femtosecond laser pulses in semiconductor ZnTe crystals. We have demonstrated a strong dependence of the efficiency of the generation process with the intensity of the laser pulse. This is evidenced by a progressive shift of the spectrum towards lower frequencies and an abnormal evolution of the THz energy with laser intensity. These behaviors result from a combination of three phenomena: the depletion of the laser pulse during its propagation in the crystal, the absorption of THz wave by free carriers created by two-photon absorption and a change of the phase matching condition. Secondly, we have developed an analytical approach to generate THz shaped pulses particularly interesting for coherent spectroscopy or coherent control. In particular, we have generated phase-locked THz pulses pairs, pulse trains and tunable narrow-band THz pulses. This technique is based on optical rectification of shaped femtosecond laser pulses generated by a liquid crystal Fourier filter. To demonstrate the validity of our approach, we have also developed a program giving results in very good agreement with experiment.

Les nanostructures de semiconducteurs II-VI ont de nombreuses applications en microélectronique, optoélectronique et photonique. Notamment, les boites quantiques II-V peuvent servir de source de photons uniques. Dans cette étude, nous nous sommes intéressés à la caractérisation chimique et structurale des nanostructures de semiconducteurs II-VI (boites quantiques (BQs) auto-organisées, nanofils II-VI et III-V …) par sonde atomique tomographique (SAT). Dans un premier temps, nous avons optimisé les conditions d’analyse des semiconducteurs III-V et II-VI par SAT. Ensuite, nous avons étudié les compositions chimiques des interfaces II-VI/III-V en montrant la formation d’un composé Ga2.7Se3 à l’interface ZnSe/GaAs et un mélange de cations (Ga, Zn) à l’interface ZnTe/InAs. Les mesures de compositions chimiques et des tailles des boites quantiques en trois dimensions par SAT ont permis de faire une corrélation avec les mesures optiques. Nous nous sommes aussi intéressés à l’étude des mécanismes de croissance des nanofils GaAs et ZnTe ainsi que des BQs (CdTe) insérés dans des nanofils ZnTe en analysant la composition chimique des catalyseurs, les BQs dans les nanofils aussi que la base des nanofils. Ces mesures montrent que les boites quantiques sont formées d’un fort mélange CdxZn1-xTe. Un scénario basé sur la diffusion de surface a été proposé pour expliquer la croissance ainsi que le mélange entre Zn/Cd pour les BQs insérées dans les nanofils. / Nanostructures of II-VI nanostructure have many applications in microelectronics, optoelectronics and photonics. For example, II -V quantum dots have shown the ability to be a source of single photons. In this work, we performed in the chemical and structural characterization of nanostructures of II-VI semiconductors (self- organized quantum dots (QDs), nanowires II-VI and III- V ...) by atom probe tomography (APT). Firstly, the analysis conditions of III-V and II- VI semiconductors by APT were optimized. Then, we studied the chemical composition of II-VI/III-V interfaces and showed the formation of a Ga2.7Se3 compound at the ZnSe/GaAs interface and the (Ga, Zn) cations mixing at the ZnTe/InAs interface. The measurements of the chemical composition and the sizes of quantum dots in three dimensions by APT allowed making a correlation with optical measurements. We studied also growth mechanisms of GaAs, ZnTe nanowire and the CdTe QDs inserted in ZnTe nanowires by analyzing the chemical composition of the catalysts QDs and nanowires basis. These measurements show that the quantum dots are formed of a strong mixing of CdxZn1-xTe. A scenario based on surface diffusion has been proposed to explain the growth and the mixing between Zn/Cd for the QDs.

This thesis reports a passive method for Fermi level regulation in quantum dot assemblies through ground state transfer between QDs. Here, ZnTe/CdS, and PbSe/CdSe core/shell QDs were used as valence band electron donors, while Cu containing CdS or ZnSe acts as electron acceptor QDs. Prior to study of ground state charge transfer process, this report discusses the synthesis of ZnTe/CdS, and PbSe/CdSe core shell QDs, which are later used to study charge transfer. Since ZnTe QDs are unstable and prone to oxidation, a CdS coated ZnTe QDs were used. Growing a CdS shell on ZnTe core is difficult because high reduction potential of Te. To overcome this problem, partially reduced sulphur is used for the synthesis of ZnTe/CdS. The peculiar optical properties exhibited by ZnTe/CdS also have been discussed. Even though the synthesis of Lead chalcogenide nanoparticles has been investigated previously, certain inconsistencies between the behavior expected from known mechanisms and empirical observations. An anion exchange mechanism is proposed and demonstrated to be involved in PbSe formation. Both ZnTe and PbSe based QDs are extensively used to study hole injection and copper containing QDs were used as acceptors. The charge transfer has been studied using optical spectroscopy. The structure and composition of the assemblies was identified using powder crystallography, electron-microscopy and composition analysis. The unique physical and chemical properties of these materials are exciting both fundamentally as well as from the point of view of applications.