Submitted by MARCEL MIYAMURA BONILHA null (marcelmb@gmail.com) on 2016-09-08T10:39:53Z No. of bitstreams: 1 Versão Final.pdf: 4946368 bytes, checksum: 4675d9707acb564334a7be7744822ddf (MD5) / Approved for entry into archive by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2016-09-09T12:47:33Z (GMT) No. of bitstreams: 1 bonilha_mm_me_bauru.pdf: 4946368 bytes, checksum: 4675d9707acb564334a7be7744822ddf (MD5) / Made available in DSpace on 2016-09-09T12:47:33Z (GMT). No. of bitstreams: 1 bonilha_mm_me_bauru.pdf: 4946368 bytes, checksum: 4675d9707acb564334a7be7744822ddf (MD5) Previous issue date: 2016-08-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Heteroestruturas que combinam camadas supercondutoras (SC) com ferromagnéticas (FM), e, em alguns casos, ainda com camadas isolantes (I) entre as primeiras, possuem propriedades que não são observadas para os filmes isolados devido a interação entre elas. Este trabalho busca contribuir para a elucidação das propriedades de super-redes do tipo SC/I/FM feitas por PLD (Pulsed Laser Deposition), utilizando o YBa2Cu3O7-δ (YBCO) como supercondutor, o La2/3Ca1/3MnO3 (LCMO) como ferromagneto e o PrBa2Cu3O6+δ (PBCO), o SrTiO3 (STO) e o CeO2 (CEO) como isolantes. A sequência utilizada foi (YBCO20nm/Idi/LCMO20nm)x20, e as espessuras do isolante PBCO foram de 0,5, 2, 4, 8 e 20nm, e de 2nm para o STO e para o CEO. Também foi analisada uma amostra com a inversão da ordem de deposição do supercondutor e ferromagneto. As análises de difração de raios X confirmaram o crescimento preferencial na direção 00ℓ e parâmetros de rede comparáveis com os valores encontrados na literatura para o YBCO. A microscopia eletrônica de varredura evidenciou poros na superfície do filme de LCMO e crescimento extra de cristais na superfície do filme de YBCO. As análises de AFM mostraram que a textura da superfície depende da espessura e da natureza do isolante assim como a distribuição de cargas observada por EFM. Medidas elétricas realizadas pelo método de Quatro Pontas comprovou temperatura crítica em torno de 90K, e somente para a amostra com 20nm de PBCO foi notado grande distinção do comportamento da resistividade ao longo do aquecimento a partir de 25K com a aplicação de campo magnético de 0,8T, o que pode indicar uma interação diferenciada entre as camadas de LCMO e YBCO nesta configuração. Pequenos picos de magnetorresistência foram observados sempre próximos da transição supercondutora. A intensidade e sinal do pico sofreram algumas variações com a direção do campo. / Heterostructures that combine superconducting (SC) layers with ferromagnetic (FM) layers, and in, some cases, with insulating (I) layers between the first ones, have properties that are not observed in single layers due the interaction between them. This work aims to contribute for elucidation of properties of SC/I/FM superlattices made by PLD deposition, using YBa2Cu3O7-δ (YBCO) as superconductor, La2/3Ca1/3MnO3 (LCMO) as ferromagnetic and PrBa2Cu3O6+δ (PBCO), SrTiO3 (STO) and CeO2 (CEO) as insulator materials. The used sequence was (YBCO20nm/Idi/LCMO20nm)x20, and the thicknesses of the insulating PBCO were 0.5, 2, 4, 8 and 20nm, and 2nm for STO and CEO. It was also analyzed a sample with inverted deposition order of superconductor and ferromagnetic. The analysis of X-ray diffraction confirmed the preferential growth toward 00ℓ and network parameters comparable to the values found in the literature for the YBCO. The scanning electron microscopy showed pores on the surface of LCMO film and extra crystal growth on the surface of the YBCO film. The AFM analyzes showed that the surface texture depends on the thickness and nature of the insulation as well as the charge distribution observed by EFM. Electrical measurements made by Four Probe method proved critical temperature around 90K, and only for the sample with 20nm PBCO has been noted large distinction of the resistivity behavior over heating from 25K with the application of magnetic field of 0.8T, which may indicate a different interaction between the layers of YBCO and LCMO for this configuration. Small magnetoresistance peaks were observed always close to the superconducting transition. The intensity and peak signal underwent some changes with the direction of the applied field.

O objetivo principal deste projeto foi o estudo das propriedades ópticas de estruturas semicondutoras do tipo super-redes, formadas a partir da heteroestrutura de GaAs/AlAs, crescida através da técnica de Epitaxia por Feixe Molecular. No trabalho apresentamos estudos feitos em super-redes do tipo (GaAs)n/(AlAs)n, crescidas em substratos semi-isolantes e orientados nas direções (100) e (nl1) com n=1,2,3,5,7 e nas polaridades A e B. Para cada periodicidade (n x n), as estruturas foram crescidas simultaneamente num mesmo porta amostra e sob as mesmas condições. As amostras foram estudadas através das técnicas de fotoluminescência a baixa temperatura e em função da temperatura. São apresentados também resultados preliminares de um estudo feito com a técnica de fotoluminescência de excitação. A técnica de difração de elétrons de alta energia foi utilizada durante o crescimento epitaxial para aferição da periodicidade da estrutura. A eficiência quântica, a posição do pico de luminescência estão fortemente correlacionados com a direção de crescimento. As medidas de fotoluminescência em função da temperatura mostram também um decréscimo anômalo da largura de linha. A partir dos resultados ópticos foi proposta a formação de microestruturas de mais baixa diemnsionalidade nos poços, formadas por flutuações nas interfaces (microrugosidades), e originárias do modo de crescimento adotado (sem interrupção nas interfaces). O comportamento óptico observado é semelhante ao de estruturas de mais baixa dimensionalidade (pontos quânticos). Este efeito é acentuado nas direções (311) e (21l), devido a própria morfologia da superfície / The main objective of this work was the study of the optical properties of semiconductors superlattices, formed by the (GaAs)n,/(AlAs)n, heterostructure, and grown by technique of Molecular Beam Epitaxy. In the work, we presented studies in (GaAs)n/(AlAs)n, superlattices, grown on semi-insulating substrates oriented in planes (100) and (n11) with n=l, 2, 3, 5, 7 and in the polarities A and B. For each periodicity (n x n), the structures were simultaneously grown in a same sample holder, and under the same conditions. The samples were studied by the photoluminescence techniques at low temperature and in function of the temperature. Preliminares results of a study done with the technique of excitation photoluminescence are also presented. The technique of high energy eletron difraction was used during the epitaxial growth for the monitoring of the periodicity of the structure. The quantum efficiency and the positions of the luminescence peak are strongly correlated with the growth direction. The photoluminescence measures in function of the temperature also show an anomalous decrease in linewidth. The analyses of the optical results shown the possibility of low dimension microstructures formation in the wells, due to the interfaces fluctuations, and related with the growth mode (without interruption at the interface). The observed optical behavior is similar to the observed in the structures of lower dimensionality (quantum dots). This effect is accentuated in the plans (311) and (211), due to the morphology of the surface

Ce manuscrit de thèse porte sur l'étude et la réalisation de photodétecteurs infrarouges à superréseaux (SR), constitués d'hétérostructures de semiconducteurs InAs/GaSb. Ces superréseaux InAs/GaSb sont considérés, depuis le milieu des années 2000, comme des structures pouvant satisfaire les besoins de la prochaine génération de photodétecteurs infrarouges (IR). À l'Institut d'Electronique du Sud (IES) de l'Université Montpellier 2, nous maîtrisons la réalisation des structures périodiques à SR par Epitaxie par Jets Moléculaires sur substrat GaSb et la fabrication technologique des photodiodes pin. Ces composants présentent des performances à l'état de l'art mondial dans le MWIR.L'objectif de mon travail de thèse, financé par la DGA et en collaboration étroite avec l'ONERA, était de contribuer à une meilleure compréhension de la physique du composant et d'améliorer les performances de cette nouvelle filière de détecteur IR. Cette étude s'effectua sur des monoélements (pixels), éléments de base du système imageur IR. En comparant trois structures InAs/GaSb différentes, conçus pour la même gamme spectrale de détection dans le MWIR mais de composition et d'épaisseur différentes (riche en GaSb, symétrique et riche en InAs), nous avons exploité la flexibilité offerte par cette technologie de détecteurs. Cette approche nous a permis de mettre en évidence la dépendance des performances avec la proportion en GaSb. Les résultats obtenus sur les structures asymétriques plus riches en InAs sont à l'état de l'art pour des photodiodes : densité de courant d'obscurité de 5x 10-8 A/cm2 à 77K pour une polarisation inverse de 50 mV. En complément, nous avons réalisé, en collaboration avec le CEA-LETI, la première matrice à SR française. Ces résultats ont contribué à une meilleure compréhension des détecteurs à superréseaux et esquissent des voies d'optimisation prometteuses. / This thesis focuses on the study and implementation of infrared photodetectors with InAs/GaSb superlattices (SL). Since the mid-2000s, InAs/GaSb SL are considered as new technology that can meet the criteria of the next generation of infrared (IR) photodetectors. At the Institut d'Electronique du Sud (IES) of the University of Montpellier 2, we control the fabrication of SL periodic structures by Molecular Beam Epitaxy on GaSb substrate and the technological process of pin photodiodes. These devices have performances at the state of the art in the midwave infrared spectral domain.The aim of my thesis work, funded by the DGA and in close collaboration with ONERA, was to contribute to a better understanding of the device physics and improve the performance of this IR detector. This study was carried out on mono-element (pixel), the basic elements of IR imaging system. Comparing three different InAs/GaSb structures, designed for the same detection spectral range (MWIR) but different composition and thickness (GaSb-rich, symmetric and InAs-rich), we used the flexibility offered by this technology detectors. This approach has allowed us to highlight the dependence of performances with the proportion on GaSb in the SL structure. The results obtained on InAs-rich asymmetric photodiodes are at the state of the art: dark current density of 5×10-8 A/cm2 at 77K for a reverse bias of 50 mV. In addition, the first French SL Focal Plane Array has been fabricated and tested. These results contributed to a better understanding of superlattice detectors and outline ways of promising optimization.

All-optical time-resolved measurement techniques provide a powerful tool for investigating critical parameters that determine the performance of infrared photodetector and emitter semiconductor materials. Narrow-bandgap InAs/GaSb type-II superlattices (T2SLs) have shown great promise as next generation materials, due to superior intrinsic properties and versatility. Unfortunately, InAs/GaSb T2SLs are plagued by parasitic Shockley-Read-Hall recombination centers that shorten the carrier lifetime and limit device performance. Ultrafast pump-probe techniques and time-resolved differential-transmission measurements are used here to demonstrate that "Ga-free" InAs/InAs₁₋xSbx T2SLs and InAsSb alloys do not have this same limitation and thus have significantly longer carrier lifetimes. Measurements of unintentionally doped MWIR and LWIR InAs/InAs₁₋xSbx T2SLs demonstrate minority carrier (MC) lifetimes of 18.4 µs and 4.5 µs at 77 K, respectively. This represents a more than two order of magnitude increase compared to the 90 ns MC lifetime measured in a comparable MWIR and LWIR InAs/GaSb T2SL. Through temperature-dependent differential-transmission measurements, the various carrier recombination processes are differentiated and the dominant recombination mechanisms identified for InAs/InAs₁₋xSbx T2SLs. These results demonstrate that these Ga-free materials are viable options over InAs/GaSb T2SLs and potentially bulk Hg₁₋xCdxTe photodetectors. In addition to carrier lifetimes, the drift and diusion of excited charge carriers through the superlattice layers (i.e. in-plane transport) directly aects the performance of photo-detectors and emitters. All-optical ultrafast techniques were successfully used for a direct measure of in-plane diffusion coeffcients in MWIR InAs/InAsSb T2SLs using a photo-generated transient grating technique at various temperatures. Ambipolar diffusion coefficients of approximately 60 cm²/s were reported for MWIR InAs/InAs₁₋xSbxT2SLs at 293 K.

As the world strives to adapt to the increasing demand for electrical power, sustainable energy sources are attracting significant interest. Around 60% of energy utilized in the world is wasted as heat. Different industrial processes, home heating, and exhausts in cars, all generate a huge amount of unused waste heat. With such a huge potential, there is also significant interest in discovering inexpensive technologies for power generation from waste heat. As a result, thermoelectric materials have become important for many renewable energy research programs. While significant advancements have been done in improving the thermoelectric properties of the conventional heavy-element based materials (such as Bi2Te3 and PbTe), high-temperature applications of thermoelectrics are still limited to one materials system, namely SiGe, since the traditional thermoelectric materials degrade and oxidize at high temperature. Therefore, oxide thermoelectrics emerge as a promising class of materials since they can operate athigher temperatures and in harsher environments compared to non-oxide thermoelectrics. Furthermore, oxides are abundant and friendly to the environment. Among oxides, crystalline SrTiO3 and ZnO are promising thermoelectric materials. The main objective of this work is therefore to pursue focused investigations of SrTiO3 and ZnO thin films and superlattices grown by pulsed laser deposition (PLD), with the goal of optimizing their thermoelectric properties by following different strategies. First, the effect of laser fluence on the thermoelectric properties of La doped epitaxial SrTiO3 films is discussed. Films grown at higher laser fluences exhibit better thermoelectric performance. Second, the role of crystal orientation in determining the thermoelectric properties of epitaxial Al doped ZnO (AZO) films is explained. Vertically aligned (c-axis) AZO films have superior thermoelectric properties compared to other films with different crystal orientations. Third, additional B-site doping of A-site doped SrTiO3 films leads to a prominent reduction in the lattice thermal conductivity without limiting the electrical transport, and hence an improvement in the figure of merit is noticed. Fourth and last, the enhancement of thermoelectric properties of thermally robust, high quality SrTiO3-based superlattices is discussed. Beside the randomly distributed oxygen vacancies and extrinsic dopants, the structure of SrTiO3-based superlattices increases the scattering of phonons at the interfaces between the alternative layers, and hence reducing the thermal conductivity, which leads to a notable enhancement in the figure of merit.

In this work, ordered all-binary short-period strained InAs/GaAs superlattice quantum wells were studied as an alternative to strained ternary alloy InGaAs/GaAs quantum wells. InGaAs quantum wells QWs have been of great interest in recent years due to the great potential applications of these materials in future generations of electronic and optoelectronic devices. The all binary structures are expected to have all the advantages of their ternary counterparts, plus several additional benefits related to growth, to the elimination of alloy disorder scattering and to the presence of a higher average indium content.

No description available.

Undoped and doped Pb(Zr_xTi_1-x)O₃, i. e. PZT, ferroelectric thin films were prepared by chemical solution deposition and spin-coating method. The precursors for making the undoped PZT films were derived from lead acetate, zirconium n-propoxide, and titanium iso-propoxide. In addition, lanthanum acetylacetonate, neodymium acetate, and niobium ethoxide were introduced into the precursor solution to accomplish doping of the corresponding elements. Both doped and undoped PZT films were coated onto Pt/Ti/SiO₂/Si, RuO_x and single-crystal sapphire substrates of various thickness and annealed at a range of temperatures and times. The effects of dopants were studied in terms of the Curie temperature, crystal distortion, transformation temperature, microstructure, optical properties, and electrical properties. In addition to the dopant effect, the effects of substrates were also investigated with regard to crystallization and preferred orientation. The Curie temperature of the doped and undoped PZT films was determined by in-situ hot-stage transmission electron microscope (TEM) and compared with those of bulk ceramics. Lattice distortion and phase transformation were determined by x-ray diffraction (XRD). Microstructure of the films was characterized by using optical microscopy, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). Optical properties were characterized by a UV-VIS-NIR scanning spectrophotometer and electrical properties and fatigue testing were measured on a standardized RT66A using a Virtual-Ground circuit. It was observed that the addition of Nd and La dopants tends to enhance perovskite phase formation and improve electrical properties of PZT films. Higher refractive indices in La and Nd-doped PZT films imply that packing densities of PZT films are improved by adding dopants. Furthermore, the lower leakage currents and improved fatigue properties in PZT films were also observed by the addition of Nb dopants. / Master of Science

Thesis (Ph.D.)--McMaster University (Canada), 1993. / Source: Dissertation Abstracts International, Volume: 55-06, Section: B, page: 2345. Adviser: G. C. Weatherly.

The present research work focuses on the fabrication and characterization of epitaxial heterostructures of 0.7 Pb(Mg1/3N2/3)O3 – 0.3 PbTiO3 (PMN-PT) and La0.6Sr0.4MnO3 (LSMO) using multi target pulsed laser ablation technique. Different heterostructures such as bilayered thin films with different individual layer thickness; symmetric and asymmetric superlattices of different periodicities were fabricated. Roles of individual layer thickness, elastic strain and interfaces between PMN-PT and LSMO layers on different physical properties were studied. An attempt has been made to understand the influence of the charge depleted interface states in addition to the probable strain mediated elastic coupling effect on the observed magneto-dielectric response in these engineered heterostructures. Chapter 1 provides a brief introduction to the multiferroic materials, occurrence of magnetoelectric (ME) coupling in them, their possible technological applications and the challenges involved. A short historical account of the multiferroic research is discussed to emphasize the importance of artificial multiferroics, particularly the engineered thin film heterostructures. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 addresses the phase formation, structural and microstructural features of the engineered heterostructures fabricated epitaxially on single crystalline LaAlO3 (100) substrates. A thin layer of LaNiO3 used as the bottom electrode material for electrical characterizations was grown on the bare substrate prior to the fabrication of the PMN-PT/LSMO heterostructures. The structural and microstructural features of different individual layers were also studied by fabricating single layer thin films of the materials. The effects of individual layer thicknesses on the surface roughness, grain size and lattice strain of the heterostructures are discussed. Chapter 4 deals with the ferroelectric studies of the PMN-PT/LSMO epitaxial heterostructures. Polarization hysteresis (P-E), capacitance – voltage (C-V) and pulsed polarization (PUND) measurements were carried out as functions of applied voltage, frequency and delay time to characterize the ferroelectric properties of the heterostructures. All the bilayered heterostructures exhibited robust ferroelectric response and contribution of non – remnant components to their polarization behaviour were observed from the P-E studies. The symmetric superlattices did not exhibit any ferroelectricity due to high leakage current conduction. After optimizing the LSMO and PMN-PT layer thicknesses ferroelectricity was observed in the asymmetric superlattices accompanied by substantial reduction in the leakage current conduction. The P-E loops were found to be asymmetrically shifted along the electric field axis in all the superlattices indicating the presence of dielectric passive layers and strong depolarizing fields at the interfaces between PMN-PT and LSMO. Chapter 5 deals with the ferromagnetic studies of the PMN-PT/LSMO heterostructures. All the heterostructures exhibited ferromagnetic behaviour in the temperature range of 10 K – 300 K with an in plane magnetic easy axis ([100]) compared to the out of plane ([001]) direction. The magnetization behaviour of the bilayers and superlattices as a function of magnetic field strength, temperature and different individual layer thickness of PMN-PT and LSMO are discussed in terms of the oxygen deficiency, magnetic dead layers and lattice strain effects in these engineered epitaxial heterostructures. Chapter 6 addresses the magneto-dielectric response, dielectric properties and ac conduction properties of the engineered biferroic heterostructures. In order to investigate the manifestation of strain mediated ME coupling in these heterostructures their dielectric response as a function of ac electric signal frequency have been studied under different static magnetic fields over a wide range of temperatures. The appearance of magneto-capacitance and its dependence on magnetic field strength and temperature along with the magnetoresistive characteristics of the heterostructures suggested that the charge depleted interfaces between PMN-PT and LSMO can have an effect on the observed dielectric response in addition to the probable strain mediated ME coupling. Dielectric characterization of the heterostructures performed over a wide range of temperature indicated a Maxwell-Wagner type relaxation mechanism. The manifestation of Maxwell-Wagner effect and the very low activation energy of ac conductivity obtained from the ac conduction studies revealed the strong influence of the charge depleted interfaces between PMN-PT and LSMO on the dielectric properties of the heterostructures. Chapter 7 deals with the dc leakage current conduction characteristics of the heterostructures. The leakage current characterization was performed over a wide range of temperature and analyzed in the framework of different models to investigate the leakage mechanism. All the heterostructures were found to obey the power law I∝Vα over the entire range of temperature with different values of α at different applied voltages. The bilayered heterostructures exhibited ohmic conduction in the lower electric field region and space charge limited conduction was observed at higher electric fields. On the other hand the low field dc conduction behaviour of the superlattices could not be attributed unambiguously to a single mechanism. Depending on the superlattice periodicity the low field conduction behaviour was dominated by either Poole-Frenkel (PF) emission or a combined contribution from the PF effect and ohmic conduction. At higher electric fields all the superlattices exhibited space charge limited conduction. Chapter 8 gives the summary and conclusions of the present study and also discusses about the future work that could give more insight into the understanding of the engineered epitaxial biferroic heterostructures.