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abstract: There has been recent interest in demonstrating solar cells which approach the detailed-balance or thermodynamic efficiency limit in order to establish a model system for which mass-produced solar cells can be designed. Polycrystalline CdS/CdTe heterostructures are currently one of many competing solar cell material systems. Despite being polycrystalline, efficiencies up to 21 % have been demonstrated by the company First Solar. However, this efficiency is still far from the detailed-balance limit of 32.1 % for CdTe. This work explores the use of monocrystalline CdTe/MgCdTe and ZnTe/CdTe/MgCdTe double heterostructures (DHs) grown on (001) InSb substrates by molecular beam epitaxy (MBE) for photovoltaic applications. Undoped CdTe/MgCdTe DHs are first grown in order to determine the material quality of the CdTe epilayer and to optimize the growth conditions. DH samples show strong photoluminescence with over double the intensity as that of a GaAs/AlGaAs DH with an identical layer structure. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a carrier lifetime of up to 179 ns for a 2 µm thick CdTe layer, which is more than one order of magnitude longer than that of polycrystalline CdTe films. MgCdTe barrier layers are found to be effective at confining photogenerated carriers and have a relatively low interface recombination velocity of 461 cm/s. The optimal growth temperature and Cd/Te flux ratio is determined to be 265 °C and 1.5, respectively. Monocrystalline ZnTe/CdTe/MgCdTe P-n-N DH solar cells are designed, grown, processed into solar cell devices, and characterized. A maximum efficiency of 6.11 % is demonstrated for samples without an anti-reflection coating. The low efficiency is mainly due to the low open-circuit voltage (V_oc), which is attributed to high dark current caused by interface recombination at the ZnTe/CdTe interface. Low-temperature measurements show a linear increase in V_oc with decreasing temperature down to 77 K, which suggests that the room-temperature operation is limited by non-radiative recombination. An open-circuit voltage of 1.22 V and an efficiency of 8.46 % is demonstrated at 77 K. It is expected that a coherently strained MgCdTe/CdTe/MgCdTe DH solar cell design will produce higher efficiency and V_oc compared to the ZnTe/CdTe/MgCdTe design with relaxed ZnTe layer. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2014

abstract: The development of high efficiency III-V solar cells is needed to meet the demands of a promising renewable energy source. Intermediate band solar cells (IBSCs) using semiconductor quantum dots (QDs) have been proposed to exceed the Shockley-Queisser efficiency limit [1]. The introduction of an IB in the forbidden gap of host material generates two additional carrier transitions for sub-bandgap photon absorption, leading to increased photocurrent of IBSCs while simultaneously allowing an open-circuit voltage of the highest band gap. To realize a high efficiency IBSC, QD structures should have high crystal quality and optimized electronic properties. This dissertation focuses on the investigation and optimization of the structural and optical properties of InAs/GaAsSb QDs and the development of InAs/GaAsSb QD-based IBSCs. In the present dissertation, the interband optical transition and carrier lifetime of InAs/GaAsSb QDs with different silicon delta-doping densities have been first studied by time-integrated and time-resolved photoluminescence (PL). It is found that an optimized silicon delta-doping density in the QDs enables to fill the QD electronic states with electrons for sub-bandgap photon absorption and to improve carrier lifetime of the QDs. After that, the crystal quality and QD morphology of single- and multi-stack InAs/GaAsSb QDs with different Sb compositions have been investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The TEM studies reveal that QD morphology of single-stack QDs is affected by Sb composition due to strain reducing effect of Sb incorporation. The XRD studies confirm that the increase of Sb composition increases the lattice mismatch between GaAs matrix and GaAsSb spacers, resulting in increase of the strain relaxation in GaAsSb of the multi-stack QDs. Furthermore, the increase of Sb composition causes a PL redshift and increases carrier lifetime of QDs. Finally, the spacer layer thickness of multi-stack InAs/GaAsSb QDs is optimized for the growth of InAs/GaAsSb QD solar cells (QDSCs). The InAs/GaAsSb QDSCs with GaP strain compensating layer are grown and their device performances are characterized. The increase of GaP coverage is beneficial to improve the conversion efficiency of the QDSCs. However, the conversion efficiency is reduced when using a relatively large GaP coverage. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

abstract: InAs/InAsSb type-II superlattices (T2SLs) can be considered as potential alternatives for conventional HgCdTe photodetectors due to improved uniformity, lower manufacturing costs with larger substrates, and possibly better device performance. This dissertation presents a comprehensive study on the structural, optical and electrical properties of InAs/InAsSb T2SLs grown by Molecular Beam Epitaxy. The effects of different growth conditions on the structural quality were thoroughly investigated. Lattice-matched condition was successfully achieved and material of exceptional quality was demonstrated. After growth optimization had been achieved, structural defects could hardly be detected, so different characterization techniques, including etch-pit-density (EPD) measurements, cathodoluminescence (CL) imaging and X-ray topography (XRT), were explored, in attempting to gain better knowledge of the sparsely distributed defects. EPD revealed the distribution of dislocation-associated pits across the wafer. Unfortunately, the lack of contrast in images obtained by CL imaging and XRT indicated their inability to provide any quantitative information about defect density in these InAs/InAsSb T2SLs. The nBn photodetectors based on mid-wave infrared (MWIR) and long-wave infrared (LWIR) InAs/InAsSb T2SLs were fabricated. The significant difference in Ga composition in the barrier layer coupled with different dark current behavior, suggested the possibility of different types of band alignment between the barrier layers and the absorbers. A positive charge density of 1.8 × 1017/cm3 in the barrier of MWIR nBn photodetector, as determined by electron holography, confirmed the presence of a potential well in its valence band, thus identifying type-II alignment. In contrast, the LWIR nBn photodetector was shown to have type-I alignment because no sign of positive charge was detected in its barrier. Capacitance-voltage measurements were performed to investigate the temperature dependence of carrier densities in a metal-oxide-semiconductor (MOS) structure based on MWIR InAs/InAsSb T2SLs, and a nBn structure based on LWIR InAs/InAsSb T2SLs. No carrier freeze-out was observed in either sample, indicating very shallow donor levels. The decrease in carrier density when temperature increased was attributed to the increased density of holes that had been thermally excited from localized states near the oxide/semiconductor interface in the MOS sample. No deep-level traps were revealed in deep-level transient spectroscopy temperature scans. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2015

La seconde génération de rubans supraconducteurs utilisant une couche déposée et épitaxiée de supraconducteur YBa2Cu3O7-x (YBCO) est étudiée de façon intense (coated conductors). Une architecture simple à bas coût a été choisie, avec une seule couche tampon de La2Zr2O7 (LZO), épitaxiée sur un ruban de Ni95W5 texturé bi-axialement. L’objectif de cette thèse était d’identifier les paramètres pertinents du processus de texturation sur des rubans d’alliage Ni95W5 laminés par APERAM à partir d’une coulée industrielle et ceux contrôlant l’épitaxie de la couche tampon de (LZO). Le rôle de la fonctionnalisation du substrat pour l’épitaxie de cet oxyde sur la surface du métal a été en particulier étudié. Parallèlement, la faisabilité d’un procédé de recuit de texturation et de fonctionnalisation du substrat métallique au défilé, en vue d’un traitement continu pour un développement industriel, a été étudiée. / The second generation of superconducting tapes using a deposited layer and epitaxialsuperconductor YBa2Cu3O7-x (YBCO) is studied intensively (coated conductors). A simple lowcostarchitecture was chosen, with one buffer layer La2Zr2O7 (LZO), epitaxially grown on aribbon of biaxially textured Ni95W5. The aim of this thesis was to identify the relevant processparameters texturing ribbons Ni95W5 alloy rolled by APERAM from an industrial casting, andthose controlling the epitaxial buffer layer (LZO). The role of the functionalization of thesubstrate for epitaxy of the oxide on the metal surface has been particularly studied. Meanwhile,the feasibility of an annealing process and functionalization texturing of the metal substrate rollto roll, in view of a continuous treatment for an industrial development, was studied.

The market for renewable energy sources, and solar cells in particular is growing year by year, as a result there is a large interest in research on new materials and new technologies for solar power applications. In this thesis the photovoltaic properties of cubic silicon carbide (3C-SiC) has been investigated. The research includes material growth using the sublimation epitaxy method, both n-type and p-type SiC have been investigated. 3C-SiC pn junctions have been produced and their electrical properties have been characterized, this is the first time 3C-SiC pn junctions have been studied in the research group. Photoresponse has been demonstrated from a 3C-SiC pn junction with Al and N used as p- and ntype dopants. This is the first demonstrated solar cell performance using 3C-SiC, to our knowledge.

abstract: III-Nitride nanostructures have been an active area of research recently due to their ability to tune their optoelectronic properties. Thus far work has been done on InGaN quantum dots, nanowires, nanopillars, amongst other structures, but this research reports the creation of a new type of InGaN nanostructure, nanorings. Hexagonal InGaN nanorings were formed using Metal Organic Chemical Vapor Deposition through droplet epitaxy. The nanorings were thoroughly analyzed using x-ray diffraction, photoluminescence, electron microscopy, electron diffraction, and atomic force microscopy. Nanorings with high indium incorporation were achieved with indium content up to 50% that was then controlled using the growth time, temperature, In/Ga ratio and III/N ratio. The analysis showed that the nanoring shape is able to incorporate more indium than other nanostructures, due to the relaxing mechanism involved in the formation of the nanoring. The ideal conditions were determined to be growth of 30 second droplets with a growth time of 1 minute 30 seconds at 770 C to achieve the most well developed rings with the highest indium concentration. / Dissertation/Thesis / M.S. Materials Science and Engineering 2012

abstract: This thesis describes the fabrication of several new classes of Ge1-x-ySixSny materials with the required compositions and crystal quality to engineer the band gaps above and below that of elemental Ge (0.8 eV) in the near IR. The work initially focused on Ge1-x-ySixSny (1-5% Sn, 4-20% Si) materials grown on Ge(100) via gas-source epitaxy of Ge4H10, Si4H10 and SnD4. Both intrinsic and doped layers were produced with defect-free microstructure and viable thickness, allowing the fabrication of high-performance photodetectors. These exhibited low ideality factors, state-of-the-art dark current densities and adjustable absorption edges between 0.87 and 1.03 eV, indicating that the band gaps span a significant range above that of Ge. Next Sn-rich Ge1-x-ySixSny alloys (2-4% Si and 4-10% Sn) were fabricated directly on Si and were found to show significant optical emission using photoluminescence measurements, indicating that the alloys have direct band gaps below that of pure Ge in the range of 0.7-0.55 eV. A series of Sn-rich Ge1-x-ySixSny analogues (y>x) with fixed 3-4% Si content and progressively increasing Sn content in the 4-10% range were then grown on Ge buffered Si platforms for the purpose of improving the material's crystal quality. The films in this case exhibited lower defect densities than those grown on Si, allowing a meaningful study of both the direct and indirect gaps. The results show that the separation of the direct and indirect edges can be made smaller than in Ge even for non-negligible 3-4% Si content, confirming that with a suitable choice of Sn compositions the ternary Ge1-x-ySixSny reproduces all features of the electronic structure of binary Ge1-ySny, including the sought-after indirect-to-direct gap cross over. The above synthesis of optical quality Ge1-x-ySixSny on virtual Ge was made possible by the development of high quality Ge-on-Si buffers via chemical vapor deposition of Ge4H10. The resultant films exhibited structural and electrical properties significantly improved relative to state-of-the-art results obtained using conventional approaches. It was found that pure Ge4H10 facilitates the control of residual doping and enables p-i-n devices whose dark currents are not entirely determined by defects and whose zero-bias collection efficiencies are higher than those obtained from samples fabricated using alternative Ge-on-Si approaches. / Dissertation/Thesis / Ph.D. Physics 2013

Orientadores: Luiz Fernando Zagonel, Mônica Alonso Cotta / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2017-05-11T13:05:53Z (GMT). No. of bitstreams: 1 Silva, Bruno Cesar da_M.pdf: 3758085 bytes, checksum: 042d8ff5a8d495ad70b0f10c888cd418 (MD5) Previous issue date: 2016 / Made available in DSpace on 2017-06-14T17:38:47Z (GMT). No. of bitstreams: 1 Silva_BrunoCesarda_M.pdf: 3758085 bytes, checksum: 042d8ff5a8d495ad70b0f10c888cd418 (MD5) Previous issue date: 2016 / Resumo: Neste trabalho, estudamos a dinâmica de crescimento de nanofios de GaP crescidos pelo método VLS (Vapor-Líquido-Sólido) via Epitaxia por Feixe Químico (CBE), usando nanopartículas catalisadoras de ouro. Investigando o efeito da temperatura na dinâmica de crescimento de nanofios de GaP encontramos uma grande variedade de nanofios em cada amostra, caracterizadas por diferentes direções de crescimento e/ou morfologias, apresentando sempre uma população dominante. Com o aumento da temperatura (510°C) observamos uma transição drástica na morfologia da população dominante, de nanofios típicos para uma nova e inesperada morfologia assimétrica. Mostramos ainda que modificando o tamanho da nanopartícula catalisadora de 5nm para 20nm os nanofios assimétricos ainda são favorecidos a alta temperatura. Procurando compreender o mecanismo de formação dos nanofios assimétricos, mostramos que estas estruturas cristalizam-se na fase WZ, com baixa densidade de defeitos, comparadas às outras amostras. Além disso, mostramos que a assimetria destes nanofios não é oriunda de diferenças de polaridade nas facetas laterais ou formação de defeitos cristalográficos que pudessem modificar a dinâmica de crescimento de modo a levar à morfologia assimétrica. Desta forma, propomos um cenário de crescimento simplificado, no qual a estrutura assimétrica é formada pela combinação do crescimento de nanofios ordinários, via VLS, junto com estruturas que crescem livre de catalizador via mecanismo VS (Vapor-Sólido), estas duas estruturas então se juntam, transferindo material e dando lugar a facetas de menor energia, resultando na formação da estrutura assimétrica. Por fim, medidas de fotoluminescência mostram a emissão característica no verde do GaP WZ para os nanofios assimétricos, mesmo com a estrutura não estando passivada, confirmando a boa qualidade cristalina das nossas amostras / Abstract: In this work, we study the growth dynamics of GaP nanowires grown by VLS (Vapor-Liquid-Solid) method via Chemical Beam Epitaxy (CBE) using gold nanoparticles as catalyst. Investigating the effect of temperature on the growth dynamics of GaP nanowires we find a wide variety of nanowires in each sample, characterized by different growth directions and/or morphologies, always having a dominant population. With increasing temperature (510°C), we observed a dramatic transition in the morphology of the dominant population of typical nanowires to a new and unexpected asymmetric morphology. We also show that modifying the size of the catalyst nanoparticle from 5nm to 20mn the asymmetric nanowires are still favored at high temperature. Looking forward to understand the mechanism of formation of the asymmetric nanowires, we show that these structures crystallize in the WZ phase with low defect density when compared to the other samples. Furthermore, we show that the asymmetry of these nanowires is not derived from differences in polarity in side facets or the formation of crystallographic defects which might modify the growth dynamics so as to bring the asymmetric morphology. Thus, we propose a simplified growth scenario, in which the asymmetric structure is formed by growth combination of ordinary nanowires via VLS, along with structures that grow catalyst-free via VS (vapor-solid) mechanism, these two structures joined and by transferring materials facets with lower energy facets appear, resulting in the formation of asymmetrical structure. Finally, photoluminescence measurements show the characteristic emission in the green of WZ GaP for asymmetric nanowires, even with no passivation, which confirms the good crystalline quality of our samples / Mestrado / Física / Mestre em Física

Orientador: Monica Alonso Cotta / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-04T02:49:32Z (GMT). No. of bitstreams: 1 Bortoleto_JoseRobertoRibeiro_D.pdf: 16802950 bytes, checksum: 1ef2bf92db0b00f30e5db9c5e3e3f34d (MD5) Previous issue date: 2005 / Resumo: Neste trabalho estudamos os mecanismos de crescimento durante a epitaxia de estruturas III-V baseadas na liga InP. Em particular, o principal objetivo foi correlacionar os mecanismos cinéticos durante a nucleação de nanoestruturas auto-formadas com as propriedades estruturais da camada que serve de substrato. Todas as amostras foram crescidas usando um sistema de epitaxia por feixe químico (CBE). De forma geral as amostras foram caracterizadas usando microscopia de força atômica (AFM), microscopia eletrônica de transmissão (TEM), difração de elétrons de alta energia (RHEED) e difração de raios-X. Na primeira parte deste trabalho correlacionamos as mudanças morfológicas nos filmes homoepitaxiais de InP com o padrão de RHEED exibido durante o crescimento epitaxial. Mostramos que as mudanças morfológicas de 3D para 2D com os parâmetros de crescimento estão diretamente relacionadas com as reconstruções superficiais 2x1 e 2x4, respectivamente. Além disso, indicamos que a formação de defeitos morfológicos é devido à dimerização In-P, através da ativação local do mecanismo de bias na difusão. Por outro lado, também investigamos o efeito dos parâmetros de crescimento (temperatura, taxa de crescimento e quantidade de material) na nucleação e auto-formação de ilhas de InP sobre InGaP/GaAs. Na segunda parte desta tese concentramos nossa atenção no efeito das propriedades da camada buffer de InGaP sobre nanoestruturas auto-formadas, principalmente sobre a sua organização espacial. Para tanto, em primeiro lugar, investigamos as propriedades de bulk da liga de InGaP e a dependência com os parâmetros de crescimento. Nossos resultados mostram que o InGaP exibe tanto ordenamento atômico de rede quanto modulação de composição. Estes dois fenômenos estão correlacionados com o tipo de reconstrução superficial. Em específico, a liga de InGaP apresenta ordenamento CuPtB quando a superfície exibe reconstrução superficial 2x1, conforme descrito na literatura. Por outro lado, a reconstrução superficial 2x4 desempenha um papel importante no fenômeno de modulação de composição. De fato, tanto a modulação de composição quanto a morfologia superficial do filme depende dos mecanismos cinéticos de superfície, que envolvem tanto os átomos adsorvidos de In quanto de Ga. Por fim mostramos que a modulação de composição na liga de InGaP pode organizar espacialmente a nucleação de ilhas de InP em uma rede quadrada. Além disso, mostramos que é possível criar redes bidimensionais de pontos quânticos de InAs/GaAs a partir do arranjo espacialmente ordenado das ilhas de InP/InGaP / Abstract: In this work we study the growth mechanisms during epitaxy of III-V structures based on InP. The main goal was to correlate the kinetic mechanisms during nucleation of self-assembled nanostructures with the bulk properties of the buffer layer. All samples were grown by chemical beam epitaxy (CBE) and characterized using atomic force microscopy (AFM), transmission electron microscopy (TEM), reflection high-energy electron diffraction (RHEED) and X-ray diffraction. In the first part of this work we correlate the morphological changes in homoepitaxial InP films with the RHEED pattern during growth process. We show that the morphological transition from 3D to 2D with growth parameters is related to changes in surface reconstruction, from 2x1 to 2x4. Moreover, we point out that the formation of morphological defects is due to mixed In-P dimerization, via the local activation of the diffusion bias mechanism. On the other hand, we also investigate the influence of the growth parameters (temperature, growth rate and amount of deposited material) on the nucleation and selfassembly of InP islands grown on InGaP/GaAs layers. In the second part of this work we concentrate our attention on the InGaP bulk properties, and their effect on the self-assembled InP nanostructures, mainly regarding their spatial ordering. In this way, we first investigate the dependence of InGaP bulk properties with the growth parameters. Our results show that our InGaP layers exhibit atomic ordering as well as compositional modulation. Both phenomena are correlated to the surface reconstruction exhibited by the InGaP surface during growth process. The InGaP alloy presents CuPtB atomic ordering when the RHEED pattern shows 2x1 reconstruction, in agreement with reports in literature. On the other hand, the 2x4-type reconstruction plays an important role in the compositional modulation phenomena. Actually, both compositional modulation and surface morphology of InGaP films depend on surface kinetic mechanisms, and thus on In and Ga adatom mobilities. At last we show that the compositional modulation in the InGaP alloy can be used to organize spatially the InP islands in a square lattice. Moreover, we point out that it is possible to produce bidimensional lattices of InAs/GaAs quantum dots starting from a template of laterally organized InP/InGaP nanostructures / Doutorado / Física da Matéria Condensada / Doutor em Ciências