Made available in DSpace on 2014-06-11T19:31:04Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-02-29Bitstream added on 2014-06-13T20:21:46Z : No. of bitstreams: 1 martins_mr_dr_bauru.pdf: 576235 bytes, checksum: cae5dc5f897a536d60d112303dafaf34 (MD5) / Secretaria de Educação do Estado de São Paulo / Neste trabalho investigamos pontos quânticos de InAs sobre um substrato de GaAs crescidos pela técnica de epitaxia por feixe molecular (MBE, Molecular Beam Epitaxy). Esses pontos quânticos emitem radiação no intervalo de 1,3 üm a 1,5 üm (0,95 eV a 0,83 eV), que corresponde à janela óptica onde ocorre a mínima atenuação do sinal em redes de transmissão por fibras ópticas. Realizamos dois tipos de estudo em dois conjuntos de amostras. No primeiro caso analisamos a influência de alguns parâmetros de crescimento nas propriedades ópticas desses pontos quânticos. No segundo caso, analisamos a influência de um tratamento térmico nas propriedades ópyicas. Resultados de fotoluminescência (PL - photoluminescence) para o primeiro estudo mostraram uma grande influência da velocidade de crescimento nos espectros de emissão que apresentaram múltiplos picos, muito provavelmente associados com o estado fundamental e seus respectivos estados excitados dos pontos quânticos. Para o segundo estudo os resultados de PL mostraram que a emissão óptica consistia de uma larga banda situada entre 1,3 a 1,5 üm. Entretanto, observou-se que, após tratamento térmico durante 3 horas a uma temperatura de 550 ºC, a intensidade da PL aumentou por um fator 3. Além disso, a larga banda observada tornou-se um conjunto de pelo menos 5 picos discretos. O efeito de tratamentos térmicos em poços quânticos é bem conhecido e foi bem explorado na literatura. Em pontos quânticos, os mesmos efeitos também existem, porém, outros de igual importância tembém se apresentam. Dentre os mais importantes podemos citar a redistribuição dos tamanhos dos pontos quânticos, que podem em alguns casos limites fazer com que o ponto quântico desapareça, e a redistribuição das tensões entre a interface ponto quântico/matriz. Neste trabalho... / This study investigated InAs large quantum dot on GaAs substrate grown by the techique of molecular beam epitaxy (MBE). These quantum dots emit in the spectral range of 1.3 üm and 1.5 üm (0.95 eV to 0.83 eV), which corresponds to the window of minimal signal attenuation on transmission networks by optical fiber. We have performed two kinds of study into two different sets of samples. In the first case, we have analyzed the influence of some growth parameters on the optical properties of these quantum dots. In the second one, we have analyzed the influence of a thermal treatment on the optical properties. Results of photoluminescence (PL) on the first study showed a great influence of growth velocity in the PL spectra line shape. For the second study the results of PL on an as grown sample showed that the emission signal was a large optical band in the wave length range of 1.3 üm and 1.5 üm. However, it was observed that after the thermal treatment of 3 hours at a temperature of 550 ºC, the intensity of these PL emissions increased by a factor 3. Moreover, the observed large band has become a series of at least 5 discrete peaks. The effect of heat treatments in quantum wells is well known and has been well explored in literature. In quantum dot, the same effects are expected; however, other equally important effects are also present. The most important is the size redistribution of the quantum dots, which can in some limit cases, vanish these quantum dot. Our study identified the origin of these multiple peaks, and found emissions of PL at room temperature in the optical window between 1.3 and 1.5 üm.

Estruturas semicondutoras com dopagem planar são sistemas de considerável interesse tanto para a a pesquisa básica como para a aplicação em dispositivos. Neste trabalho caracterizamos estruturas semicondutoras com dopagem planar tipo n ou p, utilizando técnicas de espectroscopia ótica tais como fotoluminescencência (PL) e fotoluminescência-excitação (PLE). As amostras foram crescidas com a técnica de Epitaxia por Feixe Molecular (BEM, Molecular Beam Epitaxy) no Laboratório de Novos Materiais Semicondutores (LNMS) do IFUSP, com exceção das amostras com dopagem planar tipo p que foram crescidas nos laboratórios do Departamento de Física da Universidade Federal de Minas Gerais. Investigamos as propriedades eletrônicas de super-redes de GaAs com dopagem planar de silício, em função da concentração dos átomos dopantes, mantendo-se fixa a distância entre os planos de dopagem. Através da comparação de nossos resultados experimentais com os de cálculos autoconsistentes da estrutura eletrônica das super-redes, identificamos a origem de todas as emissões observadas nos espectros de PL. As emissões principais (denominada bandas B) foram identificadas como oriundas do processo de recombinação radiativa dos portadores do gás bidimensional de elétrons (2DEG) com buracos fotogerados na banda de valência. Outras emissões (denominadas bandas A) foram associadas com o processo de recombinação dos elétrons do 2DEG com impurezas de Carbono. Analisamos também amostras de poços quânticos de InGaAs/GaAs com dopagem planar de Silício. Nestes sistemas, devido à presença de impurezas (que atuam como centros de espalhamento) e variações na composição da liga de InGaAs (que dão origem à localização de buracos), transições com e /ou sem conservação de quase-momento envolvendo estados de buraco estendidos e/ou localizados constituem os possíveis processos de recombinação radiativa entre os elétrons do 2DEG e os buracos fotogerados. Neste trabalho, investigamos os processos de recombinação dos elétrons do 2DEG com os buracos gerados por excitação ótica comparando a forma de linha dos espectros experimentais e teóricos de PL. Estruturas semicondutoras de GaAs contendos um único plano de átomos de Berílio (dopagem tipo p ) também foram analisadas neste trabalho. Os resultados de nossas investigações evidenciam a existência de um potencial fotoinduzido, que confina os elétrons fotogerados. O processo de formação deste potencial é discutido neste trabalho. / Delta-doped semiconductor structures are systems of considerable interest for basic research and device applications. In this work, we performed the characterization of n or p-type semiconductor structures, using spectroscopic tecniques as photoluminescence (PL), photoluminescence-excitation (PLE) and selective photoluminescence (SPL). The samples were grown by Molecular Beam Epitaxy (BEM) at LNMS (Laboratório de Novos Materias Semicondutores) of IFUSP anda t Physical Department of UFMG. The electronic structure of Silicon delta-doped GaAs super-lattices with different donor concentrations in the delta-doped layer and a fixed distance between adjacent Si-doped layers was investigated. Though the comparison o four experimental results with the superlattices electronic structure calculated self-consistently we identified the origino f all observed in PL spectra structures. The principal emissions (denominated as bands B) are due to recombination of two-dimensional electron gás (due to delta doping) with photocreated holes in Valence band. Other spectral features (denominated as bands A) were associated with recombination of two dimensional electron gás and Carbon impurity. We analyzed PL spectra of InGaAs/GaAs quantum well samples with Silicon delta doping. In this systems recombination of electrons from two-dimensional gas with photocreated holes through the transitions with or without quase-momentum conservation were observed in PL spectra. Comparing experimental and theoretical lineshape, we are able to determine optical transitions in which holes in localize dor extended states took part. Localization of holes in Valence band is due to fluctuations in dopant distribution in the delta-doped layer. Moreover, GaAs with Beryllium delta doping (p-type) were analyzed in this work . Results o four investigation shown the existence of a photoinduced potential, which confine photocreated electrons in strutures of this type. Formation processo f this potential is discussed in this work.

<p>Molecular Beam Epitaxy (MBE) is a well-established method to grow low-dimensional structures for research applications. MBE has given many contributions to the rapid expanding research-area of nano-technology and will probably continuing doing so. The MBE equipment, dedicated for Silicon/Germanium (Si/Ge) systems, at Karlstads University (Kau) has been studied and started for the first time. In the work of starting the system, all the built in interlocks has been surveyed and connected, and the different subsystems has been tested and evaluated. Service supplies in the form of compressed air, cooling water and electrical power has been connected. The parts of the system, their function and some of the theory behind them are described.</p><p>The theoretical part of this master’s thesis is focused on low-dimensional structures, so-called quantum wells, wires and dots, that all are typical MBE-built structures. Physical effects, and to some extent the technical applications, of these structures are studied and described.</p><p>The experimental part contains the MBE growth of a Si/Ge quantum well (QW) structure and characterisation by Auger Electron Spectroscopy (AES). The structure, consisting of three QW of Si0,8Ge0,2 separated by thicker Si layers, was built at Linköpings University (LiU) and characterised at Chalmers University of Technology (CTH). The result of the characterisation was not the expected since almost no Ge content could be discovered but an extended characterisation may give another result.</p><p>Keywords: Silicon, Germanium, Molecular Beam Epitaxy, MBE, Quantum wells</p> / <p>Molecular Beam Epitaxy (MBE) är en väl etablerad metod när det gäller tillverkning av låg-dimensionella strukturer för forskningsändamål och lämpar sig väl för användning inom det expanderande forskningsområdet nanoteknologi. MBE utrustningen vid Karlstads universitet (Kau), som är avsedd för kisel/germanium (Si/Ge) strukturer, har studerats och startats för första gången. Under studien av systemet har alla inbyggda förreglingar utretts och anslutits och de olika delsystemen har testats och utvärderats. Tryckluft, kylvatten och el har utretts och anslutits. Systemets delar, deras funktion och i viss mån den bakomliggande teorin har studerats.</p><p>Den teoretiska delen av detta arbete är inriktad mot låg-dimensionella strukturer d.v.s. kvant brunnar, kvanttrådar och kvantprickar, som alla är strukturer lämpliga för framställning i MBE processer. De fysikaliska effekterna och i viss mån de tekniska tillämpningarna för dessa strukturer har studerats.</p><p>Den experimentella delen består av MBE tillväxt av en Si/Ge kvantbrunn-struktur och karakterisering m.h.a. Auger Electron Spectroscopy (AES). Tillväxten av strukturen, som består av tre kvantbrunnar av Si0,8Ge0,2 separerade av tjockare Si-lager, utfördes på Linköpings Universitet (LiU) och karakteriseringen utfördes på Chalmers Tekniska Högskola (CTH). Resultatet av karakteriseringen var inte det förväntade då knappast något Ge innehåll kunde detekteras men en utökad undersökning skulle kanske ge ett annat resultat.</p><p>Sökord: Kisel, germanium, Molecular Beam Epitaxy, MBE, kvantbrunn</p>

Electroabsorption is the physical phenomenon by which the absorption of light in a medium can be controlled by applying an electric field. The Quantum–Confined Stark Effect, which makes the absorption band–edge in quantum wells very field–dependent, together with the strong absorption peak provided by excitons, are the physical foundations for the success of electroabsorption modulators based on quantum well structures in telecommunication networks. This thesis describes the design and fabrication of surface–normal electroabsorption modulation devices. The techniques needed to understand the design and fabrication of surface–normal multiple quantum well optical modulators are introduced, as are the various characterisation techniques used during and after the fabrication. Devices for several types of applications have been designed, fabricated, characterised and in some cases integrated into optical systems: – Two–dimensional arrays of 128´128 pixel amplitude modulators grown on GaAs substrates have been fabricated and characterised. Speeds of up to 11700 frames per second were demonstrated, limited by the output electronics of the computer interface. – Large–area modulators grown on GaAs substrates for free–space optical communication were developed, with an active area of 2cm2 and a modulation speed of several megahertz. Contrast ratios up to 5:1 on full modulator areas were measured. Problems limiting the yield and modulation speed of such devices have been studied, and solutions to overcome them have been demonstrated. – Large–area devices grown on InP substrates for free–space optical communication have been developed. Contrast ratios of up to 2:1 for transmissive types have been demonstrated. – Devices consisting of two rows of pixels, grown on GaAs substrates, with an active area of 22mm´5mm, divided into 64 or 128 pixels per row have been developed. These amplitude modulation devices were designed for optical signal processing applications. – One variant of these optical signal processing devices was also characterised as a ternary, binary amplitude and binary phase modulator array. – The use of GaAs multiple quantum well optical modulators in a free–space optical retro–communication system has been studied. An opto–mechanical design for a modulating retro–reflector is described, allowing a large field of view in one direction using reflecting, resonant–cavity modulators for high contrast ratios. / QC 20100802

Molecular Beam Epitaxy (MBE) is a well-established method to grow low-dimensional structures for research applications. MBE has given many contributions to the rapid expanding research-area of nano-technology and will probably continuing doing so. The MBE equipment, dedicated for Silicon/Germanium (Si/Ge) systems, at Karlstads University (Kau) has been studied and started for the first time. In the work of starting the system, all the built in interlocks has been surveyed and connected, and the different subsystems has been tested and evaluated. Service supplies in the form of compressed air, cooling water and electrical power has been connected. The parts of the system, their function and some of the theory behind them are described. The theoretical part of this master’s thesis is focused on low-dimensional structures, so-called quantum wells, wires and dots, that all are typical MBE-built structures. Physical effects, and to some extent the technical applications, of these structures are studied and described. The experimental part contains the MBE growth of a Si/Ge quantum well (QW) structure and characterisation by Auger Electron Spectroscopy (AES). The structure, consisting of three QW of Si0,8Ge0,2 separated by thicker Si layers, was built at Linköpings University (LiU) and characterised at Chalmers University of Technology (CTH). The result of the characterisation was not the expected since almost no Ge content could be discovered but an extended characterisation may give another result. Keywords: Silicon, Germanium, Molecular Beam Epitaxy, MBE, Quantum wells / Molecular Beam Epitaxy (MBE) är en väl etablerad metod när det gäller tillverkning av låg-dimensionella strukturer för forskningsändamål och lämpar sig väl för användning inom det expanderande forskningsområdet nanoteknologi. MBE utrustningen vid Karlstads universitet (Kau), som är avsedd för kisel/germanium (Si/Ge) strukturer, har studerats och startats för första gången. Under studien av systemet har alla inbyggda förreglingar utretts och anslutits och de olika delsystemen har testats och utvärderats. Tryckluft, kylvatten och el har utretts och anslutits. Systemets delar, deras funktion och i viss mån den bakomliggande teorin har studerats. Den teoretiska delen av detta arbete är inriktad mot låg-dimensionella strukturer d.v.s. kvant brunnar, kvanttrådar och kvantprickar, som alla är strukturer lämpliga för framställning i MBE processer. De fysikaliska effekterna och i viss mån de tekniska tillämpningarna för dessa strukturer har studerats. Den experimentella delen består av MBE tillväxt av en Si/Ge kvantbrunn-struktur och karakterisering m.h.a. Auger Electron Spectroscopy (AES). Tillväxten av strukturen, som består av tre kvantbrunnar av Si0,8Ge0,2 separerade av tjockare Si-lager, utfördes på Linköpings Universitet (LiU) och karakteriseringen utfördes på Chalmers Tekniska Högskola (CTH). Resultatet av karakteriseringen var inte det förväntade då knappast något Ge innehåll kunde detekteras men en utökad undersökning skulle kanske ge ett annat resultat. Sökord: Kisel, germanium, Molecular Beam Epitaxy, MBE, kvantbrunn

Condensed matter systems exhibit a variety of dynamical phenomena at low energy scales, from gigahertz (GHz) to terahertz (THz) frequencies in particular, arising from complex interplay between charge, spin, and lattice. A large number of collective and elementary excitations in solids occur in this frequency range, which are further modified and enriched by scattering, interactions, and disorder. Recent advancements in spectroscopic methods for probing low-energy dynamics allow us to investigate novel aspects of charge and spin dynamics in solids. In this dissertation work, we used direct current (DC) conductivity, GHz, THz, and mid-infrared (MIR) techniques to provide significant new insights into interaction and disorder effects in low-dimensional systems. Specifically, we have studied temperature-dependent magnetoresistance (MR) and electron spin resonance (ESR) in single-wall carbon nanotubes (SWCNTs), intra-exciton scattering in InGaAs quantum wells, and high-field MIR-induced band gaps in graphene. Temperature-dependent resistance and MR were measured in an ensemble of SWCNTs from 0.3 to 350 K. The resistance temperature behavior followed a 3D variable range hopping (VRH) behavior from 0.3 to ~100 K. A positive MR was observed at temperatures above 25 K and could be fit with a spin-dependent VRH model; negative MR was seen at low temperatures. In the GHz regime, the ESR linewidth for SWCNTs was observed to narrow by as much as ~50% as the temperature was increased from 3 to 300 K, a phenomenon known as motional narrowing, suggesting that we are detecting the ESR of hopping spins. From the linewidth change versus temperature, we find the hopping frequency to be 285 GHz. For excitons in InGaAs quantum wells, we demonstrate the manipulation of intra-excitonic populations using intense, narrow-band THz pulses. The THz radiation temporarily quenches the 1s emission, which is then followed by an enhancement and subsequent decay of 2s emission. After the quenching, the 1s emission recovers and then eventually becomes enhanced, a demonstration of energy storage in intra-exciton states known as excitonic shelving. We show that the diffusive Coulomb scattering between the 2p and 2s states produces a symmetry breaking, leading to a THz-field-induced 1s-to-2s exciton population transfer.

The quantum Spin Hall Insulator (QSHI) is a two-dimensional variant of a novel class of materials characterized by topological order, whose unique properties have recently triggered much interest and excitement in the condensed matter community. Most notably, the topological properties of these systems hold great promise in mitigating the difficult problem of decoherence in implementations of quantum computers. Although QSHI has been theoretically predicted in a few different materials, prior to the work presented in this thesis, only the HgTe/CdTe semiconductor system has shown direct evidence for the existence of this phase. Ideally insulating in the bulk, QSHI is characterized by one-dimensional channels at the sample perimeter, which have a helical property, with carrier spin tied to the carrier direction of motion, and protected from elastic back-scattering by time-reversal symmetry. In this thesis we present low temperature transport measurements, showing strong evidence for the existence of proposed helical edge channels in InAs/CaSb quantum wells, which thus emerge as an important alternate to HgTe/CdTe quantum wells in studies of two-dimensional topological insulators and superconductors. Surprisingly, edge modes persist in spite of comparable bulk conduction of non-trivial origin and show only weak dependence on magnetic field in mesoscopic devices. We elucidate that the seeming independence of edge on bulk transport comes due to the disparity in Fermi wave-vectors between the bulk and the edge, leading to a total internal reflection of the edge modes. Furthermore, low Schottky barrier of this material system and good interface to superconductors allows us to probe topological properties of helical channels in Andreev reflection measurements, opening a promising route towards the realization of topologically superconducting phases hosting exotic Majorana modes.

Some interesting electronic and dynamic properties of semiconductor nanocrystal heterostructures have been investigated using various spectroscopic methods. Semiconductor nanocrystal heterostructures were prepared using colloidal synthesis techniques. Ultrafast transient absorption spectroscopy was used to monitor the relaxation of hot electrons in CdS/HgS/CdS quantum dot quantum wells. Careful analysis of the hot electron relaxation in CdS/HgS/CdS quantum dot quantum wells reveals an energy dependent relaxation mechanism involving electronic states of varying CdS and HgS composition. The composition of the electronic states, combined with the layered structure of the nanocrystal permits the assignment of CdS localized and HgS localized excited states. The dynamic effect of surface passivation is then shown to have the strongest influence on excited states that are localized in the HgS layer. New quantum dot quantum well heterostructures of different sizes and compositions were also prepared and studied. The dynamic properties of CdS/CdSe/CdS colloidal quantum wells suggest simultaneous relaxation of excited electrons within the CdS core and CdSe shell on the sub-picosecond time scale. Despite the very different electronic structure of CdS/CdSe/CdS compared to CdS/HgS/CdS, the time scales of the relaxation and electron localization were very similar. Enhancement of trap luminescence was observed when CdS quantum dots were coated with silver. The mechanism of the enhancement was investigated using time-resolved spectroscopic techniques.

With rapid development of light emitting device, the detection techniques of semiconductor are more and more important, which include time-resolved photoluminescence (TRPL) and optical beam induced current (OBIC) microscopy. In this thesis, we realize the carrier behaviors of active region with multiple quantum wells (MQWs) by these microscopies, and the samples are light emitting diodes (LEDs). However, PL intensity of LEDs increase but OBIC not due to external field compensates, on the other hand, reducing PL lifetime indicates the response time of device shorter with higher reverse bias.

The nonlinear optical properties of semiconductor heterostructures, such as GaAsAl/GaAs alloys, are studied with analytic and numerical methods on the basis of quantum mechanics. Particularly, second and third-order nonlinear optical properties of quantum wells described by the various types of confining potentials are considered within the density matrix formalism. We consider a P&ouml / schl-Teller type potential which has been rarely considered in this area. It has a tunable asymmetry parameter, making it a good candidate to investigate the effect of the asymmetry on the nonlinear optical properties. The calculated nonlinear quantities include nonlinear absorption coefficient, second-harmonic generation, optical rectification, third-harmonic generation and the intensity-dependent refractive index. The effects of the DC electric field on the corresponding nonlinearities are also studied. The results are in good agreement with the results obtained in other types of quantum wells, such as square and parabolic quantum wells. The effects of the Coulomb interaction among the electrons on the nonlinear intersubband absorption are considered within the rotating wave approximation. The result is applied to a Si-delta-doped, square quantum well in which the Coulomb interaction among the electrons are relatively important, since there has been no work on the nonlinear absorption spectrum of the Si-delta-doped quantum well. The results are found to be new and interesting, especially when a DC electric field is included in the calculations.