Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 1992.

Orientador: Roberto E. Lagos Monaco / Banca: José Antonio Roversi / Banca: Bernardo Laks / Resumo: Neste trabalho estudamos o modelo do movimento Browniano de uma partícula carregada sob a ação de campos elétrico e magnético, externos e homogêneos, no formalismo de Langevin. Calculamos a energia cinética média através do teorema da flutuação-dissipação e obtivemos uma expressão para a temperatura efetiva das partículas Brownianas em função da temperatura do reservatório e dos campos externos. Esta temperatura efetiva mostrou-se sempre maior que a temperatura do reservatório, o que explica a expressão "portadores quentes". Estudamos essa temperatura efetiva no regime assintótico, ou seja, no estado estacionário atingido em tempos muito longos (quando comparado com o tempo de colisão) e a utilizamos para escrever as equações de transporte em semicondutores, denominadas equações de Shockley generalizadas sendo que incluem nesse caso também a ação do campo magnético. Uma aplicação direta e relevante foi a modelagem para o já conhecido efeito Gunn para portadores assumidos como Brownianos. A temperatura efetiva calculada por nós no regime transiente permitiu estudar também os efeitos do reservatório na relaxação da temperatura efetiva à temperatura terminal (de não equilíbrio e estacionária). Nossos resultados no que diz respeito ao efeito Gunn, embora seja o modelo mais simples de um portador Browniano, mostrou uma surpreendente concordância com resultados experimentais, sugerindo que modelos mais sofisticados devam incluir os elementos apresentados neste estudo / Abstract: We present a Brownian model for a charged particle in a field of forces, in particular, electric and magnetic external homogeneous fields, within the Langevin formalism. We compute the average kinetic energy via the fluctuation dissipation and obtain an expression for the Brownian particle's effective temperature. The latter is a function of the heat bath temperature and both external fields. This effective temperature is always greater than the heat bath temperature, therefore the expression "hot carriers". This effective temperature, in the asymptotic regime, the stationary state at long times (greater than the collision time), is used to write down the transport equations for semiconductors, namely the generalized Shockley equations, now incorporating the magnetic field effect. A direct and relevant application follows: a model for the well known Gunn effect, assuming a Brownian scheme. In the transient regime the computed effective temperature also allow us to probe some features of the heat bath, as the effective temperature relaxes to its terminal stationary value. As for our results in the Gunn effect model, the simplest of all in a Brownian scheme, we obtain a surprisingly good agreement with experimental data, suggesting that more involved models should include our minimal assumptions / Mestre

The global consumption of fossil fuels continues to increase due to the rapid growth of energy demand, as a consequence of expanding population and human activities. Fast climate change is another inescapable issue caused by humans that need to be addressed. The development of solar energy conversion technologies is widely considered as one of the most promising solutions to sustainably maintain a modern lifestyle of the society and create a carbon-neutral social development operation mode. The solar energy is carried and delivered in the form of electromagnetic fields. Therefore, the efficiency of photon collection is the primary factor to create any solar energy conversion systems. Through the inspiration from nature, the functionalized disorder, with a specific design and engineering, can introduce unusual light-matter interaction behaviors, and thus offer a potential capability to achieve perfect light harvesting. In my thesis, we develop complex Epsilon-Near-Zero (ENZ) metamaterials that can be used either as light capturing networks or the photoactive media by turning the energy damping ratio between radiative and non-radiative channels. We successfully integrate it into thin-film photovoltaic modules with showing an excellent performance enhancement led by broadband light localization effect. Thanks to universal of such complex ENZ metamaterials, with combining a thin layer of dielectric, we further develop efficient hot-carriers driven plasmonic photo-catalysts for artificial green chemical fuel synthesis. The detailed theoretic analysis is presented in this work.

Graduation date: 1993

Understanding changes in materials properties as a function of size is crucial for both fundamental science development and technological applications. Size restriction results in quantum confinement effects that modify both energy level structures and electron dynamics of solid materials. This study investigates individual quantum states in a single nanocrystal. Single electron charging effects in gold and semiconductor nanocrystals are observed. Charging effects are found to be dominant in samples, where the nanocrystals are weakly coupled to the substrate. For nanocrystals strongly coupled to the substrate, nanocrystal-substrate tunneling rate is larger than tip-nanocrystal tunneling rate. Therefore, the resulting peaks in the dI/dV spectrum are attributed to tunneling through the energy levels of the nanocrystal. A newly developed nanocrystals BEES technique is used successfully to further explore quantized energy levels and electron dynamics in single gold nanocrystals. BEES samples were grown successfully by depositing $unit[10]{nm}$ thick gold on silicon substrates. Nanocrystals are chemically attached to the gold substrate using a self assembled monolayer (SAM) of xyelendithiol molecules. Immobile and single isolated nanocrystals were imaged at low temperature. A BEES turn-on voltage of $unit[0.84]{V}$ was found on nanocrystal-free region of the substrate. The BEES spectrum acquired on a single gold nanocrystal is found to be attenuated by a factor of 10 when compared with BEES acquired on the substrate. The attenuation is attributed to electron relaxation to lower energy states before tunneling out of the nanocrystal. The measured hot electron lifetimes from experimental data were found to be on the order of $unit[16]{picoseconds}$, which is a long time compared to lifetimes in bulk metals or large nanocrystals. The long measured lifetimes result from the molecular-like energy level structures of these small nanocrystals.

Thesis (Ph. D. in Electrical Engineering)--Vanderbilt University, 2003. / Title from PDF title screen. Includes bibliographical references.

Just after discovery of porous silicon (PSi) there was clarified that its wide application in various fields opens new unexpected possibilities. One of the possibilities of products of porous silicon in microwave (MW) technique is carried out in the USA now. The propagation of MWs in PSi layers is under investigation. It has been shown that radio and optoelectronic connectors made from this material have low losses and can be applied to improve technique of cellular phone communication as well as other high frequency technique. It is obvious that the next element following the connector has to be the sensor of microwave radiation. The most practicable way would be to use porous silicon in the production of it. There are known MW detectors of crystal silicon for operating under the effects of hot charge carriers. Sensitivity of the sensors usually depends on the dimensions of separate parts of it. In general, sensitivity increases while reducing the mentioned above dimensions. The technology of porous silicon presents the advantage since the specific dimensions of PSi stem could be reduced up to the nanometre sizes. After having introduced PSi technology in production of sensors which require certain diminutive dimensions, it is possible to expect significant increase of the sensitivity of such sensors. Additional advantages are expected to be achieved from the quantum confinement effect. To realize promises of application of PSi in MW technique it is of relevance to... [to full text] / Atradus akytąjį silicį (ASi) paaiškėjo, kad daugelyje sričių jo panaudojimas atveria naujas netikėtas galimybes. Viena galimybių panaudoti akytojo silicio gaminius mikrobangų technikoje tiriama JAV. Tiriamas mikrobangų sklidimas ASi sluoksniuose. Parodyta, kad radijo- ir optoelektroninės jungtys iš šios medžiagos yra mažų nuostolių ir tinka panaudojimui tobulinant mobilaus ryšio ir kitą superaukštų dažnių techniką. Sekantis po jungties elementas turėtų būti superaukšto dažnio spinduliuotės jutiklis. Patogiausiai būtų jį gaminti iš tos pačios medžiagos – akytojo silicio. Yra žinomi kristalinio silicio mikrobangų spinduliuotės detektoriai, kurių fizinis veikimo pagrindas – karštųjų krūvininkų efektai. Jutiklių jautris priklauso nuo tam tikrų jo dalių matmenų. Jautris didėja mažinant minėtus matmenis. Akytojo silicio technologija suteikia tą privalumą, kad ją pritaikius ASi kamieno charakteringieji matmenys gali būti sumažinami iki nanometrų dydžio. Pritaikius ASi gamybos technologiją jutikliuose, kuriuose pageidaujama kaip galima mažesnių tam tikrų matmenų, galima tikėtis žymiai padidinti tokių jutiklių jautrį. Papildomų privalumų galima laukti ir iš pasireiškiančio ASi erdvinio kvantinio ribojimo (pagavimo) efekto. Tam, kad galima būtų spręsti apie ASi darinių panaudojimo superaukšto dažnio (SAD) technikoje perspektyvą, aktualu ištirti ASi sluoksnių ir darinių fizines savybes, veikiant juos SAD spinduliuotės lauku. Nei superaukšto dažnio spinduliuotės poveikis ASi savybėms... [toliau žr. visą tekstą]

Atradus akytąjį silicį (ASi) paaiškėjo, kad daugelyje sričių jo panaudojimas atveria naujas netikėtas galimybes. Viena galimybių panaudoti akytojo silicio gaminius mikrobangų technikoje tiriama JAV. Tiriamas mikrobangų sklidimas ASi sluoksniuose. Parodyta, kad radijo- ir optoelektroninės jungtys iš šios medžiagos yra mažų nuostolių ir tinka panaudojimui tobulinant mobilaus ryšio ir kitą superaukštų dažnių techniką. Sekantis po jungties elementas turėtų būti superaukšto dažnio spinduliuotės jutiklis. Patogiausiai būtų jį gaminti iš tos pačios medžiagos – akytojo silicio. Yra žinomi kristalinio silicio mikrobangų spinduliuotės detektoriai, kurių fizinis veikimo pagrindas – karštųjų krūvininkų efektai. Jutiklių jautris priklauso nuo tam tikrų jo dalių matmenų. Jautris didėja mažinant minėtus matmenis. Akytojo silicio technologija suteikia tą privalumą, kad ją pritaikius ASi kamieno charakteringieji matmenys gali būti sumažinami iki nanometrų dydžio. Pritaikius ASi gamybos technologiją jutikliuose, kuriuose pageidaujama kaip galima mažesnių tam tikrų matmenų, galima tikėtis žymiai padidinti tokių jutiklių jautrį. Papildomų privalumų galima laukti ir iš pasireiškiančio ASi erdvinio kvantinio ribojimo (pagavimo) efekto. Tam, kad galima būtų spręsti apie ASi darinių panaudojimo superaukšto dažnio (SAD) technikoje perspektyvą, aktualu ištirti ASi sluoksnių ir darinių fizines savybes, veikiant juos SAD spinduliuotės lauku. Nei superaukšto dažnio spinduliuotės poveikis ASi savybėms... [toliau žr. visą tekstą] / Just after discovery of porous silicon (PSi) there was clarified that its wide application in various fields opens new unexpected possibilities. One of the possibilities of products of porous silicon in microwave (MW) technique is carried out in the USA now. The propagation of MWs in PSi layers is under investigation. It has been shown that radio and optoelectronic connectors made from this material have low losses and can be applied to improve technique of cellular phone communication as well as other high frequency technique. It is obvious that the next element following the connector has to be the sensor of microwave radiation. The most practicable way would be to use porous silicon in the production of it. There are known MW detectors of crystal silicon for operating under the effects of hot charge carriers. Sensitivity of the sensors usually depends on the dimensions of separate parts of it. In general, sensitivity increases while reducing the mentioned above dimensions. The technology of porous silicon presents the advantage since the specific dimensions of PSi stem could be reduced up to the nanometre sizes. After having introduced PSi technology in production of sensors which require certain diminutive dimensions, it is possible to expect significant increase of the sensitivity of such sensors. Additional advantages are expected to be achieved from the quantum confinement effect. To realize promises of application of PSi in MW technique it is of relevance to... [to full text]

Reliability of sub-micron analog circuits is directly related to impact ionization and the subsequent changes in threshold voltage and drain current of n-MOSFET devices. This thesis presents theory of the hot-electron effects on the device characteristics and circuit performance, explores several approaches to improve performance at both the device and circuit level, and finally shows a new composite n-MOSFET device which significantly suppresses substrate current - an indication of hot-electron degradation. By using the composite device in the output gain stage of a CMOS differential amplifier with 1p.m technology, the normalized substrate current of the n-channel device is reduced by eight orders of magnitude for a sloping input waveform. The reduction in device substrate current is achieved at the cost of increased area and reduced frequency response. Replacing conventional n-channel devices with composite n-MOSFETs provides a simple way to improve device and circuit reliability without modification of the device structure and/or fabrication process. / Graduation date: 1993