We investigate ionization and high harmonic generation (HHG) from the interaction of a mid infra-red laser pulse with a solid state system confined to nano-dimensions. The theory of strong field processes in solids is developed for confined quantum systems in general. Here it is applied to two-dimensional quantum wires with a driving field linearly polarised along the axis of the wires. Our findings indicate that that we are able to control the ionization and high-harmonic output by altering the width of the wire. Control of ionization leads to an increased damage threshold which has important implications for nano-engineering and realizing all solid state coherent XUV sources.

No description available.

<p>We study the spin polarization and conductance in infinitely long quasi one-dimensionalquantum wires under various conditions in an attempt to reproduce and to explain some of theanomalous conductance features as seen in various experiments. In order to accomplish thistask we create an idealized model of a quantum wire in a split-gate semiconductorheterostructure and we perform self-consistent Hartree-Fock calculations to determine theelectron occupation and spin polarization. Based on those results we calculate the currentthrough the wire as well as the direct and differential conductances. In the frame of theproposed model the results show a high degree of similarity to some of the experimentallyobserved conductance features, particularly the 0.25- and 0.85-plateaus. These results lead usto the conclusion that those conductance anomalies are in fact caused by the electronsspontaneously polarizing due to electron-electron interactions when an applied potentialdrives a current through the wire.</p>

We study the spin polarization and conductance in infinitely long quasi one-dimensionalquantum wires under various conditions in an attempt to reproduce and to explain some of theanomalous conductance features as seen in various experiments. In order to accomplish thistask we create an idealized model of a quantum wire in a split-gate semiconductorheterostructure and we perform self-consistent Hartree-Fock calculations to determine theelectron occupation and spin polarization. Based on those results we calculate the currentthrough the wire as well as the direct and differential conductances. In the frame of theproposed model the results show a high degree of similarity to some of the experimentallyobserved conductance features, particularly the 0.25- and 0.85-plateaus. These results lead usto the conclusion that those conductance anomalies are in fact caused by the electronsspontaneously polarizing due to electron-electron interactions when an applied potentialdrives a current through the wire.

BATISTA JÚNIOR, Francisco Florêncio. Propriedades de transportes em fios e poços quânticos. 2009. 73 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2009. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-04T19:05:12Z No. of bitstreams: 1 2009_dis_ffbatistajunior.pdf: 2157799 bytes, checksum: 21fa9e91409293ec4b1914c4cd297c44 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-07T14:47:19Z (GMT) No. of bitstreams: 1 2009_dis_ffbatistajunior.pdf: 2157799 bytes, checksum: 21fa9e91409293ec4b1914c4cd297c44 (MD5) / Made available in DSpace on 2015-05-07T14:47:19Z (GMT). No. of bitstreams: 1 2009_dis_ffbatistajunior.pdf: 2157799 bytes, checksum: 21fa9e91409293ec4b1914c4cd297c44 (MD5) Previous issue date: 2009 / Semiconductor materials are responsible for the large development in electronic industry, what made it possible the creation of new devices. The heterostructures gave a large impulse to the solid-state physics. Semiconductors study is nowadays concentrated in the low-dimensional systems, as quantum wells, quantum wires, quantum dots and quantum rings. In this work, we investigate the transport properties of heterostructured quantum wires of double barrier. We begin with calculation of radial confinement energy in a quantum wire InAs/InP of double barrier. We use a cylindrical model of wire with gradual and abrupt nterfaces. Transmission coefficients are calculated. We study its behavior varying barriers width, distance between them and the wire radius. In the future, we will use these results to calculate electric current through the device. We also investigate transport properties of bidimensional systems with self-energy potential. We use heterostructures of Si/SiO2 and Si/HfO2. We solve Poisson’s equation with epsilon depending on z, expanding the potential in a Fourier-Bessel series, finding the image potential of the barriers. We calculate the electric current through this potential in function of the applied voltage, varying temperature and the distance between the barriers. We also consider gradual interfaces for the simple barrier case. / Materiais semicondutores são os principais responsáveis pelo grande crescimento da indústria eletrônica e pelo surgimento de novas tecnologias. A criação de heteroestruturas possibilitou um grande impulso à física do estado sólido. Atualmente, o estudo de semicondutores está concentrado em sistemas de dimensionalidade reduzida, como os poços, fios, pontos e aneis quânticos. Neste trabalho, investigamos as propriedades de transporte em fios quânticos heteroestruturados de barreira dupla e em sistemas bidimensionais de barreira simples e dupla. Iniciamos com o cálculo da energia do confinamento radial no fio quântico InAs/InP de barreira dupla. Usamos um modelo de fio cilíndrico com e sem interfaces graduais. Calculamos as transmissões através das barreiras e estudamos o comportamento das mesmas variando a largura das barreiras, a distância entre elas e o raio do fio. Futuramente utilizaremos estes resultados para o cálculo da corrente elétrica através do dispositivo. Também investigamos as propriedades de transporte em sistemas bidimensionais com potencial de auto-energia. Utilizamos heteroestruturas formadas por Si/SiO2 e Si/HfO2. Sendo as constantes dielétricas dos óxidos diferentes do silício, resolvemos a equação de Poisson com epsilon dependente de z. Expandimos o potencial em uma série de Fourier-Bessel, encontrando, por fim, o potencial imagem para as barreiras. Calculamos a corrente elétrica através deste potencial em função da voltagem, variando a temperatura, a distância entre as barreiras. Também levamos em conta as interfaces graduais para o caso de barreira simples.

<p> Semiconductor quantum wire (QWR) structure is a promising candidate for potential applications in long wavelength laser devices. In this thesis, the investigations were focused on the growth and characterization on the structural and optical properties of InAs quantum wires deposited on InGaAlAs lattice matched with InP substrate by gas source molecular beam epitaxy. </p> <P> The practical growth parameters were first determined by studying the samples containing single InAs layer embedded within Ino.s3Gll{)_37Alo.10As barrier layers. These parameters were then employed for fabricating multilayer quantum wires with different (1) spacer layer thicknesses; (2) quantum wire layer thicknesses; and (3) different Al concentrations in the spacer/barrier layer materials. </P> <P>Structural properties of the quantum wires were characterized by (scanning) transmission electron microscopy based techniques. The composition variation, elastic field and the variation of QWR stacking patterns in multilayer samples were qualitatively studied through diffraction contrast imaging. Quantification of the In distribution in individual QWRs and the QWR-induced In composition modulation in barrier layers were obtained by electron energy loss spectrometry and energy dispersive X-ray spectrometry, respectively. These experimentally observed structural features were explained through finite element simulations. </P> <P> The optical properties of the QWR structures were studied by photoluminescence. Optical emission at room temperature was achieved from selected multilayer QWR samples after etching and rapid thermal annealing. The emission wavelength ranging from 1.53 to 1.72 μm makes the QWR structure suitable candidates for laser device applications. </P> / Thesis / Doctor of Philosophy (PhD)

In der vorgelegten Arbeit wurden zwei Arten von Quantendrahtstrukturen untersucht, die mittels Molekularstrahlepitaxie (MBE) hergestellt wurden. Erstens ist dies eine laterale Quantendrahtstruktur, die sich entlang einer Mesakante durch selektives Wachstum auf strukturierten GaAs (311)A-Substraten ausbildet. Zunächst wurden vertikal gestapelte Quantendrähte mit starker elektronischer Kopplung realisiert. Weiterhin wurden, unter Nutzung des amphoteren Einbaus von Si, p-i-n-Leuchtdioden mit einem einzelnen Quantendraht in der aktiven Zone hergestellt, die sich durch selektive Ladungsträgerinjektion in die Quantendrähte auszeichnen. Die Leuchtdioden wurden weitergehend mittels Mikrophotolumineszenz(µ-PL), Kathodolumineszenz (CL) und Elektronenstrahl-induziertem Strom (EBIC) charakterisiert. Zur Erklärung der selektiven Elektrolumineszenz (EL) wurde ein Modell, basierend auf der lateralen Diffusion von Elektronen und Löchern, vorgeschlagen. Für verspannte Systeme wurde der Einfluss von atomarem Wasserstoff auf das Wachstum von (In,Ga)As auf GaAs (311)A und die Bildung von lateralen Quantendrähten untersucht. Atomarer Wasserstoff spielt dabei die Rolle eines Surfaktanden und unterdrückt deutlich die Bildung von dreidimensionalen Inseln. Zweitens wurde das Wachstum von verspannten (In,Ga)As-Schichten auf GaAs (100) untersucht. Es zeigte sich, dass die dreidimensionale Inselbildung durch die Wachstumskinetik bestimmt ist, und ein Übergang von symmetrischen zu asymmetrisch verlängerten Inseln bei Erhöhung der Wachstumstemperatur auftritt. Dieser Prozess wird durch das Zusammenspiel von Oberflächen- und Verspannungsenergie bestimmt, wobei die experimentellen Befunde in guter Übereinstimmung mit den theoretischen Arbeiten von Tersoff und Tromp sind. Ausgehend von asymmetrischen (In,Ga)As-Inseln wurden selbstorganisierte Quantendrähte hergestellt, deren Homogenität und Länge sich durch Wachstum einer Vielschichtstruktur deutlich erhöhen. Strukturell wurden die (In,Ga)As-Quantendrähte mittels Rasterkraftmikroskopie (AFM), Röntgendiffraktometrie (XRD) und Transmissionselektronenmikroskopie (TEM) untersucht. Der laterale Ladungsträgereinschluss in den Quantendrähten zeigte sich deutlich in polarisationsabhängigen Photolumineszenz- und Magnetophotolumineszenzmessungen. / The present work focuses on two types of quantum wire structures which were grown by molecular beam epitaxy (MBE). First, the sidewall quantum wires based on the selective growth on mesa stripe patterned GaAs(311)A are studied. Single stacked sidewall quantum wires with strong electronic coupling have been fabricated. p-i-n type LEDs of the quantum wires employing the amphoteric Si incorporation for p- and n-type doping on GaAs(311)A have been fabricated. Strong selective carrier injection into the quantum wires is observed in electroluminescence (EL) measurements. The samples are characterized by micro-photoluminescence (µ-PL), cathodoluminescence (CL), as well as electron beam induced current (EBIC) measurements. To account for the highly selective EL, a model is proposed, which is based on the lateral diffusion of electrons and holes resulting in self-enhanced carrier injection into the quantum wires. Atomic hydrogen effects in the growth of (In,Ga)As on GaAs(311)A and its application to the sidewall quantum wire are investigated. It is found that atomic hydrogen suppresses island formation. Atomic hydrogen delays the relaxation by islanding thus playing the role of a surfactant. Second, the growth of (In,Ga)As layers on GaAs(100) is investigated showing that the formation of coherent 3D islands is a kinetically limited process. The transition from square-shaped islands to elongated islands is observed by changing the growth temperature for the growth of (In,Ga)As single layers. The elongation of the islands is a tradeoff between the surface free energy and the strain energy. A quantitative comparison between the experimental results and the theoretical work done by Tersoff and Tromp shows a good agreement. Self-organized quantum wires based on elongated discolation-free islands have been fabricated. The uniformity of the quantum wires is greatly improved by a superlattice growth scheme which also makes the wires much longer. The structural characterization of the quantum wires is performed by atomic force microscopy (AFM), x-ray diffractometry (XRD), and transmission electron microscopy (TEM). The lateral carrier confinement in the quantum wires is confirmed by polarization dependent PL and magneto-PL measurements.

OLIVEIRA, Claudio Lucas Nunes de. Excitons em Sistemas Quânticos 0-2D. 2005. 116 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2005. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-07T17:09:25Z No. of bitstreams: 1 2005_dis_clnoliveira.pdf: 1729842 bytes, checksum: 61191dfaedb56331176b4cb1c951e7e7 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-07T17:23:51Z (GMT) No. of bitstreams: 1 2005_dis_clnoliveira.pdf: 1729842 bytes, checksum: 61191dfaedb56331176b4cb1c951e7e7 (MD5) / Made available in DSpace on 2015-05-07T17:23:51Z (GMT). No. of bitstreams: 1 2005_dis_clnoliveira.pdf: 1729842 bytes, checksum: 61191dfaedb56331176b4cb1c951e7e7 (MD5) Previous issue date: 2005 / In the last few decades the physics of low dimensional semiconductor systems have attracted much attention due to the potential applications that arise from their due to electronic and optical properties. For example, InGaAs and InGaAsP heterostructures are currently used in optoelectronic applications that operate in the infrared spectrum. In such systems, the con_nement of charges can be realized in one, two or in three dimensions. The optical properties of quantum con_nement systems are basically determined by electronic transitions. Excitons, formed by an electron-hole pair bounded by coulombic interaction, are the responsible for the emission wavelenght. The aim of this work is to computer the ground state exciton energies in quantum wells, cylindrical quantum wires and pyramidal quantum dots as a function of the their size and shape. The results show that the exciton energies of In0:4Ga0:6As/GaAs quantum wells and wires are in the range from 0.9 to 1.3 eV. The results of In0:4Ga0:6As/GaAs pyramidal quantum dots show that the e-lh (e-hh) recombination energies are approximately 1.3-1.4 (1.18-1.28) eV. / A física de sistemas semicondutores de baixa dimensionalidade tem evoluído bastante nas últimas décadas. Em parte, porque essas estruturas oferecem a oportunidade de testarmos vários modelos teóricos, mas também porque existe um grande potencial de aplicação tecnológica derivada das propriedades de tais estruturas e dos materiais que a formam. Como exemplo, temos as heteroestruturas semicondutoras formadas com os materiais InGaAs e InGaAsP que são de grande utilidade em dispositivos optoeletrônicos emitindo na região do infravermelho. Nesses sistemas podemos fazer um confinamento dos portadores de carga, como elétrons e buracos, em uma, duas ou em três direções, aos quais são chamados de poço (2D), fio (1D) e ponto quântico (0D), respectivamente. As propriedades óticas dos semicondutores são determinadas pelos autovalores e autovetores do movimento dos elétrons e buracos. Os excitons que é o par elétron-buraco interagindo entre si são os maiores responsáveis pela emissão (pico da fotoluminescência) em sistemas de confinamento em semicondutores. A interação colombiana e o tipo de confinamento imposto pela construção dessas estruturas junto com suas interfaces graduais afeta o movimento desses portadores. O nosso objetivo neste trabalho é calcular a energia de emissão dos excitons elétron-buraco leve e elétron-buraco pesado em poços, fios cilíndricos e em pontos quânticos piramidais em função de seus parâmetros de dimensionalidade. Os resultados obtidos mostram as energias do exciton no poço e no fio quântico In0.4Ga0.6As/GaAs na mesma ordem de grandeza, estando na faixa de 0.9 à 1.3 eV. Para o ponto piramidal, as energias de recombinação do par e-hh (e-lh) estão na faixa de 1.3-1.4 (1.18-1.28) eV.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / In the last few decades the physics of low dimensional semiconductor systems have attracted much attention due to the potential applications that arise from their due to electronic and optical properties. For example, InGaAs and InGaAsP heterostructures are currently used in optoelectronic applications that operate in the infrared spectrum. In such systems, the con_nement of charges can be realized in one, two or in three dimensions. The optical properties of quantum con_nement systems are basically determined by electronic transitions. Excitons, formed by an electron-hole pair bounded by coulombic interaction, are the responsible for the emission wavelenght. The aim of this work is to computer the ground state exciton energies in quantum wells, cylindrical quantum wires and pyramidal quantum dots as a function of the their size and shape. The results show that the exciton energies of In0:4Ga0:6As/GaAs quantum wells and wires are in the range from 0.9 to 1.3 eV. The results of In0:4Ga0:6As/GaAs pyramidal quantum dots show that the e-lh (e-hh) recombination energies are approximately 1.3-1.4 (1.18-1.28) eV / A fÃsica de sistemas semicondutores de baixa dimensionalidade tem evoluÃdo bastante nas Ãltimas dÃcadas. Em parte, porque essas estruturas oferecem a oportunidade de testarmos vÃrios modelos teÃricos, mas tambÃm porque existe um grande potencial de aplicaÃÃo tecnolÃgica derivada das propriedades de tais estruturas e dos materiais que a formam. Como exemplo, temos as heteroestruturas semicondutoras formadas com os materiais InGaAs e InGaAsP que sÃo de grande utilidade em dispositivos optoeletrÃnicos emitindo na regiÃo do infravermelho. Nesses sistemas podemos fazer um confinamento dos portadores de carga, como elÃtrons e buracos, em uma, duas ou em trÃs direÃÃes, aos quais sÃo chamados de poÃo (2D), fio (1D) e ponto quÃntico (0D), respectivamente. As propriedades Ãticas dos semicondutores sÃo determinadas pelos autovalores e autovetores do movimento dos elÃtrons e buracos. Os excitons que à o par elÃtron-buraco interagindo entre si sÃo os maiores responsÃveis pela emissÃo (pico da fotoluminescÃncia) em sistemas de confinamento em semicondutores. A interaÃÃo colombiana e o tipo de confinamento imposto pela construÃÃo dessas estruturas junto com suas interfaces graduais afeta o movimento desses portadores. O nosso objetivo neste trabalho à calcular a energia de emissÃo dos excitons elÃtron-buraco leve e elÃtron-buraco pesado em poÃos, fios cilÃndricos e em pontos quÃnticos piramidais em funÃÃo de seus parÃmetros de dimensionalidade. Os resultados obtidos mostram as energias do exciton no poÃo e no fio quÃntico In0.4Ga0.6As/GaAs na mesma ordem de grandeza, estando na faixa de 0.9 à 1.3 eV. Para o ponto piramidal, as energias de recombinaÃÃo do par e-hh (e-lh) estÃo na faixa de 1.3-1.4 (1.18-1.28) eV.

Materiais semicondutores sÃo os principais responsÃveis pelo grande crescimento da indÃstria eletrÃnica e pelo surgimento de novas tecnologias. A criaÃÃo de heteroestruturas possibilitou um grande impulso à fÃsica do estado sÃlido. Atualmente, o estudo de semicondutores està concentrado em sistemas de dimensionalidade reduzida, como os poÃos, fios, pontos e aneis quÃnticos. Neste trabalho, investigamos as propriedades de transporte em fios quÃnticos heteroestruturados de barreira dupla e em sistemas bidimensionais de barreira simples e dupla. Iniciamos com o cÃlculo da energia do confinamento radial no fio quÃntico InAs/InP de barreira dupla. Usamos um modelo de fio cilÃndrico com e sem interfaces graduais. Calculamos as transmissÃes atravÃs das barreiras e estudamos o comportamento das mesmas variando a largura das barreiras, a distÃncia entre elas e o raio do fio. Futuramente utilizaremos estes resultados para o cÃlculo da corrente elÃtrica atravÃs do dispositivo. TambÃm investigamos as propriedades de transporte em sistemas bidimensionais com potencial de auto-energia. Utilizamos heteroestruturas formadas por Si/SiO2 e Si/HfO2. Sendo as constantes dielÃtricas dos Ãxidos diferentes do silÃcio, resolvemos a equaÃÃo de Poisson com epsilon dependente de z. Expandimos o potencial em uma sÃrie de Fourier-Bessel, encontrando, por fim, o potencial imagem para as barreiras. Calculamos a corrente elÃtrica atravÃs deste potencial em funÃÃo da voltagem, variando a temperatura, a distÃncia entre as barreiras. TambÃm levamos em conta as interfaces graduais para o caso de barreira simples. / Semiconductor materials are responsible for the large development in electronic industry, what made it possible the creation of new devices. The heterostructures gave a large impulse to the solid-state physics. Semiconductors study is nowadays concentrated in the low-dimensional systems, as quantum wells, quantum wires, quantum dots and quantum rings. In this work, we investigate the transport properties of heterostructured quantum wires of double barrier. We begin with calculation of radial confinement energy in a quantum wire InAs/InP of double barrier. We use a cylindrical model of wire with gradual and abrupt nterfaces. Transmission coefficients are calculated. We study its behavior varying barriers width, distance between them and the wire radius. In the future, we will use these results to calculate electric current through the device. We also investigate transport properties of bidimensional systems with self-energy potential. We use heterostructures of Si/SiO2 and Si/HfO2. We solve Poissonâs equation with epsilon depending on z, expanding the potential in a Fourier-Bessel series, finding the image potential of the barriers. We calculate the electric current through this potential in function of the applied voltage, varying temperature and the distance between the barriers. We also consider gradual interfaces for the simple barrier case.