This thesis presents research on the initialization, control, and readout of electron spin states in gate defined GaAs quantum dots. The first three experiments were performed with Singlet-Triplet spin qubits in double quantum dots, while the remaining two experiments were performed with an Exchange-Only spin qubit in a triple quantum dot. / Physics

Submitted by Roberto de Aguiar Ramos Junior null (robeerto.aguiar.ramos@gmail.com) on 2018-04-03T14:25:40Z No. of bitstreams: 1 dissertaçao_final.pdf: 2202514 bytes, checksum: b29c60407d282d9848bdfc887c1a97c8 (MD5) / Approved for entry into archive by Lucilene Cordeiro da Silva Messias null (lubiblio@bauru.unesp.br) on 2018-04-04T19:31:40Z (GMT) No. of bitstreams: 1 ramosjunior_ra_me_bauru.pdf: 2202514 bytes, checksum: b29c60407d282d9848bdfc887c1a97c8 (MD5) / Made available in DSpace on 2018-04-04T19:31:40Z (GMT). No. of bitstreams: 1 ramosjunior_ra_me_bauru.pdf: 2202514 bytes, checksum: b29c60407d282d9848bdfc887c1a97c8 (MD5) Previous issue date: 2018-02-08 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho traz o estudo das propriedades óticas, elétricas e morfológicas do material TiO2 de forma individual e acoplado com SnO2 dopado com 4at%Sb, formando uma heteroestrutura. Tanto TiO2 quanto a heteroestrutura foram trabalhados na forma de filmes finos depositados pelo método sol-gel-dip-coating, e, no caso do TiO2 também foi relevante sua análise em forma de pós prensados (pastilhas). No que diz respeito ao SnO2:4%Sb, este trabalho traz uma revisão literária de suas principais propriedades, buscando apresentar um panorama geral, pois com isto pode-se entender melhor os fenômenos que ocorrem na heteroestrutura. Os resultados das pastilhas de TiO2 indicam uma transição parcial de fase anatase/rutilo para tratamentos térmicos entre 500ºC e 1000ºC, confirmadas pela fotoluminescência, que apresentou bandas relacionadas a fase anatase ou rutilo, dependendo do processamento utilizado. Filmes de TiO2 mostraram boa foto sensibilidade, com a corrente elétrica respondendo imediatamente à excitação independente da energia, além de um rápido decaimento com relação ao valor excitado. Fora isto, medidas de decaimento da corrente foto induzida, realizadas em atmosfera de O2, indicaram que o decaimento se torna ainda mais rápido na presença do gás, estando associado ao aprisionamento de portadores de carga pelas moléculas adsorvidas na amostra, além da recombinação dos pares elétron-buraco. Com relação à heteroestrutura, quando a condução ocorre preferencialmente no TiO2, a amostra apresenta resultados muito similares às dos filmes de TiO2, tendo uma rápida resposta à excitação com fonte de luz acima do bandgap do TiO2 e um rápido retorno para seu estado de equilíbrio, no escuro. Entretanto, em atmosfera gasosa, o decaimento se torna muito mais rápido, o que também está associado ao aprisionamento de portadores pelas moléculas adsorvidas de gás. Porém, o aumento na taxa com que isto acontece, está relacionado à formação da heteroestrutura e às compensações de carga na interface, que podem ocorrer quando a excitação utiliza comprimento de onda adequado. A configuração lado a lado da heteroestrutura mostrou emissão Poole – Frenkel para tensões maiores que 40V, e quando irradiada com luz que simula o espectro solar apresentou uma região de resistência negativa para algumas potências de excitação que pode estar ligada ao aprisionamento de elétrons na interface. Desta forma, este trabalho visa trazer uma contribuição importante à compreensão do mecanismo de transporte elétrico dos materiais estudados. Por fim, pode-se dizer que os materiais aqui estudados podem ser aplicados como sensores de gás ou dispositivos retificadores/amplificadores desde que seja escolhida a melhor configuração para a aplicação desejada. / This work presents a study of the optical, electrical and morphological characterization of TiO2 thin films, deposited individually or coupled with SnO2 doped with 4at%Sb, forming a heterostructure. Both sort of samples, TiO2 and the heterostructure were studied in thin film form, deposited by sol-gel-dip-coating, and, in the case of TiO2, it was relevant the analysis of samples also in the form of pellets form (pressed powders). With regards to SnO2:4%Sb, this work brings a literary revision of its principal properties, trying to present a general overview, for the better understanding of the phenomena that occur in the heterostructure. The results of TiO2 pellets indicates a partial anatase/rutile phase transition to thermal annealing between 500 and 1000ºC, confirmed by the photoluminescence that presented bands related to anatase or rutile, depending the utilized processing. TiO2 films showed fair photo-sensibility, with immediate response on the electric current to light excitation, independent on the utilized energy, along with fast decay in relation to the excited value. Moreover, photo-induced current decay measurements, performed in O2 atmosphere, indicated that the decay becomes faster in gas presence, being associated to charge carriers trapping by the adsorbed molecules on sample, besides the electron-hole recombination. Concerning the heterostructure, when the conduction occurs preferentially in TiO2 layer, the sample shows very similar results to the TiO2 films, with a fast response to light excitation above the TiO2 bandgap and fast return to the equilibrium state, in dark. However, in gas atmosphere, the decay becomes much faster, which is also associated to the carriers trapping by the gas adsorbed molecules. Nevertheless, the rate increase in this phenomenon is related to the heterostructure formation and the charge compensations at the interface, which may occur when appropriate wavelength is used for excitation. The side by side heterostructure sample showed a Poole – Frenkel emission to bias higher than 40V, and presented a negative resistance region to some irradiation power when illuminated with solar light that can be associated to electrons trapping at the interface. In summary, this work aims to bring a contribution related to the electric transport mechanism of the studied materials. The materials investigated here may be applied in gas sensors or rectifiers/amplifiers devices, according the sample configuration. / CAPES: 1578735

Magnetic random­access memory (MRAM) devices have been widely studied since the 1960s. During this time, the size of spintronic devices has continued to decrease. Conse quently, there is now an urgent need for new low­dimensional magnetic materials to mimic the traditional structures of spintronics at the nanoscale. We also require new effective mechanisms to conduct the main functions of memory devices, which are: reading, writ ing, and storing data. To date, most research efforts have focused on MRAM devices based on magnetic tun nel junction (MTJ), such as a conventional field­driven MRAM and spin­transfer torque (STT)­MRAM devices. Consequently, many efforts are currently focusing on new alterna tives using different techniques, such as spin­orbit torque (SOT) and magnetic skyrmions (a skyrmion is the smallest potential disruption to a uniform magnet required to obtain more effective memory devices). The most promising memory devices are SOT­MRAMs and skyrmion­based memories. This study investigates the magnetic properties of 1T­phase vanadium dichalcogenide (VXY) Janus monolayers, where X, Y= S, Se, or Te (i.e., monolayers that exhibit inversion symme try breaking due to the presence of different chalcogen elements). This study is developed along four directions: (I) the nature of the magnetism and the SOT effect of Janus mono layers; (II) the Dzyaloshinskii Moriya interaction (DMI); (III) investigation of stability en hancement by adopting practical procedures for industry; and (IV) study of the effect of a hexagonal boron nitride (h­BN) monolayer as an insulator on the magnetism of the VXY monolayer. This study provides a clear perspective for the next generation of memory de vices, such as SOT­MRAMs based on transition metal dichalcogenide monolayers.

Thesis advisor: Chia-Kuang Tsung / Thesis advisor: Dunwei Wang / Metal-organic frameworks (MOFs), as a type of inorganic-organic hybrid porous materials, have attracted enormous research interests over the past two decades due to their extraordinary variability and richness of their chemistry and structures. The original design on MOFs is in pursuit of and high surface area, typically for gas storage. However, the properties in a simple MOF system could not meet the needs for a wide variety of advanced applications. Therefore, it is highly desirable to introduce multiplicities and impart functionalities into MOFs through materials design. In this regard, this dissertation focuses on engineering MOFs in two strategies, constructing heterostructures, fabricating thin films, and evaluating their impact on catalysis and energy conversions. The first chapter focuses on constructing a well-defined interface between materials with vast differences in structural dimensions. Another highlight of this study lies in developing characterization protocols to characterize interfacial structures. In the second part, a MOF-74 thin film with crack-free nature serves as a promising platform for the study of ion transport. The last part of this dissertation reports a new two-dimensional (2D) structure derived from UiO-66. The 2D structure was attained by limiting the coordination number and inducing anisotropic growth. The layered material could be further exfoliated and fabricated into thin films. This work presents strategies to impart functionality to MOFs with rational material design and elucidate their positive impacts on the performance of the whole system. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are effective components in optoelectronic devices due to their tunable and attractive electric, optical and chemical properties. Combining different 2D TMDCs into either vertical or lateral heterostructures has been pursued to achieve new optical and electronic properties. Chemical treatments have also been pursued to effectively tune the properties of 2D TMDCs. Among many chemical routes that have been studied, plasma treatment is notable for being rapid and versatile. In Wang’s group earlier work, plasma treatment of MoS2 and WS2 resulted in the formation of MoO3 and WO3 nanosheets and nanoscrolls. However, plasma treatment of 2D TMDC heterostructures have not been widely studied. In this dissertation, MoS2/WS2 vertical and lateral heterostructures were grown and treated with air plasma. The result showed that the vertical heterostructure and lateral heterostructures behaved differently. For the vertical heterostructures, the top WS2 layer acts as a shield for the underlying MoS2 monolayer from oxidizing and forming transition metal oxide nanoscrolls, as shown by Raman spectroscopy and atomic force microscopy (AFM). On the contrary, for the lateral heterostructures, the WS2 that was grown surrounding the MoS2 triangular core served as a tight frame to stop the propagation of the oxidized MoS2, resulting a gradient of crack distribution. These findings provide insight into how plasma treatment can affect the formation of oxide in heterostructure, which can have further application in nanoelectronic devices and electrocatalysts. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18772号 / 理博第4030号 / 新制||理||1581(附属図書館) / 31723 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 川上 則雄, 教授 田中 耕一郎, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

The observation of voltage-tunable plasmon resonances in the terahertz range in two dimensional electron gas (2-deg) of a high electron mobility transistor (HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems is reported. The devices were fabricated from a commercial HEMT wafer by depositing source and drain contacts using standard photolithography process and a semi-transparent gate contact that consisted of a 0.5 [micro]m period transmission grating formed by electron-beam lithography. Narrow-band resonant absorption of THz radiation was observed in transmission in the frequency range 10-100 cm-1. The resonance frequency depends on the gate voltage-tuned sheet-charge density of the 2deg. The fundamental and higher resonant harmonics were observed to shift towards lower frequencies with the implementation of negative gate bias. The theory of interaction of sub millimeter waves with 2deg through corrugated structure on top has been applied to calculate and understand the phenomena of resonant plasmon excitations. The observed separation of resonance fundamental from its harmonics and their shift with gate bias follows theory, although the absolute frequencies are lower by about a factor of 2-3 in InGaAs/InP system. However, calculated values match much better with AlGaN/GaN system.

<p> InGaAs/InP heterostructure nanowires (NWs) grown by gas source molecular beam epitaxy (GS-MBE) have been analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS). The morphology and interfacial properties of these structures have been compared to pure InP NWs and lattice-matched InGaAs!InP superlattice (SL) structures, respectively. Based on high-resolution x-ray diffraction (HRXRD) and photoluminescence (PL) measurements of the SLs a detailed structural model is proposed, consisting of strained InAsP and InGaAsP mono layers due to group-V gas switching and atomic exchange at the SL interfaces. The interfaces of the heterostructure NW s were an order of magnitude larger than those of the SLs and showed a distinct bulging morphology. Both of these characteristics are explained based on the slow purging of group-III material from the Au catalyst. Growth of lnGaAs on the sidewalls of the InP base of these wires was also observed, and occurs due to the shorter diffusion length of Ga adatoms as compared to In. </p> / Thesis / Master of Science (MSc)

Two-dimensional (2D) carrier systems built from semiconductor heterostructures have been at the center of a wide variety of experimental and theoretical research over the past decades. The quality improvement of GaAs/AlGaAs systems has allowed the observation of several peculiar ground states stabilized by the subtle interplay between carrier-carrier interaction, disorder and magnetic field. More recently, 2D systems in semiconductor heterostructures have also been used as a prime substrate for further confinement of the carriers to mesoscopic systems of major interest for the emerging fields of quantum computing and spintronics. This thesis addresses both magnetotransport measurements in hole open quantum dots (QDs) and thermopower studies of 2D holes in (311)A GaAs heterostructures. In the first part of this thesis, we describe the fabrication process for hole GaAs open QDs and investigate their magnetotransport properties at very low temperature T. Below 500 mK, the magnetoconductance of the open QDs exhibits clear signatures of coherent transport, namely magnetoconductance fluctuations and weak anti-localization. From these effects, we extract a T dependence for the dephasing time, together with an upper limit for the spin-orbit scattering time using the random matrix theory. Both the dephasing time and the spin-orbit scattering time are found to be much smaller than for electrons in similar systems. In the second part of this work, we report low-T thermopower measurements in the parallel magnetic field-induced metal-insulator transition (MIT) of 2D GaAs hole heterojunctions with different interface-dependent mobilities. When the magnetic field is increased, the diffusion thermopower decreases across the MIT. The reduction of the diffusion thermopower is more pronounced for the lower mobility sample where it reverses its sign. This behaviour indicates that the system does not undergo any ground state modification through the MIT but rather that the parallel magnetic field induces a dramatic change of the dominant hole scattering mechanisms. Finally, the last part of this thesis is devoted to the thermopower study of the insulating phase (IP) observed in 2D GaAs bilayer hole systems around the total Landau level filling factor n = 1. Our measurements show that the diffusion thermopower diverges with decreasing T in the IP. This divergence of the diffusion thermopower at low T indicates the opening of an energy gap in the system's ground state and suggests the formation of a pinned bilayer hole Wigner crystal around n = 1.

Esta tese apresenta um estudo experimental em sistemas eletrônicos multicamadas formados em diversas heteroestruturas semicondutoras de alta qualidade crescidas por epitaxia de feixes moleculares. Especificamente, poços quânticos isolados baseados em InGaAs/InP e super-redes baseadas em GaAs/AlGaAs foram caraterizados por meio de medidas de fotoluminescência (PL) em função da temperatura, potência de excitação e do campo magnético. O estudo de efeitos na dinâmica de processos de recombinação destes sistemas eletrônicos é a base principal deste trabalho. Além disso, exploramos os efeitos da desordem sobre os processos de recombinação e demonstramos que o espalhamento por rugosidade interfacial é responsável pela resposta óptica destes sistemas. Nas amostras de InGaAs/InP com maior largura do espaçador observamos um novo efeito, o tempo de recombinação Auger aumenta notavelmente com a potência de excitação. Atribuímos este novo efeito à distribuição de elétrons fotoexcitados em diferentes vales da banda de condução. E em amostras de menor largura do espaçador, o relaxamento da regra de seleção do momento induzido pela desordem faz que o tempo de recombinação Auger diminua com o aumento da potência. Por outro lado, nas amostras de GaAs/AlGaAs, evidenciamos que a desordem gerada pela rugosidade interfacial afeta consideravelmente o transporte dos elétrons da banda de condução, e em poços quânticos de largura apropriada resulta em uma transição metal-isolante. A borda de mobilidade Ec, energia crítica que separa os estados estendidos dos estados localizados, foi determinada a partir das medidas do tempo de recombinação em função da energia de emissão de PL. Para uma desordem crítica, a Ec mostra uma interseção com a energia do nível de Fermi, a qual corresponde à transição metal-isolante. Além disso, realizamos medidas de PL resolvida no tempo em função do campo magnético. Observamos que a redistribuição espacial de elétrons causada pelo campo magnético afeta os tempos de recombinação. Nas amostras metálicas, os resultados mostraram deslocamento da Ec para altas energias, devido à quantização da energia dos elétrons provocada pelo campo magnético. No entanto, nas amostras isolantes, o campo magnético foi responsável pelo relaxamento significativo da regra de seleção do momento, que aumenta a probabilidade de recombinação dos elétrons localizados com os buracos fotoexcitados da banda de valência e, por consequência, diminui o tempo de recombinação. / This thesis presents an experimental study in multilayer electronic systems formed in several high quality semiconductor heterostructures grown by molecular beam epitaxy. Specifically, GaAs/AlGaAs based superlattices and isolated quantum wells based on InGaAs/InP were characterized by photoluminescence (PL) measurements as a function of temperature, pump power and magnetic field. The study of effects on the dynamics of the recombination processes of these electronic systems is the principal goal of this work. In addition, we explore the effects of the disorder on the recombination processes and show that the interfacial roughness scattering is responsible for the optical response in these systems. In the small spacer InGaAs/InP samples, we observed a new effect, the Auger recombination time becomes larger with the increasing the pump power. We propose that the distribution of photoexcited electrons over different conduction band valleys might account for this effect. In large spacer quantum wells, the non-radiative recombination time is reduced with the increasing pump power, as a consequence the disorder-induced relaxation of the momentum rule. On the other hand, in GaAs/AlGaAs samples, we showed that the disorder generated by interfacial roughness considerably affects transport of the conduction band electrons and at appropriate quantum wells width results in a metal-to-insulator transition. The mobility edge energy Ec was determined from the measurements of the recombination time as a function of energy allowed. At a critical disorder, the mobility edge energy demonstrates intersection with the Fermi level energy which correspond to the metal-insulator transition. In addition, we perform time-resolved PL measurements as a function of the magnetic field. We observed that the spatial distribution of electrons caused by the magnetic field influence on the recombination time. In the metallic samples was observed a shift of the mobility edge to higher energy due to the magnetic field quantization of conduction band electron energy. However, in the insulating samples, the magnetic field was responsible to cause a significant relaxation of the momentum selection rule which enhances the probability of recombination of the localized electrons with the photoexcited holes of the valence band, and consequently the recombination time is reduced.