Dissertations / Theses on the topic 'GaAs; Superconductors; Quantum heterostructures'

In the first structure presented a single two-dimensional electron gas is positioned in a region of graded (0 ≤ x ≤ 0.1) InGaAs composition. Through a series of MBE grown wafers the technique of successfully growing InGaAs as an InAs/GaAs superlattice was demonstrated. Varying the period of the superlattice was used to achieve the graded InGaAs region in the final device design. The exchange-enhanced g-factor was measured via thermal excitation seen to increase with the application of positive back-gate voltages is. One-dimensional conductance is observed in a graded alloy system for the first time, a stepping-stone to the implementation of single electron devices. Through analysis of the low-field resistivity, the appearance of a second subband in the two-dimensional electron gas, attributed to the zero-field spin-splitting from the Rashba interaction, was seen, and a dependence on back-gate voltage observed. Gate-voltage control of the spin-orbit interaction has only previously been observed in much higher indium concentration samples. A second structure investigated consists of two two-dimensional electron gases and forms a new kind of double quantum well device. Two-dimensional electron gases are located in separate GaAs and InGaAs quantum wells, separated by an AlGaAs barrier. Devices presented in this thesis allow two different gating schemes to be investigated. Firstly large-area front- and back-gates allow the isolation of a two dimensional electron gas in either well. This means two-dimensional conduction can be limited to either the GaAs or InGaAs layer. Secondly through use of a split-gate midline device it is possible to select the conduction pathway through the device with quasi-one-dimensional channels. This technique uses surface gates only, and again it gives the ability to select the material composition in which the electron wavefunction is situated. Such a double quantum well system gives the possibility of investigating the effect of the local g-factor and spin-orbit coupling on various low-dimensional spin-related phenomena.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.
Includes bibliographical references (leaves 189-198).
by Jody Lee House.
Sc.D.

Orientador: Fernando Iikawa
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Neste trabalho estudamos as propriedade estruturais e ópticas de pontos quânticos auto-organizados de InP crescidos sobre o substrato de GaAs. Esta estrutura apresenta o alinhamento de bandas tipo-II na interface, confinando o elétron no ponto quântico, enquanto o buraco mantém-se na barreira, próximo à interface devido à interação coulombiana atrativa. As amostras foram crescidas por epitaxia de feixe químico (CBE) no modo Stranskii-Krastanov. Os pontos quânticos apresentam raio médio de 25 nm e grande dispersão de altura (1-5 nm) e ocorre a relaxação parcial do parâmetro de rede, chegando a 2 %, em pontos quânticos superficiais. Do ponto de vista de propriedades ópticas, a fotoluminescência de pontos quânticos superficiais exibe uma eficiente emissão óptica, devido a baixa velocidade de recombinação dos estados superficiais do InP, e reflete a densidade e distribuição bimodal de tamanhos. Além disso, sua emissão óptica em função da intensidade de excitação exibe comportamento diverso em comparação com pontos quânticos cobertos com uma camada de GaAs. Em pontos quânticos cobertos, determinamos a energia de ativação térmica, que varia de 6 a 8 meV, e é associada à energia de ligação do éxciton ou energia de ionização do buraco. O decaimento temporal da luminescência de pontos quânticos é de 1,2 ns, um tempo relativamente curto para um ponto quântico tipo-II. A análise das propriedades magneto-ópticas em pontos quânticos individuais, inédita em QDs tipo-II, permitiu verificar que o fator-g do éxciton é praticamente constante, independentemente do tamanho dos QDs, devido ao fato dos buracos estarem levemente ligados. Por fim, mostramos a versatilidade do sistema acoplando-o a um poço quântico de InGaAs. Este acoplamento introduz mudanças na superposição das funções de onda do par elétron-buraco que permitem a manipulação do tempo de decaimento da luminescência e da energia de ligação excitônica
Abstract: We have investigated structural and optical properties of InP self-assembled quantum dots grown on GaAs substrate. This system presents a type-II band lineup where only electrons are confined in the InP quantum dots. The InP/GaAs quantum dots were grown by chemical beam epitaxy in the Stranskii-Krastanov mode. Our quantum dots present a mean radius of 25 nm and large height dispersion, 1-5 nm, and a partial relieve of the strain up to 2 % is observed. The photoluminescence spectra of surface quantum dots show an efficient optical emission, which is attributed to the low surface recombination velocity in InP. We observed a bimodal dispersion of the dots size distribution, giving rise to two distinct emission bands. A remarkable result is the relatively large blue shift of the emission band from uncapped samples as compared to those for capped dots. In capped quantum dots, we obtained the thermal activation energy, from 6 to 8 meV, which is associated to the exciton binding energy or hole ionization energy. The observed luminescence decay time is about 1.2 ns, relatively short decay time for type II system. We investigated magneto-optical properties using single-dot spectroscopy. The values of the exciton g factor obtained for a large number of single InP/GaAs dots are mainly constant independent of the emission energy and, therefore, of the quantum dot size. The result is attributed to the weak confinement of the holes in InP/GaAs QDs. We have also investigated structures where InP quantum dots are coupled to a InGaAs quantum well. This system permits the manipulation of the wave function overlap between electron-hole in order to control the optical emission decay time and exciton binding energy
Doutorado
Física
Doutor em Ciências

Le suivi de la viabilitié, la croissance et le métabolisme cellulaire des bactéries peut contribuer de manière significative au diagnostic précoce de la maladie, mais peut aussi aider à améliorer le rendement des produits bactériens dans des expériences industrielle ou à petite echelle. Les méthodes conventionnelles utilisées pour l'étude de la sensibilité des bactéries aux antibiotiques sont basées principalement sur la culture, une technique qui prend au moins 12 heures pour rendre un résultat. Ce retard conduit au surtraitement d'un large éventail d'infections par des antibiotiques à large spectre, ce qui est coûteux et peut conduire à l'apparition de résistance à ces antibiotiques précieux, tandis que la détection rapide d'une infection virale ou l'absence de bactéries pourrait prévenir de tels traitements et, dans le cas d'une infection bactérienne, l'identification de la sensibilité aux antibiotiques pourrait permettre l'utilisation d'antibiotiques à spectre étroit. Le projet décrit dans le présent document vise à surveiller les activités biologiques des bactéries vivantes immobilisées sur les surfaces biofonctionnalisées de microstructures composées de semi-conducteurs quantiques (QS). Le procédé dépend de la sensibilité de la photoluminescence (PL) émise par des semi-conducteurs à la perturbation du champ électrique induit par la charge électrique des bactéries immobilisées sur la surface de ces structures. Dans la première phase du projet, nous avons étudié une méthode innovante impliquant la surveillance par PL de l'effet de photocorrosion dans des hétérostructures GaAs/AlGaAs. Le maintien d'un équilibre entre la sensibilité et la stabilité du biocapteur dans l'environnement aqueux nous a permis de détecter Escherichia coli K12 dans des solutions salines tamponnées au phosphate (PBS) avec une limite de détection attrayante de 103 UFC/ml en moins de 2 heures. Suite à cette recherche, nous avons émis l'hypothèse que ces hétérostructures pourraient être utilisés pour développer une méthode à faible coût et quasiment en temps reel de la croissance et de la sensibilité des bactéries aux antibiotiques. L'un des éléments clés dans le développement de cette plate-forme de biocapteurs était de démontrer que le GaAs (001), normalement utilisé pour recouvrir les hétérostructures de GaAs/AlGaAs, ne nuira pas à la croissance des bactéries. Dans la deuxième phase du projet, nous avons exploré la capture et la croissance de E. coli K12 sur des surfaces nues et biofonctionnalisées de GaAs (001). Il a été déterminé que la couverture initiale et les taux de croissance de bactéries dépendent de l'architecture de biofonctionnalisation utilisée pour capturer les bactéries: les surfaces biofonctionnalisées avec d'anticorps présentaient une efficacité de capture significativement plus élevée. En outre, on a trouvé que pour des suspensions contenant des bactéries à moins de 105 UFC/ml, la surface des plaquettes de GaAs ne supportait pas la croissance des bactéries, quel que soit le type d'architecture de biofonctionnalisation. Dans la troisième phase du projet, nous avons suivi la croissance et la sensibilité aux antibiotiques de E. coli K12 et E. coli HB101. Tandis que la présence de bactéries retardaient d’apparition du maximum de PL, la croissance des bactéries retardaient encore plus ce maximum. Par contre, en presence d’antibiotiques efficaces, la croissance des bactéries était arrêtée et le maximum de PL est arrivé plus tôt. Ainsi, nous avons pu distinguer entre des E. coli sensibles ou résistantes à la pénicilline ou à la ciprofloxacine en moins de 3h. En raison de la petite taille, du faible coût et de la réponse rapide du biocapteur, l'approche proposée a le potentiel d'être appliquée dans les laboratoires de diagnostic clinique pour le suivi rapide de la sensibilité des bactéries aux antibiotiques.
Abstract : Monitoring the viability, growth and cellular metabolism of bacteria can contribute significantly to the early diagnosis of disease, but can also help improve yield of bacterial products in industrial- or small-scale experiments. Conventional methods applied for investigation of antibiotic sensitivity of bacteria are mostly culture-based techniques that are time-consuming and take at least 12 h to reveal results. This delay leads to overtreatment of a wide range of infections with broad spectrum antibiotics which is costly and may lead to the development of resistance to these precious antibiotics, whereas rapid detection of a viral infection or absence of bacteria could prevent such treatments and, in the case of bacterial infection, identification of antibiotic susceptibility could allow use of narrow spectrum antibiotics. The project outlined in this document aims at monitoring biological activities of live bacteria immobilized on biofunctionalized surfaces of quantum semiconductor (QS) microstructures. The method takes advantage of the sensitivity of photoluminescence (PL) emitting semiconductors to the perturbation of the electric field induced by the electric charge of bacteria immobilized on the surface of these structures. Our hypothesis was that bacteria growing on the surface of biofunctionalized QS biochips would modify their PL in a different, and measurable way in comparison with inactivated bacteria. In the first phase of the project, we investigated an innovative method involving PL monitoring of the photocorrosion effect in GaAs/AlGaAs heterostructures. Maintaining the balance between device sensitivity and stability in the biosensing (aqueous) environment allowed us to detect Escherichia coli K12 in phosphate buffered saline solutions (PBS) at an attractive limit of detection of 103 CFU/mL in less than 2 hours. Following this research, we hypothesised that these heterostructures could be employed to develop a method for inexpensive and quasi-real time monitoring of the growth and antibiotic susceptibility of bacteria. One of the key elements in the development of this biosensing platform was to demonstrate that GaAs (001), normally used for capping PL emitting GaAs/AlGaAs heterostructures, would not inhibit the growth of bacteria. In the second phase of the project, we explored the capture and growth of E. coli K12 on bare and biofunctionalized surfaces of GaAs (001). It has been determined that the initial coverage, and the subsequent bacterial growth rates are dependent on the biofunctionalization architecture used to capture bacteria, with antibody biofunctionalized surfaces exhibiting significantly higher capture efficiencies. Moreover, for suspensions containing bacteria at less than 105 CFU/mL, it has been found that the surface of GaAs wafers could not support the growth of bacteria, regardless of the type of biofunctionalization architecture. In the third phase of the project, we used PL to monitor the growth and antibiotic susceptibility of E. coli K12 and E. coli HB101 bacteria. While immobilization of bacteria on the surface of GaAs/AlGaAs heterostructures retards the PL monitored photocorrosion, growth of these bacteria further amplifies this effect. By comparing the photocorrosion rate of QS wafers exposed to bacterial solutions with and without antibiotics, the sensitivity of bacteria to the specific antibiotic could be determined in less than 3 hours. Due to the small size, low cost and rapid response of the biosensor, the proposed approach has the potential of being applied in clinical diagnostic laboratories for quick monitoring of antibiotic susceptibility of different bacteria.

Orientador: Fernando Iikawa
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Neste trabalho, estudamos o efeito de diferentes condições de interface de InP/GaAs nas propriedades ópticas de pontos quânticos auto-organizados, crescidos por epitaxia de feixe químico, no modo Stranskii-Krastanov. Espera-se que os pontos quânticos de InP/GaAs apresentem alinhamento de bandas do tipo II, e somente os elétrons ficam confinados, enquanto os buracos ficam localizados nas camadas de GaAs em volta do ponto quântico, atraídos pelo elétron. No entanto, devido ao efeito de mistura de átomos nas interfaces o perfil de potencial nas interfaces pode ser alterado significativamente, afetando, com isso, as propriedades ópticas dos pontos quânticos. Foram estudadas amostras com as seguintes condições de interface entre a camada de InP e as camadas de GaAs: inclusão ou não de uma camada de InGaP em uma ou nas duas interfaces. O InGaP gera uma barreira para ambos os tipos de portadores de carga em uma junção tanto com o GaAs como InP e evita a difusa de As das camadas de GaAs para a de InP. Através de medidas de fotoluminescência resolvida no tempo, observamos a variação do tempo de decaimento da emissão óptica associada aos pontos quânticos de acordo com as diferentes condições de interface. Foi observado um tempo curto de decaimento em amostras sem a inclusão de InGaP e com a inclusão apenas na interface superior, enquanto foi observado um tempo longo quando incluímos camadas de InGaP em ambas as interfaces. O tempo de decaimento curto é incompatível com o alinhamento de bandas do tipo II, que deveria separar espacialmente o elétron do buraco. A partir desses resultados e estudos anteriores a esse trabalho, pudemos concluir que o tempo curto se deve à mistura de átomos nas regiões de ambas as interfaces, gerando ligas que localizam os portadores próximos um ao outro. O tempo longo na amostra contendo InGaP nas duas interfaces é atribuído à separação espacial do elétron e do buraco. O efeito de mistura de átomos nas interfaces, neste caso, não forma uma liga na interface que localize os dois tipos de portadores próximos um ao outro. Isso pode ser uma alternativa de preparação de pontos quânticos de InP/GaAs onde se mantém separados espacialmente o elétron e o buraco
Abstract: We studied the effect of different interface conditions on the optical properties of InP/GaAs self-assembled quantum dots grown by chemical beam epitaxy in the Stranskii-Krastanov mode. InP/GaAs quantum dots is expected to present type II band alignment, and only electrons are confined, whereas the holes are localized in the GaAs layers around the quantum dot, attracted by the electron. However, due to the atomic intermixing effect in the interface the potential profile can be strongly changed, affecting the optical properties of the quantum dots. We studied samples with the following conditions at the interfaces between the InP layer and GaAs layers: the inclusion, or the lack of, a InGaP layer at one of or both interfaces. InGaP generates a barrier for both types of carriers in a junction with GaAs and InP, and avoid the diffusion of As from the GaAs layers to the InP one. Using time-resolved photo-luminescence, we observed a change of the optical emission decay times associated to the quantum dots as the interface condition is changed. We observed a short decay lifetime in samples without InGaP layers and with the inclusion in the top interface only, whereas we observed a long decay time when we included InGaP layers in both interfaces. The short decay lifetime is incompatible with the type II band alignment, where the electron and the hole should be spatially separated. Using these and other previous results, we concluded that the short decay lifetime is due to the atomic intermixing in both interfaces regions, forming alloys that localize the carriers near each other. The long lifetime observed for sample containing InGaP in both interfaces is attributed to the large electron-hole spatial separation. In this case intermixing effects at the interfaces do not form a potential well to localize the carries near each other
Mestrado
Física
Mestre em Física

博士
國立中央大學
電機工程研究所
89
This thesis covers the growth and characterization of In(Ga)As quantum dot (QD) heterostructures and lasers. QD lasers with different doping schemes in the In0.5Ga0.5As QD active region are investigated. Their lasing wavelength, characteristic temperature, quantum efficiency, and internal loss are characterized and correlated with the size, uniformity, and density of the QDs revealed by atomic force microscopy. Room temperature continuous-wave operation of Be-doped quantum-dot lasers has been achieved. Undoped In0.5Ga0.5As quantum-dot lasers with a characteristic temperature as high as 125 K above room temperature have also been demonstrated. It is also found that the performance of In0.5Ga0.5As/GaAs multi-stack QD lasers is sensitive to the GaAs spacer thickness between the dots. Reducing the spacer thickness from 30 nm to 10 nm leads to narrow photoluminescence linewidth, low threshold current, high characteristic temperature and high internal quantum efficiency. This behavior is attributed to inhomogeneous broadening caused by dot size fluctuation related to spacer thickness. We then study the matrix-dependent strain effect in self-assembled InAs QD heterostructures using photoluminescence measurements. A series of samples is prepared to examine the effect of QD position with respect to the so-called strain-reducing layer (SRL). Since the SRL reduces the residual hydrostatic strain in the QDs, long emission wavelength of 1.34 mm is observed for the InAs QDs with an In0.16Ga0.84As SRL. The dependence of the emission wavelength on the thickness of the cap layer on SRL also indicates the critical role of the matrix that plays in the strain relaxation process of the dots. This is further confirmed by using In0.16Al0.84As instead of In0.16Ga0.84As as the SRL. A blue shift in wavelength is observed because the elastic stiffness of In0.16Al0.84As is higher than that of In0.16Ga0.84As and less strain is removed from the dots with In0.16Al0.84As SRL. The effects of the arrangement between InAs QDs and InGaAs SRL on the optical properties of QD light emitting diodes are also investigated. Electroluminescence wavelength longer than 1.3 mm is obtained as InAs QDs are covered with a thin InGaAs SRL. For the same sample, the full width at half maximum of the ground state emission peak is as narrow as 19 meV at low injection current, and less than 40 meV even at saturation condition. It is also found that the slope efficiency of the diode is higher than that of other samples in the linear region and its light output saturation level is higher because of its higher density of QD. Finally, we present the lasing properties of InAs/GaAs QD lasers with InGaP cladding layers grown by solid-source molecular beam epitaxy. These Al-free lasers exhibit a threshold current density of 138 A/cm2, an internal loss of 1.35 cm-1 and an internal quantum efficiency of 31 % at room temperature. At low temperature, a very high characteristic temperature of 425 K and very low threshold current density of 30 A/cm2 are measured.

碩士
長庚大學
電機工程研究所
90
In this thesis, the InAs/GaAs quantum dots have been studied by a photoluminescence, electroluminescence, and fourier transform infrared spectrometer spectra system. All samples were grown on the dopped GaAs substrates by solid sources molecular beam epitaxy (MBE). Owing to the difference exciting process between each measurements, we can find some interesting results. From the photoluminescence (PL) spectrums, we found that actived carriers tunneling through dots with different size do influencing the PL spectrum FWHM under various temperatures. In addition, the same sample also measured by the electroluminescence system spectrums, and we found that due to the heat effect by the current injection in EL the difference between the ground states energy and exciting states energy is almost the same as by the PL spectrum. When the current input the device, increasing the temperature the internal part of device. So the spectrum of the EL peak energy is lower than the PL peak energy. Finally from the FT-IR measurement results, the absorption spectrums can be compared with the PL spectrums and clearly realized the dots intraband transition energy.

博士
國立中央大學
電機工程研究所
94
This dissertation is devoted to growing and characterizing high quality long-wavelength InAs quantum dot heterostructures and lasers by an ultra-high vacuum molecular beam epitaxy system. Since the growth of QD active layers is the key factor to the development of QD lasers, it is most important to study the epitaxy growth of a high quality QD active layer. First, we focus on the modification of the QD growth recipe, including growth rate, temperature, nominal thickness and growth interruption. In the optimization of the QD growth recipe for fine optical properties, the importance of dot-height uniformity to the optical properties is disclosed through the employment of atomic force microscopy and photoluminescence measurement. Long-wavelength QD lasers with different dot-height uniformity are also fabricated to demonstrate the significance of dot-height uniformity in achieving high performance QD lasers. The results reveal that a ridge-type QD laser with high dot-height uniformity shows better characteristics than its counterpart, and can be operated at 1328 nm under room temperature. The threshold current density of this as-cleaved laser is 250 A/cm2 , which is comparable to the reported results obtained with high-reflectivity facet coating and similar ridge sizes, indicating the potential of the QD laser in this work for optical-fiber communication. In order to further improve the characteristics of the long-wavelength QD laser, we systemically study and clarify the mechanisms of elongated QD emission wavelength within different matrices. It is found that the thickness of the InGaAs overgrown layer would alter the wavelength-extension mechanisms and that strain-reducing effect is more dominant in extending the emission wavelength of QDs when the overgrown layer is thin. We also overgrow an InAlAs layer on the InAs QDs to act as a strain-reducing layer (SRL). Since the InAlAs layer suppresses indium segregation and increases potential barrier height in InAs/GaAs band diagram, uniform dot size and large state separation results are obtained by QDs with an InAlAs overgrown layer. The mechanisms of red-shifted wavelength discussed here could be constructive in realizing long-wavelength QD LDs with high characteristic temperature when operated above room temperature. After optimizing the QD growth parameters and studying the mechanisms of elongated QD emission wavelength, multi-stack QDs are grown for increasing the modal gain of QD lasers. However, the surface stress caused by lattice mismatch between the InAs and (In)GaAs overgrown layers often results in defects that are detrimental for optical devices. The surface stress would also lead to the formation of pinhole-like defects in the growth of multi-stack QDs. These not only impede the formation of uniform QDs, but also deteriorate the crystal as well as the optical quality of the multi-stack QDs. Growth interruptions during GaAs spacer layer formation and thermal annealing after the GaAs growth are employed to lead to indium-flush behavior and increase the diffusion length of GaAs adatoms for a smooth GaAs surface, and multi-stack quantum dot structures without pinhole-like defects are thus obtained. Based on the investigation, the demonstration of high performance 1.3 um quantum dot lasers with a 5-stack QD active region proves the effectiveness of the novel method in eliminating the pinhole-like defects. Additionally, the employment of an InGaAs strain-reducing layer is now the most favorable approach for long-wavelength (1.3 um) QD lasers. However, further extending the emission wavelength to 1.5 um always induces misfit dislocations and degrades the radiative efficiency of the QD structure because of lattice mismatch. Besides, since the InGaAs strain-reducing layer acts as a transit channel for facilitating the carriers’ thermal escape from InAs/GaAs QD structure, the thermal stability of this kind of laser at elevated temperatures is therefore far from expected. For improving the characteristics of QDs, we employ InGaAsSb or InAlAsSb instead of typical InGaAs or InAlAs strain-reducing layers, and find it can actually improve the band structure of QD active layer. In this investigation, the influence of Sb incorporation in SRL on QD optical properties and thermal stability is comprehensively studied. Enhanced emission intensity, thermal stability and extended emission wavelength of InAs/GaAs QDs is shown by the use of an Sb-contained strain-reducing layer. The superior optical characteristics of this novel structure make it a promising candidate for high-performance QD optoelectronic devices.

碩士
長庚大學
光電工程研究所
98
We investigate the carrier dynamics of the self-assembled InAs/GaAs quantum-dot (QD) heterostructures experimentally and theoretically. Theoretical discussions that take into account the dot size distribution, the random population of density of states, and the important carrier transferring mechanisms of a QD system, are proposed. The carrier capturing and relaxing, radiative and nonradiative recombination, thermal emission and retrapping, and the thermal induced electron-phonon scattering, are all considered. Mechanisms of carrier dynamics in QD system related to the thermal redistribution and lateral transition of excitons, and the filling effect on density of states, are discussed in detail. The temperature and incident-power dependent photoluminescence spectra from QD samples with different dot size distributions are measured and studied. In addition, the phonon-assisted activations of excitons in the QD system are analyzed. Quantitative discussion of the correlation between thermal redistribution and electron-phonon scattering effects on QD system provides distinct explanation for the different behaviors with increasing temperature that observed in the photoluminescence spectra from QD heterostructures.

博士
長庚大學
電子工程研究所
95
We investigate the carrier dynamics of the self-assembled InAs/GaAs quantum-dot (QD) heterostructures experimentally and theoretically. Theoretical discussions that take into account the dot size distribution, the random population of density of states, and the important carrier transferring mechanisms of a QD system, are proposed. The carrier capturing and relaxing, radiative and nonradiative recombination, thermal emission and retrapping, and the thermal induced electron-phonon scattering, are all considered. Mechanisms of carrier dynamics in QD system related to the thermal redistribution and lateral transition of excitons, and the filling effect on density of states, are discussed in detail. The temperature and incident-power dependent photoluminescence spectra from QD samples with different dot size distributions are measured and studied. In addition, the phonon-assisted activations of excitons in the QD system are analyzed. Quantitative discussion of the correlation between thermal redistribution and electron-phonon scattering effects on QD system provides distinct explanation for the different behaviors with increasing temperature that observed in the photoluminescence spectra from QD heterostructures.

This thesis presents an experimental and theoretical study of some of the electronic properties and device applications of GaAs/Al_xGa_1-xAs single and double barrier tunnel structures. In Chapter 2, energy band diagrams are calculated for heterostuctures in which tunneling occurs between two degenerately doped electrodes separated by a single quantum barrier. When a bias voltage is applied to a structure, the energy band profile gives the voltage drop distribution in the cladding layers as well as in the barrier. This distribution may differ significantly from that based on the commonly made assumption that the entire applied voltage drops linearly across the barrier layer. It is shown that band bending effects become more important for larger applied voltages, thicker barriers, smaller electrode doping densities and larger barrier doping concentrations. Energy band diagrams are found to be useful for calculating tunneling currents and determining what the dominant low temperature current transport mechanisms occurring in these structures are. In some cases, they reveal that these mechanisms are different from those predicted when band bending is neglected.

In Chapter 3, elastic and inelastic tunneling processes are investigated in GaAs-AlAs-GaAs single barrier heterostructures grown on [100]-oriented substrates. The GaAs electrodes are degenerately doped n-type with Se, and the AlAs quantum barriers are doped either p-type with Mg or n-type with Se. In p-type barrier structures, low temperature current transport is found to be dominated by elastic and inelastic electron tunneling through the AlAs band gap at the Γ-point and at the X-point. Anomalous zero-bias conductances obtained from several of the samples are also discussed. A theoretical model, which treats trap levels in the AlAs barrier as intermediate states for two-step tunneling processes shows that impurity-assisted tunneling becomes more important as the tunnel barrier is made thicker. In heterostructures in which the n-type barrier layers are thick enough and/or sufficiently doped, the AlAs conduction band at the X-point is not totally depleted of electrons. The dominant low temperature current transport mechanism is then tunneling through two reduced AlAs X-point barriers separated by a bulk region of AlAs. When the n-type AlAs barrier layer is sufficiently thin, the AlAs conduction band remains fully depleted of carriers. As a result, electrons tunnel through the AlAs band gap at the X-point and/or at the Γ-point in a one-step process. In these structures, it is found that plasmons located near the GaAs/AlAs interfaces interact with GaAs and AlAs longitudinal optical (LO) phonons when the doping density in the n-type GaAs electrodes is such that the plasma frequency becomes comparable to the LO phonon frequencies.

Chapter 4 presents a study of resonant tunneling in GaAs/Al_xGa_1-xAs double barrier heterostructures grown epitaxially in the [100]-direction. In these structures, electrons tunnel through two Al_xGa_1-xAs quantum barriers separated by a thin GaAs layer forming a quantum well. The resonant energy levels in the GaAs well which produce negative differential resistances in the experimental I-V characteristics are identified by calculating the energy band diagrams of the structures. In samples having pure AlAs barrier layers, tunneling via resonant states confined in the well by the AlAs Γ-point potential energy barriers is often inconsistent with experimental results. However, the experimental data can usually be explained by tunneling via quasi-stationary levels confined in the well by the AlAs X-point potential energy barriers as well as the AlAs Γ-point barriers. The relative contributions of tunneling via resonant Γ- and X-states in the well are found to depend upon the samples studied and sometimes upon the sign of the applied bias. Resonant tunneling is also investigated in double barrier heterostructures in which a low doped GaAs buffer layer is grown before the first Al_xGa_1-xAs barrier. As a result of this structural asymmetry, the peaks in current corresponding to a given resonant state in the quantum well may be observed in the experimental I-V characteristics at very different applied voltages in reverse bias than in forward bias.

In Chapter 5, we propose and analyze two types of three-terminal devices based upon resonant tunneling through quantum well and quantum barrier heterostructures. The first type includes two configurations in which a base voltage controls the emitter-collector tunneling current by shifting the resonances in a quantum well. In the proposed devices, the relative positions of the base and collector are interchanged with respect to the conventional emitter-base-collector sequence as a means for obtaining negligible base currents and large current transfer ratios. The second type of three-terminal devices includes three configurations in which the current through a double barrier structure is modulated by a Schottky barrier gate placed along the path of the electrons. These devices feature, in their output current-voltage (I_D-V_D) curves, negative differential resistances controlled by a gate voltage.

Chapter 6 presents a growth uniformity study performed on several of the heterostructures discussed in the thesis. First, the reproducibility and uniformity of the electrical characteristics of GaAs/AlAs tunnel structures are used to show that the doping concentrations and layer thicknesses are uniform across the samples under test. Secondly, discrete fluctuations in layer thicknesses are discussed in GaAs/Al_0.35Ga_0.65As double barrier heterostructures. These fluctuations are manifested by non-uniform experimental results and by sequences of negative differential resistances in the I-V characteristics of many devices.

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Nanometer scale wire structures are fabricated by selective disorder of a GaAs/AlGaAs quantum well. These structures are investigated by cathodoluminescence (CL). Spectrally resolved CL images of the structures as well as local CL spectra of the structures are resented. The effects of fabricational variations on quantum wire laser gain spectra and performance are discussed. A new technique for determining carrier diffusion lengths by cathodoluminescence measurements is presented. The technique is extremely accurate and can be applied to a variety of structures. The ambipolar diffusion length and carrier lifetime are measured in [...] for several mole fractions in the interval 0 < [...] < 0.38. These parameters are found to have significantly higher values in the higher mole fraction samples. These increases are attributed to occupation of states in the indirect valleys, and supporting calculations are presented.

GaAs-based nanowires are attractive building blocks for the development of future (opto)electronic devices owing to their excellent intrinsic material properties, such as the direct band gap and high electron mobility. A pre-requisite for the implementation of novel functionalities on a single Si chip is the monolithic integration of the nanowires on the well-established Si complementary-metal-oxide-semiconductor (CMOS) platform with precise control of the nanowire growth process. The self-catalyzed (Ga-assisted) growth of GaAs nanowires on Si(111) substrates using molecular beam epitaxy has offered the possibility to obtain vertical nanowires with predominant zinc blende structure, while potential contamination by external catalysts like Au is eliminated. Although the growth mechanism is fairly well understood, control of the nucleation stage, the nanowire number density and the crystal structure has been proven rather challenging. Moreover, conventional growth processes are typically performed at relatively high substrate temperatures in the range of 560-630 °C, which limit their application to the industrial Si platform. This thesis provides two original methods in order to tackle the aforementioned challenges in the conventional growth processes. In the first part of this thesis, a simple surface modification procedure (SMP) for the in situ preparation of native-SiOx/Si(111) substrates has been developed. Using a pre-growth treatment of the substrates with Ga droplets and two annealing cycles, the SMP enables highly synchronized nucleation of all nanowires on their substrate and thus, the growth of exceptionally uniform GaAs nanowire ensembles with sub-Poissonian length distributions. Moreover, the nanowire number density can be tuned within three orders of magnitude and independent of the nanowire dimensions without prior ex situ patterning of the substrate. This work delivers a fundamental understanding of the nucleation kinetics of Ga droplets on native-SiOx and their interaction with SiOx, and confirms theoretical predictions about the so-called nucleation antibunching, the temporal anti-correlation of consecutive nucleation events. In the second part of this thesis, an alternative method called droplet-confined alternate-pulsed epitaxy (DCAPE) for the self-catalyzed growth of GaAs nanowires and GaAs/AlxGa1-xAs axial nanowire heterostructures has been developed. DCAPE enables nanowire growth at unconventional, low temperatures in the range of 450-550 °C and is compatible with the standard Si-CMOS platform. The novel growth approach allows one to precisely control the crystal structure of the nanowires and, thus, to produce defect-free pure zinc blende GaAs-based nanowires. The strength of DCAPE is further highlighted by the controlled growth of GaAs/AlxGa1-xAs axial quantum well nanowires with abrupt interfaces and tunable thickness and Al-content of the AlxGa1-xAs sections. The GaAs/AlxGa1-xAs axial nanowire heterostructures are interesting for applications as single photon emitters with tunable emission wavelength, when they are overgrown with thick lattice-mismatched InxAl1-xAs layers in a core-shell fashion. All results presented in this thesis contribute to paving the way for a successful monolithic integration of highly uniform GaAs-based nanowires with controlled number density, dimensions and crystal structure on the mature Si platform.
GaAs-basierte Nanodrähte sind attraktive Bausteine für die Entwicklung von zukünftigen (opto)elektronischen Bauelementen dank ihrer exzellenten intrinsischen Materialeigenschaften wie zum Beispiel die direkte Bandlücke und die hohe Elektronenbeweglichkeit. Eine Voraussetzung für die Realisierung neuer Funktionalitäten auf einem einzelnen Si Chip ist die monolithische Integration der Nanodrähte auf der etablierten Si-Metall-Oxid-Halbleiter-Plattform (CMOS) mit präziser Kontrolle des Wachstumsprozesses der Nanodrähte. Das selbstkatalytische (Ga-unterstützte) Wachstum von GaAs Nanodrähten auf Si(111)-Substrat mittels Molekularstrahlepitaxie bietet die Möglichkeit vertikale Nanodrähte mit vorwiegend Zinkblende-Struktur herzustellen, während die potentielle Verunreinigung der Nanodrähte und des Substrats durch externe Katalysatoren wie Au vermieden wird. Obwohl der Wachstumsmechanismus gut verstanden ist, erweist sich die Kontrolle der Nukleationsphase, Anzahldichte und Kristallstruktur der Nanodrähte als sehr schwierig. Darüber hinaus sind relativ hohe Temperaturen im Bereich von 560-630 °C in konventionellen Wachstumsprozessen notwendig, die deren Anwendung auf der industriellen Si Plattform begrenzen. Die vorliegende Arbeit liefert zwei originelle Methoden um die bestehenden Herausforderungen in konventionellen Wachstumsprozessen zu bewältigen. Im ersten Teil dieser Arbeit wurde eine einfache Prozedur, bezeichnet als surface modification procedure (SMP), für die in situ Vorbehandlung von nativem-SiOx/Si(111)-Substrat entwickelt. Die Substratvorbehandlung mit Ga-Tröpfchen und zwei Hochtemperaturschritten vor dem Wachstumsprozess ermöglicht eine synchronisierte Nukleation aller Nanodrähte auf ihrem Substrat und folglich das Wachstum von sehr gleichförmigen GaAs Nanodraht-Ensembles mit einer sub-Poisson Verteilung der Nanodrahtlängen. Des Weiteren kann die Anzahldichte der Nanodrähte unabhängig von deren Abmessungen und ohne ex situ Vorstrukturierung des Substrats über drei Größenordnungen eingestellt werden. Diese Arbeit liefert außerdem ein grundlegendes Verständnis zur Nukleationskinetik von Ga-Tröpfchen auf nativem-SiOx und deren Wechselwirkung mit SiOx und bestätigt theoretische Voraussagen zum sogenannten Nukleations-Antibunching, dem Auftreten einer zeitlichen Anti-Korrelation aufeinanderfolgender Nukleationsereignisse. Im zweiten Teil dieser Arbeit wurde eine alternative Methode, bezeichnet als droplet-confined alternate-pulsed epitaxy (DCAPE), für das selbstkatalytische Wachstum von GaAs Nanodrähten und GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen entwickelt. DCAPE ermöglicht das Nanodrahtwachstum bei unkonventionell geringeren Temperaturen im Bereich von 450-550 °C und ist vollständig kompatibel mit der Standard-Si-CMOS-Plattform. Der neue Wachstumsansatz erlaubt eine präzise Kontrolle der Kristallstruktur der Nanodrähte und folglich das Wachstum von defektfreien Nanodrähten mit phasenreiner Zinkblende-Struktur. Die Stärke der DCAPE Methode wird des Weiteren durch das kontrollierte Wachstum von GaAs/AlxGa1-xAs axialen Quantentopf-Nanodrähten mit abrupten Grenzflächen und einstellbarer Dicke und Al-Anteil der AlxGa1-xAs-Segmente aufgezeigt. Die GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen sind interessant für den Einsatz als Einzelphotonen-Emitter mit einstellbarer Emissionswellenlänge, wenn diese mit gitterfehlangepassten InxAl1-xAs-Schichten in einer Kern-Hülle-Konfiguration überwachsen werden. Alle Ergebnisse dieser Arbeit tragen dazu bei, den Weg für eine erfolgreiche monolithische Integration von sehr gleichförmigen GaAs-basierten Nanodrähten mit kontrollierbarer Anzahldichte, Abmessungen und Kristallstruktur auf der industriell etablierten Si-Plattform zu ebnen.