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This thesis describes a series of experiments aimed at understanding the low-temperature electrical transport properties of semiconductor nanowires. The semiconductor nanowires (1-100 nm in diameter) are grown from nanoscale gold particles via a chemical process called vapor-liquid-solid (VLS) growth. The huge versatility of this material system (e.g. in size and materials) results in a wide range of potential applications in (opto-)electronics. During the last few years many important proofs of concept have already been provided like lasers, field-effect transistors, light emitting diodes, and biochemical sensors. Simultaneously, the versatility of semiconductor nanowires creates new opportunities for the study of quantum transport phenomena. The quantum mechanical properties of semiconductor nanowires become visible at low temperatures (below a few Kelvin) and can be very different from room-temperature transport properties. For instance, the confinement of electrons in a small nanowire segment results in a discrete electronic energy spectrum forming a quantum dot, or artificial atom.

For the first time, an analytical model of arbitrarily shallow p-n junctions is presented. Depending on the junction depth, electrical characteristics of ultrashallow p-n junctions can vary from the characteristics of standard Schottky diodes to standard deep p-n junctions. This model successfully unifies the standard Schottky and p-n diode expressions. In the crossover region, where the shallow doping region can be totally depleted, electrical characteristics phenomenologically substantially different from typical diode characteristics are predicted. These predictions and the accuracy of the presented model are evaluated by comparison with the MEDICI simulations. Furthermore, ultrashallow n+-p diodes were fabricated, and the anomalous behavior in the crossover regime was experimentally observed.

The morphological evolution of both pits and SiGe islands on patterned Si(001) substrates is investigated. With increasing Si buffer layer thickness the patterned holes transform into multifaceted pits before evolving into inverted truncated pyramids. SiGe island formation and evolution are studied by systematically varying the Ge coverage and pit spacing and quantitative data on the influence of the pattern periodicity on the SiGe island volume are presented. The presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes.

In high-tech industries, projects play a central role in the development of new products and processes. Since these projects can be quite complex, it would be useful to look at where complexity in projects comes from and how these complexities influence these projects. The research project described in this thesis aims to increase the understanding of this topic in a company in the semiconductor industry, NXP Semiconductors. Products that are produced in the semiconductor industry play an increasingly important role in our lives. Products ranging from mobile telephones to cars to lighting are all equipped with semiconductor products and the performance of these products is steadily increasing with time. Therefore, the development of these products and the processes, which are needed to produce the products, are becoming more complex. The research project To describe the complexities that are encountered in the process engineering industry, a framework (the TOE framework, where TOE stands for Technical, Organizational and External) was developed earlier. The TOE framework consists of 47 elements (which are categorized into the three aforementioned categories) that describe aspects that can contribute to a project’s complexity. To understand the complexities that are encountered in the semiconductor industry, this research project has applied the TOE framework to this industry. The goal of this research project is twofold: firstly, it aims to understand where complexities in projects in the semiconductor industry come from; secondly, it would be useful for the company if the company would be able to understand which complexities could play a role in a future project and this could be used to come with the right measures to cope with these complexities. The main research question that this research project aims to answer is: What benefits does the application of the TOE framework provide for projects at NXP? This question is answered by doing a combination of desk research and case studies on projects in the company. In the desk research phase, the current practice related to development projects at NXP is investigated and a tool is described that calculates the design complexity of a new product design: Numetrics. Case studies To understand what complexities play a role in NXP projects, 16 projects have been investigated. The projects that are studied are from a wide variety of departments within NXP, but all projects (except for one) involve development of a new product or process. From each project, one person (the project manager) was interviewed. During the interviews, these project managers were asked to explain what the project entailed, which complexities were encountered during the projects, what influence these complexities had on the project and how the project managers coped with the complexities. The project managers were asked to indicate too which degree the complexities in the TOE were applicable to the project and if any complexities were missing in the framework. The complexities in the TOE framework that scored highest on the TOE scoring list are: - Involvement of different technical disciplines - Technical risks - High project schedule drive - Level of competition These high scoring complexities reflect the image that development projects in the semiconductor industry require multidisciplinary teams, that technical risks are often high (since it is not always known whether certain solutions will provide the required functionality), that there is high drive to develop new products quickly and that there is a high level of competition on the market. Next to the complexities that are directly related to the TOE framework, interviewees were also asked to share their views and experiences with the Numetrics system. A number of observations and recommendations with respect to this system are presented in this thesis. Adaptation and application of TOE From the original TOE framework, five elements were considered to be not applicable to the projects by the interviewees. These elements are not present the adapted version of the TOE framework for use at NXP. The interviewees also indicated that a number of complexities were missing or not precisely enough described in the TOE framework. In total 13 elements are added to the existing framework (4 technical, 5 organizational and 4 external elements). These adaptations lead to a new version of the TOE framework, that is modified to meet the situation at NXP. A preliminary version of a score chart is made, on which a project manager can indicate which complexities could be present in a project that is under planning. However, further research would be needed to successfully implement the application of TOE in practice. A possible way of applying TOE would be to use a similar approach that is taken by Numetrics – a tool that is currently being used at NXP to assess the design complexity of product developments. Since the relevance of Numetrics is limited to product development projects, the TOE approach would have an added value by also addressing other types of projects, such as process developments. Next to the version of TOE that is adapted to meet the needs of NXP, a suggestion is made for the inclusion of four new elements, which were the result of the case studies that were performed in this research project. Conclusions The research project shows that the possible application of the TOE framework could have benefits for NXP. As the case studies show, complexities can have a large influence on the project’s execution and success. Therefore, a tool that can assess the complexity and sources of complexity of multiple types of (development) projects would be of value to the company. Recommendations Towards the company, a number of recommendations are given. Firstly, although the BCaM framework is of much value to the company, one of the outcomes of the interviews was that the time between gates can be quite long. This can have a negative effect on the focus in the project team and including more steps into the system would increase the focus in the team. Secondly, we believe that the implementation of TOE in the planning phase of projects can add value by giving the project manager insight into the types of complexities that are expected to be encountered in the project under planning.

Small signal gain measurements of optically excited terahertz silicon lasers are reported. Two types of lasers, Si:P and Si:Bi, were investigated. They were optically excited with radiation from a free electron laser or a CO2 laser. The experiments were performed with an oscillator-amplifier scheme where one sample serves as a laser while the other one is an amplifier. In case of the free electron laser the pump frequency corresponds to intracenter excitation of the 2p0 or 2p± states of the P and Bi Coulomb centers, and the gain was determined for the 2p0→1s(E), 2p0→1s(T2) transitions in Si:P and the 2p±→1s(E) transition in Si:Bi. Pumping with a CO2 laser leads to photoexcitation of the Coulomb centers. In this case the gain was determined for the 2p0→1s(T2) of Si:P transition. The gain for intracenter pumping is in the range 5−10 cm−1 while for photoexcitation the gain is considerably less, namely ∼ 0.5 cm−1. The experimental results are analyzed and found to be in good agreement with theoretical calculations based on balance equations.

An expression is derived for the perturbative response of a lumped resonance circuit to a sudden change in the circuit parameters. This expression is shown to describe also the photo-induced conductivity of a semiconductor mounted in a single-mode microwave cavity. The power dissipated in the cavity is measured in the two dimensions corresponding to time (after photo-excitation of the sample) and frequency (of the microwave driving source). Analysis of the experimental data for different semiconductor materials demonstrates the general applicability of the presented analytical expression here, by retrieving the time dependence of the sample's photo-induced conductivity.

We report voltage dependent photoluminescence experiments on single indium arsenide phosphide (InAsP) quantum dots embedded in vertical surround-gated indium phosphide (InP) nanowires. We show that by tuning the gate voltage, we can access different quantum dot charge states. We study the anisotropic exchange splitting by polarization analysis, and identify the neutral and singly charged exciton. These results are important for spin addressability in a charge tunable nanowire quantum dot.

A procedure has been implemented for a quantitative aluminum-doping profiling of µm-scale aluminum-induced solid-phase-epitaxy (SPE) Si islands formed at 400°C. The aluminum concentration was measured to be 1–2×1019 cm−3, which is about 10 times higher than previously reported electrical activation levels. The elemental concentration was measured by secondary-ion-mass-spectroscopy (SIMS) on arrays of SPE Si islands grown by a recently developed process that allows control of the island geometry.

We report the experimental demonstration of single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires is fabricated by epitaxial growth on an electron-beam patterned substrate. Photoluminescence spectra taken on single quantum dots show narrow emission lines. Superconducting single photon detectors, which have a higher sensitivity than avalanche photodiodes in the infrared, enable us to measure auto and cross correlations. Clear antibunching is observed [g(2)(0) = 0.12] and we show a biexciton–exciton cascade, which can be used to create entangled photon pairs.

In this thesis we study charge transport in organic semiconductors. We do this by focusing on the physical characterization of disordered organic field-effect transistors. It will be made clear that the disorder in the polymer films is crucial for the interpretation of the data. The field-effect transistor geometry allows variation of the charge carrier density in the semiconductor, without the presence of counter ions. Therefore, the transistor allows a rather clean study of the charge transport in organic semiconductors as a function of the charge carrier density and temperature. In the experiments we find that the organic transistors are in several respects not comparable to silicon MOSFETs. Therefore, in this thesis we redefine and re-evaluate basic transistor parameters, such as the threshold voltage, the field-effect mobility, the contact resistance and the dopant density. Subsequently, we study the charge transport as a function of charge density, temperature and electric field, giving insight into the charge transport mechanism. Based on our observations we propose as the main charge transport mechanism: multiphonon hopping of polaronic charge carriers in a Gaussian density of states. We investigate the electrical stability of the polymer layer in metal-insulator-semiconductor diodes, where we determine and analyse the dopant density changes as a function of oxygen and light exposure. The presence of contact resistances in the transistors is addressed by analysing the scaling behavior of the electrical characteristics as a function of the transistor channel length, and an empirical relation between the contact resistance and the charge carrier mobility in the polymer layer is observed. Finally, we discuss why typically only unipolar transistor behavior is observed experimentally, and we demonstrate ambipolar transistor behavior in organic field-effect transistors based on blends of organic semiconductors and on low band gap organic semiconductors.

The catalyst-assisted growth of semiconductor nanowires heterostructures offers a very flexible way to design and fabricate single photon emitters. The nanowires can be positioned by organizing the catalyst prior to growth. Single quantum dots can be formed in the core of single nanowires which can then be easily isolated and addressed to generate single photons. Diameter and height of the dots can be controlled and their emission wavelength can be tuned at the optical telecommunication wavelengths by the material composition. The final morphology of a wire can be shaped by the radial/axial growth ratio, offering the possibility to form single mode optical waveguides with a tapered end for efficient photon collection.

We report on the fabrication and characterization of field-effect transistors based on single crystals of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF). We demonstrate that it is possible to prepare very-good-quality DB-TTF crystals from solution. These devices show high field-effect mobilities typically in the range 0.1–1 cm2/V s. The temperature dependence was also studied revealing an initial increase of the mobility when lowering the temperature until it reached a maximum, after which the mobility decreased following a thermally activated behavior with activation energies between 50 and 60 meV. Calculations of the molecular reorganization energy and intermolecular transfer integrals for this material were also performed and are in agreement with the high mobility observed in this material.

Using single-crystal transistors, we have performed a systematic experimental study of electronic transport through oxidized copper/rubrene interfaces as a function of temperature and bias. We find that the measurements can be quantitatively reproduced in terms of the thermionic emission theory for Schottky diodes, if the effect of the bias-induced barrier lowering is included. Our analysis emphasizes the role of the coupling between metal and molecules, which in our devices is weak due to the presence of an oxide layer at the surface of the copper electrodes.

We investigated transport properties of organic heterointerfaces formed by single-crystals of two organic donor-acceptor molecules, tetramethyltetraselenafulvalene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Whereas the individual crystals have unmeasurably high resistance, the interface exhibits a resistivity of few tens of megohm with a temperature dependence characteristic of a small gap semiconductor. We analyze the transport properties based on a simple band diagram that naturally accounts for our observations in terms of charge transfer between two crystals. Together with the recently discovered tetrathiafulvalene–TCNQ interfaces, these results indicate that single-crystal organic heterostructures create functional electronic systems with properties relevant to both fundamental and applied fields.

Semiconductor metal oxide films of copper-doped tin oxide (Cu-SnO2), tungsten oxide (WO3) and indium oxide (In2O3) were deposited on a platinum coated alumina substrate employing the electrostatic spray deposition technique (ESD). The morphology studied with scanning electron microscopy (SEM) and atomic force microscopy (AFM) shows porous homogeneous films comprising uniformly distributed aggregates of nano particles. The X-ray diffraction technique (XRD) proves the formation of crystalline phases with no impurities. Besides, the Raman cartographies provided information about the structural homogeneity. Some of the films are highly sensitive to low concentrations of H2S (10 ppm) at low operating temperatures (100 and 200 °C) and the best response in terms of Rair/Rgas is given by Cu-SnO2 films (2500) followed by WO3 (1200) and In2O3 (75). Moreover, all the films exhibit no cross-sensitivity to other reducing (SO2) or oxidizing (NO2) gases.

We characterize a heterodyne receiver based on a surface-plasmon waveguide quantum cascade laser (QCL) emitting at 2.84 THz as a local oscillator, and an NbN hot electron bolometer as a mixer. We find that the envelope of the far-field pattern of the QCL is diffraction-limited and superimposed onto interference fringes, which are similar to those found in narrow double-metal waveguide QCLs. Compared to the latter, a more directional beam allows for better coupling of the radiation power to the mixer. We obtain a receiver noise temperature of 1050 K when the mixer is at 2K, which, to our knowledge, is the highest sensitivity reported at frequencies beyond 2.5 THz.

Crystalline silicon is probably the best studied material, widely used by the semiconductor industry. The subject of this thesis is an intriguing form of this element namely amorphous silicon. It can contain a varying amount of hydrogen and is denoted as a-Si:H. It completely lacks the neat long range order of the crystal, yet its structure is not random. Almost all silicon atoms have four neighbors and the average bond angle is identical to the tetrahedral angle in the crystal. Order is thus preserved over several bond lengths. The motivations to study a-Si:H are two-fold. Firstly some of its properties are different from the crystalline form and we do not understand them completely. For example, the electronic properties degrade after exposure to intense light, but can be recovered reversibly by heat treatment. The microscopic process of this is not known. Secondly, research on a-Si:H is motivated by its applications. These are mostly large area devices such as liquid crystal displays and solar cells. The latter are in use already today, the former are waiting to be widely used in future. Amorphous semiconductors can be deposited over large areas from vapor. On the other hand, the size of c-Si devices is limited by the much smaller size of the wafers. The production of a-Si:H is also cheaper and consumes less energy. Unlike its crystalline counterpart a-Si:H has a direct band gap, leading to an increased light absorption. Consequently, a-Si:H solar cells are ~ 1000 times thinner than c-Si cells, resembling more a foil than a semiconductor device. The methods used in the thesis are computational, largely relying on algorithms and powerful computers. The structural models are atomistic, where the interaction between electrons and nuclei is treated on the level of Density Functional Theory. This is a first-principles methods, meaning that it does not use any adjustable parameters. The chemical bonding, even of complex structures is described accurately. Calculation of total energies and forces allows us to find equilibrium structures and perform molecular dynamics calculations. The models of a-Si:H are prepared by cooling a melt to room temperature. This method resembles the preparation of glasses. We find that the structure is strongly in influenced by the cooling rate. Using slower cooling rates we improved existing models that contained excessive strain and a high defect concentration. Using a cooling rate of ~ 0.02 K/fs we were even able to prepare small defect-free models. The structure was in good agreement with available neutron scattering data. Calculated density of states shows a pronounced band gap. After the generation of structural models we turn our attention to defects. Defects in an amorphous solid are defined as atoms that deviate from the normal coordination. We find 3-fold and 5-fold coordinated Si atoms and 2-fold coordinated H atoms. We focus only on the 3-fold coordinated Si, also called the dangling bond (DB), that is believed to be the major defect in a-Si:H. We have calculated formation of the DB defect in the negative, neutral and positive charge state. By averaging over 25 distinct DB models we find a considerable spread in the energies of 0.2 eV. Another related property of a defect is its correlation energy U. A positive value of U means that we have to invest energy to add an extra electron to the defect. The size and sign of U are still a subject of controversy. On average we find a positive U value of 0.1 eV. Four models, however, have a negative correlation energy, suggesting large relaxations in the defect structure. Amorphous silicon readily forms compounds with nitrogen and carbon. We have investigated silicon-rich nitride (a-SiN:H) at two different densities of 2.0 and 3.0 g/cm3. Features in the pair-distribution functions can be related to "square structures". These are planar structures consisting of two Si in opposite corners of a square and two N in the remaining corners. The dense phase shows signs of phase separation into silicon and stoichiometric nitride. Both valence and conduction band edges are dominated by Si states. This is corroborated by the fact that by increasing the nitrogen content the band gap of the nitride can be varied from 1.8 to 5.3 eV. Recently there has been a considerable interest in man-made materials. Examples are multilayers (ML) formed by two semiconductors with a different band gap. By adjusting the thickness of the small band gap material (the well) one can tune the band gap of the ML due to quantum confinement effects. This concept is well established in crystalline semiconductors. The existence of quantum confinement in amorphous structures is, however, being still debated. Using models prepared previously we have constructed a model of a silicon/nitride ML. This allowed us to study confinement effects directly without using transport or optical measurements that can obscure the observations. Comparing our model to an experimental system with the same composition gave almost identical band gaps. This confirmed the existence of quantum confinement in a amorphous multilayer. The calculation of band offsets between the materials revealed that there is almost no barrier for the electrons and the confinement originated solely from holes.

Semiconductor nanocrystals (NCs), which can have a variety of sizes, shapes and chemical compositions, will be a large and important family of future advanced materials.This thesis focuses on colloidal semiconductor NC solids, also called quantum-dot (QD) solids, which are promising materials for many applications, such as photo-detectors, field-effect transistors, solar cells, light-emitting diodes, and lasers. The thesis presents studies on the charge carrier properties of PbSe QD solids, going through the charge carrier photogeneration, thermalization, diffusion and decay, which together are the ``life and fate'' of the charge carriers. Diverse tools have been utilized to reveal the whole picture of the ``life and fate''. The most important ones are: femtosecond transient absorption spectroscopy (TA) (Chapter 2, 3, 4), picosecond Terahertz spectroscopy (THz) (Chapter 2), the nanosecond time-resolved microwave conductivity technique (TRMC) (Chapter 2-5), and Monte Carlo simulations (Chapter 4).