Epitaxial Growth, Processing and Characterization of Semiconductor Nanostructures

This thesis deals with the growth, processing and characterization of nano-sized structures, eg., self-assembled quantum dots and nano-wires. Such structures are promising candidates for the realization of nano-scale electronic and optical devices, like for instance single electron transistors, resonant tunneling devices, and single photon emitters. For such purposes, the main focus of this work has been on the controlled growth of self-assembled quantum dots. For epitaxy, which is the fundament of this work, low-pressure metal organic vapor phase epitaxy (MOVPE) and ultra high vacuum chemical vapor deposition (UHV-CVD) were used. The structures grown were composed of III/V materials, and SiGe/Si was used for some experiments. For the first group of structures, fundamental investigations on quantum dot growth enabled in-situ growth of InAs/InP self-assembled quantum dot samples in MOVPE. These studies were carried out on freestanding as well as epitaxially overgrown dots. Topography and photo-luminescence were measured with atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FTIR) respectively. InAs/InP low-density quantum dot samples were grown in single or multiple layers, suitable for electrical measurements. These structures were studied by electrical characterization (IV), transmission electron microscopy (TEM), and cross sectional scanning tunneling microscopy (STM). Resonant tunneling through these quantum dots was observed, with peak-to-valley ratios as high as 1300 and negative differential resistance up to a point above the temperature of liquid nitrogen. For the second, more complex, group of structures, patterns on semiconductor surfaces were created, either by electron beam lithography and wet chemical etching, or by the partial overgrowth of electron beam induced carbonaceous material. Spatially ordered growth of III/V and SiGe/Si quantum dots on such patterns was studied by AFM. For the InAs/InP system, conditions were found for which dots could be grown selectively in the patterns by the use of As-P exchange reactions. For the SiGe/Si system, commonly quadruples of islands were observed around each pit. The third group of structures was grown from size selected gold particles, deposited in-house in an aerosol machine, or from Au colloids that were dispersed on the semiconductor surface. These gold particles enabled vapor-liquid-solid (VLS) growth of highly anisotropic one-dimensional structures that were characterized by scanning electron microscopy

Towards high efficiency nanowire solar cells

Semiconductor nanowires are a class of materials recently gaining increasing interest for solar cell applications. In this article we review the development of the field with a special focus on the III-V materials due to their potential to reach high power conversion efficiencies. After introducing basic concepts of nanowire synthesis, we discuss important aspects of nanowire design for high power conversion efficiencies; first in terms of light absorption, then in terms of charge carrier separation and collection. Further, we examine methods to assess and understand the materials quality and the solar cell performance. We end the review by a discussion of strategies and challenges in achieving efficiencies above the Shockley-Queisser limit, and the potential for cost efficient production

InxGa1-xP Nanowire Growth Dynamics Strongly Affected by Doping Using Diethylzinc

Semiconductor nanowires are versatile building blocks for optoelectronic devices, in part because nanowires offer an increased freedom in material design due to relaxed constraints on lattice matching during the epitaxial growth. This enables the growth of ternary alloy nanowires in which the bandgap is tunable over a large energy range, desirable for optoelectronic devices. However, little is known about the effects of doping in the ternary nanowire materials, a prerequisite for applications. Here we present a study of p-doping of InxGa1-xP nanowires and show that the growth dynamics are strongly affected when diethylzinc is used as a dopant precursor. Specifically, using in situ optical reflectometry and high-resolution transmission electron microscopy we show that the doping results in a smaller nanowire diameter, a more predominant zincblende crystal structure, a more Ga-rich composition, and an increased axial growth rate. We attribute these effects to changes in seed particle wetting angle and increased TMGa pyrolysis efficiency upon introducing diethylzinc. Lastly, we demonstrate degenerate p-doping levels in InxGa1-xP nanowires by the realization of an Esaki tunnel diode. Our findings provide insights into the growth dynamics of ternary alloy nanowires during doping, thus potentially enabling the realization of such nanowires with high compositional homogeneity and controlled doping for high-performance optoelectronics devices

Diameter-dependent conductance of InAs nanowires

Electrical conductance through InAs nanowires is relevant for electronic applications as well as for fundamental quantum experiments. Here we employ nominally undoped, slightly tapered InAs nanowires to study the diameter dependence of their conductance. Contacting multiple sections of each wire, we can study the diameter dependence within individual wires without the need to compare different nanowire batches. At room temperature we find a diameter-independent conductivity for diameters larger than 40 nm, indicative of three-dimensional diffusive transport. For smaller diameters, the resistance increases considerably, in coincidence with a strong suppression of the mobility. From an analysis of the effective charge carrier density, we find indications for a surface accumulation layer.Comment: 9 pages, 5 figure

Doping Incorporation in InAs nanowires characterized by capacitance measurements

Sn and Se doped InAs nanowires are characterized using a capacitance-voltage technique where the threshold voltages of nanowire capacitors with different diameter are determined and analyzed using an improved radial metal-insulator-semiconductor field-effect transistor model. This allows for a separation of doping in the core of the nanowire from the surface charge at the side facets of the nanowire. The data show that the doping level in the InAs nanowire can be controlled on the level between 2×1018 to 1×1019 cm−3, while the surface charge density exceeds 5×1012 cm−2 and is shown to increase with higher dopant precursor molar fraction

Road Haulier Competition: Implications for Supply Chain Integration

Road freight competition is playing out in deregulated markets. The EU single market is a market with abundant responses in terms of haulier strategic actions. This chapter situates the crucial role of road haulier strategies in the logistics service supply chain and industrial supply chain to achieve sustainability. Competitive and sustainable transport depends on effective transport services, vehicles and transport infrastructure, and conditions that foster the development of transport and logistics services. By examining how four case firms develop competences and make use of available resources we develop insights into road haulier competition and its implications. The chapter contributes to understanding how road hauliers are part of logistics service chains as well as industrial supply chains and how the many links and relationships increase the magnitude and implications of hauliers’ performances

A Fully Abstract Symbolic Semantics for Psi-Calculi

We present a symbolic transition system and bisimulation equivalence for psi-calculi, and show that it is fully abstract with respect to bisimulation congruence in the non-symbolic semantics. A psi-calculus is an extension of the pi-calculus with nominal data types for data structures and for logical assertions representing facts about data. These can be transmitted between processes and their names can be statically scoped using the standard pi-calculus mechanism to allow for scope migrations. Psi-calculi can be more general than other proposed extensions of the pi-calculus such as the applied pi-calculus, the spi-calculus, the fusion calculus, or the concurrent constraint pi-calculus. Symbolic semantics are necessary for an efficient implementation of the calculus in automated tools exploring state spaces, and the full abstraction property means the semantics of a process does not change from the original

Implementing an Insect Brain Computational Circuit Using III–V Nanowire Components in a Single Shared Waveguide Optical Network

Recent developments in photonics include efficient nanoscale optoelectronic components and novel methods for sub-wavelength light manipulation. Here, we explore the potential offered by such devices as a substrate for neuromorphic computing. We propose an artificial neural network in which the weighted connectivity between nodes is achieved by emitting and receiving overlapping light signals inside a shared quasi 2D waveguide. This decreases the circuit footprint by at least an order of magnitude compared to existing optical solutions. The reception, evaluation and emission of the optical signals are performed by a neuron-like node constructed from known, highly efficient III-V nanowire optoelectronics. This minimizes power consumption of the network. To demonstrate the concept, we build a computational model based on an anatomically correct, functioning model of the central-complex navigation circuit of the insect brain. We simulate in detail the optical and electronic parts required to reproduce the connectivity of the central part of this network, using experimentally derived parameters. The results are used as input in the full model and we demonstrate that the functionality is preserved. Our approach points to a general method for drastically reducing the footprint and improving power efficiency of optoelectronic neural networks, leveraging the superior speed and energy efficiency of light as a carrier of information.Comment: 28 pages, 6 figures; supplementary information 15 pages, 8 figure