Fabrication, characterisation and modelling of electronic devices based on amorphous metal-oxide and carbonaceous materials

This thesis describes the deposition, characterisation and device applications of amorphous zinc tin oxide (a-ZTO) and carbon-based materials using energetic deposition techniques. The study aimed to explore the suitability of this growth method in producing highquality carbon and a-ZTO films for device applications. Technology Computer Aided Design (TCAD) simulations were also conducted in tandem with experiments to identify critical parameters affecting device behaviour and to provide guidance for improved device performance. Firstly, electrical carbon contacts to n-type 6H-SiC were energetically deposited from a filtered cathodic vacuum arc (FCVA) at room temperature and elevated temperature with low and high biases. Lower energy (< 100 eV) in the carbon flux resulted in resistive amorphous carbon contacts. As the deposition energy and sp2 bonding (graphitic) fraction were increased, an oriented graphitic microstructure developed and rectifying electrical characteristics emerged. TCAD simulations revealed the effects of interfacial layers and contact work functions on the device performance and suggested that the rectification ratios of C/6H-SiC Schottky diodes could be increased by improving the lateral homogeneity of the junctions and/or by controlling the thickness of interfacial layers. Secondly, thin films of amorphous zinc tin oxide (a-ZTO) were energetically deposited using high power impulse magnetron sputtering (HiPIMS). HiPIMS and DC magnetron sputtering modes were enabled to co-deposit an a-ZTO layer with Zn:Sn ratio that varied laterally across a 4-inch diameter sapphire substrate. Electrical, structural and optical properties of the films were investigated as a function of composition. The as-deposited films were found to be amorphous, transparent and highly resistive with little variation in the Zn:Sn ratios. Annealing in the presence of hydrogen yielded improved film conductivity and measured carrier concentrations of ~ 1017 cm-3 . Hall mobilities of up to 13 cm2 /V.s were also measured in the ntype films. These findings suggest that HiPIMS can produce dense, high quality a-ZTO suitable for device applications. As a transparent amorphous conducting oxide with high transparency and good electron mobility, a-ZTO has proven applications in interconnects and thin film transistors. In this thesis, the potential for this material in ‘next-generation’ signal processing devices is discussed. Specifically, the ability of the material to support resistive switching and memristive phenomena was investigated in lateral memristors on HiPIMS a-ZTO. The transport mechanisms and conductance of Ag/a-ZTO memristors were found to depend on prior activity and on the imposed current limit, mimicking biology synaptic plasticity. After microscopy, the switching mechanism was attributed to nanoscale filaments formed between the electrodes. These filaments were subject to Rayleigh instability and exhibited relaxation times determined by their effective radii

Self-catalyzed and catalyst-free III-V semiconductor nanowire grown by CBE

In this thesis, the growth dynamics and mechanisms of III-V semiconductor nanowires (NWs) and their heterostructures are studied. III-V NWs are realized by self-catalyzed and catalyst-free growth methods on Si (111) substrates by means of chemical beam epitaxy. The Au-free growth approach is particularly important for the integration of III-V semiconductors on silicon toward a CMOS-compatible electronics. The morphological and structural properties of the grown NWs are investigated by scanning (SEM) and transmission electron microscopy (TEM). These NWs exhibit very high aspect ratio and good material quality, which makes them useful to be employed for fundamental studies as well as for application in electronics and optoelectronics. The first part of the thesis is focused on the growth of InAs/InP/GaAsSb core-dual-shell (CDS) NWs. Detailed morphological, structural, and compositional analyses of the NWs as a function of growth parameters are carried out by SEM, TEM, and by energy-dispersive X-ray spectroscopy. Furthermore, by combining the scanning transmission electron microscopy-Moir\ue9 technique with geometric phase analysis, we studied the residual strain and the relaxation mechanisms in this system. We found that InP shell facets are well-developed along the crystallographic <110> and <112> directions only when the nominal thickness is above 1 nm, suggesting an island-growth mode. Moreover, the crystallographic analysis indicates that both InP and GaAsSb shells grow almost coherently to the InAs core along the \u27e8112\u27e9 direction and elastically compressed along the \u27e8110\u27e9 direction. For an InP shell thickness above 8 nm, some dislocations and roughening occur at the interface. This study provides useful general guidelines for the fabrication of high-quality devices based on these CDS NWs. Indeed, we investigated the tunnel coupling between the outer p-type GaAsSb shell and the n-type InAs core in InAs/InP/GaAsSb CDS NWs. Low-temperature (4.2 K) transport measurements in the shell-shell configuration in CDS NWs with 5 nm-thick InP barrier reveal a weak negative differential resistance. Differently, when the InP barrier thickness is increased to 10 nm, this negative differential resistance is fully quenched. The electrical resistance between the InAs core and the GaAsSb shell, measured in core-shell configuration, is significantly higher with respect to the resistance of the InAs core and of the GaAsSb shell. The field effect, applied via a back-gate, has an opposite impact on the electrical transport in the core and in the shell portions. Our results show that electron and hole free carriers populate the InAs and GaAsSb regions respectively and indicate InAs/InP/GaAsSb CDS NWs as an ideal system for the investigation of the physics of interacting electrons and holes at the nanoscale. The second part of this thesis is dedicated to the growth of self-catalyzed InAs/InSb axial heterostructures. The growth mechanisms of these heterostructures are thoroughly investigated as a function of the In and Sb line pressures, and growth time. Some interesting phenomena are observed and analysed. In particular, the presence of an In droplet on top of the InSb segment is shown to be essential to form axial heterostructures in the self-catalyzed vapor-liquid-solid mode. Axial versus radial growth rates of InSb segments are investigated under different growth conditions and described within a dedicated model containing no free parameters. It is shown that a widening of the InSb segment with respect to the InAs stem is caused by the vapor-solid growth on the nanowire sidewalls rather than by the droplet swelling. The In droplet can even shrink smaller than the nanowire facet under Sb-rich conditions. The third part of the thesis is focused on the realization of self-catalyzed InSb quantum dot (QD) embedded into InAs NW. A systematic study on the influence of the growth parameters on the morphology of such NWs is performed. Radial and axial growth rates are studied as a function of growth parameters in order to realize InSb QD NW with controlled morphology. In particular, we have explored different growth conditions to minimize the InAs shell around the InSb QD. We found that the shell thickness around the InSb QD decreases with increasing growth temperature while it increases with an increase of the As line pressure. Furthermore, from the high resolution-TEM analysis, we observed that InAs-stem and InAs-top segment have a wurtzite (WZ) crystal structure with several defects such as stacking faults and twins perpendicular to the growth direction. It is commonly observed that the InAs NWs grown by catalyst-free and self-catalyzed growth methods show highly defective (or mixed WZ/ZB) crystal structure. By contrast, here the InSb QD shows a defect-free zincblende (ZB) crystal structure without any stacking faults, consistently with the energetically preferred cubic structure of the InSb crystals generally attributed to the low ionicity of group III to Sb bonds. This study gives useful information for the realization of InSb QDs with controlled morphology and optimized quality embedded in InAs NWs in the self-catalyzed regime

Quantum Transport in Mesoscopic Systems

Mesoscopic physics deals with systems larger than single atoms but small enough to retain their quantum properties. The possibility to create and manipulate conductors of the nanometer scale has given birth to a set of phenomena that have revolutionized physics: quantum Hall effects, persistent currents, weak localization, Coulomb blockade, etc. This Special Issue tackles the latest developments in the field. Contributors discuss time-dependent transport, quantum pumping, nanoscale heat engines and motors, molecular junctions, electron–electron correlations in confined systems, quantum thermo-electrics and current fluctuations. The works included herein represent an up-to-date account of exciting research with a broad impact in both fundamental and applied topics

The development of micropillars and two-dimensional nanocavities that incorporate an organic semiconductor thin film

Photonic crystals (PC) are periodic optical structures containing low and high refractive index layers that influence the propagation of electromagnetic waves. Photonic cavities can be created by inserting defects into a photonic crystal. Such structures have received significant attention due to their potential of confining light inside volumes (V) smaller than a cubic wavelength of light (λ/n)3 which can be used to enhance light-matter interaction. Cavity quality factor (Q) is useful for many applications that depend on the control of spontaneous emission from an emitter such quantum optical communication and low-threshold lasing. High Q/V values can also result in an enhancement of the radiative rates of an emitter placed on the surface of the cavity by means of the Purcell effect. This thesis concerns the fabrication and study of two types of optical cavity containing an organic-semiconductor material. The cavities explored are; (1) one-dimensional micropillar microcavities based on multilayer films of dielectric and organic materials, and (2) two-dimensional nanocavities defined into a photonic crystal slab. Firstly, light emission from a series of optical micropillar microcavities containing a thin fluorescent, red-emitting conjugated polymer film is investigated. The photoluminescence emission from the cavities is characterized using a Fourier imaging technique and it is shown that emission is quantised into a mode-structure resulting from both vertical and lateral optical confinement within the pillar. We show that optical-confinement effects result in a blue-shift of the fundamental mode as the pillar-diameter is reduced, with a model applied to describe the energy and distribution of the confined optical modes. Secondly, simulation, design, and analysis of two dimensional photonic crystal L3 nanocavities photonic crystal are presented. Nanocavities were then prepared from silicon nitride (SiN) as the cavity medium with the luminescence emitted from an organic material at red wavelengths that was coated on the cavity surface. To improve the quality factor of such structures, hole size, lattice constant and hole shift are systematically varied with their effect as cavity properties determined. Finite Difference Time Domain (FDTD) modelling is used to support the experimental work and predict the optimum design for such photonic crystal nanocavity devices. It is found that by fine-tuning the nearest neighbour air-holes close to the cavity edges, the cavity Q factor can be increased. As a result, we have obtained a single cavity mode having a Q-factor 938 at a wavelength of 652 nm. Here, the cavity Q factor then increases to 1100 at a wavelength of 687 nm as a result of coating a red-emitting conjugated polymer film onto the top surface of the nanocavity. We propose that this layer planarizes the dielectric surface and helps reduce optical losses as a result of scattering

Cavity quantum electrodynamics with quantum dots in microcavities

Quantum dots (QDs) are nm-size semiconductor structures that hold promise for applications in quantum information. One important requirement, however, is to achieve near-unity interaction between photons and (singly charged) QDs. For this purpose, we make use of oxide-apertured micropillars that confine light in a small volume and thereby enhance the interaction. A QD transition coupled to the cavity mode can turn a transmittive cavity into a reflective one, and this property can be used to create entanglement between the spatial state of a photon and the spin state of a charge in the QD. This thesis consists of two parts: 1) in the first part we demonstrate techniques to monitor and fine tune the oxide aperture size and shape. By controlling the oxide shape we show we can fabricate polarization degenerate microcavities. 2) In the second part, perform cryogenic experiments with such a QD-cavity system. We investigate neutral and singly charged QDs as function of polarization and find this offers a way to assess the QD coherence. Next, we demonstrate a novel effect where charges around the QDs have a strong feedback with the QD properties. Finally, we present a homodyne detection technique of the QD coherence and phase shift.This work is part of the research programme of the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organisation for Scientific Research (NWO)Quantum Matter and Optic

Microscopy and Analysis

Microscopes represent tools of the utmost importance for a wide range of disciplines. Without them, it would have been impossible to stand where we stand today in terms of understanding the structure and functions of organelles and cells, tissue composition and metabolism, or the causes behind various pathologies and their progression. Our knowledge on basic and advanced materials is also intimately intertwined to the realm of microscopy, and progress in key fields of micro- and nanotechnologies critically depends on high-resolution imaging systems. This volume includes a series of chapters that address highly significant scientific subjects from diverse areas of microscopy and analysis. Authoritative voices in their fields present in this volume their work or review recent trends, concepts, and applications, in a manner that is accessible to a broad readership audience from both within and outside their specialist area

Charakterisierung funktionaler Nanomaterialien für biomagnetische Sensoren und Atemanalyse

The presented thesis is covering materials aspects for the development of magnetoelectric sensors for biomagnetic sensing and solid state sensors for breath monitoring. The electrophysiological signals of the human body and especially their irregularities provide extremely valuable information about the heart, brain or nerve malfunction in medical diagnostics. Similar and even more detailed information is contained in the generated biomagnetic fields which measurement offers improved diagnostics and treatment of the patients. A new type of room temperature operable magnetoelectric composite sensors is developed in the framework of the CRC1261 Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics. This thesis focuses on the individual materials structure-property relations and their combination in magnetoelectric composite sensors studied by electron beam based techniques, at lengths scales ranging from micrometers to atomic resolution. The first part of this thesis highlights selected studies on the structural and analytic aspects of single phase materials and their composites using TEM as the primary method of investigation. With respect to the piezoelectric phase, alternatives to AlN have been thoroughly investigated to seek for improvement of specific sensor approaches. In this context, the alloying of Sc into the AlN matrix has been demonstrated to yield high quality films with improved piezoelectric and unprecedented ferroelectric properties grown under the control of deposition parameters. Lead-free titanate films with large piezo-coefficients at the verge of the morphotropic phase boundary as alternative to PZT films have been investigated in terms of crystal symmetry, defect structure and domains of cation ordering. New morphologies of ZnO and GaN semiconductors envisioned for a piezotronic-based sensor approach were subject of in-depth defect and analytical studies describing intrinsic defects and lattice strains upon deposition as well as hollow composite structures. When the dimensions of a materials are reduced, novel exciting properties such as in-plane piezoelectricity can arise in planar transition-metal dichalcogenides. Here, the turbostratic disorder in a few-layered MoSe2 film has been investigated by nanobeam electron diffraction and Fast Fourier Transformations. From the perspective of magnetic materials, the atomic structure of magnetostrictive multilayers of FeCo/TiN showing stability up to elevated temperatures has been analyzed in detail regarding the crystallographic relationship of heteroepitaxy in multilayer composites exhibiting individual layer thicknesses below 1 nm. Further, magnetic hard layers have been investigated in the context of exchange spring concepts and ME composites based on shape memory alloy substrates have been studied regarding structural changes implied by different annealing processes. The second part of this thesis introduces materials aspects and sensor studies on gas detection in the clinical context of breath analysis. The detection of specific vapors in the human breath is of medical relevance, since certain species can be enriched depending on the conditions and processes within the human body. Hence, they can be regarded as biomarkers for the patients condition of health. The selection of suitable materials and the gas measurement working principle are considered and selected studies on solid state sensors with different surface functionalization or targeted application on basis of ZnO or CuO-oxide and Fe-oxide species are presented