Effect of Dopants at the Grain Boundary on Thermal Transport in β-SiC at High Temperatures

Advanced semiconductor materials for thermoelectric applications often comprise of nanostructured grains in order to take advantage of phonon scattering phenomenon at the grain boundaries and thus increase the thermoelectric figure of merit for the material. Opportunities for further improvements in the figure of merit are available via usage of appropriate dopants. In this report, thermal transport across low-angle, symmetric tilt grain boundaries in β-SiC is studied and the influence of dopants, introduced at these grain boundaries, on the phononic transmission across the grain boundary is investigated.Non-equilibrium molecular dynamics (NEMD) simulation are used to gain insights into the impact of grain-boundary segregation on Kapitza resistance of doped β-SiC at high-temperature. In particular, the role of dopant concentration and dopant/matrix interaction strength in determining the resistance is assessed. Dopants that adhere to the matrix material with the same strength as they adhere to other dopant atoms are determined to spread out across the grain boundary cross-section forming a layered structure and resulted in a concomitant gradual increase in resistance with increase in dopant concentration. Whereas, for relatively weak dopant/matrix interaction strengths, dopant clustering predominates, and the Kapitza resistance increases significantly for small changes in dopant concentration. The different interaction strength regimes are investigated by mapping the spatial distribution of temperature at the grain boundary cross-section and calculating the degree of structural disorder. It was found that the dopant clusters lead to a heat flux parallel to the grain boundary plane and a significant increase in boundary disorder, partly explaining the observed increase in Kapitza resistance at the boundary. A comparison of the local vibrational density of states for the weak and strong dopant/matrix interaction strength cases is performed and a subset of modes that are significant for thermal transport in this system are identified. It is determined that for the nano-structures studied, the loss of optical phonon modes that have typically been ignored for thermal transport analyses, resulted in a more significant increase in Kapitza resistance at the grain boundary. This analysis is complemented by calculations of the projected density of states and a corresponding eigenmode analysis of the dynamical matrix that highlight important phonon polarizations and propagation directions. We also examine the dependence of the Kapitza resistance on temperature, dopant mass and dopant/matrix interaction strength, the latter parameter affecting grain-boundary structure and, hence, phonon scattering.The study concludes with an investigation into the effects of grain boundary orientation and the local grain boundary energy on phonon scattering at the boundaries. More specifically, the impact of dopants on the interface resistance is examined for these boundaries. It is observed that for the methodology used to create the grain boundary systems, the interface resistance was independent of the grain boundary orientation irrespective of the dopant concentration. However, grain boundary energy had distinct effects on the interface resistance. Layering dopant caused an increase in disorder at the grain boundaries in higher energy system resulting in an increase in phonon scattering and therefore higher interface resistance

Innovative Growth Techniques and Heterogeneous Structures for PbSe-Based MWIR Photodetectors

Lead-chalcogenide semiconductor materials such as PbSe are an attractive class of narrow band-gap semiconductors due to their unique optical and electrical properties, finding their way into numerous mid-wave infrared (MWIR) optoelectronic and topological device applications. While PbSe is a mature material system with research efforts dating back almost a century, the full potential of its physical properties has yet to be realized in fabricated devices. Much of the difficulty lies in the unique bonding nature and crystal structure of PbSe films, which gives both advantageous MWIR optoelectrical properties along with a limited selection of suitable substrates for high-quality growth. Further, PbSe homojunction devices on dissimilar substrates face issues with doping, where diffusion of PbSe dopants through defect channels inhibit the formation of an abrupt junction resulting in degraded device performance. Further, p-n heterojunction formation with PbSe is difficult due to the small bandgap of PbSe, resulting in a shortlist of suitable materials with the corresponding band alignment to form a type-II heterojunction. Even for material systems that share a suitable band alignment, issues often arise with dissimilar crystallinity, lattice constant, and thermal expansion coefficient. The dissimilarity between these films introduces large stress/ strain relations at the interface, resulting in the formation of cracks or dislocations which ruin the interface and bulk electrical properties. For these reasons, PbSe-based MWIR devices have been surpassed by more competitive material systems such as II-VI HgCdTe, and Sb-based type-II superlattices. In this work, new methods for improving PbSe film quality are explored, along with the growth and design of new heterogeneous structures and MWIR photodetectors which may improve the PbSe-based MWIR sensing platform. Presented here will be a new approach for creating heterogeneous material structures with PbSe by molecular beam epitaxy (MBE). Demonstration of a new heterogenous p-n junction structure between mismatched germanium substrates and epitaxial lead selenide thin-films will be introduced utilizing a vicinal growth surface. Extending from this, epitaxial PbSe films with enhanced surface morphology will also be demonstrated on vicinal silicon substrates, showcasing record low surface defect densities compared to traditional growth on nominal silicon. Regarding the former, germanium will also serve as an active layer in the formation of a p-n heterojunction structure due to its suitable type-II band alignment with PbSe. However, large differences in lattice constant and thermal expansion coefficient will need to be addressed to form such a structure. These challenges are tackled by optimizing the surface kinetics of PbSe adatoms through high-temperature surface treatment, along with utilizing misfit accommodation steps induced by the periodic atomic step edges from the high degree of vicinal miscut of the germanium growth surface. Further, different structures and device applications of PbSe-based material systems will be explored, including the growth of a new PbOSe complex oxide thin-film via oxygen-plasma assisted MBE deposition. Fabrication of a single phase (cubic) all-epitaxial n-CdSe/p-PbSe heterojunction structure with room temperature MWIR detection capabilities will also be presented, along with the fabrication of MWIR transparent contacts using cadmium oxide thin-films for enhanced photodetector device design. The combined results of these efforts provide multiple avenues for the development of state-of-the-art MWIR PbSe-based photodetectors, enabling future commercial MWIR sensing capabilities with reduced size, weight, power consumption, and cost

Novel Metamaterials and Their Applications in Subwavelength Waveguides, Imaging and Modulation

The development of metamaterials has opened the door for engineering electromagnetic properties by subwavelength artificial atoms , and hence accessing new properties and functionalities which cannot be found among naturally occurring materials. In particular, metamaterials enable the flexibility of independently controlling the permittivity and permeability to be almost any arbitrary value, which promises to achieve deep subwavelength confinement and focusing of electromagnetic waves in different spectrum regimes. The next stage of this technological revolution will be focused on the development of active and controllable metamaterials, where the properties of the metamaterials are expected to be tuned by external stimuli. In this sense, some natural materials are also promising to provide the tunable capability, particularly in the near infrared and terahertz domains either by applying a voltage or shining light on the materials. The objective of this dissertation is to investigate novel metamaterials and explore three important applications of them: subwavelength waveguiding, imaging and modulation. The first part of this dissertation covers the theory, design and fabrication of several different types of metamaterials, which includes artificially designed metamaterials and some naturally existing materials. The second part demonstrates metal gratings functioning as designer surface plasmonic waveguides support deep subwavelength surface propagation modes at microwave frequency. The third part proposes multilayered metal-insulator stack as indefinite metamaterial that converts evanescent waves to propagating waves, hence deep subwavelength image can be observed. The fourth part explores the tunability of several natural materials - gallium (Ga), indium tin oxide (ITO) and graphene, and demonstrates electro-optical (EO) modulators based on these materials can be achieved on nano-scale. The final part summarizes the work presented in this dissertation and also discusses some future work for photodetection, photovoltaics, and modulation

Hybridization of Surface Plasmon Polaritons and Molecular Excitations

Starke Kopplung von Molekülen mit einem räumlich begrenzten Lichtfeld führt zur Bildung neuer polaritonischer Eigenzustände des Systems, die sowohl molekulare als auch photonische Eigenschaften erhalten und somit ein großes Potenzial für Anwendungen in der Chemie und Optoelektronik besitzen. In dieser Arbeit wird die Kopplung zwischen Oberflächenplasmonen Polaritonen (SPPs), die als das räumlich begrenzte Lichtfeld agieren, und molekularen Anregungen wie Schwingungen und polaronischen Resonanzen untersucht. Das starke Kopplungsregime zwischen einer Molekülschwingung und einem SPP wird zum ersten Mal im mittleren Infrarot unter Verwendung der Carbonylschwingung von Poly(vinylmethylketon) Polymer und Silber als Ausbreitungsmedium von SPPs demonstriert. Die neu gebildeten Hybridmoden werden durch Experimente und numerische Modellierung untersucht, wobei Messungen der abgeschwächten Totalreflexion und der thermischen Emission sowie Berechnungen mittels der Transfermatrix und der linearen Dispersionstheorie verwendet werden. Ein Anticrossing in der Dispersion der Polariton-Zweige mit einer Energieaufspaltung bis zu 15 meV, was die Hauptsignatur des starken Kopplungsregimes ist, wird beobachtet. Die starke Kopplung mit Zinkgalliumoxid, einem hochdotierten Halbleiter als Alternative zu Edelmetallen, wird auch untersucht. Experimentelle und simulierte Reflektometrie-Spektren sowie Dispersionsrelationen werden diskutiert, um Rückschlüsse auf die Eigenschaften des Systems zu ziehen. Außerdem wird ein Ansatz zur Verbesserung der Leitfähigkeit organischer Halbleiterpolymere durch starke Kopplung ihrer polaronischen Zustände an SPPs vorgestellt und Leitfähigkeitsmessungen durchgeführt. Ziel ist es, die Delokalisierung der Hybridzustände auszunutzen, um die Leitfähigkeit zu verändern. Die präsentierten Ergebnisse bieten neue Einblicke in den Nutzen der Eigenschaften der Licht-Materie-Hybridisierung, um ihr volles Potenzial für verschiedene Bereiche und Anwendungen zu erforschen.Strong coupling of molecules with a confined light field results in the formation of new polaritonic eigenstates of the system called polaritons that inherit both molecular and photonic characteristics and thus holds strong potential for applications in chemistry and optoelectronics. In this work, coupling between propagating surface plasmon polaritons (SPPs), as confined light field, and molecular excitations, such as vibrational resonances and polaronic features, is investigated. The strong coupling regime between a molecular vibration and a propagating SPP is demonstrated for the first time in the mid-infrared spectral range using the carbonyl stretch vibration of Poly(vinyl methyl ketone) polymer and silver as metallic medium for SPPs propagation. The newly formed hybrid modes are investigated through experiments and numerical modelling, employing attenuated-total-reflection and thermal emission measurements as well as transfer-matrix and linear dispersion theory calculations. An anticrossing behavior in the dispersion of the polariton branches with an energy splitting up to 15meV, which is a key signature of the strong coupling regime, is observed. Strong coupling involving zinc gallium oxide, which is a highly doped semiconductor, as an alternative to noble metals is also investigated. Experimental and simulated reflectometry spectra as well as the dispersion relations are discussed so as to draw conclusions about the properties of the system. Furthermore, an approach to enhance the conductivity of organic semiconductor polymers by strongly coupling their polaronic states to SPPs is presented and four-point probe measurements are conducted. The goal is to exploit the delocalization of the hybrid states to alter the conductivity of the organic semiconductor. The results presented in this thesis provide new insights into the profit from the properties of light-matter hybridization in order to explore its full potential for several areas and applications

Dielectrophoresis Control of Semiconductor Nanowires for Sensing Technology

Semiconductor nanowires (NWs) synthesis successes have given keys to unprecedented nano-scale sensitivity opening up new opportunities in device applications. NWs’ potential relies upon the possibility of engineering and modifying properties such as sensitivity and carrier transport, by tailoring the NWs’ morphology and conductivity. New challenges have taken place with the downscaling of electronics for NWs integration and assembly techniques. Amongst a large variety of integration techniques, dielectrophoresis (DEP) is a powerful tool for the precise manipulation of NWs of different compositions and sizes. However, experimental implementation and analysis with DEP often lack depth regarding the optimisation of the technique and the effects of the parameters on the performances of the final devices, which is crucial for the understanding of NWs’ electric transport properties and technology improvement. Consequently, this thesis presents a comprehensive study of the experimental implementation of DEP which is of paramount importance to obtaining optimum conditions for NWs alignment. With this aim in mind, the presented work demonstrates a detailed investigation of the electrical and optical properties of germanium (Ge) and gallium-arsenide-bismuth (GaAsBi) NWs-based devices by DEP as a function of the collection frequency. The Ge and GaAsBi NWs were obtained by MOVPE and MBE respectively as a collaborative work with the Institute of Material for Electronic and Magnet (Italy) for the Ge NWs, and with the research team of Dr Robert Richard at the University of Sheffield (Department of Electrical Engineering and Electronics) for the GaAsBi NWs. Firstly, to maximise NWs alignment precision, optimum DEP parameters are for the first time thoroughly extracted by testing the effects of different mediums (chemical inertia, volatility and contact angle) and electrode designs (gradients and electric field). Secondly, fabricated with a DEP frequency range of 500 kHz to 10 MHz the devices were electrically characterised using voltage-current response. An asymmetric diode-like behaviour was found to be originating from heterostructured Ge NWs specifically orientated by electrophoresis combined with DEP. This result is particularly promising for orientation control demonstrated for the first time, tuning and altering current response from chemically heterostructured nanowires. A particular focus was given to the effect of increasing frequency on the device performances such as carrier transport. Despite a decrease of aligned NWs corroborating theoretical analysis, increasing frequency collected higher conductivity Ge NWs with carrier mobility improving from 2θ at 500 kHz to 4.38θ at 10 MHz demonstrated using Mott-Gurney, and GaAsBi NWs with carrier mobility increasing from 5.29 ± 0.027 to 100 ± 0.70 cm2 V−1.s−1 at 500 kHz and 10 MHz demonstrated using the Fermi-velocity law. Such selectivity is of great potential to improve sensing technology transduction. Using optimum parameters previously found, a low-cost and simple voltage divider system joined to DEP is demonstrated for the first time to improve the alignment technique of a single Ge nanowire. The resulting spectral response was consistent with optical characterisations found in the literature for a single Ge nanowire and demonstrated high sensitivity near-infrared and communication wavelength as confirmed with a high responsivity of 6.2 x 105 A/W at 1550 nm. Such high resistivity is amongst the highest ever obtained for NWs. Furthermore, a NWs-based biosensor for the spike protein of the SARS-CoV-2 was fabricated by multilayered surface functionalisation evidenced by Raman spectroscopy. Upon exposure to increasing concentration of the protein, the biosensors transduced increasing current response with a working range of at least 1 aM to 100 fM. Selectivity to the spike protein was testified using bovine serum albumin as a negative control reference. The GaAsBi NWs were for the first time fully characterised and implemented as devices by DEP. The NWs surface roughness showcased the importance of surface properties that influenced DEP collection and carrier transport. Spectral responses from the devices brought to light the different bismuth content at the origin of reduced band-gap energy shown by the cut-off energies of the spectrum. With a Bi content increase of roughly 1% in GaAs the photodetectors presented high responsivity from 1.3 x 104 A/W to 5.6 x 104 A/W. Effective NWs-based biosensors and photodetectors were proof of concept devices that corroborate NWs and dielectrophoresis functionality paving the way to future nanotechnology improvement

Excitation of local magnetic moments by tunnelling electrons

The advent of milli-kelvin scanning tunneling microscopes (STM) with inbuilt magnetic fields has opened access to the study of magnetic phenomena with atomic resolution at surfaces. In the case of single atoms adsorbed on a surface, the existence of different magnetic energy levels localized on the adsorbate is due to the breaking of the rotational invariance of the adsorbate spin by the interaction with its environment, leading to energy terms in the meV range. These structures were revealed by STM experiments in IBM Almaden in the early 2000's for atomic adsorbates on CuN surfaces. The experiments consisted in the study of the changes in conductance caused by inelastic tunnelling of electrons (IETS, Inelastic Electron Tunnelling Spectroscopy). Manganese and Iron adatoms were shown to have different magnetic anisotropies induced by the substrate. More experiments by other groups followed up, showing that magnetic excitations could be detected in a variety of systems: e.g. complex organic molecules showed that their magnetic anistropy was dependent on the molecular environment, piles of magnetic molecules showed that they interact via intermolecular exchange interaction, spin waves were excited on ferromagnetic surfaces and in Mn chains, and magnetic impurities have been analyzed on semiconductors. These experiments brought up some intriguing questions: the efficiency of magnetic excitations was very high, the excitations could or could not involve spin flip of the exciting electron and singular-like behavior was sometimes found at the excitation thresholds. These facts called for extended theoretical analysis; perturbation theories, sudden-approximation approaches and a strong coupling scheme successfully explained most of the magnetic inelastic processes. In addition, many-body approaches were also used to decipher the interplay between inelasComment: Review article to appear in Progress of Surface Scienc

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

ICCG-10: Tenth International Conference on Crystal Growth. Oral presentation abstracts

Oral presentation abstracts from the tenth International Conference on Crystal Growth (ICCG) (Aug. 16-21, 1992) are provided. Topics discussed at the conference include superconductors, semiconductors, nucleation, crystal growth mechanisms, and laser materials. Organizing committees, ICCG advisory board and officers, and sponsors of the conference are also included