Physics - Doctoral Theseshttps://hdl.handle.net/10468/4302024-03-28T12:04:18Z2024-03-28T12:04:18Z1011A study of the elastic and electronic properties of III-nitride semiconductorsTanner, Daniel Stephen Patrickhttps://hdl.handle.net/10468/54592023-04-04T06:57:14Z2017-01-01T00:00:00Zdc.title: A study of the elastic and electronic properties of III-nitride semiconductors
dc.contributor.author: Tanner, Daniel Stephen Patrick
dc.description.abstract: In this work, a theoretical study of the elastic and electronic properties of III-N semiconductors is made. Particular attention is given to wurtzite quantum wells (QWs) based on the technologically important InxGa1_xN alloy system. An investigation of the effects of structural inhomogeneities in these systems is made; specifically, the impact of random alloy fluctuations and well width fluctuations on the electronic and optical properties is investigated. This investigation comprises atomistic tight binding calculations on large ( 80,000 atoms) supercells. The theoretical framework used accounts for local strain and polarisation fluctuations arising from random alloy effects and possesses the computational efficiency needed for the calculation of many electronic states for realistic QW sizes. An accurate account is thus given of the interplay between nanoscopic and atomistic features of InGaN quantum wells, which was beyond the reach of previous ab-initio or continuum studies. The analysis is performed for excited states as well as ground states for InGaN quantum wells of varying indium content and growth direction. The calculations reveal that random alloy fluctuations are sufficient to bring about carrier localisation effects in both c- and m-plane InGaN based QWs. For the case of zincblende InGaN, shortcomings in the currently used semi-empirical methods of strain and atomic relaxation are pointed out and quantified. The development of an improved framework for the analysis of the structural and elastic properties of zincblende InGaN systems is thus undertaken. Elastic properties are extracted from density functional theory (DFT) data, and a new valence force field (VFF) implementation is introduced which makes explicit use of elastic properties neglected by ubiquitous VFF models. Additionally, third order elastic properties are determined from the results of DFT calculations using finite-strain theory. Finally, the utilisation of these properties in a valence force field model is discussed, and an appropriate functional form is suggested.
2017-01-01T00:00:00ZA tight-binding analysis of the electronic properties of III-nitride semiconductorsCoughlan, Conor Terencehttps://hdl.handle.net/10468/27612023-04-04T06:54:59Z2016-01-01T00:00:00Zdc.title: A tight-binding analysis of the electronic properties of III-nitride semiconductors
dc.contributor.author: Coughlan, Conor Terence
dc.description.abstract: This thesis divides into two distinct parts, both of which are underpinned by the tight-binding model. The first part covers our implementation of the tight-binding model in conjunction with the Berry phase theory of electronic polarisation to probe the atomistic origins of spontaneous polarisation and piezoelectricity as well as attempting to accurately calculate the values and coefficients associated with these phenomena. We first develop an analytic model for the polarisation of a one-dimensional linear chain of atoms. We compare the zincblende and ideal wurtzite structures in terms of effective charges, spontaneous polarisation and piezoelectric coefficients, within a first nearest neighbour tight-binding model. We further compare these to real wurtzite structures and conclude that accurate quantitative results are beyond the scope of this model but qualitative trends can still be described. The second part of this thesis deals with implementing the tight-binding model to investigate the effect of local alloy fluctuations in bulk AlGaN alloys and InGaN quantum wells. We calculate the band gap evolution of Al1_xGaxN across the full composition range and compare it to experiment as well as fitting bowing parameters to the band gap as well as to the conduction band and valence band edges. We also investigate the wavefunction character of the valence band edge to determine the composition at which the optical polarisation switches in Al1_xGaxN alloys. Finally, we examine electron and hole localisation in InGaN quantum wells. We show how the built-in field localises the carriers along the c-axis and how local alloy fluctuations strongly localise the highest hole states in the c-plane, while the electrons remain delocalised in the c-plane. We show how this localisation affects the charge density overlap and also investigate the effect of well width fluctuations on the localisation of the electrons.
2016-01-01T00:00:00ZAdvanced processing strategies for site-controlled pyramidal quantum dotsVaro, Simonehttps://hdl.handle.net/10468/124752023-04-04T10:57:54Z2021-10-07T00:00:00Zdc.title: Advanced processing strategies for site-controlled pyramidal quantum dots
dc.contributor.author: Varo, Simone
dc.description.abstract: Today, the world is on the verge of a new technological breakthrough that has been called the “second quantum revolution”: much like the understanding of semiconductor physics paved the way for the development of integrated circuits and ushered the age of information technology, the development of the first working quantum computers might start a profound change in the way we process information, and pave the way for much anticipated technological breakthrough.
In my thesis, I have investigated the applications of Pyramidal Quantum Dots (PQDs) as potential sources of single and entangled photons, with a particular focus on improving their quality and brightness, and explored new approaches for their processing that would make them suitable for integration in future quantum devices.
In order to address the limits of the conventional InGaAs in GaAs PQDs in relation to resonant pumping of the biexcitonic state, I contributed to the development of two new families of dots: GaAs in AlGaAs PQDs and GaAs dots confined by superlattice barriers. For the former, only results of growths on templates of smaller size were reported in the literature, while the latter is a completely new family of PQDs altogether. Results reported in this thesis indicate a surprising uniformity in spectral features for either family, and demonstrate that resonant excitation can indeed be achieved due to the binding biexcitonic state.
In collaboration with Prof. Di Falco’s group in the University of St. Andrews (UK), we have demonstrated potential applications in the field of semiconductor non-classical light sources of the Electron Beam Induced Deposition technique, which allowed us to fabricate complex dielectric nanostructures directly above the quantum emitters. Our work showed how this approach doesn’t cause any significant degradation in the emission spectra, and was complemented by a large number of simulations to elucidate the underlying mechanisms that allow SiO_2 nanostructures to boost light extraction efficiency form the semiconductor matrix depending on their shape.
A more promising approach to boost the brightness of our dots was also developed, in the form of a self-aligning technique that allows to fabricate micro and nanopillars using dry etching, and that allowed us to achieve our own record of brightness.
Furthermore, an approach to compensate for the effects of the excitonic fine structure splitting on the level of entanglement was devised: the model we have developed complements similar proposals that have previously appeared in the literature, but is also extremely intuitive due to its sequential logic gate structure.
Finally, for the first time transmission electron microscopy of PQDs was achieved in a joint effort with Queen’s University Belfast, a success in large part made possible by the new processing strategies devised in this thesis, and whose preliminary results are here reported.
2021-10-07T00:00:00ZAll optical systems for terabit network eraFabbri, Simonhttps://hdl.handle.net/10468/38592023-04-04T07:09:10Z2016-01-01T00:00:00Zdc.title: All optical systems for terabit network era
dc.contributor.author: Fabbri, Simon
dc.description.abstract: The continuous growth of the network capacity demand drives the development of the optical fiber networks. Since optical super-channels carrying multi- Terabit/s transmissions are the next evolution of the optical links, future transmitters and receivers systems will be required to handle vast volume of information while maintaining reasonable power consumption and cost. In addition, when point-to-point links are approaching the fundamental limit of standard fiber, the efficient use of the entire transmission window for optical networks will only be achieved through flexible superchannels compatible with flexible optical nodes. This thesis describes the research work carried out to investigate the future optical systems that will support the Terabit era networks. All-optical systems are presented, allowing for high spectral efficiency in future networks, from the comb sources to the optical nodes. The following manuscript firstly reports on the development of optical combs based on external modulators to provide transmitters with a large number of optical carriers. Then, the implementation of an experimental all-optical super-channel through the use of optical signals and wavelength manipulations is described. Finally, a novel all-optical node called Terabit Interferometric add, Drop, and Extract (TIDE) is presented. With a management of the super-channel components in the optical domain, the optical node offers transparency and flexibility while maintaining the high spectral efficiency of the super-channel carrying links. An optical frequency comb source was developed, generating a high quality 9-line comb with a spectral flatness < 0.3 dB and side-mode suppression ratio > 20 dB. A single Mach-Zehnder modulator was driven with a low power multi- harmonic electrical signal. Furthermore, a large 36 line comb was obtained by the addition of a two cascaded modulators comb source driven with a 90 GHz signal The implementation of an alloptical Orthogonal Frequency Division Multiplexing (OFDM) super-channel, using a high quality single sideband modulation scheme based on a dual-parallel Mach-Zehnder modulator, allowed for the development and test of the all-optical TIDE node. The interferometer-based structure was capable of managing (add, drop, and extract) channel from a super-channel strictly in the optical domain. In this manuscript, we prove that it is possible to extract channels even with overlapping spectrum super-channel aggregation for both single and dual quadrature modulation formats.
2016-01-01T00:00:00ZAnalysis and optimisation of semiconductor reflective modulators for optical networksNaughton, Alan J.https://hdl.handle.net/10468/19192023-04-04T07:39:59Z2014-01-01T00:00:00Zdc.title: Analysis and optimisation of semiconductor reflective modulators for optical networks
dc.contributor.author: Naughton, Alan J.
dc.description.abstract: Reflective modulators based on the combination of an electroabsorption modulator (EAM) and semiconductor optical amplifier (SOA) are attractive devices for applications in long reach carrier distributed passive optical networks (PONs) due to the gain provided by the SOA and the high speed and low chirp modulation of the EAM. Integrated R-EAM-SOAs have experimentally shown two unexpected and unintuitive characteristics which are not observed in a single pass transmission SOA: the clamping of the output power of the device around a maximum value and low patterning distortion despite the SOA being in a regime of gain saturation. In this thesis a detailed analysis is carried out using both experimental measurements and modelling in order to understand these phenomena. For the first time it is shown that both the internal loss between SOA and R-EAM and the SOA gain play an integral role in the behaviour of gain saturated R-EAM-SOAs. Internal loss and SOA gain are also optimised for use in a carrier distributed PONs in order to access both the positive effect of output power clamping, and hence upstream dynamic range reduction, combined with low patterning operation of the SOA Reflective concepts are also gaining interest for metro transport networks and short reach, high bit rate, inter-datacentre links. Moving the optical carrier generation away from the transmitter also has potential advantages for these applications as it avoids the need for cooled photonics being placed directly on hot router line-cards. A detailed analysis is carried out in this thesis on a novel colourless reflective duobinary modulator, which would enable wavelength flexibility in a power-efficient reflective metro node.
2014-01-01T00:00:00ZArrays of quantum-light-emitting diodes with site-controlled pyramidal quantum dotsChung, Tung-Hsunhttps://hdl.handle.net/10468/39882023-04-04T07:19:31Z2016-01-01T00:00:00Zdc.title: Arrays of quantum-light-emitting diodes with site-controlled pyramidal quantum dots
dc.contributor.author: Chung, Tung-Hsun
dc.description.abstract: Quantum information technology is an interdisciplinary subject, merging quantum mechanics and information science. In this field, the building blocks are quantum bits (qubits), which are superposition quantum states of simple two-level systems. Among all the platforms for the preparation of qubits, the polarization directions of single-photons are attractive as information carriers for practical realizations, due to reduced photon decoherence as well as the fact that they can be manipulated by convenient optical components. Moreover, the request for entangled sources from quantum communication and quantum computation can be satisfied by polarization-entangled photon emitters. In combination of their atomic-like energy structure and mature development in foundries/labs, epitaxial semiconductor quantum dots (QDs) have been exploited to deliver quantum light sources, such as indistinguishable single-photons and polarization-entangled photon pairs, by both optical and electrical injection (with triggering on demand). However, conventional self-assembled QDs nucleate randomly and possess finite values of finestructure splitting (FSS) in the excitonic states, mainly due to low crystal symmetry, which adds hurdles to entanglement reconstruction and significantly limits their scalability and potential for further integration. In this thesis, pyramidal QDs (PQDs) were grown by metalorganic vapor phase epitaxy (MOVPE), starting from site- controlled pyramidal recesses photolithographydefined on GaAs (111)B substrates. Their inherent properties enable them to be controlled spatially and provide them with high crystal symmetry, i.e., close-to zero FSS, suggesting an alternative for the solution of the abovementioned difficulties encountered by selfassembled QDs. However, the non-planar feature of PQDs makes it challenging to embed them into light-emitting diodes (LEDs). Here, we developed and designed a fabrication process which successfully overcame the configuration-induced processing complexity for the preparation of on demand single-photon and entangled-photon sources by electrical injection. xiv Indeed, the main achievement in this thesis is the fabrication of quantum LEDs with site-controlled PQDs which are able to generate single-photons and polarizationentangled photon pairs triggered on demand. In single-photon emission, the value of g(2) (0) could be reduced to 0.078 0.066 when combined with a time-gating technique under pulse exciation. On the other hand, the fidelities to the expected maximally polarizationentangled state were 0.85±0.04 under continuous excitation and 0.823±0.019 under pulse excitation by assistance of time-gating, with 75 % of the intensity maintained in a 1.5 ns window. The prepared entangled source was also importantly proved to violate Bell’s inequalities. Moreover we worked around finding solutions to some challenging issues concerning PQDs. The system in this study was an In0.25Ga0.75As QD sandwiched by GaAs barriers. One issue in our MOVPE-grown PQDs is a dominant negatively-charged environment. We explored a number of methods as detailed in the text which proved effective in suppressing at specific conditions the probability of capturing excess electrons, strongly improving the polarization-entangled photon pair emission via the biexcitonexciton decay process, and improving our sources. Also, a previous growth model on the AlGaAs/GaAs system developed in our group was expanded to the current system to understand the mechanism of indium segregation on both InGaAs V-grooved quantum wires and PQDs. The simulation successfully suggested consistent growth temperature-dependent emission energy evolution with the reported experimental results, in which an unexpected QD redshifting paired by a lateral quantum wires blueshifting with increasing growth temperature was observed. In addition, a new faceting at the pyramidal recess base during MOVPE growth was observed and reported for the first time. Altogether our results justify the PQD system as a promising platform to generate quantum light sources meeting a number of important requirements: e.g. spatial control, high fidelity, trigger on demand, and electrical injection.
2016-01-01T00:00:00ZAtomistic simulation and analysis of novel group IV semiconductor alloys and devicesDunne, Michael D.https://hdl.handle.net/10468/144662023-06-08T02:05:10Z2023-01-01T00:00:00Zdc.title: Atomistic simulation and analysis of novel group IV semiconductor alloys and devices
dc.contributor.author: Dunne, Michael D.
dc.description.abstract: A long held goal of the semiconductor community is the development of a direct gap silicon (Si) compatible material to enable the seamless integration of optical and electronic components on a single chip. The dominant elemental group-IV semiconductors silicon and germanium are the mainstays of current microelectronics, but their fundamental indirect gaps pose a roadblock to the development of active photonic components. The alloying of germanium with other group-IV elements, such as tin or carbon, has come into focus in recent years in pursuit of developing a direct gap alloy. Band engineering of germanium is attractive owing to the small difference between the indirect L6c-Γ8v and direct Γ7c-Γ8v band gaps of germanium which is only 140 meV. Alloying opens the possibility of reducing the Γ7c state below that of the L6c state leading to a direct gap alloy. Initial work on Ge1−xCx alloys have predicted the formation of a direct gap upon incorporation of dilute quantities of C (<1%), though there has not yet been an experimental demonstration of direct gap behaviour. Ge1−xSnx alloys have attracted greater research interest owing to the experimental demonstration of direct gap behaviour for a range of Sn compositions. Previous theoretical work suggested the transition from indirect to direct band gap occurs in a composition range of 6-11% Sn, while recent research indicates that a direct band gap emerges continuously with increasing x due to alloy band mixing. This atomistic effect, which is neglected in the widely-employed virtual crystal approximation (VCA), results in the alloy conduction band (CB) edge possessing hybridised character that evolves continuously from indirect (Ge L6c-like) to direct (Ge Γ7c-like) with increasing x. In this thesis we present a theoretical analysis of electronic structure evolution in the highly- mismatched dilute carbide group-IV alloy Ge1−xCx by adopting an atomistic approach encompassing calculations of the electronic structure using the semi-empirical tight-binding method. We demonstrate that C incorporation strongly perturbs the conduction band (CB) structure by driving hybridisation of A1-symmetric linear combinations of Ge states lying close in energy to the CB edge. These calculations describe the emergence of a “quasi-direct” alloy band gap, which retains a significant admixture of indirect Ge L-point CB edge character. The trends identified by our calculations are markedly different to those expected based on a recently proposed interpretation of the CB structure based on the band anti-crossing model.
For Ge1−xSnx alloys we are interested in the impact of the previously overlooked alloy effects have on the band to band tunneling in the alloy. We achieve this using non-equilibrium Green’s function (NEGF) band-to-band tunneling (BTBT) calculations based on atomistic tight-binding electronic structure calculations. We then extend this analysis to look at the effect of Sn incorporation on the
current characteristics of TFET devices. We demonstrate that CB mixing strongly modifies the complex band structure, driving complex band anti-crossing that – for Sn compositions at which the band gap is assumed indirect in the VCA – strongly increases the BTBT generation rate G. Our results highlight the importance of atomistic effects in determining the electrical properties of Ge1−xSnx alloys
2023-01-01T00:00:00ZAxial symmetry and transverse trace-free tensors in numerical relativityConboye, Rory P. A.https://hdl.handle.net/10468/5272023-04-04T07:10:41Z2012-02-01T00:00:00Zdc.title: Axial symmetry and transverse trace-free tensors in numerical relativity
dc.contributor.author: Conboye, Rory P. A.
dc.description.abstract: Transverse trace-free (TT) tensors play an important role in the initial conditions of numerical relativity, containing two of the component freedoms. Expressing a TT tensor entirely, by the choice of two scalar potentials, is not a trivial task however. Assuming the added condition of axial symmetry, expressions are given in both spherical and cylindrical coordinates, for TT tensors in flat space. A coordinate relation is then calculated between the scalar potentials of each coordinate system. This is extended to a non-flat space, though only one potential is found. The remaining equations are reduced to form a second order partial differential equation in two of the tensor components. With the axially symmetric flat space tensors, the choice of potentials giving Bowen-York conformal curvatures, are derived. A restriction is found for the potentials which ensure an axially symmetric TT tensor, which is regular at the origin, and conditions on the potentials, which give an axially symmetric TT tensor with a spherically symmetric scalar product, are also derived. A comparison is made of the extrinsic curvatures of the exact Kerr solution and numerical Bowen-York solution for axially symmetric black hole space-times. The Brill wave, believed to act as the difference between the Kerr and Bowen-York space-times, is also studied, with an approximate numerical solution found for a mass-factor, under different amplitudes of the metric.
2012-02-01T00:00:00ZBiosensors using photonic crystal fibresRutowska, Monika S.https://hdl.handle.net/10468/15112023-04-04T07:13:14Z2014-01-01T00:00:00Zdc.title: Biosensors using photonic crystal fibres
dc.contributor.author: Rutowska, Monika S.
dc.description.abstract: In this thesis, we present the unique properties of hollow-core photonic crystal fibres (HC-PCFs) for sensing applications in terms of viscosity detection and DNA sensing using a special poly(ethylene) glycol (PEGDA) hydrogel. The low loss HC-PCFs ensure a long interaction length between the sample and the optical signals. Thus in this thesis, we report the characterisation of filled HC-PCFs and the development of a selective filling process. For the first time, we report the investigation of a new viscometer device, and a new device for DNA sensing development, and also the chemical process for hydrogel growth was adapted to the fibres. By combining HC-PCFs with the hydrogel we enable 3D volumetric sample confinement within the HC-PCF, further increasing the interaction between the sample and the optical signal. However, the hydrogel has a large influence on the guidance properties of the HC-PCF and the HC-PCF has a strong influence on the growth process for the hydrogel itself. When we integrate the hydrogel and HC-PCFs we detect concentration levels as low as 400 nM of labelled DNA. However, using our technology for fluorescence detection we can achieve results two orders of magnitude better than those previously reported.
2014-01-01T00:00:00ZCarrier momentum relaxation in highly doped polar semiconductors and semiconductor heterostructuresHauber, Anna Miriamhttps://hdl.handle.net/10468/34172023-04-04T06:55:49Z2016-01-01T00:00:00Zdc.title: Carrier momentum relaxation in highly doped polar semiconductors and semiconductor heterostructures
dc.contributor.author: Hauber, Anna Miriam
dc.description.abstract: Highly doped polar semiconductors are essential components of today’s semiconductor industry. Most strikingly, transistors in modern electronic devices are polar semiconductor heterostructures. It is important to thoroughly understand carrier transport in such structures. In doped polar semiconductors, collective excitations of the carriers (plasmons) and the atoms (polar phonons) couple. These coupled collective excitations affect the electrical conductivity, here quantified through the carrier mobility. In scattering events, the carriers and the coupled collective modes transfer momentum between each other. Carrier momentum transferred to polar phonons can be lost to other phonons through anharmonic decay, resulting in a finite carrier mobility. The plasmons do not have a decay mechanism which transfers carrier momentum irretrievably. Hence, carrier-plasmon scattering results in infinite carrier mobility. Momentum relaxation due to either carrier–plasmon scattering or carrier–polar-phonon scattering alone are well understood. However, only this thesis manages to treat momentum relaxation due to both scattering mechanisms on an equal footing, enabling us to properly calculate the mobility limited by carrier–coupled plasmon–polar phonon scattering. We achieved this by solving the coupled Boltzmann equations for the carriers and the collective excitations, focusing on the “drag” term and on the anharmonic decay process of the collective modes. Our approach uses dielectric functions to describe both the carrier-collective mode scattering and the decay of the collective modes. We applied our method to bulk polar semiconductors and heterostructures where various polar dielectrics surround a semiconducting monolayer of MoS2, where taking plasmons into account can increase the mobility by up to a factor 15 for certain parameters. This screening effect is up to 85% higher than if calculated with previous methods. To conclude, our approach provides insight into the momentum relaxation mechanism for carrier–coupled collective mode scattering, and better tools for calculating the screened polar phonon and interface polar phonon limited mobility.
2016-01-01T00:00:00Z