Physics - Doctoral Theses

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    Theory of carrier transport in III-Nitride based heterostructures
    (University College Cork, 2023-05-01) O'Donovan, Michael; Schulz, Stefan; O'Reilly, Eoin P.; Science Foundation Ireland; Sustainable Energy Authority of Ireland
    Wurtzite III-nitride materials and their alloys have attracted significant interest for solid state lighting applications. This is due to the direct band gaps of InN, GaN, and AlN crystals, which span a wide range of emission wavelengths. Due to the importance of these systems, the goal of optimising device performance has been an extremely active field of research. An important aspect of this is the development of improved modeling techniques. More recently, an emphasis has been put on understanding the impact the disordered alloy microstructure has on the electronic structure, however models focusing on transport properties are less mature. This is in part due to the challenges of connecting a random alloy description of the underlying microstructure with transport models. This thesis addresses this difficult problem by developing and utilizing different simulation frameworks, focusing on transport properties of (In,Ga)N/GaN quantum well systems. More specifically, the non-equilibrium Green's function (NEGF) formalism has been employed to study ballistic transport in a fully quantum mechanical setting. This builds on a tight-binding description of the electronic structure which ensures an atomistic description of the alloy is achieved. Our results indicate that while the alloy microstructure is of secondary importance for electrons, the transmission of holes is strongly perturbed by the presence of disorder. This is attributed to the breakdown of the translational symmetry of the system, which opens up new channels not present when fluctuations in local alloy content are neglected (using a virtual crystal approximation). Moreover, we have developed a new semi-classical multi-scale drift-diffusion model. This allows simulation of full devices due to a reduced computational demand compared to the NEGF formalism, while still keeping a microscopic resolution and accounting for important quantum corrections. The starting point is again the tight-binding model, which is used as a foundation to describe the alloy microstructure: A 3-dimensional energy landscape is extracted which includes an atomistic description of alloy fluctuations, local strain, and local polarization. This can be used as a confining potential for electrons and holes, and quantum corrections can be included in a numerically efficient manner via the recently developed localization landscape theory. This landscape, including or excluding quantum corrections, is used to study both uni-polar electron and hole transport. Our results show that, when quantum corrections are accounted for, the virtual crystal approximation is again a good approximation for electron transport, whereas hole transport is reduced due to carrier localization effects in the quantum well region. Finally this framework is extended to a p-i-n junction, where carrier (and thus recombination) distribution across a multi-quantum well system is studied. This system allows for a comparison between our in-house model and a commercial software package. Without including disorder in the alloy microstructure both schemes fail to reproduce literature experimental results. However, the situation changes when the random alloy microstructure is accounted for using our newly developed approach: The predicted behaviour is consistent with literature experimental results, without changing any other simulation parameters. These results highlight the importance of the treatment of the alloy microstructure in simulations, and indicate that our developed framework is an ideal starting point for modeling III-N systems to understand fundamental properties and guide device design.
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    Micro-transfer printing of micro-structured, ultra-thin light-emitting devices
    (University College Cork, 2023-03) Shaban, Zeinab; Corbett, Brian; Parbrook, Peter James; Science Foundation Ireland
    3D integration of optoelectronic devices is a crucial future technology, which can be applied in the areas of photonic integrated circuits, flexible displays, communication and more. Among the various technologies, micro-transfer printing has emerged as a precise and cost-effective way to assemble devices for 3D integration. To enable this technology, devices must be released from their native substrates, which open up a lot of possibilities. It can achieve integration with flexible or heat-conductive backplanes, as well as heterogeneous integration of multiple materials on a common platform, resulting in miniaturised chips. Also one can benefit from reclaiming and reusing the original substrates to reduce the production cost significantly. On the other hand, GaN devices exhibit unique optical properties in optoelectronics compared to other semiconductors, and GaN-based LEDs have established themselves in a variety of applications, due to their low power consumption, long lifetime, short response time, and high brightness. This thesis has focused on releasing high performance GaN LEDs and addressing their associated issue for micro-transfer printing. The first part of this work is focused on releasing and transfer-printing of GaN LEDs grown on Si substrate. There are several factors that limit the performance and manufacturing of GaN LEDs on Si. One issue is related to the deformation of the released coupons due to their high inbuilt strain, which could result in transfer-printing failures as well as challenges during the post-print integration process. To address this issue, COMSOL software was used to study the stress effect on the devices. Experimentally, the intrinsic deformation of the released LEDs was compensated by using compressed SiNx layers that resulted in flat devices after release. Another issue is related to the low light extraction for GaN LEDs on Si. To solve this problem, the underside of the released LEDs is roughened during the coupon preparation process prior to transfer printing. Furthermore, using the unique properties of transfer printing, the roughened LEDs are printed inside a fabricated reflective trench with 10 μm depth to direct the light to the surface normal. Results showed that roughening along with the reflective trench increased the collected power by a factor of ∼ 7 compared with LEDs on the original substrate. A second part of this study examines the release of GaN-based structures from substrates (i.e. sapphire or bulk GaN) by photoelectrochemical (PEC) etching when pure chemical etching is not possible. A sacrificial layer which can obtain smooth etch surfaces and uniform etching with high selectivity is needed. Also, from the perspective of transfer printing, thick rather than thin sacrificial layers are preferred to facilitate the releasing and picking process. In this work, 300 nm-thick releasing layers comprising of InGaN/AlInN stacks are proposed for PEC etching. The presence of two-dimensional hole gas at the interface of InGaN/AlInN due to the strong polarization field are indicated by modelling and capacitance-voltage measurement. This resulted in a smoother surface with a three times higher etch rate compared to the conventional InGaN/GaN superlattice structures used for PEC etching. Moreover, various electrolytes and post-PEC treatment were studied to improve the surface smoothness. Further work should be done to determine the impact of the adhesion layer in transfer printing on heat generation and device performance. Using the optimized sacrificial layer to release other structures like lasers should also be investigated.
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    Enhanced shortcuts to adiabaticity
    (University College Cork, 2022) Whitty, Chris; Ruschhaupt, Andreas; Irish Research Council
    Dynamical control of quantum systems is a fundamental requirement for designing and engineering quantum technologies. Adiabatic control methods are used extensively to control quantum systems in many settings. However, adiabatic control methods require long operation times. To address this issue, a collection of techniques called “shortcuts to adiabaticity" (STA) have been developed. STA have been applied in many settings, and they can offer significant improvement over adiabatic schemes. However, a major limitation of STA is that fully analytic STA schemes are known only for several specific families of quantum systems. Motivated to overcome this restriction, in this thesis we derive an analytic method called “enhanced shortcuts to adiabaticity" (eSTA) that extends STA techniques to systems that do not admit STA methods exactly. We first derive the eSTA formalism and demonstrate its utility in designing control schemes for several practical quantum control settings. We then investigate the robustness of eSTA against several types of systematic error and environmental noise, using the setting of neutral atom lattice transport. We also derive an alternative eSTA technique that naturally includes higher order terms, at the expense of further calculation. Both the alternative and original eSTA schemes are applied to fast anharmonic trap expansion. Finally, transport of two ions with Coulomb interaction in an anharmonic trap is considered and eSTA is shown to be robust against the effect of amplitude noise in this setting.
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    Atomistic simulation and analysis of novel group IV semiconductor alloys and devices
    (University College Cork, 2023-01-01) Dunne, Michael D.; O'Reilly, Eoin P.; Broderick, Christopher; Schulz, Stefan; Science Foundation Ireland
    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
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    Optical spectroscopy for biological and biomedical applications: potentially impacting future of healthcare with research, clinical translation and education
    (University College Cork, 2021-11-10) Saito Nogueira, Marcelo; Andersson-Engels, Stefan; O'Riordain, Micheal; Science Foundation Ireland; Mercy Hospital Foundation
    Clinical interventions have been significantly improved by recent advances in devices for disease detection, monitoring, diagnosis and surgical guidance. However, most of the current imaging technologies provide primarily morphological/structural information on analyzed subjects or samples, which is insufficient to detect early-stage diseases and identify tissue structures with enough contrast. Optical spectroscopy and imaging can solve this insufficiency by providing molecular-sensitive tools for applications involving screening, diagnosis, monitoring vital signs, treatment planning and guidance, treatment-outcome prediction, and others. These tools provide non-invasive, real-time, cost-effective, and in situ interrogation of biological samples. Key advantages can be added to optical tools upon their miniaturization and integration into existing medical instruments, wearables and portable test kits, as patient prognosis can be improved by increasing the contrast of identification between vital structures and tissues to be resected, as well as by enabling telehealth solutions via remote monitoring individual health status. Such identification and monitoring can be achieved by extracting information on sample microstructure, biochemistry, and associated quantities. The thesis investigates the feasibility of using optical spectroscopy as an optical technique for application in cancer detection and delineation, specifications of next generation optical devices for this application, as well as tissue microstructural and biochemical features associated with carcinogenesis and organ viability for transplantation. Previous studies cover the selection and optimization optical techniques for a limited number of applications and parameters. Most studies of this thesis optimize diffuse reflectance spectroscopy (DRS) parameters for colorectal and organ preservation applications, while instruments developed for other optical techniques and associated analysis methods can be used for a wide range of applications. In particular, the clinical need for CRC diagnosis, surgery and therapy is accurately locating and completely removing premalignant lesions and surrounding compromised tissue. Since clinical procedures have limited time, tools providing endoscopy and surgical guidance must enable real-time detection of CRC and premalignancies. Therefore, we have validated DRS and fluorescence spectroscopy (FS) as optical techniques which could fulfil the clinical need for CRC applications, as well as be integrated into medical devices via fiber optic probes with <4mm source-detector distances (SDD). This validation was performed through pre-clinical and clinical studies and required accuracies and/or area under the receiver operating characteristic curve (AUC) >90% to be used in the clinic. Thesis resources could potentially be used to develop a probe for CRC detection during colonoscopy, laparoscopy, intestinal anastomosis and CRC surgery with the aim of real-time automated tissue classification by using machine learning models coupled with DRS instruments capable of displaying the results of a single reading in 2-3 seconds, and potentially coupled with methods to quantify early-stage cancer biomolecules. To the best of our knowledge, this thesis shows (1) the first DRS study to investigate the potential of probing tissue layers up to 2mm deep by using larger (>600 μm) SDD probes for CRC detection during colonoscopy. (2) the first study evaluating the benefit of the extended DRS wavelength range between 350-1919 nm in colorectal tissues through intra- and inter-study comparison of achieved tissue classification performance metrics, (3) the first study to evaluate biomolecule concentrations and scattering properties of superficial and deeper tissue layers for CRC detection in the luminal wall, (4) the first study evaluating solely the cooling effect on multiple tissue microstructural and biochemical parameters such as blood oxygenation, concentrations of blood, methemoglobin, water, lipid, and bile as well as scattering amplitude, Mie scattering power and fraction of Rayleigh scattering, (5) one of the first studies extracting tissue microstructural and biochemical parameters over extended wavelength ranges between 450-1590 nm by using a spectral fitting based on Monte Carlo simulations of light propagation in complex media, (6) some of the scarce broadband DRS studies investigating tissue measurables such as reflectance and optical properties over wavelengths ranging from 350-1919 nm, and (7) some of the scarce broadband DRS studies combining machine learning methods with algorithms determining tissue microstructural and biochemical parameters.