Physics - Doctoral Theses

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Surface dynamics in III-V epitaxy and its device implications
(University College Cork, 2024) Ozcan Atar, Ayse; Pelucchi, Emanuele; Juska, Gediminas; Science Foundation Ireland
Metalorganic vapour phase epitaxy (MOVPE) is a well-established, industry-compatible, compound semiconductor crystal growth technique, allowing for efficient and controllable material deposition. A wide range of semiconductor devices, both from III-V and nitrides families, are commonly fabricated for a broad range of applications in photonics, electronics and related fields, due to the reproducibility, scalability and overall excellent control over the growth process. Nevertheless, despite the technique’s popularity, there persists a large number of unresolved issues (mostly related to growth process/dynamic understanding) effectively hindering some of potential developments of III-V devices. A major unresolved technological issue, is the reported long range “leakage” of the dopant Zn into intrinsic layers during (and post) epitaxy, including the device processing steps. Zn is used as a typical p-type dopant for III-V materials and devices, but it is reportedly highly diffusive and historically very problematic. To bypass the Zn related problems, the large majority of InP based photonic devices, such as lasers and modulators, are fabricated with an n-i-p design, using p-type dopant at the top of the device. This approach essentially limits the design degree of freedom and stands in the way of the novel advanced stacked device architectures. This work reevaluated the Zn doping issues with unprecedented and surprising findings on Zn dopant behavior. The secondary ion mass spectrometry (SIMS) experiments show that Zn (or its precursors) can behave as a surfactant; accumulating on the sample surface during the growth of intentional doping layer and gradually incorporating into the nominally undoped layers even after the Zn source is shut off. Experimental findings are modelled by combining the surfactant and diffusion behavior with good qualitative agreement. Also, we demonstrated that this phenomenology can be suppressed and controlled either by introducing growth interruption steps or, even more effectively, introducing a competing surfactant species (Sb or its precursors). Our results highlight the relevance of (often overlooked) multi-faceted surface processes during MOVPE epitaxy, and help the implementation of robust solutions for novel device designs, crucially enabling next-generation integrated III-V applications.
Dynamically reconfigurable long-reach PONs for high capacity access
(University College Cork, 2019) Carey, Daniel; Townsend, Paul D.; Talli, Giuseppe; Science Foundation Ireland; Seventh Framework Programme
Fibre-to-the-Premises (FTTP) is currently seen as the ultimate in high-speed transmission technologies for delivering ubiquitous bandwidth to customers. However, as the deployment of network infrastructure requires a substantial investment, the main obstacle to fibre deployment is that of financial viability. With this in mind, a logical strategy to offset network costs is to optimise the infrastructure in order to capture a greater amount of customers over larger areas with increased sharing of network resources. This approach prompted the design of a long-reach passive optical network (LR-PON) in which the physical reach and split of a conventional PON is significantly increased through the use of intermediate optical amplification. In particular, the LR-PON architecture effectively integrates the metro and access networks enabling the majority of local exchange sites to be bypassed resulting in a substantial reduction in field equipment requirements and power consumption. Furthermore, the extension in physical reach and split can be coupled with an increased information capacity through the use of time- and wavelength division multiplexing (TWDM) which serve to exploit the large bandwidth capabilities offered by single-mode fibre. In this project, reconfigurable TWDM LR-PON architectures which dynamically exploit the wavelength domain are proposed, assembled and characterised in order to establish an economically viable ‘open access’ environment that is capable of concurrently supporting multiple operators offering converged services (residential, business and mobile) to support diverse customer requirements and locations. The main investigations in this work address the key physical layer challenges within such wavelength-agile networks. In particular, a range of experimental analysis has been carried out in order to realise the critical component technologies which include low-cost, 10G-capable, wavelength-tuneable transmitters for mass-market residential deployment and the development of gain-stabilised optical amplifier nodes to support the targeted physical reach (≥ 100km) and split (≥ 512). Finally, the feasibility of the proposed dynamically reconfigurable LR-PON configurations as a flexible and cost-effective solution for future access networks is verified through full-scale network demonstrations using an experimental laboratory test-bed.
Design and fabrication of single-mode tunable lasers for regrowth-free monolithically integrated photonic circuits
(University College Cork, 2019) Caro, Ludovic; Peters, Frank H.; Science Foundation Ireland
In this communication age, the bandwidth requirements are increasing exponentially with the development of more and more data-intensive services, from high-definition video streaming and cloud-based computing to the Internet of Things and machine-to-machine communication. This rapid expansion is powered by a rapidly growing fibre-based optical communication network. This growth, both in geographical extension and density of terminals, results in a large-scale need for the photonic components that are at the core of the optical communication network. In order to satisfy the demand, photonics industries need to increase their production capabilities and adopt more efficient fabrication processes. Streamlining the fabrication implies the removal of slow or costly processes. In the case of the photonics fabrication, epitaxial regrowth and advanced lithography steps are slow and expensive parts of the fabrication, and are one of the first targets for streamlining the process. In addition to reducing the cost of the fabrication itself, the integrated electronics approach can be a source of inspiration with the monolithic integration of multiple photonic components fabricated at the same time to create highly complex circuits while limiting the fabrication complexity. Focusing on a component at the core of photonic circuits: the tunable single-mode laser, this work is a contribution to the development of components that can be fabricated without requiring any regrowth or advanced lithography. Based on multi-cavity geometries enabled by multimode interference couplers and on a cleave-free approach to facilitate integration, a portfolio of tunable lasers is presented, showing tuning ranges of up to 51nm in the C and L optical windows, and side-mode suppression ratio levels of up to 35dB. Inspired by integrated electronics, proofs of concept for monolithically integrated electroabsorption modulated lasers and comb sources are presented, showing up to 19.5dB static absorption ratios for the modulated lasers, and a 4GHz optical comb tuned over a 28nm discrete tuning range. These results validate the proposed lasers as suitable candidates for the development of monolithically integrated photonic circuits where the fabrication complexity was kept minimal, making them attractive devices for the large-scale, streamlined production processes necessary to meet the increasing need for photonic components.
Site-controlled quantum dots as sources of quantum light
(University College Cork, 2018) Moroni, Stefano T.; Pelucchi, Emanuele
Quantum information is at its infancy. Though several different approaches are being pursued, the ability of manipulating a quantum state and make two quantum systems interact in a controlled way is still a great challenge, especially in terms of reproducibility and fidelity to the expected theoretical state. Among the possible implementations of quantum information, photonics seems to offer many technological advantages, the biggest challenge being the availability of a pure, scalable and integrable source of photons with all the required properties. Epitaxial semiconductor quantum dots (QDs) have been exploited to deliver such quantum light, e.g. indistinguishable single-photons and polarization-entangled photon pairs, by both optical and electrical injection, generated on demand. However, conventional self-assembled QDs are necessarily characterized by random positioning and randomly distributed optical properties, which limit the feasibility of a technology based on this kind of system. In this context, pyramidal quantum dots (PQDs) are one very valid alternative to conventional semiconductor QD-based quantum light sources. In fact, the precise control over the position of the PQDs, together with the uniformity of properties granted by the combination of lithographic methods and metalorganic vapor-phase epitaxy (MOVPE), make this source one of the few scalable systems which have been proven in recent years to emit photons with very interesting properties, polarization-entangled photons, for instance, upon optical excitation. In this work, all the main relevant aspects regarding PQDs are addressed through the most recent results obtained studying the system, starting from fundamental aspects regarding the epitaxy step. A growth model is presented which was also employed in the past as a practical tool to predict the result of the MOVPE in terms of geometry and composition of AlGaAs and GaAs structures inside a pyramidal recess. Here the model is extended in its simulation capabilities in order to include also the epitaxy of InGaAs, the actual material of which the optically active QD layer is made. This is then employed to simulate and understand the physical reason for a previously observed experimental behavior, henceforth confirming the applicability of the extended model to the simulation of InGaAs. Segregation, one fundamental epitaxy-related phenomenon which is predicted as well by the abovementioned growth model, is the key element allowing selective injection of carriers into a PQD, when its structure is embedded into a PIN junction. The whole fabrication process is described, including a selective-contacting technique that allows the realization of the electrical excitation of PQDs, one of the requirements for an efficient integration of PQDs on a photonic platform. On-demand generation of both single photons and entangled photon pairs is demonstrated from this device, the latter also importantly reaching a record fidelity to the ideal entangled state of 0.82 (upon the application of an appropriate time-filtering technique) and violating Bell’s inequalities. Among the unique possibilities offered by the PQD system is the capability of precisely stacking one quantum dot over another, allowing the formation of interacting multiple-QD systems. A first study of the statistics resulting from the fabrication of multiple PQDs with different distance and number of QD is here presented, showing how the QDs affect each other and offer further “tuning-knobs” for controlling their optical properties. For example, stacking two PQDs at the right distance can result in the generation of two subsequent photons with the same energy, which was previously reported only in a specific case of self-assembled QDs. However, limitations in the quality of the optical properties of PQDs are still in place for actual technological implementations. Among these is the distribution of emission energies and the residual fine structure splitting (FSS) affecting the quality of the entangled photon pairs. A piezoelectric stress-based tuning technique is used to tune both the emission energy of PQDs and their FSS, demonstrating also restoral of entanglement upon the application of the proper stress. Finally, a new method for releasing PQDs is shown, which allows their manipulation on several substrates and opens up new possibilities for the integration of the QDs onto functional platforms. As an example, integration over a multimode fiber is demonstrated as well as the emission of single-photons directly through the fiber.
Role of sulfur in vibration spectra and bonding and electronic structure of GeSi surfaces and interfaces
(University College Cork, 2016) Hartnett, Mark C.; Fahy, Stephen B.; Photonic Integration from Atoms to Systems (PiFAS), Tyndall National Institute, Cork
A quantum mechanical density functional theory approach was used to investigate the structural atomic configuration, vibration mode frequencies and electronic structure of surfaces and interfaces using germanium. Initially, we investigated the H2S and H2Opassivated germanium surfaces. A supercell approach is used with the local density (LDA), generalized gradient (GGA) approximations and van der Waals (vdW) interactions. The frozen phonon method was used to calculate the vibrational mode frequencies of these surfaces. The calculated frequencies produce stretch, bond bending and wag modes. The differences between the functionals including vdW terms and the LDA or GGA are less than the differences between LDA and GGA for the vibrational mode frequencies. Some of these modes provide useful vibrational signatures of bonding of both sulfur and oxygen on germanium surfaces, which may be compared with vibrational spectroscopy measurements. A bare germanium surface is bonded to a bare silicon surface to form a Ge-Si interface. As germanium has a 4% larger lattice constant than silicon this implies there are regions on the interface where the germanium and silicon match perfectly (aligned) and are completely mismatched (misaligned). The atomic structure of the GeSi aligned interface shows the original crystal structure and the projected band structure (PBS) shows no interface states in the band gap. The GeSi misaligned structure forms a (2x1) configuration. The electronic PBS shows interface states in the band gap. To remove the interface states seen in the GeSi interface, sulfur with its six valence electrons and its flexible chemical bonds is suggested to improve the interface bonding and remove interface states. The PBS in both the aligned and misaligned GeSSi interfaces shows states around the germanium and silicon interface atomic layers and a charge density localised around the sulfur interface atoms. A sulfur terminated germanium surface results in a (1x1) configuration with surface states present in the band gap. However, a H2S terminated germanium surface results in a (2x1) configuration with symmetric Ge-Ge dimers and pushes the surface states into the bulk region, implying the presence of hydrogen results in no surface states. Including hydrogen on our GeSSi interfaces, the atomic configuration remains the same with the hydrogen molecule in the channels. However, upon looking at the PBS, states are clearly visible in the band gap and when we investigate the charge density contour plots, interface states do exist. Therefore, the presence of hydrogen here does not influence the interfaces.

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