Physics - Doctoral Theses
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Item Surface dynamics in III-V epitaxy and its device implications(University College Cork, 2024) Ozcan Atar, Ayse; Pelucchi, Emanuele; Juska, Gediminas; Science Foundation IrelandMetalorganic 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.Item Towards a GaN-on-sapphire photonic integrated circuit via micro-transfer printing(University College Cork, 2024) O'Brien, Megan; Corbett, Brian; Peters, Frank H.; Science Foundation IrelandPhotonic integrated circuits (PICs) can alleviate the pressure on existing infrastructure for increasing bandwidth requirements by potentially offering faster data transmission with low energy consumption all on the same chip. The ability to integrate active lasing sources to waveguide platforms while also maintaining low-loss coupling is key for efficient PICs.This research focuses on the development of an AlGaN/GaN-on-sapphire platform and the integration of 1.3 µm edge-emitting InP-based multi-quantum well (MQW) and GaAs-based quantum dot (QD) lasing devices to the platform via micro transfer printing (µTP). Gallium nitride (GaN) can be used alternatively to traditional Si-based photonics due to its broad spectral transmission, birefringence, refractive index suitable for fiber coupling, and the possession of both the linear electro-optic effect and non-linear properties. First, we design an AlGaN/GaN structure that creates a single-mode, broad passive spot size for relaxed alignment tolerance of the transfer-printable laser-to-waveguide. Coupling losses as low as 0.6 dB were calculated for the optimal positioning of the laser. We also outline a double-tapered mode adapter that converts the dimensions to more suitable structures for light-routing. The integration of LiNbO3 coupon to the AlGaN/GaN platform is explored through simulations in order to enhance the electro-optic property, and a Mach-Zehnder modulator (MZM) design is proposed. Additionally, the simulations of (i) directional couplers, (ii) polarization converters and (iii) grating couplers on an AlGaN/GaN platform are presented in this work. Next, the fabrication of AlGaN/GaN waveguides devices through BCl3-based etching recipes, along with the challenges of achieving smooth and vertical waveguide sidewalls, are discussed. A BCl3 dry etch for forming a trench into sapphire is also developed for µTP alignment purposes. The AlGaN/GaN waveguides were then characterized, and we distinguish the differences in propagation losses between the TE and TM polarizations.The average propagation losses ranged between 2.4-14.7 dB/cm for both polarizations. Following on from this, both the InP-based MQW and the GaAs-based QD laser coupons were characterized and transfer-printed and coupled to the fabricated GaN waveguides. A coupling output of 2.3 mW at 80 mA was achieved for the QD laser, giving a coupling efficiency of 10.3 %. The results show promise for GaN to be a suitable platform for integrated photonics.Item Micro-transfer print integration of high-speed photodetectors to SOI platform(University College Cork, 2024) Muthuganesan, Hemalatha; Corbett, Brian; Peters, Frank H.; Science Foundation IrelandPhotonic integrated circuits (PICs) offer a pivotal solution to meet the escalating demands of data bandwidth and used in data centres, LIDAR and optical sensing. Among various approaches, micro-transfer printing emerges as a rapid and universally compatible integration technology to generate PICs. To implement this technology, it is necessary to separate the devices from their original substrate using a release layer and transfer print them to designated location on target wafer. This enables the heterogeneous integration of numerous active devices from multiple wafers onto a single wafer, resulting in a compact and highly functional PIC. In addition to being a room temperature process, it is also cost-effective allowing reuse of expensive III-V substrates. This thesis demonstrates the application of micro-transfer printing technology in integrating ultra-thin, high-speed InGaAs photodetectors with silicon waveguides through various coupling mechanisms. The thesis begins with optimizing the optical power coupled between the waveguide and the photodetector (PD), as even a small gap between them leads to optical mode loss at the interface. A co-planar in-fill idea is experimented by transfer printing the PD’s absorber region at the same height as silicon waveguide. To fill the gap, evaporated amorphous silicon (a-Si) with a refractive index of 3.1 in the telecom wavelength range is utilized. This strategy aims to achieve maximum coupling. In transfer printing process, it is crucial to efficiently release the PD coupons along with smooth interface, for successful printing on target wafer. This work introduces for the first time, a combination of InGaAs and AlInAs as release layers for InP based devices yielding double the etch rate and almost isotropic etch compared to individual release layers (InGaAs or AlInAs). This facilitates direct bonding of the PD coupons to the target wafer since the interface roughness is extremely low as 0.2 nm over an area of 10 µm x 10 µm, contributing to high-speed of the PD. An excellent selectivity of 5410 is obtained with InP, which is 3.4 times higher than AlInAs and 7 times higher than InGaAs as release layers. These outcomes are accomplished at room temperature thus saving time, cost, and energy for the entire process. The above optimized processes are used in integration of high-speed InGaAs photodetector to SOI platform, by direct bonding, with 100 % yield. An ultra-thin 675 nm epitaxial stack is grown with dual release layer on InP substrate, where a PD of size 21 µm x 57 µm is fabricated. Such small dimension has the potential to fit 1 million devices on a 75 mm InP wafer. The same photodetectors are coupled to silicon waveguides via evanescent, grating and butt coupling mechanisms on the same target wafer. These PDs exhibit a maximum responsivity of 0.6 A/W, 47 nA dark current and with a data communication rate of 50 Gbps with on-off keying. In future, this unique integration approach can be universally applied to any III-V active device like lasers and modulators to realise compact and high performance photonic integrated circuits.Item Toward single-growth monolithically integrated electro-absorption modulated lasers(University College Cork, 2023) Mulcahy, Jack; Peters, Frank H.; Corbett, Brian; Science Foundation Ireland; Rockley PhotonicsEvery year the demand for bandwidth is growing exponentially due to the emergence of data-intensive services such as high-definition video streaming, cloud-based computing, and machine-to-machine communication. This rapid expansion is primarily driven by the extensive deployment of fibre-based optical communication networks. Consequently, there is an increasing need for photonic components to meet the requirements of these networks, which are expanding both in geographical coverage and terminal density. To satisfy this demand, the photonics industry must enhance its production capabilities and adopt more efficient fabrication processes. A crucial aspect of streamlining fabrication involves eliminating slow and costly processes. In photonics fabrication, epitaxial regrowth and advanced lithography steps are typically time-consuming and expensive, making them prime targets for process optimisation. Moreover, the integrated electronics approach provides valuable insights by enabling the monolithic integration of multiple photonic components fabricated simultaneously. This integration technique allows for the creation of highly complex circuits while reducing overall fabrication complexity. This research focuses on a key component at the heart of photonic circuits: the tunable single-mode laser. The aim is to contribute to the development of components that can be fabricated without the need for regrowth or advanced lithography. Additionally, the study emphasises the importance of monolithic integration, specifically with electro-absorption modulators (EAMs). By integrating EAMs with tunable lasers, the resulting devices can offer enhanced functionality and performance, leading to more efficient and compact photonic systems. The issue at hand, however is the varied epitaxial requirements of lasers and EAMs, which provides a noted barrier to a monolithic, regrowth-free integration process. This thesis aims to advance the development of single-growth monolithically integrated externally modulated lasers (EMLs) based on electro-absorption modulators (EAMs). The design of quantum well structures is explored, revealing the significance of introducing an imbalance in the position of the quantum wells to optimise the transit times of carriers in EAMs, thus maximising the bandwidth. Simulation studies on epitaxial structures led to the identification of an optimal material that balances the performance of lasers and EAMs, providing an ideal platform for EML fabrication. Different laser designs are investigated, including slotted Fabry P\'erot lasers and snails, with a focus on achieving a redshifted single-mode laser. Simulation models are developed to predict laser reflectivity and spectral output, which were verified through fabrication and testing. The optimal laser design for integrated EMLs was determined through critical evaluation with a laser being produced with $>$ \SI{40}{dB} SMSR and a tuning range of \SI{60}{nm}. A high-speed process for fabricating EAMs is developed, featuring optimised lithographic mask layers for the isolation of contact pads and metal bridges to reduce parasitic capacitance. The resulting EAMs exhibited a predicted bandwidth of approximately \SI{80}{GHz}. Drawing upon the knowledge gained from laser and EAM simulation, fabrication, and characterisation, a new high-speed process for EMLs is devised. The o-band lasers and EAMs were designed based on optimal principles determined in previous chapters. The fabricated single-mode lasers were successfully matched to simulated models. Further analysis identified potential avenues for improving future EML fabrication yields. In summary, this thesis provides valuable insights and tools for the creation of single-growth monolithically integrated electro-absorption modulated lasers. The journey spans from material design to device outputs, with the aim of enabling readers to replicate and enhance the development of EMLs.Item Evaluation and properties of site-controlled pyramidal quantum dots for quantum information processing(University College Cork, 2023) Ranjbar Jahromi, Iman; Pelucchi, Emanuele; Townsend, PaulSemiconductor quantum dots (QDs) are a promising platform for optical quantum information processing (QIP) due to their unique optoelectronic properties, such as their discrete energy levels and strong light-matter interactions. These properties allow for the manipulation and control of the optical and spin states of individual QDs, making them suitable for applications such as quantum cryptography,quantum computing, and quantum simulation.One of the key advantages of QDs for QIP is, aside of being embedded in asemiconductor matrix, their small size, which allows for the confinement of carriers within a few nanometers. This results in a discrete energy spectrum and a strong confinement of the carrier wavefunctions, which leads to a strong light-matter interaction. This strong interaction allows for the manipulation of the optical and spin states of individual QDs, enabling the implementation of quantum gates and other quantum operations.In particular, site-controlled pyramidal QDs have emerged as a promising candidate for optical quantum information processing, due to their unique structuraland optical properties. Pyramidal QDs are formed by patterning a semiconductor material into a pyramid shape recesses, which results in a confinement of the electronic and optical states in the inverted pyramid apex. This confinement allows for a high level of control over the electronic and optical properties of the dot, such as the energy level spacing and the optical transition dipole moment. Furthermore,pyramidal QDs (intrinsically) could exhibit a high degree of symmetry, which makes them well suited for implementing various quantum operations.One of the key advantages of pyramidal QDs is their ability to achieve high optical quality, which is crucial for implementing high-fidelity quantum operations. In addition, pyramidal QDs can be integrated with other semiconductor technologies, such as waveguides and electro-optic modulators, which allows for the implementation of more complex quantum operations. This integration can be done using various techniques such as epitaxial growth, nano-fabrication and transfer printing or other heterogeneous integration techniques.Overall, site-controlled pyramidal quantum dots have emerged as a promising candidate for optical quantum information processing, due to their unique structuraland optical properties, which allow for high-fidelity quantum operations and the integration with other semiconductor technologies. This research field is still in the early stage and a lot of work is currently ongoing to improve the performance and scalability of pyramidal QD devices.In this work, we evaluate and optimize some features of this specific type of QDs for optical quantum information processing. One of the main prominent factor/feature, called fine structure splitting (FSS), which is detrimental to the quality of entanglement (essential to quantum information processing), is widely studied and different strategies are discussed to minimize its pernicious effect in the GaAs QD family. Resonant excitation of anti-binding InGaAs QDs, bindingGaAs QDs and charge distribution of GaAs QDs sandwiched between super-lattice structure are other topics covered in this work.