Exploring and exploiting charge-carrier confinement in semiconductor nanostructures:heterodimensionality in sub-monolayer InAs in GaAs and photoelectrolysis using type-II heterojunctions

In this thesis, semiconductor nanostructures are studied, both experimentally and theoretically, to help aid the development of two diverse and important technologies. Firstly, charge-carrier confinement in stacked sub-monolayer (SML) InAs in GaAs: SML deposition results in the formation of In-rich agglomerations within a lateral InGaAs quantum well (QW) with lower In content. Low-temperature photoluminescence (PL) and magneto-PL reveals strong vertical and weak lateral confinement, indicative of a two-dimensional (2D) excitons. Paradoxically, high-temperature magneto-PL allows excited-state peaks to become resolved, which can be fitted by a Fock-Darwin spectrum, characteristic of a zero-dimensional (0D) system. To solve this contradiction, we postulate that stacked SML InAs in GaAs forms a heterodimensional system, in which electrons are 2D, and see only the lateral InGaAs QW, whilst the heavier holes are 0D, and are confined in the In-rich agglomerations. This description is fully supported by single-particle effective-mass and eight-band k · p calculations, which show heterodimensional confinement is probable for a large variation in In content. SML vertical-cavity surface-emitting lasers (VCSELs) — which prove to be one of the most promising candidates for datacoms applications — have been demonstrated at >20 Gb/s, and we postulate that heterodimensionality is fundamental to this high-speed operation. Efficient carrier injection is achieved by the lack of a wetting layer, along with the 2D electrons coupling to several In-rich agglomerations, making them quickly available to states that are lasing. Furthermore, the shallow confining potential of the In-rich agglomerations means that excess holes cannot build up in states that aren't lasing. Secondly, semiconductor photoelectrolysis for the solar-powered generation of renewable hydrogen by water splitting is researched. The novel use of nanostructures at the semiconductor-electrolyte interface (SEI) in a photoelectrochemical cell (PEC) is proposed to help increase the maximum potential that can be photo-generated, thus increasing the likelihood of a given PEC being able to split water. By solving the Schrödinger, Poisson and drift-diffusion equations, we simulate the band alignment, confined carrier energy states and carrier densities for a variety of different material systems. ZnO quantum dots on InxGa1-xN show the most promising band alignment, with electron-donating and -accepting states straddling the hydrogen- and oxygen-production potentials (respectively) for small x (x < 0.3), indicating an ability to split water

State-of-the-art InAs/GaAs quantum dot material for optical telecommunication

This thesis reports on the characterization of the state-of-the-art In(Ga)As/GaAs quantum dot (QD) material grown by molecular beam epitaxy for optical telecommunication applications. A wide variety of characterization methods are employed to investigate the material properties and characteristics of a number of QD-based devices enabling future device optimization. The motivation that prompted this study was predicated mainly upon two technological advantages. First, that the QDs gain spectra exhibits a symmetric gain shape and thus the change of refractive index with respect to gain is negligible at the lasing wavelength. This is therefore expected to result in a zero or a very small linewidth enhancement factor (LEF), which is desirable for instance, for high-speed modulation purposes where frequency chirp under modulation, which is directly proportional to the LEF, may be substantially reduced. Second, the fact that not only QDs exhibit a damped frequency response attributed to the carrier relaxation dynamics but also as the resilience of a laser to optical feedback is inversely proportional to the fourth power of the LEF, QD lasers are expected to demonstrate a relatively higher feedback insensitivity. This bodes well for operating these devices isolator free, which would be greatly cost-effective. The absorption and gain spectra of the QD active material are investigated in chapters 2 and 3, respectively. The LEF of QD lasers at a range of temperatures is studied in chapter 3, which confirms the expectation for the first time for In(Ga)As/GaAs QD lasers from -10 oC to 85 oC. Subsequently, the findings of chapters 2 and 3 are employed in chapter 4 with an electro absorption modulator device in mind which would be able to operate with chirp control. In chapter 5, the modulation response of QD lasers is investigated through examining the relative intensity noise (RIN) spectra in the electrical domain. The resilience of the devices to optical feedback is subsequently studied through the RIN characteristics at a range of temperatures. Chapter 6 provides a summary of the thesis findings and possible future works that may be carried out as continuation to this project, which fell outside of the remit of this work

InAs quantum dot vertical-cavity lasers

Edge-emitting semiconductor lasers with self-assembled InAs quantum dot (QD) active regions have demonstrated excellent device performance, including low sensitivity to operating temperature and record-low thresholds. In this dissertation, the application of QDs in vertical-cavity lasers (VCLs) is investigated. QDs can reach an emission wavelength up to 1300 nm on GaAs substrate. Key design and device processing issues are discussed and vertical-cavity surface emitting lasers (VCSELs) with both optical and electrical excitation are fabricated. VCSEL diodes with distributed Bragg reflectors (DBRs) formed by selective wet oxidation of AlAs, as well as standard GaAs/AlGaAs mirrors were processed. The latter performed better due to an increased number of QD layers in the cavity. Continuous wave (CW) operation of InAs QD VCSEL diodes with 1 mW output power and threshold current densities below 500 Acm^−2 were achieved. Replacing one of the DBRs with an external spherical mirror, vertical-external cavity surface-emitting lasers (VECSELs) allow the lateral dimensions of the device active region to be increased significantly, yielding high output power while still retaining single mode operation. Pumped by widely available high power diode lasers, QD VECSELs with CW output powers close to 400 mW were demonstrated with threshold pump power densities below 1 kWcm^−2. Since the VECSEL cavity extends into free space, additional optical components can be integrated. By using a non-linear β-Barium Borate (BBO) inside the cavity, we were able to frequency-double the QD emission to produce visible red light, which could be utilized for the red channel of full-color laser projection applications. Despite suboptimal cavity design and minimal heatsinking, output powers over 10 mW at a wavelength of 630 nm were achieved

Recent Progress in Optical Fiber Research

This book presents a comprehensive account of the recent progress in optical fiber research. It consists of four sections with 20 chapters covering the topics of nonlinear and polarisation effects in optical fibers, photonic crystal fibers and new applications for optical fibers. Section 1 reviews nonlinear effects in optical fibers in terms of theoretical analysis, experiments and applications. Section 2 presents polarization mode dispersion, chromatic dispersion and polarization dependent losses in optical fibers, fiber birefringence effects and spun fibers. Section 3 and 4 cover the topics of photonic crystal fibers and a new trend of optical fiber applications. Edited by three scientists with wide knowledge and experience in the field of fiber optics and photonics, the book brings together leading academics and practitioners in a comprehensive and incisive treatment of the subject. This is an essential point of reference for researchers working and teaching in optical fiber technologies, and for industrial users who need to be aware of current developments in optical fiber research areas

Surface plasmons for enhanced metal-semiconductor-metal photodetectors

Surface Plasmon Polaritons (SPPs) are quantized charge density oscillations that occur when a photon couples to the free electron gas of the metal at the interface between a metal and a dielectric. The extraordinary properties of SPP allow for sub-diffraction limit waveguiding and localized field enhancement. The emerging field of surface plasmonics has applied SPP coupling to a number of new and interesting applications, such as: Surface Enhanced Raman Spectroscopy (SERS), super lenses, nano-scale optical circuits, optical filters and SPP enhanced photodetectors. In the past decade, there have been several experimental and theoretical research and development activities which reported on the extraordinary optical transmission through subwavelength metallic apertures as well as through periodic metal grating structures. The use of SPP for light absorption enhancement using sub-wavelength metal gratings promises an increased enhancement in light collection efficiency of photovoltaic devices. A subwavelength plasmonic nanostructure grating interacts strongly with the incident light and potentially traps it inside the subsurface region of semiconductor substrates. Among all photodetectors, the Metal-Semiconductor-Metal photodetector (MSM-PD) is the simplest structure. Moreover, due to the lateral geometry of the MSM-PDs, the capacitance of an MSM-PD is much lower than capacitances of p-i-n PDs and Avalanche PDs, making its response time in the range of a few tens of picoseconds for nano-scale spacing between the electrode fingers. These features of simple fabrication and high speed make MSM-PDs attractive and essential devices for high-speed optical interconnects, highsensitivity optical samplers and ultra-wide bandwidth optoelectronic integrated circuits (OEIC) receivers for fibre optic communication systems. However, while MSM-PDs offer faster response than their p-i-n PD and avalanche PD counterparts, their most significant drawbacks are the high reflectivity of the metal fingers and the very-low light transmission through the spacing between the fingers, leading to very low photodetector sensitivity. This thesis proposes, designs and demonstrates the concept of a novel plasmonicbased MSM-PD employing metal nano-gratings and sub-wavelength slits. Various metal nano-gratings are designed on top of the gold fingers of an MSM-PD based on gallium arsenide (GaAs) for an operating wavelength of 830 nm to create SPP-enhanced MSM-PDs. Both the geometry and light absorption near the designed wavelength are theoretically and experimentally investigated. Finite Difference Time Domain (FDTD) simulation is used to simulate and design plasmonic MSM-PDs devices for maximal field enhancement. The simulation results show more than 10 times enhancement for the plasmonic nano-grating MSM-PD compared with the device without the metal nano-gratings, for 100 nm slit difference, due to the improved optical signal propagation through the nano-gratings. A dual beam FIB/ SEM is employed for the fabrication of metal nano-gratings and the sub-wavelength slit of the MSM-PD. Experimentally, we demonstrate the principle of plasmonics-based MSM-PDs and attain a measured photodetector responsivity that is 4 times better than that of conventional single-slit MSM-PDs. We observe reduction in the responsivity as the bias voltage increases and the input light polarization varies. Our experimental results demonstrate the feasibility of developing high-responsivity, low bias-voltage high-speed MSM-PDs. A novel multi-finger plasmonics-based GaAs MSM-PD structure is optimized geometrically using the 2-D FDTD method and developed, leading to more than 7 times enhancement in photocurrent in comparison with the conventional MSM-PD of similar dimensions at a bias voltage as low as 0.3V. This enhancement is attributed to the coupling of SPPs with the incident light through the nano-structured metal fingers. Moreover, the plasmonic-based MSM-PD shows high sensitivity to the incident light polarization states. Combining the polarization sensitivity and the wavelength selective guiding nature of the nano-gratings, the plasmonic MSM-PD can be used to design high-sensitivity polarization diversity receivers, integrating polarization splitters and polarization CMOS imaging sensors. We also propose and demonstrate a plasmonic-based GaAs balanced metalsemiconductor- metal photodetector (B-MSM-PD) structure and we measure a common mode rejection ratio (CMRR) value less than 25 dB at 830nm wavelength. This efficient CMRR value makes our B-MSM-PD structure suitable for ultra-high-speed optical telecommunication systems. In addition, this work paves the way for the monolithic integration of B-MSM-PDs into large scale semiconductor circuits. This thesis demonstrates several new opportunities for resonant plasmonic nanostructures able to enhance the responsivity of the MSM-PD. The presented concepts and insights hold great promise for new applications in integrated optics, photovoltaics, solidstate lighting and imaging below the diffraction limit. In Chapter 10 we conclude this thesis by summarizing and discussing some possible applications and future research directions based on SPP field concentration