III-V Compound Semiconductor Nanowire Terahertz Detectors

III-V semiconductor nanowires have emerged over the past decade as promising nano-components for future electronic and optoelectronic devices and systems, including field-effect transistors, light-emitting diodes, photodetectors, lasers and solar cells. Recently, III-V semiconductor nanowires have been considered as ideal candidates for photoconductive terahertz (THz) detection, as they possess many desirable properties, such as a direct and tunable band gap, good carrier mobility and short carrier lifetime (on the picosecond to nanosecond timescale). Due to the one-dimensional structure and nanoscale size, such III-V nanowire THz detectors are promising as building blocks for advanced THz systems with compact configuration and enhanced functionalities (i.e. sub-wavelength resolution and high polarisation sensitivity). This dissertation presents the first attempt to examine the suitability of III-V semiconductor nanowires for their applications as photoconductive THz detectors. At first, a series of GaAs/AlGaAs core-shell nanowires were grown (using Au-catalyst metalorganic vapour phase epitaxy technique), characterised and compared for selection to detect the THz signal in a THz time-domain spectroscopy (THz-TDS) system. The fabricated GaAs/AlGaAs single nanowire THz detectors exhibited a pA-level THz response with good signal-to-noise ratio and high polarisation sensitivity however a narrow detection bandwidth (in the range of 0.1-0.6 THz). The origin of the narrow bandwidth for single nanowire detectors was thoroughly investigated using finite-difference time-domain (FDTD) simulations, revealing that the limited bandwidth arose from the strong low frequency resonance caused by the specific device geometry design (rather than the nanowire itself). By adjusting and optimising the nanowire detector geometry, broadband (0.1-1.6 THz) GaAs/AlGaAs single nanowire THz detectors were demonstrated. Furthermore, due to the nanoscale active material fabricated on an insulating substrate (z-cut quartz), single nanowire photoconductive THz detectors showed a very low dark current and a resultant low-noise nature when compared with the traditional (bulk) photoconductive THz detectors. This relaxes the ultra-short carrier lifetime requirement for the (semiconductor) detection material for photoconductive THz detection, since in traditional photoconductive detector the detection material has to have a carrier lifetime of a few picoseconds to minimise the noise current. Therefore, nanowires with longer carrier lifetime can also be used for photoconductive THz detection. Based on above findings, the high-quality core-only InP nanowires (grown by selective-area metalorganic vapour phase epitaxy technique) were investigated for photoconductive THz detection. With previously optimised device geometry and superior optoelectronic properties of InP nanowires, InP single nanowire THz detectors were fabricated and found to exhibit a broadband response (0.1-2.0 THz) and excellent sensitivity, which were then used to measure the transmission spectra for real material characterisation with performance comparable to the traditional (bulk) detectors. A longer time-domain sampling window (compared to the traditional bulk detectors) and thus a higher spectral response resolution were obtained for the InP single nanowire THz detectors, which have been ascribed to the small active material volume and thick THz-transparent z-cut quartz substrate, enabling the single nanowire detector to have less Fabry-Pérot reflections in measured signal. Furthermore, it was found that the contact quality significantly affects THz detector performance and is particularly crucial for the performance and reliability of single nanowire detectors due to their large surface-to-volume ratio. In the final part of this work, an axial n+-i-n+ InP nanowire structure was designed and investigated for use in nanowire THz detectors. The improved contact quality (due to contact doping) has led to further improvement of the nanowire THz detector performance particularly in its signal-to-noise ratio. In summary, this thesis demonstrates a series of photoconductive THz detectors fabricated from different III-V semiconductor nanowire materials and structures, showing excellent bandwidth and sensitivity, approaching that of the conventional THz detectors. The nanowire device design, fabrication, characterisation and related optical simulations described in this work have provided deep insights into the characteristics of the single nanowire THz detectors, which may serve as a useful guidance for future development of nano-device based THz systems

Photonic platform and the impact of optical nonlinearity on communication devices

It is important to understand properties of different materials and the impact they have on devices used in communication networks. This paper is an overview of optical nonlinearities in Silicon and Gallium Nitride and how these nonlinearities can be used in the realization of optical ultra-fast devices targeting the next generation integrated optics. Research results related to optical lasing, optical switching, data modulation, optical signal amplification and photo-detection using Gallium Nitride devices based on waveguides are examined. Attention is also paid to hybrid and monolithic integration approaches towards the development of advanced photonic chips

Flexible and substrate-free optoelectronic devices based on III-V semiconductor nanowires

III-V nanowires have been the subject of intense research interest for the past 20 years, as their unique optical and electronic properties, which arise from their nanoscale dimensions and composition, make them particularly suited for high-performance opto-electronic devices. Since epitaxial growth is on expensive, brittle, crystalline substrates, the field of flexible devices has been little explored in the context of III-V nanowires. In order to fully exploit these properties and move away from conventional wafer based electronics to flexible electronics, hybrid devices consisting of organic and inorganic components must be developed to harness the benefits from both materials systems. Embedding high performance vertically aligned III-V nanowires in a flexible matrix enables applications where there is a need for substrate-free, flexible devices. The work in this thesis looks to address this by (1) developing a repeatable method of producing nanowire-polymer thin films and (2) demonstrating how these thin films could be fabricated into different opto-electronic devices. The thin films are made by encapsulating the nanowires in Parylene C, which are then be peeled off from the growth substrate, thus retaining the vertical alignment of the nanowires. These thin films are used to fabricate a THz modulator and a solar cell. Single and multi-layer THz modulators are fabricated from nanowire-Parylene C thin films laminated together. 1,2,4,8, and 14-layer modulators are compared, with the 14-layer modulator displaying the best performance. A high switching speed (<5 ps), modulation depth (-8 dB), extinction (13%) and dynamic range (-9 dB) and broad bandwidth operation (0.1 THz–4 THz) are obtained. This surpasses the performance of several devices in the literature and presents the first THz modulator which combines a large modulation depth, broad bandwidth, picosecond time resolution for THz intensity and phase modulation, which makes it an ideal candidate for ultrafast THz communication. In addition to the THz work, the fabrication process towards a flexible solar cell is also developed. This consists of optimising the dry etching, and annealing-free contacting processes to give nanowire devices that show good ohmic IV characteristics. Following this work, a proof-of-concept Schottky barrier solar cell is fabricated using the knowledge gleaned from this development work. This preliminary device gives a conversion efficiency of 0.02% and a fill factor of 0.3, with scope for device performance improvement by using nanowires that are grown and optimised specifically for solar cell operationEPSRC - Photonics CD

Semiconductor photodetectors for photon-starved applications in the short-wavelength infrared spectral region

The design, fabrication and characterisation of planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors is described in this Thesis. These devices utilise a Si avalanche multiplication layer, and an adjacent Ge layer to absorb short-wave infrared incident photons. The innovative planar geometry design ensures the confinement of the high electric field to the centre of the detector away from the exposed sidewalls resulting in significantly reduced dark count rate (DCR). Planar Ge-on-Si SPADs were fabricated and characterised in terms of single-photon detection efficiency (SPDE), DCR, and timing jitter. These devices exhibited SPDE of almost one order of magnitude greater than previously reported, with the highest SPDE measured being 38%. The dark count rates per unit area were approximately 4 orders of magnitude less than equivalent mesa devices. A record-low noise equivalent power of 4 × 10-17 WHz-1/2 was obtained, more than two orders of magnitude lower than the previous best reported value. The lowest timing jitter of 26 µm diameter devices was 150 ps. These devices exhibited lower afterpulsing when compared to a commercial InGaAs/InP SPAD detector, illustrating the potential for high count rate operation. An investigation of an SPDE spectral dependence at different operating temperatures revealed that efficient single-photon detection of 1550 nm wavelength light will require an operating temperature of 245 K. Laboratory-based light detection and ranging (LIDAR) experiments using the time-offlight approach were performed using an individual Ge-on-Si SPAD detector. This approach allowed depth and intensity profiles of scanned targets to be reconstructed. Based on these results, a parametric LIDAR model was used to estimate LIDAR performance at long distances. For example, eye-safe sub-mW average laser power levels would be sufficient for imaging at kilometre distances. It was demonstrated that by employing appropriate image processing algorithms the total acquisition time can be reduced down to a few seconds for a 10000 pixels image at kilometre range, illustrating the potential for rapid three-dimensional imaging for automotive applications.School of Engineering and Physical Science

Near infra-red single-photon detection using Ge-on-Si heterostructures

This Thesis investigates the design of Ge-on-Si single-photon avalanche diode (SPAD) detectors combining the many advantages of low-noise Si single-photon avalanche multiplication with the infrared sensing capability of germanium. The devices were simulated by using electric field modelling software to predict key aspects of the device behaviour in terms of the current-voltage characteristic and electric field. The devices were then characterised in terms of their single-photon performance. A 25 m diameter device showed a single-photon detection efficiency of ~ 4 % at a wavelength of 1310 nm and a temperature of 100 K when biased at 10 % above the breakdown voltage. In the same condition, a dark count rate of ~ 6 Mcs-1 was measured. This resulted in the lowest noise equivalent power of ~ 1 × 10-14 WHz-1/2 of Ge-on-Si SPADs reported in the scientific literature. At the longer wavelength of 1550 nm, the single-photon detection efficiency was reduced to ~ 0.1 % at 125 K and 6 % of relative excess bias. Although further investigation needs to be carried out, a potential major advantage of these devices compared to the InGaAs/InP SPADs could be that of reduced afterpulsing since a small increase (a factor of 2) in the normalised dark count rate was measured when the repetition rate was increased from 1 kHz to 1 MHz. Finally, the fill-factor enhancement of 32 × 32 Si CMOS SPAD arrays resulting from the integration of high efficiency diffractive optical microlens arrays was investigated. A full characterisation of SPAD arrays integrating microlens arrays in terms of improvement factor and spatial uniformity of detection is presented for the first time in the scientific literature in a large spectral range (500-900 nm) and different f-numbers (from f/2 to f/22) by using a double telecentric imaging system. The highest improvement factor of ~16 was measured for a SPAD array integrating microlens arrays, combined with a very high spatial efficiency uniformity of between 2–6%

Superconducting nanowire single-photon detectors for advanced photon-counting applications

The ability to detect infrared photons is increasingly important in many elds of scienti c endeavour, including astronomy, the life sciences and quantum information science. Improvements in detector performance are urgently required. The Superconducting Nanowire Single-Photon Detector (SNSPD/SSPD) is an emerging single-photon detector technology o ering broadband sensitivity, negligible dark counts and picosecond timing resolution. SNSPDs have the potential to outperform conventional semiconductor-based photon-counting technologies, provided the di culties of low temperature operation can be overcome. This thesis describes how these important challenges have been addressed, enabling the SNSPDs to be used in new applications. A multichannel SNSPD system based on a closed-cycle refrigerator has been constructed and tested. E cient optical coupling has been achieved via carefully aligned optical bre. Fibre-coupled SNSPDs based on (i) NbN on MgO substrates and (ii) NbTiN on oxidised Si substrates have been studied. The latter give enhanced performance at telecom wavelengths, exploiting the re ection from the Si=SiO2 interface. Currently, the detector system houses four NbTiN SNSPDs with average detection e ciency >20% at 1310 nm wavelength. We have employed SNSPDs in the characterisation of quantum waveguide circuits, opening the pathway to operating this promising platform for optical quantum computing for the first time at telecom wavelengths

Photonic Technology for Precision Metrology

Photonics has had a decisive influence on recent scientific and technological achievements. It includes aspects of photon generation and photon–matter interaction. Although it finds many applications in the whole optical range of the wavelengths, most solutions operate in the visible and infrared range. Since the invention of the laser, a source of highly coherent optical radiation, optical measurements have become the perfect tool for highly precise and accurate measurements. Such measurements have the additional advantages of requiring no contact and a fast rate suitable for in-process metrology. However, their extreme precision is ultimately limited by, e.g., the noise of both lasers and photodetectors. The Special Issue of the Applied Science is devoted to the cutting-edge uses of optical sources, detectors, and optoelectronics systems in numerous fields of science and technology (e.g., industry, environment, healthcare, telecommunication, security, and space). The aim is to provide detail on state-of-the-art photonic technology for precision metrology and identify future developmental directions. This issue focuses on metrology principles and measurement instrumentation in optical technology to solve challenging engineering problems

Shape engineering of InP nanostructures for optoelectronic applications

III-V semiconductor nanostructures have been the research focus in the past two decades thanks to the superior properties of the materials themselves and the unique properties produced by reducing the dimension to the nanoscale. In particular, III-V nanowires have drawn much more attention and have been extensively applied in a wide range of devices including solar cells, light-emitting diodes, lasers, transistors, and photodetectors. Despite these great successes, these one-dimensional (1D) nanostructures still face many challenges in terms of synthesis, assembly, fabrication and the ability to form complex nanoarchitectures. Surprisingly, it has been demonstrated that nanostructures with more complex two- and three-dimensional (2D and 3D) shapes can provide possible solutions. These shape-engineered nanostructures can improve material properties and device functionality. Furthermore, recent intensive investigation of nanostructure networks shows a great demand on the shape flexibility, uniformity, structural and optical qualities of epitaxially grown III-V nanostructures. This dissertation presents the shape engineering of InP-based nanostructures grown by metal organic chemical vapour deposition (MOCVD) from 1D nanowires to more sophisticated 2D and 3D shapes. Shape transformation mechanism, crystal structure and optical properties were thoroughly investigated to understand the growth mechanism of these complex III-V nanostructures for electronic and optoelectronic applications. Among the various growth techniques, selective area epitaxy (SAE) has the advantages of producing uniform nanostructure arrays with a high degree of controllability in pattern geometry, such as shape, dimension, site position and spacing, and thus has been used in this work. InP nanostructures grown on {111}A InP substrates was first investigated. Two key growth parameters, temperature and V/III ratio, were studied to optimise the growth conditions. We found that a higher growth temperature was crucial to obtain high quality nanostructures. Under optimal growth conditions, the highly uniform arrays of wurtzite (WZ) InP nanostructures with tunable shapes, such as nanowires, nanomembranes, prism- and ring-like nanoshapes, were simultaneously achieved. Their side facets can be dominated by {101̅0} and/or {112̅0}, making WZ InP a good candidate for tailoring the shape of the nanostructures. In-depth investigation of shape transformation with time and opening geometry/dimension revealed that the shape was essentially determined by pattern confinement and minimisation of total surface energy. A theoretical model was proposed to explain the observed behaviour. Structural and optical characterisation results demonstrated that all the different InP nanostructures grown under optimal conditions have perfect wurtzite (WZ) crystal structure regardless of their shape and strong and homogeneous photon emission. Moreover, we investigated the shape evolution from branched nanowires to vertical/inclined nanomembranes and crystal structure of InP nanostructures grown on InP substrates of different orientations, including {100}, {110}, {111}B, {112}A and {112}B. Two growth models were proposed to explain these observations regarding shape transformation and phase transition. A strong correlation between the growth direction and crystal phase was revealed. Specifically, WZ and zinc-blende (ZB) phases form along the A and B directions, respectively, while crystal phase remains the same along other low-index directions. The polarity-induced crystal structure difference was explained by thermodynamic difference between the WZ and ZB phase nuclei on the {111}A and {111}B planes. Growth from the openings was essentially determined by pattern confinement and minimisation of the total surface energy, similar to growth on {111}A InP substrates. Accordingly, a novel type-II WZ/ZB nanomembrane homojunction array was obtained by tailoring growth directions through alignment of the openings along certain crystallographic orientations. Finally, the incorporation of InAsP quantum well was carried out on pure WZ InP nanostructure templates grown on {111}A InP substrates with two different shapes, i.e. nanowires and nanomembranes. InAsP quantum wells grew both axially and laterally on the InP nanowires and nanomembranes. While the axial quantum well was of ZB phase, the lateral one grown on side facets had a WZ phase. When sidewalls of nanowires and nanomembranes were the nonpolar {11̅00} facets, the radial quantum well selectively grew on the sidewall located at the semi-polar A side of the axial quantum well, leading to the shape evolution of nanowires from hexagonal to triangular cross section and destroying the symmetry of nanomembranes. In comparison, nanomembranes with {112̅0} sidewalls are shown to be an ideal template for growing InP/InAsP heterostructures thanks to the high symmetry and uniformity of quantum well nanomembrane array. EDX results showed that quantum well composition was highly dependent on the crystal facet. Moreover, quantum well nanomembranes with {112̅0} sidewalls gave strong and uniform photon emission around 1.3 µm, showing superior optical properties compared with quantum wells incorporated in InP nanowires and nanomembranes with {11̅00} facets

Photodetectors for graphene-based integrated photonics

The development of integrated optical circuits has enabled a diverse portfolio of chipscale photonic applications—ranging from data communication over sensing to imaging—that is set to grow further as new device concepts in, for example, quantum information processing and optical neural networks mature. While silicon photonics has emerged as a viable candidate to translate proof-of-principle demonstrations to mass-manufacturing, the fabrication of photonic integrated circuits (PICs) and their subcomponents remains highly heterogeneous, which drives up cost and slows down progress towards faster and more power-efficient performance. In this dissertation, I demonstrate how single-layer graphene (SLG) can bring active functionality to arbitrary passive waveguide platforms, offering a more universal approach to developing integrated photonic components. SLG can be transferred to PICs without the limitations or complexity of traditional deposition or bonding processes. It also stands to outperform conventional semiconductors in terms of speed and spectral operating range due to its ultra-fast carrier dynamics, high carrier mobility, and gapless bandstructure. Using the key component for optical-to-electrical signal conversion—the photodetector—as an example, here I demonstrate graphene-based components on three different PIC platforms. First, I show plasmonic enhanced graphene photodetectors (GPDs) on silicon nitride waveguides, which generate a voltage from optically generated hot-carrier distributions in SLG via the photo-thermoelectric (PTE) effect. Then, I demonstrate PTE-GPDs—fabricated from layered material heterostructures—on a silicon-on-insulator microring resonator. Both detectors operate at Telecom wavelengths (λ = 1.55 μm), are compatible with high-speed (> 10 GHz) operations, and hold the current records in voltage responsivity—R ∼ 12 V/W and R ∼ 90 V/W—for waveguide-integrated GPDs fabricated from chemical vapour deposited and mechanically exfoliated SLG, respectively. I then go on to show how established GPD concepts can be translated to an integrated mid-infrared (λ = 3.8 μm) platform based on sub-wavelength grating waveguides in silicon and study how light-graphene interaction under in-plane incidence can be further optimised to improve GPD performance. Finally, I develop new approaches towards high-quality, scalable SLG on PICs, critically required to advance graphene-based integrated photonics towards industrial production