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Modern semiconductor physics research is often carried out at low temperatures where the carrier mobility is high and the thermal broad- ening of quantised energy levels is minimised. 4.2 K, the temperature of liquid helium, is thought of as relatively warm. Temperatures of 300 mK and below are readily accessible with sorption-pumped helium-3 and 3He/4He dilution refrigerators. At such low tempera- tures, the phonons that normally facilitate thermal equilibrium be- tween the charge carriers and the lattice are greatly suppressed. This can lead to the electron system thermally decoupling from the lattice. Conventional thermometers such as ceramic oxide or ruthenium ox- ide resistance thermometers thermally couple to the lattice, meaning that in this regime, they only effectively measure the lattice temper- ature, TL. The effect is further pronounced in low dimensional car- rier systems such as two-dimensional electron gases (2DEGs). This is problematic because the electron temperature, Te, is often of far greater importance than TL in such systems. Recent efforts have been made to develop thermometry techniques that are directly sensitive to Te. These techniques include using the thermopower of quantum point contacts and the Coulomb blockade of quantum dots. However, these techniques have issues with the ease and reliability of fabrication of the thermocouples, and the complexity of their subsequent oper- ation. This thesis presents a direct electron thermometry technique called bar-gate thermometry (BGT) that utilises the diffusion ther- mopower of the 2DEG as a thermocouple. BGT uses simple metal- lic bar-gates, fabricated directly onto the semiconductor wafer, and requires no particularly sophisticated electronic instruments. BGT proves to be a reliable and relatively simple thermometry technique. A device featuring a BGT is presented which allows a determination of the thermal relaxation length, l, of the 2DEG. This comes hand in hand with effective measurements of the inelastic scattering time, τi, and the inelastic scattering exponent, αi, of the system, which are not otherwise easily measured. Finally, an example of how BGTs can be used to measure the thermal conductivity of mesoscopic 2DEGs is given, which is just one possible use case of the technique. An overview of all the necessary and relevant theory needed to under- stand the techniques and results, as well as information on the design and fabrication of the devices used in the experiments is also included., The Leverhulme Trust

Semiconductor quantum dots (QD)s have been well established as sources of single photons and entangled photon-pairs, both of which are essential for realising quantum networks. To be compatible with real-world applications, such a source must further conform to basic requirements including operation at telecom wavelengths and high clock rates. Today, most quantum dot experiments are carried out with sophisticated and bulky research-grade equipment, restricting their use to within laboratories. In this work, a high clock rate entangled photon-pair source for deployment in an existing fibre network is developed. Alternatives to components within the experimental system which are incompatible with deployment are also investigated. To remove the requirement for laser excitation, the quantum dot entangled LED (ELED) is electrically pulsed. The ELED, which is fabricated using standard semiconductor processing techniques, is designed for high clock rate operation at 1GHz, with an in-laboratory entanglement fidelity of 89%. As the entangled photons emitted by the ELED are at telecom wavelengths, 1GHz clocked entanglement distribution over an installed fibre network in Cambridge is demonstrated between two points 4.6km apart. Over the course of a 14 hour experiment, an average entanglement fidelity of 79% is achieved showing excellent stability of the system. In order for entangled photon-pair experiments to be performed outside of a laboratory environment, both the source and detection systems are re-examined. In the detection system, the research-grade single-photon detectors are replaced by self-differencing avalanche photodiodes which have been used in quantum key distribution systems, with an entanglement fidelity of 71% achieved. In the source system, the free space spectral filter which required precise alignment is replaced by a fibre-based spectral filter. As the cryostat in which the ELED was previously cooled required a supply of liquid helium, thus confining the ELED to a laboratory, the operation of the ELED inside a compact closed-cycle cryocooler module is demonstrated.

An electron is usually considered to have only one type of kinetic energy, but could it have more, for both its spin and charge, or by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the linear Tomonaga- Luttinger Liquid (TLL) model, with hallmark predictions, such as spin-charge separation, having already been observed. Recent theoretical work has focused on extending the theory to deal with more realistic curved dispersions. However, experimental realisations have remained elusive until recently. Here, we report on measurements of many-body modes using a momentum-resolved tunnelling spectroscopy technique in gated 1D wires connected via air-bridges. We map the 1D dispersion in a variety of devices, both in and out of equilibrium, and observe the formation of two separate Fermi seas, associated with spin and charge excitations, at energies up to five times that of the Fermi energy, which cannot be accounted for by the noninteracting model. By reducing the length of the system, we also observe the emergence of higher-order ‘replica’ parabolic dispersions with higher momenta or negative effective mass, which is consistent with one of the leading nonlinear Luttinger theories. Determining the bare electron mass m_0 in crystals is also another problem often hindered by many-body effects. Here, Fermi-liquid physics renormalises the band mass, making the observed values density-dependent. In 1D geometries, however, the effect of interactions is strongly amplified, and they naturally decouple from the mass. By changing the level of confinement in the wires, we are able to tune the electron density down to about 18 electrons per micron or, equivalently, an interaction parameter of rs~4. This allows us to extract a constant bare mass value of m_0 = 0.0545m_e, about 20% lighter than observed in GaAs in geometries of higher dimensionality. Finally, by progressively occupying more 1D subbands, our system also allows us to change the amount of inter-subband screening by over 50%, consequently varying the effective interaction strength, in situ, all the way from the weakly to the strongly interacting regime. This ability is the first of its kind for 1D solid-state system. Our spectroscopy technique therefore offers itself as an important and powerful tool for probing strongly correlated systems where new emerging phenomena are occurring. This knowledge is now being used to develop systems for improved energy efficiency and could prove useful in designing the next generation of sustainable materials., Clare-Yale Travel Grant

Quantum computers are on the verge of revolutionising modern technology by providing scientists with unparalleled computational resources. With quantum-mechanical phenomena such as the superposition principle and entanglement, these computers could solve certain computational problems that are otherwise impossible for even the most powerful classical supercomputers. One of the major challenges standing in the way of this computing revolution is the accurate control of quantum bits. Quantum systems are extremely fragile and, by their nature, cannot be measured without destroying their quantum state. I wrote a numerical program to solve the time-dependent Schr��dinger equation, the differential equation that describes the evolution of wave functions. The advantage of my code over other solvers is its speed. I used graphics processing units (GPUs), a technology that has only recently matured, to accelerate high-performance computing. Hardware- acceleration allows me to solve complex time-evolution problems within days rather than years. Such an exceptional speedup has enabled me to calculate the behaviour of single electrons in semiconductor devices. Electrons are particularly interesting because they are ubiquitous in modern technology, as well as being fundamental quantum particles. Using the simulations produced by my code, I track the time evolution of an electron wave function as it propagates along quantum circuits. By animating the evolution of the wave function, I am able to visualise the wave function of electrons propagating in space and time. This is a remarkable tool for studying the behaviour of quantum particles in nanodevices. I focused my thesis on the realistic modelling of devices that are readily available in a laboratory or on designs that could be fabricated in the near future. I began by modelling single electrons as quantum bits. I provide a definition for an optimal qubit and lay out the set of operations required to manipulate the quantum information carried by the electron. In all my simulations, I aim to model experimentally realistic devices. I calculated the electrostatic potential of a real nanodevice and simulated the time-evolution of a single electron. I show that it is possible to create a single-electron beam splitter by tuning the voltages applied to various parts of the device and I calculate the range of voltages in which quantum information is preserved and manipulated accurately. These results were verified experimentally by collaborators at the Institut N��el and were published in Nature Communications 10, 4557 (2019). Using my code, I developed a framework for general measurements of electron qubits and provided a design for a semiconductor device capable of performing positive-operator valued measures (POVMs). A POVM is a powerful measurement technique in quantum mechanics that allows quantum information to be manipulated in interesting ways. The proposed setup is suggested as an implementation of entanglement distillation, which is a useful error correction tool that transforms an arbitrary entangled state into a pure Bell pair. Entanglement is one of the most fascinating aspects of quantum mechanics and it remains a challenge to generate perfectly entangled particle pairs. An experimentally viable method for distilling ��� or perfecting ��� entanglement is crucial for the design of quantum computers or quantum communication systems. Using this design, I introduced a protocol to use electrons, rather than photons, in quantum-optics-like systems. These results were published in Phys. Rev. A 96, 052305 (2017). Going beyond single-particle behaviour, I compare different methods for generating entanglement between electron-spin qubits using the power-of-SWAP operation. By using realistic experimental parameters in my simulations, I demonstrate that generating entan- glement via electron-electron collisions in a harmonic channel cannot be implemented for multidimensional systems. These findings go against what researchers thought was possible and put forward the need for new solutions to particle entanglement. I provide an alternative by demonstrating that a method based on the exchange energy is more viable than previously thought. I present a semiconductor device structure and an electrostatic potential that experi- mental groups can use in order to obtain the most efficient entangling quantum logic gates. These findings were published in Phys. Rev. A 101, 022329 (2020). The results presented in this thesis provide a comprehensive description of the control of single electrons in a surface-acoustic-wave-based quantum circuit. However, work in this field is far from over. I present various research paths for future projects. These include going beyond the time-dependent Schr��dinger equation to capture more complicated dynamics, using different hardware solutions to further accelerate numerical problem solving, and studying new systems of interest to extend this project beyond semiconductor physics.In all my simulations, I aim to model experimentally realistic devices. I calculated the electrostatic potential of a real nanodevice and simulated the time-evolution of a single electron. I show that it is possible to create a single-electron beam splitter by tuning the voltages applied to various parts of the device and I calculate the range of voltages in which quantum information is preserved and manipulated accurately. These results were verified experimentally by collaborators at the Institut N��el and were published in Nature Communications 10, 4557 (2019). Using my code, I developed a framework for general measurements of electron qubits and provided a design for a semiconductor device capable of performing positive-operator valued measures (POVMs). A POVM is a powerful measurement technique in quantum mechanics that allows quantum information to be manipulated in interesting ways. The proposed setup is suggested as an implementation of entanglement distillation, which is a useful error correction tool that transforms an arbitrary entangled state into a pure Bell pair. Entanglement is one of the most fascinating aspects of quantum mechanics and it remains a challenge to generate perfectly entangled particle pairs. An experimentally viable method for distilling ��� or perfecting ��� entanglement is crucial for the design of quantum computers or quantum communication systems. Using this design, I introduced a protocol to use electrons, rather than photons, in quantum-optics-like systems. These results were published in Phys. Rev. A 96, 052305 (2017). Going beyond single-particle behaviour, I compare different methods for generating entanglement between electron-spin qubits using the power-of-SWAP operation. By using realistic experimental parameters in my simulations, I demonstrate that generating entan- glement via electron-electron collisions in a harmonic channel cannot be implemented for multidimensional systems. These findings go against what researchers thought was possible and put forward the need for new solutions to particle entanglement. I provide an alternative by demonstrating that a method based on the exchange energy is more viable than previously thought. I present a semiconductor device structure and an electrostatic potential that experi- mental groups can use in order to obtain the most efficient entangling quantum logic gates. These findings were published in Phys. Rev. A 101, 022329 (2020). The results presented in this thesis provide a comprehensive description of the control of single electrons in a surface-acoustic-wave-based quantum circuit. However, work in this field is far from over. I present various research paths for future projects. These include going beyond the time-dependent Schr��dinger equation to capture more complicated dynamics, using different hardware solutions to further accelerate numerical problem solving, and studying new systems of interest to extend this project beyond semiconductor physics., The Institute of Physics Horizon 2020 Marie Sk��odowska Curie Actions Fonds de Recherche du Qu��bec ��� Nature et technologies St Edmund���s College, Cambridge Canadian Imperial Bank of Commerce Canadian Centennial Scholarship Fund Institute of Engineering and Technology

Sub-Poissonian entangled photon-pair sources based on semiconductor quantum dots are promising devices for the construction of a secure quantum network, thanks to their natural lack of multi photon-pair generation. In the previous decades, we have witnessed the achievements on a large variety of quantum dot light sources. However, they are still far from being used in an established standard fibre network. On the one hand, most of the sources are still operated in an environment that requires the installation of bulky laser setups, preventing their integration in a network. On the other hand, most of the experiments on entangled qubits generated from quantum dot sources are performed in the laboratory, resulting in a lack of work on the system supporting the remote operation of the device and high-fidelity transmission of entangled qubits over a long-distance in a large geographic scale. In this work, we demonstrate the integration of a quantum dot device in an optical network. To achieve this, both the source and the systems have been developed that favour the remote operation in a non-laboratory environment. For the source, a fully electrically operated telecom entangled on-chip pumping quantum dot device is designed and fabricated. It supports the operation at a low current of 1.6 mA and simultaneously achieves high wavelength tuneability of more than 25 nm by changing the applied bias voltage from 0 to -3.8 V. A in-laboratory entanglement fidelity of 97.3��0.2% is achieved with this on-chip pumping quantum dot light source. Several systems are developed in this work, supporting the integration of this developed source. We have proposed an innovative method for the precise alignment of the detection system to the quantum dot eigenbasis based on analysing time-resolved photon-pair correlations in a set of randomly oriented detection bases. It eliminates the restriction on the placement of the free-space polariser before the light is coupled from the quantum dot emitter into the first single-mode fibre. To support the classical traffic for sending the control signal to required components for source and system operation, we have developed a multiplexing system that allows the transmission of qubits at the telecom O-band with classical traffic at the telecom C-band over the same optical fibre without generating severe background at the quantum channel. To overcome changes of birefringence introduced by the deployed optical fibres, which causes the variation of the polarisation states during qubit transmission, we have developed a polarisation stabilisation system making use of the commercial polarisation controlling components. The system has an extremely low loss and a high duty cycle. The above development has enabled us to achieve a successful deployment and integration of the quantum dot device in the Cambridge Fibre Network. We demonstrate multiplexing of true single entangled photons tuned to 1310 nm from the on-chip pumping device with classical data traffic and achieve entanglement fidelity above 94% for over 40 hours., Cambridge Trust - CSC Scholarship Toshiba Research Europe Limited

The development of scalable, on-chip multiplexing technology, capable of electrically characterising an array of nanomaterial field-effect transistors at cryogenic temperatures, is described. The channel in each transistor in the array is a nanomaterial which is transferred onto the multiplexer device, and each transistor in the array can be measured individually. The underlying multiplexer device is fabricated from a GaAs / AlxGa1−xAs heterostructure; nanomaterials successfully incorporated into the multiplexer circuit are monolayer graphene and InAs nanowires. Two device designs are presented: the first can characterise up to 16 field-effect transistors; the second, up to 128. A 16-output multiplexer, with 11 functioning graphene field-effect transistors is the primary focus of this thesis. This device is characterised by multiple magneto-transport experiments, which demonstrate that the multiplexing technique can be used to collect reproducible data that is consistent with existing results, at a rate higher than what would otherwise be possible. In the absence of a magnetic field, transistors are characterised by calculating commonly used metrics, such as the carrier mobility and the intrinsic carrier density, which are found to be consistent over multiple device cool-downs. In weak magnetic fields, the scattering processes are investigated by analysing weak localisation signals and reproducible conductance fluctuations. In strong magnetic fields, Landau quantisation is observed in some of the graphene channels. The level spacing is used to calculate the Fermi velocity of carriers and the cyclotron mass of carriers in each Landau level, which provides conclusive evidence that the intrinsic properties of graphene are not significantly altered by the multiplexing technique. From the magneto-transport experiments, it is evident that many-body electron interactions are relevant to the observed transport phenomena. At low temperature and strong magnetic field, a transition into an insulating state is observed in one of the channels, which also is likely to be caused by many-body effects. Finally, initial results from ongoing work are presented, which includes extending the device design so that it can operate at both room temperature and cryogenic temperature, and the measurement of InAs nanowire field-effect transistors.

This thesis presents an exploration of how electrochemical etching (ECE) can be used to create pores in nitride semiconductors in order to engineer their properties. ECE offers a simple, flexible approach to pore formation, driven by an electric field between the material surface and a suitable electrolyte. This means that the material must be conductive in order to form pores. This makes doping density and potential the two key parameters for controlling the process. A particular focus of this work has been to understand how pores can form in a subsurface doped layer, below a non-porous non-intentionally doped capping layer. It is found that this is enabled by nanopipes formed at dislocations, which allow the electrolyte to reach the doped layer with minimal damage to the capping layer. Large birefringence of 0.14 has been produced in porous GaN structures and this has been thoroughly studied through optical measurements, finite element modelling and 3D structural data obtained using focussed ion beam milling combined with scanning electron microscopy. Characterisation of structures with periodic layers of porous GaN has also been demonstrated, using x-ray diffraction measurements. This has been used to measure the layer thickness and porosity of these samples and predicts their optical reflectivity with higher accuracy than the standard method for such characterisation. Forming pores in nitride semiconductors other than GaN has also been demonstrated. Porous AlGaN UV reflectors have been created, with a peak reflectivity of 89% at 324 nm and ECE has been applied to InGaN quantum wells with the aim of enhancing the luminescence. Measurements indicate that ECE creates both enhancement and a blueshift in the emission peak. Controlling and characterising pore formation at this scale is a huge challenge and further study of the pore morphology is required to verify the mechanism causing this.

This dissertation reports on several experimental projects studying electronic transport in thin-flm electronic devices. Self-assembly methods and graphene were used to realise devices contacting films of self-assembled PbS quantum dots. The devices have exhibited single-electron tunnelling with a high yield. The electrical properties of the junctions are studied individually and collectively using statistical tools to extract correlations between device geometries and electrical data. The dissertation includes discussion of the theory of relevant electronic transport including numerical simulations. Several initiated projects deriving from this work are introduced. A second device reported in this thesis is a memristive switch. Contacting thin films of Al2O3 with graphene delivered junctions which exhibit memristive behaviour with an ultrahigh on-off conductance ratio. The conduction state of the junctions is correlated with morphological changes in the devices, whereby conductive flament formation in the junction is found to lead to electrically-controllable and reversible gas encapsulation in bubbles in the structure. The device is measured electrically and topographically, and the correlation between the two aspects is studied. A discussion of memristive conduction is included with numerical simulations. A third section reports on a project studying thermoelectricity in self-assembled molecular junctions, as they show potential for improved thermoelectric efficiency for energy harvesting; this is discussed in the dissertation. Strategies to benchmark the studies are presented with relevant devices fabricated and measured. These include the development of a measurement protocol to study thermoelectricity in devices, studies of electrical coupling between various molecular structures and graphene electrodes, molecular-structure dependence of electrical and thermal conductance of junctions. Preliminary results and on-going work are discussed., I want to thank the Semiconductor Physics Group of the Cavendish Laboratory, the Semiconductor Physics Group of the Institute of Physics, the Cambridge NanoDTC, and Fitzwilliam College, Cambridge for their support. I want also to thank the collaborators acknowledged in the thesis for contributions in materials and services.

Lead halide perovskites have shown remarkable device performance in both solar cells and LEDs. Whilst the research efforts so far have been mainly focussed on device optimisation, little is known about the photophysical properties. For example, the nature of the bandgap is still debated and an indirect bandgap due to a Rashba splitting has been suggested. In this thesis, we study the early-time carrier relaxation and its impact on photoluminescence emission. We first study ultrafast carrier thermalization processes using 2D electronic spectroscopy and extract characteristic carrier thermalization times from below 10 fs to 85 fs. We then investigate the early-time photoluminescence emission during carrier cooling. We observe that the luminescence signal shows a rise over 2 picoseconds in CH3NH3PbI3 while carriers cool to the band edge. This shows that luminescence of hot carriers is slower than that of cold carriers, as is found in direct gap semiconductors. We conclude that electrons and holes show strong overlap in momentum space, despite the potential presence of a small band offset arising from a Rashba effect. Recombination and device performance of perovskites are thus better described within a direct bandgap model. We finally study carrier recombination in perovskites and the impact of photon recycling. We show that, for an internal photoluminescence quantum yield of 70%, we measure external yields as low as 15% in planar films, where light out-coupling is inefficient, but observe values as high as 57% in films on textured substrates that enhance out-coupling. We study the photo-excited carrier dynamics and use a rate equation to relate radiative and non-radiative recombination events to measured photoluminescence efficiencies. We conclude that the use of textured active layers has the ability to improve power conversion efficiencies for both LEDs and solar cells., Winton Programme for Physics of Sustainability, EPSRC studentship, Cambridge Trust Scholarship

This thesis presents the development of a selective-etch fabrication process to create sub-10 nanometre metallic gaps and the subsequent use of the gaps to study the electronics of nanocrystals and molecules. A complete picture of the success of the process required both examination by scanning electron microscopy as well as probing the current response to an applied bias at low temperature. The empty gaps were fully characterised before self-assembling 7 nm CdSe nanocrystals onto the metal with the help of linker molecules. The I-V characteristics of the empty gaps showed a reduction of the tunnelling barrier height from the expected value (~5.1 eV, the work function of Au) when the results were fitted to the Simmons tunnelling model for a metal-insulator-metal system. Results indicate that after the barrier height is surpassed, a transition from direct to field-effect (Fowler-Nordheim) tunnelling occurs. After CdSe assembly, the collected I-V characteristics of the system at 77 K showed varied results. Many devices displayed conductance peaks at low voltages comparable to the results of the shadow evaporation process for 4.2 nm nanocrystals (also documented in this thesis). Several devices revealed switching between multiples of fundamental curves, suggesting conduction through multiples of nanocrystals.

This thesis describes electrical measurements performed on low dimensional p- type devices, fabricated from GaAs/AlGaAs heterostructures. The Coulomb interaction between holes is similar to that between electrons. However, the kinetic energy is suppressed, which makes interaction effects particularly important. Holes may also be used to study band structure effects which arise from spin-orbit coupling in the valence band. The effects of Coulomb interactions in low dimensional electron systems are currently being studied extensively. Experiments presented in this thesis indicate the possible importance of Coulomb exchange interactions in both one and two dimensional hole systems (1DHSs,2DHSs). Tilted magnetic field studies of 2DHSs in the quantum Hall regime indicate that Landau levels at even filling factors will not cross. For high filling factor, this is attributed to a spin-orbit mixing effect which arises from the low symmetry of the system. At lower filling factor, activation-energy measurements verify that the energy gaps decrease and then increase as the field is tilted. However, the energy gap versus field dependences do not exhibit the curvature that might be expected from a perturbative anticrossing. It is speculated that the origin of this effect is a phase transition driven by the exchange interaction. Balanced arguments contrasting the relative strengths of the mixing and interactions theories are provided. The second part of this thesis describes a new method for the fabrication of ballistic 1DHSs, which exhibit clear conductance quantization. The quantization changes from even to odd multiples of e2/h as a function of the magnetic field in the plane of the heterostructure, as "spin splitting" causes the 1D subbands to cross. Measurements of the 1D subband energy spacings are used together with the magnetic fields at which the crossings occur to calculate the in-plane g factors of the 1D subbands. These are found to increase as the number of occupied 1D subbands decreases. This enhancement of the g factor is attributed to exchange interactions; possible mixing explanations are also discussed. At higher magnetic fields, the pattern of quantization features shows that the subbands have crossed many times, and that the 1DHS can be strongly magnetized., EPSRC