Scalable Control and Measurement of Gate-Defined Quantum Dot Systems

There is currently a worldwide effort towards the realisation of large-scale quantum computers that exploit quantum phenomena for information processing. While these computing systems could potentially redefine the technological landscape, harnessing quantum effects is challenging due to their inherently fragile nature and the experimentally demanding environments in which they arise. In order for quantum computation to be viable it is first necessary to demonstrate the operation of two-level quantum systems (qubits) which have long coherence times, can be quickly read out, and can be controlled with high fidelity. Focusing on these key requirements, this thesis presents four experiments towards scalable solid state quantum computing using gate-defined quantum dot devices based on gallium arsenide (GaAs) heterostructures. The first experiment investigates a phonon emission process that limits the charge coherence in GaAs and potentially complicates the microwave control of multi-qubit devices. We show that this microwave analogy to Raman spectroscopy can provide a means of detecting the unique phonon spectral density created by a nanoscale device. Experimental results are compared to a theoretical model based on a non-Markovian master equation and approaches to suppressing electron-phonon coupling are discussed. The second experiment demonstrates a technique involving in-situ gate electrodes coupled to lumped-element resonators to provide high-bandwidth dispersive read-out of the state of a double quantum dot. We characterise the charge sensitivity of this method in the few-electron regime and benchmark its performance against quantum point contact charge sensors. The third experiment implements a low-loss, chip-level frequency multiplexing scheme for the readout of scaled-up spin qubit arrays. Dispersive gate-sensing is realised in combination with charge detection based on two radio frequency quantum point contacts to perform multiplexed readout of a double quantum dot in the few-electron regime. Demonstration of a 10-channel multiplexing device is achieved and limitations in scaling spin qubit readout to large numbers using multiplexed channels discussed. The final experiment ties previously presented results together by realising a micro-architecture for controlling and reading out qubits during the execution of a quantum algorithm. The basic principles of this architecture are demonstrated via the manipulation of a semiconductor qubit using control pulses that are cryogenically routed using a high-electron mobility transistor switching matrix controlled by a field programmable gate array. Finally, several technical results are also presented including the development of printed circuit board solutions to allow the high-frequency measurement of nanoscale devices at cryogenic temperatures and the design of on-chip interconnects used to suppress electromagnetic crosstalk in high-density spin qubit device architectures

Noise characterization of nanoelectronic devices

In recent years, noise characterization has emerged as an extraordinarily powerful tool to investigate aspects of transport phenomena at a very basic level. Through noise it is possible to obtain information about the structure and the transport properties of nanoscale devices that are complementary to those given by the DC characteristics and the small signal AC response. Time-dependent fluctuations in the measured current due to the granularity of charge lead to the so-called shot noise. Such statistical fluctuations show up much stronger in nanosize electronic devices, compared to macroscopic classical devices, due to the small number of electrons involved in device operation. The experimental challenge in the measurement of shot noise consists of the elimination of other sources of noise, such as thermal noise and low frequency 1/f noise due both to the device under test and to the external environment. In such measurements on semiconductor devices, it is generally very difficult to detect in a direct way noise levels associated with bias currents below a few hundred picoamperes. This is due to the fact that noise power spectral densities, corresponding to such current levels, are of the same order of magnitude or much lower than those which are characteristic of common low noise amplifiers. To overcome this problem, many techniques for reducing the noise of the measurement system have been implemented, as described in this thesis

Reliability-Driven Experimental and Theoretical Study of Low-Frequency Noise Characteristics of AlGaN/GaN HFETs

Silicon technology, which is the most mainstream semiconductor technology, poses serious limitations on fulfilling the market demands in high-frequency and high-power applications. In response to these limitations, wide bandgap III-nitride devices, including AlxGa1-xN/GaN heterojunction field effect transistors (HFETs), were introduced at about two decades ago to satisfy these rapidly growing market demands for high-power/high-frequency amplifiers and high-voltage/high-temperature switches. The most appealing features of III-nitride technologies, and particularly AlxGa1-xN/GaN HFETs, in these applications, are the polarization-induced high sheet-carrier-concentration, high breakdown-voltage, high electron saturation-velocity, and high maximum operating temperature. Therefore, the development of enhancement-mode AlGaN/GaN HFETs is one of the most important endeavours in the past two decades. Low-frequency noise (LFN) spectroscopy, empowered by a proper physics-based model, is received as a capable tool for reliability studies. As a result, devising a physics-based LFN model for AlGaN/GaN HFETs can be capable of not only evaluating the alternative techniques proposed for realization of enhancement-mode AlGaN/GaN HFETs, but also more importantly forecasting the reliability, and noise performance of these devices. In this dissertation, for the first time, a physics-based model for the low-frequency drain noise-current of AlGaN/GaN HFETs is proposed. The proposed model, through including the thermally-activated and quantum tunneling processes of trapping/de-trapping of electrons of channel into and out of the trap-sites located both in the barrier- and buffer-layer of these HFETs, provides a descriptive picture for the LFN behavior of these devices. This work also aims to experimentally investigate the low-frequency noise-current characteristics of both conventional and newly-proposed devices (i.e., fin-, and island-isolated AlGaN/GaN HFETs) at various temperatures (i.e., 150, 300, and 450 K) and bias points in order to address the possible difficulties in performance of these devices. Matching of the trends proposed by the physics-based model to the experimentally recorded LFN spectra of AlGaN/GaN HFETs designed according to a newly-proposed technological variant for positive-shifting the threshold-voltage, confirms the accuracy and predicting power of the proposed model. The insights gained from this model on the latter group of devices provide evidence for the challenges of the aforementioned technological variants, and as a result offer assistance in proposing remedies for those challenges. In formulating the LFN model, a massive discrepancy between the predictions of the existing analytical relationships used by others in evaluating the subband energy levels of AlGaN/GaN HFETs and the realities of the polarization-induced electron concentration of these HFETs was spotted. Careful evaluation of the polarization properties of these heterostructures unmasked the inaccuracy of the assumption of zero penetration of the electron wave into both the AlGaN barrier-layer and the GaN buffer-layer as the culprit in this discrepancy. In response to this observation, a model based on the variational-method for calculating the first and second subband energy levels of AlGaN/GaN HFETs is developed. On the basis of this model, more accurate analytical frameworks for calculating these subband energy levels in AlGaN/GaN HFETs for a variety of barrier thicknesses and Al mole-fractions in the barrier-layer are proposed

Cryogenic Ultra-Low Noise InP High Electron Mobility Transistors

Indium phosphide high electron mobility transistors (InP HEMTs), are today the best transistors for cryogenic low noise amplifiers at microwave frequencies. Record noise temperatures below 2 K using InP HEMT equipped cryogenic low noise amplifiers (LNAs) were demonstrated already a decade ago. Since then, reported progress in further reducing noise has been slow. This thesis presents new technology optimization, modeling, measurements and circuit implementation for the cryogenic InP HEMT. The findings have been used to demonstrate a new record minimum noise temperature of 1 K at 6 GHz. The thesis considers aspects all the way from material, process and device design, to hybrid and monolithic microwave integrated circuit (MMIC) LNAs. The epitaxial structure has been developed for lower access resistance and improved transport characteristics. By investigating device passivation, metallization, gate recess etch, and circuit integration, low-noise InP HEMT performance was optimized for cryogenic operation. When integrating the InP HEMT in a 4-8 GHz 3-stage hybrid LNA, a noise temperature of 1.2 K was measured at 5.2 GHz and 10 K operating temperature. The extracted minimum noise temperature of the InP HEMT was 1 K at 6 GHz. The low-frequency 1/f noise in the 1 Hz to 1 GHz range and gain fluctuations in the 1Hz to 100 kHz range have been measured for six different types of HEMTs, and compared to two different SiGe heterojunction bipolar transistors (HBTs). The results showed that radiometer chop rates in the kHz range are needed for millimeter wave radiometers with 10 GHz bandwidth. A comparative study of GaAs metamorphic HEMTs (mHEMTs) and InP HEMTs has been performed. When integrated in a 4-8 GHz 3-stage LNA, the InP HEMT LNA exhibited 1.6 K noise temperature whereas the GaAs mHEMT LNA showed 5 K. The observed superior cryogenic noise performance of the InP HEMT compared to the GaAs MHEMT was related to a difference in quality of pinch-off as observed in I-V characteristics at 300 K and 10 K. To demonstrate the low noise performance of the InP HEMT technology, a 0.5-13 GHz and a 24-40 GHz cryogenic monolithic microwave integrated circuit (MMIC) LNA was fabricated. Both designs showed state-of-the-art low noise performance, promising for future radio astronomy receivers such as the square kilometer array

Characterisation and modelling of graphene FET detectors for flexible terahertz electronics

Low-cost electronics for future high-speed wireless communication and non-invasive inspection at terahertz frequencies require new materials with advanced mechanical and electronic properties. Graphene, with its unique combination of flexibility and high carrier velocity, can provide new opportunities for terahertz electronics. In particular, several types of power sensors based on graphene have been demonstrated and found suitable as fast and sensitive detectors over a wide part of the electromagnetic spectrum. Nevertheless, the underlying physics for signal detection are not well understood due to the lack of accurate characterisation methods, which hampers further improvement and optimisation of graphene-based power sensors. In this thesis, progress on modelling, design, fabrication and characterisation of terahertz graphene field-effect transistor (GFET) detectors is presented. Amajor part is devoted to the first steps towards flexible terahertz electronics.The characterisation and modelling of terahertz GFET detectors from 1 GHz to 1.1 THz are presented. The bias dependence, the scattering parameters and the detector voltage response were simultaneously accessed. It is shown that the voltage responsivity can be accurately described using a combination of a quasi-static equivalent circuit model, and the second-order series expansion terms of the nonlinear dc I-V characteristic. The videobandwidth, or IF bandwidth, of GFET detectors is estimated from heterodyne measurements. Moreover, the low-frequency noise of GFET detectors between 1 Hz and 1 MHz is investigated. From this, the room-temperature Hooge parameter of fabricated GFETs is extracted to be around 2*10^{-3}. It is found that the thermal noise dominates above 100 Hz, which sets the necessary switching time to reduce the effect of 1/f noise.A state-of-the-art GFET detector at 400 GHz, with a maximum measured optical responsivity of 74 V/W, and a minimum noise-equivalent power of 130 pW/Hz^{0.5} is demonstrated. It is shown that the detector performance is affected by the quality of the graphene film and adjacent layers, hence indicating the need to improve the fabrication process of GFETs.As a proof of concept, a bendable GFET terahertz detector on a plastic substrate is demonstrated. The effects of bending strain on dc I-V characteristics, responsivity and sensitivity are investigated. The detector exhibits a robust performance for tensile strain of more than 1% corresponding to a bending radius of 7 mm. Finally, a linear array of terahertz GFET detectors on a flexible substrate for imaging applications is fabricated and tested. The results show the possibility of realising bendable and curved focal plane arrays.In summary, in this work, the combination of improved device models and more accurate characterisation techniques of terahertz GFET detectors will allow for further optimisation. It is shown that graphene can open up for flexible terahertz electronics for future niche applications, such as wearable smart electronics and curved focal plane imaging

Investigation of AuNiGe-based superconducting ohmic contacts and hydrodynamic transport effects in GaAs/AlGaAs-based two-dimensional electron gases

This thesis describes an investigation of AuNiGe-based ohmic contacts as well as a series of differential resistance and magnetic field measurements performed on GaAs/AlGaAs-based two-dimensional electron gases (2DEGs) alongside all the required background information. The aim of the AuNiGe ohmic contact study was to discover the cause of the superconductivity occurring below 1 K. The result is a list of the most probable superconducting compounds, likely AuAl or AuGe-based but there could be a combination of several compounds. The main compound identified, Au7Ga2, is not a superconductor and no other compound was found in sufficient amounts to cause bulk superconductivity. As a result, it is possible that the observed superconductivity could be the result of a percolating network existing throughout the ohmic contact. If true, then a much more granular TEM study is required. The interface between the ohmic contact and the semiconductor was also found to be very inhomogeneous and the existence of the 2DEG below it is questionable. The differential resistance and magnetic field measurements performed on 2DEG Hall bars of various widths showed that applying a magnetic field to a narrow Hall bar recovers the Bloch-Grüneisen transition observed in wider Hall bars. Without a magnetic field the behaviour is hidden by hydrodynamic effects which only occur in narrow Hall bars due to the interplay between the width of the Hall bar and the electron-electron scattering length. Primary thermometry based on cross-correlated Johnson noise was also performed to establish that the transition was the Bloch Grüneisen transition; with all wide Hall bars measured having a transition temperature in line with the Bloch Grüneisen temperature