School of Physics - Theses
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ItemCharacterization of silicon and diamond semiconductor devices in the low temperature regime( 2015)At ultra low temperatures, materials reveal interesting behaviours that become evident due to the freezing out of thermal vibrations. We report studies of two important group IV material systems using a newly-commissioned dilution refrigerator at temperatures of less than 50 mK, and under axial magnetic fields of up to 7 Tesla and 1 Tesla in the lateral plane. We realized these magnetic fields with a 3D vector magnet and calibrated the system using well-known test samples. The first material studied in detail was nitrogen enhanced grown ultranano- crystalline diamond (N-UNCD). Diamond displays a combination of many extreme physical properties such as a high thermal conductivity, the highest number density of atoms, and a wide optical band gap, and is in addition to these properties also biocompatible [1]. The ultra-nanocrystalline form of diamond is composed of small, 3-5 nm diameter grains of diamond and shares many of the desirable properties of the single crystal form, but is much easier to produce in thin form and can be used in wide variety of applications [2], especially for nanomechanics due to its strength and high Young's modulus [3]. When doped with boron, nano-crystalline diamond (B-NCD) displays superconducting properties below a critical temperature of less than a few Kelvin [4, 5]. The nitrogen doped form has found application in biomedical devices [6], but its superconducting behaviour at very low temperatures has not yet been demonstrated [7]. We fabricated N-UNCD thin films using microwave-enhanced CVD growth and used optical lithography to create Hall bar designs. We found that the conductivity of N-UNCD decreased with decreasing temperature, and between 36 mK and 4.9 K, a negative magneto-resistance was observed. Fitting the temperature and magnetic field dependent data with the 3D weak localization model developed by Kawabata [8], we found that 3D weak localization indeed plays a main role in the conduction mechanism of N-UNCD films even at ultra low temperatures. The second project was on the characterization of erbium doped silicon (Si:Er) semiconductor devices. Erbium has long been known to be important for use in optical fibre amplification in silica [9], and it also shows strong luminescent properties when it is added to Si as a dopant [10]. As such, Er is also of interest as optoelectronic semiconductor material [11]. Since the recent demonstration that it is possible to optically address single erbium ions in the silicon lattice [12] interest in Si:Er has increased. We studied silicon doped with erbium using ion implantation and report on our attempts to create CMOS devices with Er doped channels. The implanted material was characterized by optical spectroscopy, deep level transient spectroscopy, and measured both electrically and magnetically at low temperatures in the dilution refrigerator. Deep level transient spectroscopy performed on devices with varying anneal temperatures showed the emergence of electronically active traps with the minimum trap density occurring at annealing temperatures above 700 °C. Our results reveal that a rapid thermal anneal at 900 °C activates the luminescence from the implanted erbium ions. This effect remains, even if the sample is subjected to subsequent high temperature treatments. MOS devices co-doped with Er and P were fabricated and characterised to the extent they could be, based on the processing issues that arose.
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ItemAn all-diamond hermetic encapsulation for a high-acuity retinal prosthesis( 2014)Bionic vision through electrical stimulation of the retina is fast becoming a reality. To date, clinical trials have allowed blind patients to see a lover’s smile and navigate night scenes. (K Stingl et al, 2013) This kind of data has encouraged an abundance of research activity. Bionic Vision Australia, among others, is developing a retinal prosthesis to restore high visual acuity. One of its flagship technologies is a diamond electrode array, which will form part of the encapsulation for the implanted electronics. The remainder of the encapsulation also needs to be constructed from leak-proof, or hermetic, materials. The aim of this work was to design and test feasibility of a hermetic encapsulation that incorporated the diamond electrode array. An all-diamond hermetic encapsulation design was proposed, in which a diamond box-shaped capsule was bonded to the diamond array, with the electronics contained inside. Diamond capsules were made from polycrystalline diamond. Laser micromachining was found to be the optimal fabrication method. Hermetic joints were made in diamond using vacuum brazing with precious metal braze alloys. Several brazes were investigated for their ability to wet and form strong bonds with diamond. Bond interfaces were studied for morphology, chemical composition and hermeticity. Brazed diamond capsules were sealed at room temperature using laser microwelding. Welds were optimised for smooth surface morphology and hermeticity. The results demonstrated a hermetic all-diamond encapsulation. Combining the hermetic capsule, the brazing technique, and the welding technique with the diamond electrode array formed a retinal prosthesis technology that can protect against degradation for the lifetime of the patient.
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ItemPrinciples and applications of quantum decoherence in biological, chemical, and condensed matter systems( 2013)This thesis focuses on the use of the Nitrogen-Vacancy (NV) defect centre in diamond as a single spin sensor of nanoscale magnetic fields. The NV system has attracted considerable interest in recent years due to its unique combination of sensitivity, nanoscale resolution, room temperature operation, and stable fluorescence, together with the inherent biocompatibility of diamond; making it ideal for measuring coherent quantum processes in biological, chemical and condensed matter systems. Existing NV-based sensing techniques, however, are ultimately limited by sources of magnetic noise that act to destroy the very resource required for their operation: the quantum phase coherence between NV spin levels. We address this problem by showing that this noise is a rich source of information about the dynamics of the environment we wish to measure. We develop protocols by which to extract dynamical environmental parameters from decoherence measurements of the NV spin, and a detailed experimental verification is conducted using diamond nanocrystals immersed in a MnCl2 electrolyte. We then detail how sensitivities can be improved by employing sophisticated dynamic decoupling techniques to remove the decoherence effects of the intrinsic noise, whilst preserving that of the target sample. To characterise the effects of pulse errors, we describe the full coherent evolution of the NV spin under pulse-based microwave control, including microwave driven and free precession intervals. This analysis explains the origin of many experimental artifacts overlooked in the literature, and is applied to three experimentally relevant cases, demonstrating remarkable agreement between theoretical and experimental results. We then analyse and discuss two important future applications of decoherence sensing to biological imaging. The first involves using a single NV centre in close proximity to an ion channel in a cell membrane to monitor its switch-on/switch-off activity. This technique is expected to have wide ranging implications for nanoscale biology and drug discovery. The second involves using an array of NV centres to image neuronal network dynamics. This technique is expected to yield significant insight into the way information is processed in the brain. In both cases, we find the temporal resolution to be of millisecond timescales, effectively allowing for real time imaging of these systems with micrometre spatial resolution. We analyse cases in which environmental frequencies are sufficiently high to result in a mutual exchange of energy with the NV spin, and discuss how this may be used to reconstruct the corresponding frequency spectrum. This analysis is then applied to two ground-breaking experiments, showing remarkable agreement. Protocols for in-situ monitoring of mobile nanodiamonds in biological systems are developed. In addition to obtaining information about the local magnetic environment, these protocols allow for the determination of both the position and orientation of the nanocrystal, yielding information about the mechanical forces to which it is subjected. These techniques are applied in analysing a set of experiments in which diamond nanocrystals are taken up endosomally by human cervical cancer cells. Finally, we focus our attention on understanding the microscopic dynamics of the spin bath and its effect on the NV spin. Many existing analytic approaches are based on simplified phenomenological models in which it is difficult to capture the complex physics associated with this system. Conversely, numerical approaches reproduce this complex behaviour, but are limited in the amount of theoretical insight they can provide. Using a systematic approach based on the spatial statistics of the spin bath constituents, we develop a purely analytic theory for the NV central spin decoherence problem that reproduces the experimental and numerical results found in the literature, whilst correcting the limitations and inaccuracies associated with existing analytical approaches.
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ItemTEM and structural investigations of synthesized and modified carbon materials( 1999-08)Due to the extreme properties of diamond, such as extreme hardness, high thermal conductivity, high electrical breakdown strength, high electron and hole mobilities and large band gap, it is of interest to study this material in detail. Before advantage can be taken of diamond’s properties for high-temperature, high-power electronic applications successful doping/ion implantation of diamond must be achieved. This requires an understanding of the types of defects produced during ion irradiation. In the present work, type IIa diamond has been irradiated with various doses of 320keV Xe ions at room temperature. Analytical techniques used are electron spin resonance spectroscopy, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. Previous models have suggested that upon ion impact, amorphous and/or graphitized clusters are formed in diamond, which will overlap at a critical dose to form a semi-continuous graphitized layer. (For complete abstract open document)
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ItemThe study of defect and trapping levels in CVD polycrystalline diamond with applications to ultraviolet dosimetry( 2004-05)The unique properties of diamond make it an excellent material for electronic and optical applications. It is particularly attractive for ultraviolet radiation dosimetry due to its intrinsic properties, which include biological tissue equivalence and visible blindness. Importantly, the advent of synthetic diamond, especially Chemical Vapour Deposition (CVD) diamond, has made it more economically viable for such applications. A thorough understanding of the electronic properties of diamond is needed before these applications can be fully explored. Consequently, this thesis investigates charge carrier trapping states in CVD polycrystalline diamond for the optimization of ultraviolet radiation dosimetry. The technique of Thermally Stimulated Conductivity (TSC) is used to probe electrically active defects and is also applied for dosimetric read-out. A range of as-grown CVD polycrystalline diamond films are studied to determine attributes that favour dosimetric-related TSC. In doing so, we establish that dosimetric TSC in these films originate from defects at the grain boundaries with a correlation to high crystalline quality. With this finding in hand, we then investigate the possibility of optimising diamond for dosimetry by controllably introducing extrinsic dosimetric defects using ion implantation. However, it is shown that these defects are not suitable for dosimetry and have a detrimental effect on the indigenous TSC signal. This study verifies the importance of crystalline quality on the indigenous dosimetric properties of CVD polycrystalline diamond. The possibility of doping CVD diamond during growth is also investigated as a means for intentionally introducing extrinsic dosimetric defects. Sulphur is selected as the dopant based on the theoretical energy levels formed by this defect, and because the prospect of S doping in diamond remains an actively debated issue in the literature. We report for the first time defect levels extracted from TSC analysis of S-doped CVD diamond and find consistency with theoretical predications. In addition, the dominant TSC trap level in S-doped diamond shows promise for radiation dosimetry with certain properties exceeding many current radiation dosimeters. The experimental results in this thesis lead to a deeper understanding of defect and trapping mechanisms in CVD polycrystalline diamond and establish attributes that favour TSC and related dosimetric properties in such films. This knowledge is fundamental to the realisation of diamond for ultraviolet dosimetry.
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ItemPhysics of low-dimensional nanostructures( 2012)Nanoscale constructs are offering access to the quantum mechanical regime due to their constrained size. The unusual, and often counterintuitive, behaviours of such constructs are of considerable interest to those developing new devices across several fields, including (but not limited to) quantum computing, communications, in vivo applications such as the bionic eye and bio-sensors, standard electronics and computing, and magnetometry. The physics of zero-, one-, and two-dimensional nanostructures comprised of various dopants or arrays of dopants in either diamond or silicon are presented and discussed. In particular, the zero-dimensional Xe-related defects in diamond are considered theoretically, via density functional theory, lattice dynamics, and thermodynamics. Xe defects have also been characterised experimentally via the probe-enhanced Ra- man spectroscopy (PERS) technique. In silicon, a one-dimensional nanowire consisting of P donors is studied with density functional theory. This wire is monatomically thin in one direction, and two donors wide in the other, with the donors spaced at the currently realisable sheet density of 25%. The two-dimensional case of infinite monatomically thin sheets of P donors is considered, both with effective mass theory and density functional theory (which is again undertaken for the most common experimental sheet density, 25%). The effective mass theory model has been applied to several sheet densities, agrees well with literature calculations of sheets with in-plane disorder, is far more rapid in execution, and offers an analytic scaling theory to describe the dependence of several key results on the sheet density. The density functional theory approach is then extended to the quasi-two dimensional case of bilayers of monatomically thin P sheets, in order to address the approach to minimal two-dimensional confinement.
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ItemThe theory of the nitrogen-vacancy colour centre in diamond( 2012)The nitrogen-vacancy (NV) colour centre in diamond is a model system for many quantum technologies including, metrology, information processing and communications. The NV centre is also highly suitable for employment in various nanotechnology applications, such as biological and sub-diffraction limit imaging, and in tests of fundamental physics, such as cavity quantum electrodynamics and the quantum entanglement of mesoscopic systems. The remarkable properties of the centre are however, not currently fully understood, with several unresolved issues limiting the performance of the centre in its many important applications. As the unresolved issues are interrelated and concern different aspects of the centre's properties, they may only be resolved by the development of a single self-consistent theory of the NV centre. The aim of this work has been to develop such a theory. The theory has been developed using a combination of the molecular model of deep level defects in semiconductors, group theoretical methods and ab initio calculations. The highly structured nature of the theory will enable its future use in the systematic identification of other colour centres that possess properties that exceed those of the NV centre.
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ItemNanoscale quantum sensing using nitrogen-vacancy centres in diamond( 2012)Devices that detect and spatially image magnetic fields are important in many areas of study including chemistry, electronics, materials science and biology. By extending the boundaries of what is currently achievable we may begin to explore areas previously inaccessible to science, such as wide-field imaging of neuron signaling or structural determination of single molecules. Here we present experimental progress towards the development of a nanoscale magnetic sensor operating under ambient conditions using the nitrogen-vacancy (NV) centre in diamond. This thesis describes the construction of a lab-built, confocal microscope capable of detecting single NV centres, with additional microwave control for coherent manipulation of the NV spin. The feasibility of using NV-diamond for real-time detection of the action potential generated by a neuron, and with high spatial resolution is experimentally demonstrated. The quantum coherence of single NV spins is monitored in various chemical solutions, and detection of nanoscale magnetic environments external to the diamond are demonstrated. This work sets the experimental foundation for using manufactured single quantum systems as sensitive probes of external chemical environments. In addition, the first measurements of single quantum coherent spins inside living cells are performed with NV centres in nanodiamonds. These studies on NV centres inside living cells demonstrate their promise as magnetic sensors for biology. Furthermore, alternative quantum sensing technologies emerge including rotational tracking of nanodiamonds and enhanced identification of fluorescent nanoparticles.
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ItemSurface engineering for quantum information processing in NV diamond( 2011)Due to an extensive list of extreme and often complementary materials properties, diamond has become a leading candidate in a range of advanced mechanical, thermal, optical and electronics applications. These include devices designed to operate under extraordinarily harsh conditions such as the low-earth orbit environment, and applications with highly demanding performance requirements such as high power electronics. Perhaps the most demanding of these areas in which diamond shows great promise is the pursuit of scalable quantum information processing (QIP) devices, with the ultimate goal being the production and integration of a large number of interacting and controllable quantum bits (Qubits), within a single device. Although the production of this so called quantum computer is theoretically compatible with diamond’s ideal intrinsic properties, experimental realization of this type of device will require absolute control over material characteristics, both in terms of the bulk material used and following the various processing steps required for device construction. Most currently successful processing techniques can be seen to leave some form of crystal damage either near the surface or within the bulk of the material and due to the extreme sensitivity of any form or qubit this residual damage can be very hard to measure, and even harder to prevent or subsequently remove. Specifically for optical QIP devices, the Nitrogen- Vacancy (NV) centre in diamond holds enormous potential in a variety of qubit architectures, most of which require coupling of this defect to a photonic cavity or waveguide system. The biggest roadblock to realization of this lofty goal however is an apparent lack of NV centres found close enough to any diamond sample surfaces, without significant degradation in their spectral qualities. In this thesis we have primarily addressed the issue of near-surface NV by conducting a set of experiments aimed at understanding whether NV centres can be feasibly created close to diamond surfaces (<100nm), whilst maintaining their desirable stable electronic/spectral properties. To this end we have completed a number of interrelated studies on single crystal and nanocrystalline diamond samples, focusing on clarifying the effect of near-surface diamond characteristics, both intrinsic and affected by fabrication procedures, on the quantum and electronic properties of near-surface diamond colour centres. We show that high crystalline quality diamond can be grown using chemical vapour deposition techniques, including the controllable growth of nanocrystalline diamond particles, and that this material is a compatible host for commensurably high spectral quality NV centres. We also show that novel colour centres can be deliberately grown using these techniques, with advantageous quantum optical properties, possibly rivaling those of the NV centre itself. Utilizing optical spectroscopy measurements, and following a range of thermal annealing steps, we show that near-surface NV centres produced by ion-implantation exhibit significant inhomogeneous and homogeneous spectral instability, making them unsuitable for optical QIP applications and that post-growth fabrication techniques, such as mechanical polishing, have also been shown to produce significant near-surface crystal damage, which is likely to further degrade the performance of near-surface NV centres. We utilize advanced surface analysis techniques, such as synchrotron based x-ray absorption, to show that nanodiamonds (<30nm) are subject to enhanced thermal and chemical reactivity, which can be significantly alleviated with the application of surface hydrogen termination schemes. Conversely, the diffusion of hydrogen into CVD grown diamond materials has been shown to passivate near-surface NV centres and mitigation of this effect is likely to be the only significant remaining hurdle in the production of near-surface NV qubits for optical QIP devices. We have seen that hydrogen plasmas do provide a useful surface processing technique, capable of removing crystal damage, which is otherwise resistant to standard wet-chemical etching, and that nitrogen dosed hydrogen plasmas form a new and advantageous plasma-based planarization technique for diamond surfaces, which may be capable of replacing the ubiquitous mechanical polishing methods. Further, we have seen that pure nitrogen plasmas have the potential to form a thermally stable surface termination barrier for diamond, offering a possible alternative to hydrogen surface passivation. Finally, we have directly measured the effect of surface termination on nearsurface unoccupied electronic states, confirming that NV centres within 100nm of the surface have a positive future in optical QIP devices.
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ItemFrom geometric phases to intracellular sensing: new applications of the diamond nitrogen-vacancy centre( 2010)This thesis consists of two parts, each of which proposes a new application of the diamond nitrogen-vacancy (NV) centre. We first consider the NV centre as a device to detect geometric phases. We show that the Aharonov-Casher phase and Berry’s phase may be produced in the NV centre’s spin sublevels and observed using existing experimental techniques. We give the background theory to geometric phases, then show how these phases apply to the NV system. Finally, we outline a number of realistic experiments to detect these phases. The second part considers the behaviour of an NV centre within a diamond nanocrystal which rotates, in a Brownian sense, in a fluid. Our aim is to understand the effect of rotational motion on the initialisation, evolution and readout of an NV centre, motivated by the idea of using colloidal nanodiamonds for biological imaging. We first develop a model to describe the quantum evolution of a rotationally diffusing nanocrystal. The model uses theory developed in NV magnetometry and also the geometric phase theory developed in the first part of this thesis. We then explore the consequences of this model for nanoscale sensing. We show that the tumbling NV system may be used as a sensitive magnetometer with nanoscale resolution and also as a probe of its own rotational motion.