Publication Search Results

Now showing 1 - 8 of 8
  • (2014) Buech, Holger
    In this thesis we present single-shot spin readout of precision placed phosphorus donors in silicon. The spin states of an electron bound to phosphorus donors in silicon make promising building blocks for the realisation of a solid state quantum computer due to their remarkably long coherence and relaxation times. Recent progress in scanning tunnelling microscope (STM) lithography has made it possible to place these donors with atomic accuracy, opening the way to individual qubit control and hence scalability. We present spin readout from two different STM-patterned devices where the electron spin qubit is hosted by either a cluster of about four phosphorus donors or a single phosphorus donor and tunnel-coupled to an atomically planar single electron transistor (SET). We demonstrate high fidelity (approx. 90%) single shot spin readout of the cluster qubit via spin-to-charge conversion and show long spin life times of T1 = 1.3 s (B=1.5 T) despite the multi-donor, multi-electron character of the spin qubit. For the single P donor we extrapolate a slightly larger T1 of approx. 6 s at B=1.5 T, consistent with previous measurements of single donor spin life times. Using atomistic tight-binding calculations, we confirm the dependency of the relaxation rate on both donor and electron numbers, where we show that multi-donor single-electron systems provide the highest T1. We also demonstrate the concept of using the single donor as a spectrometer to analyse the energy level structure of the SET island. Finally, the successful spin readout of the cluster qubit with long measured T1 provides an opportunity to use cluster qubits in conjunction with single donor qubits to achieve qubit addressability by a global microwave field with very low error rates. We show by atomistic tight binding modelling, that the electron spin resonance (ESR) frequency of a double donor qubit can be separated by up to 350 MHz from the ESR frequency of a single donor qubit due to the difference in the hyperfine coupling, allowing qubit rotations with error rates as low as approx. 10^(-5). Together, these results advance STM device fabrication technology towards the realisation of scalable donor based qubit architectures in silicon.

  • (2013) Weber, Bent
    In this thesis, we present atomically precise donor-based electronic devices, fabricated using STM hydrogen lithography, combined with a gaseous doping source (PH3) and low-temperature silicon homoepitaxy. Using this technique we address three key issues towards the scale-up of planar Si:P donor-based quantum computing (QC) architectures towards multiple qubits: (i) The fabrication of atomic-scale low-resistive control-electrodes with diameter and pitch comparable to qubit separations. (ii) The independent electrostatic control of donor qubits in predictively modeled device architectures. (iii) The investigation of Pauli-spin blockade towards control and read-out of two-spin states. We demonstrate the formation of ``interface-free'' wires with lithographic widths down to 1.5nm. At T = 4.2K, these wires retain a low diameter-independent resistivity, ρW= (0.3 ± 0.2) mΩcm, comparable to that of bulk-doped silicon and the lowest ever reported for doped silicon wires. Investigating the wire conductance as a function of in-plane gates, we find that Si:P wires remain highly conductive down to millikelvin temperatures, allowing us to use them in more complex device architectures. We subsequently demonstrate the independent electrostatic control of two serially tunnel-coupled quantum dots which are only ~4nm in diameter, separated by ~10nm. Incorporating 4.6nm wide wires as source and drain electrodes, the dots have been embedded within a planar device architecture, optimized using predictive capacitance modeling. With excellent agreement between modeled and measured charge stability data we achieve a similar level of electrostatic control compared to much larger double quantum dots. Finally, we demonstrate independent gate control down to the atomic limit as we investigate Pauli spin blockade of the first few electrons bound to two tunnel-coupled donor-clusters of only 2P and 3P donor atoms. A fundamental manifestation of the spin degree of freedom, Pauli blockade is a main pre-requisite for the control and read-out of two-spin states in double quantum dots and coupled donors. This work therefore demonstrates the potential of STM hydrogen lithography to realize the fundamental building block of Si:P donor-based QC architectures – two precision-placed donor qubits, individually addressable with local electric fields.

  • (2011) Wilkinson-Thompson, Daniel
    This thesis demonstrates the successful development of surface-gated, highly phosphorus doped single electron transistors, defined by scanning probe lithography and low-temperature silicon molecular beam epitaxy. In order to fabricate these devices, a custom ultra-high vacuum technique was developed to grow silicon dioxide as a gate dielectric at low temperatures to prevent thermal diffusion of the buried STM patterned dopants. This technique combined atomic oxygen generated using an RF plasma source with a coincident flux of sublimated silicon to grow silicon dioxide at temperatures down to 160 degrees C at growth rates of 0.3nm.min^−1. Using aluminium electrodes deposited on the dielectric, aligned to our buried STM-patterned dopants, we were able to form atomically-abrupt, surface-gated single electron transistors. We performed chemical and structural analyses of the low temperature oxide using STM, TEM, XPS, and ellipsometry. These analyses indicated the oxide had low suboxide content and a sharp interface with the silicon substrate (< 1nm) comparable to high quality thermal oxide control samples. In addition there were no observable crystal defects induced within the underlying silicon, known to enhance dopant diffusion. However, we observed a high density of macroscopic surface defects (> 1.25 x 10^−12cm^−2) — believed to arise from spitting of silicon particles from the Si cell. These defects created leakage paths in C-V and MOSFET devices and, despite reducing the device size to 2 x 10^−4cm^2, inhibited electrical optimisation of the oxide. Nevertheless, electrical characterisation of the oxide was possible for several samples and indicated a trap density of Nit < 4.3 x 10^11cm^−2, consistent with that of un-annealed thermal oxide control samples (Nit < 3 −6 x 10^11cm^−2). The low temperature UHV silicon dioxide was then incorporated into a surface gated single electron transistor with 200 P donors, whose small size (< 1 x 10^−8cm^2) reduced the likelihood of overlap with macroscopic defects. The results were compared to an inplane gated SET of the same size, which did not have a surface gate. The surface gated SET showed gating up to electric fields of^−1 —exceeding the range of all-epitaxial in-plane gates by around one order of magnitude (<−1). Using the surface gate, we were able to tune the number of electrons on the dot by 160e, compared to 30e using a comparable in-plane gated device. Low-frequency noise measurements showed similar charge noise using the two gating schemes (Qd = 0.5%e surface gated vs. 0.2%e in-plane gated), however there was severe hysteresis (4000e) in the gate action of the surface gated device. These results emphasise the greater tunability afforded by surface gated devices but highlight the need for further improvement of the low temperature dielectric.

  • (2017) Dixit, Anubhav
    In this thesis, the introduction of top gates over buried phosphorus donor devices in silicon patterned with the atomic precision of a Scanning Tunneling Microscope (STM) is investigated. To achieve this a low temperature 200 ALD grown Al2O3 dielectric is introduced and a process strategy is developed to both contact the buried nanostructure and align surface gates to the devices. The thesis describes the impact of the Al2O3 dielectric on the electrical proper- ties of the buried nanostructure and successful efforts to maintain the integrity of the dielectric whilst contacting the buried donor layers in silicon. Dose rates are optimized for the EBL resist to contact the buried donor device. Moreover, with the optimization of the etch rate of the deposited Al2O3 to minimize the dielectric undercut, the top gate was successfully implemented on 3 types of donor based devices. In the first device a 2D δ-layer, the implementation of top gate shows a effective gate range of -4 V to 4 allowing ∼ 3 change in the carrier density of the highly doped (∼ 1014 cm−2) δ-layer in silicon. In the second device an Al2O3 dielectric and a top gate was integrated onto a precision STM-patterned SET. An increase in the overall tunability of the SET iii quantum dot was observed using the top gate owing to its large lever arm (∼ 4 times that of the in-plane gate) with no significant reduction in the charging energy of the quantum dot. An enhanced gating range of -4 V to 4 V was observed for the SET quantum dot device. Additionally, the patterned top gated device demonstrated exceptional stability with the lowest noise and drift of all devices and a successful dynamic frequency response up to 1 MHz. In the third device the top gate and an Al2O3 dielectric was incorporated into a precision STM-patterned 4 quantum dot device with the aim to capacitively couple two singlet-triplet qubits. Without the top gate the limited in-plane gate range did not allow any singlet-triplet inter-dot transitions in the gate space. However, after successful patterning of a top gate, two new inter-dot transitions were accessible in the gate range, with one of them being a singlet-triplet type inter-dot transition. An enhanced gate range of -4 V to 4 V was again observed for the top gate patterned on top of the 4 quantum dot. In conclusion a successful integration of an Al2O3 dielectric on precision STM- patterned donor devices in silicon was demonstrated important for the continuing success of silicon based atomic electronics.

  • (2014) Yang, Xiaohong
    Titanium dioxide (TiO2) nanoparticles have exhibited excellent properties and applications in many fields of clean energy, environment protection and biotechnology. This thesis aims to study on TiO2 and its hybrid nanostructures. A brief introduction and literature review on the titanium dioxide synthesis, modifications as well as potential applications which will be presented in Chapters 1 and 2, respectively. Chapter 3 presents an acetone assisted sol-gel synthesis method for preparing monodispersed TiO2 nanoparticles under mild conditions. The experimental parameters, colloidal growth mechanism, and role of acetone in the synthesis process were systematically studied. The photocatalytic activities of the as-prepared particles were compared with commercial titanium dioxide P25 powder under the same conditions. To enhance the functional properties of photocatalysis, the nanocomposites of silver and titanium dioxide with different nanostructures were fabricated in Chapter 4, in which silver@titanium dioxide core-shell nanostructures were found to exhibit excellent performance in photo degradation of organic molecules, nearly double high efficiency to the pure titanium dioxide. The mechanism was studied by density functional simulation (DFT) method. To further explore the multi-functionality of silver-titanium dioxide composites, Chapter 5 assessed the bacterial growth inhibition and bactericidal ability between silver@titanium dioxide core-shell nanostructures and silver doped on the surface of titanium dioxide nanostructures. In chapter 6, a general applied titanium dioxide coating strategy was developed on a variety of core particles (such as Au, Ag, Fe2O3, V2O5, SiO2) with different functionalities. Finally, the summary was presented in Chapter 7.

  • (2011) Zhang, Huiming
    The blast furnace is the traditional ironmaking process and the most important technology to produce the liquid iron. However, as the natural resource has been rapidly depleted and the awareness of environmental protection has risen, new ironmaking technologies were developed in the last two decades. FINEX, the smelting reduction process, is one of the promising technologies that could solve these problems. Based on fluidized bed reducing technologies, fine ore can be directly used in FINEX process rather than cokes or sinter which leads to the reduction of the costs and adverse effect on environment caused by pellets or sinter preparation. To improve the FINEX technology, the multiphase flow in the Melter&Gasifier needs to be recognized. Mathematical modeling is an efficient way to achieve this especially the coupling approach of discrete particle simulation (DEM) and computational fluid dynamics (CFD). Gas-solid flow and heat transfer phenomenon were investigated at a microscopic level by CFD-DEM. The results reveal how variables like particles charging angles, solid flow rate, gas flow rate and particles properties influence particle flow patterns. Microscopic information such as individual particle velocity, porosity, coordination number and force structure acquired from the simulation process is crucial for us to understand the mechanism of particle flow patterns.

  • (2010) Tan, Kuan-Yen
    This thesis demonstrates the fabrication and the measurement of single-electron tunnelling through deliberately implanted individual phosphorus (P) donors in double-gated silicon nanoscale field-effect-transistors (nanoFET). These structures conveniently allow the control of the number, depth, and tunnel coupling of the donors. For each P donor, three possible charge states have been observed, in which two are successively occupied by spin-down and spin-up electrons in a magnetic field. These states are separated by a charging energy consistent with those between the D0 and D− charge states of a P donor coupled to the surrounding electrodes. These experiments provide important understanding of the electric and magnetic properties of individual phosphorus donors needed for the realization of Si:P nanoelectronics. The demonstrated ability to resolve spin states of donors in these devices is the first step towards the measurement of electrically-detected magnetic resonance for a single-donor electron spin. Due to the tunability of the Fermi levels of the reservoirs of the nanoFET, the density of states (DOS) in the source and drain reservoirs are found to be quasi-1-dimensional. These reservoir DOS modulate the transport current through the nanoFET, consequently manifesting themselves as conductance peaks in bias stability diagrams. Further investigations showed that in a externally applied magnetic field, these DOS peaks shift in magnetic field at precisely half the Zeeman-splitting energy compared with peaks related to P-donor bound electrons, in good agreement with theoretical predictions. This result enables convenient discrimination between spin and orbital excited states from other features attributed to DOS modulations in the leads. Another important feature observed in the bias stability diagrams is the mutual charging effect of adjacent parallel donors in the conducting channel of the nanoFET. Charging a single donor in parallel with other active donors gives rise to a potential shift which is significant compared with the D0 and D− charging energy. In addition, the mutual charging energy also provides a good indication of the proximity of the active donors in the conducting channel. The successful demonstration of resonant tunnelling through locally-doped P donors in silicon in these nanoFET structures indicates that they can provide a convenient platform for the investigation of single-atom nanoelectronics and spintronics in silicon.

  • (2011) Pok, Wilson
    In this thesis we investigate electron transport between ultra-thin, atomically abrupt, buried, nanoscale gaps in silicon, formed between coplanar source and drain leads consisting of highly phosphorus doped silicon. The leads are patterned using scanning probe lithography, dosed with phosphine gas and encapsulated with silicon using low temperature molecular beam epitaxy. These unique tunnel gaps have leads with low sheet resistivities of <1 kohms, are atomically abrupt in all three dimensions, and are scaled down to sub-20 nm dimensions. The overall tunneling resistance of these devices depends exponentially on the aspect ratio (lead width/gap separation) such that, by engineering the gap dimensions, we control the resistance over seven orders of magnitude from <10 kohms to >500 Gohms. This relationship allows us to predictively engineer gap resistance with device geometry. We also demonstrate an asymmetric barrier shape, forming an atomic-scale diode with clear rectifying behaviour. We explore six commonly used models to estimate the barrier heights in these nanoscale gaps (rectangular and parabolic barrier models, transition voltage, Fowler-Nordheim field emission and thermionic emission), and where appropriate, adapt these models to include the complex silicon bandstructure. Although these models provide reasonable estimates, no single model describes the conductance over the complete range of gaps studied, and highlight the need for more accurate models that self-consistently account for both tunneling within the electrostatic potential of the exact lead geometry and Thomas-Fermi screening. We develop three different methods of top-gating these devices using a Schottky barrier, a native oxide, or a silicon dioxide dielectric grown at low temperature. The low temperature oxide exhibits the largest gating range, and the overall tunnel gap resistance can be gated by three orders of magnitude. Finally, we investigate the possibility of engineering the dopants between the source and drain leads. We confirm that we can order dopants with sub-10 nm precision in the channel and demonstrate their overall effect on device conductance. This thesis shows that across a statistical number of devices, we can engineer the lead geometry, the tunneling resistance, the barrier height, and the level of disorder, between atomically abrupt source/drain leads.