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An important challenge in silicon quantum electronics in the few electron regime is the potentially small energy gap between the ground and excited orbital states in 3D quantum confined nanostructures due to the multiple valley degeneracies of the conduction band present in silicon. Understanding the “valley-orbit” (VO) gap is essential for silicon qubits, as a large VO gap prevents leakage of the qubit states into a higher dimensional Hilbert space. The VO gap varies considerably depending on quantum confinement, and can be engineered by external electric fields. In this work we investigate VO splitting experimentally and theoretically in a range of confinement regimes. We report measurements of the VO splitting in silicon quantum dot and donor devices through excited state transport spectroscopy. These results are underpinned by large-scale atomistic tight-binding calculations involving over 1 million atoms to compute VO splittings as functions of electric fields, donor depths, and surface disorder. The results provide a comprehensive picture of the range of VO splittings that can be achieved through quantum engineering.

This thesis describes a series of experiments on the electronic properties of individual shallow dopant atoms in silicon. Shallow dopants are impurity atoms that bind either a single electron or hole and can therefore be con- sidered as the solid state analogue to the hydrogen atom. As the transistor density increases, critical device dimensions are fast approaching the effective Bohr radius of shallow dopant atoms. This offers the compelling possibility to utilize the quantum nature of dopant atoms to enhance the functionality if semiconductor nano-devices. The emphasis of the experimental work described in this thesis is on the interaction between single dopant atoms and their environment. When dopant atoms are embedded within a nano-structure quantum confinement at the interface will strongly perturb the wavefunction of the dopant-bound electron (or hole), as a consequence the wavefunctions and energies will no longer be that of hydrogenic states. Moreover, dielectric mismatch between the semicon- ductor and its surroundings will influence the energy spectra of dopant atoms near the interface. Generally speaking, the presence of interfaces will break the tetrahedral symmetry of the silicon crystal and will cause degeneracies to be lifted, drastically shifting the electronic states of dopant atoms with respect to dopant-bound states in bulk silicon. Scanning tunneling spectroscopy (STS) is a unique method that allows for the spatially resolved investigation of the electronic structure of sub-surface dopant atoms. Unlike in other transport measurements, both lateral position and depth of single dopant atoms can unambiguously be determined. Chapters 3 and 4 of this thesis describe experiments aimed at studying the energy spectra as a function of depth of individual sub-surface dopant atoms by means of electron transport through the localized dopant states. Interface enhancement of the ionization energy, and as a consequence deactivation of dopants near the interface, is a major concern for doping nano-structures. Chapter 3 describes the experimental investigation of the effect of the interface on the ionization energy of single sub-surface acceptors. It is worthwhile mentioning here that the vacuum-silicon interface yields the largest possible dielectric mismatch attainable for silicon. The depth of individual ac- ceptors is measured by the influence of the ionized acceptor nucleus on the local density of valence band states. The ionization energy is determined from the voltage at which resonant tunneling through the localized acceptor state occurs. An absolute energy scale is provided by the thermal broadening of the conductance peaks. It is explicitly demonstrated that acceptors in silicon less than a Bohr radius away from the interface maintain a bulk-like ionization energy. Building on the methods described in Chapter 3, measurements of the excited state spectra of single sub-surface acceptors are presented in Chapter 4. Interface induced spin-orbit splitting of the four-fold degenerate ground state of boron in silicon results in the formation of two Kramers doublets. The observed enhancement of this splitting for acceptors close to the interface, and moreover the ability to controllably tune this splitting will have strong implications for quantum computation schemes based on the spin of acceptor-bound holes. One of the key challenges in single atom electronics is the strict require- ments for dopant placement. Recent developments in scanning tunneling microscopy (STM) based bottom-up fabrication have paved the way for atomically precise dopant based electronic devices. Chapter 5 illustrates, for the first time, how low temperature scanning tunneling spectroscopy can be used in conjunction with bottom-up dopant engineering. Transport measurements on single phosphorus donors deliberately placed five monolayers beneath the surface of a p-type silicon substrate serve as a proof-of-principle for STS studies on atomically precise dopant structures. Chapter 6 describes an experiment where, for the first time, the quantum states of a single arsenic donor embedded in a nano-scale field-effect transistor are utilized to increase the device functionality of the transistor. By integrating two single-atom transistors in a circuit a classical logic operation, namely a full addition, is performed using only a fraction of the transistors required in a conventional complementary-metal-oxide-semiconductor circuit.

The use of time-of-flight (TOF) information in positron emission tomography (PET) enables significant improvement in image noise properties and, therefore, lesion detection. Silicon photomultipliers (SiPMs) are solid-state photosensors that have several advantages over photomultiplier tubes (PMTs). SiPMs are small, essentially transparent to 511 keV gamma rays and insensitive to magnetic fields. This enables novel detector designs aimed at e.g. compactness, high resolution, depth-of-interaction (DOI) correction and MRI compatibility. The goal of the present work is to study the timing performance of SiPMs in combination with LaBr3:Ce(5%), a relatively new scintillator with promising characteristics for TOF-PET. Measurements were performed with two, bare, 3 mm × 3 mm × 5 mm LaBr3:Ce(5%) crystals, each coupled to a 3 mm × 3 mm SiPM. Using a 22Na point source placed at various positions in between the two detectors, a coincidence resolving time (CRT) of ~100 ps FWHM for 511 keV annihilation photon pairs was achieved, corresponding to a TOF positioning resolution of ~15 mm FWHM. At the same time, pulse height spectra with well-resolved full-energy peaks were obtained. To our knowledge this is the best CRT reported for SiPM-based scintillation detectors to date. It is concluded that SiPM-based scintillation detectors can provide timing resolutions at least as good as detectors based on PMTs.

A dopant atom in a semiconductor, the solid state analogue of a hydrogen atom, has a Bohr radius of several nanometers. Because this length scale is close to being accessible by modern nanolithography, detection and control of charge and spin in a semiconductor down to the level of individual dopant atoms is within reach and provides the unique opportunity to study, manipulate, and utilize a single atom's wave function. We have performed electrical transport measurements across epitaxial defect-free nanometer-sized Schottky diodes. These were formed by self-assembled CoSi2-islands on Si(111) and contacted with the tip of a scanning tunneling microscope (STM). Greatly enhanced conductance was observed in diodes which were small compared to the Debye length in the semiconductor. The observed behavior can be understood qualitatively from a decreased barrier width for smaller diodes. On highly doped substrates, we find that individual dopant atoms even dominate the transport characteristics of our nanometer sized devices, due to their random distribution in the space charge region. The ability to observe the energy levels of single dopant atoms is essential for experimental studies of individual wave functions in a semiconductor. Preliminary results in a fabrication method for nano-devices approaching the size regime necessary for the observation of single dopants demonstrate the feasibility of our STM-based measurement method for this purpose. The most straightforward means to address an individual impurity is manipulation of its wave function with a gate. As a first approach to this problem, we theoretically studied the effect of a homogeneous electric or magnetic field on the energy levels of shallow impurities in silicon, taking the bandstructure into account. Furthermore, we used a description as hydrogen-like impurities for accurate computation of energy levels and lifetimes up to large electric fields. A similar description was used in a realistic device geometry, in which a small nearby gate influences a single dopant atom. This knowledge is particularly important for the development of a dopant-atom based quantum computer.