Quantum dots in Si/SiGe have long spin decoherence times, due to the low density of nuclear spins and weak coupling between nuclear and electronic spins. Because of this, they are excellent candidates for use as solid state qubits. The initial approach towards creating controllable Si/SiGe quantum dots was to fabricate them in delta doped heterostructures. We provide evidence that the delta doping layer in these heterostructures provides a parallel conduction path, which prevents one from creating controllable quantum dots. Instead, it may be more favorable to supply electrons in the 2DEG through capactive gating, instead of a delta doping layer. We therefore discuss efforts to fabricate Si/SiGe quantum dots from undoped heterostructures and the difficulties encountered. A new method for fabricating ohmics in undoped heterostructures is discussed. We also discuss parallel conduction which occurs in the Si cap layer of these undoped heterostructures, which appears to be a major obstacle towards achieving workable devices in undoped Si/SiGe heterostructures.

Silicon/silicon-germanium (Si/SiGe) heterostructures are useful as hosts for gated quantum dots. The quality of the as-grown Si/SiGe heterostructure has a large impact on the final quality of the quantum dot as a qubit host. For many years, quantum dots have been fab- ricated on strain-graded heterostructures. Commonly used strain-graded heterostructures inevitably develop plastic defects that lead to interface roughness, crosshatch, and mosaic tilt. All of these factors are sources of disorder in Si/SiGe quantum electronics. In this dissertation, I report the fabrication of Hall bars and gated quantum dots on heterostruc- tures grown on fully elastically relaxed SiGe nanomembranes, rather than strain-graded heterostructures. I report measurements of Hall bars demonstrating the creation of two- dimensional electron gases in these structures. I report the fabrication procedures used to create pairs of Hall bars and quantum dots on individual membranes. In addition, I explain a general process flow for the creation of Si/SiGe quantum devices. I focus especially on an ion-implantation technique I implemented for the fabrication of Hall bars and quantum dots in Si/SiGe heterostructures without modulation doping layers.

Quantum size effects are becoming increasingly important in microelectronics, as the dimensions of the structures shrink laterally towards 100 nm and vertically towards 10 nm. Advanced device concepts will exploit these effects for integrated circuits with novel or improved properties. Keeping in mind the trend towards systems on chip, this book deals with silicon-based quantum devices and focuses on room-temperature operation. The basic physical principles, materials, technological aspects, and fundamental device operation are discussed in an interdisciplinary manner. It is shown that silicon-germanium (SiGe) heterostructure devices will play a key role in realizing silicon-based quantum electronics.

Since being proposed almost 40 years ago, scientists across many disciplines have made great progress in the fields of quantum computation and quantum information. Instead of a classical bit (0 or 1), a quantum computer uses a two-level quantum system as a quantum bit or qubit. By controllably manipulating the quantum-mechanical properties of these qubits, a quantum computer could, for example, be used to simulate other, less well understood quantum systems, or to run certain classes of quantum algorithms that cannot be run on classical hardware. In order to build a quantum computer, certain basic requirements must be met. As with a classical computer, logic gates are necessary to controllably manipulate qubits to perform calculations. One such requirement for a universal quantum computer is a two-qubit logic gate. This is an inherently quantum mechanical gate, which has no classical analog. For example, the controlled-not two-qubit gate will perform a not operation on the target qubit if and only if the control qubit is in the one state, else it does nothing to the target qubit. In either case, the control qubit is left unchanged and unmeasured. Being able to perform this gate with high fidelity is critical to creating a quantum computer. In this dissertation, I present progress towards fabricating, characterizing, and manipulating two-qubit devices in Si/SiGe heterostructures. First, I motivate the use of quantum dot qubits hosted in Si/SiGe as a suitable platform for quantum computing. Then, I present characterization of Si/SiGe substrates and discuss fabrication of a quantum dot device. Next, I outline the electronics set up for measuring a quantum dot device in a dilution refrigerator. I then present results of two, published experiments which explore multi-qubit systems: one which demonstrates controllable tunnel coupling between a quantum dot an a nearby localized impurity, and the other which demonstrates state-conditional Landau-Zener-Stückelberg oscillations between capacitively coupled double quantum dots in a quadruple quantum dot device. Next I discuss fabrication and characterization of micromagnets for spin qubit applications. I finally conclude by discussing future research avenues towards realizing a robust, multi-qubit device in silicon.

The potential power of quantum algorithms to solve problems that are currently computationally intractable has launched investigations into a wide range of quantum systems with the goal of developing quantum hardware. Among these candidate systems for encoding quantum bits (qubits), electron spins localized in gate-defined quantum dots are promsing. In particular, quantum dots defined in Si/SiGe heterostructures have a number of advantages, including a low defect density and a low intrinsic density of interfering nuclear spins. This work presents a series of investigations into gate-defined quantum dots formed in Si/SiGe heterostructures. A key parameter for the performance of quantum dot qubits hosted in Si is the valley splitting energy. In Si/SiGe heterostructures, controlling the valley splitting remains an outstanding challenge. Here we present measurements of valley splitting in Si/SiGe heterostructures with varied Ge concentration profiles. The measured scaling of the valley splitting with vertical electric field is compared with tight-binding simulations of heterostructures with interfacial steps, and the agreement in scaling provides evidence for the important role played by interfacial disorder in setting the valley splitting in these samples. Additionally, for gate-defined quantum dots, a key device element is the dielectric layer which mediates the electric fields between the gate electrodes and the gate-defined dots. We discuss experiments to test a novel method for generating a gate dielectric for Si/SiGe quantum dots using thermal oxidation at 700 degrees C. Another critical component towards developing quantum processors based on quantum dots is the engineering of strong and controllable inter-qubit coupling. For double quantum dot qubits with an effective charge dipole moment, a capacitive dipole-dipole interaction can generate coherent coupling between neighboring qubits. Here we also discuss investigations into the capacitive interaction between double quantum dots in a quadruple quantum dot device. This includes a demonstration of the tuning of the capacitive coupling energy over a wide range using barrier gate voltages and analysis of the dependence of that coupling energy on device geometry. Finally, we present procedures for fabricating quadruple quantum dot devices in Si/SiGe using an architecture based on overlapping self-oxidized Al gates.

This book comprehensively covers the areas of materials growth, characterisation and descriptions for the new devices in siliconheterostructure material systems. In recent years, the development of powerful epitaxial growth techniques such as molecular beam epitaxy (MBE), ultra-high vacuum chemical vapour deposition (UHVCVD) and other low temperature epitaxy techniques has given rise to a new area of research of bandgap engineering in silicon-based materials. This has paved the way not only for heterojunction bipolar and field effect transistors, but also for other fascinating novel quantum devices. This book provides an excellent introduction and valuable references for postgraduate students and research scientists.