Quantum computing has become a thriving field over the past several decades. Although many candidate systems exist, this dissertation will focus on quantum dots as a quantum computing implementation, specifically lateral quantum dots in silicon based heterostructures. Lateral quantum dots use trapped electrons in semiconducting heterostructures to form qubits, the basic building block of a quantum computer. There are several potential qubit implementations using quantum dots and new qubit schemes, such as the valley qubit presented in Chapter 4, are still being investigated. Many of these implementations have already been successfully demonstrated. In this sense, research into quantum dots is a maturing field, having successfully demonstrated proof of concept for multiple qubit implementations. If quantum dots are to succeed as a quantum computing platform research needs to focus on improving the qubits themselves. Decoherence and dephasing need to be improved, but also yield and reproducibility. In this work I describe experiments intended to help understand and improve the performance of lateral quantum dots. I fabricated multiple lithographically identical devices on Si/SiO2 and Si/SiGe heterostructures to compare charge noise on the two Silicon based substrates. I describe the first conclusive observation and characterization of a valley based qubit. The noise characteristics of the valley qubit are particularly attractive as it's operation is resistant to charge noise, the primary source of noise in Silicon based qubits. Finally I present the ongoing development of a novel gate architecture for lateral quantum dots. Called a hybrid architecture, this design possesses good tunability along with simple fabrication and a reduced number of total gates relative to other leading architectures; this has the potential to dramatically improve yield and scalability.

Growth of Self Organized Quantum Dots / J.-S. Lee / - Excitonic Structures and Optical Properties of Quantum Dots / Toshihide Takagahara / - Electron-Phonon Interactions in Semiconductor Quantum Dots / Toshihide Takagahara / - Micro-Imaging and Single Dot Spectroscopy of Self Assembled Quantum Dots / Mitsuru Sugisaki / - Persistent Spectral Hole Burning in Semiconductor Quantum Dots / Yasuaki Masumoto / - Dynamics of Carrier Relaxation in Self Assembled Quantum Dots / Ivan V. Ignatiev, Igor E. Kozin / - Resonant Two-Photon Spectroscopy of Quantum Dots / Alexander Baranov / - Homogeneous Width of Confined Excitons in Quantum Dots - Experimental / Yasuaki Masumoto / - Theory of Exciton Dephasing in Semiconductor Quantum Dots / Toshihide Takagahara / - Excitonic Optical Nonlinearity and Weakly Correlated Exciton-Pair States / Selvakumar V. Nair, Toshihide Takagahara / - Coulomb Effects in the Optical Spectra of Highly Excited Semiconductor Quantum Dots / Selvakumar V. Nair / - Device ...

With the rapid progress in nanofabrication, scientists and researchers are now able to make lateral quantum dots in semiconductor materials. The few electrons confined in these quantum dots provide the possibility of realizing a qubit, the building block of a quantum computer. Tremendous effort has been put in the solid state quantum information field in the last ten years of making single electron spin qubit or singlet triplet qubit based on two electron spin. However, the operation of these types of qubit requires additional engineering by either integrating a microwave loop or an external magnet to creat field difference. This thesis project was inspired by DiVincenzo's proposal of developing qubit based on three electrons controlled by Heisenberg exchange interactions only, which is called "exchange-only" qubit. All the qubit operation can be done in principle via electrical pulses only. We proposed to make the triple quantum device in silicon system. This type of device will have small qubit decoherence, easy integration to industry infrustructure and great chance of scaling up to a real quantum computer. We developed and fabricated the electrostatically defined triple quantum dot (TQD) device in a silicon metal-oxide-oxide structure. We characterized its electrostatic properties using a quantum point contact charge sensing channel nearby. We are be able to obtain the charge stability diagram in the last few elelctron regime that provides the experimental basis of forming a exchange only qubit. We demonstrated the tunability of the TQD by acheiving the quadruple points where all three dots are on resonance. This is the first experimental demonstration of well controlled triple quantum dot device in silicon system. The constant interaction model and the hubbard model for triple quantum dot system are developed to help understand the electrostatic dynamics. Tunnel couplings between quantum dots, which determines the exchange interactions, are extracted using various fitting methods. We implemented the qubit manipulation with three quantum dots in both a linearly and a triangularly arranged geometry. For the first time, we observed coherent oscillation in the Si MOS based triple quantum dot device with oscillation frequency of 2MHz and 7MHz. We suspect the these oscillations are related with spin dynamics in our system. These experimental investigations demonstrate that we have the ability to develop triple quantum dot device for exchange ony qubit and the potential to perform qubit operation in the future.

The possibility of building a computer that takes advantage of the most subtle nature of quantum physics has been driving a lot of research in atomic and solid state physics for some time. It is still not clear what physical system or systems can be used for this purpose. One possibility that has been attracting significant attention from researchers is to use the spin state of an electron confined in a semiconductor quantum dot. The electron spin is magnetic in nature, so it naturally is well isolated from electrical fluctuations that can a loss of quantum coherence. It can also be manipulated electrically, by taking advantage of the exchange interaction. In this work we describe several experiments we have done to study the electron spin properties of lateral quantum dots. We have developed lateral quantum dot devices based on the silicon metal-oxide-semiconductor transistor, and studied the physics of electrons confined in these quantum dots. We measured the electron spin excited state lifetime, which was found to be as long as 30 ms at the lowest magnetic fields that we could measure. We fabricated and characterized a silicon double quantum dot. Using this double quantum dot design, we fabricated devices which combined a silicon double quantum dot with a superconducting microwave resonator. The microwave resonator was found to be sensitive to two-dimensional electrons in the transistor channel, which we measured and characterized. We developed a new method for extracting information from random telegraph signals, which are produced when we observe thermal fluctuations of electrons in quantum dots. The new statistical method, based on the hidden Markov model, allows us to detect spin-dependent effects in such fluctuations even though we are not able to directly observe the electron spin. We use this analysis technique on data from two experiments involving gallium arsenide quantum dots and use it to measure spin-dependent tunneling rates. Our results advance the understanding of electron spin physics in lateral quantum dots, in silicon and in gallium arsenide.

Semiconductor Quantum Dots presents an overview of the background and recent developments in the rapidly growing field of ultrasmall semiconductor microcrystallites, in which the carrier confinement is sufficiently strong to allow only quantized states of the electrons and holes. The main emphasis of this book is the theoretical analysis of the confinement induced modifications of the optical and electronic properties of quantum dots in comparison with extended materials. The book develops the theoretical background material for the analysis of carrier quantum-confinement effects, introduces the different confinement regimes for relative or center-of-mass motion quantization of the electron-hole-pairs, and gives an overview of the best approximation schemes for each regime. A detailed discussion of the carrier states in quantum dots is presented and surface polarization instabilities are analyzed, leading to the self-trapping of carriers near the surface of the dots. The influence of spin-orbit coupling on the quantum-confined carrier states is discussed. The linear and nonlinear optical properties of small and large quantum dots are studied in detail and the influence of the quantum-dot size distribution in many realistic samples is outlined. Phonons in quantum dots as well as the influence of external electric or magnetic fields are also discussed. Last but not least the recent developments dealing with regular systems of quantum dots are also reviewed. All things included, this is an important piece of work on semiconductor quantum dots not to be dismissed by serious researchers and physicists.

This book describes the full range of possible strategies for laterally aligning self-assembled quantum dots on a substrate surface, beginning with pure self-ordering mechanisms and culminating with forced alignment by lithographic positioning. The text addresses both short- and long-range ordering phenomena and introduces future high integration of single quantum dot devices on a single chip. Contributions by well-known experts ensure that all relevant quantum-dot heterostructures are elucidated from diverse perspectives.

Spurred on by the promise of a quantum “speed-up” for certain classes of computational problems, interest in quantum computation has seen a great surge over the past decades. One of the approaches is confining carriers in “quantum dots”, small regions in a semiconductor, such that quantum properties become observable. The first step in creating quantum dots in semiconductors is having exact control over the confinement potential. However, defects, coming about by disorder or other fabricational issues are of great influence on quantum dot formation. This work concerns the properties of hole quantum dots under the influence of, and the charge transport through, these defects. We thoroughly explore the silicon-based heterostructure of these devices and most prominently look at the Si/SiO2 interface and its associated defects. We explore the origin of these defects, and devise methods of eliminating them. We find that the ability to passivate defects at the Si/SiO2 interface hinges primarily on controlling the dewetting at higher temperatures. We introduce an ALD-grown Al2O3 layer, which strikes two birds with one stone. The layer both provides hydrogen for the annealing process and prevents dewetting. The introduction of Al2O3 introduces a layer of negative fixed charge, which can be eliminated or controlled by exposure to a UV-ozone oxidation process. We also find that by supplanting Al by Pd as the electrode material, the formation of an interfacial layer between the metal and the oxide is prevented We show that we can indeed make a hole quantum dot in intrinsic silicon, by demonstrating single-hole tunneling through a two-dimensional hole gas. Furthermore, by using Al2O3 grown by atomic-layer deposition in an annealing process, we passivate the majority of the electrically active defects. Using this oxide, and the passivating properties of the hydrogen contained therein, we are able to create intentional quantum dots of at least 180 nm. These quantum dots show many charge transitions, indicating the low level of disorder in the devices. We also study the g-factor anisotropy a hole quantum dot in silicon. We do this by studying the Zeeman splitting in a magnetic field capable of rotating 360∘ over all 3 degrees of freedom. Using two methods of fitting the data to our model, we extract similar anisotropies for the g-tensor (g∥*≈ 2.2, g⊥*≈ 4), indicating that the g-factor is roughly twice as strong out-of-plane than in-plane. This is consistent with the prediction that light-holes and heavy-holes are oriented preferentially in a 2D quantum well. A proof-of-principle of a single-layer depletion-mode hole quantum dot is demonstrated. The negative fixed charge in ALD-grown Al2O3 enables the operation in depletion-mode by inducing a 2D hole gas. Characterization of the charge-offset stability of this device indicates that the device is extremely stable, with the charge-offset stability having an upper bound at Q0 = 0.04e and a lower bound of Q0 = 0.005e. This compares favourably to previously known results for all-Si based devices (Q0