Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    ATOMICALLY PRECISE FABRICATION AND CHARACTERIZATION OF DONOR-BASED QUANTUM DEVICES IN SILICON
    (2019) Wang, Xiqiao; Silver, Richard M; Appelbaum, Ian; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Atomically precise donor-based quantum devices in silicon are a promising candidate for scalable solid-state quantum computing and analog quantum simulation. This thesis demonstrates success in fabricating state-of-the-art silicon-phosphorus (Si:P) quantum devices with atomic precision. We present critical advances towards fabricating high-fidelity qubit circuitry for scalable quantum information processing that demands unprecedented precision and reproducibility to control and characterize precisely placed donors, electrodes, and the quantum interactions between them. We present an optimized atomically precise fabrication scheme with improved process control strategies to encapsulate scanning tunneling microscope (STM)-patterned devices and technological advancements in device registration and electrical contact formation that drastically increase the yield of atomic-precision fabrication. We present an atomic-scale characterization of monolayer step edges on Si (100) surfaces using spatially resolved scanning tunneling spectroscopy and quantitatively determine the impact of step edge density of states on the local electrostatic environment. Utilizing local band bending corrections, we report a significant band gap narrowing behavior along rebonded SB step edges on a degenerately boron-doped Si substrate. We quantify and control atomic-scale dopant movement and electrical activation in silicon phosphorus (Si:P) monolayers using room-temperature grown locking layers (LL), sputter profiling simulation, and magnetotransport measurements. We explore the impact of LL growth conditions on dopant confinement and show that the dopant segregation length can be suppressed below one Si lattice constant while maintaining good epitaxy. We demonstrate weak-localization measurement as a high-resolution, high-throughput, and non-destructive method in determining the conducting layer thickness in the sub-nanometer thickness regime. Finally, we present atomic-scale control of tunnel coupling using STM-patterned Si:P single electron transistors (SET). We demonstrate the exponential scaling of tunnel coupling down to the atomic limit by utilizing the Si (100) 2×1 surface reconstruction lattice as a natural ruler with atomic-accuracy and varying the number of lattices counts in the tunnel gaps. We analyze resonant tunneling spectroscopy through atomically precise tunnel gaps as we scale the SET islands down to the few-donor quantum dot regime. Finally, by combining single/few-donor quantum dots with atomically defined single electron transistors as charge sensors, we demonstrate single electron charge sensing in few-donor quantum dots and characterize the tunnel coupling between few-donor quantum dots and precision-aligned single electron charge sensors.
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    Measurements of charge motion in silicon with a single electron transistor: toward individual dopant control
    (2005-12-02) Brown, Kenton Randolph; Kane, Bruce E; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    I present the results of experimental investigations into single electron transistors made on doped silicon substrates, with the ultimate goal of individual dopant manipulation at millikelvin temperatures. The sensitivity of single electron transistors to local charge motion should enable observations of single donor ionization. Here I formulate a model for the electrostatic control of a donor electron near an oxide interface and describe a device geometry that should enable its measurement. I give data from several Al-AlOx-Al single electron transistors below 100 mK that provide evidence for field-induced dopant ionization, as well as for the motion of individual charges whose origins are not yet understood. I also describe a cryogenic scanning force microscope that I built to measure large arrays of single electron transistors.