Epitaxial Nanotemplates for Strain Modulated Carrier Confinement in Silicon

Petz, Christopher, Materials Science - School of Engineering and Applied Science, University of Virginia
Floro, Jerrold, Department of Materials Science and Engineering, University of Virginia

The fundamental goal of this thesis is to characterize and assess the effects of strain, chemistry, and growth orientation on the structure of complex epitaxial heterostructures. In this dissertation, we discuss artificial patterning, formation, and encapsulation of Ge quantum dots (QDs).

Precise spatial ordering of QDs may enable predictable quantum states due to direct exchange interactions of confined carriers. The realization of anticipated quantum states may lead to unique functionalities such as cluster qubits and spintronic bandgap systems. In order to define exemplary quantum architectures, one must develop control over QD size and spatial arrangement on the sub-35 nm length scale. We employ fine-probe electron-beam irradiation to locally decompose ambient hydrocarbons onto a bare Si (001) surface. These carbonaceous patterns are annealed in UHV, forming ordered arrays of nanoscale SiC precipitates that may template subsequent epitaxial Ge and Si growth to form ordered hybrid Ge/SiC or Si/SiC QD arrays on ultra-small length scales.

First, we investigate the templated feature evolution during UHV annealing and subsequent Ge epitaxial overgrowth to identify key mechanisms that must be controlled in order to preserve pattern fidelity and reduce broadening of the nanodot size distribution. We find that to obtain the narrow size distribution required for spintronic applications, one must precisely control the total thermal budget.

Next, we show that sub-10 nm 3C-SiC nanodots form, in cube-on-cube epitaxial registry with the Si substrate. The SiC nanodots are fully relaxed by misfit dislocations, and exhibit small lattice rotations with respect to the substrate. Ge overgrowth at elevated deposition temperatures, followed by Si capping, results in expulsion of the Ge from SiC template sites due to the large chemical and lattice mismatch between Ge and C. Preliminary magnetotransport measurements of our templated nanostructures show significant promise for local strain-based confinement of carriers in ordered arrays.

Lastly, we have investigated low temperature epitaxial breakdown of inhomogeneously strained Si capping layers. By growing Si films on coherently strained GeSi quantum dot surfaces, we differentiate effects of surface roughness, strain, and growth orientation on the mechanism of epitaxial breakdown. Using atomic force microscopy and high resolution cross-sectional transmission electron microscopy we find that while local lattice strain up to 2% has a negligible effect, growth on higher-index facets such as {113} significantly reduces the local breakdown thickness. Nanoscale growth mound formation is observed above all facet orientations. Since diffusion lengths depend directly on the surface orientation, we relate the variation in epitaxial thickness to low temperature stability of specific growth facets and on the average size of kinetically limited growth mounds.

These experiments elucidate two technologically significant results: (1) a metric for selection of growth temperature in order to achieve high epitaxial quality Si encapsulation and (2) SiC based patterning routes can provide modulated carrier confinement potentials on relevant length scales for realization of high quality spintronic devices.

PHD (Doctor of Philosophy)
epitaxy, Si, Ge, SiGe, MBE, molecular beam epitaxy
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