Electrostatic and Strain-Induced Quantum Dots in Silicon Nanostructures
Zimmerman, Neil M.
Lobb, Christopher J.
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Quantum dots (QDs) are nanometer scale regions that can trap charges. In this dissertation I describe my work on understanding the reproducibility of silicon QDs, and why unintentional QDs are so common. I studied both the reproducibility and predictability of gate capacitances to intentional QDs. I found that, in our devices, electrostatic QDs have gate capacitances that are reproducible to within 10% and predictable using a capacitance simulator to within 20%. I describe a technique that uses the gate capacitances to determine the locations of the unintentional QDs in a nanowire with a precision of a few nanometers. I do this by comparing the measured gate capacitances to simulated gate capacitances. I suggest that strain from the gates or contacts may be the cause of many of the observed unintentional QDs. Strain can cause QDs because it changes the band structure, thus changing the energy of the conduction band and the valence band. I discuss the effects of strain in three common device architectures: a mesa-etched nanowire with poly-silicon gates, metal-gated bulk silicon, and chemically grown nanowires with metal contacts. Because strain can affect these very different architectures, I suggest that the strain in a QD can be as important as the electrostatics to understanding how a device works.