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Graduate Defense: Kevin Vallejo
October 19 @ 3:00 pm - 5:00 pm MDT
Title: Dreams of Molecular Beams: Indium Gallium Arsenide Tensile-Strained Quantum Dots and Advances Towards Dynamic Quantum Dots
Program: Doctor of Philosophy in Materials Science and Engineering
Advisor: Dr. Paul Simmonds, Materials Science and Engineering
Committee Members: Dr. David Hurley, Materials Science and Engineering, Dr. Bernard Yurke, Materials Science and Engineering, and Dr. Dmitri Tenne, Physics
Through the operation of molecular beam epitaxy (MBE) machine, I worked on developing the homoepitaxy of high quality InAs with a (111)A crystallographic orientation. By tuning substrate temperature, we obtained a transition from a 2D island growth mode to step-flow growth. Optimized MBE parameters (substrate temperature = 500°C, growth rate = 0.12 ML/s and V/III ratio >40) lead to growth of extremely smooth InAs(111)A films, free from hillocks and other 3D surface imperfections. We see a correlation between InAs surface smoothness and optical quality, as measured by photoluminescence spectroscopy. This work establishes InAs(111)A as a platform for future research into other materials from the 6.1 A, family of semiconductors grown with a (111) orientation.
Continuing this work, we also have determined a reproducible set of growth conditions for the self-assembly of tensile-strained In1-xGaxAs quantum dot nanostructures on InAs(111)A surfaces. During molecular beam epitaxy, In1-xGaxAs islands form spontaneously on InAs(111)A when the Ga content x>50%. We analyze the structure and composition of InGaAs/InAs(111) samples using atomic force microscopy, transmission electron microscopy, electron energy loss spectroscopy, and photoluminescence spectroscopy. We demonstrate control over the size and areal density of the islands as a function of In1-xGaxAs coverage, In1-xGaxAs composition, and substrate temperature.
Furthermore, we also present a study aimed to determined the growth conditions of InGaAs self-assembled tensile-strained QDs on GaSb(111)A surfaces. From previous work we determined that a larger band gap barrier was necessary to ensure the confinement of charge carriers in the InGaAs nanostructures. Through a series of temperature, V/III ratio, and growth rate we determined the best parameters for GaSb(111) homoepitaxy. We then studied the nucleation of optimal-morphology \ingaas QDs by locking the compositions at In0.5Ga0.5As, studying the critical pause for group V element transition and V/III ratio prior and post QD growth. Several photoluminecesnce techniques are employed to determine the light emission properties of these structures.
Finally, we did preliminary studies on how to achieve the dynamic lateral confinement of charge carriers in 2D and 3D using near-THz surface acoustic phonon pulses in polar semiconductors. Using the acousto-electrical effect, we measure the degree to which surface acoustic waves (SAWs) confine electrons and holes limiting the number of recombination processes. Applications for this technological development include the external modulation of lateral confinement size in the SAWs and subsequent photon emission wavelength, as well as potential quantum logical gate design using acoustic pulses to drive electrons in a circuit.