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Graduate Defense: Katie Sautter
April 16 @ 12:00 pm - 2:00 pm MDT
Title: Tensile-Strained Germanium Quantum Dots Grown on Indium Aluminum Arsenide (111)A and (110) by Molecular Beam Epitaxy
Program: Doctor of Philosophy in Materials Science and Engineering
Advisor: Dr. Paul Simmonds, Materials Science and Engineering
Committee Members: Dr. Eric Jankowski, Materials Science and Engineering, Dr. Christian Ratsch, Materials Science and Engineering, and Dr. Michael Scheibner, Materials Science and Engineering
Molecular beam epitaxy (MBE) enables the growth of semiconductor nanostructures known as tensile-strained quantum dots (TSQDs). The highly tunable nature of TSQD properties means that they are of interest for a wide variety of applications including for infrared (IR) lasers and light-emitting diodes (LEDs), improved tunnel junction efficiency in multijunction solar cell technology, quantum key encryption, and entangled photon emission. In this project, I focus on one of the most technologically important semiconductors, germanium (Ge). Ge has a high gain coefficient, high electron mobility, and low band gap: all excellent properties for optoelectronic applications. Until recently, these technological advantages were unattainable for light-emitting purposes due to Ge’s indirect band gap. Placing Ge under tensile strain changes this semiconductor’s fundamental electronic structure by turning its indirect band gap into that of either a semimetal or a direct band gap semiconductor, depending on the choice of surface orientation. The primary objective of this dissertation was therefore to explore whether we can use tensile-strained self-assembly to synthesize Ge TSQDs under large tensile strains, and in doing so, transform the fundamental properties of this technologically important element. TSQD self-assembly is one of the only ways in which we can induce the very large tensile strains needed for dramatic changes to Ge’s band structure without producing crystalline defects.
I used TSQD self-assembly to explore the first known Ge TSQDs on two non-traditional (i.e. non-(001)) surface orientations: (111)A and (110). I discovered proper growth conditions for the first-ever reported Ge TSQDs grown on InAlAs(111)A and (110). During this process, I not only found an unusual growth mode transition for Ge/InAlAs(111)A TSQDs, but I also increased the body of knowledge on the fundamental kinetics behind their growth. This knowledge will be a boon for future electronic device development using semimetallic Ge TSQDs. For the first time, I report room-temperature, direct band gap light emission from Ge(110) TSQDs, suggesting that an indirect-to-direct band gap transition occurred, and provide a robust methodology for their successful growth. This breakthrough could enable the future use of these nanostructures for an entirely new type of IR light emitter.