Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Materials Science and Engineering
Materials Science and Engineering
Paul J. Simmonds, Ph.D.
David Estrada, Ph.D.
William B. Knowlton, Ph.D.
Olga Goulko, Ph.D.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
The use of molecular beam epitaxy (MBE) to create quantum dots (QDs) embedded in solid-state semiconductor media has been at the forefront of novel and record-breaking optoelectronic device development for many years. However, the wide range of semiconductor fabrication capabilities and the non-equilibrium growth parameters inherent to MBE mean that there are still many QD research frontiers that are yet to be explored.
This work focuses on a recently discovered method that permits, for the first time, the growth of QDs under tensile strain on non-(100) surfaces. My research explores the first (and currently only) optically active materials system for tensile-strained QD (TSQD) growth on (111) surfaces: GaAs/InAlAs(111)A TSQDs. The use of MBE for the self-assembly of (111)-oriented GaAs TSQDs is of particular interest for quantum information science due to several properties inherently favorable for quantum light emission and quantum device integration.
In Chapter 1, I provide the background necessary to understand the self-assembly and properties of TSQDs. This background includes the basics of MBE operation and material growth, structural properties of III-V semiconductors (comparing the (100), Ga-terminated (111)A, and As-terminated (111)B planar surfaces), the nucleation and growth of the well-established gallium arsenide (GaAs) (100) III-V system and of indium arsenide QDs grown on that surface, the essential factors for entangled photon emission, and the research on TSQDs that preceded this work.
In Chapter 2, I follow this with a comprehensive analysis of the growth-parameter phase-space of GaAs(111)A TSQDs. Growth parameters include deposition amount, substrate temperature, growth rate, and V/III flux ratio. I discuss the boundaries of these parameters and the effects they have on QD height, diameter, volume, areal density, and photon emission wavelength and intensity. This study provides the first ever guide for customizing TSQD properties for future research and device applications. Using this guide, I outline the best route for optimization of GaAs(111)A TSQD entangled photon emission. In the course of this analysis I discuss several interesting and unique properties of TSQD nucleation and growth, including evidence for an equilibrium TSQD size and a TSQD nucleation rate sensitivity to arsenic concentration.
In Chapter 3, I present on an unusual and impactful discovery: a deviation from the conventional Stranski-Krastanov (SK) growth (in which a 2D wetting layer precedes QD formation and then remains fixed at given thickness). In contrast, I show that GaAs(111)A TSQD self-assembly occurs via an anomalous SK growth mode in which the WL continues to grow after QD formation. I use experimental and computational analyses of the GaAs(111)A WL and TSQDs to confirm this anomalous SK growth. No previous reports of this growth mode exist. This novel growth mode could prove to be valuable to future device designers, since research indicates that varying WL thickness can have significant impact on QD optical properties. This provides a unique and useful addition to growth-parameter tuning of TSQD properties.
In Chapter 4, I explore the use of dimeric arsenic (As2) versus the tetrameric As4 traditionally used for (111)-oriented growth. I discovered several differences between As2 and As4 grown TSQDs, which provides a greater ability to tailor TSQDs and reveals different nucleation and growth kinetics. I also uncovered that GaAs(111)A has three distinct morphologies that depend on the substrate temperature and arsenic species used, these include high symmetry hexagon TSQDs and two orientations of triangular TSQDs. For the hexagonal and both types of triangular TSQDs, growth with As2 exhibits higher photon emission intensity compared to As4 grown TSQDs, an indication of improved crystal quality (essential for reliable optoelectronic devices).
Finally, in Chapter 5 I present a complete roadmap for tuning TSQD structural and optical properties, and reveal the growth-parameter conditions for optimized TSQD emission and entangled photon emission. I also provide a discussion of the future work that will be required to complete the quantum optical analysis and device integration of GaAs(111)A TSQDs.
This comprehensive analysis of GaAs(111)A TSQD growth provides an essential foundation for future (111)-surface and TSQD research and applications. My exploration of the many unique and interesting properties of GaAs(111)A TSQD growth provides new insights into the physics of the nucleation and growth of tensile-strained (111)-surface. The additional investigation to the science behind TSQD formation and optical properties provides an essential foundation for understanding (111)-oriented TSQD capabilities, with an eye toward the many yet unexplored TSQD materials systems. The use of this guide to optimize TSQDs for entangled photon emission cements the utility of this roadmap. The many promising device applications of (111)-oriented TSQDs, including robust and easily integrated entangled photon LED materials made in a single processing step, are now a real possibility.
Schuck, Christopher, "(111)-Oriented Gallium Arsenide Tensile-Strained Quantum Dots Tailored for Entangled Photon Emission" (2019). Boise State University Theses and Dissertations. 1547.