Tensile-Strained Germanium Quantum Dots Grown on Indium Aluminum Arsenide (111)A and (110) by Molecular Beam Epitaxy
Authors: Sautter Kathryn Eva 
Published: August 2021

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 materials, 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 direct band gap semiconductor or a semimetal, depending on the choice of surface orientation. However, it is extremely difficult to use bulk Ge, because the propensity for dislocation formation and strain relaxation is high under the tensile strains required for this band gap transition. In contrast, we can store large amounts of tensile strain in TSQDs without detrimental effects on the crystal quality. The primary objective of this dissertation is 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.

In this project I used TSQD self-assembly to create Ge TSQDs on two non-traditional (i.e. non-(001)) surface orientations: (111)A and (110). My research explores the first known Ge TSQDs on these systems. Because of this, I wrote much of this dissertation trying to understand the impact of MBE parameters on Ge TSQD growth/formation and how the properties of Ge TSQDs compare to other quantum dot (QD) systems. I focus on this, because 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, making it a new and exciting area of study. Additionally, TSQD self-assembly is a recent advancement, leaving this area of science relatively unexplored. For the first time, I am able to report light emission from tensile-strained Ge(110) TSQDs, suggesting that we have transformed this important semiconductor into a direct band gap material with efficient light emission.

In Chapter 1, I provide the background needed to understand the work within this dissertation. I describe the motivation for the work, the basics of MBE growth, the characterization tools I employed, the relevant crystallography of these structures, the mechanisms for TSQD self-assembly, and finally the optoelectronic background needed to better understand Ge TSQDs.

In Chapter 2, I expand on many of the concepts from Chapter 1. Chapter 2 was an invited paper, wherein we wrote a tutorial-style guide for quantum dot (QD) growth by MBE and provide methodologies for many different QD self-assembly systems commonly investigated in literature. We discuss the premise for QD development, where it fits into quantum applications, how QD self-assembly works, how to grow various compressively strained systems, and finally how to grow tensile-strained systems. This particular paper was designed as an introduction to new QD growers, helping them by giving all the information they would need to start growing QDs in general, but particularly TSQDs. Since TSQDs are still a relatively new field, gathering all the information about how to grow several different materials systems all in one place and providing clear, step-by-step instructions about how to grow them is valuable. This Chapter is thus a guidebook for other researchers.

In Chapter 3, I apply this background information and use it to create the first self-assembled Ge TSQDs on InAlAs(111)A. I provide a comprehensive study of how the structural properties of Ge/InAlAs(111)A TSQDs change with growth parameters, providing a robust platform for future work in embedded, low-resistivity tunnel junctions and contacts. I discovered an extremely unusual phase transition for these Ge/InAlAs(111)A TSQDs from a Stranski-Krastonov (SK) growth mode at low temperatures, to a Volmer-Weber (VW) growth mode at higher temperatures. This characteristic is highlighted in the paper, because this work provides the clearest evidence to-date of the ability to switch between different growth modes for quantum dot self-assembly based simply on MBE parameters. Being able to choose between mixed one-and three-dimensional (3D) quantum confinement (e.g., from the wetting layer and QDs in the SK growth mode) or just 3D quantum confinement (e.g., QD-only VW growth) with a high degree of tunability opens up the door to new electronic device applications.

In Chapter 4, I compare Ge/InAlAs(111)A TSQDs to an analogous purely III-V TSQD system: namely GaAs/InAlAs(111)A TSQDs. These two TSQD systems, while seemingly similar from both a surface (InAlAs(111)A) and a strain perspective (both have ~3.7% tensile strain), have entirely different shapes, nucleation behaviors, and areal densities. We use potential energy surfaces, radial distribution scaling, and island scaling analyses to compare the two TSQD systems. In the process, we obtain a much deeper understanding of the kinetic behavior during self-assembly for both Ga and Ge adatoms on an InAlAs(111)A surface. This will allow us to more effectively tailor these TSQDs for specific optoelectronic applications.

In Chapter 5, I demonstrate growth of the first Ge(110) TSQDs grown on InAlAs. The (110) surface is essential to this project, because theory suggests that tensile strain should produce a direct band gap transition in the Ge, transforming it into an efficient light-emitting semiconductor. I use a variety of experimental techniques and surface symmetry/diffusion anisotropy arguments to explain the unusual shapes of the resulting TSQDs. Initial photoluminescence data indicates strong light emission from Ge(110) TSQDs for the first time, indicating that a strain-driven indirect-to-direct transition has occurred. This breakthrough could enable the future use of these nanostructures for an entirely new type of IR light emitter.

In Chapter 6, I discuss several avenues for researchers to continue this project. The light emission I report in Chapter 5 needs to be investigated in greater detail. Once we learn more about the light emission properties of Ge(110) TSQDs, we can make LED and lasing devices out of them, optimizing their growth for these purposes and test their overall performance. We can also investigate the possible topological insulating characteristics of Ge/(111)A TSQDs and test how they change tunnel junction efficiency. Additionally, there are several other surface orientations that could work for defect-free tensile-strained Ge growth, either from the point of known, successful surface orientations (i.e. (111)B) or from the perspective of antiphase-domain (APD)-free capping on (211) surfaces. Tensile-strained self-assembly on miscut (111)B surfaces often leads to quantum wire formation, which in the case of Ge would enable novel quantum wires for embedded semimetallic tunnel junctions, contacts, and even topological insulators. Meanwhile, the tensile-strained self-assembly on the (211) surface is entirely untouched, remaining a vast area for exploration in tunable, tensile-strained self-assembly for the development of novel electronic devices.

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/(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. I also proved efficient, direct band gap light-emission from Ge(110) TSQDs and provided a robust methodology for their successful growth.