III-V self-assembled quantum dots (QDs) and quantum dashes (Q-dashes) grown by epitaxy have numerous applications for optoelectronics and quantum information. Such nanostructures are most commonly formed through strain-driven self-assembly on (001) surfaces. In this process, a thin layer of material deposited under compressive strain reorganizes into three dimensional islands. While compressive self-assembly on the (001) face produces QDs across a wide range of semiconductor materials, few successful reports have addressed QD growth under tensile strain or on other low-index surfaces. Growth of tensile-strained QDs tends to produce dislocations that impair their optical properties. This problem likewise occurs for QD attempts on (111) or (110) surfaces.
QDs grown under tensile strain or alternative surface orientations would exhibit previously unavailable properties, while providing access to new QD materials for novel optoelectronic devices. Most prominently, tensile strain strongly reduces the bandgaps of nanostructures, allowing them to emit light at much lower energies than they could under compressive strain for long wavelength optoelectronics. Secondly, QDs grown on (111) surfaces are promising candidates for generating polarization-entangled photons. The high electronic symmetry achievable in (111) QDs produces an ideal exciton fine structure for the emission of entangled-photon-pairs. Alternative techniques have been proposed to produce tensile nanostructures and (111) QDs, but these often involve complex processing requirements that lack the simplicity of strain-driven self-assembly.
To achieve dislocation-free growth of the desired QDs, a growth model is employed that describes the relationship between dislocation nucleation, surface orientation, and strain direction (tensile or compressive). This model shows that both tensile growth on the (001) surface and compressive growth on (111) or (110) surfaces suffers from low dislocation nucleation energy. Instead, dislocation-free QD growth can be achieved by combining the use of tensile strain with a (111) or (110) substrate.
Using this principle, the present work demonstrates the growth of tensile strained GaAs QDs and Q-dashes, using In0.52Al0.48As barriers, grown on IP (110), (111)B, and (111)A substrates by molecular beam epitaxy (MBE). The effects of growth conditions on self-assembly are investigated for each surface orientation, and these trends are utilized to tune the size, shape, and density of the nanostructures. Observations of dislocation-free tensile QDs or Q-dashes on each surface orientation confirm the predictions of the growth model. As a result, strong room temperature luminescence is visible from the nanostructures grown on each surface.
Due to the high tensile strain, the GaAs nanostructures emit photons as low as 240 meV below the normal bandgap of GaAs. The large bandgap reductions achievable under tension are anticipated to extend QD and Q-dash devices into longer wavelength ranges that are difficult to achieve by other means. Next, by achieving highly symmetric QDs on the (111)A surface, very low exciton tine structure splitting values are observed – a key requirement for producing entangled photons. Tensile self-assembly thus offers a simple approach for the growth of entangled photon emitters on (111) surfaces. Finally, the results of these QD investigations are anticipated to apply broadly to zinc-blende and diamond-cubic semiconductors, enabling novel devices with a wide range of properties.
The growth of lattice-matched InAlAs epilayers on InP (110), (111)B, and (111)A substrates is also extensively studied in this work to produce high quality buffer and barrier layers for quantum nanostructure growth. In addition, the development of (110) and (111) semiconductors would allow access to their unique properties, including different alignments of the internal polarization field, compatibility with growth of hexagonal materials, access to different zones of the electronic bandstructure, and long spin lifetimes. Due to these properties, such epilayers are under current investigation to support spintronics, topological insulators, transition metal dichalcogenides, and novel MOSFETs. However, epitaxy on these surface orientations is very challenging due to the formation of hillocks and rough surfaces. Little information is available for growing these semiconductors, which limits the material quality that can be achieved.
To support emerging (111) and (110) applications, the effects of growth conditions on the morphological, electrical, and optical properties of InAlAs, InGaAs, and InP, grown on InP wafers, are systematically studied for each substrate orientation. Growth parameters are identified that either eliminate or strongly reduce morphological defects on each surface. Conditions for optimizing photoluminescence, carrier mobility, and background doping are also reported. This work therefore offers a comprehensive guide to overcoming material challenges for both epilayers and QDs grown on (110) and (111)-oriented InP substrates.
|Advisor:||Lee, Minjoo Lawrence|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 76/11(E), Dissertation Abstracts International|
|Subjects:||Electrical engineering, Nanotechnology, Materials science|
|Keywords:||Band engineering, Entangled photons, III-V semiconductors, Molecular beam epitaxy, Quantum dot, Strain engineering|
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