Spectroscopic properties of self-assembled lateral quantum dot molecules

Zhou, Xinran
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University of Delaware
In the 20th century the first transistor was invented and the first computing device based on it was built. Since then, people have been looking for various ways to reduce the size and cost of the electronic devices used in computers, cell phones and other electronic products. When the feature sizes of these devices reach the nanometer scale, the deterministic properties of materials are replaced by the uncertainty caused by quantum effects. This brings challenges to the improvement of traditional devices but also presents opportunities for the development of electronics based on quantum mechanics. Quantum dots (QDs), semiconductor materials with quantum confinement in all three dimensions, are a very important material platform for the implementation of quantum mechanical devices. III-V semiconductor self-assembled quantum dot molecules (QDMs), consisting of two closely-spaced QDs, are of great interest as potential components for next-generation optoelectronic devices. One of the attractive features of QDMs is the ability to manipulate, in-situ, the formation of delocalized molecular states with unique optoelectronic and spin properties. The structure, geometry and compositional profile of a QDM together determine the electronic and optical properties of that QDM. Lateral QDMs (LQDMs), in particular, consist of two or more QDs placed close to each other with a molecular axis perpendicular to the growth direction of the heterostructure, creating a QD complex structure with outstanding properties. LQDMs have good scalability and provide the opportunity to independently control charge occupancy and quantum coupling. LQDMs grown by molecular beam epitaxy (MBE) using partial GaAs capping and emph{in-situ} annealing of single InAs QDs create LQDMs with a small inter-dot spacing and relatively homogeneous geometry. However, there has been substantially less work on LQDMs than in Vertical QDMs (VQDMs) because the growth control in LQDMs is less precise and the energy level structure in LQDMs is more complex than in VQDMs. This dissertation focuses on the spectroscopic characterization of the optoelectronic properties of these LQDMs under electric and magnetic fields. It covers the experimental and theoretical foundation of the energy structure and optical properties of LQDMs and techniques for manipulating the delocalized states in a single LQDM. In chapter 5, a quantitative study of Coulomb interactions and charging effects in the LQDMs' ground states, combining both experimental and theoretical results. By measuring the photoluminescence (PL) emitted by LQDMs as a function of both electric field along the growth direction and excitation laser power density we verify the existence of delocalized molecular ground states under certain charge occupation. In chapter 6 I present results and analysis that experimentally verify the existence of delocalized molecular states in the first excited electron states of InGaAs LQDMs. In chapter 7 I report the behavior of single LQDM photoluminescence from different charge states and energy shells as a function of applied magnetic field. The para-magnetic shift in the first excited states of LQDMs and the variation of g-factors in different charge states suggests the electrons in the excited states can be localized in individual dots by the magnetic fields. In chapter 8, I proposed a design for a four-terminal device, in which a controllable vector electric field (including electric fields along and perpendicular to the growth direction of the LQDMs) can be applied.