PROGRAM | Materials Science and Engineering

By: Xinran Zhou Chair: Matthew Doty

ABSTRACT

In 20 century, the first transistor was invented and the first computing device was built based on it. Since then, people started to look for various ways to reduce the size and cost of electronics devices which are used in computers, cell phones and other electronic products. When the feature sizes of the devices reduced to 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 determine the electronic and optical properties of that QDM. Lateral QDMs (LQDMs) is a QD complex structure consisting two or more QDs close to each other with a molecular axis perpendicular to the growth direction of the heterostructure. 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 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 Vertical QDMs (VQDMs) because the growth control is less precise and the energy level structure in LQDMs is more complex then the VQDMs.

My research project 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 technique for manipulating the delocalized states in a single LQDM. We firstly experimentally verify the existence of delocalized molecular states in the ground and first excited electron states of InGaAs LQDMs by measuring photoluminescence (PL) of LQDMs as as functions of both electric field along the growth direction and excitation laser power density. Then, a quantitative research of Coulomb interactions and charging sequence in LQDMs’ ground states combining experimental and theoretical results are introduced. In the next chapter of this thesis, we demonstrate the anomalous behavior of single LQDMs’ photoluminescence as a function of applied magnetic field in different charge states and energy shells. The para-magnetic shift in the first excited states of LQDMs and the change of g-factors suggests the electrons in the excited states can be localized in each dots by the magnetic fields. In the last part of my thesis, I proposed a design of 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 to the LQDMs.

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