Exploring single hole states in InAs/GaAs quantum dots and quantum dot molecules under 2-D electric fields
Date
2018
Authors
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Publisher
University of Delaware
Abstract
InAs/GaAs quantum dots (QDs) and quantum dot molecules (QDMs) are self-assembled semiconductor nanostructures that can trap a single electron or hole in a 3-D potential-well. Grown by molecular beam epitaxy (MBE), they have excellent optical qualities that can be used in applications for quantum information processing and quantum computing. Specifically, hole spins in a single QD/QDM have longer decoherence time than electron spins due to the lack of hyperfine interaction, making hole spins a great qubit candidate. However, many challenges such as QD growth, device integration, and spin manipulation inhibit the scalability of a hole-spin-based quantum information platform. A deeper understanding of the hole spin physics and the QD/QDM material system is needed to advance device engineering opportunities. ☐ In this dissertation work, we explore a hole spin in an InAs/GaAs QD/QDM under a 2-D electric field. We develop a hybrid computational method that combines a tight-binding atomistic simulation and a finite matrix approximation. This hybrid method can quickly explore the properties of a single hole spin state under a variety of electric field conditions. We discover that a hole spin in a single QD can be polarized in the vertical direction with an in-plane (lateral) electric field and a Voigt (lateral) direction magnetic field. We show that this effect persists with different QD shape, composition, and orientation. We also demonstrate the ability to control hole spin states using a lateral electric field in a QDM, particularly the ability to induce hole spin mixing with a gradient 2-D electric field. We will discuss the spin texture concept that explains these exotic spin effects using an animated visualization algorithm. ☐ We also present the experimental effort to apply 2-D electric field to a single QD/QDM. We use COMSOL semiconductor simulation to design a 3-electrode device that can apply 2-D electric field in GaAs. We discuss a range of device parameters and their influence on the device performance. We grow a single QD sample in an intrinsic GaAs matrix using MBE. We fabricate the 3-electrode device on this sample using electron beam lithography, ICP etching, Ion Milling and angled e-beam metal evaporation. The device is then characterized by low-temperature micro-photoluminescence. We show that we are able to control the charging of a single hole state in a QD using two different voltage geometries. Combining the experimental data with the COMSOL simulation results, we show the charging comes from the induced 2-D electric field around a single QD. We will discuss the improvements required to continue on the path to full 2-D field control of a single QD.