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Title:Semiconductor quantum dots for mid-IR light emission
Author(s):Yu, Lan
Director of Research:Wasserman, Daniel M
Doctoral Committee Chair(s):Wasserman, Daniel M
Doctoral Committee Member(s):Eden, James Gary; Li, Xiuling; Lee, Minjoo Lawrence
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Quantum Dots
Type-II material
Mid-IR emitters
InAs quantum well lasers
Nano-sphere lithographically defined quantum dots
III-V material growth
Abstract:Mid-infrared wavelength range (3 μm to 30 μm) is important in many applications such as environmental monitoring, industrial process control, bio- medical imaging, security and defense. The mid-IR is the spectral home of the distinct vibrational and rotational absorption resonance signatures of a wide range of molecular species, giving mid-IR sensing systems the potential to enable the monitoring and identification of molecules for gas sensing or chemical and biological imaging applications. To enable many of the above applications, compact, high efficiency, and low-cost mid-IR emitters and detectors are required. The goal of this project is to develop highly efficient and low-cost mid-IR emitters. The first part of this thesis gives an introduction to why mid-IR light is important and the state-of-the-art mid-IR sources. We discuss the working theory, structure, advantages and disadvantages of quantum cascade laser (QCL), interband cascade laser and type-I quantum well lasers, which together cover the wavelength range from 2 μm to 11 μm. Then, in the last part of chapter 1, we provide an introduction to quantum dots and a discussion as to why we might want to use quantum dots to improve the performance of QCLs. Here we will discuss primarily two types of quantum dots: those that are generally referred to as patterned (either etched top down, or site-selectively grown), and self-assembled QDs (SAQD). In this project, we studied studied mechanisms for control of energy states in both top-down nanolithographically defined QD and bottom-up InAs submono-layer QD grown by MBE. We demonstrate strong carrier confinement in, and electroluminescence (EL) from, quantum nanostructures fabricated from epitaxially grown quan- tum wells (QWs) using a top-down nanosphere lithography (NSL), dry-etch, mass-transport, and overgrowth fabrication process. Optically active nano- pillars with diameters as small as 90 nm are fabricated, and narrow linewidth(18 meV) electroluminescence from a fabricated diode structure is observed, with an emission blue-shift of over 37 meV from the original quantum well sample luminescence. The results presented offer the potential for low-cost, large-area patterning of quantum nanostructures for optoelectronic applications. However, the NSL defined QD density is limited by the size of the NS used. For 200 nm diameter size NS, the QD density can only reach to 2.5 ∗ 109/cm2 which is likely not nearly high enough for most optoelectronic applications. For this reason, we started to study InAs submonolayer quantum dots (in the next chapter), aiming to use InAs SML QD in the QCL active region. The thesis then goes on to discuss work demonstrating control of energy states in epitaxially-grown quantum dot structures formed by stacked sub- monolayer InAs depositions via engineering of the internal bandstructure of the dots. Transmission electron microscopy of the stacked submonolayer re- gions shows compositional inhomogeneity, indicative of the presence of quantum dots. The quantum dot ground state is manipulated not only by the number of deposited InAs layers, but also by control of the thickness and material composition of the spacing layers between submonolayer InAs de- positions. In this manner, we demonstrate the ability to shift the quantum dot ground state energy at 77K from 1.38 eV to 1.88 eV. The results pre- sented offer a potential avenue towards enhanced control of dot energies for a variety of optoelectronic applications. The SML QD structures were then integrated into QCL-like structures, with our SML deposition in the active region. We also demonstrate infrared light emission from thin epitaxially grown In(Ga)Sb layers in InAs(Sb) matrices across a wide range (3-8 μm) of the mid- infrared spectral range. Our structures are characterized by x-ray diffraction, photo-electron spectroscopy, atomic force microscopy and transmission electron microscopy. Emission is characterized by temperature- and power- dependent infrared step-scan photoluminescence spectroscopy. The epitaxial In(Ga)Sb layers are observed to form either quantum wells, quantum dots, or disordered quantum wells, depending on the insertion layer and substrate material composition. The observed optical properties of the monolayer scale insertions are correlated to their structural properties, as determined by transmission electron and atomic force microscopy. In this work, we employ time resolved PL to study the carrier recombination mechanism in a thin type II material system. With the experimental system we set up and the analysis process developed, we are able to resolve the Shockley-Read-Hall and radiative rates from our materials. This provides a powerful way to study the emitter quality. According to our TRPL study as well as the optical study, we find that the sample with obvious nano- structure formation has the best optical performance and material quality, which makes the QD structure the best candidate for mid-IR emitter and laser applications. Finally, the thesis ends with a study of the growth of InGaSb QDs using MBE by systematically changing growth parameters such as substrate temperature, Ga/In ratio and layer thickness. A high density and high uniformity QD sample is grown and studied at the end of chapter 6. This sample shows a better temperature performance and a better material quality than any other samples without QD formation. From this work we are able to draw the conclusion that the type-II QD structure has the best potential in the future to be made into a low-cost, simple structure and high-performance room-temperature mid-IR emitters.
Issue Date:2016-11-29
Rights Information:Copyright 2016 Lan Yu
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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