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Title:Atomic structure and defects of III-V compound semiconductor strained-layer-superlattices for infrared detection
Author(s):Kim, Honggyu
Director of Research:Zuo, Jian-Min
Doctoral Committee Chair(s):Zuo, Jian-Min
Doctoral Committee Member(s):Shim, Moonsub; Wasserman, Daniel M.; Shoemaker, Daniel P.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):III-V semiconductor
Scanning Transmission Electron Microscopy
Point defect
Abstract:There are considerable interests in the use of infrared (IR) photodetectors, in particular mid- and long-wavelength IR detection, for diverse scientific, civil, and military applications. Devices based on the InAs/GaSb or InAs/InAsSb strained-layer-superlattices (SLSs) have gained special attention as the next generation IR photodetectors for replacing current technology based on mercury cadmium telluride. When in contact, InAs and GaSb (or InAs and InAsSb) forms the broken-band alignment (type-II), which gives rise to a narrow effective energy band gap in a short period superlattice, thereby making it suitable for detecting IR radiations of various wavelengths. The advances in epitaxial thin-film growth techniques, such as molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD), have brought dramatic reductions in structural defects, such as misfit dislocations, that are strong carrier scatters and major device failure factors. However, despite the remarkable success in the growth of Type-II SLSs (T2SLs) in the form of dislocation-free thick layers, theoretical promises of high device performance of these structures have yet to be realized, primarily due to the Shockley-Read-Hall recombination process which shortens the minority carrier lifetime. The microscopic origin responsible for the short carrier lifetime is still unknown, which is a major impediment for the further improvements of SLS technologies. Thus, this thesis is motivated by the need of microscopic understanding of structure and defects in T2SLs. A SLS is typically comprised of alternating lattice-mismatched layers, with thicknesses of the constituent layers on a few nanometer scale. Structural analysis at high spatial resolution is therefore mandatory in order to develop a detailed understanding of SLS structures. Here, scanning transmission electron microscopy (STEM) is used as a major tool for structural characterization of SLSs. In a STEM with a high angle annular dark field (HAADF) detector, the recorded real space image can be readily interpreted based on the peak intensities and peak positions as the intensity scales with the atomic density and atomic numbers to a good approximation. In particular, a probe forming spherical aberration corrector improves the spatial resolution of STEM to 1 Å or better, thus providing the resolving power for imaging individual atomic columns. Here, HAADF-STEM, together with the advancement in image processing methods, are employed for quantitative structural analysis of InAs/GaSb and InAs/InAsSb T2SLs. Among the results obtained by this research, first of all, a newly developed pattern recognition method revealed asymmetric interfacial sharpness and chemical intermixing in InAs/GaSb T2SL. A correlative study with atom probe tomography (APT) demonstrates segregation of Ga, In, and Sb, which are crucial information for the optimization of interface design. Next, a combination of two dimensional Gaussian peak fitting and template matching based image processing is used to accurately determine atomic column positions in the recorded HAADF-STEM images and use this information to obtain two dimensional strain map with a measurement precision at picometer scale. Atomic resolution strain map provides a variety of structural information, such as relationship to composition (including interface chemistry and atomic segregation), and strain uniformity. Using this approach, the effects of interfacial engineering in the InAs/GaSb T2SL are studied. The results demonstrate that interface engineering improves the strain uniformity for both InAs and GaSb and reduces Ga and As concentrations at interfaces, which was further supported by chemical profiles obtained with electron energy loss spectroscopy (EELS). Furthermore, the ability to quantify atomic column position at picometer precision has led to the investigation of point defects and their detection. The first principle study performed in this thesis demonstrates that vacancy defects induce >10 pm displacement of atomic columns in simulated STEM images while anti-site defects induce only a few pm displacements. Applying statistics analysis, both cation and anion vacancies as well as their locations and strain values within the InAs/GaSb T2SL, are identified from the experimental atomic resolution strain maps. Lastly, strain mapping was also performed for the InAs/InAsSb T2SL. Recently, InAs/InAsSb, or Ga-free, T2SLs have gained significant interests since such devices show longer carrier lifetime and lower dark current level compared to the InAs/GaSb T2SL. Since the constituent layers share common cation (In), the measured strain directly relates to anion variations, which allows for obtaining spatially resolved anion distribution across T2SL.
Issue Date:2015-07-17
Rights Information:Copyright 2015 Honggyu Kim
Date Available in IDEALS:2015-09-29
Date Deposited:August 201

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