title: The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem.
abstract: We have used high resolution transmission electron microscopy (HRTEM), aberration-corrected quantitative scanning transmission electron microscopy (Q-STEM), atom probe tomography (APT) and X-ray diffraction (XRD) to study the atomic structure of (0001) polar and (11-20) non-polar InGaN quantum wells (QWs). This paper provides an overview of the results. Polar (0001) InGaN in QWs is a random alloy, with In replacing Ga randomly. The InGaN QWs have atomic height interface steps, resulting in QW width fluctuations. The electrons are localised at the top QW interface by the built-in electric field and the well-width fluctuations, with a localisation energy of typically 20meV. The holes are localised near the bottom QW interface, by indium fluctuations in the random alloy, with a localisation energy of typically 60meV. On the other hand, the non-polar (11-20) InGaN QWs contain nanometre-scale indium-rich clusters which we suggest localise the carriers and produce longer wavelength (lower energy) emission than from random alloy non-polar InGaN QWs of the same average composition. The reason for the indium-rich clusters in non-polar (11-20) InGaN QWs is not yet clear, but may be connected to the lower QW growth temperature for the (11-20) InGaN QWs compared to the (0001) polar InGaN QWs.
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: colin.humphreys@msm.cam.ac.uk.
authors.forename: C J
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authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: jg641@cam.ac.uk.
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authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: ft274@cam.ac.uk.
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authors.affiliation: CNRS/C2N, Paris Sud University, Route de Nozay, 91460 Marcoussis, France. Electronic address: fabrice.oehler@lpn.cnrs.fr.
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authors.lastname: Oehler
authors.affiliation: School of Physics and Astronomy, Monash University, Victoria 3800, Australia. Electronic address: scott.findlay@monash.edu.
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authors.affiliation: Monash Centre for Electron Microscopy, Monash University, Victoria 3800, Australia. Electronic address: changlin.zheng@monash.edu.
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authors.affiliation: Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Electronic address: tomas.martin@materials.ox.ac.uk.
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authors.affiliation: Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Electronic address: paul.bagot@materials.ox.ac.uk.
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authors.affiliation: Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Electronic address: michael.moody@materials.ox.ac.uk.
authors.forename: M P
authors.initials: MP
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authors.affiliation: School of Physics and Astronomy, Photon Science Institute, University of Manchester, Manchester M13 9PL, UK. Electronic address: danny.sutherland@manchester.ac.uk.
authors.forename: D
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authors.affiliation: School of Physics and Astronomy, Photon Science Institute, University of Manchester, Manchester M13 9PL, UK. Electronic address: philip.dawson@manchester.ac.uk.
authors.forename: P
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authors.affiliation: Tyndall National Institute, Lee Maltings Complex, Dyke Parade, Cork, Ireland. Electronic address: stefan.schulz@tyndall.ie.
authors.forename: S
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authors.lastname: Schulz
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: siyuan.zhang@mpie.de.
authors.forename: S
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authors.lastname: Zhang
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: wyfu@hku.hk.
authors.forename: W Y
authors.initials: WY
authors.lastname: Fu
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK.
authors.forename: T
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authors.lastname: Zhu
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: mjk30@cam.ac.uk.
authors.forename: M J
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authors.lastname: Kappers
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Electronic address: rao28@cam.ac.uk.
authors.forename: R A
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keywords: Aberration-corrected electron microscopy; Atomic structure; Gallium nitride; Quantitative STEM; Quantum wells