The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem

•We have studied the atomic structure of polar and non-polar InGaN quantum wells.•The non-polar (11-20) InGaN quantum wells contain indium-rich clusters, unlike the polar (0001) quantum wells.•The electrons and holes in the quantum wells are localised by different mechanisms. We have used high resol...

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Published in:Ultramicroscopy 2017-05, Vol.176, p.93-98
Main Authors: Humphreys, C.J., Griffiths, J.T., Tang, F., Oehler, F., Findlay, S.D., Zheng, C., Etheridge, J., Martin, T.L., Bagot, P.A.J., Moody, M.P., Sutherland, D., Dawson, P., Schulz, S., Zhang, S., Fu, W.Y., Zhu, T., Kappers, M.J., Oliver, R.A.
Format: Article
Language:English
Subjects:
Aberration-corrected electron microscopy
Atomic structure
Gallium nitride
Quantitative STEM
Quantum wells
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Summary:•We have studied the atomic structure of polar and non-polar InGaN quantum wells.•The non-polar (11-20) InGaN quantum wells contain indium-rich clusters, unlike the polar (0001) quantum wells.•The electrons and holes in the quantum wells are localised by different mechanisms. 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.
ISSN:0304-3991
1879-2723
DOI:10.1016/j.ultramic.2017.01.019