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© 2016 Author(s). In this paper, we compare and contrast the experimental data and the theoretical predictions of the low temperature optical properties of polar and nonpolar InGaN/GaN quantum well structures. In both types of structure, the optical properties at low temperatures are governed by the effects of carrier localisation. In polar structures, the effect of the in-built electric field leads to electrons being mainly localised at well width fluctuations, whereas holes are localised at regions within the quantum wells, where the random In distribution leads to local minima in potential energy. This leads to a system of independently localised electrons and holes. In nonpolar quantum wells, the nature of the hole localisation is essentially the same as the polar case but the electrons are now coulombically bound to the holes forming localised excitons. These localisation mechanisms are compatible with the large photoluminescence linewidths of the polar and nonpolar quantum wells as well as the different time scales and form of the radiative recombination decay curves. ; This work was carried out with the support of the United Kingdom Engineering and Physical Sciences Research Council under grant Nos. EP\J001627\1, EP/I012591/1 and EP/H011676/1 and EP\J003603\1, Science Foundation Ireland (SFI) under project numbers 13/SIRG/2210 and 10/IN.1/I2994 and the European Union 7th Framework Programme project DEEPEN (grant agreement Number 604416). S. S. also acknowledges computing resources at Tyndall provided by SFI and the SFI and Higher Education Authority funded Irish Centre for High End Computing.

Abstract The use of pulsed mode scanning electron microscopy cathodoluminescence (CL) for both hyperspectral mapping and time-resolved measurements is found to be useful for the study of hybrid perovskite films, a class of ionic semiconductors that have been shown to be beam sensitive. A range of acquisition parameters is analysed, including beam current and beam mode (either continuous or pulsed operation), and their effect on the CL emission is discussed. Under optimized acquisition conditions, using a pulsed electron beam, the heterogeneity of the emission properties of hybrid perovskite films can be resolved via the acquisition of CL hyperspectral maps. These optimized parameters also enable the acquisition of time-resolved CL of polycrystalline films, showing significantly shorter lived charge carriers dynamics compared to the photoluminescence analogue, hinting at additional electron beam-specimen interactions to be further investigated. This work represents a promising step to investigate hybrid perovskite semiconductors at the nanoscale with CL. ; J.F.O and C.D. acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) Nano Doctoral Training Centre (EP/L015978/1). J.FO. and S.D.S. acknowledge the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (HYPERION, grant agreement no. 756962). E.M.T. thanks the ERC Horizon 2020 research and innovation programme (Marie Skłodowska-Curie, grant agreement no. 841265). S.D.S. and E.M.T. acknowledge funding from the EPSRC (EP/R023980/1), from the EPSRC Centre for Advanced Materials for Integrated Energy Systems (CAM-IES, EP/P007767/1), and the Cambridge Royce facilities grant (EP/P024947/1). CL studies were supported by the EPSRC (EP/R025193/1). Dr. Christian Monachon from Attolight is thanked for his ongoing support of the CL system. Yu-Hsien Chiang from the Stranks group is thanked for his support in the sample preparation.

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an $m$-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems. ; This work was supported by Science Foundation Ireland (project numbers 13/SIRG/2210 and 10/IN.1/I2994), the United Kingdom Engineering and Physical Sciences Research Council (grant agreement nos.: EP\J001627\1 and EP\J003603\1), the European Union 7th Framework Programme DEEPEN (grant agreement no.: 604416), and in part by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no 279361 (MACONS). We like to acknowledge the help of D. Hanley, University of Oxford, on the APT results. S. S. acknowledges computing resources provided by Science Foundation Ireland (SFI) to the Tyndall National Institute and by the SFI and Higher Education Authority Funded Irish Centre for High End Computing. ; This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.92.235419