Optical Materials 100 (2020) 109709 Available online 24 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved. Study of structural and optical properties of MBE grown nonpolar (10-10) ZnO/ZnMgO photonic structures M. Stachowicz a, * , J.M. Sajkowski a , S. Kryvyi a , A. Pieniazek a , A. Reszka a , A. Wierzbicka a , M. A. Pietrzyk a , E. Przezdziecka a , D. Jarosz a , K. Gw� o� zd� z b , E. Placzek-Popko b , E. Alves c , S. Magalh~ aes c , A. Kozanecki a a Institute of Physics of the Polish Academy of Sciences, Al. Lotnik� ow 32/46, PL-02-668, Warsaw, Poland b Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370, Wroclaw, Poland c IPFN, Instituto Superior T� ecnico, Universidade de Lisboa, 2695-066, Bobadela, Portugal A R T I C L E INFO Keywords: MBE technology ZnO ZnO/ZnMgO photonic structures SEM-CL mapping X-ray diffraction RSM mapping Distributed bragg refectors ABSTRACT Two types of simple monolithic photonic ZnO/ZnMgO structures, differing by a factor of two in thicknesses of individual components and grown on m-ZnO substrates, are tested to determine limitations in obtaining their good quality at Mg concentrations close to the limit for phase segregation (~40–45%). The studied structures were composed of 5.5 pairs of ZnO/ZnMgO bilayers (20/22 and 40/45 nm), the central ZnO microcavity (70 and 140 nm) and 5 bilayers on top. Actual layer thicknesses were verifed using Scanning Electron Microscopy. Rutherford Backscattering Spectrometry was used to determine the Mg content. The X-ray rocking curves revealed high periodicity and proved that the wurtzite structure was retained without precipitates of foreign phases. Reciprocal space maps and the calculated lattice constants indicated that the layers were strained. Excitonic lines in cross-sectional cathodoluminescence (CL) spectra from individual ZnO layers are blue-shifted compared to emissions from ZnO substrates, thus confrming the presence of strain. The CL intensity of the frst ZnMgO layers deposited on ZnO substrates and on both interfaces of the central microcavities was low thus suggesting an effective generation of misft defects, especially in thicker structures. Refectivity measurements confrmed the existence of cavity resonance at 385 nm and stop band at 370–400 nm in the photonic structure with a 140 nm microcavity. The results show that defect engineering at the initial stages of deposition of ZnMgO layers with high Mg is critical for the optimization of all-ZnO photonic structures. 1. Introduction Zinc oxide is one of the most intensively studied semiconductors due to its potential importance for optical and optoelectronic devices [1] including solar blind [2], wavelength selective UV detectors [3], trans- parent electronics [4], and many others. ZnO has a direct 3.37 eV band gap at room temperature [5] comparable to that of GaN and a large free exciton binding energy of 60 meV that can increase up to 100 meV in ZnMgO/ZnO/ZnMgO quantum wells (QWs) [6]. Alloying of ZnO with MgO allows tailoring energy gaps between ~3.37 eV (ZnO) up to 7.5 eV for cubic MgO, which means that barrier heights in ZnO/ZnMgO quantum structures can be greater than 2 eV. As a result, exciton luminescence can be observed above room temperature. It is also important that the technology of growth of bulk ZnO crystals is mature, so ZnO substrates with different orientations are easily available. Epitaxy on bulk crystalline ZnO substrates reduces lattice mismatch between the substrate and ZnMgO layers. This will lead to better crys- tallographic quality of heterostructures and better performance of de- vices than in the case of epitaxy on sapphire substrates. The excellent optical properties of ZnO have stimulated intensive research on light-emitting diodes [7,8], optically pumped lasers [9,10], and ZnO-based quantum structures [11,12]. Polariton lasers are a new feld of generation of coherent emission in ZnO-based heterostructures. Polariton lasing and exciton-polariton condensates were successfully generated in ZnO microcavities by a few groups of researchers [13–17]. However, to obtain effcient laser emission it is necessary to optimize the crystal quality of complex structures such as Distributed Bragg Re- fectors (DBRs) and minimize the disorder caused primarily by the * Corresponding author. E-mail address: stachow@ifpan.edu.pl (M. Stachowicz). Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2020.109709 Received 22 November 2019; Received in revised form 16 January 2020; Accepted 18 January 2020