Full Length Article Optical investigations of ZnO/ZnMgO quantum wells in self-assembled ZnMgO nanocolumns grown on Si (111) by MBE M.A. Pietrzyk, M. Stachowicz, A. Reszka, A. Kozanecki Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46 PL-02668, Warsaw, Poland article info Article history: Received 8 April 2016 Received in revised form 29 June 2016 Accepted 3 August 2016 Keywords: Zinc oxide Photoluminescence Nanomaterials abstract Epitaxial growth of semiconductor nanocolumns has attracted large interest in recent years as means to fabricate nano-sized devices. Nanostructures grown on Si (111) substrates are very useful for applications in optoelectronics and integrated optoelectronic circuits. We present the catalyst-free ZnMgO nano- columns growth under high temperature conditions with asymmetric ZnO double quantum wells separated by ZnMgO barriers of different thickness (3 nm and 100 nm) deposited on Si (111) substrate using a molecular beam epitaxy method. The high quality quantum structures based on ZnMgO nano- columns were characterized using scanning electron microscopy and optical spectroscopy (photo- and cathodoluminescence). Their luminescent characteristics were investigated using different techniques. Study of excitons in ZnMgO/ZnO/ZnMgO quantum wells is thus important to understand the optical properties of these semiconductor structures. & 2016 Elsevier B.V. All rights reserved. 1. Introduction In last two decades extensive research efforts have been put into wide band gap semiconductors due to the large demand for optoelectronic devices covering the spectral range from ultraviolet to green. Zinc oxide (ZnO) is drawing special attention for its unique and very promising physical properties. High electron mobility, high thermal conductivity and high exciton binding energy at room temperature (60 meV at room temperature, com- pared to 25 meV for GaN [1]) are just the most important prop- erties that can be utilized in optoelectronics, photovoltaic, piezo- electric devices, transparent and spin electronics and in chemical sensor applications [2–5]. Apart from its wide bandgap (3.3 eV), ZnO is characterized by its high optical transmission in the visible spectrum [6], as well as by easily controllable electrical parameters (a wide range of electron concentrations and mobility) depending on the deposition conditions. ZnO-based QWs structures have attracted much interest due to the possible modulation of the band gap by using ZnMgO as a barrier layer. ZnMgO alloy maintains all favorable characteristics of wide band gap semiconductors, including transparency in the visible region and the ability to undergo phase evolution from hexagonal structure to NaCl-type cubic structure with the increase of Mg fraction because of the cubic structure of MgO crystal (a ¼ 0.424 nm). In addition, the ionic radii of Mg and Zn atoms are similar which results in a small lattice mismatch between the ZnMgO and ZnO lattice constants [7]. One dimensional (1-D) ZnO nanostructures such as nanotubes, nanowires, nanorods, nanobelts, and nanoribbons stimulate con- siderable interests for scientific research due to their importance in fundamental physics studies and their potential applications in nanoelectronics and flat panel displays [8–10]. In particular, application of 1-D ZnO nanostructures in optoelectronic devices became an important driving force for recent nanoscience research [11,12]. Also, 1-D ZnMgO nanocolumns with embedded ZnO quantum wells (or quantum dots) with atomically abrupt interfaces open up many new applications, in the fields of optoe- lectronics, photonics and sensor devices [13–15]. The high quality ZnMgO-based 1-D structures can be employed for the fabrication of high performance devices including field-effect transistors (FETs), Schottky diodes and light emitting devices [16,17]. The quantum size effects due to the low dimensional nanos- tructure may enhance a radiative recombination because of spatial confinement of carriers [18]. Studies of the optical properties of simple quantum structures such as two wells separated by a thin barrier may bring important information about inter-well inter- action, useful for more complex systems. It can be done by var- iation of the barrier thickness between the wells. The overlap between two QW – with built-in electric field, along c-direction- separated by penetrable barrier, leads to degeneration of single energy state into symmetric and antisymmetric doublet states in such a way, that split levels of each of the coupled electronic states Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2016.08.009 0022-2313/& 2016 Elsevier B.V. All rights reserved. E-mail address: pietrzyk@ifpan.edu.pl (M.A. Pietrzyk). Journal of Luminescence 179 (2016) 610–615