Optical and excitonic properties of ZnO films M. Mihailovic a, * , A.-L. Henneghien a , S. Faure b , P. Disseix a , J. Leymarie a , A. Vasson a , D.A. Buell c , F. Semond c , C. Morhain c , J. Zu `n ˜iga Pe ´rez c a LASMEA, UMR 6602 UBP/CNRS, 24 Avenue des Landais, 63177 Aubie `re Cedex, France b GES, UMR 5650 CNRS-Universite ´ Montpellier II, Place Euge ` ne Bataillon, 34095 Montpellier Cedex, France c CRHEA-CNRS, Rue Bernard Gre ´gory, 06560 Valbonne, France Received 15 July 2007; accepted 1 October 2007 Available online 14 February 2008 Abstract Optical and excitonic properties of ZnO heterostructures are studied in order to observe the strong light-matter coupling in this mate- rial as it has been recently demonstrated in GaN. The optical index of ZnO is first studied as a function of wavelength through spectro- scopic ellipsometric and reflectivity experiments on ZnO layers grown by molecular beam epitaxy and deposited on two different substrates: sapphire and silicon with an AlN buffer layer. The main features of the excitons: energy, oscillator strength and broadening are deduced. From the knowledge of these properties, a microcavity is then designed. The ZnO active layer is embedded between AlGaN/ AlN and dielectric Bragg mirrors. The calculation of the reflectivity spectra as a function of the incident angle attests the strong coupling and a large Rabi splitting of 110 meV is expected in such a cavity. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO) is a II–VI semiconductor that has already been used as a transparent conductive material and for piezoelectric applications [1,2]. With a wide direct band-gap of about 3.3 eV at room temperature, some opto- electronic applications of ZnO overlap that of GaN. Since excitons are stable at room temperature, ZnO and ZnMgO materials are very attractive for the realisation of optoelec- tronics devices based on cavity-coupled polariton lasers operated at room temperature [3]. Compared to (Al)GaN, Zn(Mg)O benefits from low cost substrates and higher oscillator strength of excitons, but there are only few studies of their optical properties and unfortunately large discrepancies among various results are reported [4,2]. In this work, the optical index of ZnO is first studied through spectroscopic ellipsometry and reflectivity experi- ments on ZnO layers. The main properties of the excitons: energy, oscillator strength and broadening are investigated. In a second step, a microcavity in which the strong light- matter coupling should be present is designed using the present determination of optical index and excitonic prop- erties of ZnO and taking advantage from the accurate knowledge of AlN and AlGaN based Bragg mirrors [5,6]. 2. Growth and experimental details The samples studied are grown by MBE on a (111) Si substrate with an AlN buffer layer (Z1 and Z2 samples) and on a sapphire substrate (Z3 sample). The nominal val- ues of thickness are listed in Table 1. The index of AlN which is transparent in the whole energy range investigated is deduced from previous studies and the complex index of Si together with the index of Al 2 O 3 are taken from the lit- erature [6]. Spectroscopic ellipsometry (SE) measurements were car- ried out at room temperature, at the incidence angle 72° with a rotating analyser ellipsometer [7]. 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.10.023 * Corresponding author. E-mail address: Martine.MIHAILOVIC@univ-bpclermont.fr (M. Mihailovic). www.elsevier.com/locate/optmat Available online at www.sciencedirect.com Optical Materials 31 (2009) 532–536