Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin Transformations in the photoluminescent, electrical and structural properties of Tb 3+ and Eu 3+ co-doped ZnO films under high-temperature annealing N. Korsunska a , L. Borkovska a,* , L. Khomenkova a , O. Gudymenko a , V. Kladko a , O. Kolomys a , V. Strelchuk a , Z. Tsybrii a , C. Guillaume b , C. Labbe b , X. Portier b , O. Melnichuk c , L. Melnichuk c a V. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine, 45 Prospect Nauky, 03028, Kyiv, Ukraine b CIMAP, Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, 6 Blvd. Maréchal Juin, 14050, Caen, France c Mykola Gogol State University of Nizhyn, 2 Hrafska Str., Nizhyn, 16600, Ukraine ARTICLE INFO Keywords: ZnO films Rare-earth ions Luminescence Energy transfer ABSTRACT The effect of thermal annealing on optical, electrical and structural properties of Tb and Eu co-doped ZnO films grown by magnetron sputtering on Si and Al 2 O 3 substrates was investigated by X-ray diffraction, photo- luminescence, micro-Raman and IR reflection methods. It is shown that incorporation of rare earth ions in ZnO is accompanied by the formation of intrinsic defects. The as-deposited and annealed at 600 °C films demonstrate Tb 3+ emission and no Eu 3+ one. Higher intensity of Tb 3+ photoluminescence in the films on Al 2 O 3 substrate as compared with that on Si is ascribed to higher content of Tb 3+ emitting centers. The model of these centers including the substitutional Tb and interstitial oxygen is proposed. In the excitation spectra of Tb 3+ emission, no features connected with light absorption in ZnO are observed. An annealing at 900 °C is found to result in the formation of crystalline terbium oxide and silicate phases. In the photoluminescence spectra, the decrease of Tb 3+ emission and the appearance of two sets of Eu 3+ related bands caused by energy transfer from Tb 3+ to Eu 3+ ions are found. This is ascribed to segregation of rare earth ions in the additional phases and the decrease of the distance between the ions. 1. Introduction Zinc oxide (ZnO) is a promising material for semiconductor device applications due to its unique electrical, optical and piezoelectric properties [1, 2, 3]. Rare earth (RE)-doped ZnO exhibiting emission of specific color is receiving great attention for potential applications in light emitting diodes (LEDs), plasma displays and fluorescent lamps [4, 5, 6, 7]. In particular, incorporation of Tb 3+ ions into ZnO host allows producing the sources of green emission [4,5], while Eu 3+ ions in ZnO are used for obtaining the red one [6,7]. The specific emission of Tb and Eu ions embedded into ZnO can be obtained by using both resonant excitation of RE ions and the near-UV excitation. The latter is usually ascribed to light absorption in ZnO followed by energy transfer from the host to RE ions. However, different conclusions on the efficiency of photoluminescence (PL) of RE ions under such indirect excitation as well as on the mechanisms of energy transfer from ZnO to RE ions were considered. Usually [8, 9, 10], resonant photoexcitation of RE ions re- sults in larger PL intensities than excitation of the ZnO host. At the same time, in [11, 12] an indirect excitation of Eu-related emission was found to be more efficient than the direct one. It was assumed that one of the reasons of low efficiency of energy transfer from ZnO host to RE ions is the short lifetime of exciton in ZnO and/or non-radiative energy dis- sipation that depends on host crystallinity [13]. On the other hand, the enhancement of the intensity of RE ion luminescence under non-re- sonant excitation was supposed to be due to defect-mediated energy transfer from ZnO to RE ions. Several models of the energy transfer from ZnO host to RE ions were proposed. One of them supposes the defect-mediated process including levels of intrinsic radiative defects [14, 15, 16]. The second one as- sumes that the electron hole pairs produced by the above bandgap excitation of ZnO form free excitons which then recombine through the UV near-band-edge emission or transfer their energy to RE ions [17]. In another model, the generated free carriers are trapped by RE-related impurity state and form a bound exciton which, when recombined, excites 4f core levels through resonant or non-resonant Auger scattering [17, 18]. https://doi.org/10.1016/j.jlumin.2019.116739 Received 1 August 2019; Received in revised form 23 August 2019; Accepted 10 September 2019 * Corresponding author. E-mail address: bork@isp.kiev.ua (L. Borkovska). Journal of Luminescence 217 (2020) 116739 Available online 13 September 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved. T