Journal of Alloys and Compounds 518 (2012) 96–100 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds j our na l ho me p ag e: www.elsevier.com/locate/jallcom Ellipsometric studies of optical properties of copper doped zinc oxide films on glass substrates A. Kostruba a,c,∗ , B. Kulyk b , B. Turko b a Lviv Institute for Physical Optics, Dragomanova Street, 23, UA-79005, Ukraine b Scientific-Technical and Educational Center of Low Temperature Studies, Physics Department, Ivan Franko National University of Lviv, Dragomanova Street, 50, UA-79005, Lviv, Ukraine c Lviv Academy of Commerce, Samtshuka Street, 9, UA-79005, Lviv, Ukraine a r t i c l e i n f o Article history: Received 7 November 2011 Received in revised form 24 December 2011 Accepted 27 December 2011 Available online 3 January 2012 Keywords: Copper-doped ZnO film RF-magnetron sputtering Ellipsometry Optical properties AFM a b s t r a c t Copper-doped ZnO films with thickness about 1200 nm were prepared by radio-frequency (RF)- magnetron sputtering on glass substrates. The influence of dopant on the optical properties was investigated using ellipsometry technique. It has been found that only three layer model of surface can be satisfactory fitted with experimental data of ellipsometric measurements obtained in wide range of incident angles. Presence of additional layers within the model can be explained by the interface rough- ness. The atomic force microscopy (AFM) analysis indicates that thickness of additional layers and value of interface roughness are commensurable. The optical parameters of copper-doped ZnO structures with different concentration of copper dopant were obtained using the fitting procedure within the three-layer model. Absence of copper clusters in the ZnO:Cu layer microstructure was confirmed using X-ray diffrac- tion and the effective media approximation (EMA) analysis. Refractive indices of doped ZnO:Cu layers (n = 2.002. . .2.04) are very close to the index of ZnO film (n = 1.98) at 632.8 nm. Extinction coefficient increase gradually to (k = 0.002) with the dopant concentration increasing. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO) is a wide-band-gap semiconductor and is also a piezoelectric material which has a reasonably large piezoelec- tric coefficient. Such unique property has been widely studied for its practical applications [1]. A rich family of ZnO nanostructures provides numerous opportunities for researchers to explore their properties and find potential applications. For semiconductors, doping is a powerful tool to tailor the elec- trical and optical properties, facilitating the construction of many electronic and optoelectronic devices. Cu doping modifies the pho- toluminescent transitions in ZnO by creating localized impurity levels [2,3]. Additionally, Cu behaves as an acceptor in ZnO with its energy level locating at 0.17 eV below the bottom of the con- duction band, making itself a good candidate for creating p-type ZnO [4]. More interestingly, previous studies, both theoretical and experimental, showed that ZnO doped with appropriate transition metals are diluted magnetic semiconductors [5] which attracted a lot of interest due to their potential applications in spintronics. Coexistence of the ZnO and Au nanoparticles gives a substantially ∗ Corresponding author at: Lviv Academy of Commerce, Samtshuka Street, 9, UA- 79005, Lviv, Ukraine. Tel.: +380 676038722. E-mail address: andriykostruba@yahoo.com (A. Kostruba). enhancement of photoinduced second harmonic generation [6] that can be explained by the contribution of the electron–phonon interactions [7]. Therefore, optical properties of ZnO thin film tailored by Cu dop- ing can be informative to predict the structural, electrical and other characteristics of material. Ellipsometry is a powerful and attractive technique that recently has found much favor in the non-destructive and in situ charac- terization of solids, thin surface films, structure of interfaces at nanometer scale [8]. Structural changes and optical constants mod- ification of solid induced by doping procedure can be estimated using ellipsometric method too [9]. 2. Materials and experiment Optical characteristics of ZnO:Cu thin films with different atomic amount of Cu dopant were explored using multiple angle of incidence (MAI) ellipsometry at single wavelength ( = 632.8 nm). The ZnO:Cu thin films were deposited on glass substrates by means of radio-frequency magnetron sputtering system using ZnO-powder tar- gets together with oxide CuO in argon-oxygen atmosphere at the gas pressure of 10 -3 Torr. In order to minimize the thermal stresses the substrate temperature was fixed at 300 ◦ C. The target-to-substrate distance was 60 mm and RF-power – 100 W. The atomic amount of Cu dopant in the film material (taken from energy dis- persive X-ray (EDX) analysis using a REMMA-102-02 scanning electron microscope, working at 20 kV dc acceleration voltages in vacuum) changed from 0.5% to 5%. The thickness of ZnO:Cu thin films was estimated by the deposition time and was about 1200 nm for all the specimens. 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.12.141