Surface segregation of W doped in ZnO thin films T.T. Suzuki ⁎, Y. Adachi, N. Saito, M. Hashiguchi, I. Sakaguchi, N. Ohashi, S. Hishita National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan abstract article info Article history: Received 6 December 2013 Accepted 11 February 2014 Available online 24 February 2014 Keywords: Surface segregation Ion scattering spectroscopy Surface segregation Tungsten Zinc oxide We observed surface segregation of W (0.05–4 mol%) doped in ZnO films by annealing above 900 K. The segregation coefficient was related to the crystal quality of the film, where slower segregation occurs in higher-quality crystalline films. Using low-energy He + ion scattering spectroscopy for structure analysis, we found that the W–ZnO surface terminates with an O-layer, and W is located in a substitutional site of Zn at the second surface layer as a consequence of segregation. On the other hand, we observed no indica- tion that W occupies certain sites in the ZnO lattice at the subsurface. Ultraviolet photoelectron spectrosco- py (He I) on the W-segregated ZnO surface indicates that W is hexavalent at the Zn site. The segregation of the W atom is likely accompanied by two Zn vacancies. Ion beam mixing followed by annealing of the ZnO surface deposited with W provides a surface electronic structure similar to that of W-segregated ZnO. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Features of ZnO such as photonic and optoelectronic properties arising from the wide and direct band gap of 3.37 eV with a large ex- citon energy, chemical and thermal stability, transparent conductivity, and non-toxicity are suitable for practical applications for environment and energy issues [1]. These applications include solar cells, light- emitting diodes, sensors, and photocatalysis. For instance, ZnO nanobundles have higher photocatalytic efficiency as compared with TiO 2 for the destruction of several organic contaminants [2]. As a photocatalyst, ZnO is advantageous because of its high quantum efficiency [3–5]. However, the limited photoactivity of ZnO to ultra- violet light and its quick electron-hole recombination have hindered ZnO from wide adoption in industry. Significant efforts are in prog- ress to improve ZnO properties, typically by doping the materials with transition metals, introduction of oxygen vacancies, or by for- mation of coupled photocatalysts. Some studies have reported superior photocatalytic properties with ZnO coupled with WO 3 . This enables visible-light absorption for photooxidative degradation of organic pollutants [6–13]. For ex- ample, Changlin et al. reported almost double the photocatalytic ac- tivity with the WO 3 –ZnO composite compared to pure ZnO for degradation of acid orange II under ultraviolet light irradiation [13]. The results from the WO 3 –ZnO composite have stimulated many studies on the basic mechanism for the incorporation of W into ZnO substrates. Incorporation has been attempted by various methods such as pulsed laser deposition, sputtering, and the sol – gel method [14–23]. Some studies have pointed out enhanced photo- catalytic activity of W-doped ZnO compared with undoped ZnO and discussed the origin [23]. Aside from photocatalysis, W doping in ZnO and the consequently formed ZnO–WO 3 binary system are also important as a phosphor in vacuum fluorescent displays and field-emission displays [24]. Japanese Patent Application Laid-open No. Sho 58-40746 proposes W addition to ZnO:Zn as a light emitter of uniform luminance with improved tem- perature characteristics [25]. It is useful for increasing initial luminance and for decreasing drop of luminance with time. In practice, the addition of W in ZnO phosphors is widely realized in fluorescent display lamps or tubes using ZnO as a fluorescent material on their anodes. The uninten- tional deposition of W on the anode should affect the performance of their luminescence. However, the detailed mechanism of the effect of W addition in ZnO for fluorescence still remains unclear [14]. In addition to photocatalysis and fluorescence, transition-metal doped ZnO is a subject of focus for oxide-based-diluted magnetic semi- conductor research [1,26,27]. The system has been shown to exhibit room-temperature ferromagnetism which is a promising feature for possible applications in spintronics. Motivated by the above-mentioned various applications of W–ZnO, a number of studies have been reported on this system. However, the detailed characteristics such as atomic arrangement and electronic structure are still controversial. For example, Zhang et al. reported W doped in ZnO exists only in the oxidized state of W 6+ with Zn in a mix- ture of oxidized and metallic formations [21]. Can et al. suggest W 6+ at Zn sites with better insulating properties due to lower carrier concen- tration and higher resistivity [22]. These are consistent with first- principle density functional calculations by Singh et al., which show Surface Science 625 (2014) 1–6 ⁎ Corresponding author. E-mail address: suzuki.taku@nims.go.jp (T.T. Suzuki). http://dx.doi.org/10.1016/j.susc.2014.02.014 0039-6028/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc