Optical transmittance enhancement and bandgap widening of ZnO:Al powders by W codoping
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Optical transmittance enhancement and bandgap widening of ZnO:Al powders by W codoping H.-Y. He • Z. He • Q. Shen • J. Lu
Received: 14 May 2012 / Accepted: 15 July 2012 / Published online: 3 August 2012 Ó Springer Science+Business Media, LLC 2012
Abstract (Al, W)-codoped ZnO powders with a constant molar ratio of Zn:Al (99:1) and various molar ratios of W:Al (0–0.02) were synthesized by a sol–gel process and characterized by X-ray diffraction, field emission scanning electron microscopy, and UV–vis and luminescent spectrophotometry. The experiments indicated that the powders illustrated increased c-orientation and a decreased average particle size with increasing W content. The W doping also resulted in the enhancement of transmittance in the UV–vis light range and the widening of the bandgap. Optimal efficiency appeared at molar ratio of W:Al = 0.01. Moreover, the W doping led to the formation of a new emission center and the enhancement of the emissions in range of visible light.
Introduction Zinc oxide (ZnO) is a wide-bandgap II–VI semiconductors (3.2–3.37 eV) with very attractive properties, including high transparency in the visible wavelength, a high piezoelectric constant, a large electro-optic coefficient [1], and a large exciton binding energy (*60 meV) at room temperature. Many studies have recently focused on ZnO materials because of their potential application in solar cells, gas sensors, piezoelectric transducers, and varistors [2–7]. Some dopants, such as Al, Si, In, and Ga, often can improve the electrical and optical properties of ZnO films
H.-Y. He (&) Z. He Q. Shen J. Lu Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science and Technology, Xi’an 710021, China e-mail: [email protected]
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[8–10]. Al-doped ZnO (AZO), especially, has shown good electrical and optical properties, and so became an alternative material to tin oxide and indium tin oxide (ITO) which are commonly used as transparent conducting oxides (TCOs). Due to their unique combination of interesting piezoelectric, electrical, and optical properties, AZO nanomaterials also possess great promise for multifunctional applications. Recently, widening in the bandgap of ZnO produced by Ti, Ca, Si, Sc, and Mo dopants has been reported in literature [11–16]. This widening in the bandgap is generally attributed to the known Moss–Burstein shift resulted from the various dopings including Al doping. Lin et al. [17, 18] investigated (Al, Sc)-codoped ZnO film based on similar electrical and optical properties of Sc-doped ZnO film with AZO film [14]. Results indicated that (Al, Sc)-codoped ZnO film shown better electric conductivity, higher transmittance, and wider bandgap than the AZO films in an appropriate amount of Sc proportion. Similar results of (Al, Sc)-codoped ZnO film was also reported by Chen et al. ˚ ) close to Zn2? [19]. W6? has an ion radius (0.68 A ˚ ). Furthermore, the incorporation of the W6? ion (0.74 A into the ZnO lattice can increases carrier conc
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