Abstract
Tungsten oxide WO3-x is a transition metal oxide and a wide bandgap semiconductor, with a wide range of possible optical and photonic applications. In dependence on the fabrication techniques different stoichiometric ratios (x) and crystalline phases are obtained, which end up with an overall polymorph and extremely versatile material, characterized by tailorable dielectric properties. In particular, WO3-x thin film deposition by Radio-Frequency (RF) sputtering techniques provides a precise control of thickness, composition and nanostructure. In this work we introduce and discuss a specific process of deposition, that is magnetron RF-sputtering as a suitable way to grow WO3-x thin films with selected properties. Possibility of integrating WO3-x thin film on to one-dimensional (1D) photonic crystal structures is also explored. Films are transparent in the near and short-wavelength infrared optical spectral range. Their quality is assessed by morphological, structural and compositional characterizations. Dielectric properties are characterized by optical spectroscopy and ellipsometry, the latter also evaluates the degree of optical anisotropy of thin films in their crystalline phase. An 1D photonics bandgap structure is designed, formed by a SiO2–TiO2 multilayer and capped with a 450 nm-thick transparent WO3-x film, so that surface confinement and local enhancement of the optical field at 1416 nm in the topmost WO3-x layer is obtained.
Highlights
The research interest for tungsten oxide (WO3) as a wide-bandgap semiconductor has increased over the latest decades, prompted by its extraordinarily broad range of applications [1]
Just to list some among the main applications of WO3-x, crystalline nanorods [9] and nanowires behave as electrical field emitters [10], compact and smooth optical thin films are used in smart windows [11,12,13,14,15,16] and UV optical detectors [17,18,19] thanks to their chromogenic qualities, nano-porous films are exploited in sensors [20,21,22,23,24,25] and photo-electro-catalytic devices [26,27,28,29]
Ra values measured by Atomic force microscopy (AFM) for sam ples annealed at 400 ◦C are found to be higher than those of samples annealed at 300 ◦C and as-deposited films, with particular regard to S3 when compared to S1 and S2
Summary
The research interest for tungsten oxide (WO3) as a wide-bandgap semiconductor has increased over the latest decades, prompted by its extraordinarily broad range of applications [1]. Annealing in air provides an effective and direct means of adjusting the compositional ratio, manipulating the structure and dielectric properties of the material, whilst keeping the optical grade. We have correlated deposition and annealing of the films to a compre hensive characterization of their morphology, structure, chemical configuration and dielectric properties. In their as-deposited form, RF-sputtered thin films are sub-stoichiometric and optically opaque in the visible and near-IR range. In present work, starting from the results provided by the spectroscopic characterization of dielectric function in WO3-x films, we have designed a resonant structure capable to effectively confine incident light in WO3-x layer at surface
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