Dielectric Properties of SnO2 Thin Film Using SPR Technique for Gas Sensing Applications

Focus has been made on the determination of dielectric constant of thin dielectric layer (SnO2 thin film) using surface plasmon resonance (SPR) technique and exploiting it for the detection of NH3 gas. SnO2 thin film has been deposited by rf-sputtering technique on gold coated glass prism (BK-7) and its SPR response was measured in the Kretschmann configuration of attenuated total reflection using a p-polarised light beam at 633？nm wavelength. The SPR response of bilayer film was fitted with Fresnel’s equations in order to calculate the dielectric constant of SnO2 thin film. The air/SnO2/Au/prim system has been utilized for detecting varying concentration (500？ppm to 2000？ppm) of NH3 gas at room temperature using SPR technique. SPR curve shows significant shift in resonance angle from 44.8° to 56.7° on exposure of fixed concentration of NH3 gas (500？ppm to 2000？ppm) with very fast response and recovery speeds. 1. Introduction Surface Plasmon Resonance (SPR) is a powerful and sensitive technique to study the dielectric and optical properties of metal-dielectric interface, thus it can be exploited for the detection of harmful and toxic gases [1]. There are two configurations for exciting surface plasmon (SP) wave at the metal-dielectric interface: one is Otto configuration [2] and the other is Kretschmann [3] configuration. Kretschmann configuration is the most commonly used, where a light wave is totally reflected at the interface between the prism coupler and the thin metal layer (deposited on the prism surface) and excites a SP wave at the outer boundary of the metal by evanescently tunnelling through the thin metal layer [3]. The changes in the plasmon dispersion relations due to variation in refractive index of the sensing film will cause changes in the SPR reflectance curve [4]. The changes in the SPR curve are used to systematically calibrate the physical factors and hence the system can be exploited as a sensor. Transparent conducting (metal) oxide thin films have been the subject of research interest over a number of decades specially for gas detection. SnO2 based systems, in particular, have been the focus of many of these investigations worldwide because it is a naturally nonstoichiometric prototypical transparent semiconducting oxide having rutile structure [5]. It has a high band gap of ~4？eV and when suitably doped, can be used both as a p-type and n-type semiconductor [6]. Although a lot of work has been carried out on SnO2 based semiconductor gas sensors based conductometric detection technique. However, integration of SPR technique is yet