Preparation and microstructural characterization of nanosized Mo–TiO2 and Mo–W–O thin films by sputtering: tailoring of composition and porosity by thermal treatment

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Abstract

Nanosized Mo doped TiO2 and mixed W–Mo oxide thin films were deposited via RF-magnetron sputtering, in either reactive or inert atmosphere, to study the influence of the plasma gas on the film growth. Mixed targets (Ti (97 at.%)–Mo (3 at.%) and W (12 at.%)–Mo (88 at.%)) were used for the purpose. Thermal treatments were performed to promote structural and chemical changes. Experimental evidences highlighted the dependence of structural, morphological and compositional evolution of the films on the annealing temperature. In particular, coalescence inhibition to titania nanograins induced by Mo was observed in the Mo doped TiO2 films. The different volatility of WO3 and MoO3 in the W–Mo mixed oxides greatly affected the evolution of the film through annealing. This method proved useful to tailor the composition and porosity of the layers. Mo doped TiO2 films were tested as chemoresistive sensors towards ethanol; mixed W–Mo oxide were found to respond to CO.

Introduction

In the last years, a great interest has been addressed in the characterization of the structural and electrical properties of thin films for gas sensing [1], [2] and electrochromic devices [3], [4]. In fact the mechanisms related to the sensing properties and electrochromic qualities are strictly connected to the structure of the film. As an instance, in the metal-oxide-semiconductors (MOS), the gas sensitivity is related to the surface reactivity and to the total surface of the film exposed to the gas. Enhanced surface reactivity can be found in substoichiometric oxides, in which oxygen vacancies induce an n-type semiconducting behavior. A complete oxidation results in very high resistance of the films, with poor response as gas sensors. Thus, nanostructured substoichiometric oxides combine the request of a high surface-to-volume ratio and the semiconducting behavior suitable for technological applications. A stable electrical response is needed, since the continuative oxidation process tends to degrade the sensitivity of the layer.

In this paper, we have studied Mo doped TiO2 films and mixed W–Mo oxides deposited via RF-magnetron sputtering. Thermal treatments induced a Selective Sublimation Process (SSP) based on the preferential sublimation of one of the oxides constituting the film [5]. The evolution under annealing of the structural, morphological, electrical characteristics and the related gas sensing properties were investigated.

Section snippets

Experimental

Film deposition was performed by RF-magnetron sputtering from a Ti/Mo mixed target (97 at.% Ti and 3 at.% Mo) and from a Mo/W alloy target (88 at.% Mo and 12 at.% W). Ti/Mo films were grown in either inert Ar or reactive Ar (50%)/O2 (50%) atmosphere, while only reactive sputtering (Ar (50%)/O2 (50%)) was applied for W/Mo samples. During deposition, the substrate was always maintained at 300 °C. Thin monocrystalline 100-oriented silicon substrates were used for compositional and morphological

Mo doped TiO2 samples

The stoichiometric composition obtained from RBS analysis of the I- and R-series of the Mo doped TiO2 samples was reported in Table 1. As far as the films deposited in inert atmosphere are concerned, RBS signal of Ti (Fig. 1(a–c)) revealed the Ti oxidation promoted by annealing and the consequent thickening of the film. A complete oxidation was deduced for the I-800 sample, while annealing at 600 °C resulted in a partial oxidation. The samples grown in reactive atmosphere look as

Conclusions

Mo doped TiO2 and mixed W–Mo oxide thin films were deposited by the means of RF-magnetron sputtering. Annealing promoted changes of structural, compositional and electrical features of the layers. In the deposition of TiO2 layers, a metallic film was obtained with inert sputtering, while a reactive atmosphere resulted in a nearly amorphous Ti–Mo–O layer. Thermal treatment induced TiO2 formation and exaggerated coalescence of grains in the metallic layer, while coalescence inhibition resulted in

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