On the Crystal Structural Control of Sputtered TiO2 Thin Films.

Junjun Jia,Toshihiro Okajima,Yuzo Shigesato,Haruka Yamamoto

doi:10.1186/s11671-016-1531-5

Junjun Jia, Toshihiro Okajima + Show 2 more

Open Access

https://doi.org/10.1186/s11671-016-1531-5

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Abstract

In this study, we focused on the origin on the selective deposition of rutile and anatase TiO2 thin films during the sputtering process. The observation on microstructural evolution of the TiO2 films by transmission electron microscopy revealed the coexistence of rutile and anatase TiO2 phases in the initial stage under the preferential growth conditions for the anatase TiO2; the observations further revealed that the anatase phase gradually dominated the crystal structure with increasing film thickness. These results suggest that the bombardment during the sputtering deposition did not obviously affect the TiO2 crystal structure, and this was also confirmed by off-axis magnetron sputtering experiments. We also investigated the mechanism of the effect of Sn impurity doping on the crystal structure using first-principles calculations. It is found that the formation energy of Sn-doped rutile TiO2 is lower than that of Sn-doped anatase TiO2; this suggests that the Sn-doped TiO2 favours the rutile phase. These results offer a guideline for the utilization of selective deposition of rutile and anatase TiO2 thin films in various industrial applications.

Highlights

Rutile and anatase TiO2 films are widely used in various industrial applications [1–3]
The sketch images demonstrate that many small crystallites were formed on the substrate in the initial growth stage and that these crystallites gradually decreased in number and increased in grain size with increasing film thickness; we attributed this behaviour to the growth competition among the crystallites [24]
The diffraction patterns indicate that the anatase and rutile phases coexist in the bottom region, whereas the anatase phase dominates the crystal structure in the surface region

Summary

Introduction

Rutile and anatase TiO2 films are widely used in various industrial applications [1–3]. Rutile TiO2 films are used as an optical coating material because of their high refractive index whereas anatase TiO2 films are utilised as photocatalysts or transparent electrodes [3]. Rutile TiO2 is the most common phase in nature, and anatase TiO2 transforms to rutile at temperatures above 400–600 °C [4]. Conventional wet processes such as the sol–gel method can be used to produce pure-phase TiO2 films; fabricating dense TiO2 films is difficult by this method [5]. TiO2 films deposited by magnetron sputtering are often a mixture of anatase and rutile phases. As a practical measure, controlling the phase content of TiO2 films is necessary for films used in precise optical applications.

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