Study the target effect on the structural, surface and optical properties of TiO2 thin film fabricated by RF sputtering method

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Highlights

  • TiO2 films deposited by RF sputtering.

  • Ti or TiO2 target used.

  • Ti target yields films with smaller crystallites and better transparency.

Abstract

The effect of target (Ti metal target and TiO2 target) on Titanium Dioxide (TiO2) thin films grown on ITO coated glass substrate by RF magnetron sputtering has been investigated. A comparative study of both the films was done in respect of crystalline structure, surface morphology and optical properties by using X-ray diffractometer (XRD), Atomic Force Microscopy (AFM) studies and ellipsometric measurements. The XRD results confirmed the crystalline structure and indicated that the deposited films have the intensities of anatase phase. The surface morphology and roughness values indicated that the film using Ti metal target has a smoother surface and densely packed with grains as compared to films obtained using TiO2 target. A high transmission in the visible region, and direct band gap of 3.67 eV and 3.75 eV for films derived by using Ti metal and TiO2 target respectively and indirect bandgap of 3.39 eV for the films derived from both the targets (Ti metal and TiO2 target) were observed by the ellipsometric measurements.

Introduction

A significant research interest in the synthesis of semiconducting TiO2 thin film as it is an important semiconductor in II–VI transition metal oxide family has been witnessed in the recent part. TiO2 has many important features such as a wide direct bandgap, high refractive index/dielectric constant, excellent optical transmittance in the visible range and a fairly good thermal and chemical stability [1], [2], [3], [4], [5], [6]. In view of its high transparency and chemical stability the material is attractive for use as optical coating protective layer [7], [8]. Its excellent insulating properties and high dielectric constant can make it a prospective dielectric material in future generation integrated circuits [9], [10]. TiO2 has been deployed for developing sensors for hazardous gases like H2 and NO [11], [12], humidity and ethanol and different photocatalytic devices [13], [14], [15], [16]. Ti interstitials and/ or oxygen vacancies in TiO2 thin film are responsible for n-type characteristics behavior of developed TiO2 film. TiO2 has been investigated in the recent part for possible uses as active channel layer in thin film transistor [17], [18].

TiO2 films generally grow in three main crystal phases, rutile, brookite and anatase. Generally rutile phase is stable at all temperature, and it is relatively easy to grow. On the other hand, it is difficult to grow anatase phase. However, the low resistance and high response time makes anatase TiO2 suitable for gas sensing applications. Most importantly it can be used as a photocatalyst to degrade organic compounds and hence can be used as an organic pollutant remover [19].

TiO2 thin film can be deposited by number of methods including Sol–Gel [20], Chemical Vapor Deposition (CVD) [21], Pulse Laser Deposition (PLD) [22], Low Temperature Arc Vapor Deposition [23], anodizing Ti metal [24], [25], and RF-sputtering. RF-Sputtering method has many advantages over other methods. By this method, thin films can be uniformly deposited with controlled thickness over a large area with highly crystalline nature. The composition of the deposited film is very close to that of the target used.

The characteristic of nano devices depend mainly on interface and surface properties which are largely governed by the deposition process and its ambiance, post-deposition annealing, annealing ambience and nature of the substrate used. In this paper, we have reported the fabrication and characterization of TiO2 thin films grown on ITO coated glass substrate by using RF-sputtering method. TiO2 thin films have been deposited by two different processes using Ti metal target and TiO2 target respectively. The structural and optical properties of films grown by the above methods have been studied. The results obtained in the two cases are compared and contrasted.

Section snippets

Preparation of polycrystalline TiO2 thin films

TiO2 thin films were deposited on ITO coated glass substrate with size of 50 mm × 25 mm × 1 mm using RF magnetron sputtering. Before deposition ITO substrate were ultrasonically cleaned with acetone, DI water and isopropyl alcohol sequentially for 15 min. Then the cleaned substrates were statically placed on a sample holder facing the target at a fixed distance of 50 mm in a vacuum chamber. An RF power supply operating at 13.56-MHz with 300 W power was used to excite the plasma. Ti metal target (99.99%

XRD analysis

The parameters obtained from the analysis of XRD spectra are listed in Table 1. For both the targets the film has anatase phase and orientations along (1 0 1) and (0 0 4) plane. For films grown by using Ti metal target, there is a high intensity peak at 25.26° and a relatively low intensity peak at 37.84°. On the other hand, the films by using TiO2 target exhibit high intensity peak at 25.32° followed by a low-intensity peak at 37.44° which corresponds to the orientations along (1 0 1) and (0 0 4)

Conclusions

TiO2 thin films were deposited on ITO coated glass with the help of RF sputtering method by using two separate approaches using Ti metal target and TiO2 target respectively. The deposited films exhibit anatase phase. The film grown with Ti metal target has smaller crystallites, larger FWHM and larger strain as compared to these exhibited by the films with TiO2 target. The surface morphology study by AFM shows that the film grown with Ti metal target has smaller grains, larger surface density of

Acknowledgements

The Authors gratefully acknowledge the facilities provided by the Centre for Interdisciplinary Research (CIR), Motilal Nehru National Institute of Technology Allahabad, Allahabad-211004.

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