Effect of Cr incorporation on the structural and optoelectronic properties of TiO2:Cr deposited by means of a magnetron co-sputtering process

doi:10.1016/j.apsusc.2011.06.072

Applied Surface Science

Volume 257, Issue 24, 1 October 2011, Pages 10351-10357

https://doi.org/10.1016/j.apsusc.2011.06.072 Get rights and content

Abstract

In this work, we report on the effect of Cr incorporation on the microstructural and optical properties of TiO₂:Cr thin films deposited by the RF-magnetron sputtering method. The structural, morphological, chemical bonding and optoelectronic properties of the sputter-deposited TiO₂:Cr films were systematically investigated, as a function the incorporated Cr content, by means of various techniques including X-ray diffraction (XRD), atomic force microscopy (AFM), Fourier-Transform Infra-Red (FTIR) absorption, X-ray Photoelectron Spectroscopy (XPS) and ellipsometry. The Cr incorporation into the TiO₂ films was controlled by adjusting the RF power (P_Cr) on the Cr target during the co-sputtering process of TiO₂ and Cr. We were thus able to demonstrate that by varying P_Cr from 8 W to 150 W, the Cr content of the TiO₂:Cr films can be fairly controlled from ∼2 at.% to ∼18 at.% and their associated bandgap engineered from 3.3 eV to 1.5 eV. The room-temperature deposited TiO₂:Cr are mainly amorphous with the presence of some TiO₂ nanocrystallites, and their density increases as their Cr content is increased. The Cr inclusions were found to coexist under both metallic and oxidized forms in the films. By subjecting the TiO₂:Cr films to post-annealing treatment (at 550 °C), their crystalline structure was found to be sensitive to their Cr content. Indeed, an anatase-to-rutile phase transformation has been pointed out to occur at a Cr content of ∼7 at.%. Likewise, the Cr-content dependence of the bandgap of annealed TiO₂:Cr films undergoes a transition around the 7 at.% of Cr. Our results demonstrate the ability to control the Cr-content of TiO₂:Cr films, which leads to tune their optoelectronic properties, such as bandgap or optical absorption edge.

Highlights

► In this work, we report on the effect of Cr incorporation on the microstructural and optical properties of TiO₂:Cr thin films deposited by the RF-magnetron sputtering method. ► The structural, morphological, chemical bonding and optoelectronic properties of the sputter-deposited TiO₂:Cr films were systematically investigated, as a function the incorporated Cr content ► We were thus able to demonstrate that by varying P_Cr from 8 W to 150 W, the Cr content of the TiO₂:Cr films can be fairly controlled from ∼2 at.% to ∼18 at.% and their associated bandgap engineered from 3.3 eV to 1.5 eV.

Introduction

Titanium dioxide (TiO₂) is a large bandgap semiconductor with many interesting properties. It presents a high refractive index and a low absorption coefficient in visible light, which makes it an excellent optical coating material [1]. TiO₂ films have many other unique properties which make them highly attractive for a variety of applications such as photovoltaic devices [2], gas sensors [3], electrochromic displays and photocatalysts [4]. Optical gap of TiO₂ is 3.2 eV that belongs in the ultraviolet region of solar spectrum. However, the peak of solar spectrum is in the visible region. Many methods have been proposed to solve these problems, but doping TiO₂ with foreign ions, such as Fe, Cr and Ni, is one of the most promising strategies for sensitizing TiO₂ to visible light and also for forming charge traps to keep electron–hole pairs separate [4], [5]. In photovoltaic, the ability of tuning the bandgap energy (E_g) of the absorbing films is of prime importance in order to cover the maximum of the solar spectrum. Moreover, detailed knowledge of the optical properties (i.e., refractive index (n) and extinction coefficient (k)), of the absorbing passive (or active) oxide layer is a prerequisite for the design of highly efficient photovoltaic.

Titanium dioxide films have been prepared by various vacuum techniques such as chemical vapor deposition [6], pulsed laser deposition [7] and sputtering [8]. In this work, we report on the use of magnetron co-sputtering technique to achieve TiO₂:Cr films of which Cr content can be controlled at will. The structural and optoelectronic properties of the developed TiO₂:Cr films were systematically investigated as a function of their Cr content and following their subjection to post-deposition annealing treatment. The Cr incorporation is found to not only enable effective bandgap engineering of the developed TiO₂:Cr films, but also triggers their anatase-to-rutile phase transition.

Section snippets

Experiment

Pure TiO₂ and Cr-doped titania (TiO₂:Cr) thin films were deposited by means of RF-magnetron sputtering (13.56 MHz) on both Si and quartz substrates. The substrates were mounted on a holder and placed at a distance of ∼20 cm from the sputtering-target. The sputtering deposition chamber was first cryo-pumped to a base pressure of ∼10⁻⁸ Torr, and then filled with a mixture of high purity Ar and O₂ gases (to a ratio of 80/20) to reach a working pressure of ∼1.2 mTorr. Before deposition on substrates,

Results and discussion

Fig. 1a shows a typical XPS broad scan spectrum of the TiO₂:Cr films after subjecting their surface to Ar⁺ ion cleaning for 60 s. This spectrum shows the four major elements forming the films, namely Ti, O, and Cr (a very small C 1s peak is also present as a consequence of some C contamination of the air-exposed samples surface). From high-resolution XPS spectra (Fig. 1b) different bonding states of Cr atoms in the films can be derived after appropriate self-consisting deconvolution procedure.

Conclusion

The co-sputtering process has been successfully used to deposit TiO₂:Cr films while controlling their Cr content. By investigating the effect of Cr incorporation on the structural and optoelectronic properties of the TiO₂:Cr films, we have shown that their bandgap can be tuned at will over a range as wide as 3.3–1.5 eV. The annealing of TiO₂:Cr films were found to lead to their crystallization with a anatase-to-rutile phase transition occurring at a specific Cr content of 7 at.%. This structural

Acknowledgments

The authors (and particularly MAE) acknowledge financial support from both NSERC (the Natural Science and Engineering council) of Canada and from the FQRNT (le Fonds Québécois de la Recherche sur la Nature et les Technologies). The support form the Ministry of Higher Education, Scientific Research and Technology of Tunisia, is also acknowledged.

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Effect of Cr incorporation on the structural and optoelectronic properties of TiO2:Cr deposited by means of a magnetron co-sputtering process

Abstract

Highlights

Introduction

Section snippets

Experiment

Results and discussion

Conclusion

Acknowledgments

Solar Energy

Sens. Actuators B

Hydrogen Energy

Surf. Sci. Rep.

Thin Solid Films

Prog. Solid State Chem.

Plasma deposition of optical films and coatings: a review

J. Vac. Sci. Technol. A.

TiO2 based gate insulators

IBM J. Res. Dev.

Electrical and optical properties of TiO2 anatase thin films

J. Appl. Phys.

Effect of Cr incorporation on the structural and optoelectronic properties of TiO₂:Cr deposited by means of a magnetron co-sputtering process

TiO₂ based gate insulators

Electrical and optical properties of TiO₂ anatase thin films