Thin Films of Titanium Dioxide Prepared by Chemical Routes using Novel Precursors
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Thin Films of Titanium Dioxide Prepared by Chemical Routes using Novel Precursors K. Shalini1, S. Chandrasekaran2, and S.A. Shivashankar1 1 Materials Research Centre, 2Department of Organic Chemistry, Indian Institute of Science, Bangalore – 560012, India. ABSTRACT Novel, volatile, stable, oxo-β-ketoesterate complexes of titanium, whose synthesis requires only an inert atmosphere, as opposed to a glove box, have been developed. Using one of the complexes as the precursor, thin films of TiO2 have been deposited on glass substrates by metalorganic chemical vapor deposition (MOCVD) at temperatures ranging from 400°C to 525°C and characterized by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. All the films grown in this temperature range are very smooth; those grown above 480°C consist of nearly monodisperse, nanocrystals of the anatase phase. Optical studies show the bandgaps in the range 3.4-3.7 eV for films grown at different temperatures. Thin films of anatase TiO2 have also been grown by spin-coating technique using another ketoesterate complex of titanium, demonstrating that the newly developed complexes can be successfully used for thin film growth by various chemical routes. INTRODUCTION Thin films of anatase TiO2 have been investigated extensively in recent years for application in photovoltaics and as catalytic surfaces [1,2]. Titanium is also an inevitable component of technologically important materials such as BaTiO3, SrTiO3, [Pb(Zr,Ti)O3] (PZT), and [(Pb, La)(Zr,Ti)O3] (PLZT) [3,4]. An important part of thin film technology is the development of methods of thin film deposition that operate at reduced temperatures, yet able to produce films of complex oxides with compositional and phase uniformity over large substrate areas. Chemical methods such as chemical vapor deposition (CVD) and sol-gel processing are therefore being investigated extensively for the purpose. These methods call for the availability of chemical precursors that are safe, stable, and reactive at sufficiently low temperatures. The CVD of thin films of TiO2 has most often been carried out using its volatile tetrachloride, TiCl4 [5] or a titanium alkoxide, Ti(OR)4 [6] as the precursor. TiCl4 is air-sensitive and can lead to chlorine contamination of the films, and requires relatively high deposition temperatures. Alkoxide precursors such as Ti(OR)4, R= Et, Me, i-Pr, t-Bu, may be used for the deposition of TiO2 films at lower substrate temperatures, but are highly reactive to air and moisture. Alternatively, heteroleptic complexes Ti(OR)2(L-L)2 [L-L is a β-diketone) and βketoiminate complexes, which are resistant to hydrolysis have been investigated as precursors in MOCVD processes [7-10]. But these precursors require stringent conditions of synthesis. The relatively easy method of synthesis and the suitability of the oxo-β-diketonate complex for the deposition of thin films of Ti-containing oxides [11], and the recent success [12] in obtaining high quality films using the vanadyl diketon
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