Gas-phase diffusion and surface reaction as limiting mechanisms in the aerosol-assisted chemical vapor deposition of TiO
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tanium dioxide thin films were deposited on crystalline silicon (100) substrates by delivering a liquid aerosol of titanium-diisopropoxide. The evidence of a metalorganic chemical vapor deposition process observed in the crystalline and morphological features of the films is strongly supported by the behavior of the growth rate rg as a function of the deposition temperature. The rg line shape indicates that in a wide range of temperatures (∼180–400 °C), the film formation is limited by both gas-phase diffusion of some molecular species toward the substrate surface and the thermal reaction of those species on that surface. The activation energy EA that characterizes the surface reaction depends somewhat on the precursor concentration; a fitting procedure to an equation that takes into account both limiting mechanisms (gas-phase diffusion + surface reaction) yields EA ⯝ 27.6 kJ/mol.
I. INTRODUCTION
Recently, the ultrasonic spray pyrolysis (SP) technique has been explored for growth of TiO2 thin films by using titanium acetylacetonates or titanium diisopropoxide as precursor materials.1,2 The results indicate that SP is a suitable technique for producing high-quality TiO2 films. Generally speaking, it is well known that depending on the precursors and deposition conditions, the SP technique represents a deposition process similar to that obtained by chemical vapor deposition (CVD). Actually, sometimes the SP technique is classified as “aerosolassisted” chemical vapor deposition (AA-CVD) and the important point for distinguishing between SP and AACVD is the thermodynamic phase (liquid or vapor) in which the initial aerosol droplets, or spray, reach the substrate surface.1,3,4 For example, by using precursors of low boiling point, like the metalorganic compounds, an AA-CVD process can be promoted.5,6 In fact, this could be referred to as an “aerosol-assisted metalorganic chemical vapor deposition (AA-MOCVD)” process.7 The book by Kodas3 gives a good review of the possible behavior modes of the SP technique. The good surface and crystalline qualities of the TiO2 films reported in Refs. 1 and 2 indicate that in those cases SP behaves like an AA-MOCVD process. However,
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0391 J. Mater. Res., Vol. 21, No. 12, Dec 2006
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when the precursors are delivered in aerosol form, the best way to investigate the mechanisms that control the film formation is to study the behavior of the deposition rate as a function of temperature.3 Indeed, such study gives clues not only on the thermodynamic phases in which the precursors reach the substrate surface, but also on the physical and chemical phenomena occurring during a MOCVD process,8,9 namely, thermodynamics of gas phase reaction, mass transport, and reaction at the substrate surface. In the present work, by correlating the crystalline and morphology properties with the behavior of the deposition rate, studied at two different solutio
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