Parameter effect on the crystallization of Nd:yttrium aluminum garnet laser-ablated TiO 2 thin film
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Process parameter dependency on the phase transition from anatase to rutile phase of laser-ablated TiO2 films was investigated. Lower ambient argon pressure, longer deposition time, higher laser fluence, and smaller target–substrate separation give rutile phase from anatase phase at comparatively lower temperature. The relationship between thickness and onset temperature of anatase–rutile transformation can be comprehensively explained in terms of film thickness. Thinner films have higher phase transition temperature. The presence of helium gas during deposition favors the anatase–rutile transition at a temperature lower than that expected from the above relationship.
I. INTRODUCTION
The importance of TiO2 films is ever increasing due to their widespread application as materials for optical coatings and protective layers because of their high refractive index and high dielectric constant.1–3 TiO2 has three crystalline polymorphs: rutile, anatase, and brookite. Rutile is the most stable phase and forms at high temperature, while anatase is less stable and forms at lower temperature; brookite phase occurs only in extreme conditions and is rare. Depending upon the preparation methods and conditions, as-prepared TiO2 film can be amorphous4 or crystalline either in anatase5 or in rutile phase.6 The anatase-to-rutile phase transition is an irreversible process and generally occurs within a range of 500 to 1000 °C. Particle size,7 impurity content, and some additives can affect this transition temperature.8 Among various thin film deposition techniques known to date, pulsed laser deposition (PLD) is proving to be the most rapid and efficient technique for fabricating highquality film with controlled thickness and composition.9–11 It is generally accepted that the laser-ablated species have larger kinetic energies that make them electronically excited or even ionized, while thermally evaporated species are in equilibrium at ambient temperature. These energetic ablated species are impinging on a growing film with enhanced surface mobility and thereby improving the crystalline quality. Various research groups have reported the crystalline growth, microstructure and properties of laser-ablated TiO2 films.8–12 Most of the investigation is concentrated on the substrate heating and ambient pressure effect on a)
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J. Mater. Res., Vol. 16, No. 11, Nov 2001
structural change. Nevertheless, there are only few reports on the effect of all parameters involved in the laser ablation process. In this report, we will try to throw some light on the influence of process parameters and postdeposition annealing on crystallization and phase transition of the Nd:yttrium aluminum garnet (YAG) laser-ablated TiO2 films. II. EXPERIMENTAL
The pellet (15 mm in diameter and 3 mm thick) used as a target was prepared from 99.9% pure TiO2 anatase powder (Wako Pure Chemical Industries Ltd., Osaka, Japan) by the standard conventional route. The target was sintered at 900 °C for 5 h, which completely conv
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