Femtosecond Pulses as a New Photonic Source for Growing Thin Films by Pulsed-laser Deposition
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Femtosecond pulses as a new photonic source for growing thin films by pulsed-laser deposition Eric Millon1,2, Jacques Perrière1, Olivier Albert3, Jean Etchepare3, and Chantal BoulmerLeborgne4, 1 GPS, CNRS UMR7588,University Paris VI, 2 pl Jussieu, 75251 Paris Cedex 05, FRANCE, 2 LSMCL, University of Metz, Metz, FRANCE; 3 LOA, CNRS UMR 7639 and ENSTA, Ecole Polytechnique, Palaiseau, FRANCE; 4 GREMI, CNRS UMR 6606, University of Orléans, Orléans, FRANCE. ABSTRACT The femtosecond (fs) lasers display noticeable specificities compared with the nanosecond (ns) ones operating in the UV domain, and classically used for the pulsed-laser deposition (PLD) technique. The ultra-short laser pulses offer the feature of minimal thermal damage induced in the target material, and the very high intensities (1012-14 W/cm2) available with fs lasers are likely to allow the ablation of any kind of materials, even the wide band gap insulators. The morphology, structure, composition and properties of the films obtained by fs PLD are studied according to the experimental growth conditions, the nature of the target material, and the dynamic expansion of plasma plume. In the case of ZnO, smooth, dense and nanocrystalline films (10 to 30 nm crystallites) can be epitaxially grown on adequate substrates (i.e. sapphire). On the contrary, BaTiO3 films are formed by the random stacking of aggregates (10 to 200 nm) leading to a non negligible surface roughness,. In addition, the chemical composition of fs PLD thin films of multicomponent compound (i.e. BaTiO3) is not homogeneous, an enrichment in the lighter element being observed in the central part of the film. These properties are related to the phenomena taking place during the various steps of the process (laser-matter interaction, plasma formation, expansion) through time resolved emission spectroscopy and plume optical imaging measurements.
INTRODUCTION The pulsed-laser deposition (PLD) technique is now currently used in research laboratories to obtain thin films from a broad range of materials even with a complex composition [1]. For this growth method, lasers in the nanosecond temporal regime and UV wavelength domain have been most commonly used, i.e. ArF (193 nm) and KrF (248 nm) excimer lasers with 10 to 30 ns pulse duration, or frequency quadrupled Nd:YAG lasers (266 nm) with 5 to 15 ns pulse duration. The main drawback of this deposition method which limits its use for industrial applications, is the presence of macroscopic particles (a few micrometers in diameter) on the surface of the films. Their density is particularly important when the optical absorption coefficient of the target is small at the wavelength used for the ablation [2]. The origin of these droplets is still a matter of discussion, but is generally assumed to be the consequence of thermal effects during the laser-matter interaction: liquid material being ejected as droplets from the molten zone of the irradiated target. The use of femtosecond lasers has been thought to be a possible solution to this problem. Ac
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