Effect of Substrate Surface Structure and Deposition Conditions on the Microstructure of Tin Dioxide Thin Films Synthesi
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EFFECT OF SUBSTRATE SURFACE STRUCTURE AND DEPOSITION CONDITIONS ON THE MICROSTRUCTURE OF TIN DIOXIDE THIN FILMS SYNTHESIZED BY FEMTOSECOND PULSED LASER DEPOSITION J. E. Dominguez,* L. Fu,* P. A. Van Rompay,** Z. Y. Zhang,** J. A. Nees,** P. P. Pronko,** and X. Q. Pan,* * Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109 **Center for Ultrafast Optical Science and Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109 ABSTRACT Tin oxide films were deposited on sapphire and silicon substrates using reactive femtosecond pulsed laser deposition at temperatures ranging from room temperature to 700°C. The effect of electrical discharge and background oxygen pressure on the thin film microstructure was studied. The microstructure of the films was characterized by transmission electron microscopy and x-ray diffraction. SnO2 films fabricated consist of different textures in microstructures that depend on the deposition conditions and substrate surface structures. For instance, films deposited on the (1012) sapphire (R-cut) are amorphous if deposited at room temperature, whereas films deposited at 700°C were epitaxial, single crystalline. Discharge and oxygen pressure had a strong effect on the ion/neutral ratio of the ablated plasma plume of SnO2. INTRODUCTION Tin oxide films have a wide array of electronic properties that make them very versatile. As a result, tin oxide films are used for transparent electrodes, energy saving coatings for buildings, components for fuel cells and gas sensors.1 Many techniques are in place for deposition of tin oxide, the most widely used being r.f. sputtering and sol-gel deposition.2 Other deposition techniques include molecular beam epitaxy (MBE), spray pyrolysis, and spin coating. A very promising new technique for oxide film deposition is pulsed laser deposition (PLD). Using this method, even ternary and quaternary oxides can be deposited with excellent stoichiometry and repeatability.3 PLD uses short laser pulses from nanoseconds (ns) to femtoseconds (fs) to ablate target material. The ablated material is ejected as plasma of the constituent atoms or clusters of atoms (plume) which condenses on a substrate. Research on tin oxide films deposited using PLD is increasing, however, challenges remain to make high quality thin films which have well-controlled stoichiometry, surface roughness, defect configurations, and crystallinity. In this paper we investigate the effect of deposition conditions on such thin films. EXPERIMENTAL A femtosecond Ti-sapphire laser with repetition rate of 10 Hz and another with 1kHz repetition rate were used to deposit tin oxide films. The wavelength of the pulses was centered at 780 nm and the pulse width was about 100fs for the 10 Hz laser system and 150 fs for the 1 kHz system. The plasma was analyzed using an optical emission spectrometer and an ion energy AA3.45.1
spectrometer. The target was made of pressed and sintered undoped (99.99%) ceramic tin oxide. Subs
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