Ultraviolet Assisted Pulsed Laser Deposition of Thin Oxide Films
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ABSTRACT The properties of Y 20 3, ITO (indium tin oxide), and TaSi 2 thin layers grown using a new in-situ ultraviolet (UV)-assisted pulsed laser deposition (UVPLD) technique have been studied. X-ray diffraction investigations showed that with respect to conventional PLD grown films under similar conditions, but without UV illumination, UVPLD grown films exhibited better crystallinity, especially for growth at low substrate temperatures, from 200 'C up to 450 'C, depending on the material. X-ray photoelectron spectroscopy investigations showed that UVPLD layers contained less physisorbed oxygen than the conventional PLD layers, exhibiting a better overall stoichiometry. These results suggest that during the ablation-growth process, UV radiation increases the surface mobility of adatoms and provides more reactive gaseous species. Both factors contribute to the crystalline growth and are especially effective at moderate processing temperatures, where the thermal energy available for the process is comparatively low.
INTRODUCTION Pulsed Laser Deposition (PLD) has become a very successfully thin film growth technique [I]. To improve the properties of the grown films, the deposition parameters, such as laser fluence, wavelength, pulse duration, repetition rate, target-substrate distance, substrate temperature and pressure of the ambient gas are usually finely adjusted. Several reports have indicated that the use of a more reactive deposition atmosphere than the usual molecular oxygen, 02, was beneficial for the crystalline quality of grown oxide films [2]. However, the use of very reactive oxidizing species for PLD such as ozone and atomic oxygen has not been thoroughly investigated so far [3]. We have previously shown that the crystallinity and optical properties of oxide films grown by PLD at moderate substrate temperatures can be significantly improved by a further annealing treatment in oxygen under ultraviolet (UV) illumination [4, 5]. Thus, it has become interesting to study whether the use of an in-situ UV source, which will expose each deposited layer to the action of high energy UV photons and reactive oxygen species would be even more advantageous. We report here the properties of thin films grown using this new UVassisted PLD (UVPLD), concentrating on Y20 3 layers, where reliable data from conventional PLD growth for comparison are readily available [6, 7].
EXPERIMENT The PLD system employed for the growth of oxide layers is extensively presented elsewhere [6, 71 and is only briefly sketched here. An excimer laser (KrF, X-=248 nm) emitting 25 ns long pulses was used to ablate targets of Y 20 3, indium tin oxide (ITO), ZnO as well as a non-oxide material, TaSi 2 . The laser fluence was varied in the 1.0-3.0 J/cm 2 range by changing the energy delivered by the laser and the repetition rate was set at 5 Hz. (100) Si wafers, cleaned in acetone, sonicated in alcohol, and then blown dry with high purity nitrogen were used as substrates. The substrate temperature, measured with a thermocouple attached to the subs
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