Room Temperature Growth of Indium Tin Oxide Films by Ultraviolet-Assisted Pulsed Laser Deposition
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INTRODUCTION Indium tin oxide (ITO) thin films are widely used for optoelectronic devices as they combine a good electrical conductivity with high transparency in the visible range. There are a number of interesting applications such as anode contact in organic light-emitting diodes [ 1, 2] or coating of flexible polymer substrates for ultralight mobile display panels [3] where the use of a low processing temperature is very important. The use of the pulsed laser deposition (PLD) technique, which has several important advantages [4], has allowed the growth of good quality indium tin oxide (fTO) thin films at relatively low temperatures and even room temperature [58]. Other techniques such as synchrotron radiation ablation [9] or plasma-ion assisted evaporation [10] were also employed to deposit ITO films at room temperature. We investigated the use of an in situ ultraviolet-assisted PLD technique (UVPLD) for the growth of ITO films at room temperature. The UV source photodissociates molecular oxygen and provides ozone and atomic oxygen during the growth [ 11]. These more reactive gases have been shown to promote the crystalline growth at lower temperatures than those normally used during conventional PLD [ 12]. Moreover, UV+ozone is known to be an effective way to clean organic contaminants from the substrate [13, 14], a fact that can also improve the quality of the deposited layers. EXPERIMENT The PLD system employed is presented elsewhere in much more detail [15, 16] and it is only briefly described here. An excimer laser (KrF, X=248 nm, laser fluence -2 J/cm 2 , repetition rate 5 Hz) was used to ablate ITO targets (99.99% purity). The oxygen pressure was
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Mat. Res. Soc. Symp. Proc. Vol. 623 ©2000 Materials Research Society
varied from 0.1 mtorr to 50 mtorr. A vacuum compatible, low pressure Hg lamp, which allows for in-situ UV irradiation of the substrate during the laser ablation-growth process, was fitted to the PLD system. Films were deposited onto (t00) Si wafers and Corning glass substrates that were placed at 10.5 cm in front of the target. The crystalline structure of the grown films was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The chemical composition was determined by xray photoelectron spectroscopy (XPS, Perkin Elmer 5100, Mg Kou radiation) and Auger electron spectroscopy (AES). The optical properties of films grown on Si substrates were investigated by spectroscopic ellipsometry (VASE, Woollam Co.) at 70' while those of film deposited on Corning glass were analyzed by optical spectrophotometry. The sheet resistance was measured by a four point probe method and the surface morphology by atomic force microscopy (AFM). RESULTS XRD and TEM investigations showed that all films deposited, regardless of the oxygen pressure used, were amorphous. Crystalline films were deposited by UVPLD at a substrate temperature of only 100 'C. The effect of the oxygen pressure on the resistivity of the grown ITO films is shown in Fig. 1. Values measured for several films, wh
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