Transparent Conducting Indium-Tin-Oxide Thin Film with Extremely Flatted Surface
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Transparent Conducting Indium-Tin-Oxide Thin Film with Extremely Flatted Surface Hiromichi Ohta1, Masahiro Orita1, Masahiro Hirano1, Hideo Hosono2 1 Hosono Transparent ElectroActive Materials, ERATO, JST, KSP C-1232, 3-3-1 Sakado, Takatsu, Kawasaki 213-0012, JAPAN 2 Materials and Structures Laboratory,Tokyo Institute of Technology Nagatsuda,Midori-ku,Yokohama 226-8503,JAPAN ABSTRACT Indium-tin-oxide films were grown hetero-epitaxially on YSZ surface at a substrate temperature of 900 oC, and their surface microstructures were observed by using atomic force microscopy. ITO films grown on (111) surface of YSZ exhibited very high crystal quality; full width at half maximum of out-of-plane rocking curve was 54 second. The ITO was grown spirally, with flat terraces and steps corresponding to (222) plane spacing of 0.29 nm. Oxygen pressure during film growth is another key factor to obtain atomically flat surfaced ITO thin film. INTRODUCTION Tin-doped indium oxide (ITO) with C-rare earth crystal structure (Crystal system: cubic, Space group: Ia3, a0=1.0118 nm (In2O3)) is widely applied as transparent electrode for flat panel displays such as liquid crystal displays (LCDs) and solar cells. Recently, ITO films have also been used as cathode electrodes for UV-emitting diodes, in which the ITO film acts as a cladding layer and substrate for n-type conductive layers and electrode [1]. For use in such optoelectronic device application, device properties would depend sensitively on the surface microstructures and crystal quality of ITO film. An important aspect in understanding the surface microstructure and crystal quality is to study the film growth mechanisms on single crystal substrate. Yittria-stabilized-zirconia (YSZ) with fluorite crystal structure (Crystal system: cubic, Space group: Fm3m, a0=0.5139 nm (Y0.15Zr0.85O1.93)) is preferred substrate material for hetero-epitaxial growth of ITO film due primary to similar crystal structure. Lattice mismatch between a unit cell of ITO and two unit cells of YSZ is less than 1.6 % [2-5]. We recently reported that very low resistive ITO thin films with resistivity of less than 1 x 10-4 Ωcm may be grown hetero-epitaxially on a single crystal of YSZ (100) surface by pulsed-laser deposition (PLD) technique [5]. Large grains with 100 - 200 nm diameters were observed, which would indicate typical columnar growth. Our approaches of obtaining further information about surface microstructure of hetero-epitaxial ITO thin film are as follows. First, ITO thin films were grown at higher temperatures to enhance surface diffusion of atoms, changing the growth mode from columnar growth to spiral and/or step flow growth. Second, two different (111) and (100) surfaces of YSZ were used for ITO film growth. YSZ cleaves readily along [111] planes. Thus, one would expect that atomic migration of relevant ions on the (111) plane would differ markedly from that on the (100) plane of YSZ. Further ITO films were grown in various oxygen pressures. In general, oxygen pressures during oxide film growth effect
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