DOI: 10.1021/cg901384d
Highly Conducting, Transparent Rhombic/Cubic Indium Tin Oxide Nanocomposite Thin Films
2010, Vol. 10 1730–1735
Luke A. Dunlop,* Ahmed Kursumovic, and Judith L. MacManus-Driscoll Department of Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom Received November 4, 2009; Revised Manuscript Received February 11, 2010
ABSTRACT: Polycrystalline tin-doped indium oxide (ITO) films were deposited at growth rates up to 0.8 nm/s on glass using pulsed nebulization CVD. The precursor solutions were nebulized into droplets with a modal size significantly smaller than predicted by theory. Rhombohedral distortion of the structure was observed in a number of films leading to nanocomposites of the rhombic/cubic structures. The rhombic phase is rarely observed without applying high pressure. The average mobility (carrier concentration) for the partially rhombic films was 27 cm2/(V s) (5.7 1020 per cm3), whereas for the purely cubic films it was 11 cm2/(V s) (4.0 1020 per cm3). These values are higher than for sputtered films. The significantly improved mobility is explained by lower strain/disorder in the presence of the rhombic phase. The higher carrier concentration may relate to increased tin solubility in rhombohedral phase. Median film optical transparency was 89%, and the median rms roughness value was 6 nm, comparable to sputtered films.
Introduction Because of its high transparency, low resistivity, and high chemical stability, tin-doped indium oxide is a technologically important coating material. It is utilized extensively as a transparent conducting layer in the majority of electronic display technologies, in photovoltaics, and as an infrared absorbing window coating. It is typically grown by sputtering, which requires vacuum growth conditions and for oxide materials has relatively low growth rates (on the order of 0.1 nm/s). Indium oxide with a corundum (rhombohedral) structure was initially reported by Shannon as part of a series of compounds containing large cations which could be formed as a result of high-pressure (65 kbar) synthesis.1 The change observed between the cubic (Figure 1a) and the high-pressure rhomobohedral structures (Figure 1b) is primarily the result of rearrangement of the oxygen anions while leaving cation positions relatively unchaged.2 It has been argued that the conversion should allow stable intermediate structures between the cubic and rhombic materials.3 Rhombohedral indium oxide is stabilized in nanopowder form at room temperature by fabrication at elevated temperatures with solution precipitation reactions from alcohols.4,5 This indicates that under nonequilibrium conditions it is possible to form the rhombic phase without extreme pressures. However, it appears that sample stability relies on the presence of OH groups, and hence it is found that undoped rhombohedral samples are generally metastable.3 In some cases, stress induced by ions dissolved in the indium oxide can induce the corundum type structure, e.g., an amorphous 10% zinc-doped indium film annealed in air at 500 C became rhombic.6 However, because tin has a more closely matched ionic radius than zinc, a similarly annealed 10% tin-doped sample retains the cubic structure.
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Published on Web 02/24/2010
For thin films, the literature abounds with papers on bixbyite indium oxide (doped and undoped) while reports of the rhombic phase are rare. By employing metal organic chemical vapor deposition combined with sapphire singlecrystal substrates to produce epitaxial matching stabilization, rhombohedral In2O3 thin films have been successfully grown.7 The films have mobilities of 55 cm2/(V s) and carrier concentrations of 6.5 1018 cm-3, which contrasts with lower values from the comparative epitaxial bixbyite samples (grown on In2O3 single crystals) that have mobilities between 11 and 35 cm2/(V s) and carrier concentrations around 5 1018 cm-3.8 The rhombic films that have been made are too resistive to be transparent conducting materials (apart from one spraydeposited film discussed later). It has also been possible to form rhombic indium tin oxide (ITO) on glass by coprecipitation of particulates followed by calcining at 600 C to obtain a complete film.9 Rhombohedral films grown by this method had good mobilities up to 23 cm2/(V s) but relatively low carrier concentrations up 1 1016 cm-3 with comparable cubic films had lower mobilities, 3 cm2/(V s), but higher carrier concentrations of around 2 1017 cm-3.10 Malik et al. reported a single rhombic indium oxide thin spray film deposited on Pyrex glass using indium and tin chlorides in methanol-acetic acid solution.11 To the best of our knowledge, this is the only report of a rhombic indium oxide film with resistivity 90% up to 400 nm, which is comparable to sputtered films. The transparency at 550 nm and the optical band gap are not correlated. However, lower transparency is clearly related to high tin concentrations, possibly because of the formation of Sn3O4 or related phases (Figure 8c).31 One the basis of these results, the best value for the performance index T10/R0 is 1.9 10-1 Ω-1, superior to most sputtered films.32
Article
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Without recourse to toxic or expensive solvents (a particular advantage for open operations systems which coat large areas), the method represents a novel and cost-effective means to transparent conducting films with properties comparable to vacuum deposited sputtered films. Acknowledgment. Dr. P. Williams of the Centre for Atmospheric Science at the University of Manchester provided significant assistance through on-site aerosol size measurements. Supporting Information Available: S1 illustrates the optical scattering data obtained concurrently with the SMPS data presented in Figure 3; S2 is a table outlining the deposition conditions and results obtained for each sample (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 8. Optical properties of the ITO films. (a) Transparency and reflectance as a function of wavelength for the three samples of highest conductivity. (b) Optical bandgap versus carrier concentration. (c) Transparency for a light wavelength of 550 nm versus percentage of tin in solution. The four data points relating to rhombic/cubic composites are shown with red stars.
Conclusions Nanocomposite thin films consisting of rhombic indium tin oxide grains in a cubic indium tin oxide matrix have been grown using an advanced ultrasonic chemical vapor deposition. Pulsed direct nebulization significantly reduces particle size compared to conventional methods and allows atomization of solutions that strongly attenuate at ultrasonic frequencies. The partial conversion of the cubic to rhombic phase arises to relieve stress. This leads to improved mobility at similar carrier concentrations. Film transparency is also high.
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