Growth, microstructure, charge transport, and transparency of random polycrystalline and heteroepitaxial metalorganic ch
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lissa A. Lane, Paul R. Brazis, and Carl R. Kannewurf Department of Electrical and Computer Engineering and the Materials Research Center, Northwestern University, Evanston, Illinois 60208 (Received 24 April 2002; accepted 16 September 2002)
Gallium–indium–oxide films (Gax In2−xO3), where x ⳱ 0.0–1.1, were grown by low-pressure metalorganic chemical vapor deposition using the volatile metalorganic precursors In(dpm)3 and Ga(dpm)3 (dpm ⳱ 2,2,6,6-tetramethyl-3,5-heptanedionato). The films were smooth (root mean square roughness ⳱ 50–65 Å) with a homogeneously Ga-substituted, cubic In2O3 microstructure, randomly oriented on quartz or heteroepitaxial on (100) yttria-stabilized zirconia single-crystal substrates. The highest conductivity of the as-grown films was found at x ⳱ 0.12, with ⳱ 700 S/cm [n-type; carrier density ⳱ 8.1 × 1019 cm−3; mobility ⳱ 55.2 cm2/(V s); d/dT < 0]. The optical transmission window of such films is considerably broader than that of Sn-doped In2O3, and the absolute transparency rival or exceeds that of the most transparent conductive oxides known. Reductive annealing, carried out at 400–425 °C in a flowing gas mixture of H2 (4%) and N2, resulted in increased conductivity ( ⳱ 1400 S/cm; n-type), carrier density (1.4 × 1020 cm−3), and mobility as high as 64.6 cm2/(V s), with little loss in optical transparency. No significant difference in carrier mobility or conductivity is observed between randomly oriented and heteroepitaxial films, arguing in combination with other data that carrier scattering effects at high-angle grain/domain boundaries play a minor role in the conductivity mechanism.
I. INTRODUCTION
Transparent conducting oxides (TCOs) are widely used as key components of numerous display and photovoltaic technologies.1 Tin-doped indium oxide (ITO) thin films with typical conductivities of (1–5) × 103 S/cm and optical transparencies of 85–90% are employed on a massive scale in numerous optoelectronic device applications (e.g., flat panel displays and solar cells),1–3 although the physical properties of ITO are far from optimum. Meanwhile, the development of new device technologies increasingly demands improvements in TCO film electrical, optical, and chemical properties for implementation as transparent electrodes. In the search for improved TCO materials, a number of studies1,4–15 have focused on processing and doping various
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 12, Dec 2002
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oxide combinations of In, Sn, Zn, and Ga. Gallium– indium–oxide is the reported parent phase of a promising new TCO family, and the layered compound GaInO3 reported by Phillips, Cava, and co-workers16,17 (assigned the -gallia crystal structure) can be doped with Sn and Ge to yield films with conductivities less than, but transparencies exceeding those of, typical ITO films. However, detailed structural studies14 of phase relationships in the bulk Ga2O3–In2O3 system at 1250 °C reveal t
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