Tuning the Properties of Transparent Oxide Conductors. Dopant Ion Size and Electronic Structure Effects on CdO-Based Transparent Conducting Oxides. Ga- and In-Doped CdO Thin Films Grown by MOCVD
A combined experimental and theoretical/band structure investigation is reported of Ga-doped CdO (CGO) and In-doped CdO (CIO) thin films grown on both amorphous glass and single-crystal MgO(100) substrates at 410 °C by metal-organic chemical vapor deposition (MOCVD). Film phase structure, microstructure, and electrical and optical properties are systematically investigated as a function of doping stoichiometry and growth conditions. XRD data reveal that all as-deposited CGO and CIO thin films are phase-pure and polycrystalline, with features assignable to a cubic CdO-type crystal structure. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited CGO and CIO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Ga and in doping widens the optical band gap from 2.85 to 3.08 and 3.18 eV, respectively, via a Burstein-Moss shift. On MgO(100), room temperature thin film conductivities of 11 500 and 20 000 S/cm are obtained at an optimum Ga and in doping levels of 1.6% and 2.6%, respectively. Together, the experimental and theoretical results reveal that dopant ionic radius and electronic configuration have a significant influence on the CdO-based TCO structural, electronic, and optical properties: (1) lattice parameters contract as a function of dopant ionic radius in the order Y (1.09 Å) < in (0.94 Å) < Sc (0.89 Å), Ga (0.76 Å), with the smallest radius ion among the four dopants only shrinking the lattice marginally and exhibiting low doping efficiency; (2) carrier mobilities and doping efficiencies decrease in the order in > Y > Sc > Ga; (3) the Sc and Y dopant d states have substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.
S. Jin and Y. Yang and J. E. Medvedeva and L. Wang and S. Li and N. Cortes and J. R. Ireland and A. W. Metz and J. Ni and M. C. Hersam and A. J. Freeman and T. J. Marks, "Tuning the Properties of Transparent Oxide Conductors. Dopant Ion Size and Electronic Structure Effects on CdO-Based Transparent Conducting Oxides. Ga- and In-Doped CdO Thin Films Grown by MOCVD," Chemistry of Materials, American Chemical Society (ACS), Oct 2008.
The definitive version is available at http://dx.doi.org/10.1021/cm702588m
National Science Foundation (U.S.)
Keywords and Phrases
MOCVD; Amorphous Glass; Meta organic chemical vapor deposition; Thin films
Article - Journal
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