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

摘要

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 A) < In (0.94 A) < Sc (0.89 A),Ga (0.76 A),with the smallest radius ion among the four dopants only slirinking 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.