title: CdO as the archetypical transparent conducting oxide. Systematics of dopant ionic radius and electronic structure effects on charge transport and band structure.
abstract: A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 degrees C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. XRD data reveal that all as-deposited CYO 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 CYO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Y doping widens the optical band gap from 2.86 to 3.27 eV via a Burstein-Moss shift. Room temperature thin film conductivities of 8,540 and 17,800 S/cm on glass and MgO(100), respectively, are obtained at an optimum Y doping level of 1.2-1.3%. Finally, electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 A) < In (0.94 A) < Sc (0.89 A); (2) the carrier mobilities and doping efficiencies decrease in the order In > Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.
authors.affiliation: Department of Chemistry, Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Yu
authors.initials: Y
authors.lastname: Yang
authors.forename: Shu
authors.initials: S
authors.lastname: Jin
authors.forename: Julia E
authors.initials: JE
authors.lastname: Medvedeva
authors.forename: John R
authors.initials: JR
authors.lastname: Ireland
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Metz
authors.forename: Jun
authors.initials: J
authors.lastname: Ni
authors.forename: Mark C
authors.initials: MC
authors.lastname: Hersam
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Freeman
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks