Highly Conductive and Transparent Cadmium Oxide Thin Films Grown by MOCVD - Epitaxial Growth and Doping EffectsPublic Deposited
Four series of doped CdO thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties investigated. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited doped CdO thin films exhibit excellent optical transparency, with an average transmittance of > 80 % in the visible range. Doping widens the optical band gap up to 3.4 eV via a Burstein-Moss shift. Epitaxial doped CdO films on single-crystal MgO(100) exhibit significantly higher mobilities (up to 236 cm2/Vs) and carrier concentrations than that of films on glass, arguing that the epitaxial CdO films possess fewer scattering centers and higher doping efficiencies due to the highly textured microstructure. Room temperature thin film conductivities of 20,000 S/cm on MgO(100), is obtained at an optimum In-doping level of 2.6%, which is the highest up to date grown by CVD technique. 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 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 slightly and exhibiting low doping efficiency; (2) carrier mobilities and doping efficiencies decrease in the order In > Y > Sc>Ga; (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. Highly conductive and transparent CdO thin films have been grown on glass and on single-crystal MgO(100) by MOCVD at 400 oC, and were used as transparent anodes for fabricating small-molecule organic-light emitting diodes (OLEDs). Device response and applications potential have been investigated and compared with those of control devices based on commercial ITO anodes.