Amorphous transparent conductive oxide InGaO3(ZnO)m (m ≤ 4):: a Zn 4s conductor

With the purpose of creating ZnO-based amorphous transparent conductors, a range of amorphous films InGaO3(ZnO)(m) (where m less than or equal to 4) was prepared using a pulsed-laser deposition method. The resulting films exhibited an optical bandgap of 2.8-3.0eV, and an n-type electric conductivity of 170-400S cm(-1) at room temperature, displaying a slight dependence on the value of m, in which the carrier density was 10(19)-10(20) cm(-3) the electron mobility was 12-20 cm(2) V-1 s(-1) showing no p-n anomaly between Hall and Seebeck coefficients. The conductivity; displayed no significant dependence on the temperature ranging from 10 to 300 K. X-ray diffraction, transmission electron microscopy and extended X-ray absorption fine structure measurements confirmed that the films were amorphous phases. A combined use of bremsstrahlung isochromat spectroscopy and ultraviolet photoelectron spectroscopy revealed that the conduction band tail had a large dispersion and that the Fermi level was located at the conduction band edge. The percolation theory and overlap integral calculations suggest that the extended conduction band of the amorphous metal oxides are formed when the (n - 1)d(10) ns(0) metal ions (n being the principal quantum number) occupy more than 20% of the atoms and the overlap integral between the vacant ns orbitals exceeds a threshold value of approximately 0.4. The present system is the first amorphous oxide semiconductor in which Zn 4s orbitals form the extended conduction band.