Characterization of the single and double films consisting of Al, Sc-co-doped ZnO/Al-doped ZnO and Al-doped ZnO/Al, Sc-co-doped ZnO

Single and double films consisting of Al, Sc-co-doped ZnO (denoted as S) and Al-doped ZnO (denoted as A) layers were deposited on the Super Twisted Nematic (STN) glass by dc-sputtering on the pure aluminum (99.999%) or Al-0.8 wt.% Sc eutectic alloy target (99.999%) combined with rf-sputtering on the pure zinc oxide (99.99%). The thickness of each layer in the double film was controlled at roughly 230 nm but the depositing order changed. A double film fabricated by depositing first with a layer of Al-doped ZnO on STN then with a layer of Al, Sc-co-doped ZnO on the top was expressed as SA. The film deposited with the same layers but inverse the order was represented as AS. The characterization of the films revealed that the electrical resistivity is lower (i.e., 1.43 × 10^{- 3}-1.58 × 10^{- 3} Ω-cm) for the double layers (both AS and SA) than the single layer (i.e., 3.8 × 10^{- 3} Ω-cm for A and 1.5 × 10^{- 3} Ω-cm for S). The transmission of visible light exceeds 80% for all the films deposited on the glass but the transmission decreases substantially at short wavelength near the ultraviolet range. X-ray diffraction (XRD) revealed that both AS and SA specimens all belong to hexagonal wurtzite textured at (002) and (103). Transmission electron microscopy (TEM) of the cross-sections depicted the films are composed of columnar grains. The electrical resistivity for all the films decreased to a minimum with increasing the annealing temperature from 200 to 300 °C, and increased beyond 300 °C. No sacrifice to the optical transparency (more than 80 % transmittance for the visible light) but a blue-shift of optical transmittance was found for the double-layered films, especially for AS specimen. Annealing of the double films at 300 °C intensified their blue-shift. Based on X-ray photoelectron spectroscopy (XPS) on the surface film we inferred that a decrease of electrical resistivity of the double films is due to an increase in the relative strength of O (III).