Synthesis and some surface studies of laminated ZnO/TiO2 transparent bilayer by two-step growth

A laminated bilayer was prepared by first depositing titanium dioxide (TiO2) nanocrystals on indium tin oxide (ITO) coated glass by a two-electrode cell. Zinc oxide (ZnO) thin film was thereafter deposited on the TiO2 by two different techniques: electrochemical deposition and vacuum evaporation. The films were characterized by some surface probing techniques. Morphological study revealed that particle size of the TiO2 underlayer increases between 110 and 138 nm with increase in deposition voltage. It also showed that ZnO thin film (overlayer) completely covered the underlying TiO2 without chemical interaction between constituents of both layers. Cross-sectional FESEM study gave values of layered film thickness below 55 µm. Exhibition of strong diffraction peak at plane (121) indicated preference of TiO2 film's growth orientation. It also suggested a feature of phase-pure brookite. Optical studies showed that each film exhibited strong absorption edge at λ=~330 nm and transmitted fairly across visible light region. Energy band gap lied between 3.24 and 3.43 eV. This study demonstrated successive layer deposition of transparent metal oxide structures from inorganic reagents. It also reaffirmed TiO2 as a recipe for barrier layer that can hinder transition of holes from absorber to transparent front contact of nanostructured photonic devices.