Plasma treatment of zinc oxide thin film and temperature sensing using the zinc oxide thin film

摘要

Zinc oxide is a direct and wide bandgap, II-VI semiconductor. It has large exciton binding energy, large piezoelectric constant, strong luminescence, and high thermal conductivity. These properties make zinc oxide as a suitable material for various optoelectronic applications. Vacuum based processes of fabrication of zinc oxide thin film dominate the market for their better electrical and optical properties. In this work, zinc oxide thin films were prepared by easy and low cost solution method with oriented crystal growth along (002) plane. To improve electrical and optical property of the fabricated zinc oxide thin films, films were treated with oxygen, hydrogen, and nitrogen plasmas. Oxygen plasma treatment improved the crystallinity of zinc oxide thin film. Hydrogen plasma treatments were found very effective in improving the electrical conductivity of the film sacrificing film's transmittance. Nitrogen plasma treatment following hydrogen plasma treatment could restore the transmittance maintaining the improved electrical property. Sequential oxygen, hydrogen, and nitrogen plasma treatment decreased the resistivity of zinc oxide thin film by more than two order maintaining transmittance close to the as deposited film. This work also reports a temperature sensor based on the temperature-dependent bandgap of zinc oxide semiconductors. Transmittance measurement of the ZnO films at different temperatures showed sharp absorption edge at around 380 nm and red shift characteristics. An optical temperature sensor was established using the zinc oxide coated glass as sensing element, ultra-violet light emitting diode as light source, and a ultra-violet photodiode as light detector. Short circuit current of the photodiode was measured over a range of the zinc oxide film's temperature. The short circuit current decreased linearly with the increase of the temperature and the sensitivity was ∼0.1 muA/°C.