From transparent to black amorphous zinc oxide thin films through oxygen deficiency control

Despite the fact that zinc oxide is a well-known transparent oxide, several recent studies on “black” ZnO have renewed its potential for photocatalytic applications. We report on the control of oxygen deficiency in ZnO thin films grown at 300 °C on c-cut sapphire single-crystal substrates by pulsed electron beam deposition (PED) through a slight variation of argon pressure in PED. At a pressure of 2 × 10−2 mbar transparent, stoichiometric (ZnO) and crystalline films are obtained, while at 9 × 10−3 mbar black, oxygen-deficient (ZnO0.85) and amorphous films result. Stoichiometry, structural, and optoelectronic properties of transparent and black ZnO thin films were comparatively analyzed as a function of oxygen deficiency. Black ZnO thin films exhibit enhanced absorption in the visible and near-infrared due to oxygen deficiency, thus extending the range of applications of zinc oxide thin films from transparent electronics to solar absorbers and photocatalysis.