Structure and optical property of Cr-O films deposited by pulsed bias arc ion plating

Abstract

A series of uniform and transparent Cr-O films were synthesized on the silicon and quartz glass substrates at different bias voltages by pulsed bias arc ion plating. Effects of bias voltage on surface morphology, phase structure, composition, chemical valence states, hardness and optical property of the films were investigated by field emission scanning electron microscopy, grazing incident X-ray diffraction, X-ray photoelectron spectroscopy, nanoindentation and ultraviolet-visible spectrophotometer, respectively. Results indicate that the bias voltage can improve the quality of the films significantly and plays an important role in the film properties. Macroparticles and holes are observed on the surface of the films if without application of bias voltage, while the films prepared with bias voltage are uniform and smooth. The crystalline phase of the film is of amorphous structure if without bias voltage. While the bias voltage applies and increases from -100 V to -500 V, the Cr2O3 phase appears and changes into CrO phase. The crystal plane (104), (116) of the Cr2O3 phase and (200) of the Cr phase are observed in the film at the bias voltage of -100 V. When the bias voltage is above -200 V, the crystal planes (311) and (400) of the CrO phase can be observed. In order to further obtain the structure information, a detailed XPS study is performed. Chromium in the films shows different valence states, namely metallic Cr, Cr2+, Cr3+ and Cr6+. Thereby, the main components of the polycrystalline films are Cr2O3 and CrO phases, meanwhile, and the films also contain a small amount of CrO3 and metal Cr phases. The films under different bias voltage show good mechanical properties and the hardness of all the films is above 19 GPa. With the increase of bias voltage the hardness first increases and then decreases, reaching a maximum value of 24.4 GPa at the bias voltage of -300 V. The films show good optical transmittance and its highest value can be up to 72%. As the bias voltage rises, it is observed first the red shift and then blue shift of the absorption edge. And the optical band gap reaches the maximum value of 1.88 eV when the bias voltage is -200 V. Therefore, Cr-O functional films can be synthesized by pulsed bias arc ion plating and the phase structure and properties can be effectively adjusted.