Structural, morphological, and optical properties of Bi2O3 thin films grown by reactive sputtering

Bi2O3 thin films were grown using reactive RF sputtering from a metallic Bi target. The influence of various deposition parameters (substrate temperature, applied power on target and oxygen content in the working gas) on the morphology, structure and optical properties of films was investigated. Depending on the O2/(Ar + O2) ratio of the working gas, bismuth, δ-Bi2O3, α-Bi2O3 or a mixture of these phases can be deposited, with a narrow window for growth of [111]-oriented δ-Bi2O3 thin films. The δ-Bi2O3 phase is stable from room temperature up to 350 °C (in air), where an irreversible transition to α-Bi2O3 occurs. This phase transformation is also shown to occur during TEM sample preparation, because of the inherent heating from the ion-milling process, unless liquid-nitrogen cooling is used.