Growth temperature-dependent phase evolution and photoactivities of sputtering-deposited crystalline Bi2O3 thin films

Abstract

Crystalline Bi₂O₃ thin films were grown through radio-frequency magnetron sputtering deposition using a bismuth metal target in an Ar/O₂ mixed atmosphere and in an Ar atmosphere with further post-annealing procedures in ambient air. The crystal structures, surface morphologies, and photoactive performance of various as-synthesized Bi₂O₃ thin films were manipulated by controlling the in situ sputtering growth temperature and post-annealing temperature. Structural analysis revealed that the in situ sputtering-grown Bi₂O₃ thin films had dual monoclinic α-/tetragonal β-phase structures below the growth temperature of 425 °C with an unexceptional impurity phase of Bi₄O₇ in the film; the Bi₂O₃ thin film consisted of a pure single β phase when the growth temperature was increased above 425 °C. In contrast, the sputtering deposited metallic bismuth thin films transformed into pure α/β phase-structured Bi₂O₃ thin films without any impurity phase when the annealing temperature was below 425 °C; however, Bi₄O₇ was formed in the α/β Bi₂O₃ film with annealing temperatures above 425 °C in this study. For the Bi₂O₃ thin-film crystal grown via in situ sputtering or annealing, the growth temperature effects on impurity phase generation showed the opposite trend. Furthermore, the formation of the impurity Bi₄O₇ phase in the Bi₂O₃ films may deteriorate the photoactive performance of the Bi₂O₃ thin film. The efficient photoexcited charge separation and transfer across the α-β phase heterojunctions in the polymorph α/β Bi₂O₃ films accounted for their higher photoactivity among the various Bi₂O₃ thin films in this study. The results reported herein show that the α/β dual phase Bi₂O₃ film without any impurity phase formed via precise process control is promising for applications in photoactive devices or as a coupling oxide layer in heterogeneous devices.