Paper EN+NS-ThA9
Optimization of the Delta Phase in Bismuth Oxide Thin Films

Thursday, November 3, 2011, 4:40 pm, Room 103

Bismuth oxide Bi₂O₃ has interesting technological applications, which have not been largely used due to the particular polymorphism of the material. Bismuth Oxide has five polymorphic forms: α, β, γ, δ and ω -Bi₂O₃. Among them, the low-temperature α and the high-temperature δ phases are stable and the others are metastable phases, as has been established by bulk solid-state studies. Each polymorph possesses different crystalline structures and various electrical, optical and mechanical properties. The face-centered cubic δ -Bi₂O₃ is stable over a narrow temperature range 729–825 ^oC (melting point) and it has the peculiarity of being among the few materials presenting high ionic conductivity at moderate temperatures (600-700^oC). In this research, we aim to obtain δ -Bi₂O₃ thin films as possible ionic conductors for the development of micro solid state fuel cells. However, the first challenge is to find the deposition conditions of the magnetron sputtering system to ensure the formation of the desired δ -Bi₂O₃, which is only thermodynamically stable at high temperatures. Based on previous results of Fan et al. (Fan 2006), we choose as the deposition variables the substrate temperature (room temperature to 300 ^oC) and the power (100-200 W). Our target was pure Bi₂O₃, but the first results indicated that it was necessary to compensate oxygen losses; therefore the atmosphere was a mixture of Argon and Oxygen, where the Oxygen flow was 20% of the total. The results from the different characterization techniques suggested that substrate temperatures between 150 and 200^oC are appropriate to obtain the δ -Bi₂O₃ phase at a high deposition rate, between 1.5 to 2 nm/s. X-ray diffraction (XRD) as a single technique to identify the film crystalline structure demonstrated to be rather difficult, since there is a large overlapping between the diffraction peaks corresponding to the α, β, γ and δ phases. However, we showed that by combining XRD and Raman spectroscopy, it was possible to clearly prove the presence of the δ-phase. The explanation for the stabilization of the high temperature phase might be related to the 2-dimensional confinement and/or then effect of the small crystalline size. The physical properties of the δ -Bi₂O₃ thin films were further investigated; optical properties by transmission spectroscopy and ellipsometric spectroscopy in the ultraviolet-visible range, surface resistivity by the four-points method, composition by X-ray photoelectron spectroscopy and X-ray energy dispersion.

Fan H. T, et al. Thin Solid Films 513 (2006) 142.