Full Length ArticleStudy of nanostructural bismuth oxide films prepared by radio frequency reactive magnetron sputtering
Introduction
Bismuth oxide (Bi2O3) thin films have attracted intensive interest because of their unique characteristics of wide optical band gap, high refractive index and dielectric permittivity, good photoconductive response, and ionic conductivity. These interesting physical properties make it suitable for applications in optoelectronics, sensor technology, optical coatings, solid oxide fuel cells and ceramic glass manufacturing [1], [2], [3], [4], [5]. Recently, Bi2O3 material has become the hot topic in the study of visible-light photocatalysts due to its unique crystal structure and electronic properties [6], [7], [8], [9], [10], [11]. The reported values of energy band gap of Bi2O3 thin films vary from 1.5 to 2.9 eV [12], [13] which are significant lower than the value of energy band gap of a TiO2 film, suggesting that excitation by visible light can generate electron-hole pairs more efficiently in Bi2O3 than in TiO2 [14]. There are four main polymorphs of bismuth oxide which are labeled as monoclinic α-Bi2O3, tetragonal β-Bi2O3, body-centered cubic γ-Bi2O3 and face-centered cubic δ-Bi2O3. There are still two more metastable Bi2O3 phases which have been obtained under special conditions: triclinic ω-Bi2O3 and orthorhombic ε-Bi2O3. For the body material, α-Bi2O3 is stable from room temperature up to 730 °C. At 730 °C, this phase transforms into the δ-Bi2O3 phase, which remains stable up to its melting point at 825 °C. β-Bi2O3 and γ-Bi2O3 are obtained as metastable phases during the cooling of δ-Bi2O3. Below 640 °C, these metastable phases are transformed into α-Bi2O3 [15]. However, for the deposited Bi2O3 thin films, the situation is very different. For example, δ-Bi2O3 phase films can be obtained by magnetron sputtering technique at low substrate temperature and even at room temperature [12], [13], [16], [17], [18]. The phase of the Bi2O3 films is very sensitive to both the deposition technique and deposition conditions. Except for δ-Bi2O3, α-Bi2O3 and β-Bi2O3 are also obtained by magnetron sputtering technique. Therefore, it is necessary to perform a detailed study on the effects of the deposition conditions on Bi2O3 films properties. Magnetron sputtering is a very important industrial large area production technique. The deposition parameters are very easy to control and the deposited films generally are very dense and have a good adhesion with the substrate. In this work, the Bi2O3 films have been prepared on the quartz substrate at different substrate temperature by rf reactive magnetron sputtering technique and the optical and structural properties of the deposited Bi2O3 films were studied.
Section snippets
Experimental procedures
The Bi2O3 films were deposited onto the quartz substrates, which were subsequently cleaned ultrasonically using ethanol and de-ionized water for 5 min, and then dried in flowing N2 gas. The cleaned substrates were then transferred to the vacuum chamber and coated with Bi2O3 layer using the radio frequency (rf) magnetron sputtering technique. The sputtering target was a bismuth metal disk with a diameter of 60 mm and a purity of 99.99%. Before the depositions, the vacuum chamber was pumping down
Results and discussion
Fig. 1 shows the deposition rate of the Bi2O3 films deposited at different substrate temperatures. It can be seen that the deposition rate decreases as the substrate temperature is increased. Although the deposition rate mainly depends on the number of sputtered atoms which subsequently reach the substrate, the other processes, such as local pressure reduction in the sputtering plasma, variation in the sticking coefficients of bismuth or oxygen and re-evaporation from the substrate also have
Conclusions
In this work, bismuth oxide films have been prepared by rf reactive magnetron sputtering from a metallic bismuth target at different substrate temperature. XPS study reveals that all the Bi ions exist in the films as Bi3+ and the deposited films are very closed to their chemical stoichiometry. The film deposited at room temperature shows an amorphous structure and the films prepared at the temperature higher than 300 °C show a δ-phase predominated Bi2O3 structure. The high substrate temperature
Acknowledgment
This work was supported by the Dalian University of Technology, China through the program of the Sea-sky Scholar.
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