Vanadium pentoxide (V 2 O 5 ) system has considerably attracted much attention in recent years because of its excellent electrochemical , electrochromic , semiconducting [4] properties.
V 2 O 5 has some polymorphs, such as a-V 2 O 5 (orthorhombic), b-V 2 O 5 (monoclinic or tetragonal) [5], g-V 2 O 5 (orthorhombic) . Since b-V 2 O 5 was obtained by the phase transition of a-V 2 O 5 using high pressure and high temperature (HP-HT) method , the significant research interest has been generated for studying the phase transition of V 2 O 5 system and the properties of b-V 2 O 5 . In the reports of Filonenko et al. , b-V 2 O 5 was prepared at P = 6.0 GPa and T = 1073K using V 2 O 5 (a phase) as starting material, and b-V 2 O 5 can transform back to a-V 2 O 5 at 643-653 K under ambient pressure. At present, the reports of pure b-V 2 O 5 are few except those using HP-HT method. According to the previous reports, the substrate temperature plays an important role on the structure of V 2 O 5 film prepared by sputtering [13] or vacuum evaporation [14], thus, under a high substrate temperature, the metastable phases of V 2 O 5 such as b-V 2 O 5 , will probable be obtained by various deposition technologies. Talledo et al. reported that b-V 2 O 5 film was obtained on glass by reactive sputtering, but the formation mechanism and the structural property of sputtered b-V 2 O 5 film have not been investigated.
In this study, b-V 2 O 5 films were successfully prepared on silicon substrates by DC reactive magnetron sputtering using a vanadium metal target. The structure change of V 2 O 5 films with increasing deposition temperature was investigated and the formation mechanism of b-V 2 O 5 film was discussed. Furthermore, the thermal stability of b-V 2 O 5 film was also studied.
2. Experimental
V 2 O 5 films were prepared on silicon (1 1 1) wafers by DC magnetron sputtering with vanadium metal target (purity of 99.97%) at the deposition temperatures of 300, 400, 500, 550 8C. The sputtering power was 42 W and deposition time was 60 min. For reactive deposition, pure oxygen and argon gas were introduced into the chamber, and the flow ratio of O 2 /Ar was 2/ 3. The preparation pressure was 7.2 Pa and the total gas flow during sputtering was maintained at 30 sccm. To investigate the effect of thermal stability of as-deposited films, V 2 O 5 films deposited at 550 8C were annealed in the temperature range of 500-700 8C for 60 min in air.
The sputtered films were investigated using X-ray diffraction (XRD), Raman spectra and field emission scan electron spectroscopy