Raman fingerprints of different vanadium oxides as impurity phases in VO2 films

The relation between the structure of VO2 and the process of the semiconductor-to-metal phase transition (SMT) is an important applied issue that is not completely understood yet. Raman spectroscopy is one of the most frequently used techniques in such studies. It allows not only monitoring SMT in situ but also detection of minor inclusions of other vanadium oxides that may form initially or as a result of the SMT. Raman analysis relies on the fingerprint vibration patterns of different oxide phases, but for some of the vibrational modes reported so far, the attribution to certain oxide compounds is still ambiguous. In the present work, we establish the origin of the modes between 800 and 1100 cm-1 based on the detailed study of a series of thin films fabricated by magnetron sputtering, with different ratios of oxide phases. By studying Raman spectra at different excitation wavelengths, we conclude that the difference in phase composition derived from Raman and XRD is not solely due to different sensitivities of these techniques to different oxides, including resonant effects in Raman, but is also the matter of interpretation of the vibrational patterns of mixed oxide phases. Importantly, the presence of other VxOy phases (V2O5, V3O7, V4O9) does not affect qualitatively the SMT of the main VO2 phase. More difference in SMT is observed between Si and quartz substrates, indicating that the SMT mechanism in such films is governed primarily by the characteristics of the initial VO2 phase, rather than by "impurity" phases VxOy.