Correlation of Valence State and Transport Properties in VO$_{2}$ Films*

Vanadium dioxide (VO$_{2})$ exhibits a metal semiconductor transition (MST) that is accompanied by the abrupt change in the electrical conductivity, optical transmittance and reflectance in infrared region, which can be used in electronic devices such as temperature sensors and electric switches. VO$_{2}$ thin films were grown on c-plane Al$_{2}$O$_{3}$ substrates with different O$_{2}$ flow rates. The XRD scans have been performed to confirm the single phase and highly textured VO$_{2}$ films despite the change in the growth parameter, i.e. the oxygen partial pressure. The valence state of vanadium on different films was investigated by soft x-ray spectroscopy. The Hall bars were fabricated for electrical transport and Hall measurements. As the O$_{2}$ flow rate increases, the XRD results show decreasing lattice parameter, hence increasing compressive strain along b-axis of monoclinic VO$_{2}$; the transport measurements also show the increasing transition temperature (T$_{MST})$ and the increasing change in resistivity associated with the strain and valence state of vanadium. Hall measurements reveal a sudden increase in carrier concentration from 10$^{20}$ to 10$^{24}$ cm$^{-3}$ around T$_{MST}$, while the mobility remains constant before and after MST. The correlation among the valence state, the strain and MST in VO$_{2}$ will be discussed.