Electronic Properties of VO2near the Semiconductor-Metal Transition

Optical, thermal, magnetic, and transport properties of V${\mathrm{O}}_2$ in the vicinity of the semiconductor-metal transition temperature of 68°C are discussed in terms of a simple one-electron band picture. Particular emphasis is placed on the variation in these properties from sample to sample. It is shown that all of the available experimental data can be qualitatively explained in terms of such a model. In the metallic state, there are at least two overlapping and partially occupied bands, resulting in a complex Fermi surface characterized by a conductivity effective mass near unity and carrier mobilities in the range between 1 and 10 ${\mathrm{cm}}^2$/V sec. There is no consistent evidence of a very narrow high-mass band near the Fermi level. In the semiconductor state, it is probable that the band overlap described above is removed by the lattice distortion, resulting in an energy gap of approximately 0.6 to 0.7 eV. This energy gap is filled to a density of typically ${10}^{19}$ to ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ with donorlike and acceptorlike states, the energy distribution and density depending on the sample, so that it is impossible to obtain an accurate measure of the energy gap. The charge density associated with these donorlike and acceptorlike states is normally larger than that associated with the carriers in the band, resulting in electronic properties characteristic of heavily compensated semiconductors. The conduction-band effective mass and the electron mobility are both near unity in the semiconducting phase of V${\mathrm{O}}_2$. Some implications of these experimental results with respect to the application to V${\mathrm{O}}_2$ of several of the published theories on semiconductor-metal transitions in solids are pointed out.