Dielectric Spectroscopy of Strongly Correlated Electronic States of Vanadium Dioxide

The thermal evolution of the conductivity of a VO2 film and database-obtained band gap E (g) of film nanocrystallites is traced in the temperature range of -196A degrees C < T < 100A degrees C (77 K < T < 273 K); the level position of donor impurity centers is determined to be E (d) = 0.04 eV. It is shown that energy E (g) decreases from 0.8 to similar to 0 eV with an increase in temperature in the range of 273 K < T < 300 K, which is caused by the narrowing of the energy gap due to correlation effects and considered as the temperature-extended Mott "insulator-metal" electron phase transition with the monoclinic lattice symmetry retained. The subsequent jump in the symmetry from monoclinic to tetragonal with a further increase in temperature is considered as the Peierls structural phase transition, the temperature of which is in the vicinity of 340 K and determined by the size effects, nonstoichiometry of VO2 film nanocrystallites, and degree of their adhesion to the substrate.