Effects of crystal orientation on electronic band structure and anomalous shift of higher critical point in VO₂ thin films during the phase transition process

The phase transition behaviour of vanadium dioxide (VO₂) with different thicknesses has been investigated by temperature-dependent optical transmittance and Raman spectra. It is found that the crystal orientation has a great effect on the metal-insulator transition (MIT) of VO₂ films. The x-ray diffraction (XRD) analysis shows that the films are polycrystalline and exhibit the characteristics of the monoclinic phase. The preferential growth crystal orientation (0 2 0) is converted to the (1 1 1) plane with the film thickness increasing. It is believed that the (1 1 1) plane is the reflection of a twinned structure with (0 1 1) crystal orientation, which will lead to the arrangements of oxygen atoms and vanadium atoms deviating from the pure monoclinic structure. It is found that the highest order transition (E ₃) is highly susceptible to the crystal orientation, whereas the lowest order transition (E₁) is nearly unaffected by it. The E ₃ exhibits an anomalous temperature dependence with an abrupt blue-shift (∼0.5 eV) in the vicinity of the metal-insulator transition (MIT) for VO₂ film with a thickness of 84 nm. The findings show that the empty σ∗ band can be driven close to the Fermi level when the (0 2 0) orientation is converted to the (1 1 1) orientation. Compared to the VO₂ films with thicknesses of 39 and 57 nm, the E₃ decreases by 0.8 eV and the E₂ increases by about 0.1 eV at the insulator state for the VO₂ film with a thickness of 84 nm. The abnormal electronic transition and the variation of energy band is likely caused by the lattice distortion and V-V dimerisation deviation from the monoclinic a_m axis.