Observation of M2 phase coming from the charge density wave for the insulator-to-metal transition in VO₂.

In strained VO₂/AlN/Si films, insulator-to-metal transition (IMT) upshifted for T_IMT ≈ 350 K higher than the bulk value (T_IMT ≈ 340 K) [1]_. Due to the strain, the intermediate monoclinic phase, known as the M2 phase, was clearly seen through the Raman phonon mode near 650 cm^-1 [2]. The M2 phase has two substructures of both the charge density wave (CDW) responsible for the Peierls insulator and zig-zag chain for the Mott insulator [3]_. However, the mechanism of the upshifted T_IMT does not yet revealed.
In this talk, through simultaneous measurements of temperature dependence of micro-Raman spectra and resistance, as well as in-situ XRD and resistance, we observed that T_IMT is prior to temperature of structural phase transition, and that M2 Raman peak still remains from screening effect despite high temperature of 370 K. Therefore, we suggest that the observed M2 Raman peak is caused by the remained CDW structure. Moreover, this can be a reason of upshifted T_IMT. [1]APL Mater. 4, 026101 (2016), [2]APL 109, 233104 (2016), [3]Scientific Reports 7, 16038 (2017)