Hydrogen incorporation induced the octahedral symmetry variation in VO2 films

Understanding the Phase Transition Evolution Mechanism of Partially M2 Phased VO2 Film by Hydrogen Incorporation

“…As expected, in the as-grown and the annealed film under N 2 , the amounts of M1 and M2 phases are almost same (i.e., M1/M2 = 0.5:0.5), while the M2 phase in the films annealed under H 2 /N 2 decreases to about 10%, as shown in Figure d. Previously, many researchers have reported that VO 2 films annealed under the H 2 environment exhibited a structural phase transition from M1 to R phases owing to the incorporation of hydrogen. ,− , However, the films used in this work retain the monoclinic phonon peaks with the highly improved asymmetric V–V bonding even after hydrogen annealing. This is different from the results reported so far.…”

“…For decades, in addition to studies of understanding the mechanism of the phase transition, many research studies have been performed to tune the phase transition temperature of the VO 2 . − Among many works, hydrogen incorporation is one of the effective ways of lowering the insulator–metal transition (IMT) temperature. − In general, the reduction of the phase transition temperature due to hydrogen incorporation in M1-VO 2 is explained as follows. First, a hydrogen ion modifies the zigzag V–V dimer to linear by entering the interstitial sites of the VO 2 lattice. , As a result, the lattice distortion of M1 to R phases reduces the bandgap and lowers the phase transition temperature. Second, when the hydrogen ion dissociates on the VO 2 surface, the additional electrons are effectively supplied to VO 2 . ,, The additional electrons reduce the electron–electron correlation in VO 2 by filling the V 3 d band, causing a phase transition from insulating M1 to the metallic R phase.…”

“…All films were postannealed at 400 °C for 10 min (RTA-1200, NEXTRON) under N 2 (0.2 L/min), N 2 (0.1 L/min)/H 2 (0.1 L/min), and H 2 (0.2 L/min) environments. The postannealing environment-dependent electrical property was measured using an in-line 4-point probe technique . Room-temperature Raman spectroscopy (RAMANtouch, Nanophoton) and high-resolution X-ray diffraction at an in-house X-ray diffractometer (Cu K α , λ = 1.5406 Å, Empyrean, PANalytical, Korea Basic Science Institute, Korea) and at synchrotron radiation 3D-XRS beamline (Pohang Accelerator Laboratory, Korea) were implemented for the structural analysis.…”

“…The in-plane compressive strain induces the M2 phase by compressing the zigzag V–V dimer, transforming it into the linear V–V chain. , Therefore, the M2 phase, shown in Figure c in the as-grown VO 2 film, is due to the in-plane compressive strain induced. After postannealing, the in-plane compressive strain decreases while the out-of-plane tensile strain increases due to the hydrogen incorporation. ,, However, the in-plane compressive strain for the hydrogenated VO 2 film (approx. −2.7%) is still large enough to induce the M2 phase .…”

“…This suggests that the film annealed under H 2 has a metallic electronic structure irrespective of IMT. However, it is difficult to distinguish from XPS whether the metallic electronic structure is due to oxygen vacancies or electron doping because both of these are similar in the O 1 s spectra. , Nevertheless, the number of conduction electrons increases in the VO 2 film after hydrogen annealing because the resistivity at 30 °C becomes lower for the films annealed under hydrogen environments (see Figure ).…”