Comprehensive studies have been performed on the effects of radio frequency (rf) substrate biasing on insulator–metal transition (IMT) properties of VO₂ thin films grown on Al₂O₃ (001) substrates. As the rf substrate bias power increased, the ion energy during deposition increased, inducing a strong in-plane stress conversion and a modification of the rutile c-axis (c_R-axis) length in the VO₂ films. The reduced c_R-axis length significantly lowered the IMT temperature. Increased IMT sharpness via improved crystallinity was obtained with rf substrate biasing at an appropriate power. Temperature coefficient of resistance at room temperature was comparable with reported values for impurity-doped VO₂ films. Thus, the rf substrate biasing during reactive sputtering has great potential to control stress in thin films, which could finally control the IMT in oriented VO₂ films.

We demonstrate self-sustaining electrical oscillations with frequency of MHz range based on out-of-plane voltage-triggered switching in VO₂ thin films grown on conductive layers. VO₂ films deposited by a reactive sputtering method at a low temperature of 250 °C on conductive TiN layers showed thermally induced out-of-plane insulator-metal transition with two orders of change in resistance. By applying dc voltage to the layered device in a point contact configuration, self-sustaining electrical oscillations were triggered and the highest frequency of 9 MHz was achieved. Dependence of the frequency on the film thickness, as well as on the source voltage and on the series resistance, was examined in order to clarify the oscillation mechanism and the factors that affect the frequency. The oscillation frequency, which is dominated by recovering time from metallic to insulating state, decreased with increasing film thickness, indicating that the resistance of VO₂ film determines the time constant for the recovery path. Self-sustaining oscillation phenomena achieved in point contact devices has great potential for applications in MHz band generators and micro-inverters.

${\text{VO}}_2$ thin-film planar-type junctions with the size of $1500\mu \text{m}$ in width and $10\mu \text{m}$ in length were fabricated on ${\text{Al}}_2{\text{O}}_3$ (0001) substrates, and the electric-field-induced resistance switching (EIRS) characteristics were investigated at hydrostatic pressures up to 2 GPa and room temperature. The resistance in the electric-field-induced low resistance state (E-LRS) was revealed to be insensitive to the hydrostatic pressure, in contrast to the apparent suppression of resistance in the temperature-induced low resistance state. The critical current to induce E-LRS is increased as the pressure is increased, while the voltage at the transition is independent of the pressure. These results indicate a picture in ${\text{VO}}_2$ that not a Joule heating but a reversible breakdown triggers EIRS, and that the electric current of huge density maintains filamentary E-LRS regions.

We have demonstrated pressure-induced transition in a c-axis oriented vanadium sesquioxide (V₂O₃) thin film from a strongly correlated metal to a Mott insulator in a submicrometric region by inducing a local stress using contact atomic force microscopy. To have an access to a pressure range of sub-gigapascal, a tip with a large radius of 335 nm was prepared by chemical vapour deposition of platinum onto a commercial tip with a focused ion beam (FIB). The FIB-modified tip gives a good electrical contact at low working pressures (0.25–0.4 GPa) allowing unambiguously to evidence reversible metal-insulator transition in a pulsed laser-deposited V₂O₃ thin film by means of local investigations of current-voltage characteristics. A finite element method has confirmed that the diminution of the c/a ratio under this tip pressure explains the observed phase transition of the electron density of states in the film.

We report the first observation of large crystalline domains of several μm-size in VO₂ films deposited on Al₂O₃ (001) substrates by rf-biased reactive sputtering technique. The large crystalline domains, dominated with random in-plane oriented growth of (011)_M1-orientation, appear only under adequate substrate biasing, such as 10 W, while most biasing conditions result in conventional nanosized grains of highly oriented (010)_M1-orientation. Two temperature-controlled analyses, x-ray diffraction and micro-Raman spectroscopy, have revealed that some parts of large crystalline domains undergo intermediate monoclinic (M2) phase during the thermally-induced structural phase transition from monoclinic (M1) to rutile-tetragonal (R) phase. As an effect of the appearance of large crystalline domains, the film showed in-plane tensile stress, resulting in high T_IMT of 69 °C due to the elongation of the V-V distance in its low-temperature monoclinic phase.

We report on the first observation of the persistent M2 phase in strongly strained (011)-oriented grains in VO₂ films grown on Al₂O₃ (001) substrates by means of conventional rf reactive sputtering under adequate deposition conditions. Spatially resolved micro-Raman spectra clearly showed that (011)-oriented large crystalline grains with the c_R-axis parallel to the substrate resulted in the appearance of the M2 phase over a wide temperature range of 30 °C. A close correlation of the appearance range of the M2 phase with the in-plane tensile stress of (011)-oriented grains was revealed by X-ray diffraction. We present a phase diagram for the M1, M2, and R phases in relation to the stress of (011)-oriented grains and temperature. It was shown that (011)-oriented micrometer-sized long grains play a crucial role in the emerging structural phase transition (SPT) via an M2 phase even in a film grown on Al₂O₃ (001), which is ordinarily reserved for the (020)-oriented VO₂ growth. The results shown here will contribute to make clear the conditions for obtaining VO₂ films with the appearance of the M2 phase in their SPT process.