Joe Sakai

Electric-Field-Induced Multistep Resistance Switching in Planar VO2/c-Al2O3 Structure

The electric-field-induced resistance switching of vanadium dioxide (VO2) films grown on c-Al2O3(001) substrates was studied in planar devices with two terminal electrodes. We demonstrated multistep resistance switching in a device with dimensions of 5/1500 µm (electrode gap/electrode length), while only a single-step resistance switching was observed in a device with dimensions of 10/10 µm. Each step in the multistep resistance switching occurred within 100 ns and exhibited a time-independent constant value. Optical microscope observations of a filamentary current path in a VO2 layer imply that initial filamentary current path formation and its development are responsible for the multistep resistance switching. Temperature-controlled X-ray diffraction (XRD) measurements suggested the coexistence of regions with a variety of transition temperatures, which is considered to be closely related to the appearance of the multistep resistance switching. The multistep resistance switching in the VO2-based planar device is promising not only for functional electronic device applications but also as a fundamental research for switching mechanism in oxide materials.

Changes in Lattice Parameters of VO2 Films Grown on c-Plane Al2O3 Substrates across Metal–Insulator Transition

We demonstrated lattice parameters of vanadium dioxide (VO2) films grown on Al2O3(001) substrates across metal–insulator transition using temperature-controlled X-ray diffraction measurements. Changes in lattice length for both the monoclinic and tetragonal phases were shown against temperature. Films prepared by pulsed laser deposition (PLD) and reactive sputtering were examined to discuss the dependence of the transition properties on lattice parameters. The expansion of cmsin β due to smaller angle β was revealed to be a characteristic in the low-temperature monoclinic phase and to be transformed to a larger in-plane at length in the high-temperature tetragonal phase. We discussed the dependence of transition temperature on ct length in the tetragonal phase in relation to the am length in the monoclinic phase across metal–insulator transition.

Raman Spectroscopy Study of Oxygen Vacancies in PbTiO3 Thin Films Generated Heat-Treated in Hydrogen Atmosphere

Raman spectroscopy is utilized to study the evaluation of vacancies in PbTiO3 thin films subjected to hydrogen atmosphere heat treatment. The B1 mode consisted of only oxygen ion vibration was shifted to lower frequency with increasing heat treatment temperature. It was considered that the oxygen ions were lacking from PbTiO3, and that oxygen vacancies were generated. It was found that the frequency of the B1 mode is proportional to oxygen ion content. Pb ions were also lacking in the PbTiO3 thin films with oxygen ions and the number of oxygen vacancies was larger than that of Pb ions. We suggest that the Raman spectroscopy is a suitable tool for the evaluation of oxygen vacancies in PbTiO3 thin films.

Self-oscillation up to 9 MHz based on voltage triggered switching in VO2/TiN point contact junctions

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 recove...

Large modification in insulator-metal transition of VO2 films grown on Al2O3 (001) by high energy ion irradiation in biased reactive sputtering

High energy ion irradiation in biased reactive sputtering enabled significant modification of insulator-metal transition (IMT) properties of VO2 films grown on Al2O3 (001). Even at a high biasing voltage with mean ion energy of around 325 eV induced by the rf substrate biasing power of 40 W, VO2 film revealed low IMT temperature (TIMT) at 309 K (36 °C) together with nearly two orders magnitude of resistance change. Raman measurements from −193 °C evidenced that the monoclinic VO2 lattice begins to transform to rutile-tetragonal lattice near room temperature. Raman spectra showed the in-plane compressive stress in biased VO2 films, which results in shortening of V–V distance along a-axis of monoclinic structure, aM-axis (cR-axis) and thus lowering the TIMT. In respect to that matter, significant effects in shortening the in-plane axis were observed through transmission electron microscopy observations. V2p3/2 spectra from XPS measurements suggested that high energy ion irradiation also induced oxygen vacan...

Pump–probe STM light emission spectroscopy for detection of photo-induced semiconductor–metal phase transition of VO2

We attempted to observe pump-probe scanning tunneling microscopy (STM)-light emission (LE) from a VO2 thin film grown on a rutile TiO2(0 0 1) substrate, with an Ag tip fixed over a semiconducting domain. Laser pulses from a Ti:sapphire laser (wavelength 920 nm; pulse width less than 1.5 ps) irradiated the tip-sample gap as pump and probe light sources. With a photon energy of 2.7 eV, suggesting phase transition from semiconducting monoclinic (M) to metallic rutile (R) phases in relation to the electronic band structure, faint LE was observed roughly 30 ps after the irradiation of the pump pulse, followed by retention for roughly 20 ps. The incident energy fluence of the pump pulse at the gap was five orders of magnitude lower than the threshold value for reported photo-induced M-R phase transition. The mechanism that makes it possible to reduce the threshold fluence is discussed.

Selective scanning tunneling microscope light emission from rutile phase of VO2.

We observed scanning tunneling microscope light emission (STM-LE) induced by a tunneling current at the gap between an Ag tip and a VO2 thin film, in parallel to scanning tunneling spectroscopy (STS) profiles. The 34 nm thick VO2 film grown on a rutile TiO2 (0 0 1) substrate consisted of both rutile (R)- and monoclinic (M)-structure phases of a few 10 nm-sized domains at room temperature. We found that STM-LE with a certain photon energy of 2.0 eV occurs selectively from R-phase domains of VO2, while no STM-LE was observed from M-phase. The mechanism of STM-LE from R-phase VO2 was determined to be an interband transition process rather than inverse photoemission or inelastic tunneling processes.

Pump–probe scanning-tunneling-microscope light-emission spectroscopy of Sb2Te3

We have investigated the temporal behavior of Sb2Te3 after irradiation with picosecond (ps) pump-laser pulses using scanning-tunneling-microscope (STM) light emission synchronized with ps-probe-laser pulses delayed by times t after individual pump pulses. We determined the gap energy Δ E at the F point in the band diagram of Sb2Te3 from the STM light-emission spectra as a function of the delay time t. We found that Δ E increased monotonically with t from the original value of 1.62 eV for a delay time of 0 to 15.3 ps and it decreased toward that original value (1.62 eV) for t in the time span between 15.3 and 28 ps; we saw no change in Δ E for t ≥ 28 ps. By comparing this t-dependence of Δ E with the dielectric functions of the crystalline and amorphous phases of Sb2Te3, we have concluded that the phase transition from the crystalline phase toward the amorphous phase is induced by the pump pulses. The phonon energy of the A2u mode is clearly seen in the STM light-emission spectra when Δ E is 1.62 eV, but not when Δ E is increased above 1.62 eV by pump-pulse irradiation. This feature appears to be consistent with the Raman signals, which are much stronger for the crystalline phase than for the amorphous phase.We have investigated the temporal behavior of Sb2Te3 after irradiation with picosecond (ps) pump-laser pulses using scanning-tunneling-microscope (STM) light emission synchronized with ps-probe-laser pulses delayed by times t after individual pump pulses. We determined the gap energy Δ E at the F point in the band diagram of Sb2Te3 from the STM light-emission spectra as a function of the delay time t. We found that Δ E increased monotonically with t from the original value of 1.62 eV for a delay time of 0 to 15.3 ps and it decreased toward that original value (1.62 eV) for t in the time span between 15.3 and 28 ps; we saw no change in Δ E for t ≥ 28 ps. By comparing this t-dependence of Δ E with the dielectric functions of the crystalline and amorphous phases of Sb2Te3, we have concluded that the phase transition from the crystalline phase toward the amorphous phase is induced by the pump pulses. The phonon energy of the A2u mode is clearly seen in the STM light-emission spectra when Δ...