Md. Suruz Mian

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...

Effects of energetic substrate-incident ions on the growth of crystalline vanadium dioxide films in inductively coupled plasma-assisted sputtering

We report on the effects of energetic ions incident to a substrate on the growth of vanadium dioxide (VO2) films on conductive layers by inductively coupled plasma (ICP)-assisted sputtering (ICPS). Ion energy distributions (IEDs) of Ar+ were measured using an electrostatic energy analyzer consisting of three meshed plates. Ions with kinetic energies up to 150 eV with peak positions corresponding to the plasma space potential were observed in ICP-assisted sputtering, in contrast with lower energies in conventional sputtering. Crystalline VO2 films with an insulator-to-metal transition (IMT) exhibiting resistance change over 2 orders of magnitudes at around 70 °C were successfully grown on conductive layers of titanium (Ti) and indium tin oxide (ITO) even at a low substrate temperature of 250 °C. Characteristic IMTs with multistep abrupt changes and hysteresis width of around 30 °C were first realized for polycrystalline VO2 film with the M2 phase grown on ITO layers on glass substrates. Through the analyses of surface morphology and stress states, we elucidated that high-energy ion irradiation strongly assists the low-temperature (<250 °C) crystalline growth of VO2, while it is, at the same time, accompanied by high growth stress.

Effect of conductive TiN buffer layer on the growth of stoichiometric VO2 films and the out-of-plane insulator–metal transition properties

A TiN buffer film is used with a conductive interfacial layer for stoichiometric vanadium dioxide (VO2) film growth, creating a layered device with a VO2 insulator–metal transition. Low-temperature growth (<250 °C) of the VO2 film on a Ti layer on a Si substrate is achieved using inductively coupled plasma-assisted sputtering. It is found that Ti diffusion and oxidation degrades the VO2 film quality at higher temperatures, but the introduction of a TiN buffer layer suppresses the degradation and enables growth of a stoichiometric VO2 film even at 400 °C. The high resistance of the VO2 film grown on the TiN layer suggests the benefit of using the intrinsic insulator–metal transition of VO2. The voltage-triggered switching properties of the layered devices are examined, and the cause of the high out-of-plane resistance in this layered structure is discussed based upon the dependence of the initial resistance as a function the electrode area.

Low-temperature oriented growth of vanadium dioxide films on CoCrTa metal template on Si and vertical metal–insulator transition

The authors achieved oriented growth of vanadium dioxide (VO2) films on CoCrTa metal template grown on an Si substrate. Low-temperature (∼250 °C) deposition of VO2 films using inductively coupled-plasma-assisted sputtering technique realized an abrupt interface between VO2 and CoCrTa layers, suppressing the oxidation and diffusion of metal components. The films revealed a metal–insulator transition with resistance change of over 2 orders of magnitude. The CoCrTa film, in which Co hexagonal crystalline grains with c-axis orientation were surrounded by segregated Cr and Ta, serves for the oriented growth of VO2 crystalline film, enabling higher orders of transition in resistance and low voltage switching, even for the vertical (out-of-plane) direction.