Abrupt metal–insulator transition observed in VO2 thin films induced by a switching voltage pulse

Abstract

An abrupt metal-insulator transition (MIT) was observed in VO2 thin films during the application of a switching voltage pulse to two-terminal devices. Any switching pulse over a threshold voltage for the MIT of 7.1 V enabled the device material to transform efficiently from an insulator to a metal. The characteristics of the transformation were analyzed by considering both the delay time and rise time of the measured current response. The extrapolated switching time of the MIT decreased down to 9 ns as the external load resistance decreased to zero. Observation of the intrinsic switching time of the MIT in the correlated oxide films is impossible because of the inhomogeneity of the material; both the metallic state and an insulating state co-exist in the measurement volume. This indicates that the intrinsic switching time is in the order of less than a nanosecond. The high switching speed might arise from a strong correlation effect (Coulomb repulsion) between the electrons in the material.Vanadium dioxide, VO 2 , possesses a first-order metalinsulator transition (MIT) making it an attractive material for switching devices.1-3 The MIT occurs near 68 • C and is accompanied by a structural phase transition. Various transition properties have been studied such as crystal structure and other physical quantities. In particular, aspects of the transition have been examined during thermal and optical inducements.3-7 It is also known that a negative differential resistance (NDR) is observed when the current-voltage characteristic of this material is controlled by a static current. 8,9 Such an experiment allows the measurement of the MIT with respect to temperature. The current-controlled NDR has been widely investigated for the various compounds of vanadium oxide. 8-10The resistance of the systems abruptly changes at the transition point in contrast to the NDR properties of a conventional semiconductor system. The MIT behavior controlled by a static current is extremely stable and reversible.9,10 Up until now, controversy over the mechanism of this transition has existed. It is not known whether the transition is due to thermal or electronic effects.Recent research favors the electronic model. Observations of the sample stability and the current injected to initiate the MIT support this view.9,10 We have reported a stable MIT induced by a constant applied electric field in highly oriented VO 2 films and revealed the mechanism of the MIT to be based upon electron-electron correlation using a Raman study of the planar devices.11 The transition speed of the MIT in VO 2 films has been reported to be below a picosecond through the use of ultrafast optical techniques.12 If the field-induced MIT occurs quickly enough to apply to high speed devices and shows a reproducible behavior, there are various applications for VO 2 in switching devices such as electrical switches, modulators, and electro-optical devices. Therefore, it is very important to observe the time dependence of the fieldinduced MIT and also to study the transient...

Electrical switching dynamics and broadband microwave characteristics of VO2 radio frequency devices

“…2a A notable aspect of the I-V curves measured on our devices is that the electrically-driven resistance change (ΔR E = R(Off)/R(On)) is about three orders of magnitude across the E-MIT, essentially equivalent to the thermally-driven resistance change (ΔR T ) measured in R-T from room temperature to 100 °C. The ΔR E observed is significantly larger than most literature reports, [2][3][4][5][6][7][8] as noted in the introduction, and the comparable ΔR E and ΔR T values suggest that the full VO 2 device volume is being switched. In the above cited references, film thicknesses and metrology for determining ΔR E are similar to our study, and thus it is reasonable to make a direct comparison.…”

“…Vanadium dioxide (VO 2 ) is one such system that exhibits a sharp metal-insulator phase transition (MIT) as a function of temperature at ~67 °C in bulk crystals. Coinciding with the electronic transition is a structural transformation from monoclinic in the room temperature insulating phase to rutile in the high temperature metallic phase.…”

Memory Metamaterials

“…VO 2 is a correlated electron material that exhibits an insulator-to-metal (IMT) phase transition which can be thermally (17), electrically (9,18), or optically (19) controlled. The IMT is percolative in nature and is initiated by the formation of nanoscale metallic puddles in the insulating host (20).…”

Temperature dependence of the first-order metal-insulator transition in VO2 and programmable critical temperature sensor

“…Thus the optical image shows that the current flows uniformly on the VO 2 film by applied voltage. From optical measurements, the transition time of the MIT in VO 2 has been measured to be in the subpicosecond regime [15,16] and in the order of nanosecond for an electronic device [17]. The heating model predicts that a delay time takes about 1 µsec for a device with L=3 µm and W=50 µm to become T d = T SP T where T d is a device temperature in a previous research [17].…”