Vanadium Dioxide Thin Films Research Articles

Impact-activated programming of electro-mechanical resonators through ferroelectret nanogenerator (FENG) and vanadium dioxide

Ferroelectret Nanogenerators (FENG) devices were introduced recently as promising flexible devices for energy harvesting and microphone/loud-speaker applications. Vanadium dioxide (VO2) thin films, on the other hand, have been demonstrated to enable large frequency tunability of miniaturized electro-mechanical structures, which are commonly integrated in transceiver and communication systems. In this work, we integrate these two technologies, to show a system where an electric pulse, supplied by the FENG can be used to tune the resonant frequency of VO2-based micro-electro-mechanical structures. Furthermore, due to the VO2's hysteretic behavior, the applied pulse also programs the tuned frequency, allowing for different frequency states in the device for a single applied DC bias. It is found that the tuning of the frequency states is determined by the supplied energy, and the programming is more efficient for larger, shorter pulses –even if the duration of the pulse is shorter than the system's thermal time constant. We explore two different mechanical structures, bridge and cantilever. A wider tuning range is found for the bridge structure (22%), which is due to the larger frequency sensitivity with stress for this configuration. The tuning/programming action uses harvested mechanical energy, which could come from the user. The potential use of the developed system as an accelerometer or impact sensor for monitoring brain injuries in contact-sports is discussed.

Electrical switching and oscillations in vanadium dioxide

We have studied electrical switching with S-shaped I-V characteristics in two-terminal MOM devices based on vanadium dioxide thin films. The switching effect is associated with the metal-insulator phase transition. Relaxation oscillations are observed in circuits with VO2-based switches. Dependences of the oscillator critical frequency Fmax, threshold power and voltage, as well as the time of current rise, on the switching structure size are obtained by numerical simulation. The empirical dependence of the threshold voltage on the switching region dimensions and film thickness is found. It is shown that, for the VO2 channel sizes of 10 × 10nm, Fmax can reach the value of 300MHz at a film thickness of ~20nm. Next, it is shown that oscillatory neural networks can be implemented on the basis of coupled VO2 oscillators. For the weak capacitive coupling, we revealed the dependence of the phase difference upon synchronization on the coupling capacitance value. When the switches are scaled down, the limiting time of synchronization is reduced to Ts ~13μs, and the number of oscillation periods for the entering to the synchronization mode remains constant, Ns ~ 17. In the case of weak thermal coupling in the synchronization mode, we observe in-phase behavior of oscillators, and there is a certain range of parameters of the supply current, in which the synchronization effect becomes possible. With a decrease in dimensions, a decrease in the thermal coupling action radius is observed, which can vary in the range from 0.5 to 50µm for structures with characteristic dimensions of 0.1–5µm, respectively. Thermal coupling may have a promising effect for realization of a 3D integrated oscillatory neural network.

Thermochromic VO2 thin films on ITO-coated glass substrates for broadband high absorption at infra-red frequencies

Thin films of vanadium dioxide (VO2) are deposited on indium tin oxide (ITO), stainless steel (SS), and glass substrates using chemical vapour deposition. X-ray diffraction and Raman spectroscopy measurements confirmed the single phase nature of the VO2, which showed a phase transition from a low conducting state at low temperature(<68 °C) to a high conducting state at high temperature (>68 °C). This was confirmed by electrical conductance and infra-red reflectance measurements. X-ray photoelectron spectroscopy was used to measure the charge states of vanadium species. The optical constants of VO2 were determined using visible and near-infra red(NIR) reflectivity and show that the VO2 film on ITO has a lowered plasma frequency compared with VO2 on glass substrates. The thin films of VO2 enable a broadband of ultra-high absorption at mid-wave infra-red frequencies due to a Fabry-Pérot (F-P) like resonance due to the dielectric properties of ITO, SS, or glass. The tunability of this absorption band via VO2 thickness and the switchability by temperature makes the system attractive for absorptive coatings with controllable emissivity.

Controlling phase separation in vanadium dioxide thin films via substrate engineering

The strong electron-lattice interactions in correlated electron systems provide unique opportunities for altering the material properties with relative ease and flexibility. In this Rapid Communication, we use localized strain control via a focused-ion-beam patterning of $\mathrm{Ti}{\mathrm{O}}_{2}$ substrates to demonstrate that one can selectively engineer the insulator-to-metal transition temperature, the fractional component of the insulating and metallic phases, and the degree of optical anisotropy down to the length scales of the intrinsic phase separation in $\mathrm{V}{\mathrm{O}}_{2}$ thin films without altering the quality of the films. The effects of localized strain control on the strongly correlated electron system are directly visualized by state-of-the-art IR near-field imaging and spectroscopy techniques and x-ray microdiffraction measurements.

Effect of Oxygen Deposition Pressure on the Structure and the Metal Insulator Transition in Pulsed Laser Deposited VO<SUB>2</SUB> Films on Soda Lime Glass

Vanadium dioxide thin films nanostructures were synthesized by pulsed laser deposition on soda lime glass at a substrate temperature of 600°C and the effects of the oxygen deposition pressure on the crystalline structure and the phase transition characteristics of VO2 nanostructured films were investigated. The structure and microstructure of the films have been examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results indicate that the crystal structure of the films is strongly sensitive to the oxygen deposition pressure; exhibiting sharp a-axis diffraction peaks, showing a texturation along (1 0 0) plane. A detailed description of the growth mechanisms and the substrate–film interaction is given, and the characteristics of the electronic transition and hysteresis of the phase transition are described in terms of the morphology, grain boundary structure and crystal orientation. The sharpness of the transition and the hysteresis upon heating and cooling are found to be strong functions of the crystal structure and microstructure (grain size).

Roles of strain and domain boundaries on the phase transition stability of VO2 thin films

The fundamental phase transition mechanism and the stability of the semiconductor-to-metal phase transition properties during multiple thermal cycles have been investigated on epitaxial vanadium dioxide (VO2) thin films via both ex situ heating and in situ heating by transmission electron microscopy (TEM). VO2 thin films were deposited on c-cut sapphire substrates by pulsed laser deposition. Ex situ studies show the broadening of transition sharpness (ΔT) and the width of thermal hysteresis (ΔH) after 60 cycles. In situ TEM heating studies reveal that during thermal cycles, large strain was accumulated around the domain boundaries, which was correlated with the phase transition induced lattice constant change and the thermal expansion. It suggests that the degradation of domain boundary structures in the VO2 films not only caused the transition property reduction (e.g., the decrease in ΔT and ΔH) but also played an important role in preventing the film from fracture during thermal cycles.

Laser-Controlled Current Switching in Two-Terminal Devices Based on Vanadium Dioxide Thin Films Using 976 nm Laser Diode

Laser-Controlled Current Switching in Two-Terminal Devices Based on Vanadium Dioxide Thin Films Using 976 nm Laser Diode

Photothermally controlled 50 mA gating in VO2-based planar device using NIR laser diode

By utilizing a near infrared laser diode as an excitation light source, we demonstrated a photothermally controlled current gating of 50 mA in a two-terminal planar device based on a vanadium dioxide (VO2) thin film grown by a pulsed laser deposition method. The photothermally controlled on/off triggering of the device current was accomplished by controlling the output power of the laser beam that illuminated the VO2 film. The transient responses of photothermally triggered currents were analyzed when periodical laser pulses from the laser diode stimulated the VO2 device at a variety of pulse widths and repetition rates. A switching contrast between off-state and on-state currents was evaluated as ∼11905, and average rising and falling times were measured as ∼45 and ∼19 ms, respectively.

Ultrafast electron-lattice coupling dynamics in VO2 and V2O3 thin films

Ultrafast optical pump - optical probe and optical pump - terahertz probe spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the electronic oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in the metallic state of both materials. The low energy Drude response of V2O3 appears more susceptible than VO2 to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold increase in V2O3 across the phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of VO2, and suggest that a similar phenomenon might be at play in the metallic phase of V2O3. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, opening the way to similar studies in other complex materials.

Voltage-induced switching dynamics based on an AZO/VO2/AZO sandwiched structure

A vanadium dioxide (VO2) thin film was prepared on an Al-doped ZnO (AZO) conductive glass substrate by DC magnetron sputtering and a post-annealing process. The AZO/VO2/AZO sandwiched structure was fabricated on the VO2/AZO composite film using photolithography and a chemical etching process. The composition, microstructure and optical properties of the VO2/AZO composite film were tested. The results showed that the VO2/AZO composite film was poly-crystalline and the AZO layer did not change the preferred growth orientation of VO2. When the voltage was applied on both of the transparent conductive layers of the AZO/VO2/AZO sandwiched structure, an abrupt change in the current was observed at different temperatures. The temperature dependence of I–V characteristic curves for the AZO/VO2/AZO sandwiched structure was analyzed. The phase transition voltage value is 7.5V at 20°C and decreases with increasing temperature.

Effects of annealing on thermochromic properties of W-doped vanadium dioxide thin films deposited by electron beam evaporation

Thermochromic vanadium dioxide (VO2) undergoes a fully reversible semiconductor-metal transition (SMT) at a critical temperature (Tt) of ~68°C with a dramatic change in electric and optical properties, which makes it an attractive candidate for its application in smart windows. Switchable VO2 and W-doped vanadium dioxide (WxV1−xO2) thin films are grown over quartz substrates via electron beam evaporation technique by using VO2/WxV1−xO2 as targets at room temperature (RT) followed by post annealing process at different temperatures. The as-deposited films are amorphous, and that transform to monoclinic VO2 (VO2(M)) with (011)-preferred orientation after annealing at 500°C under vacuum. The (011) peak of W-doped VO2 films shifts to a lower diffraction angle as compared with un-doped VO2 films which confirms the incorporation of W ions into the VO2 lattice. Temperature dependent optical transmittance measurement demonstrates the thermochromic properties, with a reduction in the phase transition temperature (Tt) as observed in W-doped VO2 films, which is attributed to the variation of electron structure in VO2 due to doping. The films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and optical transmittance measurement.

Study of Si doped VO2 thin films for solar thermal applications

A new generation of smart solar thermal collectors is based on thermochromic thin film technology. Thermochromic absorber coatings change their optical properties with temperature, particularly in the infrared range. In order to prevent overheating, the collector selectivity is deliberately hindered, through an increase of the thermal emittance above a critical temperature. The transition temperature of pure thermochromic VO2 (68°C) needs to be adjusted for solar thermal applications and to do that Si doping is attempted. Thus, pure and Si doped vanadium dioxide thin films are deposited by reactive magnetron sputtering and the films are characterized by four point probe and in-situ X-ray and UV photoelectron spectroscopic measurements. While a modest increase in the transition temperature with doping is reached (up to a critical Si concentration when the transition is lost), the insulating character of the films is also enhanced (decreased density of states at the Fermi level).

Impact of tungsten doping on the dynamics of the photo-induced insulator-metal phase transition in VO2 thin film investigated by optical pump-terahertz probe spectroscopy

The influence of tungsten (W) doping on the ultrafast dynamics of the photo-induced insulator-metal phase transition (IMT) is investigated at room temperature in epitaxially grown vanadium dioxide (VO2) thin films by means of optical pump-terahertz (THz) probe spectroscopy. It is observed that the THz transmission variation of the films across the IMT follows a bi-exponential decrease characterized by two time constants, one corresponding to a fast process and the other to a slower process. W-doping (i) reduces the photo-excitation fluence threshold required for triggering the IMT, (ii) accelerates the slow process, and (iii) increases the THz transient transmission variation for corresponding fluences. From the Drude-Smith model, it is deduced that a strong carrier confinement and an enhancement of the transient conductivity occur across the IMT. The IMT is also accompanied by an increase in the carrier concentration in the films, which is enhanced by W-doping. Our results suggest that W-doped VO2 could be advantageously exploited in applications such as ultrafast THz optical switching and modulation devices.

Natural metamaterial behavior across the phase transition for WxV1−xO2 films revealed by terahertz spectroscopy

Metamaterials, which are made of repeated patterns of appropriately arranged small discrete structures, display unusual electromagnetic properties that overwhelm those of conventional materials. The modification of their properties is generally achieved by arranging the structures mechanically or electrically and requires rather complex designs. We report on the study of the complex conductivity of epitaxially-grown tungsten-doped vanadium dioxide (WxV1−xO2) thin films through the semiconductor-to-metal phase transition (SMT) using terahertz time-domain spectroscopy. The modelling of the terahertz conductivity across WxV1−xO2 SMT provides clear insights about the gradual nucleation of VO2 metallic domains among the semiconducting host and evidences the presence of strong carrier confinement and enhanced absorption close to the transition temperature, leading to a strong capacitive response of the electrons. The evolution of the SMT is also strongly affected by W doping, which reduces the scattering time in the metallic state, lowers the transition onset temperature and extends the temperature range over which the transition occurs. The WxV1−xO2 films thus forms an effective medium in the vicinity of the SMT and display the signature of a disordered metamaterial, which has significantly enhanced functionality thanks to its readily thermally-tunable properties over a wide range of temperatures close to room temperature.

Ultrafast response flexible breath sensor based on vanadium dioxide

Real-time monitoring of breath can provide clinically relevant information about apnea syndrome and other important aspects of human physiology. Here, we introduce a flexible skin-like breath sensor developed by transfer-printing vanadium dioxide (VO2) thin films on PDMS substrates. This flexible breath sensor can conformably laminate on the skin under the nose with different curvatures and operate at different environment temperatures through day and night. Attributed to the high temperature coefficient of resistance of VO2, the enhanced breath sensing performance was demonstrated and the response time and recovery time can be as fast as 0.5 s. The excellent sensing performance and fast response time indicate that the VO2-based breath sensor is feasible in monitoring breath for prevention of apnea syndrome.

Epitaxial VO2 thin film-based radio-frequency switches with thermal activation

In this paper, we report on the demonstration of thermally triggered “normally ON” radio-frequency (RF) switches based on epitaxial vanadium dioxide (VO2) thin films with a SnO2 template on (001) TiO2 substrates. Fast insulator-to-metal phase transition of the epitaxial VO2 at a relatively low temperature allowed RF switches made of the VO2 to exhibit sharp changes in the RF insertion loss during cooling and heating at 60 °C and 66 °C, respectively. The change of RF insertion loss due to phase transition is greater than 15 dB. The VO2 RF switches also completed the transition of S21 within less than 3 °C and showed a low-loss operation frequency of up to 24.2 GHz with a low insertion loss of −1.36 dB and isolation of 17.56 dB at 12.03 GHz, respectively. The demonstration suggests that epitaxial VO2-based RF switches can be used in switching elements up to Ku-band RF circuits.

Enhancing Modulation of Thermal Conduction in Vanadium Dioxide Thin Film by Nanostructured Nanogaps

Efficient thermal management at the nanoscale is important for reducing energy consumption and dissipation in electronic devices, lab-on-a-chip platforms and energy harvest/conversion systems. For many of these applications, it is much desired to have a solid-state structure that reversibly switches thermal conduction with high ON/OFF ratios and at high speed. Here we describe design and implementation of a novel, all-solid-state thermal switching device by nanostructured phase transformation, i.e., modulation of contact pressure and area between two poly-silicon surfaces activated by microstructural change of a vanadium dioxide (VO2) thin film. Our solid-state devices demonstrate large and reversible alteration of cross-plane thermal conductance as a function of temperature, achieving a conductance ratio of at least 2.5. Our new approach using nanostructured phase transformation provides new opportunities for applications that require advanced temperature and heat regulations.

Elevated transition temperature in Ge doped VO2 thin films

Thermochromic GexV1−xO2+y thin films have been deposited on Si (100) substrates by means of reactive magnetron sputtering. The films were then characterized by Rutherford backscattering spectrometry (RBS), four-point probe electrical resistivity measurements, X-ray diffraction, and atomic force microscopy. From the temperature dependent resistivity measurements, the effect of Ge doping on the semiconductor-to-metal phase transition in vanadium oxide thin films was investigated. The transition temperature was shown to increase significantly upon Ge doping (∼95 °C), while the hysteresis width and resistivity contrast gradually decreased. The precise Ge concentration and the film thickness have been determined by RBS. The crystallinity of phase-pure VO2 monoclinic films was confirmed by XRD. These findings make the use of vanadium dioxide thin films in solar and electronic device applications—where higher critical temperatures than 68 °C of pristine VO2 are needed—a viable and promising solution.

Nanosecond polarization modulation in vanadium dioxide thin films

The polarization state of light beams reflecting off thin layers of vanadium dioxide is measured as the material undergoes an insulator-metal phase transition by optical excitation. Under certain conditions, films as thin as 50 nm are shown to rotate linear polarization by 90° in reflection on the nanosecond and potentially shorter timescales. Possible applications for high-contrast optical modulation over a wide spectral range and high bandwidths are discussed.

Effect of process parameters on phase stability and metal-insulator transition of vanadium dioxide (VO2) thin films by pulsed laser deposition

Owing to remarkable thermochromic, electrochromic, and photochromic properties which arise from a first-order phase transition from insulator to metal at 68 °C, vanadium dioxide (VO2) has an enormous range of potential applications. However, due to such narrow thermodynamic stability, there is a major challenge surrounding selective phase deposition of vanadium dioxide thin films. Here, we present a report that establishes precise conditions for the deposition of three major polymorphs of VO2 by pulsed laser deposition. By a systematic study of the synergistic effects of the deposition conditions using a robust design of experiment (Taguchi design), we are able to deposit the VO2 (M1), VO2 (T), and VO2 (A) phases onto thermal oxide (Si/SiO2) substrates with remarkable precision. By electrically probing the temperature induced phase transformation from insulator to metal of each of these polymorphs, we find not only a strong dependence on the phase, but that the nature of the deposition conditions influences the magnitude and hysteresis width of the temperature cycle.