Electrical activation of insulator-to-metal transition in vanadium dioxide single-crystal nanobeam and their high-frequency switching performances

Among the strongly correlated electron materials exhibiting insulator-to-metal transition,^1,2 vanadium dioxide (VO₂) is undoubtedly the one which has attracted the most research interest both from a fundamental point of view,³ but also for its integration potential into electronic,⁴ photonic,⁵ and neuromorphic devices.⁶ The particular attention on VO₂ comes from the large modification of its electrical and optical properties during the onset of the metal–insulator transition (MIT) occurring at relatively low temperatures (insulating behavior below $≈340$ K and metal-like characteristics above) and for the possibility to additionally trigger the state change using conveniently implemented electrical or optical stimuli.^5–11 Most of the applications reported in the scientific literature describe the use of the material in the form of thin films. Whether VO₂ thin films are epitaxial or polycrystalline, their insulator-to-metal transition (IMT) is characterized by the simultaneous coexistence of different phases and its percolative insulator–metal nature.^7,8 These peculiar properties triggered the demonstration of electrical and high-frequency devices with interesting performance.^10,11 Thus, due to their broadband electrical response, vanadium dioxide thin films were integrated as localized switching or tuning elements in devices spanning large operational frequency domains, from microwaves and millimeter waves up to terahertz waves.^10–12 Among currently employed deposition techniques for growing high-quality VO₂ thin films are pulsed laser deposition,^3,8,13 magnetron sputtering,^11,14 or reactive electron-beam evaporation.¹⁵ Chemical vapor deposition (CVD) or vapor–liquid–solid (VLS) techniques were recently applied to fabricate, in a simple way, VO₂ nano/micro-objects like wires, plates, beams, and nanoparticles, which have the particularity of being single-crystalline.¹⁶ This specificity of their crystalline structure makes it possible to overcome the random, percolative domain structures occurring in thin films, hardly compatible with production of reliable and reproducible devices.¹⁷ Initial reports on VO₂ wires or nanobeams integration in electrical devices were centered on top down approaches starting from VO₂ films and their nanometer scale structuring using electron-beam lithography.¹⁸ Still, the obtained VO₂ beams keep the multi-domain granularity of the initial thin films resulting in strain associated with grain boundaries, dislocations, or stoichiometry fluctuations.¹⁹ 

Vanadium dioxide micro- and nanowires fabricated using the bottom-up VLS-CVD method were since thoroughly investigated not only for their peculiar fundamental properties (investigation of the metal–insulator transition and the associated physics at nanoscale in pure, single-crystal objects) but also for their applications as extremely localized temperature or strain sensors,^20,21 photo-and-thermoelectric nanoscale switches,²² solid-state thermal memories,²³ or memristors with self-heating capabilities.²⁴ Since best quality VO₂ films (in terms of the magnitude of their electrical conductivity variation subsequent to the insulator-to-metal transition) are obtained on sapphire substrates at relatively high temperatures (superior to $500 °$C), this makes their CMOS (complementary metal oxide semiconductors) monolithic integration within high-frequency devices difficult. Instead, VO₂ micro and nanowires can be fabricated by the CVD method on SiO₂ layers (or substrates) and integrated afterward on a plethora of different substrates, including polymer ones.²⁵ As RF switches are key elements in reconfigurable telecommunications systems or consumer electronics, many efforts have focused lately on emerging technologies based on micro-and nanoscale devices with, potentially, improved performances than current, more conventional solutions based on CMOS or RF-MEMS technologies.²⁶ In the past, several reports studied the feasibility of using inorganic nanowires²⁷ or nanoscale memristors based on conductive filaments²⁸ as active elements for radio frequency and microwaves switches but no studies have been yet reported on VO₂ nanobeams performances at these frequency domains. Here, we report for the first time to the best of our knowledge on the evaluation of high-frequency characteristics of VO₂ nanobeams and the demonstration of simple switching devices in the microwave domain based on the thermal and electrical activation of the metal–insulator transition in vanadium dioxide single-crystal beams.

The synthesis was performed by a vapor transport method using commercial V₂O₅ powder employed as a precursor for the synthesis of VO₂ crystals, as already reported by Guiton et al.¹⁶ Nucleation and growth of nanowires takes place on the surface of a substrate of Si(100) with a thermal oxide layer (⁠ $2μ m$ thick amorphous SiO₂). The substrate and powder were placed in an alumina boat. The part of the boat containing the powder was introduced precisely in the center zone of a high temperature tube furnace. After a primary vacuum step, argon was introduced into the tube at a flow rate of 200 SCCM under a pressure of 2 Torr while the furnace was heated at $850 °$C with a speed rate of $6 °$C/min. After a 1-h hold at this temperature, the sample was naturally cooled to room temperature in the same pressure conditions. The local differences in the flow of precursor at the substrate surface lead to an inhomogeneity of the morphology and the density of VO₂ nanostructures on the same sample.²⁹ As shown in Figs. 1(a) and 1(b), synthesis can lead to the formation of nano- and micro-scale objects with shapes ranging from micro/nanowires to microplates.

XRD experiments have been performed on a Bruker D8 Discover diffractometer equipped with a parabolic multilayer mirror, a two-reflection asymmetrically cut Ge(220) monochromator (Cu $K α 1$ radiation, $λ=1.5406 A ˚$⁠) as primary optics, and a linear position sensitive detector. They were performed in a $θ−2θ$ configuration between $15 °$ and $80 °$ with a $0.01 °$ angular resolution.

Micro-diffraction Laue pattern of a VO₂ nanobeam has been obtained at the BM32-IF beamline of the European Synchrotron Radiation Facility (ESRF) using the diffraction experimental setup in which the focused electrons beam has energies in the 5–22 keV range and a spot size of $500×500$ nm $2$⁠.

To study its dc and high-frequency thermal and electrical switching properties, a single VO₂ nanobeam (14  $μ$m in length, width of 1.5  $μ$m, and height of 600 nm) was integrated between the two discontinuous parts (10  $μ$m apart) of the central signal line of a coplanar waveguide (CPW, microwave transmission line), as shown in Fig. 1(d), where the nanobeam acts as a tunable impedance in the CPW. The device designed on the SiO₂/Si substrate has a center signal conductor (S) and two outer ground conductors (G) supporting traveling electromagnetic (EM) waves, with dimensions (width of the signal line and gap between the central line and grounds) adapted to $50Ω$⁠. The device was fabricated by conventional photolithography techniques using 500 nm thick molybdenum (Mo) electrodes obtained by DC magnetron sputtering and patterned by photolithography and wet etching.

The high-frequency performances of the CPW device were evaluated in the transmission mode using a pair of ground-signal-ground (G-S-G) high-frequency probes (Cascade, with 125 mm spacing between the G-S-G individual probes) connected to a vector network analyzer (VNA) (ZVA from Rohde $&$ Schwarz).

Figure 1(c) displays the XRD data recorded for as-grown VO₂ nanowires on SiO₂/Si substrates. In the $θ−2θ$ scan, only 011 reflections from monoclinic VO₂ M1 phase and 400 reflections from Si are observed, which demonstrates that the VO₂ nanostructures grow with the (011) planes parallel to the surface of the sample. These XRD macroscale results were confirmed by structural analysis of individual nanobeams using Laue micro-diffraction measurements, which are showing the presence of both VO₂ M1 (011) and VO₂ M1 (0-11) twinning orientations as illustrated in Fig. 2.

Figure 3 displays the temperature dependence of electrical resistivity of a VO₂ nanobeam measured using the fabricated device. When the temperature rises, a first jump of resistivity is observed toward $60 °$C, corresponding to the transition between the M1 and M2 monoclinic insulating states.³⁰ Then, the resistivity gradually decreases until $100 °$C where it drops to the metallic resistivity value of the rutile R phase. For the cooling process, the path is simpler, the material is suddenly transitioning from the metallic to the insulating state M1 at a temperature of $70 °$C. The large hysteresis width of the temperature-dependent resistance and the remarkably high insulator to metal transition temperature during heating are attributed to the defect-free, single-crystalline and single domain structure of the measured nanowire.³¹ 

The high-frequency performances of the CPW device integrating the VO₂ nanobeam were evaluated in the 100 MHz–24 GHz frequency domain in the transmission mode. The transmission (⁠ $S 12$ and $S 21$⁠) and reflection (⁠ $S 11$ and $S 22$⁠) parameters at the two ports of the symmetrical CPW device (⁠ $S 12= S 21$ and $S 11= S 22$⁠) were recorded while heating the device through the VO₂ nanobeam transition temperature. Alternatively, the VO₂ material was electrically polarized at fixed, room temperature, by superposing on the RF signal a DC voltage between the two parts of the signal line using DC bias Ts.

The transmission $S 12$ parameters of the CPW device are shown in Fig. 5(a) when thermally cycled between room temperature (⁠ $24 °$C) and $105 °$C, through the insulator-to-metal transition temperature of the VO₂ single crystal. At room temperature, the device is significantly blocking the transmission of the RF signal (off state), with an isolation (attenuation of the transmitted signal) higher than 20 dB through the entire measured frequency domain. As the temperature of the device is increased over $100 °$C, the VO₂ beam becomes conductor and allows the RF waves to be transmitted through the coplanar waveguide (on state).

The hysteretic behavior of the transmission coefficient with temperature is highlighted in Figs. 5(b)–5(d) for frequencies of 5 MHz, 10 GHz, and 20 GHz, respectively. These hysteresis characteristics are similar to those obtained when measuring the resistance variation of the nanowire with temperature shown in Fig. 3.

The transmission losses in the metal-like state of the VO₂ nanobeam (better than 4 dB up to 24 GHz) can be perceived as relatively high, but this can be explained by the impedance mismatch within the gap of the CPW device and the resistance in the conductive state of the active VO₂ area, in the order of $2Ω/μ m$⁠. These losses can be further reduced to acceptable values (⁠ $<1$ dB) through more adapted CPW designs, by decreasing the distance between the two parts of the RF signal line or by stacking in parallel several VO₂ wires in order to decrease their equivalent linear resistance in the metallic state under $1Ω/μ m$⁠. The device can be modeled by a simple electrical circuit consisting of a capacitor (⁠ $C O F F$⁠) in parallel with a high-value resistance (⁠ $R O F F$⁠) when the VO₂ beam is in the insulating state and by a simple resistive impedance with a low value (⁠ $R O N$⁠) when the VO₂ material is in the conducting state.

Thus, at 24 GHz, we obtain for the single nanobeam VO₂ switching device a $FOM≈180$ fs. This value is similar or better than the state-of-the art of current semiconductor-based switching technologies.³⁹ 

In order to electrically actuate the VO₂ nanobeam within the CPW, we connected the device (via bias Ts) to a simple electrical polarization circuit (present in the inset of Fig. 6) allowing to apply a voltage between the two segments of the CPW signal line separated by the VO₂ beam.

The measured transmission $S 12$ parameters at $58 °C$ of the device submitted to a periodic square-type voltage signal (5 V amplitude, 2 Hz) are shown in Fig. 6. As the material is changing between the two states following the electrical activation, the impedance of the VO₂ beam is rapidly and periodically modified between its extreme values, reflected in the periodic modulation of the $S 12$ parameter. The switching dynamics of the device is relatively important, keeping in mind that the active part of the material is composed of only a 10  $μ$m long VO₂ nanobeam. As in the case of thermal activation, this dynamics can be further improved by using shorter VO₂ beams and decreasing the gap between the two parts of the signal line or by integrating several beams in parallel within the CPW gap for decreasing the overall $R O N$ of the device. We compare, in Table I, the performances of our device with similar RF devices integrating VO₂ thin films, extracted from relevant reports in the literature. As mentioned above, even if the design of our proposed device can be further improved to consider the limited size of the VO₂ nanowire, its switching performances compare rather well with similar VO₂-based thin film RF switches.

In conclusion, we presented the proof of concept and experimental demonstration of microwave switching using thermal and electrical activation of the insulator-to-metal transition in VO₂ single-crystal beams. Compared with VO₂ films, vanadium dioxide nanobeams have higher insulator-to-metal activation temperatures implying more stable electrical performances for larger temperature domains, up to $100 °$C. They present convenient reversible and repetitive thermal and electrical switching behavior with more than 15 dB switching amplitude between the two states in the 100 MHz–24 GHz domain. While the performances of the devices presented here can be further improved by conveniently adjusting their topologies (beam parallelization, dimensions), the demonstration of RF signals switching using VO₂ nanobeams initiates interesting opportunities for micro- and nanoscale microwave devices for efficient switching and modulation of electromagnetic signals in the microwaves and millimeter waves domains.

This work was financially supported by Agence Nationale de la Recherche (ANR) under Grant No. ANR-22-CE24-0016, CIRANO project.

The authors have no conflicts to disclose.

Ethics approval is not required.

J.-C. Orlianges: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). O. Allegret: Conceptualization (equal); Investigation (equal); Methodology (equal). E.-N. Sirjita: Investigation (equal). A. Masson: Conceptualization (equal); Investigation (equal). A. Boulle: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). V. Théry: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal). S. Tardif: Conceptualization (equal). J. S. Micha: Conceptualization (equal); Investigation (equal). A. Crunteanu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.