Insulator-to-metal Phase Transition Research Articles

Temperature-dependent optical constants of vanadium dioxide thin films deposited on polar dielectrics

Coating polar dielectrics with thin film vanadium dioxide (VO2) enables one to exploit the temperature-dependent phase transition within VO2 to actively tune and modulate surface phonon polaritons at mid-infrared. However, controlling the behavior of such systems requires intimate knowledge of the temperature-dependent optical constants of VO2, which depend greatly on the growth conditions and substrate material. Here, we accurately determine the complex optical constants of VO2 on polar dielectrics across the insulator-to-metal phase transition (IMT) using only normal incident reflectance spectra by analyzing the reflectance with Kramers–Kronig relations and thin film Fresnel equations. This non-ellipsometry technique offers an advantage in determining the refractive index using a standard infrared spectrometer.

Highly Tunable Negative Differential Resistance Device Based on Insulator-to-Metal Phase Transition of Vanadium Dioxide.

Negative differential resistance (NDR) based on the band-to-band tunneling (BTBT) mechanism has recently shown great potential in improving the performance of various electronic devices. However, the applicability of conventional BTBT-based NDR devices is restricted by their insufficient performance due to the limitations of the NDR mechanism. In this study, we develop an insulator-to-metal phase transition (IMT)-based NDR device that exploits the abrupt resistive switching of vanadium dioxide (VO2) to achieve a high peak-to-valley current ratio (PVCR) and peak current density (Jpeak) as well as controllable peak and valley voltages (Vpeak/valley). When a phase transition is induced in VO2, the effective voltage bias on the two-dimensional channel is decreased by the reduction in the VO2 resistance. Accordingly, the effective voltage adjustment induced by the IMT results in an abrupt NDR. This NDR mechanism based on the abrupt IMT results in a maximum PVCR of 71.1 through its gate voltage and VO2 threshold voltage tunability characteristics. Moreover, Vpeak/valley is easily modulated by controlling the length of VO2. In addition, a maximum Jpeak of 1.6 × 106 A/m2 is achieved through light-tunable characteristics. The proposed IMT-based NDR device is expected to contribute to the development of various NDR devices for next-generation electronics.

Origin of the current-driven breakdown in vanadium oxides: Thermal versus electronic

We report the existence of two competing mechanisms in the current-driven electrical breakdown of vanadium sesquioxide (${\mathrm{V}}_{2}{\mathrm{O}}_{3}$) and vanadium dioxide ($\mathrm{V}{\mathrm{O}}_{2}$) nanodevices. Our experiments and simulations show that the competition between a purely electronic (PE) mechanism and an electrothermal (ET) mechanism, suppressed in nanoscale devices, explains the current-driven insulator-to-metal phase transition (IMT). We find that the relative contribution of PE and ET effects is dictated by thermal coupling and resistivity, a discovery which disambiguates a long-standing controversy surrounding the physical nature of the current-driven IMT in vanadium oxides. Furthermore, we show that the electrothermally driven IMT occurs through a nanoscopic surface-confined filament. This nanoconfined filament has a very large thermal gradient, thus generating a large Seebeck-effect electric field.

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

Vanadium dioxide (VO2) is a correlated material exhibiting a sharp insulator-to-metal phase transition (IMT) caused by temperature change and/or bias voltage. We report on the demonstration of electrically triggered radio-frequency (RF) switches based on epitaxial VO2 thin films. The highly epitaxial VO2 and SnO2 template layer was grown on a (001) TiO2 substrate by pulsed laser deposition (PLD). A resistance change of the VO2 thin films of four orders of magnitude was achieved with a relatively low threshold voltage, as low as 13 V, for an IMT phase transition. VO2 RF switches also showed high-frequency responses of insertion losses of −3 dB at the on-state and return losses of −4.3 dB at the off-state over 27 GHz. Furthermore, an intrinsic cutoff frequency of 17.4 THz was estimated for the RF switches. The study on electrical IMT dynamics revealed a phase transition time of 840 ns.

Active directional switching of surface plasmon polaritons using a phase transition material

Active switching of near-field directivity, which is an essential functionality for compact integrated photonics and small optoelectronic elements, has been challenging due to small modulation depth and complicated fabrication methods for devices including active optical materials. Here, we theoretically and experimentally realize a nanoscale active directional switching of surface plasmon polaritons (SPPs) using a phase transition material for the first time. The SPP switching device with noticeable distinction is demonstrated based on the phase transition of vanadium dioxide (VO2) at the telecom wavelength. As the insulator-to-metal phase transition (IMT) of VO2 induces the large change of VO2 permittivity at telecom wavelengths, the plasmonic response of a nanoantenna made of VO2 can be largely tuned by external thermal stimuli. The VO2-insulator-metal (VIM) nanoantenna and its periodic array, the VIM metagrating, are suggested as optical switches. The directional power distinction ratio is designed to change from 8.13:1 to 1:10.56 by the IMT and it is experimentally verified that the ratio changes from 3.725:1 to 1:3.132 as the VIM metagratings are heated up to 90 °C. With an electro-thermally controllable configuration and an optimized resonant design, we expect potential applications of the active switching mechanism for integrable active plasmonic elements and reconfigurable imaging.

Surface plasmon resonance modulation in nanopatterned Au gratings by the insulator-metal transition in vanadium dioxide films.

Correlated experimental and simulation studies on the modulation of Surface Plasmon Polaritons (SPP) in Au/VO2 bilayers are presented. The modification of the SPP wave vector by the thermally-induced insulator-to-metal phase transition (IMT) in VO2 was investigated by measuring the optical reflectivity of the sample. Reflectivity changes are observed for VO2 when transitioning between the insulating and metallic states, enabling modulation of the SPP in the Au layer by the thermally induced IMT in the VO2 layer. Since the IMT can also be optically induced using ultrafast laser pulses, we postulate the viability of SPP ultrafast modulation for sensing or control.

Optical properties of vanadium oxides-an analysis

In this study, the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates, have been analyzed from infrared to vacuum ultraviolet range (up to 12 eV). Utilizing the available data of wavelength dependent optical constants of these materials in the literature, the energy corresponding to the peaks in the imaginary part of the dielectric function (e2–R spectra), have been interpreted and compared as a function of structure, polarization, and temperature. The energies corresponding to the peaks in reflectivity-energy (R–E) spectra are explained in terms of the Penn gap (Ep). Ep values for VO2 and V2O5 are close to the average of energies corresponding to the peaks (\( \bar{E} \)) while, their values are even closer in V2O3, reflecting the degree of anisotropy in the order of V2O3 < VO2 < V2O5. The first order reversible, insulator to metal phase transition (IMT) of both bulk and thin films of the V–O systems are studied as an effect of temperature change. The effective number of electrons, neff, participating in the optical transitions is described from the numerical integration using the well-known sum rule. The change in neff with respect to the energy of incident photons is also calculated and it is found that this change is consistent with the peaks observed in the e2–E spectra.