Phase Transition In VO2 Films Research Articles

Full-Space Wavefront Shaping of Broadband Vortex Beam with Switchable Terahertz Metasurface Based on Vanadium Dioxide

Currently, vortex beams are extensively utilized in the information transmission and storage of communication systems due to their additional degree of freedom. However, traditional terahertz metasurfaces only focus on the generation of narrowband vortex beams in reflection or transmission mode, which is unbeneficial for practical applications. Here, we propose and design terahertz metasurface unit cells composed of anisotropic Z-shaped metal structures, two dielectric layers, and a VO2 film layer. By utilizing the Pancharatnam–Berry phase theory, independent control of a full 2π phase over a wide frequency range can be achieved by rotating the unit cell. Moreover, the full-space mode (transmission and reflection) can also be implemented by utilizing the phase transition of VO2 film. Based on the convolution operation, three different terahertz metasurfaces are created to generate vortex beams with different wavefronts in full-space, such as deflected vortex beams, focused vortex beams, and non-diffraction vortex beams. Additionally, the divergences of these vortex beams are also analyzed. Therefore, our designed metasurfaces are capable of efficiently shaping the wavefronts of broadband vortex beams in full-space, making them promising applications for long-distance transmission, high integration, and large capacity in 6G terahertz communications.

Effect of SiO2 buffer layer on phase transition properties of VO2 films fabricated by low-pressure chemical vapor deposition

Vanadium dioxide (VO2) is a promising material with semiconductor-metal phase transition for novel electronic and optic applications. One of the most widely used substrates for VO2 films is silicon (Si) covered with a silicon dioxide (SiO2) buffer layer because such a substrate is compatible with standard silicon technology. However, little attention has been paid to the effect of SiO2 buffer layer preparation and properties of the VO2/SiO2 interface to the VO2 phase-transition parameters. In the present paper, we investigate the phase-transition properties of VO2 films grown by low-pressure chemical vapor deposition on dry and wet thermally grown SiO2 buffer layers. The x-ray diffraction technique, scanning electron microscopy, and temperature-dependent resistance measurements showed a high crystal quality of the grown VO2 films. We identified the synthesis conditions for obtaining a sharp reversible semiconductor-metal phase transition in VO2 films on both SiO2 buffer layers. It was found that the resistance jump due to phase transition in VO2 films grown on dry and wet SiO2 was 6.5 × 103 and 5.6 × 103 with the hysteresis widths being equal to 2 and 3 °C, respectively. The calculated average size of VO2 crystallites on the dry SiO2 buffer layer proved to be 1.5–2 times larger than that on the wet SiO2 buffer layer. The field-effect measurements showed that the electric-current modulation in VO2 films synthesized on dry SiO2 is almost ten times higher than on wet SiO2 due to different interface layer properties. For 250 nm VO2 film on dry SiO2, the current modulation value reached 0.6%. The results of this work may be useful for the fabrication of new VO2-based functional devices compatible with standard silicon technology.

Tunable delay line based on balanced coupled resonators cladded with VO2 films.

A delay line consisting of a balanced side-coupled integrated spaced sequence of resonators and phase change material VO2 films is employed to realize continuously tunable delays with ultrafast response and low distortion. Simulation results show that a tunable delay of up to 80 ps with a 10% broadening, 150 GHz bandwidth, and 0.087 dB/ps delay loss is achieved from this structure. Taking advantage of photoinduced phase transition of VO2 films, this device obtains a switching time of less than 0.6 ps and effective compensation for group delay dispersion. This delay line shows advantages in the high-bit-rate all-optical processing systems.

Correlation Mechanism of the Insulator–Metal Transition in V2O3 Films

A complex character of the mechanism of thermal phase transformations from the insulating phase to the metallic phase has been revealed in thin V2O3 films. The insulator–metal phase transition in V2O3 is shown to consist of two stages: the hysteresis-less temperature-extended electron Mott transition extended in temperature and the stepwise structural Peierls transition with temperature hysteresis. The features of the insulator–metal phase transition revealed for V2O3 are discussed. These features are analyzed on the base of their comparison with characteristic features of analogous phase transition in VO2 films.

Dielectric Spectroscopy and Features of the Mechanism of the Semiconductor–Metal Phase Transition in VO2 Films

In the range of 0.1–106 Hz, the temperature-induced transformation of the frequency dependences of the dielectric-loss tangent tanδ(f) as well as the Cole–Cole diagrams for undoped vanadium-dioxide films are investigated. The measurements are carried out in the temperature range T = 273–373 K. It is shown that the shape of the Cole–Cole diagrams for all films depends slightly on the temperature in the specified interval, while the frequencies f0 corresponding to the peaks of the function tanδ(f) increase with temperature. The thermal-hysteresis loops of the frequency positions f0(T) of the peaks are measured. When interpreting the data of dielectric spectroscopy, a complex equivalent electrical circuit of the sample is used; it makes it possible to detect the presence of two types of grains with different electrical properties in undoped VO2 films. The presence of two types of grains determines the features of the semiconductor–metal phase-transition mechanism in VO2 films.

Electrically triggered dual-band tunable terahertz metamaterial band-pass filter based on Si3N4–VO2–Si3N4 sandwich**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574059, 61565004, and 11774288), the National Technology Major Special Project, China (Grant No. 2017ZX02101007-003), the Natural Science Foundation of Guangxi, China (Grant Nos. 2015GXNSFDA139039 and 2017GXNSFBA198116), the Foundation from Guangxi Key Laboratory of Automatic Detection Technology and

We experimentally demonstrate an electrically triggered terahertz (THz) dual-band tunable band-pass filter based on Si3N4–VO2–Si3N4 sandwich-structured hybrid metamaterials. The insulator–metal phase transition of VO2 film is induced by the Joule thermal effect of the top metal layer. The finite-integration-time-domain (FITD) method and finite element method (FEM) are used for numerical simulations. The sample is fabricated using a surface micromachining process, and characterized by a THz time-domain-spectrometer (TDS). When the bias current is 0.225 A, the intensity modulation depths at two central frequencies of 0.56 THz and 0.91 THz are about 81.7% and 81.3%, respectively. This novel design can achieve dynamically electric–thermo–optic modulation in the THz region, and has potential applications in the fields of THz communications, imaging, sensing, and astronomy exploration.

Active macroscale visible plasmonic nanorod self-assembled monolayer

Although plasmonic nanostructure has attracted widespread research interest in recent years, it is still a major challenge to realize large-scale active plasmonic nanostructure operation in the visible optical frequency. Herein, we demonstrate a heterostructure geometry comprising a centimeter-scale Au nanoparticle monolayer and VO2 films, in which the plasmonic peak is inversely tuned between 685 nm and 618 nm by a heating process since the refractive index will change when VO2 films undergo the transition between the insulating phase and the metallic phase. Simultaneously, the phase transition of VO2 films can be improved by plasmonic arrays due to plasmonic enhanced light absorption and the photothermal effect. The phase transition temperature for Au/VO2 films is lower than that for bare VO2 films and can decrease to room temperature under the laser irradiation. For light-induced phase transition of VO2 films, the laser power of Au/VO2 film phase transition is ∼28.6% lower than that of bare VO2 films. Our work raises the feasibility to use active plasmonic arrays in the visible region.

Holes doping effect on the phase transition of VO2 film via surface adsorption of F4TCNQ molecules

The holes doping effect on the metal–insulator transition (MIT) behavior of VO2 film is investigated via the tetrafluorotetracyanoquinodimethane (F4TCNQ) molecules adsorption induced surface charge transfer. Comparing with the MIT process of pristine VO2 film, a critical temperature decrease of about 4 °C for the F4TCNQ covered VO2 sample is observed. The MIT depression mechanism is deeply investigated based on detailed experiments including synchrotron radiation photon electronic spectroscopy (SRPES), X-ray absorption near-edge structure (XANES) spectroscopy and variable temperature Raman spectroscopy. Results indicate that the electronic structures of F4TCNQ covered VO2 sample are changed clearly due to the effective holes doping. In addition, the doped holes also change the V 3d orbital occupancy and weaken the electron–electron correlation as well, lowering the crystalline stability energy. Both of the above effects are in favor of triggering the earlier occurrence of MIT, resulting in the decrease of critical temperature.

Avalanche breakdown and self-stabilization effects in electrically driven transition of carbon nanotube covered VO2 film

Electrical-driven metal–insulator transition (MIT) is quite vital and widespread for both applications and MIT mechanism of VO2. In this report, we discussed the avalanche breakdown and self-stabilization effects behind the electrically driven phase transition in macro-scale carbon nanotube covered VO2 film to further understand the phase transition behaviors as well as explore promising electrical-driven VO2 devices. It was found that the Joule heat was the main source to trigger the phase transition of VO2 film. However, the time-dependent triggering routes were quite different, since the avalanche behavior was observed under the voltage-driven mode, while the gradual self-stabilization existed in the current-driven mode. The simulation results based on the proposed thermodynamics models were in good agreement with the experimental phenomena, which were basically originated from the intrinsic first-order phase transition properties of VO2 film.

Comprehensive studies of interfacial strain and oxygen vacancy on metal–insulator transition of VO2 film

As a typical strong correlation material, vanadium dioxide (VO2) has attracted wide interest due to its particular metal–insulator transition (MIT) property. However, the relatively high critical temperature (Tc) of ~68 °C seriously hinders its practical applications. Thus modulating the phase transition process and decreasing the Tc close to room temperature have been hot topics for VO2 study. In the current work, we conducted a multi-approach strategy to control the phase transition of VO2 films, including the interfacial tensile/compressive strain and oxygen vacancies. A synchrotron radiation reciprocal space mapping technique was used to directly record the interfacial strain evolution and variations of lattice parameters. The effects of interfacial strain and oxygen vacancies in the MIT process were systematically investigated based on band structure and d-orbital electron occupation. It was suggested that the MIT behavior can be modulated through the combined effects of the interfacial strain and oxygen vacancies, achieving the distinct phase transition close to room temperature. The current findings not only provide better understanding for strain engineering and oxygen vacancies controlling phase transition behavior, but also supply a combined way to control the phase transition of VO2 film, which is essential for VO2 film based device applications in the future.

Hybrid metamaterial switching for manipulating chirality based on VO2 phase transition.

Polarization manipulations of electromagnetic waves can be obtained by chiral and anisotropic metamaterials routinely, but the dynamic and high-efficiency modulations of chiral properties still remain challenging at the terahertz range. Here, we theoretically demonstrate a new scheme for realizing thermal-controlled chirality using a hybrid terahertz metamaterial with embedded vanadium dioxide (VO2) films. The phase transition of VO2 films in 90° twisted E-shaped resonators enables high-efficiency thermal modulation of linear polarization conversion. The asymmetric transmission of linearly polarized wave and circular dichroism simultaneously exhibit a pronounced switching effect dictated by temperature-controlled conductivity of VO2 inclusions. The proposed hybrid metamaterial design opens exciting possibilities to achieve dynamic modulation of terahertz waves and further develop tunable terahertz polarization devices.

Control of the metal-insulator transition in VO₂ epitaxial film by modifying carrier density.

External controlling the phase transition behavior of vanadium dioxide is important to realize its practical applications as energy-efficient electronic devices. Because of its relatively high phase transition temperature of 68 °C, the central challenge for VO2-based electronics, lies in finding an energy efficient way, to modulate the phase transition in a reversible and reproducible manner. In this work, we report an experimental realization of p-n heterojunctions by growing VO2 film on p-type GaN substrate. By adding the bias voltage on the p-n junction, the metal-insulator transition behavior of VO2 film can be changed continuously. It is demonstrated that the phase transition of VO2 film is closely associated with the carrier distribution within the space charge region, which can be directly controlled by the bias voltage. Our findings offer novel opportunities for modulating the phase transition of VO2 film in a reversible way as well as extending the concept of electric-field modulation on other phase transition materials.

Characteristics of phase transition of VO2 films grown on TiO2 substrates with different crystal orientations

Vanadium dioxide (VO2) films were synthesized on two-side polished titanium dioxide (TiO2) substrates of five different crystal orientations, (001), (100), (101), (110), and (111), through pulsed laser deposition. X-ray diffraction measurements suggested that epitaxial VO2 films with good crystallinity were grown on TiO2. Transmission electron microscopy measurements showed the thickness of VO2 films to be about 50nm and revealed the presence of an inter-mixing layer of ~10nm thickness at the interface between VO2 and TiO2. A metal-insulator transition (MIT) showing a change in resistance of 3–4 orders of magnitude was observed in all samples. The MIT temperature (TMI) showed a significant variation with crystal orientation: the highest value of TMI was 350K in VO2/TiO2(001) and the lowest value was 310K in VO2/TiO2(110), and VO2 films for the (111), (101), and (100) orientations exhibited TMI ~315K, 330K, and 340K, respectively. For infrared light, the change in the optical transmittance by the MIT was about 60%.

高调制度光致相变特性氧化钒薄膜太赫兹时域频谱研究

高调制度光致相变特性氧化钒薄膜太赫兹时域频谱研究

Anomalous optical switching and thermal hysteresis during semiconductor-metal phase transition of VO2 films on Si substrate

We present a detailed infrared study of the semiconductor-to-metal transition (SMT) in a vanadium dioxide (VO2) film deposited on silicon wafer. The VO2 phase transition is studied in the mid-infrared region by analyzing the transmittance and the reflectance measurements, and the calculated emissivity. The reflectance has been measured in two configurations: from the side of the VO2 film and from that of Si wafer. The results show a strong asymmetry between the emissivity in the two configurations, and the fact that the emissivity dynamic range from the silicon side is twice as large than that from the VO2 side. The temperature behaviour of the emissivity during the SMT put into evidence the phenomenon of the anomalous absorption in VO2, which has been explained by applying the Maxwell Garnett effective medium approximation theory.

Photoinduced insulator-metal phase transition and the metallic phase propagation in VO2 films investigated by time-resolved terahertz spectroscopy

The particle size and film thickness dependence of the photoinduced insulator-metal phase transition in VO2 films has been studied systematically by time-resolved terahertz spectroscopy at room temperature. It is found that the dynamical photoinduced phase transition from insulator to metal consists of two processes: a 1.7 ps fast process and a slow process with a typical time constant of 40 ps. Both of the two processes show particle size independence. The 40 ps slow process is revealed to arise from the longitudinal propagation of the metallic phase from the photoexcited surface to the interior of the VO2 film. A phase boundary propagation speed with a magnitude of ∼2400 m/s is obtained, which is close to the velocity of sound in solid materials and coincides with the prediction of diffusionless phase transformation. Our experimental results clearly establish the entire procedure of photoinduced phase change in the VO2 film.

Active near infrared linear polarizer based on VO2 phase transition

We demonstrate via the numerical simulation the modulation of the transmission of visible and near-infrared light through a hybrid grating structure containing a gold metallic wire-grid grating patterned on a thermochromic vanadium dioxide (VO2) thin film. We find that the transmission through the subwavelength slits of this hybrid device is significantly modulated in the near-infrared region due to the phase transition of VO2 film from insulator to metal when the temperature increases from 30 °C to 85 °C. By optimizing the hybrid-grating architecture parameters, the best switching efficiency of our hybrid grating at 1.5 μm reaches to 40%, and extinction ratio is larger than 1013. The capability of switching on/off and fine tuning of the thick hybrid grating parameters make our hybrid-grating structure a promising candidate of an active broadband linear polarizer with high switching efficiency and extinction ratio.

Nature of the electronic component of the thermal phase transition in VO2 films

By the construction of a full grid of minor hysteresis loops and the step heating and cooling of vanadium dioxide films, it was found that the thermal semiconductor-metal phase transition in vanadium dioxide is a multistage process. It is shown that the transition consists of two inherently different transitions, i.e., electronic and structural ones. Both transitions occur simultaneously and trigger each other in a wide temperature range. More specifically, a continuous electronic transition occurs in the range 0–140°C; in the range 20–80°C, a series of discontinuous structural phase transformations occurs in film nanocrystallites at various fixed temperatures. The temperatures of the structural transformations are related to nanocrystallite sizes. An essentially new form of the elementary hysteresis loop of an individual nanocrystallite is constructed. The properties of minor hysteresis loops of the VO2 film’s reflectance are analyzed; based on these results, the electronic component of the semiconductor-metal phase transition is identified and studied.

Terahertz transmission characteristics across the phase transition in VO2 films deposited on Si, sapphire, and SiO2 substrates

Vanadium dioxide (VO2) films were deposited on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method. The effect of the substrate on the structure, morphology, and phase transition properties of the VO2 films was demonstrated. We proposed that the film-substrate interaction induced the differences in the fraction of the +4 valence state vanadium oxide phase, surface morphology, and grain size for the VO2 films. The VO2 film on the Si substrate exhibited a switching property of about 2 orders of change in electrical resistivity. By contrast, the VO2 films on the sapphire and SiO2 substrates exhibited a switching property of about 3 orders of change in resistivity. The THz transmission across the phase transition in the VO2 films was quite different in the transmission modulation ratio, the width, and the slope of the hysteresis loop. In particular, the VO2 films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46% comparing with about 35% in the VO2 film on the Si substrate. Furthermore, the VO2 film on the SiO2 substrate exhibits the widest hysteresis loop with the steepest slope.

Microspectroscopic detection of local conducting areas generated by electric-pulse-induced phase transition in VO2 films

Insulator-metal phase transitions were induced in VO2∕Al2O3 films by applying a voltage pulse to a micrometric junction. We investigated the mechanism involved both spatially and temporally using midinfrared microspectroscopy. At the phase transition, we found that multilevel current switching occurred, a behavior typical of numerous compounds showing electric-pulse-induced resistance switching. We were able to demonstrate that in the case of our VO2 film, the multilevel switching was directly related to the formation of local conducting paths, which originated from sample inhomogeneity.