Abrupt Metal-insulator Transition Research Articles

Selective modulation of electronic transport in VO2 induced by 10 keV helium ion irradiation

Vanadium dioxide (VO2) manifests an abrupt metal–insulator transition (MIT) from monoclinic to rutile phases, with potential use for tunable electronic and optical properties and spiking neuromorphic devices. Understanding pathways to modulate electronic transport in VO2, as well as its response to irradiation (e.g., for space applications), is critical to better enable these applications. In this work, we investigate the selective modulation of electronic transport in VO2 films subject to different 10 keV helium ion (He+) fluences. Under these conditions, the resistivity in the individual monoclinic and rutile phases varied by 50%–200%, while the MIT transformation temperature remains constant within 4 °C independent of irradiation fluence. Importantly, different trends in the resistivity of the monoclinic and rutile phases were observed both as a function of total He fluence as well as in films grown on different substrates (amorphous SiO2/Si vs single crystal Al2O3). Through a combination of measurements including majority carrier sign via Seebeck, low frequency noise, and TEM, our investigation supports the presence of different kinds of point defects (V in; O in), which may arise due to grain boundary defect interactions. Our work suggests the utility of He irradiation for the selective modulation of VO2 transport properties for neuromorphic, in contrast to other established but non-selective methods, like doping.

Coexistence of Both Localized Electronic States and Electron Gas at Rutile TiO2 Surfaces.

Localized electron polarons formed through the coupling of excess electrons and ionic vibrations play a key role in the functionalities of materials. However, the mechanism of the coexistence of delocalized electrons and localized polarons remains underexplored. Here, the discovery of high-mobility 2D electron gas at the rutile TiO2 surfaces through argon ion irradiation induced oxygen vacancies is reported. Strikingly, the electron gas forms localized electronic states at lower temperatures, resulting in an abrupt metal-insulator transition. Moreover, it is found that the low-temperature conductivity in the insulating state is dominated by excess free electrons with a high mobility of ≈103 cm2 V-1 s-1 , whereas the carrier density is dramatically suppressed with decreasing temperature. Remarkably, it reveals that the application of an electric field can lead to a collapse of the localized states, resulting in a metallic state. These results reveal the strongly correlated/coupled nature between the localized electrons and high-mobility electrons and offer a new pathway to probe and harvest the exotic electron states at the complex oxide surfaces.

Structural and electrical properties of high-performance vanadium dioxide thin layers obtained by reactive magnetron sputtering

Epitaxial vanadium dioxide (VO2) films exhibiting an abrupt metal-insulator transition with resistivity ratios of five orders of magnitude have been grown on (001)-oriented sapphire substrates by reactive magnetron sputtering. The influence of deposition and annealing temperature on the structure and morphology of the layers are discussed as well as their impact on the films’ electrical properties. Thus, the VO2 layers obtained using the optimal experimental conditions have electrical resistivity ratios over 105 and can be obtained on substrates as large as three-inch in diameter. The combination of the scalability of magnetron sputtering and the high quality of the films enables the large-scale industrial applications of high quality VO2 layers.

Photonic bandgap engineering in (VO2) n /(WSe2) n photonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal–insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO2) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO2 as a strongly correlated phase transition material and tungsten diselenide (WSe2) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO2 with WSe2 in a photonic superlattices structure through the near and mid-infrared (NIR–MIR) thermochromic phase transition regions. By increasing the thickness of the VO2 layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO2 thickness. For the monoclinic case of VO2, the number of the forbidden bands increase with the number of layers of WSe2. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR–MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study’s findings may help understand vanadium oxide’s role in tunable photonic superlattice for infrared switchable devices and optical filters.

Concurrent Structural and Electronic Phase Transitions in V2O3 Thin Films with Sharp Resistivity ChangeSupported by the National Key R&D Program of China (Grant No. 2017YFA0403600), and Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences Large-Scale Scientific Facility (Grant No. U1532149).

The relationship between structural and electronic phase transitions in V2O3 thin films is of critical importance for understanding of the mechanism behind metal–insulator transition (MIT) and related technological applications. Despite being extensively studied, there are currently no clear consensus and picture of the relation between structural and electronic phase transitions so far. Using V2O3 thin films grown on r-plane Al2O3 substrates, which exhibit abrupt MIT and structural phase transition, we show that the electronic phase transition occurs concurrently with the structural phase transition as revealed by the electrical transport and Raman spectra measurements. Our result provides experimental evidence for clarifying this issue, which could form the basis of theoretical studies as well as technological applications in V2O3.

Search for power-efficient wide-range reversible resistance modulation of VO2 single crystals

The abrupt metal insulator transition in VO2 is attracting considerable interest from both fundamental and applicative angles. We report on DC I–V characteristics measured on VO2 single crystals in the two-probe configuration at several ambient temperatures below the insulator–metal (I–M) transition. The insulator-mixed-metal-insulator transition is induced by Joule heating above ambient temperature in the range of negative differential resistivity (NDR). In this range the stability of I(V) is governed by the load resistance RL. Steady state I(V) is obtained for RL > |dV/dI|max in the NDR regime. For RL < |dV/dI|max there is switching between initial and final steady states associated with peaks in the Joule power, that are higher the lower RL is. The peaks caused by steep switching are superfluous and damaging the samples. On the other hand, the large RL needed for steady state is the main power consumer in the circuit at high currents. The present work is motivated by the need to avoid damaging switching in the NDR regime while reducing the power consumption in the circuit. Large resistance modulation can be obtained under steady state conditions with reduced power consumption by increasing the ambient temperature of the device above room temperature. Under steady state conditions, the transition to the mixed metal-insulator state is smooth and is followed closely by appearance of sliding domains.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor.

Vanadium dioxide (VO2) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO2 is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO2 microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe2) semiconductor by van der Waals stacking. We fabricated the WSe2 transistor using the VO2 wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe2 transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO2. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO2 is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO2-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO2) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Electrical resistivity anomaly, valence shift of Pr ion, and magnetic behavior in epitaxial (Pr1-yYy)1-xCaxCoO3 thin films under compressive strain

We have fabricated (Pr1−yYy)1-xCaxCoO3 (PYCCO) epitaxial films with various thicknesses by pulsed laser deposition on the SrLaAlO4 (SLAO) substrate that applied an in-plane compressive stress to the film, and investigated the temperature dependence of the electrical resistivity, ρ(T), of the films. An anomalous ρ(T) upturn with a broad hysteresis could be clearly observed only for the thinnest film (d = 50 nm), and the ρ(T) anomaly decreased by increasing film thickness, d. The temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at Pr L2-edge was measured for the films, and the valence states of praseodymium (Pr) ion were determined using the analysis of the XANES spectra. As a result, the average valence of the Pr ion in the d = 50 nm film slightly increases with decreasing temperature from the common value of 3.0+ around room temperature to 3.15+ at 8 K. The valence shift of Pr is thus similar to what was observed on the PYCCO polycrystalline bulks with an abrupt metal-insulator transition, accompanied by a spin-state (SS) transition of Co ions. Furthermore, the low-temperature SQUID measurements evidenced a paramagnetic behavior down to the lowest temperature, which suggests that the dominant part of Co3+ ions in the film grown on the SLAO substrate tends to be in the low spin state characteristic for the insulating ground state. These results strongly suggest that the anomalous ρ(T) upturn in the thin films on the SrLaAlO4 (SLAO) substrate is closely related to the SS transition of Co ions. On the other hand, PYCCO films grown on the LaAlO3 (LAO) substrate that applied an in-plane tensile stress showed no valence shift of Pr ions and developed a long range ferromagnetic order, which points to a complete suppression of the low-temperature transition. The behaviors of the epitaxial films are discussed in terms of the in-plane stress exerted by different substrates and accumulated elastic energy.

Polymorph separation induced by angle distortion and electron delocalization effect via orbital modification inVO2epitaxial thin films

Since Morin discovered that vanadium dioxide $(\mathrm{V}{\mathrm{O}}_{2})$ undergoes a reversible and dramatic structural phase transition coupled with an abrupt metal-insulator transition, extensive attention has been paid to $\mathrm{V}{\mathrm{O}}_{2}$ due to its importance in fundamental condensed state physics and its potential technological applications. Here, we observed that the precipitated phases of $\mathrm{V}{\mathrm{O}}_{2}$ (insulating and metallic polymorphs) could be controlled by relaxing the dimerization of the vanadium-vanadium (V-V) atomic chain. In particular, the monoclinic metallic phase can be stabilized even at room temperature with the assistance of the angle-distortion-induced $(\ensuremath{\beta}={120}^{\ensuremath{\circ}})$ metallization through symmetry matching between the $\mathrm{V}{\mathrm{O}}_{2}$ epitaxial thin films and the (0001)-oriented sapphire substrates. Concomitantly, the insulating phase $(\text{M}1,\ensuremath{\beta}=122.{6}^{\ensuremath{\circ}})$ that separates from the metallic matrix may supply another driving force for stabilizing the metallic phase, as indicated by scattering-type scanning near-field optical infrared microscopy and further confirmed by synchrotron radiation high-resolution x-ray diffraction characterizations. Soft x-ray absorption spectroscopy results showed that the orbital features of the monoclinic metallic phase are analogous to those of the high-temperature metallic rutile $\mathrm{V}{\mathrm{O}}_{2}$ (R) phase. First-principles calculations further demonstrate the angle-distortion-induced reduction of the V-V atomic dimerization, which enhances the electron delocalization and thus the conductivity. Therefore, the angle distortion results in the metallic monoclinic phase and stabilizes it with the assistance of the nanoscale insulating $\mathrm{V}{\mathrm{O}}_{2}$ (M1) domains at room temperature. These results are of great importance for understanding the contributions of various polymorphs to the metal-insulator transition and for the design of novel artificially heterointerfacial devices based on $\mathrm{V}{\mathrm{O}}_{2}$ nanoscale polymorphs.

Structural and electrical properties of large area epitaxial VO2 films grown by electron beam evaporation

Large area (up to 4 squared inches) epitaxial VO2 films, with a uniform thickness and exhibiting an abrupt metal-insulator transition with a resistivity ratio as high as 2.85×104, have been grown on (001)-oriented sapphire substrates by electron beam evaporation. The lattice distortions (mosaicity) and the level of strain in the films have been assessed by X-ray diffraction. It is demonstrated that the films grow in a domain-matching mode where the distortions are confined close to the interface which allows growth of high-quality materials despite the high film-substrate lattice mismatch. It is further shown that a post-deposition high-temperature oxygen annealing step is crucial to ensure the correct film stoichiometry and provide the best structural and electrical properties. Alternatively, it is possible to obtain high quality films with a RF discharge during deposition, which hence do not require the additional annealing step. Such films exhibit similar electrical properties and only slightly degraded structural properties.

The “self spin valve” in oxygen stoichiometric SrRu1−Fe O3−δ epitaxial thin films

Abstract SrRu 1− x Fe x O 3−δ ( x = 0.00, 0.05, 0.10, and 0.20) thin films were fabricated to study the intrinsic aspects of a “self spin valve”. Using epitaxial strain and high oxygen partial pressure during thin film growth, single phase thin films with negligible oxygen vacancies were successfully grown, and problems related to A-site disorder and grain boundaries were minimized. Under application of an external magnetic field of up to 9 T, the resistivity of all films decreased, resulting in large negative magnetoresistance (up to ∼14.4%), which was stronger at temperatures in the range 10–30 K. An abrupt metal-insulator transition at T ∼ 43 K was found in the x = 0.20 film, which was explained using a two-fluid model related to electron–electron interactions. From the model, two fitting parameters were found to be necessary for in-situ and homogenous defects, while three or unphysical fitting parameters were necessary for ex-situ and inhomogeneous defects.

Separating electric field and thermal effects across the metal-insulator transition in vanadium oxide nanobeams

We present results from an experimental study of the equilibrium and non-equilibrium transport properties of vanadium oxide nanobeams near the metal-insulator transition (MIT). Application of a large electric field in the insulating phase across the nanobeams produces an abrupt MIT, and the individual roles of thermal and non-thermal effects in driving the transition are studied. Transport measurements at temperatures (T) far below the critical temperature (Tc) of MIT, in nanoscale vanadium oxide devices, show that both T and electric field play distinctly separate, but critical roles in inducing the MIT. Specifically, at T≪Tc, electric field dominates the MIT through an avalanche-type process, whereas thermal effects become progressively critical as T approaches Tc.

High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes

We report the fabrication of VO2-based two terminal devices with ∼125-nm gaps between the two electrodes, using a simple, cost-effective method employing optical lithography and shadow evaporation. Current-voltage characteristics of the obtained devices show a main abrupt metal-insulator transition (MIT) in the VO2 film with voltage threshold values of several volts, followed by secondary MIT steps due to the nanostructured morphology of the layer. By applying to the two-terminal device a pulsed voltage over the MIT threshold, the measured switching time was as low as 4.5 ns and its value does not significantly change with device temperature, supporting the evidence of an electronically driven MIT.

Reentrant insulating state in ultrathin manganite films

The transport and magnetic properties of La0.7Sr0.3MnO3 thin-films grown by pulsed laser deposition on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 single crystal substrates have been investigated. A systematic series with various thicknesses of La0.7Sr0.3MnO3 was used to establish a phase diagram which showed a clear difference compared to films grown on SrTiO3 substrates, highlighting the importance of film thickness, and substrate strain. At 8 unit cells, the boundary between the metallic and insulating groundstates, a second abrupt metal-insulator transition was observed at low temperatures, which could be tuned by magnetic field, and was interpreted as a signature of electronic phase separation.

Cover Picture: Observation of insulating-insulating monoclinic structural transition in macro-sized VO2 single crystals (Phys. Status Solidi RRL 3/2011)

To clarify the origin of metal–insulator transition (MIT) in VO2, it is of crucial importance to under-stand the interplay among monoclinic M1, monoclinic M2, and rutile R phases. The Letter by Bongjin Simon Mun et al. (pp. 107–109) reports the unusual insula-tor–insulator structural phase transition (SPT) of VO2 single crystals, which involves monoclinic M1 and M2 phase. The VO2 crystals exhibit an extremely abrupt MIT at 67.8 °C and an insulator–insulator transition (IIT) at ∼49 °C. Using synchrotron-based X-ray microdiffraction, it is found that the IIT in this VO2 crystal is related to a SPT between the M2 and M1 phases while the MIT occurs with a SPT of M1 and R phase. Also, a stable M2 phase is found at room temperature without any presence of external stress, which has not been reported previously. The authors believe that further investigations on this intriguing system will not only enrich the fundamental understanding of the system, but also will be exploited in future technologies.

Epitaxial Growth of Phase Transition V2O3 Thin Films on c-Al2O3 in Reactive Sputtering

Vanadium sesquioxide (V2O3) is known as a crystalline phase of vanadium oxide with metal-insulator transition (MIT). This V2O3 has hexagonal corundum crystalline structure same as sapphire. In this study, highly c-axis textured V2O3 crystalline growth on c-plane sapphire substrate was realized even at film thickness around 15 nm under carefully-controlled small oxygen flow conditions in reactive magnetron sputtering. In-plane compressive strain state was found for the thin V2O3 films, while lattice relaxation was observed with increase of film thickness. Under higher growth temperature, abrupt MIT of the films with resistance change over several decades was achieved at temperature around 160 K.

Vanadium Dioxide and Vanadium Sesquioxide Thin Films Fabricated on (0001) or (1010)Al2O3 by Reactive RF-Magnetron Sputter Deposition and Subsequent Annealing Processes

Vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films showing an abrupt metal–insulator transition near their typical critical temperature (TC) were successfully fabricated by reactive RF magnetron sputter deposition and subsequent annealing processes. The annealing processes for preparing VO2 and V2O3 films were carried out in 30 mTorr O2 ambient and at a very low pressure (2 ×10-6 Torr), respectively. V2O3 films could be fabricated by the one-step annealing of as-deposited VOx films in contrast to the V2O3 fabrication by the two-step annealing of sol–gel V2O5 films. The annealing temperature for preparing V2O3 films (≥550 °C) was much lower than those reported in earlier works. The dependence of TC on the surface orientation of a single crystalline Al2O3 substrate was also shown for VO2 and V2O3 films. The films fabricated on (1010)-plane Al2O3 substrate showed superior characteristics to those on (0001)-Al2O3.

Vanadium Dioxide Films Deposited on Amorphous SiO2- and Al2O3-Coated Si Substrates by Reactive RF-Magnetron Sputter Deposition

VO2 films showing abrupt metal–insulator transition (MIT) were fabricated on amorphous-film-coated Si wafers by reactive RF-magnetron sputter deposition using a V-metal target, and postdeposition annealing. The amorphous films were thermally grown SiO2 or Al2O3 films deposited by low-temperature plasma-enhanced atomic layer deposition. The effects of the underlayer and chamber pressure governing oxidation ambient on MIT characteristics were investigated. The MIT of VO2 films deposited at 5 mTorr on SiO2 and Al2O3 induced resistance changes of 1.3×104 and 2.3×103, respectively. The resistance change due to MIT was larger than 6.3×103 in an operating pressure range as wide as 5–25 mTorr on SiO2.

Characteristics of vanadium dioxide films deposited by RF-magnetron sputter deposition technique using V-metal target

Vanadium dioxide films were deposited using reactive RF-magnetron sputter deposition technique and characterized without or with post-annealing process for the application of thermal sensors. The film thickness variation on a 4-in wafer was less than ±2%. As-deposited film showing an abrupt metal–insulator transition (MIT) could be obtained with O 2 fraction of 6%. The films deposited at 400 and 450 °C showed abrupt changes of resistance in the order of 10 3 near typical MIT temperature of VO 2 without post-annealing. The 110-nm-thick VO 2 film deposited at 450 °C on c-sapphire revealed the resistance change of 1.2×10 4 near 68 °C after annealing at 510 °C.