Session D22: Metal Insulator Transition in VO2

The band structure of VO2 measured by angle-resolved photoemission

The origin of the 340K metal-insulator transition (MIT) in VO2 is still under debate. the main reason is that no direct experimental verifications of the electronic structure of VO2 exist up to this point. The quality of the available single crystals is not sufficient for ARPES measurements, so that photoemission is limited to angle-integrated mode. New opportunities are offered by oxide films, on which data of equal or even higher quality have been reported (Saeki \textit{et al.}, PRB 2009). WIth the \textit{in situ} pulsed-laser-deposition (PLD) system available on beamline 7.0.1 at the Advanced Light Source we have grown VO2(001) films on a TiO2 substrate and measured the Fermi surface of the metallic phase. These results will permit a direct comparison with the existing band calculations and open the way to the study of the MIT as a function, e.g., of film thickness or electron doping with Cr.   

Electronic structure of strained vanadium dioxide thin films using soft X-ray spectroscopy

Despite over five decades of intense investigation, the origin of the metal-insulator transition (MIT) in VO$_2$ still presents a challenge to explain. Whether the lattice (Peierls physics) or electron-electron correlations (Mott-Hubbard physics) are responsible for the MIT has been hotly debated; more recently, the general consensus has favored a co-operative description, in which both structural and correlation effects are important and sympathetic to the transition. Key to understanding such a co-operative picture has been the behavior of VO$_2$ under doping and strain. Here, we report recent soft X-ray measurements of strained VO$_2$ thin films grown on TiO$_2$(001) and (110) substrates. We employ X-ray absorption spectroscopy and X-ray emission spectroscopy to probe the changes in both the {\em unoccupied} and {\em occupied} partial density of states across the MIT, observing distinct changes in the V $3d$-O $2p$ hybridization. Additionally, the location in energy of the unoccupied $d_{\parallel}$ state in the insulating phase is found to be dependent on the lattice strain, in agreement with the predictions of recent dynamical mean-field theory calculations. Finally, our results are discussed in the context of the origin of the MIT in VO$_2$.   

Scanning Photocurrent Microscopy of VO$_{2}$ Nanobeams

Vanadium dioxide (VO$_{2}$) is a strongly correlated material that displays a near-room temperature metal-to-insulator transition ($\sim 68^{o}C$). This transition can be explored at the single domain level in single crystalline VO$_{2}$ nanobeams, where the material dimension is smaller than the characteristic domain size. Here we investigate the metal-insulator phase transition and its domain wall physics in single VO$_{2}$ nanobeam devices through scanning photocurrent microscopy. This technique, which measures the photocurrent as a function of the local photo-injection position, allows us to determine the band bending direction and the height of the Schottky barriers at each domain wall. Our results may shed light on the charge dynamics in strongly correlated materials and the metal-insulator phase transition mechanism.   

Conductivity anisotropy in strained VO$_{2}$ thin films, probed by THz Time Domain Spectroscopy

We used THz time domain spectroscopy to measure the temperature and polarization dependent far-infrared conductivity of high quality strained VO$_{2}$ thin films epitaxially grown on (100) TiO$_{2}$ substrates. A large conductivity anisotropy is observed in the metallic phase of our VO$_{2}$ films with the conductivity along the rutile axis $\sim $30 times larger than the orthogonal direction. The MIT temperature also exhibits anisotropy with a value of 360K along the rutile c axis and 340K along the perpendicular direction. Our results are consistent with substrate induced strain modulation of the energy and bandwidth associated with the vanadium 3d orbitals.   

X-ray induced photoconductivity in Vanadium Dioxide samples

Vanadium Dioxide (VO2) goes through a first-order phase transition at approximately 340K, exhibiting both an insulator to metal transition (IMT) and a structural phase transition (SPT), with a monoclinic (M1) insulating phase at low temperatures and a rutile (R) metallic phase at high temperatures. We show an anomalous behavior of x-ray induced persistent photoconductivity (PPC) well below the temperature induced phase transition in VO2 devices. We present conductivity and X-ray Diffraction (XRD) measurements, revealing a large enhancement of conductivity due to photo-induced carriers. Moreover, with the addition of nominal electric fields, we are able to fully transition into the rutile metallic phase at room temperature. This effect is completely reversible, allowing the monoclinic insulating phase to be recovered via annealing.   

Electric field-induced breakdown of the Mott insulating state in V2O3 nanostructures

The origin of the electric field-induced breakdown of the Mott insulating state in vanadium sesquioxide (V2O3) nanostructures is of considerable interest. We have prepared high quality, epitaxial films of V2O3 on (0001)-oriented sapphire substrates by oxygen plasma-assisted thermal evaporation. Lateral, two-terminal nanostructures were patterned by electron beam lithography. The nanostructures displayed strong metal-to-insulator transitions upon cooling to below $\sim $150K. Modest voltages applied across the devices drive the films into a conducting state. We discuss the role of temperature, applied voltage, device size, and potential Joule heating effects on the switching process, as well as implications for the underlying mechanism involved.   

Gatability of vanadium dioxide single crystal nanobeams and hydrogen doping

Vanadium dioxide is famous for its dramatic metal insulator transition, exhibiting up to 4 or 5 orders magnitude change in conductivity. It is also known to be nongatable, although in the insulating phase it behaves like a semiconductor with 0.5-0.7 eV energy gap. With no sign of gating effects using conventional dielectric materials, such as SiO$_{2}$, Al$_{2}$O$_{3}$ and HfO$_{2}$, ionic liquids were used as the gating medium. Ionic liquids form electric double layers (EDL) and could possibly exert an electric field as high as 10$^{9 }$V/m on the interface of ionic liquid and single-crystal vanadium dioxide nanobeam. No gating effect was observed in the vanadium dioxide device. On the other hand, we found that under positive gate voltage the hydrogen ions originating from trace amounts of water diffuse into the vanadium dioxide crystal, acting as dopants. By controlling the gate voltage and temperature, the insulating phase's conductivity can be reversibly increased up to 2-3 orders magnitude by this process.   

Dependence of VO$_{2}$ thin-film metal-insulator transition on its intrinsic impurities

We present variation in strain, metal-insulator transition temperature ($T_{MIT})$, activation energy ($\Delta E_{a})$, and charge carrier type in the insulating phase of (011) preferred polycrystalline (Poly-) and multidomain (020) epitaxial (Epi-) VO$_{2}$ films grown at different temperature ($T_{S})$, to produce variable intrinsic impurities. Both the Poly- and Epi-VO$_{2 }$behave $n$-type conductivity when grown at relative low $T_{S}$. As $T_{S}$ increases, acceptor density of impurity increases to alter conductivity from $n$- to $p$-type in the Poly-VO$_{2}$, while conductive $n$-type still keeps in the Epi-VO$_{2}$ with increased donor density. Moreover, the strain along monoclinic $a_{m}$ axis dramatically reverses from tensile to compressive in both the Poly- (848 K$ [Preview Abstract]

Manipulation of avalanche characteristics in nanoscaled VO$_{2}$ devices

The temperature driven metal insulator transition (MIT) in nanoscaled VO$_{2}$ devices occurs through a series of resistance jumps ranging over two decades in magnitude. A power law distribution of the jump sizes, demonstrates that the transition is caused by avalanches across the percolation transition. We investigate the effect of a DC write current on the intrinsic behavior of the MIT transition in nanoscaled VO$_{2}$ devices. We find an increase in the maximum resistance jump size by as much as a factor of 10x after application of a DC write current at room temperature. Interestingly, we find no significant changes in the exponent of the power law distribution as a function of an applied DC write current. The observations suggest that the DC current changes the intrinsic properties of the VO$_{2}$ thin film and may be related to spatial confinement which leads to an increase in the maximum resistance jump size.\footnote{Hong-Ying Zhai, J.X. Ma, D.T. Gillaspie, X.G. Zhang, T.Z. Ward, E.W. Plummer, and J. Shen, Phys. Rev. Lett. \textbf{97} 167201 (2006).}   

stoichiometry engineering of metal-insulator transition in suspended single crystalline vanadium dioxide nanobeams

While the metal-insulator transition (MIT) in VO2 bulk and thin films has been investigated for several decades, recent studies of nanobeams have provided new opportunities to investigate and manipulate the metal-insulator transition and structural domain formation in a correlated manner. We will describe the electrical and structural characterization of suspended single crystal VO2 nanobeams grown/annealed under various conditions. Annealing nanobeams under reducing conditions led to the stabilization of single-crystal rutile nanobeams at room temperature, in some cases suppressing the MIT temperature from 340 K down to below 100 K. Re-annealing under oxidizing conditions led to a recovery of the transition temperature for stoichiometric VO2. Furthermore, growth under oxidizing conditions produced the Mott insulator M2 phase and an intermediate M3. Systematic annealing studies enabled the generation of a pseudo-phase diagram with dimensions of stoichiometry and temperature. The temperature dependence of the electrical resistivity of rutile nanobeams above the transition temperature will also be discussed.   

Tungsten as a substitutional dopant and its effect on ultrafast switching of vanadium dioxide

VO$_{2}$ undergoes a metal-insulator transition (MIT) at 340K accompanied by a structural change from monoclinic (M1) to tetragonal (R). We have grown W-doped VO$_{2}$ films on glass and epitaxially on sapphire substrates and have characterized them by SEM, white light transmission, RBS, XRD, and Z-STEM. These provide direct experimental evidence that W acts as a substitutional dopant in the VO$_{2}$ lattice in addition to lowering the transition temperature. From GGA+U, DFT-based simulations we have also calculated the formation energy of substitutional W in VO$_{2}$, and relative stability of M1 and R phases before and after doping. Ultrafast pump-probe measurements at 800nm with varying pump fluences show that doped VO$_{2}$ switches at substantially lower fluences than undoped VO$_{2}$, indicating that the W dopant provides additional conduction-band electrons, thus altering the photo-induced dynamics of the phase transition.   

Control of the metal-insulator transition in vanadium dioxide nanobeams

Single-crystal nanobeams of vanadium dioxide, which are smaller than the characteristic domain size, exhibit a more reproducible and controllable metal-insulator transition (at around 67 degrees C) than bulk samples. We are exploiting this fact to perform systematic studies of the intrinsic properties of the phases involved, the phase transition, and the interphase wall, as well as to control the transition temperature. For these purposes it is necessary to have high quality crystals and to apply uniform strain. We are therefore investigating and improving the procedure of VO2 small-crystal growth by vapor phase transport, while developing experimental techniques in which thin nanobeams can be suspended across adjustable-widths gaps on silicon structures. The latter will enable application of strain purely along the tetragonal c-axis, to tune the transition, while simultaneously carrying out transport, optical and scattering measurements.   

Electrical properties of vanadium dioxide devices for micro-electronic applications making use of metal-insulator phase transitions

In principle the metal-to-insulator transition offers prospects for use in an electronic switch. This study investigates the properties of VO$_{2}$ test devices to evaluate VO$_{2}$'s potential use in micro-electronic applications such as a memory, two-terminal selector or transistor device. Vanadium dioxide thin films were produced by thermal oxidation of vanadium and the physical properties of these layers were investigated. Electrical properties of concentric two-terminal vanadium dioxide structures will be discussed such as current-voltage behavior, switching behavior and contact formation to VO$_{2}$ with different metals and implications such as Fermi-level pinning and Schottky-type behavior for different metals.   

Metal-insulator transition mechanism in VO2 under electric bias

It is in controversy if metal-insulator phase transition (MIT) of VO$_{2}$ can be triggered by electric field/current. In this work, a series of two terminal devices with different gap length, width, and multiple-channel configurations were fabricated on epitaxially grown VO$_{2}$ thin films, to study its MIT mechanism under the electric bias. Micro-Raman spectroscopy was used to differentiate the rutile metallic phase from the monoclinic insulator phase. Voltage-current measurements indicated that a temperature-dependent critical current density (J$_{c}$) is required to induce MIT. Under the electric bias, the phase transition was observed to be a percolation process until a clear current path (or filament) is formed between the electrodes. Afterwards the pure metallic phase was identified along the current path, while outside of it become pure insulator phase. As current varies, current path width is proportionally changed to keep a constant current density. These observations indicate that a J$_{c}$ is necessary to maintain the metallic phase current path. Contributions of the current effect and Joule heat effect to the phase transition were discussed.   

Electric-field-driven phase transition in vanadium dioxide

In recent years, various strongly correlated materials have shown sharp switching from insulator to metallic state in their I(V) transport curves. Determining if this is purely an out of equilibrium phenomena (due to the strong electric field applied throughout the sample) or simply a Joule heating issue is still an open question. To address this issue, we have first measured local I(V) curves in vanadium dioxide (VO$_2$) Mott insulator at various temperatures using a conducting AFM setup and determined the voltage threshold of the insulator to metal switching. By lifting the tip above the surface ($>$35nm), we have then measured the purely electrostatic force between the tip and sample surface as the voltage between these two was increased. In a very narrow temperature range (below 360K), a tip height range (below 60nm) and a voltage applied range (above 8V), we observed switching in the electrostatic force (telegraphic noise vs. time and vs. voltage). This purely electric field effect shows that the switching phenomenon is still present even without Joule heating in VO$_2$.