Session J16: Vanadium Dioxide; Nano and Devices

M2 phase in free-standing single-crystalline nanostructures of VO$_2$ at ambient conditions

A significant drawback of a promising material for realization of an ultrafast switch based on a metal-insulator transition (MIT) - VO$_2$ - is the inherent linkage between the MIT and lattice transformation from tetragonal to monoclinic with a lattice contribution in the band gap formation in the stable monoclinic M1 structure. On the other hand, the metastable M2 phase exhibits a pure Mott MIT and was shown to be driven metallic without the structural change. Existence of this phase at ambient conditions was reported for Cr and Al-doped VO$_2$ and nanostructures doped with oxygen vacancies. Here we report stabilization of the M2 phase in VO$_2$ single-crystalline nanoplatelets (NPls) doped with Al during the growth process via two new methods. Development of these methods came from our recent in situ studies of the NPl growth mechanism. We reconstructed temperature-doping phase diagram for the NPls. Electrical properties of the NPls were also studied as functions of doping level and temperature.   

Photocurrent in Vanadium Dioxide

We investigate the photoresponse of VO$_{2}$~using scanning photocurrent microscopy below and above the metal-insulator transition at 67 \r{ }C. To avoid complications of nonuniform strain and twinning boundaries we focus on single-crystal suspended nanobeams, where the strain is either minimal or well controlled. At intermediate temperatures the metallic and insulating phases coexist and a photocurrent and photoconductance associated with the boundary between them is seen. The magnitude and profile of the photoresponse above and below the transition demonstrates a photothermal mechanism, with fast electron-lattice relaxation and absence of built-in electric fields in the insulating phase consistent with strong electron-electron correlations and a short screening length.   

Electronic phases in vanadium dioxide nanowires observed by scanning tunneling spectroscopy

The electronic behavior of vanadium dioxide around its metal-insulator transition has been the focus of many experimental and theoretical investigations, concerning transition mechanisms. In this work, we present our experimental investigation of the temperature driven metal-insulator transition in individual single crystal vanadium dioxide nanowires with scanning tunneling spectroscopy. The local density of states is studied at temperatures around the transition temperature. We observe three distinct electronic phases: the insulating phase and two metallic phases with different density of states, both in terms of conductivity and in the complexity of the density-of-states spectra. Vanadium dioxide nanowires doped with tungsten were also studied and will be discussed.   

Current-induced metastable states in single-crystalline VO$_2$ nanoplatelets

The metal-insulator transition (MIT) in VO$_2$ occurs close to ambient temperature, T$_c$ = 68 $^{\circ}$C, which can be reduced by doping. The phase transition results in a few orders of magnitude change of electrical conductivity and is accompanied by a change of the lattice from tetragonal to monoclinic, which is associated with lattice expansion of $\sim$1\% along the tetragonal c-axis of the metallic phase. We observed that, in suspended single-crystalline VO$_2$ nanoplatelets (NPls) carrying a sufficiently strong electrical current, Joule heating leads to formation of metal-semiconductor domains, which are self-organized in chains providing a path for the current flow. This results in NPl bending depending on the current strength, which can be used for electrically controlled actuator action. The observed domain structures should be interpreted as distinct metastable states in freestanding and end-clamped quasi-1D VO$_2$ samples. We analyze the stability conditions for the heterophase domains patterns and main prerequisites for the realization of current-controlled nanoactuators based on the proposed concept.   

Hydrogen stabilization of metallic VO2 in single-crystal nanobeams

Vanadium dioxide (VO$_{2})$ is a strongly correlated material with a metal-insulator transition at 67$^{o}$C from a high temperature, rutile metal to a lower symmetry insulating state. Substitutional doping can alter the properties of VO$_{2}$, but is irreversible. Using individual microcrystals and nanobeams, we show that spillover may dope VO$_{2}$ reversibly with atomic hydrogen. Raman and optical microscopy show a stabilized metallic state, consistent with single-crystal electron diffraction and scanning electron microscopy that demonstrate a post-hydrogenation structure similar to the rutile state. Electronic transport shows that the energy band gap of insulating phase can be reversibly tuned towards the metallic phase upon different hydrogenation conditions and the metallic state may be stabilized down to cryogenic temperatures. Electronic structure calculations agree that a hydrogen-containing distorted rutile structure is energetically favored over the monoclinic state.   

Identifying the Role of Domains in Metal-Insulator Transitions in Individual Nanowires of Tungsten-Doped $VO_{2}$

Though it is well known that the metal-insulator transition (MIT) in $VO_{2}$ can be achieved by a variety of external parameters, an understanding of how different parameters drive such a transition has remained relatively unknown. We report transport and Raman spectroscopic characteristics on voltage ($V$)- and temperature ($T$)-driven MIT in individual, single-crystal, tungsten-doped $VO_{2}$ nanowires. From transport analyses we discuss the $T$-dependent features in $I$ vs $V$ curves; specifically hysteresis gaps and resistance jump features seen in sub-micron devices. From Raman spectroscopic analyses we discuss the Raman intensity of $A_{g}$ modes while driving the temperature and voltage across the transition. We conclude that driving $T$ supports a slow transition to the rutile (R) metallic phase with a wide temperature range of mixed insulating, monoclinic (M1) and R states due to the population of metallic domains. $V$-driven transition does not appear evolve via the formation of domains, but is activated when $V$ is sufficiently large above a $T$-dependent threshold.   

Ionic Liquid Gated Vanadium Oxide Three Terminal Devices: Chemical Stability and Field Effect

Understanding electrostatic field effect in correlated oxides is one approach to uncovering mechanisms leading to metal-insulator transition and further is of great interest in oxide-based device technologies. We have fabricated ionic liquid gated VO$_{2}$ three-terminal devices. The VO$_{2}$/IL interface properties were systematically studied with emphasis on electrochemical stability, gate capacitance and charging/discharging using photoelectron spectroscopy, impedance spectroscopy and other electrical characterization. We have observed a large modulation of VO$_{2}$ channel conductance at room temperature with polarity dependence. Interestingly, the conductance modulation also exhibits a time-dependent response to external gate bias and possible mechanisms will be discussed.   

Gating Effect on VO2 nanowire by Ionic Liquid

VO2 is a well-known strongly correlated material with insulator-to-metal phase transition at 68 \r{ }C. Below this temperature, VO2 is an insulating material whose temperature dependence behaves like a semiconductor with 0.6 eV band gap. However, such material cannot be gated by traditional method. In our experiment, we applied a new technique, using ionic liquids, to provide a much stronger transverse electric field on the VO2 nanowire. We did not observe obvious gating effect on VO2, but in the meantime, we found that VO2 is sensitive to hydrogen. The hydrogen produced by electrochemistry when applying voltage on the electrolyte (an ionic liquid with water contamination, in this case) can dramatically change the conductance of VO2.   

Strain control of the metal-insulator transition in vanadium dioxide nanobeams

To be able perform systematic studies at the metal-insulator transition in VO2, we have developed a platform for applying axial strain to suspended single-crystal nanobeams while carrying out optical and transport measurements. The nanobeams are positioned on a piezo-actuated silicon structure using a nanomanipulator. The strain can be used to control the transition temperature, because the metallic and insulating phases have different c-axis lattice constants, or to move the interphase boundary between metal and insulator in coexistence. We report transport, Raman and photoconductance measurements as the transition is tuned in this way.   

Investigation of switching behavior of two-terminal devices on VO$_{2}$

Vanadium dioxide undergoes an insulator to metal transition at about 68\r{ }C. Two-terminal devices fabricated on VO$_{2}$ show a steep decrease of resistance when the current or voltage applied are large enough. This switching has been largely attributed to a field effect even in two-terminal devices but controversy still exists. We fabricate devices with an array of electrode separations and widths and study how the dc switching voltage and current depend on device size. The data obtained from these devices are most consistent with a Joule heating mechanism governing the switching. Additionally we perform an ac investigation of these devices and find that the switching to the low resistance state can happen faster than 5ns (the time resolution of the measurement set-up) while the switching to the high resistance state is of the order of hundreds of nanoseconds, consistent with the estimated heat dissipation time. In spite of the Joule heating mechanism which is expected to induce device degradation, we find that the devices can be switched for more than 10$^{10}$ cycles making VO$_{2}$ a promising material for memory applications.   

Local thermal heating in VO$_2$ electric-field-induced metal insulator transition

Over recent years, the insulator to metal transition (IMT) of the vanadium dioxide (VO$_2$) Mott insulator has been revisited revealing an electric-field-induced resistance switching. Whether this feature is purely due to an electrical field effect or due to some Joule heating is still under debate. Here we report a local temperature measurement in a 10$\mu$m and a 20$\mu$m VO$_2$ junction while going through the resistance switching. The sample was placed at $\Delta$T=15K below 340K (the thermally induced insulator to metal transition). When ramping up the voltage across the junction we find that the local heating inside the VO$_2$ junction is close to 15K. This data suggests that in these temperature, current and voltage ranges, the field induced IMT can be explained by local Joule heating. Work supported by the French ANR-09-BLAN-0388-01 and the US DOE and AFOSR.   

Electrical properties of vanadium dioxide -- dielectric -- metal structures and the metal-insulator transition

VO$_{2}$ and its metal-to-insulator transition (MIT) are of interest for memory and logic nanoelectronic devices due to its very fast transition, its full volume transition implying good scalability and reliability and relatively large resistance on/off ratio of 2-5 orders of magnitude. In this work the equivalent of the heart of MOS (metal-oxide-semiconductor) technology, the MOS capacitor is investigated for the VO$_{2}$ case in which the Si is replaced by VO$_{2}$. Thermally oxidized VO$_{2}$ with a HfO$_{2}$ or Al$_{2}$O$_{3}$ dielectric grown on top with Atomic Layer Deposition were used to form MOS structures. The MOS capacitor electrical properties are analyzed such as the gate current and capacitance behavior with special regard to the MIT. The influence of the MIT on gate dielectric tunneling is shown and modeled as well as RRAM phenomena and an evaluation of the field effect. Implications for the field-induced metal-insulator transition and for device applications are discussed.   

Field-effect modulation of conductance in VO$_2$ nanobeam transistors with HfO$_2$ as the gate dielectric

Field-effect transistors have been fabricated from VO$_2$ nanobeams using HfO$_2$ as the gate dielectric. When heated up from low to high temperatures, VO$_2$ undergoes an insulator-to-metal transition. We observe a change in conductance (6 \%) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of the hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system involving a timescale of a few minutes. The origin of such slow processes may lie in the coupling between the dipolar arrangement and the strain state of the VO$_2$ crystal.   

Metal-to-insulator transition in a columnar nanocomposite oxide

The capability of tuning lattice strain, composition, and dimensionality in epitaxial film growth provide a new avenue of exploring new functionality in correlated electron materials. Here we demonstrated a chemical ratio controlled metal-insulator transition in the nanocomposite films of V$_{2}$O$_{3}$-La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) grown on LaAlO$_{3}$ (111) substrate through alternative deposition of LSMO and V$_{2}$O$_{3}$. A V-Mn ion exchange between V$_{2}$O$_{3}$ and LSMO occurs during the growth and results in the formation of nanoscale and vertically columnar-like insulating Mn$_{2}$O$_{3}$ and metallic La$_{1-x}$Sr$_{x}$VO$_{3}$ composite, as qualitatively revealed by scanning transmission electron microscopy. As determined by transport measurement, a metal-insulator transition is found in the composite films, which depends on the ratio of deposition time of V$_{2}$O$_{3}$ to LSMO.   

Avalanches in sub-micron V$_{2}$O$_{3}$ devices

Systematic resistance versus temperature measurements were performed on micron and sub-micron V$_{2}$O$_{3}$ devices. Instead of smooth R-T curves reported previously, multiple jumps in resistance are observed through the temperature driven metal-insulator transition. These jumps range over 3 orders of magnitude in resistance. A power law distribution of the jump sizes indicates that the metal-insulator transition in V$_{2}$O$_{3}$ occurs through a series of avalanches. The power law exponent in V$_{2}$O$_{3}$ devices is very close to that found in similar VO$_{2}$ devices.\footnote{A. Sharoni, J. G. Ramirez, and I. K. Schuller, Phys. Rev. Lett. \textbf{101}, 026404 (2008).} This indicates that the phase transition in VO$_{2}$ and V$_{2}$O$_{3}$ are similar and occur through phase separation, percolation and avalanches. The effect of magnetic field on the avalanches in V$_{2}$O$_{3}$ will be discussed.