Session C19: Vanadate Experiment: Devices

Phase transition transistors based on strongly-correlated materials

The field-effect transistor (FET) provides electrical switching functions through linear control of the number of charges at a channel surface by external voltage. Controlling electronic phases of condensed matters in a FET geometry has long been a central issue of physical science. In particular, FET based on a strongly correlated material, namely ``Mott transistor,'' has attracted considerable interest, because it potentially provides gigantic and diverse electronic responses due to a strong interplay between charge, spin, orbital and lattice. We have investigated electric-field effects on such materials aiming at novel physical phenomena and electronic functions originating from strong correlation effects. Here we demonstrate electrical switching of bulk state of matter over the first-order metal-insulator transition [1]. We fabricated FETs based on VO$_{2}$ with use of a recently developed electric-double-layer transistor technique, and found that the electrostatically induced carriers at a channel surface drive all preexisting localized carriers of 10$^{22}$ cm$^{-3}$ even inside a bulk to motion, leading to bulk carrier delocalization beyond the electrostatic screening length. This non-local switching of bulk phases is achieved with just around 1 V, and moreover, a novel non-volatile memory like character emerges in a voltage-sweep measurement. These observations are apparently distinct from those of conventional FETs based on band insulators, capturing the essential feature of collective interactions in strongly correlated materials. This work was done in collaboration with K. Shibuya, D. Okuyama, T. Hatano, S. Ono, M. Kawasaki, Y. Iwasa, and Y. Tokura. \\[4pt] [1] M. Nakano et al., Nature 487, 459 (2012).   

Optical Characterization of Structural Evolution of UltrathinVO2 Films across Metal-Insulator Transition

We use ultrafast x-ray diffraction, time resolved pulsed THz spectroscopy and infrared transient absorption techniques to study the structural and electronic evolution of photo-induced ultrathin VO$_{2}$ film epitaxied on Titanium Dioxide (TiO$_{2})$ substrate. We have demonstrated the lattice structural state of VO$_{2}$ film can be tunable by the amount of excitation fluence. The structural evolution in nanosecond scale can be understood by thermal transport model simulation done by comsol. At high excitation fluence (as high as 29mj/cm$^{2})$, a new transient state is observed based on the x-ray diffraction result. The transient structure is unambiguously identified to benon-thermally induced and decoupled from metal-insulator electronic phase transition. It recovers to the tetragonal phase within one nanosecond, and further to the monoclinic phase in tens of nanoseconds. Moreover, the lattice vary properties are highly dependent on the orientation of substrate. The lattice expands along surface normal axis in VO$_{2}$/TiO$_{2}$ (100) films while contracts in VO$_{2}$/TiO$_{2}$ (001) films when going through the phase transition region.   

The field-effect in vanadium dioxide and the metal-insulator transition

VO$_2$ and its metal-insulator transition are currently of interest to enhance understanding of metal-insulator transitions and for investigating possible applications in nanoelectronic devices. Inducing the metal-insulator transition by means of an electric field, instead of by changing the temperature, could entail a major enhancement of present-day nano-electronics. Both the field induced metal-insulator transition and the regular semiconductor field-effect are investigated in this work using monocrystalline VO$_2$ field-effect structures. The field dependent VO$_2$ conduction characteristics across the metal-insulator transition are elucidated. The relation of these VO$_2$ characteristics with the VO$_2$-insulator interface is clarified by means of admittance analysis.   

Current induced Metal-Semiconductor Transition in VO2 grown on Pt

Vanadium dioxide (VO$_{2}$) exhibits a metal-semiconductor transition at 340K; this transition can also be triggered by an electric field or direct current injection. In this study VO$_{2}$ was grown on 100 nm thick Pt bottom electrodes. The top Pt contacts were added for the transport measurements. The transport behavior indicated a reduced transition temperature. We have shown that the switching voltage for a Pt/VO2/Pt structure was as low as 0.3 V, and at this voltage we observed two orders of magnitude change in the resistance. XPS will be used to determine the valence state.   

Electrical Breakdown in a V2O3 device at the Insulator to Metal Transition

We have measured the electrical properties of a V2O3 thin film micro bridge at the insulator metal transition (IMT). Discontinuous jumps to lower voltages in the current voltage characteristic (IV) followed by an approximately constant voltage progression for high currents indicate an electrical breakdown of the device. In addition, the IV curves show hysteresis and a training effect, i.e. the subsequent IV loops are different from the first IV loop after thermal cycling. Low temperature scanning electron microscopy (LTSEM) reveals that the electrical breakdown over the whole device is caused by the formation of electro-thermal domains (ETDs), i.e. the current and temperature redistribution in the device. On the contrary, at the nanoscale, the electrical breakdown causes the IMT of individual domains. In a numerical model we considered these domains as a network of resistors and we were able to reproduce the electro-thermal breakdown as well as the hysteresis and the training effect in the IVs. This work was supported by AFOSR grant number FA9550-12-1-0381.   

Strain control and the triple point of the metal-insulator transition in vanadium dioxide

We have developed an apparatus for applying controlled strain to suspended nanostructures while carrying out optical and transport measurements. This platform enables us to control and study phenomena where strain plays a key role, such as the metal-insulator transition in vanadium dioxide. The relationship between the metallic (R) phase and the two insulating (M1 and M2) phases involved in this first-order solid-state transition remains intriguing. Due to the different lattice constants of the phases, controlling the length of a VO2 nanobeam allows us to study the transitions between them methodically as a function of temperature and strain. One of our findings is that the triple point temperature of these three phases is extremely close to the transition temperature at zero strain, suggesting that the balance between M1 and M2 controls the stability of the metallic phase.   

Temperature dependence of laser induced insulator-metal transition in VO$_{2}$

We performed optical pump-probe experiments on VO$_{2}$ thin films with low laser fluence at temperatures ranging across the insulator-metal transition (IMT). At room temperature, the reflectivity of VO$_{2}$ increases in the first 400-500 fs when pumped by 150 fs laser pulses. An exponential decay of the reflectivity is observed in the following 1 ps. Interestingly, as the temperature approaches the transition temperature (340 K), the reflectivity shows a second increase on an 80 ps time scale following the exponential decay, indicating an IMT. We propose that the decay of the reflectivity is due to electron-phonon thermalization, which raises the phonon temperature and causes a superheating of the lattice. This process provides the latent heat and induces the IMT on the 80 ps time scale. The coexistence of the insulating and metallic phases is observed in the reflectivity measurements for temperatures above 340 K.   

Conductance Modulation across the Metal-Insulator Transition in Single Nanowire Devices of doped-VO$_{2}$ Gated with Ionic Liquid

Studies of the effects of charge modulation in VO$_2$ systems may provide useful insights into the microscopic mechanisms behind its metal-insulator transition (MIT). Recently, ionic liquid (IL) has become a popular material for gating nanodevices due to its superior charge accumulation capabilities. Thus, using IL to gate single nanowires of W-doped-VO$_2$, we systematically study the modulation of electrical transport across the temperature-driven and voltage-driven MIT as a function of gate voltage. We report the manifestation of hysteresis loops, which show an unprecedented modulation of resistance and current by as large as 20\%. Moreover, we show that the largest modulation loop coincides with the largest changes in resistance across the temperature-driven MIT suggesting that the memory behavior in VO$_2$ and its MIT are closely linked. Similar behavior is also observed across the voltage-driven MIT. These studies lay the ground work for an alternative approach to understanding the mechanisms behind the MIT in VO$_2$ systems when driven by different external parameters.   

Decoupling of structural and electronic phase transitions in VO$_2$

Using optical, TEM and ultrafast electron diffraction experiments we find that single crystal VO$_2$ microbeams gently placed on insulating substrates or metal grids exhibit different behaviors, with structural and metal-insulator transitions occuring at the same temperature for insulating substrates, while for metal substrates a {\it new monoclinic metal phase} lies between the insulating monoclinic phase and the metallic rutile phase. The structural and electronic phase transitions in these experiments are strongly first order and we discuss their origins in the context of current understanding of multi-orbital splitting, strong correlation effects and structural distortions that act cooperatively in this system.   

Modulation of single-crystal vanadium dioxide film by hydrogen

Vanadium Dioxide is a strongly correlated material with a bulk metal-to-insulator transition at 340 K. This transition temperature can be affected by strain, and previous experiments in single-crystal nanowires (J. Wei et al., Nature Nano. 7, 357-362 (2012)) have shown that catalytic doping with atomic hydrogen can stabilize the high temperature metallic state. In this experiment, we examine the effects of hydrogen on a 10 nm thick VO$_{\mathrm{2}}$ film grown on TiO$_{\mathrm{2}}$ (001) substrate by MBE with a transition temperature at 280K. We found the transport properties of this film can be dramatically modulated by doping and releasing hydrogen in and out of VO2 film even at room temperature. The resulting changes in the conductivity are even more dramatic than those seen in nanowires. The enhanced rate of response at room temperature is likely aided by the crystallographic orientation of the film, which has a growth direction along which hydrogen is known to diffuse rapidly in rutile TiO$_{\mathrm{2}}$, which is isostructural to the metallic VO$_{\mathrm{2}}$ high temperature phase.   

Bi-chromatic probing of the metal-insulator transition in VO2 thin film

VO2 is a correlated electron material that exhibits a metal-insulator (MIT) phase transition that can be thermally, electrically, or optically controlled. For the thermally-induced case the material undergoes a structural transition from a monoclinic insulating state to rutile metal at around 340K. The salient features of this first order phase transition are that upon the transition the material exhibits up to five orders-of-magnitude increase in conductivity and consequently also significant changes in the optical properties. Typically in these oxides, competing states can often coexist and form nano- or microscale domains of different phases while transitioning. Here we show that upon thermally inducing the MIT on epitaxial VO2 films when simultaneously probed by two very different frequencies- namely IR and THz- the onset of the MIT appears at somewhat different temperature depending on the light used to probe it, thus confirming the coexistence of nano-scale domains of different phases. We will show our correlated far field optical and transport studies on these films to investigate the percolative nature of the transition and applied mean field approximations to model the observed response.   

Role of joule heating in electrically-driven metal insulator transition in vanadium oxide nanowires

Metal to insulator transition (MIT) in vanadium oxide system can be triggered by several external stimuli such as temperature, electric field, strain and light. Electrically driven MIT in single nanowire devices of crystalline vanadium oxide (W- doped VO$_{2}$, Ag-doped V$_{2}$O$_{5}$) is the topic of current study. Recent works on realizing switching devices using these materials have discussed the importance of Joule heating near the transition as supposed to a purely electric field induced effect. We propose a novel method for identifying the individual roles of Joule heating and/or electric field by analyzing the frequency response of the AC electric signal near the MIT in these devices. The method may also be used in other strongly correlated electron system to delineate the roles of individual microscopic conduction mechanisms near MIT.   

Switchable vanadium dioxide (VO2) metamaterials fabricated from tungsten doped vanadia-based colloidal nanocrystals

We report the fabrication of thermochromic VO2-based metamaterials using solution-processable colloidal nanocrystals. Vanadium-based nanoparticles are prepared through a non-hydrolytic reaction, resulting in stable colloidal dispersions in solution. Thermochromic nanocrystalline VO2 thin-films are prepared via rapid thermal annealing of colloidal nanoparticles coated on a variety of substrates. Nanostructured VO2 can be patterned over large areas by nanoimprint lithography. Precise control of tungsten (W) doping concentration in colloidal nanoparticles enables tuning of the phase transition temperature of the nanocrystalline VO2 thin-films. W-doped VO2 films display a sharp temperature dependent phase transition, similar to the undoped VO2 film, but at lower temperatures tunable with the doping level. By sequential coating of doped VO2 with different doping concentrations, we fabricate ?smart? multi-layered VO2 films displaying multiple phase transition temperatures within a single structure, allowing for dynamic modulation of the metal-dielectric layered structure. The optical properties programmed into the layered structure are switchable with temperature, which provides additional degrees of freedom to design tunable optical metamaterials.