Metal-insulator Transition In Vanadium Dioxide Research Articles

Metal-insulator transition in vanadium dioxide

Metal-insulator transition in vanadium dioxide

The metal-insulator transition in vanadium dioxide: A view at bulk and surface contributions for thin films and the effect of annealing

Vanadium dioxide is investigated as potential oxide barrier in spin switches, and in order to incorporate VO2 layers in complex multilayer devices, it is necessary to understand the relation between bulk and surface/interface properties. Highly oriented VO2 thin films were grown on (0001) sapphire single crystal substrates with reactive bias target ion beam deposition. In the analysis of the VO2 films, bulk-sensitive methods [x-ray diffraction (XRD) and transport measurements] and surface sensitive techniques [photoelectron spectroscopy (PES) and scanning tunneling microscopy and spectroscopy] were employed. The samples were subjected to heating cycles with annealing temperatures of up to 425 and 525K. Prior to annealing the VO2 films exhibit the transition from the monoclinic to the tetragonal phase with the concurrent change in conductivity by more than a factor of 103 and their phase purity is confirmed by XRD. Annealing to 425K and thus cycling across the metal-insulator transition (MIT) temperature has no impact on the bulk properties of the VO2 film but the surface undergoes irreversible electronic changes. The observation of the valence band with PES during the annealing illustrates that the surface adopts a partially metallic character, which is retained after cooling. Annealing to a higher temperature (525K) triggers a modification of the bulk, which is evidenced by a considerable reduction in the MIT characteristics, and a degradation in crystallite morphology. The local measurement of the conductivity with scanning tunneling spectroscopy shows the transition of the surface from predominantly semiconducting surface prior to annealing to a surface with an overwhelming contribution from metallic sections afterward. The spatial distribution of metallic regions cannot be linked in a unique manner to the crystallite size or location within the crystallites. The onset of oxygen depletion at the surface is held responsible for this behavior. The onset of bulk modification at higher temperatures is most likely linked to oxygen loss and effusion along the grain boundaries and concurrent onset of sintering. Our study focuses on the comparison of the MIT in the bulk and at the surface of thin VO2 layers and establishes an irreversible modification of the crystallite structure and surface for temperatures exceeding the MIT. The surface modification impacts on the strategies which will be employed to build the metallic contacts to VO2 layers.

Tunable waveguiding in electrically programmable VO2-based photonic crystals

The feasibility of electrically programmable waveguiding in a photonic crystal (PC) is explored based on the metal-insulator transition of vanadium dioxide (VO2). Unlike the ordinary PCs, wave propagation in the desired structure may be switched on/off or redirected by applying an electrical bias on the selective electrodes by taking advantage of the electrically induced VO2 phase transition and subsequent modulation of dielectric properties. The characteristics of the two-dimensional VO2-based PCs with line defects are analyzed using the iterative plane wave and finite difference time domain methods. Particularly, the influence of the Drude relaxation on waveguiding is examined as the high rate typical for metallic VO2 can lead to the signal loss. An optimized structure is proposed to minimize the loss and simplify the fabrication.

Electrically programmable photonic crystal slab based on the metal-insulator transition in VO2

The possibility of an electrically programmable photonic crystal is investigated theoretically based on the metal-insulator transition of vanadium dioxide (VO2). Recent experiments indicate that this phase transition of the first order can be controlled via external bias and results in significant changes in the electrical and optical properties of VO2. We propose a slab structure based on VO2 whose dielectric properties are modulated periodically in the in-plane direction by selectively inducing phase transition through a lithographically defined array of gate electrodes. A two-dimensional photonic band calculation predicts the presence of a band gap, clearly illustrating the feasibility of the proposed structure. The electrically controlled nature may enable the VO2- based structure to be programmable at a very high speed.

Dynamical Singlets and Correlation-Assisted Peierls Transition inVO2

A theory of the metal-insulator transition in vanadium dioxide from the high- temperature rutile to the low- temperature monoclinic phase is proposed on the basis of cluster dynamical mean-field theory, in conjunction with the density functional scheme. The interplay of strong electronic Coulomb interactions and structural distortions, in particular, the dimerization of vanadium atoms in the low-temperature phase, plays a crucial role. We find that VO2 is not a conventional Mott insulator, but that the formation of dynamical V-V singlet pairs due to strong Coulomb correlations is necessary to trigger the opening of a Peierls gap.

Electrical switching and Mott transition in VO2

In this paper the problem of the Mott metal-insulator transition in vanadiumdioxide driven by an external electric field is considered. Delay time (td)measurements have shown that the experimental value of td is almost threeorders of magnitude lower than the theoretical value, calculated in a simpleelectrothermal model. This suggests that under non-equilibrium conditions (inhigh electric fields) electron correlation effects contribute to the developmentof the insulator to metal transition. The extra-carrier injection from Si intoVO2 was carried out in the structures Si-SiO2-VO2 on p-type siliconwith ρ = 0.1 Ω cm and a SiO2 thickness 70 nm. It has been shown thatthe metal-insulator transition in VO2 can be initiated by injection, i.e. by the increase of the electron density. The value of the critical density wasfound to be of the order of the electron density in VO2 in the semiconductingphase, approximately 1018-1019 cm-3. This confirms thatthe metal-insulator transition in VO2 is the purely electronic Mott-Hubbardtransition.

A model for electronic structure of VO2

Abstract In order to examine the metal-insulator transition in vanadium dioxide (VO2) a number of numerical calculations have been made which attempt to place a chemical emphasis on the understanding of this phenomenon. Angular overlap model (ADM) calculations have been applied to VO6 8- octahedra in the distorted and rutile phases to show the energies and characters of the perturbed V 3d orbitals. These results have then been used to construct Coulson-Fischer-type wavefunctions to model the effects of V-V bonding along the c axis. Calculations have been made on both V-V dimers and V-V chains, using complete neglect of differential overlap (CNDO)- and intermediate neglect of differential overlap (INDO)-type approximations to show the effects on the occupation of the V 3d orbitals of changing the lattice and electron correlation.

Metal-insulator transition in vanadium dioxide

The basic physical parameters which govern the metal-insulator transition in vanadium dioxide are determined through a review of the properties of this material. The major importance of the Hubbard intra-atomic correlation energy in determining the insulating phase, which was already evidenced by studies of the magnetic properties of ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{2}$ alloys, is further demonstrated from an analysis of their electrical properties. An analysis of the magnetic susceptibility of niobium-doped V${\mathrm{O}}_{2}$ yields a picture for the current carrier in the low-temperature phase in which it is accompanied by a spin cloud (owing to Hund's-rule coupling), and has therefore an enhanced mass ($m\ensuremath{\simeq}60{m}_{0}$). Semiconducting vanadium dioxide turns out to be a borderline case for a classical band-transport description; in the alloys at high doping levels, Anderson localization with hopping transport can take place. Whereas it is shown that the insulating phase cannot be described correctly without taking into account the Hubbard correlation energy, we find that the properties of the metallic phase are mainly determined by the band structure. Metallic V${\mathrm{O}}_{2}$ is, in our view, similar to transition metals like Pt or Pd: electrons in a comparatively wide band screening out the interaction between the electrons in a narrow overlapping band. The magnetic susceptibility is described as exchange enhanced. The large density of states at the Fermi level yields a substantial contribution of the entropy of the metallic electrons to the latent heat. The crystalline distortion removes the band degeneracy so that the correlation energy becomes comparable with the band width and a metal-insulator transition takes place.