V2O3 Film Research Articles

High-performance p-type V2O3 films by spray pyrolysis for transparent conducting oxide applications.

High-quality epitaxial p-type V2O3 thin films have been synthesized by spray pyrolysis. The films exhibited excellent electrical performance, with measurable mobility and high carrier concentration. The conductivity of the films varied between 115 and 1079 Scm-1 while the optical transparency of the films ranged from 32 to 65% in the visible region. The observed limitations in thinner films' mobility were attributed to the nanosized granular structure and the presence of two preferred growth orientations. The 60nm thick V2O3 film demonstrated a highly competitive transparency-conductivity figure of merit compared to the state-of-the-art.

Strong correlation effects in vanadium oxide films

In the present work the metal-insulator transition in doubly orbitally degenerated model of quasi-two-dimensional material based on V2O3 film, in which a crucial role is played by on-site Coulomb interaction and correlated hopping of electrons, has been investigated. With use of a projection procedure in the Green function method the energy spectrum of electrons has been calculated to model variations of the material properties at the temperature changes, the external pressure application and doping. The obtained expressions for thermodynamic potential and the energy gap widths allow analyzing the possible phase transitions in a system, the dependency of characteristics on the external actions for this strongly correlated material.

Texture and phase control of magnetron-sputtered VO2 thin films with an Al-doped ZnO seed layer using topotactic oxidization

We developed a technique to produce stoichiometric VO2 films with a well-defined (010) orientation, using an Al-doped ZnO (AZO) thin film and topotactic oxidation. The AZO seed layer was prepared on a glass substrate by radio frequency magnetron sputtering (RF-MS), and then the V2O3 film was deposited on the AZO film by RF-MS. Finally, the V2O3 film was post-annealed, and underwent topotactic oxidation in air to convert the V2O3 phase to the VO2 phase. The AZO seed layer converted V2O3 to VO2 while maintaining the crystallographic orientation. Consequently, well-defined (010) orientated 200-nm-thick VO2 films with large change in electrical resistivity of 1.55 × 103 times together with large change in optical transmittance of 64.3% at wavelength of 2000 nm, before and after the transition, were successfully obtained by combination with AZO seed layer and topotactic oxidation.

Giant nonvolatile resistive switching in a Mott oxide and ferroelectric hybrid

Controlling the electronic properties of oxides that feature a metal-insulator transition (MIT) is a key requirement for developing a new class of electronics often referred to as "Mottronics." A simple, controllable method to switch the MIT properties in real time is needed for practical applications. Here we report a giant, nonvolatile resistive switching (ΔR/R > 1,000%) and strong modulation of the MIT temperature (ΔTc > 30 K) in a voltage-actuated V2O3/PMN-PT [Pb(Mg,Nb)O3-PbTiO3] heterostructure. This resistive switching is an order of magnitude larger than ever encountered in any other similar systems. The control of the V2O3 electronic properties is achieved using the transfer of switchable ferroelastic strain from the PMN-PT substrate into the epitaxially grown V2O3 film. Strain can reversibly promote/hinder the structural phase transition in the V2O3, thus advancing/suppressing the associated MIT. The giant resistive switching and strong Tc modulation could enable practical implementations of voltage-controlled Mott devices and provide a platform for exploring fundamental electronic properties of V2O3.

Strain-induced resistance change in V2O3 films on piezoelectric ceramic disks

We prepared a stacked structure consisting of a quasi-free-standing functional oxide thin film and a ceramic piezoelectric disk and observed the effect of the piezoelectric disk deformation on the resistance of the thin film. Epitaxial V2O3 films were grown by a pulsed laser deposition method on muscovite mica substrates, peeled off using Scotch tapes, and transferred onto piezoelectric elements. In this V2O3/insulator/top electrode/piezoelectronic disk/bottom electrode structure, the resistance of the V2O3 film displayed a variation of 60% by sweeping the piezoelectronic disk bias. With support from x-ray diffraction measurements under an electric field, a huge gauge factor of 3 × 103 in the V2O3 film was inferred. The sizeable resistance change in the V2O3 layer is ascribed to the piezo-actuated evolution of c/a ratios, which drives the material towards an insulating phase. A memory effect on the resistance, related to the hysteretic displacement of the piezoelectric material, is also presented.

X-ray induced vanadium L3M23M45 auger electron spectra as tool to trace vanadium oxidation state: Study of surface stoichiometry recovery of V2O3 film

Ratio of two peaks in X-ray induced vanadium L3M23M45 Auger electron spectra has been used to monitor oxidation state of V2O3 thin film surface followed by vacuum annealing. The V2O3 film was grown with sol-gel method on the Al2O3 (0001) substrate. The O 1s, V 2p core-level X-ray photoelectron spectra and V L3M23M45 Auger electron spectra were measured from the film as function of annealing temperature. Brief exposure of the film to the air turned surface vanadium to 4+ oxidation state. Annealing in UHV at 455 K completely converted vanadium to 3+. The binding energy (BE) difference between O 1s and V 2p3/2 core-level changed from 14.0 eV to 14.9 eV as annealing temperature increased from room temperature to 455 K. Intensity ratio of the two peaks in V L3M23M45 Auger electron spectra, peak originated from O 2p region to that from V 3d region, changes from 0.91 to 0.63. In combination with measurement from VO2 film, relation between intensity ratio of V L3M23M45 doublet and the oxidation state of vanadium was established by comparison with BE difference between O 1s and V 2p3/2 core-level. V L3M23M45 Auger electron spectra can be useful tool for scanning Auger microscope (SAM) to get high spatial resolution vanadium ion distribution. A few points to consider for application toward SAM mapping have been discussed.

Comparison of surface structures of corundum Cr2O3(0 0 0 1) and V2O3(0 0 0 1) ultrathin films by x-ray photoelectron diffraction

Thin Cr2O3(0 0 0 1) layers are formed by oxidation of a Cr(1 1 0) single crystal. This surface is further modified by growing an epitaxial ultrathin V2O3(0 0 0 1) film by reactive vapor deposition. Synchrotron based soft-x-ray photoemission spectroscopy and x-ray photoelectron diffraction are used to characterize the surface layers of these two corundum-structured oxides. By comparison of experimental XPD patterns with simulated electron multiple scattering calculations, two distinctively different surface terminations are extracted for the two oxides. While for V2O3 we confirm the previously proposed vanadyl-terminated surface structure, we propose a new surface structure for Cr2O3 that consists of excess chromium atoms occupying interstitial sub-surface sites.

Optical and electrical performance of thermochromic V2O3 thin film fabricated by magnetron sputtering

V2O3 was recognized as one of the clearest examples of Mott-Hubbard physics but was rarely treated as transmittance modulating coatings based on the metal-insulator transition. Here, we deposited high quality epitaxial V2O3 thin films on sapphire (001) substrates as well as polycrystalline V2O3 films on glass or Si substrates, measured the optical properties during cooling and heating, and discussed the modulating ability using fully reversible optical hysteresis loops. Meanwhile, we also optimized the electrical performance of V2O3/SiNx/Si samples by W-Ti co-doping. The obtained co-doped V2O3 film shows a relatively high temperature coefficient resistance of −8.1%/K at 80 K without hysteresis loops, which exhibits great potential in high sensitivity thermal resistor devices. Our work provides a comprehensive scenario of the V2O3 thin film physics.

Time-resolved light scattering by photoexcited V2O3

ABSTRACTUsing ultrafast angle-resolved light scattering technique, we were able to trigger photoinduced phase transition processes in V2O3 film grown on a glass substrate. The phase transition is caused by photoacoustic wave in the film and appears as coherent oscillations of scattering signal at various time scales. These processes strongly depend on the size of microstructures constituting the V2O3 film. One of the key findings of our study is the presence of a size dependent phase transition threshold for V2O3 microstructures, where small size structures ($\tilde <$ 0.5μm) have lowest contribution to the phase transition. The presence of this threshold can be well described by considering uneven internal strain in the films which is one of the key parameters controlling phase transition dynamics in various vanadium oxides.

Room-temperature epitaxial growth of V2O3 films

Herein we report the room-temperature epitaxial growth of V2O3 films by laser molecule beam epitaxy. X-ray diffraction profiles show the room-temperature epitaxial V2O3 films orient in the [110] direction on α-Al2O3 (0001) substrates. Atomic force microscopy measurements reveal that the ultra-smooth surfaces with root-mean-square surface roughness of 0.11 nm and 0.28 nm for 10-nm-thick and 35-nm-thick V2O3 film, respectively. X-ray photoelectron spectroscopy results indicate the V3+ oxidation state in the films. Typical metal-insulator transition is observed in films at about 135 K. The resistivities at 300 K are approximately 0.8 mΩ cm and 0.5 mΩ cm for 10-nm-thick and 35-nm-thick V2O3 film, respectively.

Direct observation of the lattice precursor of the metal-to-insulator transition in V2O3 thin films by surface acoustic waves

A surface acoustic wave (SAW) delay line is used to study the metal-to-insulator (MI) transition of V2O3 thin films deposited on a piezoelectric LiNbO3 substrate. Effects contributing to the sound velocity shift of the SAW which are caused by elastic properties of the lattice of the V2O3 films when changing the temperature are separated from those originating from the electrical conductivity. For this purpose the electric field accompanying the elastic wave of the SAW has been shielded by growing the V2O3 film on a thin metallic Cr interlayer (coated with Cr2O3), covering the piezoelectric substrate. Thus, the recently discovered lattice precursor of the MI transition can be directly observed in the experiments, and its fine structure can be investigated.

Epitaxial Growth of V2O3 Thin Films on c-Plane Al2O3 in Reactive Sputtering and Its Transformation to VO2 Films by Post Annealing

Epitaxial growth of thin vanadium sesquioxide (V2O3) films on c-plane sapphire (c-Al2O3) substrates was achieved with reactive magnetron sputtering under restricted oxygen flow. Even with a film thickness of approximately 12 nm, highly c-axis textured growth of corundum V2O3 was realized because of the smaller mismatch of V2O3 against corundum Al2O3. Post annealing in O2 atmosphere for as-grown V2O3 films caused phase transformation to oxidized crystalline phases. At a moderate annealing temperature of 450 °C, the V2O3 thin films transformed to VO2 films, which show a resistivity change of over three orders of magnitude. The X-ray photoelectron spectroscopy spectra for the annealed VO2 film showed a single charge state of V4+, indicating a homogeneous crystalline structure, in contrast to the inhomogeneous feature with mixed charge states of V in addition to V3+ for as-grown V2O3 film. This method is promising to prepare thin VO2 films with metal–insulator transition in productive reactive sputtering and to examine crystalline phase transformation mechanisms, including phase coexistence.

Role of the V2O3(0001) Defect Structure in the Adsorption of Au Adatoms

The interaction of Au atoms with point defects in a vanadyl-terminated V2O3(0001) film has been investigated with low-temperature scanning tunneling microscopy (STM). For this purpose, oxygen and vanadyl vacancies were introduced into the oxide surface via electron bombardment and characterized with STM topographic imaging and conductance spectroscopy. In addition, water titration experiments have been performed to distinguish the different types of point defects. Oxide films with increasing defect densities have then been prepared to explore the interaction of individual Au atoms with the surface. Up to the highest defect concentration, regular bridge sites in the vanadyl lattice were identified to be the preferred Au adsorption sites. This is in conflict with recent DFT calculations that found Au binding to oxygen vacancies to be much stronger than to regular surface sites. Possible reasons for the discrepancy are discussed in the text.

Nucleation of gold atoms on vanadyl-terminated V2O3(0001)

The adsorption of Au atoms on a vanadyl-terminated V2O3 film grown on Au(111) is studied by means of low-temperature STM and DFT+U calculations. The adatoms preferentially bind in an bridge configuration between two adjacent groups. Missing vanadyl groups that have been identified as the characteristic surface defect do not offer an attractive binding environment and are only sparsely occupied with Au. On the other hand, varying concentrations of V2O3 bulk defects that modulate the oxide electronic structure are found to affect the spatial distribution of Au atoms on the oxide surface.

Early stages of the metal-to-insulator transition of a thin V2O3 film

The metal-to-insulator (MI) transition of a V2O3 thin film is studied, using a surface acoustic wave delay line. The V2O3 film covers not only the sound path but also one of the interdigital transducers (IDTs). The resulting mismatch of the IDTs detunes the delay line, until the film passes through the MI transition. This “self-retuning” behavior makes the device extremely sensitive to the initial changes of the electrical and dielectric properties of the film during the MI transition. Attenuation and sound velocity are measured between 260 and 4.2K. Both suggest a precursor to the MI transition in the paramagnetic metallic regime. The existence of a precursor is consistent with recent extended x-ray absorption fine structure measurements. An oscillatory behavior with steep reductions of the sound velocity is observed during the MI transition. The reductions of the sound velocity probably indicate the recently predicted anomaly at the Mott transition.

The metal–insulator transition inV2O3(0001) thin films: surface termination effects

Epitaxially grown V2O3(0001) thin films have been prepared with different surface terminations, as evidenced byatomically resolved scanning tunnelling microscopy and high-resolution electronenergy loss spectroscopy (HREELS) phonon spectra. The spectral changesobserved in valence band photoemission spectra and HREELS on cooling theV2O3 samples from 300 to 100 K have been associated with the metal–insulator transition (MIT) in the bulkof the V2O3 film. The reconstructed surface regions per se do not display the MIT, butaffect the MIT signature observed with surface sensitive techniques, dependingon the thickness of the reconstructions. Whereas the thermodynamically stable(1 × 1) vanadyl V = O surface termination allows the observation in photoemission and HREELS of a clearsignature of the MIT, the latter is screened on a surface formed by V = O defect structures. Doping of the surface with small amounts of adsorbed water restores reversibly the MIT spectralfingerprints. These observations are discussed in terms of the different geometrical andelectronic structures of the different surface terminations.

Preparation and Characterization of V2O3 Powder and Film

V2O3 powder and film were prepared by the reduction of sol–gel-synthesized V2O5. The V2O3 single phase was synthesized by reduction above 500°C for 2 h in hydrogen atmosphere. X-ray diffraction measurements revealed that the films were distorted by the substrate and that the antiferromagnetic insulator-metal (AFI-M) transition observed in resistivity temperature curves of the films was broader than those of powders. It was concluded that the broader transition was caused by film strain.