Thin V2O3 Films Research Articles

Epitaxial growth of V2O3 thin films on Si(1 1 1) by molecular beam epitaxy

Vanadium sesquioxide (V2O3) is a strongly correlated electron material exhibiting two distinct metal–insulator transitions that can be tuned via strain, doping, or pressure, making it an interesting material for new-generation sensors or smart devices. For this purpose, it is required to achieve well-ordered epitaxial thin film growth with high-quality electrical and optical properties on technologically relevant substrates. We report the successful growth of epitaxial thin films of V2O3 via molecular beam epitaxy, in the paramagnetic insulating (PI) phase on the (111) plane of silicon, by tailoring the growth conditions. Extensive electrical, structural, and morphological characterization both in situ and ex situ has been performed on all samples. The structural analysis reveals that temperature plays a more impactful role in affecting the thin film microstructures than the oxygen partial pressure. When the epitaxy of V2O3 occurs on the unoxidized (111) plane of silicon, four equivalent epitaxial domains begin to form, leading to twin boundaries in the bulk of the film. The considerable lattice mismatch between silicon and V2O3 induces the growth of the corundum PI phase. Lastly, small deviations from stoichiometry due to different oxygen inflow during growth alter significantly the resistivity change upon cooling.

Disentangling Structural and Electronic Properties in V2O3 Thin Films: A Genuine Nonsymmetry Breaking Mott Transition.

Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the combination of material synthesis and photoelectron spectroscopy, we demonstrate a genuine Mott transition undressed of any symmetry breaking side effects in the thin films of V2O3. In particular and in contrast with the bulk V2O3, we unveil the purely electronic dynamics approaching the metal-insulator transition, disentangled from the structural transformation that is prevented by the residual substrate-induced strain. On approaching the transition, the spectral signal evolves slowly over a wide temperature range, the Fermi wave-vector does not change, and the critical temperature is lower than the one reported for the bulk. Our findings are fundamental in demonstrating the universal benchmarks of a genuine nonsymmetry breaking Mott transition, extendable to a large array of correlated quantum systems, and hold promise of exploiting the metal-insulator transition by implementing V2O3 thin films in devices.

Correlation Mechanism of the Insulator–Metal Transition in V2O3 Films

A complex character of the mechanism of thermal phase transformations from the insulating phase to the metallic phase has been revealed in thin V2O3 films. The insulator–metal phase transition in V2O3 is shown to consist of two stages: the hysteresis-less temperature-extended electron Mott transition extended in temperature and the stepwise structural Peierls transition with temperature hysteresis. The features of the insulator–metal phase transition revealed for V2O3 are discussed. These features are analyzed on the base of their comparison with characteristic features of analogous phase transition in VO2 films.

Toward reproducible metal-insulator transition characteristics in V2O3 thin films sputter-deposited on glass

Vanadium sesquioxide (V2O3) exhibits remarkable property changes through its metal-insulator transition near 150 K and is a very promising candidate for device applications. Thin V2O3 films were deposited on SiO2 glass by reactive DC magnetron sputtering. While resistivity changes over four orders of magnitude were demonstrated, films deposited under nominally identical conditions exhibited significant differences in electrical characteristics, which would hinder reproducibility under deposition techniques appropriate for industrial scale production with standard control features. These differences were attributed to small deviations from exact stoichiometry. A post-deposition thermal treatment consistent with equilibrium temperature and oxygen pressure conditions for V2O3 applied to the samples succeeded in nearly equalizing their characteristics within a relatively short time and without negatively impacting the glass substrate or film continuity. Analysis of film structure, morphology, and resistivity measured from room temperature through the metal-insulator transitions, both before and after the thermal process, revealed information about the interplay between non-stoichiometry, residual stress, and electrical characteristics of the films. The approach employed can lead to reproducible results for V2O3 films and is applicable to similar materials which exhibit metal-insulator transitions.

Metal–insulator phase transition in hydrogenated thin films of V2O3

Temperature dependences of the electrical conductivity of thin vanadium sesquioxide V2O3 films obtained by using the laser sputtering technique have been studied. A significant decrease (by four–five orders of magnitude) in the electrical conductivity has been observed below 150 K as a result of a metal–insulator phase transition. It is shown that hydrogenation of films lowers the temperature of this phase transition.

Surface termination of an ultrathin V2O3-film on Au(111) studied via ion beam triangulation

The structure of the topmost layer of thin V2O3 films on a Au(111) substrate is studied via Ion Beam Triangulation. From electron emission induced by fast H atoms scattered from the film surface under a grazing angle of incidence as function of azimuthal rotation of the target, we find evidence for a reconstructed O3 termination as proposed from DFT-calculations and recent experimental work using methods based on large angle impact of fast ions. From our studies we derive detailed lateral positions for the topmost O atoms.

Increased metal-insulator transition temperatures in epitaxial thin films of V2O3 prepared in reduced oxygen environments

Thin films of V2O3 were grown epitaxially on c-plane sapphire substrates by oxygen plasma-assisted thermal evaporation. Reducing the amount of oxygen supplied during growth led to a nearly 50 K increase in V2O3’s metal-insulator transition temperature to a temperature as high as 184 K. By systematically varying the oxygen pressure the transition temperature monotonically increased, which was accompanied by a concomitant increase in the room-temperature resistivity. These trends are consistent with a continuous change in the stoichiometry of V2O3.

Epitaxial Growth of Phase Transition V2O3 Thin Films on c-Al2O3 in Reactive Sputtering

Vanadium sesquioxide (V2O3) is known as a crystalline phase of vanadium oxide with metal-insulator transition (MIT). This V2O3 has hexagonal corundum crystalline structure same as sapphire. In this study, highly c-axis textured V2O3 crystalline growth on c-plane sapphire substrate was realized even at film thickness around 15 nm under carefully-controlled small oxygen flow conditions in reactive magnetron sputtering. In-plane compressive strain state was found for the thin V2O3 films, while lattice relaxation was observed with increase of film thickness. Under higher growth temperature, abrupt MIT of the films with resistance change over several decades was achieved at temperature around 160 K.

Thickness dependence of electronic phase transitions in epitaxial V2O3 films on (0001) LiTaO3

Single crystal epitaxial thin films of V2O3 were grown on (0001) LiTaO3 by pulsed laser deposition. X-ray diffraction and atomic force microscopy data show that the deposits were initially pseudomorphic, that they underwent plastic relaxation at a critical thickness of ≈16 nm, and that relaxation is accompanied by the development of surface roughness, increasing with deposit thickness. These effects lead to changes in electrical properties of the films as a function of temperature. As film thickness increases the properties go from insulator-insulator to metal-insulator, then metal-metal transitions. The thickest films (>200 nm) remained metallic over the temperature range of the measurements.

Resistivity of V2O3 thin films deposited on a-plane (110) and c-plane (001) sapphire by pulsed laser deposition

Thin films of V2O3 with thickness of 215nm were grown on a- and c-plane sapphire by pulsed laser deposition with (001)V2O3∥(001)Al2O3 and (110)V2O3∥(110)Al2O3 epitaxy. The effects of the growth direction on the electrical resistivity of the films were examined. Films on c-plane sapphire displayed a metal-to-insulator transition at T=180K compared to T=160K in single-crystal V2O3. The films on a-plane sapphire, however, showed an insulator-to-insulator transition at T=186K. The variation in the phase transformation characteristics and the resistivity can be attributed to different levels of strain and commensurate changes in the film morphology.

Surface acoustic wave investigations of the metal-to-insulator transition of V2O3 thin films on lithium niobate

Thin V2O3 films were deposited on a piezoelectric substrate by electron-beam evaporation. Surface acoustic waves were generated by interdigital transducers. The attenuation and sound velocity were investigated from 260 to 10 K, providing an insight into the temperature-dependent electrical, dielectrical, and elastic properties of V2O3 at the metal-to-insulator transition.

Structural and electronic properties of epitaxialV2O3 thin films

Thin films of V2O3 with thickness 4–300 nm were grown on -oriented sapphire substrates by reactive dc magnetron sputtering. X-ray diffraction, polefigure measurements and scanning tunnelling microscopy show high crystallinity andepitaxy to the substrate with a faceted surface structure, and the absence of strain.Measurements of the electrical resistivity, scanning tunnelling and x-ray absorptionspectroscopy show a metal–insulator transition near 150 K that is connected with theopening of an energy gap and a characteristic modification of the absorption spectrum atthe vanadium-2p and oxygen-1s edges. These observations reveal that the films have bulk-like properties.

Correlation Between Structural and Electronical Properties Of Strained V2O3 Thin Films

AbstractEpitaxial thin films of V2O3 were grown on c-axis oriented sapphire substrates by reactive e-beam evaporation. The high orientation and epitaxy is shown by low energy electron and x-ray diffraction. The microstucture of the films was investigated by AFM imaging. The measurements show an island growth mode with a temperature dependent increase of the grain size. At a growth temperature of above 700°C a change of the surface structure is observed, which produces much more structured and irregular shaped crystallites. High temperature growth also causes changes in the electronical properties, due to an increased stress in the films as measured by XRD. The metal-insulator transition (MIT) and the properties of the metallic phase of the films were determined mainly by temperature dependent electrical resistivity measurements. The metallic properties are distorted in films grown at low temperature (T<500°C) probably due to defects, dislocations and grain boundaries. Favorable growth conditions for films showing a single crystal like MIT were found in a narrow temperature range around 550°C. For higher growth temperatures (T>650°C) metallic behavior at high temperatures is suppressed by an in plane tensile stress. The electronic properties of such films are similar to those of chromium doped V2O3. The relation between growth conditions, structural properties and resulting electronic properties is discussed.

Coherent V2O3 Precipitates in ɑ-Al2O3 Co-Implanted With Vanadium and Oxygen

AbstractThe oxides of vanadium VO2 and V2O3 are of fundamental and practical interest since they undergo structural phase transitions during which large variations in their optical and electronic properties are observed. In the present work, we report the formation of buried precipitates of V2O3 in sapphire by ion implantation and thermal annealing. It was found that the co-implantation of oxygen and vanadium was required in order to form nanophase V2O3 precipitates. Additionally, these precipitates, which formed only following an anneal of the co-implanted sample under reducing conditions, are coherent with the sapphire lattice. Two epitaxial relationships were observed: (0001)V2O3//(0001) ɑ-Al2O3 and (11-20)V2O3//(0001) ɑ-Al2O3. This finding is in agreement with results obtained elsewhere for thin films of V2O3 deposited on c-axis-oriented sapphire.