Structural transformation and functional properties of vanadium oxide films after low-temperature annealing (original) (raw)
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Optics & Laser Technology, 2013
Vanadium oxide thin films were grown onto quartz substrates using the pulsed DC reactive magnetron sputtering technique at room temperature and afterwards post annealed under vacuum conditions in the temperature range from 75 to 230 1C. The electrical resistance, temperature coefficient of resistance (TCR), optical energy gap and structural properties were investigated. The films are amorphous, nanoscale grained V 2 O 5 phase and the mean grain size increases with increasing temperature. Additionally, the post-annealing at 230 1C induces formation of both V 2 O 5 and V 4 O 9 phases and pinholes on the film surface. The temperature dependent variation of the electrical resistance indicates two activation energy areas corresponding to two TCR values for the films post annealed up to 180 1C, but only one activation area was found after annealing at 230 1C. Analyses of the absorption coefficient versus photon energy revealed a direct forbidden transition. The mean grain size and TCR values increase with increasing post-annealing temperature, whereas the optical energy gap and electrical resistance do not follow this tendency. The evolution of the structure and its correlation to the optical energy gap, electrical resistance, activation energy and TCR were discussed by means of the results obtained in the present study.
Synthesis and characterization of vanadium oxide thin films on different substrates
Journal of Materials Science: Materials in Electronics, 2017
In this study, the V8O15 derivative of vanadium oxide was produced on plain glass, indium tin oxide and silicon wafer substrate layers by taking advantage of wet chemical synthesis which is an easy and economical method. The structural properties of the produced films were examined by XRD and SEM analyses. Besides, Al/VOx/p-Si metal-oxide-semiconductor (MOS) structure was obtained by the same synthesis method. Doping densities of these MOS structures were calculated from frequency dependent capacitance–voltage measurements. It was determined that the interface states which were assigned with the help of these parameters vary according to frequency.
Modern Technologies for Creating the Thin-film Systems and Coatings, 2017
This chapter describes a new deposition method proposed to achieve Vanadium Oxide VO x /V 2 O 5 thin films with high temperature coefficient of resistance (TCR), intended to be used as functional material in IR microsensors (bolometers). The main aim of the work is to attain a deposition method compatible with the lift-off microstructuring technique in order to avoid the use of a reactive-ion etching (RIE) process step to selectively remove the VO x /V 2 O 5 deposited layer in the course of the definition of the bolometer geometry, preventing the harmful effects linked to the spatial variability and the lack of selectivity of the RIE process. The proposed technique makes use of a two-stage process to produce the well-controlled VO x or V 2 O 5 thin films by applying a suitable thermal annealing to a previously deposited layer, which was obtained before at room temperature by RF magnetron sputtering and patterned by lift-off. A set of measurements has been carried out with thin films attained in order to check the quality and properties of the materials achieved with this method. The results reached with V 2 O 5 pure phase films are consistent with a charge transport model based on the small polarons hopping derived from Mott's model under the Schnakenberg form.
Objective: To deposit the highly crystalline thin film of vanadium pentoxide on Si substrate. Method: In this work, we deposited vanadium oxide thin films by RF sputtering setup. These deposited thin films were annealed at 500°C for 1 hour in argon atmosphere. Grazing Incidence X-Ray Diffraction (GIXRD), Raman Spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) are used to analyze structural properties of as-deposited and annealed thin films of vanadium oxide. Finding: GIXRD spectra of annealed film revealed that highly crystalline thin film of vanadium pentoxide (V2O5) is obtained. The texture of the film is oriented along c- axis, perpendicular to the surface of the Si substrate and it’s a, b axis are parallel to the surface of substrate. Raman Spectroscopy and FTIR results confirmed the layered structure of the annealed vanadium pentoxide thin film. After post annealing, the highly crystalline and layered structure of V2O5 thin film on Si substrate is obtained. Applications: Vanadium pentoxide thin films are used in electrochromic devices, lithium batteries, and energy storage devices and as toxic gas sensors.
Journal of The Electrochemical Society, 1999
Vanadium oxides have been extensively used in a large number of scientific and technological applications. 1 Among the many compounds present in the V-O system, 2 VO 2 and V 2 O 5 are undoubtedly the most studied because of their electrical, optical, and catalytic properties. Stoichiometric vanadium (V) oxide has the d band unoccupied; however, the oxygen vacancies usually present in the lattice 3 make this oxide an n-type semiconductor with an optical bandgap of about 2.6 eV. The main technical uses of V 2 O 5 , when deposited as thin film, can be found in electrochromic devices, oxidation catalysts, and optical switches. 5 Due to the semiconductor-metal phase transition, corresponding to a distortion in the crystal structure, that results in an increase in the electrical conductivity, 6 VO x thin films can find important applications, such as thermochromic devices and variable-reflectance mirrors. Actually VO 2 can be considered as a prototype of transition metal oxides showing the first-order metalinsulator transition (MIT). Abrupt changes in electrical, optical and magnetic properties are detected across the transition temperature, which is around 68ЊC for bulk VO 2 . 7 These dramatic changes in the physical properties are reversible, but affected by hysteresis cycles depending on the microstructure and stoichiometry of the films used. 8 In order to optimize functional properties of this material it is very important to control the crystal structure and stoichiometry during thin film, growth. To this purpose it is important to remember that three different crystal phase of VO 2 are currently known: the most stable one is the rutile structure, while there are two metastable phases, A and B, characterized by a layered structure and a higher oxygen content. 9 V 2 O 5 thin films have been prepared by vacuum evaporation, by the sol-gel method, and by spin-coating from organic vanadium solutions, 10 while VO 2 films have been deposited by reactive ion beam sputtering, magnetron sputtering, chemical vapor deposition (CVD), 11 and, recently, by the inorganic sol-gel technique. 12 Oxides in the range VO 2 -V 2 O 5 can also be grown by thermal oxidation of metallic vanadium substrates. CVD of vanadium oxides thin films has been performed starting from vanadyl trichloride, 14a vanadium(III) 14d and vanadium(IV) acetylacetonate, 14f vanadyl triisopropoxide, 14b and triethoxide. 14c,e In this study we report results about the synthesis and characterization of vanadium oxide thin films, obtained by the CVD method using a series of vanadyl precursors of formula VO(L) 2 (X) where L is hfa, acac, dpm, and fod (Hhfa ϭ 1,1,1-5,5,5-hexafluoro-2,4-pentanedione; Hacac ϭ 2,4-pentanedione; Hdpm ϭ 2,2-6,6-tetramethyl-3,5-heptanedione; Hfod ϭ 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione; X ϭ H 2 O when L ϭ hfa). These complexes have the advantage, with respect to the other precursors previously used, to be easily prepared and purified (all manipulations are carried out in air and in aqueous solvents). They are characterized by different vapor pressures, the VO(hfa) 2 (H 2 O) being the most volatile, 15 and by different stability and reactivity depending on the ligand nature. The aim of the present work is to examine how different ligands, in the framework of vanadyl(IV) complexes, can influence the composition, microstructure, and morphology of the grown films. For instance, it is well known that the replacement of methyl groups by trifluoromethyls strengthens the C=O and C=C bonds, while the M-O bonds weaken. As reported in previous works, the thermal decomposition of fluorinated -diketonato VO(L) 2 complexes is different from that of nonfluorinated ones. 15 It is also expected that the reactivity toward H 2 O of the M-O bonds depends on the values of K b s of the ligands used.
Effects of microstructure and nonstoichiometry on electrical properties of vanadium dioxide films
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1989
Voided growth structures of sputter-deposited films affect strongly their optical and electrical properties. Vanadium dioxide is an interesting material to study effects of film microstructure and nonstoichiometry on electrical properties because its phase transition makes it possible to investigate electrical behavior both in a semiconducting phase and in a metallic phase. We have deposited vanadium oxide films with different vanadium/oxygen ratios for substrate temperatures between 250 and 550°C by dc reactive magnetron sputtering. The resistivity ratios between a semiconducting phase and a metallic phase are limited to 10 3 order by voided boundaries and oxygen vacancies. The voided boundaries are defined by columnar structure and agglomerated grain growth. The results emphasize the necessity of a combination of deposition to obtain the film with a favorable structure and postdeposition annealing to control the film stoichiometry.
Synthesis and characterization of vanadium oxide films by post-oxidation and reactive sputtering
Materials Science and Engineering: B, 2007
Vanadium oxide films were fabricated by two techniques-reactive deposition in oxygen-containing plasma and metal vanadium vacuum deposition with subsequent oxidation in air. It has been shown that the latter process allows formation of the films having reduced phase transition temperature ∼50 • C. The proposed capacitor design of sensitive element excludes electrical contacts to vanadium oxide, so the element is less crytical to degradation and current noise effects.
Matéria (Rio de Janeiro)
Vanadium oxide films amorphous, nonstoichiometric and highly absorbing in the optical region were deposited on ITO-coated glass and on silicon substrates, by the hot-filament metal oxide deposition technique (HFMOD) and oxidized by ex-situ annealing in a furnace at 200, 300, 400 and 500 ºC, under an atmosphere of argon and rarefied oxygen. X-ray diffraction, Raman and Rutherford backscattering spectroscopy as well as optical transmission were employed to characterize the amorphous and annealed films. When annealed at 200 and 300 ºC the as-deposited opaque films become transparent but still amorphous. Under treatments at 400 and 500 ºC a crystalline nonstoichiometric V 2 O 5 structure is formed. All the annealed films became semiconducting, with their optical absorption coefficients changing with the annealing temperature. An optical gap of 2.25 eV was measured for the films annealed at 400 and 500 ºC. The annealing in rarefied oxygen atmosphere proved to be a useful and simple ex-situ method to modulate the structural and optical properties of vanadium oxide films deposited by HFMOD technique. This technique could be applied to other amorphous and non-absorbing oxide films, replacing the conventional and sometimes expensive method of modulate desirable film properties by controlling the film deposition parameters. Even more, the HFMOD technique can be an inexpensive alternative to deposit metal oxide films.
Thin Solid Films, 2013
A simple and cost effective sol-gel process for producing vanadium dioxide thin films was developed via thermolysis of V 2 O 5 ·nH 2 O (n≈ 2) V V precursors prepared by dissolving vanadium powder or V 2 O 5 powder in 30% hydrogen peroxide solutions. After spin-coating on fused silica substrates and annealing at 750°C in vacuum, without any intermediate gas reducing step, the major phase VO 2 (M, monoclinic phase) was found in both of the films based on V-H 2 O 2 and V 2 O 5 -H 2 O 2 precursor, exhibiting large transmittance changes (>40%) in the IR region (>2000 nm) and small hysteresis loop width (b 5°C) which were comparable to reported epitaxial VO 2 films. The two films have similar metal-to-insulator transition temperature τ C =62.5°C, lower than the classical value of 68°C for VO 2 thin films. In addition, the method enables simple doping, as found for 0.56 at.% W-doped VO 2 films. This intrinsically simple solution process followed by one-step annealing makes it potentially useful in smart window applications.