Properties Of V2O5 Thin Films Research Articles

Optical, Electrical, Structural, and Thermo-Mechanical Properties of Undoped and Tungsten-Doped Vanadium Dioxide Thin Films.

The undoped and tungsten (W)-doped vanadium dioxide (VO2) thin films were prepared by electron beam evaporation associated with ion-beam-assisted deposition (IAD). The influence of different W-doped contents (3-5%) on the electrical, optical, structural, and thermo-mechanical properties of VO2 thin films was investigated experimentally. Spectral transmittance results showed that with the increase in W-doped contents, the transmittance in the visible light range (400-750 nm) decreases from 60.2% to 53.9%, and the transmittance in the infrared wavelength range (2.5 μm to 5.5 μm) drops from 55.8% to 15.4%. As the W-doped content increases, the residual stress in the VO2 thin film decreases from -0.276 GPa to -0.238 GPa, but the surface roughness increases. For temperature-dependent spectroscopic measurements, heating the VO2 thin films from 30 °C to 100 °C showed the most significant change in transmittance for the 5% W-doped VO2 thin film. When the heating temperature exceeds 55 °C, the optical transmittance drops significantly, and the visible light transmittance drops by about 11%. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to evaluate the microstructure characteristics of VO2 thin films.

Boosting the UV photo-detecting properties of V2O5 thin films by doping In3+ doping

In recent years, optoelectronic technology has demanded the development of efficient photodetectors by layered semiconductor materials at affordable prices. Herein, we put the efforts to generate V2O5 thin films on the surface of glass slides with diverse insertion of In content (0, 1, 2, 3, 4, and 5 wt%) via cost effective nebulizer spray pyrolysis technique and examined their application in U.V. photosensing. Standard characterization techniques are used for these films to identify the cause of In dopant on its morphological, structural, and opto-electrical properties. X-ray diffraction (XRD) plots confirm the development of single-phase orthorhombic structure grown along [010] direction, without the appearance of foreign phases. Further, the dopant ingredient (In) hikes the crystallinity and crystallite sizes (36–51 nm) up to a certain threshold level (2 wt%) and decreases it for further growing In doping. The Field emission scanning electron microscope (FESEM) images display homogeneous nanorods morphology formation at all doping levels. Doping improved its photon absorption in the U.V. region, and the bandgap obtained from the absorbance spectra tuned to a lower value of 1.98 eV up to 2 wt%. All the created thin films emit intense emission bands at 475 nm and 640 nm. Characterization involving I-V measurements under 365 nm illumination at 5 V bias exhibiting commendable performance in terms of photodetector metrics (Responsivity (R) = 7.96 × 10−1 AW−1, Detectivity (D*)=3.63 × 1011 Jones and External quantum efficiency (EQE) = 186 %) for 2 % In doped V2O5. The same sample exhibits a fast-rising /falling time up to 0.6s /0.7 s, respectively. Our results may widen the field of applications in high-performance U.V. photodetector.

Investigation of Optical and Structural Properties of VO2 Thin Films by Thermal Treatment

In this study, VO2 thin films were grown on SiO2 substrate using the PLD method. Then the grown VO2 thin films were subjected to heat treatments at different temperatures and times in the atmosphere environment. The structural properties of the heat-treated films were investigated using X-ray diffraction (XRD) technique and Raman spectroscopy techniques. The surface microstructures of these films were investigated by Scanning Electron Microscope (SEM) technique, and their optical properties and bond structures were investigated by Photoluminescence (PL) spectra and Fourier Transform Infrared Spectroscopy (FTIR) measurements, respectively. Especially XRD and Raman results revealed that heat treatments with high temperature values transformed the films into VO2 and V2O5 mixed-phase crystal structures. Due to the heat treatment carried out in the atmosphere and the high oxygen affinity of the vanadium metal, crystallization took place in both VO2 and V2O5 forms. In order to obtain homogeneous crystalline VO2 structures, heat treatments should be carried out for a long time in oxygen-limited environments.

Impact of the crystallographic variants of VO2 thin films on c- and r-cut sapphire on structural phase transition and radiofrequency properties

Vanadium dioxide exhibits a metal to insulator transition close to room temperature, making it very interesting in particular for radio frequency (RF) device applications. Here, we compare the structural and RF properties of VO2 thin films grown by magnetron sputtering on c-cut and r-cut sapphire substrates. The epitaxial growth of VO2 on c-cut sapphire gives rise to several crystallographic variants for the insulating M1 phase. Moreover, during the structural transition, simultaneous presence of both metallic and insulating phases is evidenced by x-ray diffraction over a large temperature range. Films grown on r-cut sapphire exhibit only two variants and present a very narrow temperature range of their structural transition. Interestingly, such structural differences of the films grown on c- and r-cut sapphire substrates have very little influence on their dc resistivity, while the transmission of the RF signal through the metallic phase was found much lower on c-cut than on r-cut sapphire. This supports the fact that r-cut sapphire is preferable for VO2-based RF device fabrication.

Characterization of the VO2 thin films grown on glass substrates by MOD

AbstractThis paper describes the deposition of Nb doped vanadium dioxide (VO2) films on a glass substrate by metal organic decomposition (MOD) and their thermochromic properties. The difference in thermochromic properties of VO2 thin films on a glass substrate was investigated with and without a buffer layer of Hf0.5Zr0.5O2 (HZO). The phase transition temperature of VO2 thin film successfully reduced from 83°C to 43°C on a glass substrate with a buffer layer of HZO. Without a buffer layer of HZO, the thermochromic properties of VO2 thin films deteriorated comparing with a buffer layer of HZO. HZO buffer layer effectively suppresses the miniaturization of VO2 crystallite size of thin film due to Nb doping. Moreover, it would block out the diffusion of Al, Na, and Ca impurity ions from a glass substrate and the partial oxidation of VO2 thin films judging from XPS O1s spectra and XPS depth profile analysis. We conclude that the insertion of the HZO buffer layer is a useful technique for controlling the transition temperature of VO2 for the smart window applications by MOD.

Tailoring the thermochromic properties of sputter-deposited VO2 films by O2+ ion irradiation

Vanadium dioxide (VO2) thin film is a well-known phase transition material showing extensive modification in their physical properties, such as electronic, magnetic, and optical characteristics when the metal–insulator transition (MIT) occurs. Many approaches (including doping, micro-structure, external stress, and defect engineering) can enhance the MIT characteristics of vanadium dioxide thin films. More recently, low-energy ion irradiation has been well-established to modify the electrical, optical, and physical properties of metal oxide thin films. In this paper, the effect of low-energy O2+ ion irradiation on the thermochromic properties of VO2 thin films was investigated. From the temperature-dependent near infrared transmittance measurements, both MIT temperature and thermal hysteresis width of VO2 thin films showed an opposite trend and decreased as the ion fluence increased. The values of solar modulation and luminous transmittance of irradiated VO2 films were significantly altered by introducing low-energy ion irradiation, compared to the pristine VO2 thin films. According to the XPS analyses, the vanadium ion are in mixed of two valence states (V4+ and V5+). Moreover, the concentration of oxygen vacancies in VO2 films increases as the fluence of low-energy O2+ ion increases. Our study concludes that low-energy O2+ ion irradiation is an effective means to optimize the MIT characteristics of VO2 thin films to a certain extent.

Enhancing the optoelectronic properties of V2O5 thin films through Tb doping for photodetector applications

Using highly economical and scalable nebulizer-aided spray pyrolysis approach, a high-quality terbium (Tb) doped vanadium pentoxide (V2O5) thin film-based optical sensor was developed. To investigate the effects of various terbium doping concentrations on its growth and optoelectronic features, the structural properties as well as its elemental composition, surface morphology, photoluminescence, optical, and current-voltage (I-V) characteristics were examined. An orthorhombic structure with orientation along the (010) plane was found to be existing in the X-ray diffraction pattern of Tb-doped V2O5 thin films with increase in the crystallite size upto 3 % Tb-doped V2O5 films. The prevalence of dense nanorods is observed in all the films while analyzing their micrograph from a field emission scanning electron microscope. Also it is noted that the addition of 3 % Tb dopants increases UV absorption by lowering its energy gap from 2.43 to 2.37 eV. A strong green emission peak at 530 nm and weak UV bands were noted in the PL spectrum due to higher crystalline quality, free from defects and native flaws for the 3% Tb-doped V2O5. Terbium-doped V2O5 films exhibited elevated UV photo response and photoelectric properties than their pristine form. Among various doping concentrations, 3 % terbium in V2O5 thin film exhibited a maximum response (4.72 × 10−1 A/W), detectivity (7.82 × 1010 Jones), external quantum efficiency (110 %), and high photo-switching characteristics. This study confirms the optimal 3 % Tb dopant concentration plays a significant role in enhancing the UV–visible photo-sensing behavior of the V2O5 thin films. Data availability statementThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Electron transport properties and free electron density evolution during the phase change in VO2 thin films

The experimental results on the phase change behavior of vanadium dioxide (VO2) as a function of temperature are invariably examined in the light of the strong electron correlation-based Mott-Hubbard model or the periodic lattice distortion based Peierls model with limited success. Further, the experimental conditions-based nanostructure imparted to the VO2 thin films also plays an important role. Hence, it has been suggested that the theoretical model that best suits the experimental conditions needs to be applied. In this work, we have examined the electron transport properties of VO2 thin films of various thicknesses all along the reversible phase change cycle. These results have led us to carefully examine the free electron density (ne) evolution all along the phase transition cycle and to establish for the first time the reversible hysteresis cycles of this important parameter. The intimate correlation between the degree of change in ne and the resistance seen in our samples leads us to believe that our results may be in favor of the Mott-Hubbard model of phase transition.

Enhancement of phase transition properties of ultrathin VO2 thin films on microsphere array by vacuum thermal pre-annealing

Vanadium dioxide (VO2) thin films were fabricated on SiO2 microsphere array surface by vacuum thermal pre-annealing and rapid thermal oxidization annealing of magnetron sputtering vanadium films. The effects of vacuum thermal pre-annealing temperature on phase transition properties of VO2 thin films were investigated. The resistance transition ranges of phase transition are improved from 7.58 to 10.23 as the increasing of vacuum thermal pre-annealing temperature. The phase transition temperature decreases and the width of thermal hysteresis loop keeps constant. We analyzed the effects of vacuum thermal pre-annealing temperature on phase transition properties of VO2 thin film in the light of vanadium thin film shrinking effects. The results are important for fabrication of ultrathin VO2 film on microsphere array and application in optical regulation field.

Interplay between boron doping and epitaxial relationships in VO2 films grown by laser ablation

We investigated the effect of boron doping on the functional and structural properties of VO2 thin films. Temperature-dependent measurements were performed on pure and boron-doped (0.5 and 1.3 at.%) VO2 films grown on Al2O3(0001) by Reactive Pulsed Laser Deposition. The transition temperature is initially reduced by the insertion of boron; decreasing from ca. 346 K in the case of the pure sample to 335 K for a 0.5 at.% boron amount. Further increase of the boron concentration leads to a slight decrease of the transition temperature (−2 K), a significantly broader hysteresis loop and a reduced resistivity contrast (less than 3 orders of magnitude). Raman spectroscopy and high-resolution X-ray diffraction were performed to elucidate the nature of the involved phases and their structure. A correlation was found between increasing the boron concentration and the formation of two peculiar in-plane epitaxial relationships, the VO2 films evolving from one to the other. We also evidenced the presence of the transient M2 phase in the highest doped sample.

Characterization of the VO<sub>2</sub> Thin Films Grown on Glass Substrates by MOD

This paper describes the deposition of Nb doped vanadium dioxide (VO2) films on a glass substrate by metal organic decomposition (MOD) and their thermochromic properties. The difference in thermochromic properties of VO2 thin films on a glass substrate was investigated with and without a buffer layer of Hf0.5Zr0.5O2 (HZO). The phase transition temperature of VO2 thin film successfully reduced from 83 to 43℃ on a glass substrate with a buffer layer of HZO. Without a buffer layer of HZO, the thermochromic properties of VO2 thin films deteriorated comparing with a buffer layer of HZO. HZO buffer layer effectively suppresses the miniaturization of VO2 crystallite size of thin film due to Nb doping. Moreover, it would block out the diffusion of Al, Na and Ca impurity ions from a glass substrate and the partial oxidation of VO2 thin films judging from XPS O1s spectra and XPS depth profile analysis. We conclude that the insertion of the HZO buffer layer is a useful technique for controlling the transition temperature of VO2 for the smart window applications by MOD.

Towards Room Temperature Phase Transition of W-Doped VO2 Thin Films Deposited by Pulsed Laser Deposition: Thermochromic, Surface, and Structural Analysis.

Vanadium dioxide (VO2) with an insulator-to-metal (IMT) transition (∼68 °C) is considered a very attractive thermochromic material for smart window applications. Indeed, tailoring and understanding the thermochromic and surface properties at lower temperatures can enable room-temperature applications. The effect of W doping on the thermochromic, surface, and nanostructure properties of VO2 thin film was investigated in the present proof. W-doped VO2 thin films with different W contents were deposited by pulsed laser deposition (PLD) using V/W (+O2) and V2O5/W multilayers. Rapid thermal annealing at 400-450 °C under oxygen flow was performed to crystallize the as-deposited films. The thermochromic, surface chemistry, structural, and morphological properties of the thin films obtained were investigated. The results showed that the V5+ was more surface sensitive and W distribution was homogeneous in all samples. Moreover, the V2O5 acted as a W diffusion barrier during the annealing stage, whereas the V+O2 environment favored W surface diffusion. The phase transition temperature gradually decreased with increasing W content with a high efficiency of -26 °C per at. % W. For the highest doping concentration of 1.7 at. %, VO2 showed room-temperature transition (26 °C) with high luminous transmittance (62%), indicating great potential for optical applications.

Structural, optical, and electrical properties of V2O5 thin films: Nitrogen implantation and the role of different substrates

This report investigates the effect of substrate and nitrogen (16 keV N+) ion implantation on the structural, morphological, compositional, and electrical properties of V2O5 thin films which are grown by thermal evaporation on various substrates, including glass, Si, and sapphire (termed V2O5:Gl, V2O5:Si, and V2O5:Sp, respectively). Structural analysis showed the formation of the mixed (α, and β-V2O5) phases on all substrates; however, the β-V2O5 phase is highly dominant in the V2O5:G and V2O5:Si samples. A deformation in the β-phase of V2O5 thin film under ion implantation-induced strain results in a change of crystallite size. Irradiation suppresses XRD peaks in relative intensities, indicating partial amorphization of the film with defect formation. Microstructural analysis confirmed the formation of uniform-sized nanorods for V2O5:Si, whereas isolated crystallites were formed for other types of substrates. Thermal conductivity may influence the size and shapes of V2O5 crystallite forms on different surfaces. Silicon absorbs heat more effectively than sapphire or glass, resulting in nanorod formation. A decrease in optical bandgap and electrical conduction has been observed due to increased oxygen vacancies, induced electron scattering, and trapping centres on N+ implantation. The present study thus offers the unique advantage of simultaneous reduction in optical band-gap and conductance of V2O5 thin films, which is important for optoelectronic applications.

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO2) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2 thin films remains a challenge. In this work, high-quality thermochromic VO2 thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO2 thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO2 thin films was studied. The shift of the VO2(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO2 thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO2 films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.

Effect of Annealing on the Microstructure, Opto-Electronic and Hydrogen Sensing Properties of V2O5 Thin Films Deposited by Magnetron Sputtering

This paper reports results of investigations on selected properties of vanadium oxide thin films deposited using gas impulse magnetron sputtering and annealed at temperatures in the range of 423 K to 673 K. Post-process annealing was shown to allow phase transition of as-deposited films from amorphous to nanocrystalline V2O5 with crystallite sizes in the range of 23 to 27 nm. Simultaneously, annealing resulted in an increase in surface roughness and grain size. Moreover, a decrease in transparency was observed in the visible wavelength range of approximately 50% to 30%, while the resistivity of formed vanadium pentoxide thin films was almost unchanged and was in the order of 102 Ω·cm. Simultaneously, the best optoelectronic performance, testified by evaluated figure of merit parameter, indicated the as-deposited amorphous films. Performed Seebeck coefficient measurements indicated the electron type of electrical conduction of each prepared thin film. Furthermore, gas sensing properties towards diluted hydrogen were investigated for annealed V2O5 thin films, and it was found that the highest senor response was obtained for a thin film annealed at 673 K and measured at operating temperature of 623 K.

Tuning of optical properties of doped vanadium pentoxide thin films

Thin films of vanadium pentoxide (V2O5) have been investigated for various technological applications. This article covers new research on the effects of transition and rare earth metal ions doping on the optical characteristics of thin films made of vanadium pentoxide. It has been observed that tuning of optical properties of V2O5 thin films is significantly influenced by size, concentration and filling of empty orbitals of dopant ions. In addition, optical transmittance of has been found to alter due to scattering from the dopants ions into the V2O5 lattice. Dopant induced stress, strain and oxygen defects notably influence the optical transitions from filled oxygen states to empty vanadium 3d states. Additionally, the creation of midgap states inside the forbidden bandgap cause bandgap narrowing at greater doping levels. Annealing of thin films has been found to reduce transmittance more than 50% as compared to pristine V2O5 thin films.

Structural, Optical and Electrical Properties of V2O5 Thin films at different Molarities by Spray pyrolysis method

Nowadays, Semiconducting thin films are the efficient candidates for good optical and electrical properties. In this study, the thin films of pure V2O5 together with 2 different molarities were prepared through the method of spray Pyrolysis. The prepared thin films were characterized by spectrographic tools such as XRD, FT-IR, UV-Vis and Hall effect to study their, crystalline nature, Functional group, band gap, resistivity, conductivity and mobility of the flow of electrons respectively. The structural morphology of the synthesized thin films was discussed through the micrographic image obtained from Scanning electron microscopy together with their surface occupancy plots. The obtained minimum crystallite size is about 26.8 nm for 4% molarities. The morphological and structural studies show enhanced results for 4% sample which makes it a viable candidature for optical and electrical applications.

Microstructural Investigations of VO2 Thermochromic Thin Films Grown by Pulsed Laser Deposition for Smart Windows Applications

The structural properties of VO2 thin films, grown on either LSAT or Si substrates by pulsed laser deposition (PLD), are elucidated by means of transmission electron microscopy (TEM) methods. The TEM observations confirmed the successful growth of VO2 by PLD in variable thicknesses, by optimizing the O2 partial pressure and growth temperature. The films adopt a columnar polycrystalline morphology with narrow columns, up to the film thickness height. Four VO2 polymorphs have been detected by electron diffraction and high-resolution TEM (HRTEM) analysis, with M1 being by far the most abundant phase. Post-experimental strain measurements in HRTEM images have revealed that the actual residual strain is minimized due to the columnar morphology of the VO2 grains, as well as intrinsic oxide layers in the VO2/Si epitaxy. The TEM outcomes confirmed the complementary electrical and magnetic measurements in the films, where a transition from a monoclinic M1 to a rutile VO2 R phase has been identified, influenced by the initial percentage of phases in thick VO2 films.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films.

Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Tailoring thermochromic and optical properties of VO2 thin films by pulsed laser deposition using different starting routes

The insulator to metal transition (IMT) of vanadium dioxide (VO2) at about 68 °C enables a variety of optical applications, including switching and modulation, and tuning of optical resonators. However, due to the multivalent nature of vanadium, controlling the synthesis of stoichiometric VO2 is still a challenge. Moreoever, the temperature dependent optical properties are strongly influenced by the microstructure of the VO2 thin film. In this paper, we report for the first time the effect on the microstructure and optical properties of two different routes implemented using the same PLD process to obtain VO2 thin films exhibiting an IMT; one route involving ablation of a metallic vanadium (V) target in an oxygen atmosphere, the other ablation of a vanadium oxide V2O5 target in a vacuum, both followed by a similar rapid thermal annealing process. Each route leads to significantly different VO2 thin films, exhibiting an IMT transition temperature of 69 °C with the V2O5 route, and of 50 °C with the V + O2 route. In addition, VO2 films obtained from V + O2 have a narrower hysteresis width, a smoother surface with very low roughness, a slightly higher refractive index, a higher absorption coefficient, and a narrower optical band gap than VO2 film obtained from V2O5. The thermochromic and optical properties of both VO2 are discussed in relation with the nanostructural properties. Our findings highlight the value and versatility of the PLD process in adapting the optical properties of VO2 films to the application concerned.