VO2 Thin Films Research Articles

Band anisotropy and effective mass renormalization in strained metallic VO2 (101) thin films

We explore how strain impacts the band structure of metallic-phase VO2 thin films deposited on TiO2(101) substrates. Employing a combination of X-ray absorption linear dichroism and valence band measurements, we demonstrate that strain can alter the intrinsic band structure anisotropy of metallic VO2. Our findings reveal that reducing the thickness of VO2 films leads to a more isotropic band structure. This observation is further supported by an analysis of the electronic population redistribution in the d||−π*\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${d}_{{||}}{-}{\\pi }^{* }$$\\end{document} bands, which affects the screening length and induces effective mass renormalization. Overall, our results underscore the potential of strain manipulation in tailoring the electronic structure uniformity of thin films, thereby expanding the scope for engineering VO2 functionalities.

Effects of Thermal Energy on the Formation of Lattice Strain in VO2 Thin Films Grown on TiO2(001).

Film thickness is a well-known experimental parameter for controlling lattice strain in oxide films. However, due to environmental and resource conservation considerations, films need to be as thin as possible, increasing the need to find alternative factors for strain management. Herein, we present the importance of thermal energy as a factor for the formation of lattice strain in the oxide films, specifically focusing on the effects of laser fluence during pulsed laser deposition (PLD) on the in-plane lattice strain of vanadium dioxide (VO2) thin films grown on titanium dioxide (TiO2) (001). VO2 thin films were deposited using a KrF excimer laser (λ = 248 nm) at laser fluences ranging from 0.88 to 1.70 J/cm2. The film thickness ranged from 10-15 nm, below the critical thickness. Films grown at higher laser fluences exhibited smooth surfaces and completely strained in-plane lattices. In contrast, films grown at lower laser fluences displayed numerous small islands and relaxed in-plane lattice strain. The metal-insulator transition (MIT) temperature was lower for films grown at higher laser fluencies compared to those grown at lower laser fluences. It was also revealed that Ti-V interdiffusion occurs, forming a solid solution (V1-xTixO2) near the interface. These observations suggest that the thermal energy of the particles, influenced by laser fluence, is a critical factor in the formation of lattice strain in metal oxide films and also that laser fluence in PLD is an effective experimental parameter for strain management in oxide films. Our findings enhance the understanding of lattice strain formation in metal oxides and offer insights for establishing effective methods for controlling lattice strain in metal oxide films.

Low Hysteresis Vanadium Dioxide Integrated on Silicon Using Complementary Metal‐Oxide Semiconductor Compatible Oxide Buffer Layer

VO2 undergoes a metal‐insulator transition (MIT) at ≈70 °C, which induces large variations in its electrical and wavelength‐dependent optical properties. These features make VO2 a highly sought‐after compound for optical, thermal, and neuromorphic applications. To foster the development of VO2‐based devices for the microelectronic industry, it is also imperative to integrate VO2 on silicon. However, high lattice mismatch and the formation of silicates at the interface between VO2 and Si degrade the quality and functionality of VO2 films. Moreover, VO2's polymorphic nature and stable VO phases pose integration issues. To address these challenges, the MIT of VO2 thin films integrated on Si with a complementary metal‐oxide semiconductor‐compatible HfxZr1−xO2 (HZO) buffer layer is investigated. Using in situ high‐resolution X‐ray diffraction and synchrotron far‐infrared spectroscopy, combined with multiscale atomic and electronic structure characterizations, it is demonstrated that VO2 on the HZO buffer layer exhibits an unusually low thermal hysteresis of ≈4 °C. In these results, the influence of strain on M2 phase nucleation, which controls the hysteresis, is unraveled. Notably, the rate of phase transition is symmetric and does not change for the heating and cooling cycles, implying no incorporation of defects during cycling, and highlighting the potential of an HZO buffer layer for reliable operation of VO2‐based devices.

Active infrared tuning of metal–insulator-metal resonances by VO2 thin film

VO2 is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO2 layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal–insulator-metal gap resonance by inserting sub-wavelength VO2 film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3–5 μm range when VO2 is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 μm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

Preparation and Performance of Ce-Doped V2O5 Thin Films Deposited on FTO Substrates

Preparation and Performance of Ce-Doped V2O5 Thin Films Deposited on FTO Substrates

In-situ XRD study on phase transition kinetics of vanadium dioxide thin film

Vanadium dioxide (VO2) has great potentials to be used in energy-related fields due to its unique reversible thermochromic metal-insulating properties. In this paper, we used in-situ XRD technique to study the phase transition kinetics of VO2 thin films, which can change phase from monoclinic VO2 (M) to tetragonal rutile VO2 (R). The VO2 thin films were prepared by a simple solution-based sol-gel method followed by spin coating to achieve the nanoscale thickness on a quartz substrate. During heating, the critical phase transition of VO2 (M) occurred at 70 °C, which dropped to 45 °C during cooling. A martensite-like crystal phase transformation kinetic model was established based on the Koistinen and Marburger (KM) technique. The relative content of the two phases was quantitatively analyzed using the Rietveld method, and the transformation rate coefficient α was obtained as 0.20345 and 0.12585, for the heating process from VO2 (M) to VO2 (R) and the cooling process from VO2 (R) to VO2 (M), respectively. This research offers an effective tool for understanding the structural characteristics of thin film VO2, paving the way for its future design and applications towards energy-related fields such as in smart windows.

Investigation on electronic and magnetic properties of Cr doped V2O5 thin films prepared by chemical solution deposition method

The present study examines the role of Cr doping on the electronic and magnetic behaviour of V2O5i.e. V2-xCrxO5 (x = 0, 0.05, and 0.10) thin films, prepared by chemical-solution deposition technique on Si (111) substrate. X-Ray diffraction technique shows the purity of the films and confirms no secondary phase formation. The atomic force microscopy was used to confirm the roughness of the films. A morphological investigation is done using High-resolution scanning electron microscopy associated with Energy dispersive X-Ray mapping of each element of the film. Fourier transform Infrared spectroscopy is utilised to identify functional groups that are present in obtained samples. The band-gap of these obtained samples is in the range of 0.8–0.6 eV, as examined by UV–Vis i.e., its value is decreasing with Cr doping. Using vibrating sample magnetometer, it is clear that samples shows ferromagnetic (FM) behaviour with saturation magnetization 0.3–0.6 μB/cc consistent with Langevin function fitting. It concludes the presence of FM behaviour of films and magnetization increases with carrier density, consistent with the carrier-induced origin of the ferromagnetism. X-ray photoemission spectroscopy (XPS) is utilised to understand its electronic properties. Core level spectra measurements suggest that V is in mixed state of 5+ and 4+. However Cr is in 3+ states. XPS and UV–Vis measurements imply that oxygen vacancies significantly contribute to the declination of the band gap and the promotion of FM-like behaviour. This discovery offers a promising pathway for engineering novel devices.

Inspection and comparative analysis of light and thermal response dynamics of Cu/V₂O₅/n-Si and Cu/La-V₂O₅/n-Si MIS diodes

Schottky Barrier Diodes (SBDs) are pivotal in modern electronics for their quick switching and low forward voltage drop, enabled by metal-semiconductor junctions. SBDs are renowned for their performance, primarily due to their metal-semiconductor intersection. This study focuses on Cu/V2O5/n-Si and Cu/La-V2O5/n-Si SBDs, examining the impact of lanthanum doping on their performance. Transition Metal Oxides (TMOs) like Vanadium Pentoxide (V2O5) offer unique electronic, magnetic and optical properties making them suitable for various applications. The fabrication process involved sol-gel spin coating and DC magnetron sputtering to create a thin V2O5 layer on n-type silicon wafers, followed by copper contact deposition. Galvanizing temperatures ranged 300° C to 500° C to study the structural, optical, and morphological changes in V2O5 thin films. Key findings include the significant reduction in ideality factor (n) with increasing annealing temperatures for Cu/V2O5/n-Si SBDs, reaching as low n value of 5.26 at 500°C, indicating improved device performances. For Cu/La-V2O5/n-Si SBDs, the ideality factor consistently decreased with higher light intensities, showcasing enhanced performance due to La doping. Barrier height (ɸB) also varied, with higher values observed for La-doped V2O5, enhancing charge recombination and increasing oxygen vacancies. Photodiode parameters were significantly enhanced by doping of the La. The Cu/La-V2O5/n-Si SBDs demonstrated high photosensitivity (PS = 3327.5 %), photo responsivity (R = 33.39 mA/W) and detectivity (D* = 9.82 × 10¹⁰ Jones), making them highly efficient for optoelectronic applications. Overall, this study highlights the potential of Cu/La-V2O5/n-Si SBDs for high-efficiency, optoelectronic applications, with optimized doping concentrations and annealing conditions significantly improving their performance.

Ultrafast spectroscopy of coherent phonons across the pressure driven insulator to metal phase transition in V2O3

Nowadays, materials science is moving towards the understanding and control of materials in nonequilibrium states by making use of perturbative techniques to investigate their dynamical responses. From this perspective, the use of ultrashort light pulses seems to be a relevant approach as it can selectively address different degrees of freedom in solid-state systems and more particularly electrons. Such a method can help to decipher the physical phenomena arising from electronic correlations and complements a more conventional methodology where the phase diagrams of materials are investigated at thermodynamical equilibrium. Here, we combine femtosecond optical spectroscopy and a high-pressure setup to monitor the ultrafast out-of-equilibrium photo response of a V2O3 thin film across the pressure driven insulator-to-metal transition. The experimental results demonstrate the possibility to use the spectroscopy of coherent phonons as a thermodynamical phase marker in V2O3 thin films. In addition, the frequency behavior of the ultrafast coherent phonon mode (A1g character) seems to reflect the manifestation of a strong coupling between the lattice and electronic degrees of freedom near first-order transition lines with a pronounced drop in frequency around the critical pressure. Published by the American Physical Society 2024

Sol-Gel Pt-VO2 Films as Selective Chemoresistive and Optical H2 Gas Sensors.

In this work, VO2 (M1/R) thin films were exploited as H2 gas sensors. A flat film morphology, obtained by furnace annealing, was compared with a laser-induced nanostructured one. The combination of the environmentally friendly sol-gel approach with the ultrafast laser crystallization allows for significant reductions in energy consumption and related emissions during the fabrication of VO2 sensors. By decorating the sensors' surface with Pt nanoparticles (NPs), the sensor response was enhanced exploiting the hydrogen spillover effect. The Pt/VO2 sensors, tested at operating temperatures between 20 and 200 °C and for concentration of H2 from few ppm to 50000 ppm, offered a dual chemoresistive and optical sensing mode. Low operating temperatures of 150 °C were achieved, along with a detection limit as low as 2 ppm and a perfect baseline recovery. Both sensors guaranteed specific selectivity toward H2, without response to NO2 or humidity, and long-term stability over 500 h. The H2 sensing mechanism, for both the monoclinic and rutile VO2 phases, was investigated through in operando X-ray Diffraction and in situ X-ray Photoelectron Spectroscopy tests. The interaction was found to be based on the reversible formation of HxVO2 bronze, along with the reversible variations in the oxidation state of V.

Preparation of V2O5-Ink for the Fabrication of V2O5 Thin Films by the Spin Coating Method.

This article reports the preparation of V2O5 ink via a novel chemical route. The prepared V2O5 ink has been spin coated for the synthesis of V2O5 thin films on glass substrates. The synthesized V2O5 thin films were annealed at 300-400 °C in air and characterized by different techniques. The X-ray diffraction data reveal the phase pure V2O5 compound with polycrystalline nature. The scanning electron microscopy micrographs unravel that the V2O5 thin films are smooth with few nanoflakes presented on the surfaces. An optical study reveals that transmittance and bandgap of the V2O5 thin films decrease with annealing temperature. The bandgap of the films varies in the range of 2.8-2.9 eV. These findings have opened a new avenue for the preparation of metal oxide ink. The prepared V2O5 ink is promising for the manufacture of low-cost V2O5-based photoelectrochemical cells.

The influence of structural defects on the electrical and infrared emission properties of VO2 thin films

Defect engineering is widely used in the modification of transition metal oxides. In this study, defects are introduced into the polycrystalline VO2 film by the energetic Ar ion irradiation, and the influence of structural defects on the electrical and infrared emission properties of VO2 thin films have been investigated. The decrease in electrical switching ratio with the increase of defect ratio (displacement per atom (dpa)) can be described as R10°C/R90°C=2.032×(dpa+0.005)−1.8069, while the infrared modulation performance is Δε=−0.119×ln(dpa+0.061). The dpa thresholds of the electrical switching ratio and the infrared modulation performance are ηdpaRR = 0.005/atom and ηdpaΔε = 0.0426/atom, respectively, indicating that the electrical switching ratio of VO2 films is more responsive to defects than the infrared modulation performance. The relationship between transition temperature and defect ratio has be determined. Concerning electrical properties, the heating phase transition temperature can be described as TMITHeating=25.88−10.93×ln(dpa+0.01); while for infrared emission modulation, the heating phase transition temperature is given by Tε−MITHeating=31.65−8.18×ln(dpa+0.01). While reducing the phase transition temperature, the VO2 thin films still exhibit an electrical switching ratio of more than two orders of magnitude and obvious infrared modulation performance. It not only offers a viable solution for broadening the application scope of VO2 but also contributes to understanding the role of defects in VO2 thin films for further research.

Electronic vs phononic thermal transport in Cr-doped V2O3 thin films across the Mott transition

Understanding the thermal conductivity of chromium-doped V2O3 is crucial for optimizing the design of selectors for memory and neuromorphic devices. We utilized the time-domain thermoreflectance technique to measure the thermal conductivity of chromium-doped V2O3 across varying concentrations, spanning the doping-induced metal–insulator transition. In addition, different oxygen stoichiometries and film thicknesses were investigated in their crystalline and amorphous phases. Chromium doping concentration (0%–30%) and the degree of crystallinity emerged as the predominant factors influencing the thermal properties, while the effect of oxygen flow (600–1400 ppm) during deposition proved to be negligible. Our observations indicate that even in the metallic phase of V2O3, the lattice contribution is the dominant factor in thermal transport with no observable impact from the electrons on heat transport. Finally, the thermal conductivity of both amorphous and crystalline V2O3 was measured at cryogenic temperatures (80–450 K). Our thermal conductivity measurements as a function of temperature reveal that both phases exhibit behavior similar to amorphous materials, indicating pronounced phonon scattering effects in the crystalline phase of V2O3.

High-quality VO2 thin films sputter-deposited on borosilicate glass substrates for modulating solar transmission and thermal emission

M1-phase VO2 exhibits a reversible phase transition between the insulating and metallic states at T = ∼341 K, and VO2-based smart windows have been extensively studied. The smart window application requires the growth of high-quality VO2 films with a large resistivity change and small hysteresis width (ΔT) on glass substrates using a scalable deposition method such as sputtering. However, the polymorphic nature of VO2 makes it challenging to obtain high-quality VO2 films on ordinary window glasses (borosilicate and soda-lime glasses). Thus far, VO2 films sputtered on glass substrates have exhibited resistivity changes of 1.5–2 orders of magnitude and ΔT = 3–19 °C. In this study, we show that high-quality VO2 films with a resistivity change of 2.5−3.0 orders of magnitude and ΔT = 2 °C can be grown on borosilicate glass substrates via radio-frequency sputtering followed by thermal annealing. The simultaneous achievement of such a large resistivity change and small hysteresis width is highly encouraging not only for smart windows but also for other applications. With respect to the modulation of solar transmission and thermal emission with temperature, the performance levels of the produced VO2 films were 70−75 % of those expected from pure M1-phase VO2 films.

Mechanisms of electrical transition using a conformal MoOx cap on Mo-doped VO2 thermochromics

Mo-doped VO2 thin-films are deposited on the soda-lime glass by a reactive high-power impulse magnetron co-sputtering technique (R-HiPIMS). Rapid thermal annealing at 500 °C for 3 min is performed to achieve the fabrication under low thermal budget. To prevent VO2 from further oxidation, a conformal and semi-conducting MoOx film is applied as the capping layer by the plasma-enhanced atomic layer deposition (PE-ALD). After MoOx film is deposited, an electrical transition in the resistance ratio of 6–10 orders of magnitude has been found, especially for MoOx thicknesses less than 20 nm. A secondary phase of V2O3 plays an important role in abrupt change of current transportation. Also, the hysteresis of optical transition in transmittance at a wavelength of 2500 nm shows that both the width and transition temperature (TC) decrease with increasing the thickness of the MoOx cap. Moreover, the effect of localized surface plasmonic resonance (LSPR) due to the Karst-like structure has been detected by the absorption spectrum. The variations in LSPR peak, such as intensity, blue-shifting, and red-shifting, originated from either excess carrier or structure change are discussed. The high aspect ratio of surface morphology is further examined by atomic force microscopy. In addition, the coverage of MoOx on Mo-doped VO2 concerning the friction behavior is evaluated by lateral force microscopy (LFM), which is helpful in clarifying the responsible mechanisms during TC transition.

Optical and Structural Properties of V2O5 Electrochromic Thin Films

The increase in global temperature has led to a significant surge in energy consumption within the air conditioning industry, resulting in environmental deterioration. Electrochromic (EC) windows have emerged as a promising solution to address these challenges. Vanadium pentoxide (V2O5) stands out among all metal oxide materials due to its remarkable EC properties, including substantial Li+ ion insertion capacity and multicolor capabilities. Despite the potential of V2O5, there remains a lack of comprehensive research on the structural and optical properties of V2O5 films with varying thicknesses. Therefore, this study aims to investigate the structural and optical properties of V2O5 thin films with thicknesses ranging from 46 to 344 nm. By employing the sol-gel spin coating method, V2O5 thin films were fabricated and analyzed using X-ray diffraction (XRD) spectroscopy and ultraviolet-visible (UV-Vis) spectrophotometry. The fabricated V2O5 thin films with thicknesses of 46-274 nm demonstrated an average film transparency of 83 %. XRD analysis further revealed that the V2O5 thin films reached their peak crystallinity at a thickness of 344 nm. Moreover, CV analysis revealed that the V2O5 device, with a thickness of 274 nm, exhibited a cathodic peak current of -1.63 mA, indicating its excellent ability to facilitate Li+ ion diffusion. Additionally, CA measurements displayed a high optical modulation of 37.78 %. Ultimately, this research contributes to the development of energy-efficient solutions for sustainable environmental practices.

Active dual quasi-BICs in a dielectric metasurface with VO2 for slow light and optical modulation.

We investigate the temperature tunable dual quasi-bound states in the continuum (qBICs) in a silicon/vanadium dioxide (Si/VO2) hybrid metasurface with Q-factor being as large as 9.3 × 106 and 2.8 × 107 by breaking the in-plane C2 symmetry. The far-field scattering of multipoles and near-field distributions confirm that the toroidal dipole and magnetic quadrupole dominate the dual qBICs resonance. The high performance of slow light with ultralarge group index exceeding 5.6 × 105 and the inverse quadratic law between the group index and asymmetric parameter are achieved. By temperature tuning of the VO2 thin film at the sub-10 K scale, a modulation depth of 90% and the ON/OFF ratio exceeding 12.8 dB are obtained. The proposed temperature tunable dual qBICs have potential applications in the fields of tunable slow light, temperature switches, and sensors.

Fabricating highly stable and reliable vanadium dioxide thin films: Insights from radio frequency magnetron sputtering

This study establishes an optimization protocol to fabricate a stoichiometric VO2 thin film onto the glass substrate using a single process step in RF magnetron sputtering. An interplay between the substrate temperature and the oxygen-to-argon ratio in process gas is shown to achieve a range of VOx composition (from oxygen-rich to oxygen-deficient phases). The re-sputtering phenomenon is found to be crucial at higher temperature (T ≥ 873 K), leading to a shift in stoichiometry from V2O5 to V2O3. Moreover, a process model for the fabrication of various VOx compounds in thin film form is also established in this study. The crystal structure transformation from M1 to R phase for these VO2 thin films deposited over a range of process conditions is found to be 339 ± 1 K, in close agreement with the reported value for the bulk VO2. In spite of the microstructural variations, these films are found to show consistent phase transition behavior, excellent stability, and reliability in repeated thermal cycles. These films show a semiconductor-to-metal transition, which is correlated to their structural phase transformation temperature. Moreover, this study also establishes a correlation between various external stimuli, widening the usage of this material in a range of applications.

Plasmonic Resonance Shifts in Gold Nanoparticles‐Thermochromic VO2 Thin Film Hybrid Platforms: A Joint Experimental and Numerical Study

AbstractThe combination of the phase transition in thermochromic vanadium dioxide (VO2) with plasmonic nanoparticles paves the way for applications in various fields, including optical sensing, advanced coatings, and dynamic optical devices. This study presents a simple fabrication method to control both the size and surface coverage of NPs combined with VO2. First, a thermochromic VO2 coating with a phase transition at 68 °C is synthesized using reactive magnetron sputtering. Then, monodisperse 30 nm diameter gold NPs are bonded to the VO2 surface using (3‐aminopropyl)trimethoxysilane (APTMS) linkers, examining the effect of immersion duration on surface coverage. Two platforms are developed: a VO2 thin film with a monolayer of NPs and a configuration with NPs between two VO2 films. The temperature‐dependent plasmonic response of these platforms is measured by extinction spectroscopy, showing a significant wavelength resonance shift of approximately 10 nm for the first platform and 20 nm for the second. Optical simulations analyze this shift over various geometries, from isolated NPs to fully covered NPs, achieving a 60 nm shift for NPs embedded in a thin VO2 film. This study demonstrates an effective approach to synthesizing thermochromic VO2 coatings with gold NPs, offering insights into the plasmonic properties of hybrid platforms.

Structural, electronic, and optical properties of Zn-doped V2O5 thin films

V2O5 shows a diverse range of applications due to its remarkable electronic and optical properties. This research is designed to tune the electronic and optical properties of V2O5 through modification in the energy band profile by varying Zn doping concentration. Density functional theory (DFT) calculations were used to investigate the Density of States (DOS) spectra for pure V2O5, exhibiting the prominent contribution of V-d and O-p orbitals, representing the p-d hybridized orbitals along with additional Zn-d orbital contribution in Zn-doped compositions. The effects of doping on the structural, morphological, elemental, and optical properties of the developed thin films were investigated employing x-ray diffraction (XRD), scanning electron microscope (SEM), x-ray dispersive spectroscopy (EDX), and spectroscopic ellipsometry (SE), respectively. x-ray diffraction analysis revealed the orthorhombic crystal structure in thin films. Surface morphology depicts the uniformly distributed compact rod-like features. The experimentally calculated band gap was found to decrease with Zn doping from 2.77 eV for pure V2O5 to 2.45 eV for maximum doping content. A significant variation is recorded in optical parameters like the increase in absorption coefficient and optical conductivity, which makes these more favorable for optoelectronic devices, particularly focusing on photovoltaics.