Polycrystalline VO2 Thin Films Research Articles

Synthesis and characterization of VO2 thin films on piezoelectric substrates

Polycrystalline VO2 thin films synthesized on two piezoelectric substrates (AT-cut quartz and GaN/AlGaN/GaN/Si) using low pressure direct oxidation technique have been characterized and compared to VO2 grown on traditional non-piezoelectric substrates sapphire and SiO2/Si. X-ray diffraction and atomic force microscopy characterization performed on the as grown films confirmed high quality of the VO2 films grown on both the piezoelectric and non-piezoelectric substrates. Changes in material properties associated with the semiconductor metal transition (SMT) of the VO2 films were investigated through resistivity and transmitted optical power changes measured across the SMT. It was observed that the VO2 films grown on the piezoelectric substrates are of high quality, and their electrical and optical properties changes are quite comparable to the best reported values on films synthesized on various substrates using different synthesis processes.

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61825102, 51872038, and 52021001) and the “111” Project, China (Grant No. B18011).

Vanadium dioxide (VO2) is a strongly correlated material, and it has become known due to its sharp metal-insulator transition (MIT) near room temperature. Understanding the thermal properties and their change across MIT of VO2 thin film is important for the applications of this material in various devices. Here, the changes in thermal conductivity of epitaxial and polycrystalline VO2 thin film across MIT are probed by the time-domain thermoreflectance (TDTR) method. The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO2 film to attenuate the impact from extra thermal interfaces. It is demonstrated that the method is feasible for the VO2 films with thickness values larger than 100 nm and beyond the phase transition region. The observed reasonable thermal conductivity change rates across MIT of VO2 thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate, which is different from the reported behavior of single crystal VO2 nanowires. The recovery of the relationship between thermal conductivity and electrical conductivity in VO2 film may be attributed to the increasing elastic electron scattering weight, caused by the defects in the film. This work demonstrates the possibility and limitation of investigating the thermal properties of VO2 thin films by the TDTR method without depositing any metal thermoreflectance layer.

Structural phase control and thermochromic modulation of VO2 thin films by post thermal annealing

Monoclinic M1 to rutile structural phase transition (SPT) in thermochromic VO2 is accompanied by the insulator to metal transition and transition from an infrared (IR) transparent to IR opaque phase. Fine control over the phase stabilization and the functional properties without additional doping can offer the ability to tailor VO2 thin films for their respective applications. In this work, post deposition thermal annealing was used to control the phase stabilization and the modulation in thermochromic performance of polycrystalline VO2 thin films. Monoclinic M1 to rutile SPT in VO2 thin films was tracked by in-situ temperature dependent synchrotron X-ray diffraction measurements and an increase in the SPT temperature was observed with increasing annealing temperature. Intermediate monoclinic M2 phase emerges near the SPT temperature at higher annealing temperature. Temperature dependent X-ray absorption spectroscopy (XAS) measurements revealed the higher electron correlation effect among the V 3d|| states across the phase transition in VO2 thin film having the intermediate phase. Spectral weight of V 3d|| states was found to increase with the annealing temperature. Lower annealing temperature drives the IR switching temperature towards room temperature while the higher annealing temperature improves the IR switching performance of VO2 thin films.

Facile Fabrication of Composite Vanadium Oxide Thin Films with Enhanced Thermochromic Properties.

In general, high-purity monoclinic VO2 (VO2(M)) was considered as a prerequisite for obtaining VO2-based thermochromic coatings with high performance. The coexistence of other vanadium oxides (such as V3O7 and V2O5) in VO2 coatings was regarded as an unfavorable issue. Here, we investigate the microstructures and thermochromic properties of the composite vanadium oxide (CVO) thin films. The results demonstrate that the proper coexistence of high valent vanadium oxides (V3O7 and V2O5) in VO2-based films can remarkably enhance the thermochromic performance of films. The CVO thin films were prepared by a room-temperature sputtering process followed by a modified rapid annealing routine in air. The structural analyses (X-ray diffraction, Raman spectroscopy, and transmission electron microscopy) reveal the coexistence of VO2(M), V3O7(M), and V2O5(O) in CVO thin films. The luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of the CVO thin film obtained by an optimal preparation process are 1.93 and 1.34 times those of the pure polycrystalline VO2 thin film, respectively. Moreover, the CVO thin film exhibits lower semiconductor-to-metal transition temperature (60.8 °C) than the pure VO2(M) thin film (67.9 °C). Furthermore, the fabrication process is well-reproducible, which is highly attractive for the mass production.

Transmittance change with thickness for polycrystalline VO2 films deposited at room temperature

In this work, polycrystalline VO2 thin films have been prepared on BK7 glass substrates by DC magnetron sputtering at room temperature with a post-annealing treatment at 450 °C, and relative thickness dependence of the semiconductor-metal transition (SMT) properties have been systematically investigated. It revealed that the prepared VO2 films exhibited unique surfaces composed of particles with various sizes, which became larger with increased film thickness. Different luminous transmittance between the metallic phase and semiconductor phase can be obtained by changing the film thickness based on the optical interference theory. The optimum thickness of VO2 films was around 90 nm with high ΔTsol of 9.7% and relatively high Tlum of 36.4% (higher than theoretical 29.1%), which is a recommendable optical property for practical application. The critical SMT temperature fluctuated when film thickness increased, and the transition speed of the heating process got slower overall with thickness decreasing, which might be caused by mechanical stress between films and substrates. In addition, the transition speed of cooling process turned slower with thickness of VO2 films increasing, which can be ascribed to the slower heat dissipation of thicker films than that of thinner films during the cooling process. These results might give an indication for creating or designing potential materials based on the thickness dependence of phase transition properties of VO2 films.

Investigation on electrically-driven semiconductor-metal transition of polycrystalline VO2 thin films on two kinds of substrates

Electrical driving is one of frequently-used stimuli for the semiconductor-metal transition (SMT) of VO2. But the driving mechanism is still under debate. We investigated the DC electrically-driven SMT features of polycrystalline VO2 thin films deposited on two kinds of substrates (quartz and silicon) with obviously-different thermal conductivity and the influence of cooling by a thermo electric cooler (TEC) on the SMT of VO2. Interestingly, the SMT doesn’t happen at a high voltage at very start, but at a relatively low one. Moreover, the SMT of VO2 thin films on silicon substrate is completely restrained by cooling through a TEC although the electric field strength across VO2 reaches 1.1×107 V/m. Our findings reveal that the Joule-heating effect plays an important role in the DC electrically-driven SMT of VO2.

Influence of Discharge Current on Phase Transition Properties of High Quality Polycrystalline VO₂ Thin Film Fabricated by HiPIMS.

To fabricate high-quality polycrystalline VO2 thin film with a metal–insulator transition (MIT) temperature less than 50 °C, high-power impulse magnetron sputtering with different discharge currents was employed in this study. The as-deposited VO2 films were characterized by a four-point probe resistivity measurement system, visible-near infrared (IR) transmittance spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The resistivity results revealed that all the as-deposited films had a high resistance change in the phase transition process, and the MIT temperature decreased with the increased discharge current, where little deterioration in the phase transition properties, such as the resistance and transmittance changes, could be found. Additionally, XRD patterns at various temperatures exhibited that some reverse deformations that existed in the MIT process of the VO2 film, with a large amount of preferred crystalline orientations. The decrease of the MIT temperature with little deterioration on phase transition properties could be attributed to the reduction of the preferred grain orientations.

Polycrystalline VO2 thin films via femtosecond laser processing of amorphous VO x

Femtosecond laser processing of pulsed laser-deposited amorphous vanadium oxide thin films was investigated. Polycrystalline VO2 thin films were achieved by femtosecond laser processing in air at room temperature. The electrical transport properties, crystal structure, surface morphology, and optical properties were characterized. The laser-processed films exhibited a metal–insulator phase transition characteristic of VO2, thus presenting a pathway for the growth of crystalline vanadium dioxide films on low-temperature substrates.

Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film

In this work, high power impulse magnetron sputtering was used to control the lattice distortion in polycrystalline VO2 thin film. SEM images revealed that all the VO2 thin films had crystallite sizes of below 20nm, and similar configurations. UV–vis-near IR transmittance spectra measured at different temperatures showed that most of the as-deposited films had a typical metal–insulator transition. Four-point probe resistivity results showed that the transition temperature of the films varied from 54.5 to 32°C. The X-ray diffraction (XRD) patterns of the as-deposited films revealed that most were polycrystalline monoclinic VO2. The XRD results also confirmed that the lattice distortions in the as-deposited films were different, and the transition temperature decreased with the difference between the interplanar spacing of the as-deposited thin film and standard rutile VO2. Furthermore, a room temperature rutile VO2 thin film was successfully synthesized when this difference was small enough. Additionally, XRD patterns measured at varied temperatures revealed that the phase transition process of the polycrystalline VO2 thin film was a coordinative deformation between grains with different orientations. The main structural change during the phase transition was a gradual shift in interplanar spacing with temperature.

Characterization of polycrystalline VO2 thin film with low phase transition temperature fabricated by high power impulse magnetron sputtering

VO2 is a unique material that undergoes a reversible phase transformation around 68[Formula: see text]C. Currently, applications of VO2 on smart windows are limited by its high transition temperature. In order to reduce the temperature, VO2 thin film was fabricated on quartz glass substrate by high power impulse magnetron sputtering with a modulated pulsed power. The phase transition temperature has been reduced to as low as 32[Formula: see text]C. In addition, the VO2 film possesses a typical metal–insulator transition. X-ray diffraction and selected area electron diffraction patterns reveal that an obvious lattice distortion has been formed in the as-deposited polycrystalline VO2 thin film. X-ray photoelectron spectroscopy proves that oxygen vacancies have been formed in the as-deposited thin film, which will induce a lattice distortion in the VO2 thin film.

Formation of VO2 by rapid thermal annealing and cooling of sputtered vanadium thin films

Sputtered vanadium-rich films were subjected to rapid thermal annealing-cooling (RTAC) in air to produce vanadium dioxide (VO2) thin films with thermochromic switching behavior. High heating and cooling rates in the thermal oxidation process provided an increased ability to control the film's microstructure. X-ray diffraction patterns of the films revealed less intense VO2 peaks compared to traditional polycrystalline samples fabricated with a standard (slower) cooling time. Such films also exhibit a high optical switching reflectance contrast, unlike the traditional polycrystalline VO2 thin films, which show a more pronounced transmittance switching. The authors find that the RTAC process stabilizes the VO2 (M2) metastable phase, enabling a rutile-semiconductor phase transition (R-M2), followed by a semiconductor–semiconductor phase transition (M2-M1).

Effect of yttrium-doping on the microstructures and semiconductor-metal phase transition characteristics of polycrystalline VO2 thin films

We investigate the effect of yttrium-doping on the microstructures and semiconductor-metal phase transition characteristics of polycrystalline VO2 thin films prepared by reactively co-sputtering process. XPS analyses indicate the existence of Y3+ in the Y-doped VO2 films, but Y-doping hardly influences the chemical states of V and O elements. X-ray diffraction patterns and Raman spectra reveal that both undoped and Y-doped VO2 thin films have a polycrystalline structure of monoclinic VO2. The introduction of Y greatly reduces the grain size of VO2 thin films as evidenced by scanning electron microscopy analyses. The relationship between the hysteresis width and doping level is not monotonic although the grain size is monotonically reduced with increasing the doping level. Y-doped VO2 films with optimal doping level (1.82at%) have a notably narrower hysteresis width (4.6°C) than undoped VO2 films (10.7°C). This is ascribed to increased heterogeneous nucleation centers due to Y in the VO2 lattice. With the further increase of doping level, the size effect gradually plays a prominent role in SMPT, and the hysteresis width of Y-doped VO2 films increases instead. The SMPT temperature of Y-doped VO2 films obviously decreases compared with undoped VO2 films due to reduced grain size and deformation of local structure around Y atom.

Role of epitaxial microstructure, stress and twin boundaries in the metal–insulator transition mechanism in VO2/Al2O3 heterostructures

The microstructures of epitaxial polycrystalline VO2 thin films grown on (001) sapphire were investigated by means of X-ray diffraction, Cs-corrected scanning transmission electron microscopy (STEM), in both plane and transverse geometry, in relation to its metal–insulator transition (MIT) properties. It is shown that the epitaxial relationship between the thin film and the substrate can be defined as out-of-plane twofold twinning symmetry {020}M//(006)S (where subscripts “M” and “S” denote the monoclinic phase of VO2 and the sapphire α-Al2O3 substrate, respectively) with in-plane threefold twinning structure (200)M//{110}S. The origin of these twinning structures is discussed: the in-plane threefold twinning structure comes from the threefold symmetry of the Al2O3 (001) plane, and the twofold twinning symmetry is induced by the MIT phase transition. The STEM planar view observations of the thin film demonstrate the presence of elongated grains down to nanoscale, with a high density of twin boundaries (TB). These TB are highly orientated into two sets of families. STEM low-angle annular dark-field imaging and STEM dark-field atomic displacement measurements evidence very different strain behaviors for these two TB families. Most of the TB and some of the smaller grains with typical dimensions of only a couple of nanometers exhibit locally an enhanced tetragonality. They are proposed to act as nucleation centers during the MIT process and then to influence the dynamics of the transition.

Modeling of the hysteretic metal-insulator transition in a vanadium dioxide infrared detector

The vanadium dioxide (VO2) thin film, usually employed as an infrared detector, exhibits hysteresis in its resistance-temperature characteristic. Considering a polycrystalline VO2 thin film as a composite medium, containing semiconducting and metallic microcrystals, the well- known effective-medium approximation theory is employed to relate the volume fraction of the semiconducting microcrystals to the effective film resistance. A phenomenological model is first proposed for describing the hysteretic dependence of volume fraction on temperature. From this, a model for hysteresis in the resistance-temperature characteristic is then derived, and a procedure for estimating the model parameters is outlined. The model reproduces the more important hysteretic character- istics such as the major, minor, and nested loops, in good agreement with the experimental characteristics. © 2002 Society of Photo-Optical Instru- mentation Engineers. (DOI: 10.1117/1.1501095) Subject terms: vanadium dioxide; microbolometer; thermal hysteresis; sensor; in- frared imaging; hysteresis model; minor loop; major loop; accommodation pro- cess; metal-insulator transition; composite medium; effective-medium approxima- tion; percolation; phase transition.