Oriented VO2 Films Research Articles

Flexible VO2 films grown on ZnO-nanorod buffered polyimide sheets with large insulator metal transition: Evaluation of flexible performance

We fabricated stand-alone flexible vanadium dioxide (VO2) films on 10 μm-thick polyimide (PI) sheets with large insulator metal transition (IMT). Zinc oxide (ZnO)-nanorods grown by the chemical synthesis method as a buffer layer between VO2 and PI realized IMT with resistance change nearly three orders of magnitude. Highly bm axis oriented VO2 films on ZnO_NR buffered PI realized switching of 1450 nm infrared-light more than 40%. The transmittance values at a metallic phase below 2% at temperatures higher than 70 °C were quite low, suggesting high potential for various applications in the infrared and terahertz wavelength region. Number densities of cracks in the VO2 films were estimated from the scanning electron microscopy (SEM) images. Quantitative relation between crack density and the bias voltage suggested the strain-induced formation of cracks in the VO2 films prepared by biased-sputtering. As for flexible performance, the stand-alone VO2 films on PI were directed to bending examinations up to 2000 times. The ZnO-nanorods buffered VO2 films on PI showed high durability for maintaining the superior IMT characteristics. The results obtained in this study show a way to realize practical flexibility of VO2 stand-alone sheets which are able to apply for a variety of fields utilizing switching of VO2.

Observation of the crystalline orientation dependence of the semiconductor–metal transition for thermal oxidation induced VO2 films over amorphous quartz glasses

Polycrystalline VO2 films were obtained through a vacuum annealing of sputtered V-rich films over quartz substrates and were characterized with x-ray diffraction, field-emission scanning electron microscopy, and x-ray photoelectron spectroscopy, respectively. The semiconductor–metal transition (SMT) was studied with the temperature-variable electric resistances. It was seen that the VO2 film crystalline orientation changes with the O2 partial pressure during the vacuum annealing. We observed a relation between the thermal hysteresis of the SMT and the crystalline orientation of the monoclinic VO2 films. The (011) oriented monoclinic film presents a narrower thermal hysteresis and a larger transition amplitude as compared to the (200) orientated films. In addition, a transition shoulder appears in the thermal hysteresis of the SMT for the (200) oriented VO2 films during the cooling process and becomes absent for the (011) orientated VO2 films.

Understanding the Phase Transition Evolution Mechanism of Partially M2 Phased VO2 Film by Hydrogen Incorporation.

Studies on the hydrogen incorporated M1 phase of VO2 film have been widely reported. However, there are few works on an M2 phase of VO2. Recently, the M2 phase in VO2 has received considerable attention due to the possibility of realizing a Mott transition field-effect transistor. By varying the postannealing environment, systematic variations of the M2 phase in (020)-oriented VO2 films grown on Al2O3(0001) were observed. The M2 phase converted to the metallic M1 phase at first and then to the metallic rutile phase after hydrogen annealing (i.e., for H2/N2 mixture and H2 environments). From the diffraction and spectroscopy measurements, the transition is attributed to suppressed electron interactions, not structural modification caused by hydrogen incorporation. Our results suggest the understanding of the phase transition process of the M2 phase by hydrogen incorporation and the possibility of realization of the M2 phased-based Mott transition field-effect transistor.

Simultaneous realization of infrared-light switching and high visible-light transmittance in extremely thin VO2 films grown on ZnO-nanorods buffered glasses

The authors demonstrate a significant effect of ZnO-nanorod buffer layers on the growth of extremely thin VO2 films. Highly c-axis oriented ZnO-nanorods were grown on glasses with sputtered ZnO-seed layers by using the chemical bath deposition method. Introducing an organic compound, (CN2)6N4 (hexamethylenetetramine), together with a Zn source, Zn(NO3)2, achieved highly c-axis oriented ZnO-nanorods with an average diameter of around 150 nm. Due to such a superior template, b-axis oriented VO2 films were grown on the ZnO-nanorod buffered glasses even with the film thickness lower than several tens of nanometers. The thinnest VO2 film emerged in insulator-metal transition, achieving high visible-light transmittance and clear infrared-light switching, which are characterized by luminous transmittance (Tlum) of 56% even at metallic state and solar modulation ability (ΔTsol) of 6.0%. The results obtained in this study propose a way to achieve practical smart windows that enable not only infrared-light switching but also maintaining high visible-light transmittance without any optical design.

Strain engineering on the metal-insulator transition of VO2/TiO2 epitaxial films dependent on the strain state of vanadium dimers

In this letter, a series of vanadium dioxide (VO2) epitaxial films were deliberately deposited on TiO2 substrates with different orientations [(001), (110), and (101)], in an attempt to gain insights into the strained VO2 epitaxial film. We found in-plane [100] and [1-10] directions, obviously anisotropic metal-insulator transition (MIT) in (110)-oriented VO2 films. In combination with synchrotron radiation high-resolution x-ray diffraction characterizations, electronic transport data reveal that the critical temperature of MIT depends on the strain state of the dimeric vanadium atomic chain along the c axis of the rutile phase. The anisotropy of MIT is closely related to the orientation of the VO2 films, which is caused by the varied orientation configuration of V-V atomic chain dimerization in the films. Soft x-ray absorption spectroscopy results further indicate that this anisotropy may be driven by the directional hybridization of O 2p and V 3d orbitals with respect to the orientation of VO2 thin films. The polarization-dependent V L-edge and O K-edge XAS data suggest that the elongation of the apical V-O bond length increases the p-d orbital overlap; thus, the energy level of the d// orbital is raised relative to that of the π* orbital. These anisotropic MIT behaviors will help us to understand how the strain engineering depends on the strain state of vanadium dimers in VO2 films.

Infrared-light switching in highly oriented VO2 films on ZnO-buffered glasses with controlled phase transition temperatures

Switching ability for infrared-light (IR) of commercial glass is of great interest for realizing practical smart windows which enable energy savings. In this article, we report on highly oriented growth of vanadium dioxide (VO2) films on ZnO-buffered glass substrates by means of biased reactive sputtering technique and their high switching ability for IR against temperature based on the insulator-metal transition (IMT). In addition, lower shift of transition temperature down to 55 °C was realized without deterioration of IMT properties in which resistance change nearly three orders of magnitude was kept. Shortening of in-plane V-V atoms distance through the effect of in-plane compressive stress responsible for energetic ion irradiation to biased substrate was supposed to contribute the achievement of controlled IMT temperatures. Large switching of around 50% for transmittance of IR with wavelength of 2.5 µm was realized under controlled phase transition temperatures. We emphasized that high biasing power enabled the shift of transition temperature for IR to nearly room temperature (～38 °C) by combination of the shortening of V-V distance and the oxygen defect formation, while the IMT reveals gradual change at biasing power higher than certain threshold value.

Low-temperature growth of VO2 films on transparent ZnO/glass and Al-doped ZnO/glass and their optical transition properties

Vanadium dioxide (VO2) is a promising compound for thermochromic applications because of its insulator-metal transition (IMT) characteristic with concomitant transmittance change for infrared-light. For practical applications, we deposited VO2 thin films on transparent ZnO films on glass substrate. Growth of (010)-oriented VO2 film was obtained on c-axis oriented hexagonal ZnO layer, however, zinc vanadate (Zn2V2O7) was formed at the interface region in conventional radio-frequency magnetron sputtering at substrate temperature of 350 °C. On the other hand, (010)-oriented VO2 films were successfully grown on ZnO/glass substrates even at lower temperature of 250 °C by introducing inductively coupled plasma (ICP)-assisted sputtering, realizing steep interface between VO2 and ZnO without undesirable compounds. The VO2 films showed resistivity drop of more than two orders of magnitude together with transmittance change for infrared-light from 45 to 10% at around 70 °C. Simultaneous achievement of steep changes for both resistivity and transmittance is a milestone for practical application of VO2 films into thermochromic devices, such as smart windows and optoelectrical devices. Further, we succeeded the low temperature growth of VO2 films on conductive Al-doped ZnO (AZO) by using ICP-assisted sputtering technique. The transmittance change of VO2 on AZO/glass from 50 to 10% was observed, realizing a basis for electrically controllable intelligent optical windows.

Electrical and optical properties of VO2 thin films grown on various sapphire substrates by using RF sputtering deposition

VO2 thin films were grown on a-, c-, m-, and r-plane sapphire and SiO2/Si substrates under identical conditions by using RF sputtering deposition from a VO2 target. The structural and the morphological properties of all VO2 films were investigated. The grain sizes of the VO2 films varied between 268 nm and 355 nm depending on the substrate’s orientation. The electrical and the optical properties of all VO2 thin films were examined in detail. The metal-insulator transition temperature (TMI) varied with the substrate’s orientation. The (200)/( $$\bar 211$$ )-oriented VO2 films on the a-plane sapphire showed the lowest TMI of about 329.3 K (56.3 °C) while the (020)/(002)-VO2 films on the c-plane sapphire displayed the highest TMI of about 339.6 K (66.6 °C). The VO2 films showed reversible changes in the resistivity as large as 1.19 × 105 and a hysteresis of ~2 K upon traversing the transition temperature. The variations observed in the TMI with respect to the substrate’s orientation were due to changes in the lattice strain and the grain size distribution. Raman spectroscopy showed that metal (rutile) - insulator (monoclinic) transitions occurred via the M2 phase for VO2 films on the c-plane substrate rather than the direct M1 to rutile transition. The shifts in the phonon frequencies of the VO2 film grown on various sapphire substrates were explained in terms of the strain along the V-V atomic bond direction (cR). Our work shows a possible correlation between the transition parameters (e.g., TMI, sharpness, and hysteresis width) and the width (σ) of the grain size distribution. It also shows a possible correlation between the TMI and the resistivities at the insulating and the metallic phases for VO2 films grown on various sapphire substrates.

Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior.

Radio frequency substrate biasing effects on the insulator-metal transition behavior of reactively sputtered VO2 films on sapphire (001)

Comprehensive studies have been performed on the effects of radio frequency (rf) substrate biasing on insulator–metal transition (IMT) properties of VO2 thin films grown on Al2O3 (001) substrates. As the rf substrate bias power increased, the ion energy during deposition increased, inducing a strong in-plane stress conversion and a modification of the rutile c-axis (cR-axis) length in the VO2 films. The reduced cR-axis length significantly lowered the IMT temperature. Increased IMT sharpness via improved crystallinity was obtained with rf substrate biasing at an appropriate power. Temperature coefficient of resistance at room temperature was comparable with reported values for impurity-doped VO2 films. Thus, the rf substrate biasing during reactive sputtering has great potential to control stress in thin films, which could finally control the IMT in oriented VO2 films.

Spectroscopic analysis of phase constitution of high quality VO2 thin film prepared by facile sol-gel method

VO2 thin films with large-area were prepared on Al2O3 substrates by a simple sol-gel method. After an annealing treatment under low vacuum condition, all the VO2 films showed a preferred growth direction and exhibited excellent semiconductor-metal transition (SMT) characteristics. The structure and electrical properties of the obtained VO2 films were investigated systematically. Raman spectra, X-ray diffraction and X-ray absorption spectra measurements pointed out that the VO2 film on ${\rm A}{\rm l}_{\rm 2} {\rm O}_{\rm 3} ( {{\rm 10}\overline {\rm 1} {\rm 0}})$Al2O3(101¯0) substrate showed a M1 phase instead of M2 phase as reported in previous studies. Based on the experiment results, it was suggested that the strained structure of oriented VO2 films could be a mechanism for the formation of the intermediate M2 phase, whereas it is difficult to access the pure M2 phase of undoped VO2 films. VO2 film on ${\rm A}{\rm l}_{\rm 2} {\rm O}_{\rm 3} \left( {{\rm 10}\overline {\rm 1} {\rm 0}} \right)$Al2O3101¯0 substrate showed a lower SMT temperature compared to VO2 film on Al2O3 (0001), which can be mostly attributed to the differences of both lattice mismatch and thermal stress. The present results confirm and make clear the relevance of the substrate orientation in the growth of VO2 film and their different contributions to the SMT characteristics in vanadate systems.

In situ x-ray diffraction studies on epitaxial VO2 films grown on c-Al2O3 during thermally induced insulator-metal transition

The structural phase transition (SPT) of VO2 films epitaxially grown on c-Al2O3 substrates by pulsed laser deposition and reactive sputtering was investigated by in situ temperature-controlled x-ray diffraction (XRD) across metal-insulator transition (MIT). An intermediate insulator phase was observed between room temperature monoclinic phase and high temperature metallic tetragonal phase. Gaussian curve fittings for measured XRD patterns revealed a significant contribution of the intermediate phase covering wide temperature range across MIT. Polycrystalline VO2 films grown on Si substrates revealed pure rutile phase after MIT in contrast with the epitaxial films on c-Al2O3 substrates. Strained structure of the VO2 films grown on c-Al2O3 substrates could be a mechanism for the formation of such intermediate phase known as monoclinic M2 phase. Presence of the intermediate insulator phase in highly oriented VO2 films may be an important factor in understanding the relation between MIT and SPT.

Semiconductor-metal transition characteristics of VO2 thin films grown on c- and r-sapphire substrates

We have made a comparative study of epitaxial growth of VO2 thin films on c-cut (0001) and r-cut (11¯02) sapphire substrates, and the semiconductor to metal transition (SMT) characteristics of these films have been correlated with their structural details. On c-sapphire, VO2 grows epitaxially in (002) orientation. These (002) oriented VO2 films have 60° twin boundaries due to three equivalent in-plane orientations. The epitaxial VO2 films on r-sapphire consisted of two orientations, namely (200) and (2¯11). The coexistence of these two orientations of VO2 has been explained on the basis of similarity of atomic arrangements in (200) and (2¯11) planes. The thermal hysteresis (ΔH), sharpness of the transition (ΔT), and the transition temperature for VO2 films on c-sapphire were found to be 4.8, 8.5, and 72.6 °C, respectively, which were higher than the corresponding values of 3.3, 5.4, and 60.3 °C for films on r-sapphire. The SMT temperature for VO2 films on c-sapphire was close to the bulk value of 68.0 °C. The significant decrease in transition temperature to 60.3 °C for VO2 films on r-sapphire has been attributed to the compressive strain along [002] direction of VO2.

On growth of epitaxial vanadium oxide thin film on sapphire (0001)

We report the characteristics of epitaxial growth and properties of vanadium oxide (VO2) thin films on sapphire (0001) substrates. Pulsed laser deposition was used to grow (002) oriented VO2 films on sapphire (0001). Transmission electron microscopy studies showed that the orientation relationship between the substrate and the thin film is: (002)f2∥(0006)sub3 and [010]f2 ∥sub. It was also established that VO2 has three different orientations in the film plane which are rotated by 60° from each other. The epitaxial growth of vanadium oxide on sapphire (0001) has been explained in the framework of domain matching epitaxy (DME). Electrical resistivity measurements as a function of temperature showed a sharp transition with a hysteresis width ˜5 °C, and large resistance change (˜1.5 × 104) from the semiconductor phase to the metal phase. It is interesting to note that in spite of large angle twin boundaries in these VO2 films, the SMT characteristics are better than those observed for polycrystalline films. The higher width of thermal hysteresis for the VO2 film on c-sapphire compared to a bulk single VO2 crystal and a single-crystal VO2 film on r-sapphire can be attributed to the existence of these large-angle twin grain boundaries. These findings can provide insight into the phase transformation characteristics of VO2, which has important applications in switching and memory devices.

Pulsed laser deposition of oriented VO2 thin films on R-cut sapphire substrates

We have deposited highly oriented VO2 thin films on R-cut sapphire substrates by the pulsed laser ablation technique. The stabilization of pure VO2 is complicated due to a large number of distinct phases in the vanadium oxide system. As many as 13 different phases ranging from oxygen stoichiometry of V4O to V2O5 can exist in this system. The deposition parameters for fabrication of phase pure, highly oriented VO2 thin films on R-cut sapphire substrates by the pulsed laser deposition technique are discussed. A KrF laser (wavelength λ=248 nm, pulse duration τ=15×10−9 s) was used to ablate a pure vanadium target in an atmosphere containing specific ratios of oxygen and argon. The partial pressure of oxygen in the chamber was critical in stabilizing the VO2 phase. X-ray diffraction and other microstructural characterization revealed that the films were single phase and strongly oriented with the (200) planes parallel to the sapphire substrate. Short transition widths of 2 K and resistivity changes of 3×104 have been observed, thus indicating the growth of highly oriented VO2 films.

Preparation of VO2 films by organometallic chemical vapour deposition and dip-coating

Low-pressure organometallic chemical vapour deposition (OMCVD) and dip-coating of VO2 films using vanadyl tri(isobutoxide) as the starting material were investigated. In OMCVD, discontinuous VO2 films, which were composed of fine needle crystals, formed under very limited conditions, around 600° C with a flow rate of oxygen gas of 0.2 to 0.5 cm3 sec−1. However, very uniform and tightly packed VO2 films were grown by deposition at 300 to 700° C in the absence of oxygen gas and subsequent annealing in nitrogen at 500° C for 2 h. The films exhibited a sharp semiconductor to metal transition at 60 to 70° C, accompanied by a change in the resistivity by four to five orders of magnitude. In dip-coating with two-step heat-treatments (300° C for 1 h in nitrogen and subsequently 500° C for 2 h in nitrogen), of the gel films formed from VO(O-i-Bu)3-H2O-i-PrOH system, uniform (0 1 1) oriented VO2 films were formed. A transition in the electrical conductivity by two to two and a half orders of the magnitude was found to occur around 60° C. Before and after the transition, no distinct variation in the XRD pattern was observed.