Search

Thin films of vanadium dioxide (VO2) are deposited on indium tin oxide (ITO), stainless steel (SS), and glass substrates using chemical vapour deposition. X-ray diffraction and Raman spectroscopy measurements confirmed the single phase nature of the VO2, which showed a phase transition from a low conducting state at low temperature(<68 °C) to a high conducting state at high temperature (>68 °C). This was confirmed by electrical conductance and infra-red reflectance measurements. X-ray photoelectron spectroscopy was used to measure the charge states of vanadium species. The optical constants of VO2 were determined using visible and near-infra red(NIR) reflectivity and show that the VO2 film on ITO has a lowered plasma frequency compared with VO2 on glass substrates. The thin films of VO2 enable a broadband of ultra-high absorption at mid-wave infra-red frequencies due to a Fabry-Pérot (F-P) like resonance due to the dielectric properties of ITO, SS, or glass. The tunability of this absorption band via VO2 thickness and the switchability by temperature makes the system attractive for absorptive coatings with controllable emissivity. [ABSTRACT FROM AUTHOR]

• Vanadium dioxide (VO 2) is deposited on glass slides by Chemical Vapor Deposition. • Structural characterization revealed the presence of phase pure monoclinic VO 2. • Electrical measurements of the as-deposited VO 2 did not show a sharp transition. • Surface analysis showed Vanadium in V3+, V4+ and V5+ states in as-deposited films. • Annealing the films enhanced the transition strength. In this work, VO 2 is deposited on borosilicate glass substrates via a single step Chemical Vapor Deposition (CVD). Though the structural characterization revealed the presence of phase pure polycrystalline VO 2 in simple monoclinic crystal structure (P2 1 /c), electrical measurements did not show a sharp transition. To understand the ambiguity, surface characterization was performed with X-ray Photoelectron Spectroscopy (XPS) on the as-deposited VO 2 thin films. Vanadium was present in V3+, V4+ and V5+ states. Following these measurements, annealing was performed in ambience at 200 °C for 10 minutes. Subsequent electrical measurements showed an improved phase transition, enhanced by an order of magnitude. We attribute the enhancement in transition to annealing of oxygen vacancies present in VO 2 thin films because of the deposition conditions. Decrease in oxygen vacancies was correlated with a decrease in the fraction of V3+ from XPS measurements. The sample annealed at 200°C for 10 minutes exhibited a higher resistance ratio (ΔA = 305) with an optimum hysteresis (4°C) and transition width(12°C), whereas the as-deposited sample exhibited a resistance ratio of ΔA = 32. The thermal treatment affected the characteristics of the transition. A narrow hysteresis of 4°C and a sharp transition width with a reasonable resistance ratio were obtained for the sample annealed at 200°C for 10 minutes. From this study, we understand that our synthesis of VO 2 thin films via CVD is prone to be off-stoichiometric and therefore, contains defects such as oxygen vacancies. We could mitigate the effect of these vacancies and engineer the stoichiometry by performing a simple annealing which in turn improved the strength of the phase transition. Drawbacks such as a poor-quality transition, resulting from synthesis parameters can be mitigated if we understand the effect of vanadium and oxygen vacancies. [ABSTRACT FROM AUTHOR]

• Comparison of two industrially scalable deposition techniques to develop large areas of VO 2. • UNSPACM is a cost-effective process that achieves electrically superior VO 2. • CVD is used to develop optically superior VO 2 over large areas. • Where optical applications are crucial, roughness is a key and CVD is preferred. • Different synthesis process alters morphology and stoichiometry. We compare the effect of two synthesis methods namely Chemical Vapor Deposition (CVD) and Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) on the Semiconductor-Metal Transition (SMT) characteristics of VO 2 thin films. The SMT characteristics of the films are measured electrically and optically. CVD films show a lower resistance ratio and SMT strength, whereas films synthesized via UNSPACM are rougher. We ascribe the reason for the observed behaviour to morphology and off-stoichiometry arising from the deposition process. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO 2) has been a much sought after candidate for many electronics and photonics based applications. These applications demand CMOS compatibility and simple but scalable synthesis procedure. Herein we report the deposition of VO 2 on CMOS compatible Si substrates via easily scalable Metal Organic Chemical Vapor Deposition (MOCVD) at different substrate temperatures between 520 and 550 °C. The morphology of the films deposited at different growth temperatures varied drastically despite retaining VO 2 in the M1 phase. This was verified by XRD and Raman experiments. The thin films deposited at 535 °C showed sharp grain boundaries with a grain size of about 200 nm. Similarly, the electrical characteristics of the films deposited at 535 °C showed superior transition compared to the films deposited at other temperatures. However the films deposited at 550 °C retained the superiority in the transition strength, the transition width and the hysteresis of the transition increased at that temperatures. We understand that the reason for this observed behavior is the fraction of surface defects. It is seen from XPS measurements that vanadium was available in V3+, V4+ and V5+ oxidation states and the fractions of these individual components varied in all the thin films deposited at different temperatures. We believe that the defects fraction can give a handle to control the nature and quality of the transition in VO 2. • Single phase VO2 was grown at different temperatures using MOCVD on Si substrates. • Morphology of the thin films changed with the growth temperature. • Surface studies by XPS showed the presence of V in V3+, V4+ and V5+ states. • For a particular growth temperature, ratio of V3+, V4+ and V5+ states is optimal. • Varying growth temperature gives a handle to tweak defects and also the transition. [ABSTRACT FROM AUTHOR]