Natural metamaterial behavior across the phase transition for WxV1−xO2 films revealed by terahertz spectroscopy

Abstract

Metamaterials, which are made of repeated patterns of appropriately arranged small discrete structures, display unusual electromagnetic properties that overwhelm those of conventional materials. The modification of their properties is generally achieved by arranging the structures mechanically or electrically and requires rather complex designs. We report on the study of the complex conductivity of epitaxially-grown tungsten-doped vanadium dioxide (W x V 1Àx O 2) thin films through the semiconductorto-metal phase transition (SMT) using terahertz time-domain spectroscopy. The modelling of the terahertz conductivity across W x V 1Àx O 2 SMT provides clear insights about the gradual nucleation of VO 2 metallic domains among the semiconducting host and evidences the presence of strong carrier confinement and enhanced absorption close to the transition temperature, leading to a strong capacitive response of the electrons. The evolution of the SMT is also strongly affected by W doping, which reduces the scattering time in the metallic state, lowers the transition onset temperature and extends the temperature range over which the transition occurs. The W x V 1Àx O 2 films thus forms an effective medium in the vicinity of the SMT and display the signature of a disordered metamaterial, which has significantly enhanced functionality thanks to its readily thermally-tunable properties over a wide range of temperatures close to room temperature.

Probing the role of thermal vibrational disorder in the SPT of VO$$_2$$ by Raman spectroscopy

“…The transition temperature ( ) of VO films were generally higher than their Cr-doped counterparts by and , respectively. The observed VO behaviour is in line with previous studies 37 , 38 as a large and sharp transition with a narrow hysteresis is observed for VO films grown on c- and r-sapphire substrates, as shown in Fig. 4 a,c, where up to 4.6 and 4.7 orders of magnitude change in resistivity were respectively obtained.…”

Complementary Vanadium Dioxide Metamaterials with Enhanced Modulation Amplitude at Terahertz Frequencies

“…where ε i is the permittivity in the insulating state, and ε m is the permittivity in the metallic state. Based on the simulated model, in both the insulating state and metallic state, we used ε i =10 and ε m =-90 between 0.1 to 1.3 THz, in agreement with the published work [24,43]. Fig.…”

Combined Role of Substrate and Doping on the Semiconductor-to-Metal Transition of VO2 Thin Films

“…Two strategies to fine-tune the above-mentioned properties are commonly considered to be effective in the literature: doping and strain engineering. First, electronic doping via inclusion of donor-type metal cations like W 6+ is known to reduce T SMT by injecting excess electrons and creating local nucleation sites for the transition in the material, thus lowering the energy required to trigger the transition. − Some dopants stabilize other phases in the film, like chromium stabilizing other monoclinic phases , by varying the pressure inside the crystal, but tungsten is known to preserve the epitaxial quality of monoclinic (M1) VO 2 films. The free carriers also contribute to increase the electrical conductivity and the IR reflectivity in the semiconducting state of VO 2 , reducing the amplitude change of these quantities upon transition with respect to the undoped case …”