Film sottili di VO2 drogata con W per il controllo della fotoemissione nel range spettrale del vicino infrarosso

In this work, polycrystalline VO$_2$ and W-doped VO$_2$ thin films were fabricated on silica and silicon substrates by DC reactive magnetron co-sputtering and subsequently annealed to obtain the monoclinic M1 phase. Morphological and structural characterizations confirmed the formation of continuous, nanocrystalline films with RMS roughness below 4~nm and crystallographic features typical of M1 VO$_2$. W incorporation was controlled by varying the tungsten target power, resulting in estimated doping levels of $\sim$1.5, 2.5 and 3.5~at.\% W/V. The optical response of pure and doped films was investigated through temperature-dependent transmittance (400–2000 nm) and temperature-dependent spectroscopic ellipsometry (400–1700 nm). Particular attention was given to the evolution of the semiconductor-to-metal transition (SMT) temperature $T_t$ and to its impact on the near-infrared (NIR) behaviour. W doping was found to significantly reduce the transition temperature while preserving a measurable optical contrast at 1500~nm, a wavelength of high technological relevance for telecommunications. In particular, the sample doped at $\sim$1.5~at.\% exhibited $T_t = 38.7 \pm 0.6~^\circ$C on heating and $31.0 \pm 0.3~^\circ$C on cooling, corresponding to a $\sim$30°C reduction with respect to undoped VO$_2$, in line with literature reported values on similar samples. A dedicated ellipsometric model was developed to account for the slightly inhomogeneous dopant distribution revealed by Rutherford Backscattering (RBS) measurements on similar samples, consisting of a thin surface layer of pure VO$_2$ above a deeper W-doped region. The extracted optical constants $n(\lambda,T)$ and $k(\lambda,T)$ for all samples showed the expected decrease of $n$ and increase of $k$ across the SMT, with a substantial contrast maintained even at moderate doping levels. For the 1.5~at.\% sample, the transition from 25$^\circ$C to 90$^\circ$C resulted in variations of $\Delta n = -1.19$ and $\Delta k = 2.31$ at 1500~nm, while at 800 nm the same transition produced $\Delta n = -0.95$ and $\Delta k = 0.34$. A progressive increase of the refractive index in the insulating phase was observed for doping levels above $\sim$1.5~at.\%, suggesting a direct influence of W on the dielectric response beyond simple transition-temperature tuning. Overall, the results demonstrate that W-doped VO$_2$ films with reduced $T_t$ and preserved NIR optical contrast are suitable candidates for active photonic and optoelectronic devices. The temperature-dependent optical constants extracted in this thesis provide realistic input parameters for the design of VO$_2$-based NIR modulators and emission-control architectures, including hybrid systems where the SMT is exploited to tune the emission properties of near-infrared emitters.