Ellipsometric characterization of VO$_2$, VO$_2$ : Mg, VO$_2$ : Ge nanocrystalline films

The spectra of the refractive index $n(\lambda)$ and the extinction coefficient $k(\lambda)$ of thin VO$_2$, VO$_2$ : Mg, VO$_2$ : Ge films were measured using the ellipsometric method. For an undoped VO$_2$ film at a wavelength $\lambda$ = 632.8 nm, near the insulator-metal phase transition, the $n(T)$ and $k(T)$ thermal hysteresis loops were studied. An interpretation of the results is given on the base of the Moss relation, the idea of a change in $n(T)$ and $k(T)$ with an impurity variation of the material density, and also on the base of the ideology of the Coulomb transformation of the density of states function in strongly correlated materials.