Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films

Elaboration of VO2 films with good thermochromic properties remains a challenge because it is mandatory to avoid other intermediate phases belonging to the vanadium-oxygen system. In this work, we propose vanadium nitride (VN) as a new precursor to obtain thermochromic VO2. VN films were reactively sputter-deposited on Si substrates by using an in-line semi-industrial machine. The films of 175 nm thickness were submitted to an annealing process implemented at different durations at 450 °C. X-ray diffraction and Raman spectrometry were performed for structural characterizations of the oxidized films showing the presence of monoclinic VO2 in a wide range of oxidation time. As the annealing time increased, V2O5 appeared to affect the performance of the oxidized films. The TEM analysis carried out on the oxidized sample for 25 min showed that there is an abrupt interface between VN and VO2. The thermal-induced properties of the studied films were analyzed in terms of their electrical resistance employing a four-point probe method and their emissivity modulation properties by infrared camera. The results showed a thermochromic behavior for the samples oxidized in the range of 15–35 min. On the contrary, the presence of residual VN layers in samples oxidized for less than 15 min and V2O5 in the ones annealed for more than 35 min hindered the thermochromic behavior of these films. Bringing together all the characterization techniques used a phase diagram of VN oxidation was plotted. The results of this work suggest that VN is an interesting new precursor to synthesize thermochromic VO2 films.