Spontaneous fluctuations in a plasma ion assisted deposition – correlation between deposition conditions and vanadium oxide thin film growth

Abstract

In this work correlations between thin film crystallinity of plasma ion assisted electron beam evaporated vanadium oxide (VOx) and fluctuations of the deposition parameters during the growth process could be observed by in situ monitoring deposition conditions and electron microscopy studies. In the presented case, unintentional fluctuations in the gas flow at the plasma source caused by inhomogeneous melting of the target material lead to an increase in discharge current and therefore a decrease of the oxygen flow in the plasma source, resulting in the formation of highly crystalline bands due to a temporary increase in energy flux. The major part of the VOx thin film consists of a large number of nanocrystals embedded in an amorphous phase. In-depth structural analysis confirms a mixture of V2O5, in different modifications, VO2, as well as the mixed-valence oxides V4O9 and V6O13, for nanocrystalline parts and crystalline bands. These differ mainly in the degree of crystallinity being influenced by variations in discharge current, and partly in the amount of higher oxidized vanadium oxides. In future, precisely controlled variation of plasma source conditions will open up pathways to control and tailor crystallinity of electron beam evaporated thin films, allowing for production methods for patterned thin films or layers with graduated crystallinity. This may give rise to a new class of coatings of nanohybrids combining amorphous VOx with low electrical conductivity and crystalline domains providing a higher electrical conductivity which is useful for electrochromic displays, smart windows, and solar cells.