Preparation And Properties Study Of Germanium Doped Vanadium Dioxide Thin Films

VO2 is a material with the metal-insulator transition(MIT),a characteristic that allows its use in fields including smart glass,laser protection,sensors and integrated circuits.The eigenstate phase transition temperature of VO2 is 68?,which,however,limits its applications.In this paper,the germanium doped VO2 films and GeO2 buffered VO2 multilayer films on Al2O3 substrate were prepared by magnetron sputtering method,with a view to increase the phase transition temperature of VO2.The effects of annealing temperature,Ge doping concentration and GeO2 buffer layer on the properties of VO2 films were studied.The main research contents of this paper are as follows:Firstly,Ge0.O53V0.947O2 thin films were prepared and annealed in Ar atmosphere at 500?,520?,550? and 600?.It showed that the crystallinity and grain size increased and the thermal hysteresis width decreased with the growing annealing temperature;the film annealed at 550? illustrated the optimum combination of high crystallinity,low thermal hysteresis width,large phase transition width and phase transition temperature.Regarding this film,its large grains increase the probability of nucleation defects and reduce the interfacial energy of the grains,which reduce the elastic energy barrier to overcome during phase transition,thereby narrowing the thermal hysteresis width.In addition,VO2 films with different germanium-doping concentrations(0 at.%,5.3 at.%,7.7 at.%,14.8 at.%,19.4 at.%,and 24 at.%)were prepared.It was found that:the phase transition temperature increased as more germanium was doped and reached the peak of 84.7? when the film doped with 19.4 at.%germanium,21.7? higher than that of the pure VO2 film;the metal-semiconductor phase transition disappeared when the germanium doping concentration was 24 at.%;the thermal hysteresis width and the phase transition amplitude decreased,and the phase transition width decreased with the increase of the germanium concentration.Analyses suggest that Germanium doping causes the VO2 lattice distortion,which on one hand increases the structural difference between VO2(M)and VO2(R)and the energy required for the phase transition;on the other hand,has the a-axis of the VO2(M)lattice stretched and the b-axis compressed,which leads to the increase in the aM/2bM value(equivalent to the cR/aR value).As a result,the lattice distortion degrees and the resulting phase transition temperatures of VO2 films vary by the doping concentration.Finally,the VO2 multi-layer films of VO2/GeO2/Al2O3 and Ge0.053V0.947O2/GeO2/Al2O3 were prepared.It was found that the phase transition temperature of the VO2 film could be increased by 2.4 ? by adding the GeO2 buffer layer;and the phase transition temperature of germanium-doped Ge0.053V0.947O2 film became 73.2 ? after the GeO2 buffer layer was added.This is because the GeO2 buffer layer compresses the aM-axis and bM-axis and stretches the cM-axis of VO2 increasing the aM/2bM value;both GeO2 buffer layer and germanium doping increase aM/2bM and further increases the phase transition temperature of the VO2 film.