Study On Increasing The Phase Transition Temperature Of VO₂ Thin Film

Vanadium dioxide(VO2)is a typical phase-change material that undergoes a reversible and ultrafast(sub-picosecond)phase transition between a low-temperature semiconductor phase(monoclinic structure)and a high-temperature metallic phase(rutile structure)at 68℃(MIT).This phase transition is accompanied by a four order-of-magnitude resistivity change,and abrupt changes in fundamental physical properties such as electrical,optical,and thermal properties.These changes allow VO2 to be made into various devices such as switches,storages,sensors,and transistors.Current research has focused on lowering the transition temperature while there are limited studies on methods and mechanisms for increasing the phase transition temperature.In this study,the doping/strain modulation were employed to enhance the phase transition temperature of VO2 thin films and the mechanisms behind the consequent temperature improvement were explored.First,VO2 films with thicknesses of 300 nm,200 nm and 100 nm were deposited on polished Al2O3 substrates when the deposition periods were 30 min,20 min and 10 min,respectively;in addition,germanium-doped VO2 films with a thicknesses of about 100 nm were prepared.The effects of film thickness and germanium doping on the physical phase,lattice parameters,microstructure and phase transition characteristics of the films were investigated.The results have showed that the lattice parameters of VO2 shrank along the a-and c-axes as the film thickness decreased,and the lattice shrinkage made the low-temperature monoclinic phase of VO2 more stable,requiring higher energy to trigger the phase transition,thus increasing the TMIT up to 70.65℃.The phase transition temperature of germaniumdoped films reached 80.15℃,mainly because the introduction of Ge4+with smaller ionic radius into the VO2 lattice transformed the V-V bond to V-Ge bond,resulting in V-V chain contraction.This compression of VO2 lattice also increased the phase transition temperature.It therefore indicates that the phase transition temperature of the films can be increased by the combination of germanium doping and strain method.Then,GeO2 buffer layers with different doping(Al3+)concentrations were deposited on Al2O3 substrates,followed by the deposition of VO2 films on the GeO2 buffer layers.The final VO2 films were obtained after annealing treatment.The effects of buffer layers doped with different concentrations of Al on the TMIT of VO2 films were investigated.The results have showed that as the concentration of Al in GeO2 buffer layer increased,the lattice mismatch between the buffer layer and VO2 film increased.This led to the decrease of lattice parameters of VO2 along the a-axis and c-axis directions,achieving lattice shrinkage as well.As a result,the TMIT was increased with the maximum phase transition temperature being 85.1℃.Finally,VO2 films with GeO2 buffer layer and Au buffer layer were deposited on single-crystal silicon,and the effects of different buffer layers on the phase transition properties of VO2 films were investigated.The results have showed that the VO2 films deposited directly on the monocrystalline silicon substrate and Au buffer layer showed minimal change in phase transition temperature which was about 68℃,and the magnitude of phase transition was small at about one order of magnitude,indicating low crystallinity of the films.The VO2 films deposited on GeO2 buffer layer showed higher phase transition temperature at about 74℃,greater magnitude of phase transition at about three orders of magnetite,and better phase transition characteristics because of the similar crystal structures of VO2 and GeO2.The small lattice mismatch during the growth of VO2 film promoted the epitaxial growth of VO2 film.In summary,this study mainly adopted the strain method/doping method to improve the phase transition temperature of VO2 films.The strain method was to adjust the lattice parameters by changing the film thickness,buffer layer type and buffer layer doping.In addition,the substitution-based doping with Ge ions was employed to achieve further enhancement in the phase transition temperature.The synergistic use of strain method and doping not only significantly improved the phase transition temperature of the VO2 films without compromising the phase transition amplitude,hysteresis width,and other phase transition parameters.This holds great significance for the development of VO2 materialbased devices.