Influence Of In³⁺ Doping On The Phase Transition Properties Of Vo₂ Thin Films

Since 1950,certain transition metal oxides which exhibit a semiconductor-to-metal transition(SMT)at a temperature T_c have attracted considerable attention.In particular,the SMT in VO₂ between a monoclinic insulating phase and a rutile metallic phase has been intensively studied over the past 50 years,both for scientific reasons seeking to clarify the nature of the phase transition and application reasons seeking to exploit the large resistance change across T_c.VO₂ thin films with different doping elements have been achieved and the T_c of the films has been tuned from less than 200 K to more than 350 K.However,introducing doping elements is usually accompanied with the decrease of?A and broadening of?T.In this paper,we prepared VO₂ thin films doped with In³⁺,which exhibit higher?A and smaller?T with neglectable change in T_c.The process parameters of VO₂ thin film prepared by pulsed laser deposition technology are optimized.For two commonly used targets,VO₂ and V₂O₅,three temperature gradients(200?,400?and 600?)were set to deposit VO₂ thin films.For the V₂O₅ target,the film obtained at 200?is VO₂(011),which is not the expected phase.At higher temperatures,the deposition rate is too high due to the loose target,there are clusters of nanowires on the surface of the films.For the denser VO₂ target,although the film did not crystallize at low temperature,VO₂(020)films were obtained when the temperature increased,and the crystallinity of the film increased at 600?.Films were deposited at 50 mtorr and 10 mtorr oxygen pressures respectively.At 50 mtorr,high oxygen pressure dramatically increased the deposition rate,the film surface roughness increased sharply,and V₂O₅ phases appeared.However,higher laser frequency will also increase the deposition rate of the film and reduce the crystallinity of the film.Closer T-S(target-to-substrate)distance will greatly increase the surface roughness of the film,but longer distance will reduce the crystallinity of the film.Finally,the VO₂ target was selected,the temperature for deposition was set at 600?,the deposition oxygen pressure was 10 mtorr,the laser frequency was 2 Hz,and the T-S distance was 45 mm.The optimized process parameters were used to prepare V_1-xIn_xO₂ film and the film quality was characterized.As the Indium content increases from 0 to 7%,the VO₂(020)diffraction peak shifts slightly to the left,and the lattice constant b increases.At the same time,the FWHM of the diffraction peak in the XRD pattern increases to 0.384°,and the crystallinity of the film decreases.The AFM image shows that the In³⁺doping decreases the surface roughness,and the crystal grains of the film are refined and arranged more closely,which is consistent with the results calculated by the Scherrer formula.The temperature-resistance curve of the doped films was tested via PPMS,and the SMT performance of the V_1-xIn_xO₂ film was obtained.The increase in the phase transition amplitude?A by doping was achieved for the first time by doping In³⁺with practically no change in T_c.When x is 0.02%,?A increases by 26.75%and the hysteresis width?H reduces by 10.35%compared with the undoped VO₂ film.The sharpness of the phase transition of VO₂ film increases significantly when the doping concentration increases.As x increases from 0 to 0.07%,?T decreased by 36%,and TCR increased by 58%.In summary,the high-quality V_1-xIn_xO₂ films were prepared by pulsed laser deposition,and the phase transition amplitude was enhanced via doping for the first time(for V_0.98In_0.02O₂ film,?A increased by 26.75%)without changing T_c.The sharpness of the phase transition also increases significantly(for V_0.93In_0.07O₂ film,?T decreased by 36%,TCR increased by 58%).This work greatly improves the SMT performance of the VO₂ film,which provides a new method for Indium-based co-doping,and is promising for the application of VO₂-based optoelectronic devices.