The Phase Stability And Electric Switching Properties Of Trivalent Ions-doped Vanadium Dioxide Thin Films

Vanadium dioxide?VO₂?is an archetypal metal-insulator transition?MIT?material with the reversible transition at 68°C.Accompanied with the MIT are a drop of conductivity in several orders of magnitude,and a drastic change in optical parameters like refractive index and extinction coefficence.Utilizing these outstanding properties we can implement VO₂ to various fields,such as smart window coatings,optical switching devices,and sensors.Besides,metastable phases of VO₂ provide additional tuning possibilities and more potential applications.A better understanding of the modulation of the MIT of VO₂ would greatly contribute to the research and development of applications based on VO₂ material.Ions-doping is a practical way to modulating the transition temperature(T_MIT)and phase stability of VO₂.Understanding the mechanism responsible for such influence on properties is of paramount importance to being able to design the composition of doped VO₂.However,the influence and mechanism of ions-doping remain ambiguous and even controversial.Therefore,in this thesis,trivalent ions-doped VO₂ were taken as examples to illustrate the role and influence of the dopants in VO₂.Fe³⁺,Al³⁺,and Cr³⁺-doped VO₂thin films were synthesized by hydrothermal method.The phase stability,T_MIT,and electric switching properties were systematically investigated.Research work has been done as the following parts of details:?1?Highly-oriented VO₂ thin films are grown on c-cut?0001?sapphire substrates using hydrothermal method.By optimizing the hydrothermal growing and post-annealing process,high-performance VO₂ films are obtained,yielding a resistance change of more than 4 orders of magnitude across the MIT.A reproductive long-time post-annealing process is further determined.Samples annealed at 480°C 1 kPa air environment give the best performance,in which the resistance changes more than 4 orders of magnitudes in the metal-insulator transition.?2?A series of Fe-doped VO₂ powders and thin films are synthesized using hydrothermal method.The effect of Fe dopants on the morphology,structures and metal-insulator transition properties of Fe-doped VO₂ are investigated by scaning electron microscope,X-ray diffraction,Raman spectroscopy and thermal resistance measurements.The transition temperature is observed to decrease first,reach its minima?⁶0°C?at¹.5 at.%Fe and then rise with increasing the doping concentration.As shown in the Raman spectra,the M₂ and T phases are stabilized by the Fe dopants.The temperature-dependent resistance measurements show that electric switching properties deteriorets with doping increases.However,the resistance change of samples with the Fe content less than 1.5 at.%remains to be⁴ orders of magnitude,demonstrating that Fe-doped VO₂ is a promising electric switching material.?3?Al³⁺-and Cr³⁺-doped VO₂ are synthesized using hydrothermal method.The influences of dopants on the electric switching properties and the transition temperature are investigated.Lattice contraction is observed in Al doped samples,and T_MITIT monotonously rises as Al doping amount increases.In Cr-doped VO₂ thin films,lattice expansion is observed at doping level lower than 2%.The T_MIT declines first when doping amount is less than²%,and then rises with further Cr doping,which is similar to the phenomenon observed in Fe-doped VO₂.The behavior of Fe³⁺-,Al³⁺-,and Cr³⁺-doped VO₂ shows that the lattice expansion induced by large-radii ions leads to the decrease of T_MIT,and the lattice effect of large-radii trivalent dopants diminishes at a specific doping level,after which the charge effect dominates the evolution of T_MIT.The electric transition properties of Fe³⁺-,Al³⁺-,and Cr³⁺-doped VO₂ also demonstrate that large-radii-trivalent doping could be an effective way to lower the T_MIT while maintaining good switching properties.