The Structure Modulation And Optoelectronic Properties Of Vanadium Dioxide

As a typical Mott transition material,vanadium dioxide(VO₂)can undergo a semiconductor-metal transition(SMT)from the monoclinic phase to the rutile phase with external stimulation like heat,light,electricity,and pressure.The SMT of VO₂ is fully reversible,and the SMT accompanies by a dramatic change in optical and electrical.When the external stimulation disappears,the VO₂ will immediately transform from the rutile phase to the monoclinic phase.The SMT characteristic makes VO₂ a potential candidate for applications in thermal detection,threshold switches,thermochromic smart windows,field-effect transistors,memristors,and tunable metasurface arrays.The phase transition of VO₂ can be belonged to structure changes in crystalline.The crystalline structure can be modulated by adjusting the crystallization process,doping,and applying stress.Thus the SMT characteristics of VO₂ can be effectively regulated by changing the technological parameters of the fabrication process,doping and stress.The effects of the changes of crystalline structure on the SMT characteristics of VO₂ were studied by two elements co-doping and changing the fabrication process.The applications of VO₂ in microbolometers,threshold switches(TS),and thermochromic smart windows are systematically investigated.The main contents are summarized as follows:1.The effects of co-doping of rare earth element Y and transition metal element Ti on the microstructure and SMT characteristics of VO₂ were investigated.With the help of structural analysis,it is found that Ti and Y co-doping has a conjugate effect on the regulation of VO₂ lattice structure.The Ti mainly affects the interaction of V and V atoms in the lattice,while the Y mainly affects the bonding of V and O elements.The films present polycrystalline monoclinic feature with mono Y or Ti doping.While the conjugated interaction between Ti⁴⁺and Y³⁺ions causes the synchronous adjustment of V-V chain and V-O bond in lattice structure,resulting a significant distort in the VO octahedral structure,even the appearance of VO₂(B)phase and amorphization.2.The effects of deposition temperatures on the crystal structure and SMT properties of Y-doped VO₂ films were deeply investigated by fixing the doping concentration of Y.Y inhibits the binding of V and O elements in VO₂ films due to the stonger bond energy of Y-O than V-O.Meanwhile,the effect of Y on VO₂ becomes more obvious with the increase of substrate temperature.When the deposition temperature increases,the average size of surface particles decreases,and the binding of V and O elements is further inhibited.The average grain size and phase transformation amplitude of VO₂ also continue to decrease with increasing deposition temperature.When the deposition temperature reaches 200?,the intensity of Raman peaks corresponding to the monoclinic VO₂ decreases,and part of the characteristic peaks disappear.SAED shows that the sample still presents a typical diffraction ring characteristic of polycrystalline VO₂.Meanwhile,a large number of amorphous regions appear in the sample,some unique amorphous regions surrounded by VO₂crystallites appear in local areas.The 200?deposited sample shows a resistivity of0.01?cm,with TCR larger than 1.5%,which is a potential candidate for thermally sensitive materials.3.A High-quality single-crystal VO₂ threshold switch was prepared by electroforming on VO₂,V₂O₅,and V₃O₇ composite film with an ITO bottom electrode and Pt top electrode.The switching performance of single-crystal VO₂ is far superior to that of polycrystal VO₂.The TS device based on single-crystal exhibits a fast switching speed of 23 ns,an excellent endurance over 10⁹ cycles,and a high I_on/I_offratio of 143.Which present certain advantages over the current reported VO₂ based switches.Besides,the electroforming process of single-crystal VO₂ with different orientations was studied.Single crystal VO₂ with different orientations can be fabricated by changing the main orientation of VO₂ in composite phase films.The((?)11)-orient single-crystal VO₂ presents a higher I_on/I_off ratio than(011)-orient one.The relation between the switching ratio and the grain orientation is due to the different stresses of grains with different orientations along the grain axis in the sample.4.Vanadium oxides films with prominent thermochromic performance were prepared by magnetron sputtering pre-deposition and rapid thermal annealing(RTA)process.This is the first report on nanocomposite VO₂ thin films which were prepared through magnetron sputtering.The RTA process gave a unique structure where VO₂ NP is dispersed in V₂O₅/V₃O₇ matrix with a dense thermodynamic stable V₂O₅ overcoat.The unique structure has substantially increased the thermochromic performance.It presents the?T_sol of 18.7%and the T_lum of 34.5%.The unique structure formed by a simple annealing process not only brings excellent thermochromic performance but also significantly improves the environmental stability of the sample.The accelerated test suggested that the expected service life of such films is over 20 years.The RTA process and W doping have good compatibility.The phase transition temperature(?_c)of the RTA processed sample was reduced to near room temperature by W doping.The W doped sample presents?T_sol of 18.7%,T_lum of 34.5%and?_c of 36.6?with over 20years expected service life.