The Mechanism Of Electronic Transitions And Infrared Switching Dynamic Of Vanadium Dioxide Film

As an important type of informational and functional oxide materials,the intrinsic physical properties and transport characteristics of phase transition material are extremely important in practical applications.Recently,due to the potential applications in smart window,memristor and infrared detector,the insulator-metal phase transition material of vanadium dioxide（VO₂）,have become one of the most popular research material in solid state physicals,condensed matter physics and optoelectronic.VO₂ is an well know transition metal oxide,which undergoes a first-order metal–insulator transition at about 340 K with an abrupt increase in resistivity by several orders of magnitude.At room temperature,the lattice in the insulator phase has the monoclinic structure（P2₁/c）and the bandgap is about 0.7 eV.Taken the strain and doping effect into consideration,the bandgap will be changed about 0.2 eV.When the temperature increased to 340 K,the insulator monoclinic structure will be changed into metal rutile structure（P4₂/mn）.Meanwhile,the bandgap value of VO₂ becomes zero due to the overlapping of valence band of a_1g and conduction band of_ge^?.In general,the transmissivity of VO₂ film at near infrared region（²650 nm）is about 80%in room temperature while the transmissivity is about 30%at metal state.The transmissivity will be changed with the thickness of the VO₂ film.Noted that the phase transition temperature is near room temperature,which is more competitive in applications than the V₂O₃ and V₂O₅.The phase transition temperature of V₂O₃ and V₂O₅ is about 160 K and 530 K,respectively.Taking advantage of the resistivity change and the variation of transmissivity in near infrared range,VO₂ can be used in energy-saving smart windows,infrared camouflage or cloaking and infrared detectors.VO₂ is a classical phase transition material,which is researched about sixty years.The mechanism of the MIT can be electronically（Mott transition）,structurally（Peierls transition）or collaborative（Mott-Peierls transition）driven,which is usually be associated with electronic ordering phenomenon.Because of the complicated relationship among the electron-lattice coupling and electron-electron interaction,the origin and fundamental mechanism driving the transition is still under controversial,which makes the research of the band structure,electronic transition and dielectric constant of VO₂ more meaningful in basic physical properties.Recently,most of the reports pay attention to the effect of strain,doping and electric field on the change of optical and electrical properties.However,the influence of oxygen vacancy,crystal orientation and carrier concentration has been rarely studied.Since the graphene was found by AndreGeim and Konstantin Novoselov in 2004,the development trend and and application prospect becomes more clear.Two-dimensional（2D）materials,includinggraphene（Gra）,boronnitride（h-BN）,blackphosphorus（BP）,transition-metal dichalcogenides（TMDs）,transition-metal carbide（TMC）and nitride（TMN）,has excellent optical and electric properties,such as ultrahigh conductivity,mobility and photoelectric response,outstanding catalyze and hydrogen production efficiency,prominent sea water desalination characteristic,which makes the 2D materials becoming the leader in future application.VO₂,as classical phase transition material,taking advantage of the phase transition to regulate the properties of 2D materials or applying with 2D materials in application will be a novelty direction.The novel condensed matter spectroscopy is nondestructive,noncontact,and sensitive to phase transition.Through the theoretical model to analyze the spectrum,the information of dielectric function,electronic transition and band structure can be obtained for the film,powder and bulk materials.More importantly,the spectrum is sensitive to phase transition extremely.The phase transition can be easily identified through the variation of spectrum.For example,the phase transition of VO₂,can be observed through the spectrum change of transmittance,Raman and ellipsometry.Furthermore,for the microcosmic samples of TiSe₂,from the peak change of Raman spectrum,the charge density wave can be distinguished clearly at about 212.5 K.If the phase transition can not be observed from the variation of spectrum,the microcosmic change can be analyzed through the electronic transition and dielectric constant,which was obtained by fitting the spectrum using theoretical model.In a word,the novel condensed matter spectroscopy is appropriate for macroscopic and microcosmic phase transition materials and non-phase transition materials.The main work and innovations of this dissertation are listed as follows:1.The VO₂ film with different V/O stoichiometry was prepared by pulsed laser deposition with different oxygen pressures.Through fitting the transmittance spectrum,the regular of electronic transition,dielectric function with temperature and oxygen pressures was obtained.In addition,the effect of oxygen vacancy to the electronic transition,dielectric function was researched comprehensively.It was found that the impurity level can be introduced by oxygen vacancy,which makes the phase transition energy of E₁ having subtle energy value when the band gap becomes zero.Optical properties and metal–insulator transition（MIT）of vanadium dioxide（VO₂）films grown by pulsed laser deposition with different oxygen pressures（5 to 50mTorr）have been investigated by temperature dependent transmittance spectra.Three interband critical points（E₁,E₂ and E₃）can be obtained via fitting transmittance spectra and the hysteresis behavior of the center transition energies E₁ and E₂ is presented.The VO₂ film grown at optimized oxygen pressure exhibits the well-defined resistivity drop（¹0³Ωcm）across the MIT process.It is found that the metal–insulator transition temperature（TMIT）increases with the oxygen pressure and the complex dielectric functions are drastically affected by oxygen pressure.It is believed that the oxygen pressure can lead to lattice defects,which introduce the donor level and the acceptor level in the forbidden gap produced by oxygen vacancies and vanadium vacancies,respectively.The donor level provides electrons for higher emptyπ*bands,which can make the energy barrier lower and decrease critical temperature.On the contrary,electrons jumping from the d_|band can be recombined by holes on the acceptor,impeding the MIT occurrence.It is claimed that the electronic orbital occupancy is closely related to oxygen pressure,which changes the energy barrier and manipulates the phase transition temperature.The present results are helpful to understand the fundamental mechanism of VO₂ films and practical applications for VO₂-based optoelectronic devices.2.The effect of crystal orientation to the band structure and high energy transi-tion was investigated by temperature dependent transmittance and Raman spectrum.It is believed that the（-111）plane is the reflection of a twinned structure with（011）crystal orientation,which will lead to the arrangements of oxygen atoms and vanadium atoms deviating from a_m axis for the pure monoclinic structure.It is found that the highest order transition（E₃）is highly susceptible to the crystal orientation and exhibits an anomalous hysteresis loop with temperature while the lowest order transition（E₁）is nearly unaffected by it.The phase transition behaviour of vanadium dioxide（VO₂）with different thicknesses has been investigated by temperature-dependent optical transmittance and Raman spectra.It is found that the crystal orientation has a great effect on the metal-insulator transition（MIT）of VO₂ films.The x-ray diffraction（XRD）analysis shows that the films are polycrystalline and exhibit the characteristics of the monoclinic phase.The preferential growth crystal orientation（020）is converted to the（-111）plane with the film thickness increasing.It is believed that the（-111）plane is the reflection of a twinned structure with（011）crystal orientation,which will lead to the arrangements of oxygen atoms and vanadium atoms deviating from the pure monoclinic structure.It is found that the highest order transition（E₃）is highly susceptible to the crystal orientation,whereas the lowest order transition（E₁）is nearly unaffected by it.The E₃ exhibits an anomalous temperature dependence with an abrupt blue-shift（～0.5 eV）in the vicinity of the metal-insulator transition（MIT）for VO₂ film with a thickness of 84 nm.The findings show that the emptyσ*band can be driven close to the Fermi level when the（020）orientation is converted to the（-111）orientation.Compared to the VO₂ film with thickness of 39 and 57 nm,the E₃decreases by 0.8 eV and the E₂ increases by about 0.1 eV at the insulator state for the VO₂ film with a thickness of 84 nm.The abnormal electronic transition and the variation of energy band may be caused by the lattice distortion and V–V dimerisation deviation from the monoclinic a_m axis.3.Through temperature dependent transmittance,resistivity and room temperature reflectance spectrum,the optical and electrical properties of VO₂film with different thickness was investigated systematically.It was found that the spectral slope in near-infrared absorption region have a positive correlations with the phase transition magnitude of the resistivity and transmittance for the VO₂ film with different thickness.It is surprised that the resistivity for the VO₂film with thickness of 27 nm is larger than that for the VO₂ film with thickness of40 nm at metal state,which is beyond the rule that the resistivity decreases at the metal state with the film thickness.Combining the energy transition of E₃ and carrier concertation,it is believed that the abnormal hysteresis loop of E₃ and the resistivity can be attributed to the band splitting of a_1g into Hubbard bands.The metal-insulator transition（MIT）is of key importance for understanding the fundamental electronic interaction that determines the physical properties of vanadium dioxide（VO₂）film.Here,the spectral slope of transmittance and reflectance at infrared absorption region（about 0.62-1.63 eV）and the interband electronic transition for VO₂ film with thickness of 27,40 and 63 nm has been investigated.The potential applications of the spectral slope was presented in detail.It is found that the variation of resistivity and transmittance increases with the spectral slope of transmittance and reflectance.It is surprised that the resistivity for the VO₂film with thickness of 27 nm is larger than that for the VO₂ film with thickness of 40nm at metal state.In addition,an anomalous counterclockwise thermal hysteresis from interband electronic transition with higher energy was also found during the MIT process for the thinnest film.It is believed that this remarkable phenomenon can be related to the correlation effects in the rutile phase,which could lead to the splitting of the a_1g band into Hubbard bands.The lower Hubbard band would result in the electronic transition blue-shift with the empty_ge^?band,which can explain the origin of the counterclockwise thermal hysteresis and abnormal resistivity at the metal state.4.The VO₂/Al:ZnO hybrid multilayer heterostructure was prepared by pulsed laser deposition successfully.The optical and electrical properties of heterostructure was studied by temperature dependent transmittance,reflectance,Raman spectrum and simple photoelectric detection system.The results indicate that the phase transition temperature of VO₂ decreases with increasing the number of the Al:Zn O layer.Affected by the double layer Al:ZnO,the abnormal Raman vibration mode was presented in the insulator region.Furthermore,by adding the external voltage on the heterostructure devices,the near infrared photoelectric switch characteristics of VO₂ can be manipulated dynamically.Active and widely controllable phase transition optical materials have got rapid applications in energy effcient electronic devices,field of meta-devices and so on.Here,we reportthe opticalpropertiesofthevanadiumdioxide（VO₂）/aluminum-doped zinc oxide（Al:ZnO）hybrid n-n type heterojunctions and the corresponding electro-optic performances of the devices.Various structures are fabricated to compare the discrepancy of the optical and electrical characteristics.It was found that the reflectance spectra presents the wheel phenomenon rather than increases monotonically with temperature at near-infrared region range.The strong interference effects was found in the hybrid multilayer heterojunction.In addition,the phase transition temperature decreases with increasing the number of the Al:ZnO layer,which can be ascribed to the electron injection to the VO₂ film from the Al:ZnO interface.Affected by the double layer Al:ZnO,the abnormal Raman vibration mode was presented in the insulator region.By adding the external voltage on the Al₂O₃/Al:ZnO/VO₂/Al:ZnO,Al₂O₃/Al:ZnO/VO₂ and Al₂O₃/VO₂/Al:ZnO thin-film devices,the infrared optical spectra of the devices can be real-time manipulated by an external voltage.The main effect of joule heating and assistant effect of electric field are illustrated in this work.It is believed that the results will add a more thorough understanding in the application of the VO₂/transparent conductive film device.