| Terahertz?THz?wave is the rarely investigated band among electronmagnetic spectra,where it locates between the infrared and microwave frequency.Due to the rare repsonse for natural materials and the short of functional devices,THz wave has become a new research area in recent years.Vanadium dioxide?VO2?films,well known by the insulator-to-metal transition?IMT?near room temperature,exhibit the promising application in THz frequency by virtue of the excellent optical modulating and switching performances.Consequently,it is of great importance that further research need to conduct on the THz optical properties,the performance optimization,and the difficiency relievation of VO2 films.On the other hand,VO2 films present well compatibility with metamaterial structures,and their hybrid devices develop novel tunability and diversity.Hence,researches also need to exploit the enhancement of the hybrid structures between VO2 films and Metamaterials,which already drew lots of attention for the related researchers.Basing on the two facts,this dissertation is focusing on the properties and applications of VO2 films with IMT performance in THz frequency,where the deposition of VO2 films employed direct-current reactive magnetron sputtering.Comparing its electrical and THz optical properties,analyzing the variation of microscale factors by different characterizations,the relationship among electrical,THz optical properties,and microscale variations steps out from the shadow,and several solutions for the property optimization are developed.In addition,the enhancement and its mechanism of the hybrid structure composing by VO2 film and Metamaterials are also revealed for the modulation application.For the convenience,the contents are divided into four parts as shown following.1.The optical properties of VO2 films starting from visible-near-infrared,mid-infrared,to far-infrared or THz frequencies are widely investigated across the IMT transition,also with the electrical properties which shows the similarity with THz optical transmittance.In visible-near-infrared region,the modulation amplitude from the transmittance difference of insulator and metal states,rises with the wavelength increasing,and different substate exhibits few influence on the modulation properties of VO2 films.In mid-infrared regime,VO2 films present the sensitivity for the partially reactive oxygen concentration during the sputtering,while have negiligble influence on the duration in annealing process.That is,the overmount or inadequate oxygen dissolving into VO2 films will result in distinct phonon vibration spectra in mid-infrared regime.In THz frequency,VO2 films exhibit markedly optical transition properties,and it shows the high agreement with elelctrical properties of sheet resistance for the hysteresis loops.In addition,the variation of IMT performance in THz frequency is consistent to the phonon-vibration changing pattern in mid-infrared regime.In specific,the transition temperature is reduced when the oxygen vacancy riched in the inadequate oxyen deposition process,is remarkable in the suitable oxygen concentration,and is disappearing for the oxygen-rich environment during deposition.Otherwise,Si doped VO2 films with the enhancement of thermochromic properties in visible-near-infrared reion are fabricated,which paved the ground for the optimization and adjustment of IMT and modulation properties in THz frequency.2.Excellent performance maintaining or enhancing in doped VO2 films are achieved by introducing dopants with same valence state of V4+in THz frequency.Firstly,Si doped VO2 films present excellent matainance for optical modulation ability in THz frequency,which is well agreeing with the enhancement behavior of thermochromic properties for Si-doped VO2 films in visible-near-infrared frequency.Meanwhile,a rebound effect of transition temperature in Si-doped VO2 films are found,where the decreasing process of transition temperature with silicon doping ratio rising meets a rebound phenomenon.Detail analysis infers that interstititial and substitutional doping forms jointly influence the transition behavior.Their different behavior and competition relationship result this rebound effect,and it is also identified in visible-near-infrared region.Secondly,the annealing temperature affects the surface morphology of Si-doped VO2 films prominently,including roughness and porosity.The specific relationship is that the rising of annealing temperature will generate a homogeneous films by decreasing the porosity and roughness,while the higher temperature also reducts the whole film.Thirdly,Ge doping exhibits the comprehensive improvement ability for VO2 films.Comparing to undoped VO2 film,Ge-doped one improves 17.6%modulation ability,and stabilizes the transition temperature around80?,narrows the hysteresis width within 3?.Such outstanding properties in Ge-doped VO2 films are closely related to the doped crystal recrystallizing and the presenting of quasi-tetragonal lattice array.3.The lower-valence-state elements introducing into VO2 films are investigated.It will rapidly degenerate the performance of VO2 films,but also introduces some unique impactions.At first,Al doping exhibits the ability to change VO2 crystalline with M1phase into M2 phase.Such behavior alleviates the modulation restriction by phonon vibration of VO2 crystal,and elevates the transmittance of phonon vibration band in THz frequency.Because of the distinct vibration source,the mid-infrared phonon vibration mode shows slightly shift while the THz phonon mode is rapidly disappearing by the crystal transformation.Meanwhile,the THz optical transition temperature is lower than the electrical one in the same Al-doped VO2 film,which is resulted from the generation and propagation behavior of phase transition.Otherwise,the element with even lower valence state,Cu,doping into VO2 films decreases transition temperatures and narrows hysteresis widths.However,Cu atoms separate out and aggregate into microscale metal particles in the heavily doping film.This phenomenon coincidently brings similar electrical and THz optical properties in different samples,and reveals the uniformity relationship betweem electrical properties and optical properties in THz range.Specifically,neither similar resistivity nor THz transmittance may achieve the similar value of the other.4.The hybrid structures of VO2 films integrating with Metamaterials present the enhancement of modulation ability.Split ring resonator?SRR?array are deposited on100-nm-thickness VO2 films.In the SRR resonanting frequency,the transmission firstly rises following by a decrease.The mechanism of this variation is that the impairment of resonant mode and the electromagnetic shielding hold the different stage across VO2transition.In the non-resonant frequency,the modulation amplitude of hybrid structure in0.72 THz is 55.9%,which increased by 31.8%comparting to the modulation ability of sole VO2 film.The enhancement derives from the alternation of resonant mode from LC mode to dipole mode in the hybrid structure.On the other hand,complementary SRR depositing on VO2 film has achieved higher modulation amplitude of 68.2%,which elevated the 62.4%modulation ability comparing to sole VO2 film.This marked elevation is owing to the status matching between complementary resonantors and VO2film.In detail,the band-pass function of complementary SRR and the insulator state of VO2 film both show high transparent state,and the metal state of VO2 films with the disappearance of band-pass resonance generate the even lower transmission.At the same time,complementary SRR exhibits high sensitivity for the material variation.Instead of the crushy resonance suppression in SRR,the sensitivity of complementary one can clearly estimate the conductivity change and the permittivity change.Moreover,the percolation process during VO2 transition are also detected,which the permittivity of VO2 film increased two magnitude from 10 to over 1000.The results of integrating structures indicate promising future for differen applications in THz area. |
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