| Lithium niobate(LiNbO3)is a traditional multi-functional crystal material with outstanding characteristics of piezoelectric,ferroelectric,electro-optic,acousto-optic and photorefractive.It is widely used in piezoelectric sensors,laser modulators,surface acoustic wave devices,etc.,and has become one of the few important materials in the field of constant development of new functional features and applications.Light-induced absorption is one of the important photoelectric properties of lithium niobate crystals.Light-induced absorption refers to that excitation light incident on the crystal excite electrons or holes in the trap level to the conduction band,resulting in the increase of the number of electrons or holes that can be excited in the quasi-steady energy level,which,macroscopically,leads to the increase of the absorption coefficient of a certain band.Since the polaron is the carrier of carrier migration,the photoluminescence phenomenon of crystals is usually closely related to the polaron properties.There are a variety of intrinsic polarons in congruent LiNbO3,such as NbNb free small polarons,NbLi bound small polarons and bipolarons,etc.,and doped metal ions with multiple valence states into LiNbO3 lattice can also trap carriers to form polarons.These polarons capture electrons or holes carriers with large lattice distortion and introduce trap energy levels into the band gap.At the same time,the external doped polarons interact with the intrinsic polarons,which can affect the distribution of trap levels and the migration process of carriers in the band gap,thus affecting the light-induced absorption properties of the crystal.Therefore,it is very important to understand the behavior of charge bound and transition by polaron and the transition and transfer process of charge between trap energy levels,to deeply understand the nature of light-induced absorption characteristics of crystals,and to develop many applications of LiNbO3 crystals.The externally doped polarons in LiNbO3 crystals are mostly in low-valence states,such as Fe,while there are few studies on polaron behavior caused by high-valence metal ion doping,such as V.High-valence metal ions generally have a variety of variable valence states,and it is easier to capture multiple electrons to form electron polarons,and the form and energy of electron occupied orbit are also easy to change,resulting in interesting optical properties.V and Nb are both the fifth subgroup transition metals,with a +5 valence.Whether V doped into the LiNbO3 lattice has similar lattice occupancy,electronic structure,polaron behavior and so on compared with NbLihas aroused our research interest and is one of the contents of this paper.In addition,Ce can be co-doped with a variety of transition metal ions,introducing different trap levels in LiNbO3 crystals band gap,which is important for regulating the absorption of light at different wavelengths and the resulting transition and migration of electrons.Therefore,we study the polaron and charge transport mechanism in V:LiNbO3,Ce:LiNbO3 and V:Ce:LiNbO3 crystals,exploring the relationship between the different trap levels introduced in the band gap,the bound behavior of the polaron to electrons,and the interaction of external doped polarons and intrinsic polarons,and then obtaining the charge migration mechanism in V:Ce:LiNbO3 crystals,which will help to understand the source of the optical properties of doped LiNbO3 crystals.Based on the first principles,we study the related theoretical work of V:LiNbO3,Ce:LiNbO3 and V:Ce:LiNbO3,including the trap levels introduced in the band gap,the form and structure of the polarons,and carrier trapping behavior,and explore the effect of V and Ce doping on optical properties of LiNbO3 crystals,revealing charge transport mechanism of V and Ce codoped LiNbO3.The main research contents and results are as follows:The chapter one,we mainly introduce the research background and significance,including the research progress and significance of the doped LiNbO3 crystals,the significance and research status of polarons as well as charge transfer mechanism in LiNbO3 crystals,and the significance and research ideas of this paper.The chapter two,we introduce the basic principle of density functional theory,the approximate functional of exchange correlation and the calculation process of self-consistent field.On this basis,we introduce the VASP quantification software package used in the calculation of this thesis.The chapter three,focusing on the polaron behavior of V:LiNbO3,we systematically studied the stable existence of V doped into LiNbO3,relaxation structure,carrier trapping behavior,and trap levels introduced in the band gap.Research have shown that when LiNbO3 crystal contain a large number of intrinsic defects VacLi,the Fermi level is near the VBM,and the V doping into congruent LiNbO3 preferentially occupies the Li site at the +4 charge state forming VLi4+ defect.The Fermi level moves to the middle of the band gap with intrinsic defect VacLi concentration decreases,and Vli4+ transfers to VLi2+ by simultaneously capturing two electrons,resulting in a negative U effect,at the same time V begin to occupy the Nb lattice to form VNb defect and it is stable in the neutral state.When V doped into the near stoichiometric LiNbO3 crystal the Fermi level is in the middle of the band gap.As the V doping concentration increases the Fermi level moves towards the conduction band.V will occupy the Nb lattice and will be able to capture electrons occuring defect state transition from 0 to-2.The two electrons captured by VLi are distributed around of VLi2+ defect center and only cause the expansion of the nearest neighbor oxygen octahedron,which is a typical small bound polaron.Similar small bound polaron behavior is observed when VLi2+ captures two additional electrons to form a neutral VLi defect.Compared with the NbLi bipolaron,the electron binding ability of the small polaron is relatively weak,the bound electrons are more easily excited,and the electron-hole recombination time is relatively shorter as well as V:LiNbO3 usually has relatively shorter photoresponse time.This is consistent with the experimental observations,which provides a theoretical basis for the practical application of LiNbO3 crystals from a microscopic perspective.The chapter four,based on the chapter three,we further study the interaction of V with the intrinsic defects NbLi and VacLi.The calculations prove that when VLi and NbLi coexist in the LiNbO3,the electrons are first bound by the VLi small polaron,and are distributed around the VLi and neighboring Nb ions in the non-polarized direction to form bound bipolarons.After further electron capture,VLi interacts with neighboring Nb ions in the non-polarization and polarization directions,and bipolaron behaviors are observed.It is concluded that VLi has stronger electron capture ability than NbLi.In addition,VLi is found to exhibit varying polaron behaviors in congruent LiNbO3 and near-stoichiometric LiNbO3,which causes a change in the depth of the trap level as well as the strength of electron binding.Since it takes longer for electrons to transfer from VLi small polaron to NbLi bipolaron,V doping in the near-stoichiometric LiNbO3 has a shorter response time than LiNbO3 crystals.On this basis,we further studied the structures and properties of clusters formed by the interaction between V and intrinsic defects.According to the experimental report,we constructed three electrically neutral defect cluster models VLi4++NbLi4++8VacLi,VLi4++4VacLi-and VLi4+ +VNb0+4VacLi-,which reflected the different defect compensation forms corresponding to V doping concentration from low to high.The most stable cluster structure was finally obtained by studying the formation energy of a series of possible cluster configurations.The size of defect clusters are about 2 nm and C3 symmetry could be maintained.We found that the formation of defect clusters has little effect on V trap levels by electronic structure analysis,but it has a great influence on the electronic state of the CBM,especially with respect to the clusters containing NbLi4+ and VNb0,which will introduce d electron state at the CBM,causing changes in electronic transitions and optical properties.The chapter five,we systematically studied the doping behavior of Ce in LiNbO3 crystal and the charge transfer mechanism of V:Ce:LiNbO3,including relaxation structure,carrier trapping behavior and trap levels introduced in the band gap.It is found that Ce prefers to substitute Li site and is stable at +3 valence in LiNbO3 crystals.CeLi3+ defect can capture an electron to form CeLi2+ defect,and it is also the case that Ce occupies Nb site and is stable at-1 charge state.The shallow trap level introduced by CeLi3+ in the band gap is mainly provided by the Ce 4f,which is a typical small bound polaron after capturing an electron.When VLi and CeLi coexist in LiNbO3,the two repel each other and tend to separate.After capturing two electrons,captured electrons are distributed around the VLi2+ defect center and only cause the expansion of the nearest neighbor oxygen octahedron,which is a typical small polaron.Compared with V:LiNbO3,when VLi and CeLi coexist in V:Ce:LiNbO3,different trap levels are introduced into the band gap.V is a deep level,and Ce is a shallow level.Under the irradiation of two different energy lights,the higher energy laser can excite the electrons in the deep energy level to the conduction band or the shallow energy level,and the electrons in the shallow energy level can also be excited to the conduction band under the action of the lower energy laser.The electrons that migrate in the conduction band are eventually captured by the deep trap center,causing changes in electronic transitions and optical properties.When VLi and CeNb coexist,the defect level of different depths is also introduced in the band gap,and the relaxation of CeNb causes the VLi trap level to move toward the conduction band as well as leads to trap level of Ce becomes shallow,which resulting light absorption and electronic transition are different from those occupying the Li lattice.Finally,the charge transfer mechanism in V:Ce:LiNbO3 crystal is discussed in detail by considering the defect energy levels introduced by V and Ce doping as well as intrinsic defects.The chapter six,we summarize the main conclusions and innovations of this work and put forward the next step. |
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