Effect Of A,B Site Doping On Physical Properties Of Ruddlesden-Popper Phase Ca₃Ti₂O₇ Film

With the rapid development of information technology,the demand for low-power,multi-functional and high integration microelectronic devices is more and more urgent in recent years.The development of new multifunctional electronic materials is one of the effective ways to solve this problem.Multiferroics materials combines two or more of the primary ferroelectric order in the single phase,so it is expected to achieve mutual regulation of more functions,improve the integration and multidimensional degree in the application of microelectronic information devices.However,the traditional generation mechanism of ferroelectricity and ferromagnetism is mutually exclusive,so it is the key to find a new generation mechanism of ferroelectric polarization for magnetoelectric coupling in single phase.Due to the special layered structure and the strong electron correlation between layers,the superlattice Rutdlesden-Popper phase exhibit excellent physical properties,such as superconductivity,giant magnetoresistance,hybrid improper ferroelectricity,multiferroic and so on.Ca₃Ti₂O₇?CTO?is a typical Ruddlesden-Popper structure.Different from the proper ferroelectrics,the ferroelectricity of improper ferroelectrics is caused by the rotation and tilt of the octahedron,which results in the net polarization along a axis.But at present,the exploration of CTO system is mostly focused on theoretical prediction and bulk materials,there are few researches on CTO thin film.In addition,A and B site doping can be used to adjust the rotation and tilt amplitude of lattice and octahedron,and then affect the physical properties of CTO films.Here,the highly oriented CTO films were grown on single crystal substrates by chemical solution method for the first time,which is simple and can be prepared in a large area.The main contents of this paper include:1.High quality epitaxial CTO films were successfully prepared by sol-gel method.The optimal growth temperature for the epitaxial growth of CTO films was determined.The results show that the film has good epitaxy with?110?STO substrate,the epitaxy relationship is[001]CTO||[001]STO,[100]CTO||[-110]STO,and the stress of CTO film partially relaxed.It is found that the dielectric constant of CTO film along b-axis increases with the decrease of temperature,which shows the quantum paraelectric behavior.The CTO film is a direct band gap material with an optical band gap of about3.9 eV.The top of valence band is formed by the O 2p orbits and the bottom of conduction band is formed by the Ti 3d orbits.In the weak absorption region,the Urbach band tail energy of CTO film is about 227 meV.2.The effects of A site substitute Sr on the structure,surface morphology,electric polarization,dielectric and optical properties of CTO films were studied.The results show that the substitution of Sr for Ca ion causes lattice expansion,and the distortion of oxygen octahedral in the perovskite layer decreases,which reduces the polarization and dielectric constant of the film.In addition,Sr doping reduced the optical band gap of the film at room temperature,and the band gap of the CTO film was 3.73 eV,and the E_g decreased with the increase of doping amount,which was related to the decrease of distortion amplitude of oxygen octahedral.3.The effects of B site Mn doping on the microstructure,dielectric response,carrier transport mechanism and optical properties of CTO films were studied.The results show that the lattice constant of Mn doped films decreases with the increase of doping amount.The ac conductivity of pure CTO film shows“nearly constant loss”behavior and Mn doped samples exhibit“universal dielectric response”behavior at low frequency,and the hopping conduction of localized carriers leads to low frequency dielectric relaxation.The hopping conduction originates from short-range hopping motion of local electrons between Mn³⁺and Mn⁴⁺ions at low doping,while under high doping condition,the relaxation is induced by the hopping of localized Jahn-Teller?JT?polarons.In addition,Mn doping reduces the optical band gap of CTO film which may due to the impurity state energy formed by oxygen vacancy,the formation of energy band expansion structure of doped Mn ion energy level in local band and the decrease of the distortion amplitude of oxygen octahedral.