Theoretical Studies On Strain Engineering And Orientation Engineering Of Ferroelectric Thin Films

Ferroelectric thin films are widely used in electronic devices,such as memory,sensors and detectors,for excellent dielectric,piezoelectric,and pyroelectric properties.It is necessary to take full advantage of ferroelectric thin film materials in order to improve the stability and integration of ferroelectric devices.Strain and orientation engineering can effectively improve the dielectric and piezoelectric properties as vital means to control the properties of ferroelectric thin films.Therefore,it has been widespread concerned in the process of ferroelectric thin films.The performance of ferroelectric thin films with different crystal orientations under the action of external fields are systematically studied in this paper based on the phenomenological nonlinear thermodynamics theory.Firstly,the transformation matrix of（111）,（110）and（310）oriented films are obtained according to the Euler’s rule.Then overall thermodynamic potentials of different oriented ferroelectric thin films are deduced from the transformation of polarization and stress tensor between different crystal orientation coordinates,which can be used to systematically investigate the effects of equiaxed in-plane strain,anisotropic in-plane strain,out-of-plane stress,electric field and temperature on the phase structure,dielectric properties,piezoelectric properties and electrocaloric properties of ferroelectric thin films.The main results of this study are as follows:（1）It is found that M_A,M_B,R and PE phase appear in the（111）oriented isotropic Pb Ti O₃ferroelectric film.Large dielectric propertiesε₁₁ appear near the M_A-PE phase boundary caused by tensile strain;high dielectric propertiesε₁₁ andε₂₂ appear the compressive strain-induced R-M_Aphase boundary below the Curie temperature;meanwhile,near the Curie temperature and R-M_Aphase boundary,there are good dielectric propertiesε₃₃ and piezoelectric properties d₃₃ in the M_Aphase.The anisotropic in-plane strain induce crystal structure to be M_A,M_B,R and T_r phase at room temperature.There are excellent dielectric propertiesε₁₁,ε₃₃ and piezoelectric property d₃₃ near the M_B-T_r¹ phase boundary induced by compressive strain u₂ and small tensile strain u₁.High dielectric propertiesε₂₂,ε₃₃ and piezoelectric property d₃₃ appear in M_A¹ phase near the multiphase-point caused by small compressive strain u₁ and u₂.Besides,T-like—R-like phase transition induce high dielectric propertiesε₂₂,ε₃₃ and piezoelectric property d₃₃ appear in T_r phase under the action of tensile strain u₁ and u₂.At same time,electric filed induce a great piezoelectric property d₃₃ response in the films caused by elastic softening.The electrocaloric properties of the（111）oriented isotropic K_0.5Na_0.5Nb O₃ ferroelectric thin films are also explored.It is found that the in-plane mismatch strain and out-of-plane stress can effectively change the Curie temperature of the ferroelectric thin films.Under the action of 30MV/m electric field,in（111）oriented isotropic K_0.5Na_0.5Nb O₃ ferroelectric thin film without out-of-plane stress and mismatch strain,the maximum electrocaloric temperature changeΔT can be as high as 18K near the Curie temperature.Although the out-of-plane stress of about-6.7GPa can effectively reduce the Curie temperature to room temperature,but the cost is that the maximum electrocaloric temperature changeΔT is reduced to 7.5K.（2）The crystal structure of（110）oriented isotropic Pb Ti O₃ ferroelectric films show M_A,M_C,O,T phases at low temperature and PE phase at high temperature.There are high dielectric propertiesε₃₃and piezoelectric properties d₃₃ near the O-PE phase boundary and good dielectric propertyε₂₂ near the O-M_C phase boundary under compressive strain.Meanwhile,tensile strain induce highε₁₁ near the PE-T phase boundary and largeε₂₂ near the M_C-T phase boundary.In addition,the anisotropic in-plane strain induce crystal structure to be M_A,M_B,M_C,O,T and R phases at room temperature.Under the action of large compressive strain u₂,good propertiesε₃₃ and d₃₃ appear near the M_A¹-T phase boundary,and high dielectric propertyε₁₁ appear near the O-M_B¹ phase boundary when O-M_Cphase transition cause a great dielectric propertyε₂₂.At same time,electric filed induce a high piezoelectric property d₃₃ response in the films caused by the decline of elastic stiffness.Besides,the best electrocaloric properties of（110）oriented isotropic Pb Ti O₃ ferroelectric films appeared near the Curie temperature and under the action of zero mismatch strain.O-PE phase transition caused by compressive strain can not significantly improve the electrocaloric properties of ferroelectric films at high temperatures.By contrast,there are great adiabatic temperature changeΔT near the M_C-T phase boundary caused by tensile strain.Under the action of an electric field of 30MV/m,the（110）oriented isotropic film has a maximum electrocaloric temperature changeΔT of 16.7K near the Curie temperature without out-of-plane stress and misfit strain.（3）In the（310）oriented isotropic Pb Ti O₃ ferroelectric thin film,temperature and strain induce phase structure to be M_C,T_r and PE.High dielectric propertyε₃₃and piezoelectric property d₃₃ appear near the M_C¹-PE phase boundary induced by compressive strain.Under the action of tensile strain,there are great dielectric propertyε₁₁near the M_C²-T_r phase boundary and highε₂₂near the M_C²-PE phase boundary.The anisotropic in-plane strain induce crystal structure to be M_C,T_r and T phases at room temperature.There are excellent dielectric propertyε₁₁near the M_C²-T_r phase boundary and high dielectric propertyε₂₂ near the T-T_r phase boundary under tensile strain u₁ and u₂.M_C¹-T phase transition induce a good dielectric propertyε₃₃ under the action of compressive strain u₂ and tensile strain u₁.Besides,there are fine propertiesε₃₃ and d₃₃ in T_r phase near the multiphase-point.The electric field can induce excellent piezoelectric d₃₃ response due to the elastic softening in films.Besides,there are great electrocaloric properties of（310）oriented isotropic Pb Ti O₃ ferroelectric thin films near the M_C¹-PE phase boundary caused by compressive strain at high temperatures.Under the action of an electric field of 30MV/m,the（310）oriented isotropic film has a maximum electrocaloric temperature changeΔT of 20K near the Curie temperature without out-of-plane stress and misfit strain.