The Preparation Of Highly Oriented CaBi₂Nb₂O₉ Films And The Study Of Their Structure-Property Relationships

At the request of devices to develop in the direction of miniaturization and integration,ferroelectric thin films have attracted significant amount of research interest because of the broad application prospects.They have many excellent electrical properties,such as ferroelectric,piezoelectric,pyroelectric properties,which can be applied in microelectronics,optoelectronics,MEMS and other fields.Bismuth layer structured ferroelectrics(BLSFs)has drawn more and more attentions because of their unique features,such as a high fatigue resistance,a low aging rate,as well as a good temperature stability of properties endowed by a high Curie temperature(Tc).These features are desirable for the development of ferroelectric-based electrical devices working under high temperature and high frequency.The spontaneous polarization of a BLSFs mainly exists in the a-b plane and is generally quite small along the c axis,while the grain grow easier along c axis.BLSFs' unique layered structure(a? b<<c)makes it difficult to re-align their crystallites with different orientations.Therefore,the electrical displacement and electromechanical responses are usually small for a BLSFs ceramics.Calcium bismuth niobate(CaBi2Nb2O9,or CBNO),which consists of alternative(CaNb2O7)2-and(Bi2O2)2+ layers in its lattice,is a member of the BLSFs.Its Curie temperature can reach up to 943?,which lead to a broad application prospects in ferroelectric-or piezoelectric-based electrical devices working under high temperature.However,to realize the practical application of CBNO films,some intractable problems need to be solved,such as low polarization and piezoelectric coefficient.In this research,we have designed the orientation and strain state of the film by combining the strain egineering with lattice mismatching theory,and successfully prepared highly oriented CBNO films via using radio frequency(rf)magnetron sputtering method.In addition,we have explored the influence of strain state,substrate orientation,film thickness,and thickness of the bottom electrode,substrate temperature,cooling atmosphere and buffer layer on the crystal structure,microstructure and electrical properties of the CBNO films.As a result,we have improved the dielectric,ferroelectric and piezoelectric properties of the CBNO films.Ultimately,we have successfully prepared CBNO films on Si substrate under middle temperatures.To summarize,we have made significant achievements in the following aspects.1.The strain engineered CBNO films(1)The orientation and strain state of the CBNO films have been designed fromthe perspective of strain engineering.As a result,highly(200)/(020)oriented CBNO films have been successfully prepared on MgO substrates.Through the microstructure analysis,we found that the grains in CBNO films were controlled both by interfacial energy and elastic energy.The c-grains grew epitaxially,while a-grain grew from the side of a neighboring c-grain.(2)There remained tensile stress in the CBNO films deposited on MgO(100)(110)substrate,which effectively improved the dielectric performance of the CBNO films.The increasing amount of a-grains and grains tilting away from the alb axis,leading to a remarkable improvement in polarization in both films(Pr?13 ?C/cm2).(3)The hysteresis loops of the CBNO films were shrinking and tilting due to high-density space charge and the leakage current density-electric voltage curve(J-V curve)of the CBNO film showed a good fitting to a modified Schottky contact model.(4)Increasing the thickness of the CBNO films,the dielectric properties have been improved.When applied electric field increased to 2750 kV/cm,the saturated polarization of the 400nm-thick CBNO film was about 90 ?C/cm2,leading to a high energy storage density(-86.8 J/cm3),which offers a new possibility to CBNO films in energy storage application.2.The highly oriented CBNO films(1)Highly a-axis oriented CBNO films have been prepared on both YSZ(100)substrate and YSZ-buffered Si(100)substrate.(2)The CBNO film deposited on YSZ-buffered Si(100)substrate showed enhanced dielectric and ferroelectric properties.High dielectric constant,large residual polarization,as well as the large poisson strain of Si substrate,make the film a significantly enhanced piezoelectric coefficient(d33-30 pm/V).3.The preparation of CBNO films under middle temperatures(1)The CBNO films have been respectively prepared under different temperatures(450?,500?,600?)on Si substrate.They all had highly(115)preferred orientation with good crystallinity.The CBNO films deposited under 500? and 600? had good ferroelectric properties,while the film with the deposition temperature of 450? need to be improved.(2)By adjusting the sputtering power to 70 W,100 W and 120 W,we found that increasing the sputtering power can optimize the electrical properties of CBNO films,especially the dielectric properties.(3)LaNiO3 buffer layer didn't make any sense to CBNO films,on the contrary,it depressed the ferroelectric and dielectric properties of CBNO films.