Modification Of Bi_3.15Nd_0.85Ti₃O₁₂ Lead-free Ferroelectric Thin Films With SrTiO₃Buffer Layer

Bi3.15Nd0.85Ti3O12（BNT） is one of bismuth-layer-structured ferroelectrics. It hasattracted great attention as a candidate memory media for nonvolatile ferroelectricrandom access memories （NvFeRAMs） applications due to its lead-free chemicalcomposition, relatively large remnant polarization, good fatigue resistance and highCurie temperature. However, ferroelectric films suffer from the degradation in theelectrical properties owing to the charged defects, an intrinsic interface dead layer andresidual stresses at the interface between ferroelectric film and electrode, which hasgreatly inhibited its commercial applications.In this thesis, BNT lead-free ferroelectric thin films were taken as research object.Aiming at those problems, SrTiO₃（STO） buffer layer was introduced betweenferroelectric film and electrode to improve the interface quality and electrical propertiesof thin films. Based on this, the influences of STO buffer layer on the microstructureand electric properties of BNT thin films were studied in details. It was found that STObuffer layer could improve the crystalline quality and electrical properties of BNT thinfilms. The physical mechanism of the improved performance was also discussed. Themain research contents and the obtained results are summarized as follows:（1） BNT films without buffer layer were prepared by a chemical solutiondeposition （CSD） method under different process conditions. Through study theinfluences of process parameters such as the annealing temperature, annealing time andannealing atmosphere on the BNT films, the preparation process parameters of the BNTferroelectric thin films could be optimized and improved. The results showed that BNTthin films presented the best ferroelectric properties when annealed10minutes at750℃in oxygen atmosphere using rapid thermal process method. The films under thisannealing condition had the largest remanent polarization and minimum coercivevoltage when tested at15V applied voltage.（2） STO thin films with different annealing temperature and annealing atmospherewere prepared under the same conditions using CSD method. The influences of theannealing temperature and annealing atmosphere on the leakage current performance ofthe STO films were studied and the optimal process parameters when the STO filmspresent the best leakage current performance were obtained. It was found that theleakage current performance of the STO thin films was not a monotonically increasing relationship with the annealing temperature, there was an optimal annealing temperatureof750℃. STO films showed the better leakage current characteristics when annealed inoxygen atmosphere.（3） The optimization of the STO buffer layer on the microstructure and electricalproperties of BNT thin films was studied. The BNT films with three different layerstructures were prepared under the optimum conditions from the previous study usingCSD method. The study found that the STO buffer layers had an important impact onthe microstructure and electrical properties of BNT thin films. After the introduction ofthe STO buffer layers, the microstructure of BNT thin films such as crystallizationdegree, flatness and density had a good improvement and the electrical properties suchas ferroelectric properties, fatigue performance and leakage current characteristics werealso improved significantly. On this basis, the morphology and properties of BNT filmswith different structure were also studied by changing the location of the buffer layer. Itwas found that the introduction of double-layer STO buffer layer could improve theperformance of the BNT films more effectively.