Influence Of Lanthanides Substitution On Structure And Electric Properties Of BiFeO₃-based Thin Films

BiFeO₃is one of the several materials which exhibit ferroelectric ordering with Curietemperature of850°C and magnetic ordering with Néel temperature of370°C at roomtemperature simultaneously. The theoretical value of remanent polarization in BiFeO₃film isas high as100μC/cm². In addition, large piezoelectric response has been observed in this film,making it promising in the application of data storage and microelectromechanical systems.However, the severe leakage problem in pure BiFeO₃films prepared using the chemicalsolution deposition method is a great obstacle for obtaining their intrinsic electric properties.It has been reported that lanthanides substitution can reduce the leakage current and thecoercive field of BiFeO₃films to a great extent. Recently, more attention has been paid on thestructure transition induced by lanthanides doping after the finding of morphotropic phaseboundary in Bi0.86Sm0.14FeO3film with a large piezoresponse of95pm/V by Fujino et al. Theeffect of different lanthanides on structure and properties of BiFeO₃film will be different dueto difference in ionic radius. In this work, we research the effect of Nd substitution on thestructure and electric properties of BiFeO₃film deposited using metal organic decompositionmethod combined with sequential layer annealing. The main contents and conclusions aresummarized as follows:In order to facilitate the observation of morphotropic phase boundary, we prepared theBi_1-xNd_xFeO₃（x=0.10⁰.16） films on LaNiO₃（100）/Si substrates firstly. The influence of Nddoped on structure and piezoelectric properties has been studied. The experimental resultsdemonstrate that all Bi_1-xNd_xFeO₃films show strong （100） and （200） peaks. The remanent ofpiezoeletric coefficient enhanced as the increases of Nd doping content ranging from10%to14%and then decreases rapidly, the mean remanent d33reaches its maximum at x=0.14withthe value of148pm/V. Additionally, the uniformity of the piezoresponse in Bi_1-xNd_xFeO₃films show a sudden deterioration with Nd doping content near14%. It can be seen thatwhereas large piezoresponse can be observed in the BiFeO₃films with high lanthanidesdoping content, the uniformity of piezoresponse in these films is really poor which mayhinder their future application in piezoelectric devices. Otherwise, it has been reported by Chen et al. that the remanent polarization of BiFeO₃film was decrease monotonically withthe increase of lanthanides doping content, indicating that it is different to obtain superiorferroelectric and piezoelectric properties simultaneously in BiFeO₃film with high lanthanidesdoping content.We prepared epitaxial Bi_1-xNd_xFeO₃（x=0.01⁰.045） films on LaNiO₃（100）/Si substratesand explored the influence of lower Nd doping content on the structure and electric propertiesof BiFeO₃film. The experimental results demonstrate that the lowest leakage current as wellas largest remanent polarization can be found in Bi_0.97Nd_0.03FeO₃film. The remanentpiezoelectric coefficient increases as the increase of Nd doping content, and thereafterdecreases rapidly. The mean remanent piezoelectric coefficient reaches its maximum at x=0.03with the value of160pm/V which is14%higher than that in Bi_0.89Tb_0.11FeO₃as reportedby Chen et al. more importantly, the Bi_0.97Nd_0.03FeO₃film exhibits the best piezoresponseuniformity. The microscopic retention behavior of Bi_0.97Nd_0.03FeO₃film shows that the meanvalue of piezoelectric coefficient for the polarized film read out20hours later is6.8%higherthan that read out immediately which in turn lead to a little decline in the uniformity ofpiezoresponse. This means that the piezoresponse of Bi_0.97Nd_0.03FeO₃film is very stable.In order to further determine the structure of Bi_0.97Nd_0.03FeO₃film, we fabricated theBi_0.97Nd_0.03FeO₃film epitaxially on LaAlO₃（001） substrate and examined its structure usingsynchrotron radiation X-ray diffraction. The results reveal that the film presents a raremulti-domain structure, and its crystal structure is monoclinic （Ma）, indicating that there wasa structural transition in this film.Based on large and stable in Bi_0.97Nd_0.03FeO₃film, we research the effect of different Biexcessive and pretreatment temperature on the structure of Bi_0.97Nd_0.03FeO₃film. Wedeposited the Bi_0.97Nd_0.03FeO₃film with the Bi excessive of0,10%,20%,30%as well aspretreatment temperature of375°C,400°C,425°C,450°C on ITO/glass. The results revealthat there is a competitive growth mode between （110）-and （012）-oriented grains. Under thesame pretreatment temperature, the crystal orientation of Bi_0.97Nd_0.03FeO₃film will transferfrom （110） to （012） direction gradually as the increase of Bi excessive. B_i2O₃phase can befound in the X-ray diffeaction patterns for all Bi_0.97Nd_0.03FeO₃film when the Bi excessivehigher than20%. For the Bi_0.97Nd_0.03FeO₃film with no Bi excessive, the intensity of （110） peak will increase with the increase of pretreatment temperature, which is contrary to （012）peak. The crystallinity went to the best in the Bi_0.97Nd_0.03FeO₃film with the Bi excessive of10%as well as a pretreatment temperature of400°C.In summary, superior ferroelectric and piezoelectric properties can be found inlanthaindes doped BiFeO₃thin film, which make it more attractive in the appliacation offuture data storage and microelectromechanical systems.