(Zn²⁺,Al³⁺,Ti⁴⁺) Single And Co-doped BiFeO₃ Spin Coating Membrane Preparation And Performance Studies

BiFeO₃(BFO)is favored by researchers in the field of multiferroic research because it has both ferroelectricity(T_C?830?)and G-type antiferromagnetism(T_N?370?)at room temperature,and has a great theoretical remnant polarization(90?150?C/cm²).The magnetoelectric coupling effect caused by the interaction between ferroelectric and antiferromagnetic order makes it attractive in the fields of multi-state memory,spintronics,magnetoelectric sensors and so on.However,due to the high temperature volatilization of Bi element and the valence change of Fe³⁺in the preparation process,the defects of Bi vacancy,oxygen vacancy and so on are caused,which lead to the problems of high leakage current and low anti-breakdown voltage of BFO,making it difficult to obtain BFO materials with excellent performance.This also seriously hinders its practical application in the field of memory devices.In this paper,a series of BFO spin coating films doped with different valence elements(Zn²⁺,Al³⁺,Ti⁴⁺)at Fe site were successfully prepared on FTO/glass substrates by sol-gel method.The phase,structure and physical properties of the samples were studied.Meanwhile,BFO spin coating films with Fe site(Zn²⁺,Ti⁴⁺)co-dopeing were prepared,and the effects of co-doping on the phase structure,microstructure,and element valence and physical properties of the spin coating films were systematically studied.The main results are as follows:(1)Three kinds of BFO spin coating films with different valence metal ions Fe-site doping were prepared on FTO/glass substrates by sol-gel method.They are Zn²⁺single doped Bi Fe_1-xZn_xO₃(BFZO,x=0-0.05),Al³⁺single doped Bi Fe_1-xAl_xO₃(BFAO,x=0-0.08)and Ti⁴⁺single doped Bi Fe_1-xTi_xO₃(BFTO,x=0-0.05).The results show that the three kinds of Fe-site doped BFO thin spin coating films have rhombic distorted perovskite structure with R3m space group,and no heterophase is produced.In addition,the doping of the three elements with different valence states can reduce the leakage density of the spin coating films,significantly improve the ferroelectric residual polarization,and improve the dielectric properties.The results show that the BFZO film with Zn²⁺doping amount 0.03 has the maximum remnant polarization(2P_r?190.8?C/cm²)and the minimum leakage density.The optimal doping amount of Al³⁺is 0.02,and the film has the maximum remnant polarization(2P_r?165.5?C/cm²)and significantly reduced leakage current.The single doping of Ti⁴⁺also significantly reduces the leakage current and improves the ferroelectric properties(2P_r?190.1?C/cm²).The essence of the ferroelectric properties enhanced by single doping of three different valence ions is to reduce the leakage current and improve the ferroelectric properties by inhibiting Fe³⁺?Fe²⁺or reducing the concentration of oxygen vacancies.(2)Based on the above single doping research result,Bi Fe_1-2xZn_xTi_xO₃(BFZTO x=0-0.05)spin coating films were prepared by using Zn²⁺and Ti⁴⁺co-doping.The effects of(Zn²⁺,Ti⁴⁺)co-doping on the phase structure,surface morphology,chemical state distribution and physical properties of BFO thin spin coating films were systematically studied.The results show that(Zn²⁺,Ti⁴⁺)co-doped BFZTO spin coating films have rhombic distorted perovskite structure with R3-m space group,and no heterophase is formed.(Zn²⁺,Ti⁴⁺)co-doping improves the surface morphology of the film.BFZTO with x=0.02(BFZTO-2)film has the smallest grain size and the densest structure.At the same time,the interface between the sample and the substrate is clear and there is no mutual diffusion.The thickness of spin coating 14 layers film is about 840 nm,which is slightly lower than that of pure BFO film.The results show that BFZTO-2 film has the lowest leakage density(J?1.13×10^-6 A/cm²)at 150k V/cm electric field,which is 5 orders of magnitude lower than that of pure BFO film.The lower leakage current makes it have the best ferroelectric properties,P_r?131.9?C/cm²,which is more than twice as large as pure BFO(P_r?52.6?C/cm²).This is mainly due to the fact that(Zn²⁺,Ti⁴⁺)co-doping suppresses the transition from Fe³⁺to Fe²⁺,thus reducing the formation of oxygen vacancies.In addition,(Zn²⁺,Ti⁴⁺)co-doping can also improve the magnetic properties of BFO spin coating films.The room temperature saturation magnetization(M_s)of BFZTO-2 film is 1.93 emu/cm³,which is significantly higher than that of pure BFO(0.23 emu/cm³).The increased magnetism may be related to the decrease of grain size and the change of lattice structure.(3)Finally,based on the above(Zn²⁺,Ti⁴⁺)co-doped preparation of BFZTO spin coating films,the best performance BFZTO-2 spin coating films were selected for the next study,and the effects of the processes such as precursor solution concentration,Bi excess,spin coating times,annealing temperature and holding time on the properties of BFZTO-2 spin coating films were investigated respectively.Through the exploration of BFZTO-2 film prepared process,we found that the best ferroelectric performance of BFZTO-2 film with the best concentration of precursor solution of0.25 M,the best excess of Bi of 4%,and the number of spin coating of 14 layers.Meanwhile,the optimal annealing process is 30 min at 550?.The resulting BFZTO-2 film has a great remnant polarization value(2P_r?265.6?C/cm²)and a significantly reduced leakage density(J?5.2×10^-7 A/cm²).