Effect Of A-site Nonatoichiometry On Microstructure And Electrical Properties Of Na_0.5Bi_0.5TiO₃-based Thin Film

Ferroelectric materials,with excellent ferroelectric,piezoelectric and pyroelectric properties,play an irreplaceable role in the electronic components such as memory,sensor and transducer.Functional electronic devices made of ferroelectrics are widely used in the many core fields including energy,national defense,aerospace and so on.And the ferroelectric in the form of thin film is small in size,light in weight,and can be integrated with semiconductor or circuit,which is beneficial to the miniaturization and integration of electronic components and in line with the trend of new material development.As a lead-free ferroelectric material,Na_0.5Bi_0.5TiO₃（NBT）is recognized as a promising substitute for lead-based materials due to its excellent performance.However,at high crystallization temperature,the volatilization of the A-site elements（Na and Bi）of NBT films will produce a large amount of oxygen vacancies.Normally,the mobile oxygen vacancies are donorlike trapping centers for electrons.The energy levels associated with oxygen vacancies are very close to the conduction band.Therefore,the electrons can be readily activated to be free for conduction by the electric field,causing an increased leakage current.At this point,an usual routine is adding excess Na/Bi-containing starting materials to compensate for the potential loss.However,the effects of nonstoichiometry of A-site elements on the microstructure and electrical properties of NBT-based films have not been systematically studied.In this paper,the effects of W and Ni doping were investigated firstly.Then,the amounts of Na and Bi in Na_0.5Bi_0.5（Ti,W,Ni）O₃ films were changed.And the influences of nonstoichiometric ratio of A-site elements on the microstructure,insulating,ferroelectric,energy storage and dielectric performances of all samples were examined and discussed systematically.The main working contents are as follows:1.The Na_0.5Bi_0.5TiO₃（NBT）,W-doped NBT（NBTW）,Ni-doped NBT（NBTN）and（W,Ni）-codoped NBT（NBTWN）thin films were deposited on ITO/glass substrates.The effects of different doping ions on the properties of NBT films were discussed.The NBTWN films have better insulating,ferroelectric and dielectric properties.The leakage current density（J）of NBTWN film at 400 kV/cm is around 4×10^-55 A/cm².Enhanced ferroelectricity is achieved in NBTWN film with a large remanent polarization（P_r）of 17.6μC/cm² at its electric break-down strength（BDS）.The NBTWN film also exhibits higher dielectric constant（ε_r）of 531 and lower dielectric loss（tanδ）of 0.05 at 100 kHz.2.Na_0.5+xBi_0.5（Ti,W,Ni）O₃（x=-3.0,-1.5,0,1.5%）thin films were prepared on ITO/glass substrates.In order to investigate the effects of Na content on the structure and electrical properties of NBT-based films.Different to the pure perovskite phase obtained in Na_0.5Bi_0.5（Ti,W,Ni）O₃ and Na_0.515Bi_0.5（Ti,W,Ni）O₃ films,the secondary phase of TiO₂ can be found in Na_0.470Bi_0.5（Ti,W,Ni）O₃ and Na_0.485Bi_0.5（Ti,W,Ni）O₃ films.The sample of Na_0.485Bi_0.5（Ti,W,Ni）O₃ has reduced J and enhanced P_r（38μC/cm²）.Meanwhile,the bigger recoverable energy-storage density（W）of 63.1 J/cm² and higher energy-storage efficiency（?）of 55.0%can be obtained due to higher BDS and larger difference value between maximum and remanent polarization(P_max-P_r).Enhanced dielectricity with higher tunability（T）of36.0%and figure of merit of 4.0 at 450 kV/cm and 500 kHz can be achieved in Na_0.485Bi_0.5（Ti,W,Ni）O₃ film.3.Na_0.5Bi_0.5+y（Ti,W,Ni）O₃（y=-2.5,0,2.5,5.0%）thin films were fabricated on ITO/glass substrates.The influences of Bi content on the microstructure and electrical properties are discussed in detail.For each film,with the increasing of the applied field,the dominant conduction mechanisms are the grain boundary limited conduction,the Ohmic conduction,the space charge limited conduction and the interface-limited Fowler Nordheim tunneling,respectively.Among all these films,the sample of Na_0.5Bi_0.525（Ti,W,Ni）O₃ exhibits obvious decreased J.And Na_0.5Bi_0.525（Ti,W,Ni）O₃ film possesses the maximum BDS（2500kV/cm）,P_r（23.1μC/cm²）,W（65.8 J/cm³）,?（52.9%）and T（44.1%）.Also,this sample shows the values ofε_r and tanδwith 630 and 0.02 at 100kHz,respectively.4.Na_0.5+xBi_0.5+y（Ti,W,Ni）O₃（x=0.0%,y=0.0%;x=0.0%,y=1.0%;x=0.5%,y=2.0%;x=1.0%,y=4.0%）films were prepared on ITO/glass substrates.And the role played by excess Na&Bi in suppressing oxygen vacancy and modifying microstructure and electrical properties were analyzed.Particularly,Na_0.505Bi_0.520（Ti,W,Ni）O₃ film shows reduced J and enhanced P_r（26.7μC/cm²）.And,the excellent energy storage performance with W（40.5J/cm³）and?（54.7%）can be obtained in this film.Meanwhile,Na_0.505Bi_0.520TWN film has superior dielectric properties.