| Bi4Ti3O12(BIT)-based layered ferroelectric thin film has always been one of the most potential ferroelectric materials to replace the commercial(Pb,Zr)TiO3(PZT)-based ferroelectric random access memory(FRAM)for its high curie temperature,large remanent polarization,and good anti-fatigue properties.Because of the anisotropy of BIT-based crystal cell whose polarization orientation is mostly along a-axis,it is better to have effective means to improve the ferroelectric properties.Recently,many inspiring works on the mechanism of polarization fatigue of Bi FeO3(BFO)thin films had been investigated,which had accomplished by means of in-situ PFM,TEM observations of domains.Based on these observations,an electron injection model induced by Schottky barrier between the electrode and the films was proposed as the main cause for polarization fatigue.However,there is no report about in-situ observations of domains of BIT-based layered perovskite thin films as yet.Nd and Mn co-doped Bi3.15Nd0.85Ti2.99Mn0.01O12(BNTM)thin film was choosed as a research subject in this dissertation.Works carried out on improving layered BNTM thin films' ferroelectric properties and exploring the mechanism of temperature-dependent polarization fatigue were concluded as follows:1.The modulation mechanisms of both bottom BIT seeding layers and upper BIT covering layer on BNTM thin films were studied.With the bottom BIT seeding layers under annealing temperatures from 550 to 700 oC,BNTM thin films show higher [117]-oriented grain growth,larger grain size,lower negative leakage,and better fatigue properties in comparison with the BNTM thin film without BIT seeding layer.Moreover,BNTM with a BIT seeding layer annealed at a temperature of 600oC(BIT-600)has the largest remanent polarization(2Pr = 114.5 ?C/cm2@270 kV/cm)and the biggest dielectric constant and tunability(r? = 575.5@ 2 V;Tunability(%)= 16.9% @ 8 V)among BNTM thin films with BIT seeding layers annealed at various different temperatures.It is found that there is a competition between the rate of grain nucleation and grain growth,and BIT-600 may have reached the best balance between them resulting in optimal performances for BNTM thin films.At the same time,the BNTM thin films with upper BIT covering layer were also prepared.Larger grain size,higher remanent polarization and better fatigue properties were also observed in comparison with the BNTM thin film without BIT covering layer.With the help of upper BIT covering layer,a part of Bi losses has been compensated and the grains have been recrystallizated at the surface of BNTM thin films.2.Polarization switching and fatigue properties of highly [117]-oriented BNTM ferroelectric thin films were studied at both low and elevated temperatures.BNTM thin filmsshow no fatigue properties below 200 K due to the fact that the rate of domain nucleation is the decisive role in this range,and oxygen vacancies or charge defects can hardly have a long-range diffusion.In the temperature range from 200 to 300 K,the polarization fatigue of BNTM films become exacerbated,which can be ascribed to the strong domain pinning effect induced by the increasing number of domain walls with temperatures.However,at elevated temperatures from 300 to 400 K,BNTM films show improved fatigue endurance with temperatures.There are three parts about the active energy fitting curves obtained by temperature-dependent ac impedance spectra analysis at elevated temperatures from 300 to475 K.At first,the values of average activation energy are 0.01-0.03 eV which indicates that the hopping electrons or trapping defects take part in the local conduction at the temperature from 300 to 375 K;Second,a part of oxygen vacancies have also participated in the local conduction from 375 to 425 K as the values of active energy increases to 0.09-0.1 eV;Third,a lot of oxygen vacancies migrate and become the decisive role for ac conduction with the active energy value of 0.18-0.2 eV.Therefore the oxygen vacancies' redistribution(less density of oxygen vacancy in the body of the film as compared to that at low temperatures)and strong domain depinning effect of the film have led to a better fatigue endurance at elevated temperatures.Moreover,non-neutral domain walls induced by head-to-head or tail-to-tail polarization configurations were obtained in PFM in-plane and out-plane phase images.These results may be useful for better understanding of polarization fatigue mechanisms of BNTM thin films.3.The differences of polarization switching and fatigue properties of a-axis-oriented,[117]-oriented and mixture-oriented BNTM thin films at both low and elevated temperatures were studied.It was found that the changed trends of the temperatue-dependent polarization fatigue of a-axis-oriented become exacerbated,while [117]-oriented BNTM thin films got better at elevated temperatures.It can be assumed that a larger number of 180 odomains,lower activate energy,and weaker domain pinning effect for a-axis-oriented BNTM thin films have caused this difference compared with BNTM thin films with other orientations.Moreover,it is the thicker interface layer between the electrode and thin films with the increasing temperatures that triggers this trend.4.The influence of the ratio of Nd doped on polarization fatigue of BNTM thin films at elevated temperatures had been studied.The polarization fatigue of BTM(not doped)and BNTM05(ratio of Nd doping is 0.5)thin films first get better and then become worse with the increasing temperatures from 300 to 400 K,while the BNTM85(ratio of Nd doping is 0.85)shows enhanced fatigue endurance.It can be concluded that a part of oxygen vacancies have been first collected by the electrodes leading to a thicker interface layer between the electrodes and thin film that made the fatigue more serious.At 350-400 K,a lot of oxygenvacancies in BTM and BNTM thin films can diffuse with a long-range way and such space charges have taken part in the polarization switching leading to improved fatigue properties.5.Pt,Au,NiFe,and Al top electrodes with different work functions on the same film were designed and temperature-dependent polarization fatigue properties of these thin film capacitors were studied.The polarization fatigue behaviors of capacitors with Pt,Au,NiFe as top electrodes are similiar at elevated temperatures: first get more serious and then become better.While the polarization fatigue of capacitor with Al top electrodes decline more seriously in comparison with others.A comparison on the fresh and fatigued samples' dielectric constants had also been done.The dielectric constant of capacitors with top Pt,Au,and NiFe electrodes had a certain proportion of reduction,which implied that the Schottky barrier could facilitate the collection of a part of oxygen vacancies and induce a thicker interface layer.While the dielectric constant of the capacitor with Al top electrodes showed no obvious changes.Through the analysis of the active energy of the four thin film capacitors,it was found that capacitor with Al top electrode had a high active energy of 0.55 eV at a temperature from 425 to 475 K,while active energies for others were 0.19 eV.It was certified that capacitor with an Al top electrode had formed a barrier for holes through the analysis of energy band diagram.A work function of the thin films was obtained to be 4.66 eV after excluding the active energy of the body of the thin film,which gives us a new method to gain the thin films' work function.Through the energy band diagram analysis one can find that a huge electron injection effect can take place in the Al and Pt as top and bottom electrode structures,implying that the electron injection is the main mechanism for the polarization fatigue extent. |
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