Investigation On The Growth Mode And Properties Of Lanthanides Doped BiFeO₃Thin Films

In recent years, as the development of the needs about information storage, sensors, andMEMS, ferroelectric materials is increasingly become a research focus in industry, technology,academic and military. Ferroelectric materials mainly used in device manufacturing is currentlylead series system (PZT) material, but this kind of material in the process of manufacturing andrecycling brings the serious problems of environmental pollution and has violated the humandemands for environmental protection, which encourage people to find a new kind of lead-freeferroelectric materials. BiFeO3(BFO) as a kind of lead-free ferroelectric materials,environmental protection, the theory of residual polarization value as high as100μC/cm2, it hasexcellent ferroelectric, piezoelectric and ferromagnetism properties, which are very promisingto be an alternative of PZT used in microelectronic device in the future.However, due to the volatilization of Bi3+and valence alternation of Fe3+, BFO film hasserious leakage problem, it becomes the key to restrict its application and development.According to recent reports, lanthanides doped can effectively inhibit the leakage current andimprove the insulation and ferroelectricity to a certain extent in BFO film. It has become one ofhot spots in the study of ferroelectric thin films. With the further study, we found that dopingbring the negative effect of reducing crystallization degree. Now the higher crystallization ofthe film is primarily by increasing annealing temperature, but the BFO thin film devices andintegrated circuits CMOS compatibility require the low prepared temperature. Therefore, howto implement BFO-based film in low temperature with higher crystallization is especiallyimportant. Due to the crystallization of the film depends on the preparation process parameters,so the preparation process parameters on the system research of lanthanides doped BFO thinfilm growth mode and the influence of performance have important theoretical and practicalapplication value.Based on the above analysis, this thesis reported lanthanides doped BFO film, mainlydiscuss the preparation of metal organic decomposition method combined with layers ofannealing process parameters on the properties of thin film growth mode and influence, themain contents and conclusions are as follows: 1, On ITO/glass substrates, we prepared the films with a series of pretreatmenttemperatures:225℃,250℃and250℃respectively,300℃,325℃and350℃Bi0.9Nd0.1FeO3(BNFO) thin film.. As a result, the pretreatment has a great influence on crystallization inBNFO films. It shows that carbonization temperature in275℃of BNFO films. Because thereare some carbide particles in grain boundaries, it leads to lower crystallization degree and thepoor ferroelectric propertiesin BNFO films, it has the minimum remanent polarization valuesand poor charge retention in275℃, and aging is obvious.In225~300℃pretreatment BNFOfilm has residual organic matter in different levels or different forms of existence, the filmcrystallization and electric properties of iron produced certain negative effect.2, On the ITO/glass substrates, the preparation of a series of the annealing temperature is450℃,475℃,500℃,475℃respectively of different pretreatment (225~350℃) in Bi0.9Nd0.1FeO3thin films, and systematic study the pretreatment and annealing temperature on thestructure and performance BNFO film.Results show that carbide is hard to eliminate byadjusting the annealing temperatures, different annealing temperature of275℃pretreatmentBNFO films have lower crystallization degree and poor ferroelectric properties, we shouldavoid to use this temperature as pretreatment temperature; pretreatment in325℃of BNFOfilm has good crystallization and ferroelectric properties of iron, which suggests that it is theoptimal pretreatment temperature in325℃.3, On the ITO/glass substrates, we adjusted the rotational speed and prepared the filmswith different single layer thickness of45nm,40nm,34nm,29nm respectively and indifferent annealing temperature (475℃,500℃,525℃) Bi0.9Nd0.1FeO3film.The resultsindicate that the crystallization of BNFO thin film show stronger dependence on the layerthickness with decreasing the annealing temperature. The crystallization degree obviouslydecreased with the increase of layer thickness of films annealed at475℃. This phenomenoncan be explained based on the nucleation and the transition of growth mode in the BNFOfilms. Morever, it can be seen that with the increase of applied voltage, the evolutions ofhyseeresis loops under differebt applied voltages were observed, the asymmetry covercivefield decreased. This should be due to the breaking of defect complexes between the acceotors(Fe2’Fe3)and oxygen vacancies(V O2). Under the same applied voltage, the asymmetry of covercive field decreased with the increase of layer thickness. This should be because thedefect content increase with the crystallization degree decrease. Combined with the hysteresisloop test principle, we explained the open direction of hysteresis loop and gap size with thechange of film layer thickness and voltage.