| As the increasing demand of sensors, micro-electro-mechanical systems and data storage devices, preparation of ferroelectric thin films with high performance and low cost is getting extensive attention from researchers. Lead zirconate titanate(PZT), the most popular ferroelectric material, is environmentally disastrous, and it fails to meet the advocacy of environmental protection and sustainable development. Environment-friendly multiferroic material BiFeO3, which exhibits ferroelectric and piezoelectric properties comparable to PZT, high Curie temperature(TC=830℃) and Neel temperature(TN=370℃), has attracted widespread attention in recent years. However, there exists large leakage current density in BiFeO3 thin films caused by oxygen vacancy and valence fluctuation of Fe3+, which makes the properties fall short of the application requirements.Experiments show that doping modification is the most effective way to suppress the leakage current of BiFeO3 thin films. The effect diversifies as the ionic radius and valence state change. According to the defect chemistry theory, high valenct elements substitution on B-site can suppress the valence fluctuation of Fe3+, and low valenct elements substitution can reduce the concentration of oxygen vacancy.In this paper, the method of sol-gel combining layer-by layer annealing process is adopted. First of all, the preparation technology of BiFeO3 thin films is optimized. Then the method of B-site heterovalent substitution is adopted to analyze the crystal structure, surface morphology, ferroelectric and leakage characteristics of Mn-doped, Zn-doped and (Mn, Ti) co-dpoed BiFeO3 thin films. Main conclusions of the study are as follows:1. The annealing temperature has great influence on the properties of BiFeO3 thin films. The grain grows larger as the annealing temperature increases, and the remnant polarization increases gradually as well. However, when the annealing temperature increases to 525℃, the grain size becomes uneven and the surface becomes porous, the ferroelectric properties deteriorates as well. This can be attributed to the abnormal growth of grains. Therefore, the optimum annealing temperature for BiFeO3 thin films should be 500℃.2. A reasonable amount of Mn substitution can refine the grains and improve the surface morphology of BiFeO3 thin films. It is also effective in suppressing leakage current and improve the breakdown resistant property. The leakage mechanism changes from space charge limited current (SCLC) to ohmic conduction mechanism as Mn content increases. At the same time, the films are seriously aged as a result of Mn substitution. So the remnant polarization decreases compared with the pure phase, and the coercive field is highly asymmetric, which can be harmful to the properties of BiFeO3 thin films.3. With the minimum leakage current density of 1.38×10-4A/cm2 in BiFe0.97Zn0.03O3 thin films, which is three orders of magnitude lower than pure phase, Zn substitution is proved to be effective in reducing the leakage current of BiFeO3 thin films. The films with Zn content of 3%～4.5% has a high (012) diffraction peak orientation, which is beneficial to the improvement of morphology and properties. The films within this doping content can be highly symmetrical in its hysteresis loops.4. Neither Mn nor Ti doping will change the crystal structure of BiFeO3 thin films. But it can greatly improve the surface morphology and reduce the leakage current. Mn or Ti substitution is effective to improve the breakdown resistant property of the films, meanwhile, the films are found to be aged. The aging problems can be solved by applying high electric field repeatedly. The hysteresis loop will saturate gradually in the process of deaging.5. Either high valence ion Ti4+ or low valence ion Mn2+ is invalid in reducing the leakage current of the films. The BFMO films have large leakage current density, and the BFTO films show diodelike behavior. However, Mn and Ti co-doping is excellent in suppressing the leakage as well as improving the ferroelectric properties.In conclusion, the leakage current is reduced and the ferroelectric property is improved by optimize the process parameters and Mn, Zn, Ti substitution, which makes the BiFeO3 thin films more promising for the data storage and micro-electro-mechanical systems. |
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