Low Temperature Preparation And Leakage Inhibition Of BiFeO₃-based Thin Films

As a lead-free perovskite multiferroic material, BiFeO₃（BFO） is a promising candidate forthe future multifunctional devices due to its coexistent ferroelectric, ferromagnetic andpiezoelectric properties at room temperature.Notwithstanding the much advancement in the multiferroic properties, the leakageproblem is still a great handicap for the commercialization of BFO especially in themicroelectronic devices based on the films prepared by the chemical solution depositionmethod. However, from the current preparation technology perspective, the chemical solutiondeposition method is the only way, which is most likely to mass production in the future. Inaddition, the preparation temperature of the BFO thin films generall at the temperatureranging from500to600oC.The higher annealing temperature make the well-known leakageproblem in epitaxial BFO-based films will become more serious compared to that of thenon-epitaxial ones as well as the compatible of the BFO film in the future large scaleintegrated circuit system is a problem. How to reduce the preparation temperature, inhibit theleakage current of BFO thin film with chemical solution deposition method has became thehotspot and focus of staff who study in the ferroelectric and piezoelectric field.In this work, we focus our attention on improving the ferroelectric properties ofBiFe_0.95Mn_0.05O3（BFMO） thin film by controlling layer thickness. In general, the degree ofCrystallization can be improved by readucing the layer thickness, which in turn achieve thepurpose of lowering the annealing temperature without sacrificing the properties. We alsoprepare the BiFe_0.97Mn_0.03O3（BFMO） thin film with different Bi excess on the ITO/glasssubstrate using chemical solution deposition method with sequential annealing process. Systemstudied the effect of excess Bi in the BFMO films on the ferroelectric properties and leakagecurrents. This is a solid foundation to reduce the leakage current of BiFeO₃-thin film. Themain research contents are as follows:In this work, we have made the BFMO thin films with the layer thickness of42,31,25,20nm on the ITO/glass substrate by controlling the concentrations of the precursor solutionsusing chemical solution deposition method with sequential annealing process. The results suggest that reducing the layer thickness moderately is an effective method for improving theferroelectric properties. But, it is also cann’t reduce the layer thickness endless. Because oncethe layer thickness exceeds the limit, continue reducing the layer thickness will bring negativeeffect. There should exist an optimal layer thickness for the BFO film sequentially annealedfor a given temperature. Reducing the layer thickness the positive effect will startpredominating if the layer thickness is larger than the optimal layer thickness. As a result, theferroelectric properties of the BFMO thin films generally being improved. We can observe thebest ferroelectric properties when the layer thickness reaches the optimal value. On thecontrary, the negative effect will start predominating when the layer thickness is thinner thanthe optimal value, which in turn leads to the ferroelectric properties begain to deteriorate. Inthis paper, the optimal layer thickness is25nm/l. The typical columnar structure can beobserved in the25nm/l film. Furthermore, the remanent polarization Prin the films of25nm/lshows weak dependence on the annealing temperature. More importantly, for a giventemperature, the Prin the films of25nm/l are much lager than those in the films of20,31and42nm/l, especially at temperatures below575oC. The results also indicate that reducing thelayer thickness moderately is an effective method for lowering the annealing temperature.In this paper, we have made the BFMO thin films on the ITO/glass substrate with Biexcess0%,5%,10%,15%,20%using chemical solution deposition method with sequentialannealing process. We focus our attention on the effect of excess Bi on the ferroelectricproperties and the structure of the BFMO films. The results suggest that the element Bi is notvolatilize when the annealing temperature below the500oC. Even if have volatile is also asmall amount of volatile. Thus, we don’t need add excess Bi to supplement the volatile. At thelow annealing temperature, the excess Bi usually exists in the grain boundaries. With theincrease of the annealing temperature, the grain boundaries decrease, the excess Bi willseparate out on the surface of the films with rich Bi phase （Bi2O3）. As a kind of leakagechannel, the Bi2O3will make the leakage current increase. Base on the discussion above, wecan deduced that the value of optimal excess Bi is different according to the differentannealing temperature. This should be due to that lower annealing temperature lead to theformation of the smaller grains, so the the grain boundaries increase. In contrast, higerannealing temperature make the grains become larger, thus the grain boundaries decrease. As well know that the decrease of the grain boundaries result in the decrease content of theexcess Bi. We can conclude that addition the excess Bi moderately may favor the growth ofthe grains, but too much excess Bi will exist in the grain boundaries, which will in turn hinderthe mobile of the boundaries. Moreover, the rich Bi phase, separate out on the surface of thefilms will increase the leakage current, which is a negative effect on the properties.In conclusion, we can effectively reduce the leakage current and the annealingtemperature of the BFMO film by controlling the layer thickness and the content of excess Bi.The lower temperature and superior properties make BFMO films more competitive in thefuture integrated ferroelectric devices.