Structural Inhomogeneity of SrBi 2 Ta 2 O 9 Thin Films Prepared by Layer-by-Layer Technique
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Structural Inhomogeneity of SrBi2Ta2O9Thin Films Prepared by Layer-by-Layer Technique J. B. Xua), G. D. Hu, and S. P. Wong Department of Electronic Engineering and Material Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong SAR, China. Grazing incidence x-ray diffraction has been employed to perform the depth-profile analysis on SrBi2Ta2O9 (SBT) thin films with different preferential orientations. For the polycrystalline SBT thin film, the change in structural orientation occurs only within the 15-nm-thick top layer, which is associated with the formation of the (200)-predominant SBT thin film prepared by the layer-by-layer annealing process. The inhomogeneity of structural orientation is more significant in the full film thickness for the (200)predominant SBT thin film. (0010) peak can only be observed for the grazing angle larger than 0.6o. A layer with the highest ratio of I(200)/I(115) is found in the top surface layer (i.e., the latest layer during deposition) of the (200)-predominant SBT thin film.
INTRODUCTION In the past several years, ferroelectric SrBi2Ta2O9 (SBT) has been intensively studied for applications in non-volatile random access memory (NvFRAM) devices because of its fatigue free nature and interesting anistropic properties [1-3]. Both the macroscopic [4] and microscopic [5] measurements have demonstrated that SBT thin films oriented along the c-axis exhibit no or poor ferroelectric properties. The c-oriented grains directly affect the polarization homogeneity of SBT thin films. According to our previous work [6-9], (200)-predominant SBT thin films can be fabricated on Pt/Ti/SiO2/Si substrates by the metal organic decomposition (MOD) technique using a layer-by-layer annealing process. The (200)-predominant SBT thin films can be uniformly polarized over large areas using a piezoelectric mode atomic force microscope (AFM) [7], although (0010) peak appears in the x-ray diffraction (XRD) pattern. Why the presence of c-oriented grains shows no obvious effects on the polarization uniformity of the (200)-predominant SBT thin films? On the other hand, why the (200)-predominant SBT thin films can be prepared using the layer-by-layer annealing method? In order to find the origins responsible for these two phenomena, depth-profile analysis was carried out on a polycrystalline SBT thin film and a (200)-predominant SBT thin film using the grazing incidence XRD. The polarization roughness of SBT thin films was detected using the piezoelectric-mode AFM to assess the structural effects. It is found that the top layer of the polycrystalline SBT thin films plays a key role in the formation of (200)-predominant SBT thin film. Besides, an explicit relationship between the structure and polarization homogeneity has been proposed.
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EXPERIMENTAL DETAILS In the geometry of grazing incidence, the incident x-ray beam impinges on the film surface at a very small angle. The penetration depth of x-ray into the film increases with the increase of t
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