Materials Compatibility between Si/SiO 2 Substrates and BaTiO 3 Thin Films
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ABSTRACT BaTiO 3 thin films, 50-60 nm in thickness, were evaporated in Ultra High Vacuum as well as under 1 mPa 02 pressure onto Si/SiO 2 (30 nm) wafers. The substrate temperature during the evaporations was approximately 70 'C. The films were flash annealed in oxygen at 500 °C. The interfacial reactions were studied by XPS and AES Ar ion assisted depth profiling, and by RBS. Interdiffusion took place even at 70 'C. The XPS/AES depth profiles suggested strong interdiffusion, with Ba being the dominant moving species. The SiO 2/BaTiO 3 interface smears out with significant Ba concentrations up to half of the SiO 2 thickness. Ti diffusion is remarkably lower. Barium is detected up to the SiO 2 /Si interface. INTRODUCTION Perovskites such as BaTiO 3 , SrTiO 3 , and Pb(Zr,Ti)0 3 (PZT) exhibit a high permittivity, and some of them (BaTiO 3 , PZT) are ferroelectric. In addition to their interesting physical properties these materials may find applications in piezoelectric transducers, pyroelectric and optical devices, nonvolatile random access memories, and sensors. These applications require thin film structures. The preparation of thin films has attracted much interest recently, with more than one hundred papers published in 1994 on the (Ba,Sr)TiO 3 system alone. It is recognized that realization of the potential applications requires the ability of growing films on useful substrates, mainly silicon-based materials that are compatible with semiconductor processing [1]. Oxygen vacancies in BaTiO 3 may accumulate at the oxide-semiconductor interface, leading to poor reproducibility of the electrical measurements in MOS structures. An interesting way of solving this problem is to use a multilayer Si/SiO 2/BaTiO 3 structure in which the silicon oxide is grown by conventional thermal oxidation. From the electrical point of view, such a structure has the advantage of minimizing the density of states at the oxide-semiconductor interface, while retaining the dielectric properties of BaTiO 3 . The SiO 2 layer should be as thin as possible, say 10 nm, so that the dielectric properties are dominated by the BaTiO 3 . The BaTiO 3 film must be highly resistive, otherwise the voltage drop would take place mainly across the SiO 2 layer. High permittivities are desired for the BT films; however, it is known that high permittivities are associated with low resistivities [2]. In the present work amorphous BaTiO 3 films were grown, of lower permittivity but higher resistivity than crystalline BaTiO 3 [3]. The main problem of these two-oxide structures is that BaTiO 3 can react with SiO 2 to form silicates. In fact, we have observed that powder mixtures of BaTiO3 and SiO 2 , compacted and heated at 1100 'C in air, lead to the formation of Ba 2 TiSi2 O 8 and BaTi2 0 5 . In the case of thin films, and because of the large contact area, chemical reactivity and associated diffusion can be a problem even at low temperatures. The present work evaluates the chemical reactivity of the SiO 2/BaTiO 3 interface by depth profiling techniques.
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