High Quality Deposition of Strontium Titanate (STO) Thin Films using Sol-Gel Method
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High Quality Deposition of Strontium Titanate (STO) Thin Films using Sol-Gel Method Kiyoshi Uchiyama, Daiki Fukunaga, and Tadashi Shiosaki Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama-cho 8916-5, Ikoma, Nara 630-0192, Japan ABSTRACT Highly oriented strontium titanate (STO) thin films were successfully deposited on sapphire substrates using a sol-gel method. Sintering temperature of 1400oC and the use of c-cut sapphire for the substrates are the keys to obtain highly oriented thin films. The sample obtained on c-cut sapphire with a sintering temperature of 1400oC showed quite high (111)-orientation and a narrow full width half maximum (FWHM) value of 0.265o, which indicates a high quality deposition of the STO thin films. We believe this high quality STO deposition technique will bring a new scope of oxide material applications for future electronic devices. INTRODUCTION In these years, approaches to obtain high quality oxide thin films have been studied for future electronic devices. For example, FET type ferroelectric RAMs (1T-FeRAMs) were proposed based on the combinations of ferroelectrics and semiconducting oxide thin films [1, 2]. In these 1T-FeRAMs, semiconducting oxide thin films, i.e. zinc oxide and indium tin oxide, were used as a channel of the FETs. However, oxide materials used in the FETs are polycrystalline and their operation speed is lower than that of FETs using single crystal substrates. Thus, improvement of the film quality is indispensable for providing high performance 1T-FeRAMs. As for high quality deposition of oxide thin films, D.-S. Yoon et al. reported quite high quality lanthanum-modified lead ziroconate titanate (PLZT ) thin films on various single crystal substrates, i.e. strontium titanate (STO), magnesium oxide (MgO), and r-cut [(012)] sapphire (αAl2O3) substrates [3,4]. Nashimoto et al. and Ishii et al. also reported PLZT thin film depositions on STO substrates [5, 6]. As mentioned above, studies for high quality oxide thin film depositions are using STO and MgO as substrates. In the point of view of lattice matching, STO substrates are preferable substrates for depositing high quality PLZTs and other provskites. In addition, doped STO possesses very high conductivity, which is suitable for electronic device applications. However, STO substrates are expensive compared to sapphire single crystal substrates. In addition, available maximum size of STO substrates is as small as 2 inches, which is much smaller than that of sapphire substrates (8 inches). These features of STO substrates may bring the production cost issue and we consider it is not suitable to use STO substrates for actual device production, even though they show fine properties. Based on this idea, we adopted sapphire substrates for PLZT depositions and achieved quite high quality, i.e. epitaxially grown, PLZT thin films with high electro-optic coefficients using a sol-gel method [7-10]. In the same manner, high quality STO deposition on sapphire substrates is achiev
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