Time-domain Terahertz Spectroscopy of Strontium Bismuth Tantalate Thin Films
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Time-domain Terahertz Spectroscopy of Strontium Bismuth Tantalate Thin Films K. Kotani, M.Misra, I. Kawayama and M. Tonouchi Research Center for Superconductor Photonics, Osaka University 2-1 Yamadaoka, Suita, Osaka 5600045, Japan. ABSTRACT We have measured the dielectric and optical properties of pulsed laser deposited SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9 thin films on MgO substrate in THz frequency region by THz time-domain spectroscopy. The imaginary parts of the dielectric constant of both the samples show broad peaks in the frequency range 0.5-1.0 THz, which may be due to the soft mode in SBT in this frequency spectrum. The difference in the real part of the dielectric constant of Sr0.8Bi2.2Ta2O9 thin films is small for MHz and THz frequencies. On the other hand, the value of real part of dielectric constant of SrBi2Ta2O9 thin films in THz frequency range is much smaller than that in MHz frequency region, indicating that SrBi2Ta2O9 is not simply displacive ferroelectric material.
INTRODUCTION Strontium bismuth tantalate (SBT) has received a considerable amount of attention as an important candidate for nonvolatile ferroelectric random access memories due to its good fatigue properties and low switching voltage [1,2]. However, the details of mechanism of ferroelectric transition and dielectric properties have not been clearly understood, because polycrystalline SBT thin films have been used in most of the studies. In recent years, it has been revealed that substitution of Sr and Bi ions in SBT strongly affects its ferroelectric and dielectric properties [3,4], and Raman scattering and far-infrared measurements of SBT [5,6] indicated that it is not simple displacive ferroelectric material. Here, we report the measurement of the optical and dielectric properties of stoichiometric and Sr-deficient Bi-excess SBT thin films in terahertz (THz) frequency region by THz time-domain spectroscopy (THz-TDS). THz-TDS has become a powerful tool for studying optical and dielectric properties of the materials in sub-THz and THz regions that are not easily accessible with other methods. The dielectric function around the soft phonon mode, which plays an important role in ferroelectric phase transition, could be well studied by this method. In this technique, an electromagnetic wave passing through sample under investigation is detected in the
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time domain by using a photo conducting sampling technique. Through Fourier transformation of the waveform, both amplitude and phase of the frequency spectrum can be obtained at the same time. Therefore, real and imaginary parts of optical parameters in the THz region are directly calculated without Kramers-Kronig analysis. In this work, we measured the transmitted electromagnetic waveforms through SBT thin films with two kinds of compositions and calculated the refractive index and the dielectric constant in the frequency range between 0.3 and 1.5 THz.
Figure 1. Experimental setup for THz time-domain spectroscopy.
EXPERIEMNTAL DETAILS SBT thin films were deposited on MgO (
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