Lanthanum-substituted bismuth titanate for use in non-volatile memories

Abstract

Non-volatile memory devices are so named because they retain information when power is interrupted; thus they are important computer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories that use ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarization-cycling characteristics. Early work focused on lead zirconate titanate (PZT) but, when films of this material were grown on metal electrodes, they generally suffered from a reduction of Pr (‘fatigue’) with polarity switching. Strontium bismuth tantalate (SBT) and related oxides have been proposed to overcome the fatigue problem3, but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-substituted bismuth titanate thin films provide a promising alternative for FRAM applications. The films are fatigue-free on metal electrodes, they can be deposited at temperatures of 650 °C and their values of Pr are larger than those of the SBT films.

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Figure 1: Lattice structure of the La-substituted bismuth titanate.
Figure 2: Results of the fatigue tests at 1 MHz.
Figure 3: Results of the fatigue tests at 1 kHz.
Figure 4: Dielectric constant and dissipation factor (loss) for the Au/BLT/Pt/Ti/SiO2/Si films as a function of frequency.

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Acknowledgements

This work was supported by the Korean Science and Engineering Foundation through the RCDAMP at Pusan National University, and by the Ministry of Science and Technology through the International Cooperative Research Project.

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Correspondence to T. W. Noh.

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