Polarization Enhancement of Nd-Modified Bismuth Titanate Prepared by Pulsed Laser Deposition at Low Temperature
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POLARIZATION ENHANCEMENT OF Nd-MODIFIED BISMUTH TITANATE PREPARED BY PULSED LASER DEPOSITION AT LOW TEMPERATURE Wu, Wenbiao; Shibuya, Akira; Noda, Minoru; Okuyama, Masanori Graduate School of Engineering Science, Osaka University, Toyonaka, JP ABSTRACT Neodymium-modified Bi4Ti3O12(BNdT) thin films have been prepared by pulsed laser deposition (PLD), aiming to realize enhanced ferroelectric properties by a low temperature treatment. When deposited at 500°C, the film shows P-E hysteresis having twice remanent polarization 2Pr of 78.8 µC/cm2, coercive force Ec of about 250 kV/cm, and leakage current of less than 10-7 A/cm2 from -50 to +50 kV/cm. Good fatigue properties of BNdT films deposited at 5 Hz have been obtained up to 1010 cycles of 500 kHz bipolar square pulses, with less than 25% decrease of +Pr and -Pr. INTRODUCTION In recent years, ferroelectric thin films have been extensively investigated due to their potential application for non-volatile memories and multi-functional devices used in microelectronics and optoelectronics. As a typical bismuth-layer-structured ferroelectric (BLSF) material, Bi4Ti3O12 (BIT) has become a key candidate for memory storage capacitor, owing to their promising ferroelectric property. The BIT has good ferroelectric properties and a relatively high Curie temperature, Tc. Nevertheless, it shows relatively high anisotropy and electrical conductivity, making BIT difficult to polarize efficiently and integrate in memory cell capacitors. Strong influence of a small amount of doping on physical properties of ferroelectrics was shown recently. The bismuth titanate doped with donors, such as Nd and La elements has reduced electrical conductivity and enhanced ferroelectric properties [1-3]. The Nd-modified bismuth titanate Bi4-xNdxTiO3(BNdT) system exhibits the most remarkable ferroelectric properties among impurity-doped Bi4Ti3O12 [4-6]. The crystal structure of Neodymium-modified Bi4Ti3O12 (Bi3.6Nd0.4TiO3) is orthorhombic and the unit cell parameters are similar to Bi4Ti3O12. The a, b and c lattice constants of Bi3.6Nd0.4TiO3 are 0.5411 nm, 0.5449 nm and 3.282 nm, respectively. As Nd ions replace a part of Bi ions in the perovskite structure, the oxygen octahedron structure is changed, because the radius of Nd3+ ion (0.127 nm) is smaller than that of Bi 3+ ion (0.14 nm). The remnant polarization, Pr, was increased because the displacement of the Ti4+ ion in the center of oxygen octahedron structure becomes easier. However, the deposition temperature is around 600°C, which is rather high for memory applications. In this paper, ferroelectric BNdT films have been prepared by PLD method, which is known to be effective in reducing the deposition temperature. The influence of various substrate temperatures, O2 gas pressure and post-annealing on the ferroelectric properties of Bi3.6Nd0.4TiO3 thin films are presented in the following sections. EXPERIMENTAL PROCEDURES FOR PREPARATION OF Bi3.6Nd0.4Ti3O12 FILMS The Bi3.6Nd0.4TiO3 thin films are prepared by PLD method using ArF excimer laser
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