Studies On The Frequency-dependent Properties In Ferroelectric Films

Compared to traditional EEPROM and FLASH memory devices, ferroelectric random access memories (FeRAMs) is currently regarded as candidates of the most potential next generation memories with clear advantages such as non-volatility, fast access time, low operate voltage and low power consumption. Although lead zirconate titanate (PZT) has some of the better ferroelectric characteristics of higher remanent polarization (Pr), it suffers from serious fatigue and lead pollution problems. As candidates instead of the lead-based materials, bismuth layer-structured ferroelectric films have the characteristics of high fatigue resistance, large remanent polarization and lead-free chemical composition. Preparation of high-quality ferroelectric thin films is the basis of the design investigation of ferroelectric memory. This paper will focus on the detail of the film preparation process and the frequency-dependent properties of ferroelectric thin films.Bi3.15Nd0.85Ti3O12 (BNT) thin films were deposited on Pt/Ti/SiO2/Si substrates by chemical solution deposition (CSD). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were carried out to exam the crystal structures and the surface morphologies, respectively. XRD shows that the BNT thin films with a bismuth-layered perovskite structure without any additional or intermediate phases, highly (117) and (00l) oriented. The film's surfaces are dense, smooth, and uniform without any cracks, composed of grains around 100-500 nm. The BNT thin films also show good ferroelectric properties with a remanent polarization (Pr) more than 22μC/cm2 and a low leakage current less than 10-8 A at applied voltage 5 V.The ferroelectric properties of BNT thin films which were deposited in different anneal atmosphere and with different doping content were investigated by Radiant Technology Precision Workstation ferroelectric test system. It was showed that BNT thin films had a maximum polarization (Pmax) nearly 41μC/cm2 at applied electric field 300 kV/cm, when the doping content was 0.85. The BNT thin films annealed in O2 atmosphere which would reduce the oxygen vacancy shows better ferroelectric properties. It was also showed that the BNT thin films annealed at 750°C had a maximum remanent polarization.The frequency dependence properties were studied in both BNT thin films and [Bi(Zn0.5,Ti0.5)O3]0.03[Pb(Zr0.4,Ti0.6)O3]0.97 (BZT-PZT) thin films. The hysteresis loops were showed highly frequency dependent. With the frequency increases, the loops showed a bit shrunk. In both of the films, the Pr decreases with the increasing frequency, which might due to uncompleted domain reversal. The coercive electric field (Ec) did not show a highly frequency dependent, when the frequency was below 5 kHz. However, while the frequency was above 5 kHz, the Ec increases with the increasing frequency. This behavior may be attributed to the domain switching. While the electric field reversal period decreases, the domain switching will be affected, which is called the frequency dependence.