Temperature induced growth away from the (00l) orientation in SrBi2Ta2O9films deposited by PLD

Abstract

The research on ferroelectric materials has been largely driven by the possibility of application in non-volatile random access memories (NVFRAM). In particular, thin films of SrBi2Ta2O9 (SBT) [1–9] have attracted great interest as good candidates for NVFRAM technology due to the excellent behavior regarding fatigue, retention and leakage current. SBT belongs to a family of Bi layered-structure perovskite oxides [10], with a highly anisotropic pseudotetragonal structure [11] (a = 0.5531 nm, b = 0.5534 nm and c = 2.4984 nm) that leads to highly anisotropic ferroelectric properties. The spontaneous polarization vector in this material is in the a-b plane but, in films grown on Pt/TiO2/SiO2/Si(111) substrates, a large fraction of the crystallites grow with the a-b planes parallel to the plane of the substrate leading to low values of the net polarization [8, 9]. It is therefore desirable to manipulate film growth to enhance the projection of the polar vector in a direction perpendicular to the substrate surface. To achieve this goal, and for the more general purpose of studying the influence of crystal orientation on the ferroelectric properties of the SBT films, deposits have been made on substrates such as SrLaGaO4(110) [3]; SrTiO3(001), (011), (111) [4]; MgO(110) [5]; and SrLaAlO4(100) [6]. It is clear, however, from a technological point of view, that these are not suitable substrates for memory devices because of their silicon-based microelectronic incompatibility. Taking this into account,the results presented in this work are based on deposits made on conventional Pt/TiO2/SiO2/Si(111) bottom electrodes that are compatible with silicon microelectronic processes. As a consequence of the deposition conditions and the heat treatment applied, after the deposition, to the SBT films deposited on Pt electrodes, the fraction of crystallites grown with the (00l) planes parallel to the plane of the substrate was reduced. This led to an enhancement of the measured polarization, reflected in the maximum values of 2Pr = 9.1 μC/cm2 and coercive field, Ec, of 52.0 kV/cm, taken at a voltage of 5 V. As a result, the combination of a higher remnant polarization with lower coercive field obtained is better than that reported in recent works [12, 13] using the same technique and depositing on the same bottom electrodes. The SBT target used in the PLD process was a stoichiometric SrBi2Ta2O9 slab obtained from reagent grade oxides by the conventional ceramic method, sintered at 1250 ◦C for 3 h. The SBT films were grown by PLD using the 248 nm radiation of a KrF excimer laser (LPX 200 by Lambda Physik). The pulse repetition rate was 10 Hz with a fluence of approximately 1 J/cm2. The films, grown on Pt/TiO2/SiO2/Si(111) substrates, were 200 nm thick. The substrate-target separation was 5 cm and the oxygen pressure was 450 mTorr. The deposits were made at temperatures of 570–715 ◦C and then were post-annealed at 750 ◦C in air for 90 min. The resulting films were analyzed in a JSM-5300 scanning electron microscope (SEM), by Jeol, and by X-ray diffraction (XRD) using the Cu Kα line (λkα1 = 1.54056 A and λkα2 =1.54439 A) of a Phillips X’pert diffractometer. Argon ion magnetron sputtering was used for