Seeding Effect by Off-Stoichiometric SrBi[sub 2]Ta[sub 2x]O[sub 9] Thin Buffer Layer on the Properties of SrBi[sub 2]Ta[sub 2]O[sub 9] Thin Films

Abstract

The seeding effect on the crystallization behaviors of the SrBi 2 Ta 2 O 9 strontium bismuth tantalite ͑SBT͒ phase on top of a thin Aurivillius buffer layer, SrBi 2 Ta 2x O 9 , was studied. A uniform microstructure and a preferred polar a-axis orientation within the SrBi 2 Ta 1.8 O 9 buffered SrBi 2 Ta 2 O 9 thin film were promoted to achieve a high remanent polarization ͑2P r ͒ value of about 19.7 C/cm 2 . Furthermore, the SBT/buffer heterostructure constructed by the two-step process showed a significantly improved leakage property compared with the nonbuffered SBT. The off-stoichiometric thin buffer layer demonstrates a simple method to improve the microstructure, polar a-axis orientation, and electric characteristics and to introduce less defects in the superposed SrBi 2 Ta 2 O 9 thin films. Strontium bismuth tantalite ͑SrBi 2 Ta 2 O 9 , SBT͒, among ferroelectric materials, has been intensively studied for nonvolatile ferroelectric random access memory ͑FeRAM͒ applications because of its relatively low operation voltage, low leakage current, and fatigue-free properties with the Pt bottom electrodes. 1 However, it has a low remanent polarization value and a high crystallization temperature, which would be the main disadvantage for high density FeRAM integration in the future. Because SBT does not exhibit any dominant self-textural growth mechanisms, it is therefore mainly governed by the underlying substrate and the interfacial properties. In most FeRAM applications, the standard substrate for SBT is platinum film. Pure platinum is not an ideal substrate for the nucleation of perovskite, and SBT films on top of Pt electrodes always show a preferential ͑00l͒ polycrystalline structure due to the match of lattices. 2 It has been reported that the ferroelectric properties of SBT thin films on the a-b plane are better than those along the c-axis. 3 Therefore, adopting a seeding layer ͑Ta, Bi-Ta, Bi 2 O 3 , Bi 4 Ti 3 O 12 , stoichiometric SBT and yttria-stabilized zirconia/SrZrO 3 , etc.͒ on the top of Pt might be beneficial for better film crystallization or superior texture of SBT. 2,4-11 The polarization properties of SBT could be improved through seeding technology by controlling the film's preferred orientation. However, employing a non-SBT seeding layer usually induced the deviation of SBT composition from stoichiometric and resulted in the formation of undesirable second phases or defects; few researchers investigated their influences on the electric properties, in addition to polarization, of SBT, e.g., leakage property. Another effective method for controlling the crystallization kinetics and the microstructure of ferroelectrics would be introducing a crystallographically preferred nucleation site in the form of many separated seeding particles ͑stoichiometric SBT͒ into the system. 12-16 The SBT films were observed to have lower crystallization temperatures and less pyrochlore or fluorite phases by the incorporation of SBT seeds or templates into SBT films. Indeed, such methods enhanced the ferroelectric pr...