铁电薄膜中应力效应的研究

Recently, ferroelectric films have attracted much attention due to their potential application in nonvolatile random access memories. However, some problems including stress, limit its commercial applications. Electroceramic thin films are subjected to residual.stress, either tensile or compressive, on the order of hundreds of MPa, which has important effects on their physical properties. Before us, less works have been down on the stress effect of ferroelectric films, systematic works are especially absent. In our work, stress.effects on ferroelectric films were studied both theoretically and perimentally. Theoretically, an uneven distribution of stress in the thickness direction was introduced into Landau theory to investigate the critical behavior of phase transition in structurally uniform and ideal ferroelectric thin films. The Curie temperature Tc was calculated to increase and decrease for compressive and tensile stress cases, respectively. The dispersion of phase transition was predicted and the diffusivity increased with stress, but showed a non-monotonic relation to film thickness. Experimentally, PbZr0.7Ti0.3O3 (PZT) and SrBi2Ta2O9 (SBT), two of the most attractive candidates and also representations of two kinds of typical structure, were studied. Metal-organic decomposition (MOD) method was employed to fabricate high quality multi-crystal films, and a self-designed instrument was used to apply in-situ mechanical stress to the films during measurements. Under applied stress, the dielectric properties of ferroelectric films showed no observable change, while ferroelectricity.changed obviously. The effect of stress in PZT films is just opposite to that in SBT films, and even more obvious. In the stress range from –100MPa to +100MPa and under electric field of about 180KV/cm, the remnant polarization Pr of PZT changed from about +4.2% to –4.2%, spontaneous polarization Ps from +2.4% to –2.4%; whereas those of SBT changed only from –1.9% to +1.9% and –0.5% to +0.5%, respectively. Fatigue and switching current measurements revealed that the applied stress up to ±120MPa was.sufficient to depress the Pr degradation effectively, but could not affect the switching properties. Based on our experimental results, a tress-induced-domain reorientation model.was put forward, which mentioned that the different behavior of PZT and SBT under stress lain in their polarities which orient along longest and shortest axis, respectively. This model was also supported by other evidences got from non-electrical measurements.As a comparison, the effect of applied electrical stress was investigated as well. Compared with mechanical stress, the applied electrical stress has the similar effect of inducing-domain reorientation, but even more pronounced. After a pretreatment under large voltage, not only the fatigue property of the film was improved distinctly, but also the.switched charge and switching time were increased by 25% and 15%, respectively. A.preliminary try showed that applying mechanical stress during film fabrication could induce a preferred orientation of the films.

应力在薄膜中普遍存在并是影响薄膜诸多性能的重要因素.本课题拟系统研究铁电薄膜中的α捌浞植级员∧さ奶缧阅?介电性质,存储记忆特性,相变行为的影响.并与热力学唯象砺巯嘟岷?揭示应力影响铁电薄膜性质的物理本质.本研究将为改善铁电薄膜存储器的性能跋乱淮呙芏燃且浯娲⑵鞯闹票柑峁┦笛榛『屠砺壑傅?