Effects of adhesion layers on the microstructure and characteristics of SrBi2Ta2O9 ferroelectric memories

• Main Authors: Ching-Chich Leu, 呂正傑
• Other Authors: Chen-Ti Hu
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/4862gd

博士 === 國立清華大學 === 材料科學工程學系 === 92 === The effects of titanium (Ti) or tantalum (Ta) adhesion layer on the ferroelectric and microstructural properties of metal-organic decomposition (MOD)-derived SrBi2Ta2O9 (SBT) films are investigated. It is found that the atoms of adhesion layer play a significant role on the resultant microstructures and physical properties of SBT films. Either the Ti or the Ta atoms of adhesion layer have the out-diffused behavior onto the surface of bottom-electrode Pt films after a thermal treatment of 750°C, 1min. However, the diffusion of Ti atom is much faster than Ta. Various out-diffused species do cause the distinct properties of SBT films, which are confirmed with the results of surface analysis and P-E measurements. By employing transmission electron microscopy (TEM), a thin nanocrystalline Pt (nano-Pt) interfacial layer was unexpectedly observed in the SBT/Pt interface region. The energy-dispersive spectrometer (EDS) analyses indicated a great amount of TiOx formed along the grain boundaries of this nanocrystalline layer. This phenomenon is strongly related to the grain-boundary diffusions of O and Ti atoms during the annealing process in oxygen ambient. It is speculated that the structural reconstruction of the Pt electrode was likely triggered by the induced stress due to the large volume expansion as the formation of TiOx. Meantime, the emergence of TiOx in nano-Pt/columnar-Pt interfacial regions would significantly deteriorate the integrality of Pt electrode, it possibly causes the peeling of SBT layer and poses a serious problem to the performance of the ferroelectric capacitors. Therefore, the employment of Ta element for the adhesion layer in SBT capacitor is more favorable. In ferroelectrics, the macroscopic properties of thin films, such as switching behavior, hysteresis, piezoelectricity and electro-optical effects, are intimately related to the microscopic domain structures. Hence, it is of paramount importance to have an insight into the structure and behavior of domains in order to better understand the characteristics of ferroelectric thin films. In the second section of this thesis, the scanning capacitance microscopy (SCM) was employed to investigate the polarization-induced micro-structural images from the MOD processed SrBi2Ta2O9 (SBT) thin films. A sharp image contrast had been induced between the nano size domains owing to the various polarities, so that the domain structure in a SBT thin film was clearly revealed. The contrast mechanism of the SCM to exhibit the ferroelectric domain structures was demonstrated. A parallel examination on the oxide-passivated SBT films verified that the SCM images truly came from the intrinsic capacitors characteristics rather than the surface properties of SBT. The two-way differential capacitance scanning over a large grain showed a hysteresis loop depicting the opposing polarities at zero bias. Subsequently the hysteretic C-V curves, as the characteristic of domain switching, were acquired by integrating the calibrated differentiation capacitance. Therefore, it is concluded that the white and black contrast in SCM images are due to the anti-parallel 180o domains whereas the gray contrast is owing to the trivial differential capacitance variation. These results strongly suggest that the local capacitance variation can be correlated very well with the polarities of polarizations in nano-scale domains. On the other hand, the impact of an illumination on the differential capacitance variation of a SBT capacitor is observed. It is found that the illumination on samples by a stray light of laser in the atomic force microscopy (AFM) can not only perturbs the dC/dV signals but also affects the switching properties of the ferroelectrics. We attribute this phenomenon to the generation of free carriers by the photon absorptions via the defect traps in a SBT thin film. As a consequence, it is suggests that the laser illumination effect shall be carefully considered whenever a field-sensitive technique is employed to analyze the properties of a ferroelectric material.