To obtain a high signal strength, fast reading-writing speed, and low energy consuming in ferroelectric random access memory (FeRAM) devices, it is of vital importance to enhance the remnant polarization (Pr) and to reduce the coercive field (Ec) simultaneously in polycrystalline ferroelectric (FE) thin films that dominate the FeRAM market due to low cost.

The PhD research developed a cheap and easy technique of thermomechanical treatment (TMT), which is able to produce nanotwins, as shown by the images of high resolution transmission electron microscopy and the patterns of electron nanobeam diffraction, and meanwhile modulate the residual stress of polycrystalline FE thin films. As a result, the measured remnant polarization (Pr) is enhanced and meanwhile the measured coercive...[ Read more ]

To obtain a high signal strength, fast reading-writing speed, and low energy consuming in ferroelectric random access memory (FeRAM) devices, it is of vital importance to enhance the remnant polarization (Pr) and to reduce the coercive field (Ec) simultaneously in polycrystalline ferroelectric (FE) thin films that dominate the FeRAM market due to low cost.

The PhD research developed a cheap and easy technique of thermomechanical treatment (TMT), which is able to produce nanotwins, as shown by the images of high resolution transmission electron microscopy and the patterns of electron nanobeam diffraction, and meanwhile modulate the residual stress of polycrystalline FE thin films. As a result, the measured remnant polarization (Pr) is enhanced and meanwhile the measured coercive field (Ec) is reduced. The enhanced Pr is attributed to the reduced residual tensile stress, while the reduced Ec is owing to the nanotwin network introduced by TMT. The developed TMT technique is superior over the interfacial engineering that is validated only for epitaxial single crystalline films. The twinned structures provide more domain nucleation sites, thereby making the polarization switching easier and thus lowering the coercive field.

Changing the TMT temperature, the TMT technique can induce the cubic to hexagonal phase transformation in the barium titanate films. The hexagonal phase was observed by high resolution transmission electron microscopy. The hexagonal phase is no-ferroelectric and has a plate shape. The mechanisms of the TMT induced nanotwins and cubic to hexagonal phase transformation have been comprehensively studied. The hexagonal phase plates embedded in ferroelectric phase lead to double hysteresis loops of polarization versus electric field. The domain mechanism of the double hysteresis loop is also discussed in this PhD study.