Influence Of Flexoelectric Effect On Leakage Current Of Ferroelectric Film

Nonvolatile ferroelectric memory is one of the most promising next memory device, owing to excellent properties, such as anti-radiation, high reading and writing speed, non-volatility. It draws lots of attention. However, ferroelectric random access memory(FeRAM) has disadvantage in low storage density. Generally, downsizing is used to improve the storage density. But this method will generate new problems,such as failure, fatigue, imprint and retention loss problems. Though fatigue and imprint problems have been solved by changing ferroelectric material or electrode,retention loss is still not be solved effectively. Leakage current is one of the main causes of retention loss. Since ferroelectric memory is multilayer structure, leakage current conductive mechanism is complex. Previous studying found strains gradient are huge due to flexoelectric effect has influence on prolarization in nanoscale ferroelectric films. The inhomogeneous strains result in inhomogeneous prolarization,and then result in internal electric field. The internal electric field will unavoidable affect the distrubution and transport of charge carrier. Therefore, the main purpose of this work is developing a theoretical model to study the influence of flexoelectric effect on leakage current in ferroelectric thin film and tune leakage current. The primary coverage of this thesis is as follows:1. We builded a model based on the phase-field method considering flexoelectric effect and carrier transport for ferroelectric domain evolution. We used the model to verify switchable diode-like current-voltage characteristics in ferroelectric Ba TiO3.The diode-like current-voltage characteristics is depended on ferroelectric bound charge.2. With typical perovskite ferroelectric Pb(Zr0.1Ti0.9)O3(PZT), we considered flexoelectric effect, and investigated the influence of the strain gradient on the leakage current in single domain film. We found that flexoelectric effect has weak influence on leakage current in single domain film, owing to the strain gradient is not big enough. When loading a local stress in ferroelectric film, there is a big strain gradient.Which lead to a remarkable flexoelectric effect having influence on leakage current.The leakage current increasing with the increasing stress, when the stress is lower than the threshold stress for domain switching.3. Films exhibit intrinsic elastic strain gradients of 107~108 m-1 at domain walls,sufficient to produce significant flexoelectric effect. We investigated the flexoelectriceffect influence on poly domain in PZT.11 f is the coupling of polarization with longitudinal strain gradient.12 f is the coupling of polarization with transverse strain gradient. They have different effects on leakage current.11 f result in electric potential reducing. It leads to hole carrier concentration and leakage current increasing. Oppositely12 f lead to electric potential increasing. It results in hole carrier concentration and leakage current reducing.