Surface And Interface Properties Of Ferroelectric Thin Films By Scanning Force Microscopy Study

Strong coupling between electrical and mechanical properties in ferroelectric materials and devices enable their widespread application. Achieving the full potential of ferroelectric thin films necessitates fundamental knowledge of surface and interface properties of ferroelectric thin films on the nanoscale level. In this thesis, Scanning Force Microscopy (SFM) was used to study the nanoscale electric phenomena of the surface and interface properties of ferroelectric thin films.The experimental setup was calibrated by measuring potential distribution of the working resistance of integrated chip. To achieve accurate detection of surface potential, the measurement of the contact potential difference of the ZnO/Si step was also carried out.Polarization-related surface properties of ferroelectric thin films were investigated by Kelvin Probe Force Microscopy (KPFM), leading to the discovery of asymmetric charge writing on the surface of Pb(Zr_0.55Ti_0.45)O₃ (PZT) thin film. It is found that the surface potential of the negative charge bits are higher than those of the corresponding positive ones. When ferroelectric polarization switching occurs, the potential difference becomes even more remarkable. The origin of this phenomenon traced to the existence of internal field within the interface layer which is near the ferroelectric/electrode interface. When external bias is applied, the internal field is enhanced or weakened by the superposition with external bias voltages, thus lead to the preference of negative charge state over the positive one.The relaxation property of the surface trapped charge on PZT and SrBi₂Ta₂O₉ (SBT) thin films were studied respectively. It is found that the trapped charge on the surface of PZT thin film is more stable than that of SBT thin film. With respect to the long-term charge storage technique, the PZT thin films are more qualified for high-density nonvolatile storage devices than SBT thin filmsThe interface of ferroelectric thin film was also characterized by SFM. The cross- section sample was prepared by polishing and cleavage. A home-made sample stage was...