Nanoscale studies of switching behavior of ferroelectric thin films by using Piezoresponse Force Microscopy

Abstract

Ferroelectrics as the special group of materials not only have piezoelectricity and pyroelectricity but also have a spontaneous polarization which can be switched by an applied external electric field. Basing on these properties there are a lot applications since the war years when BaTiO3 was discovered and broadly used as the capacitors in radio and radar equipment. At present ferroelectrics are widely used in different fields, such as the ferroelectric capacitors, transducers, actuators, and thermistors. One of the most promising applications is nonvolatile ferroelectric random access memories or FeRAMs. [14, 15]

From a fundamental aspect, we need to directly verify several different theoretical models used for studies of polarization switching in ferroelectric thin film capacitors to determine which model works for a specific class of FeRAM capacitor structure. There are several main problems addressed in the present study: (1) Direct observations of the domain dynamics in the thin film capacitors during polarization reversal need study and analysis. (2) The dependence of microstructure on the switching mechanism of ferroelectric thin film capacitors is not clear and needs to be investigated. (3) The capacitor scaling and time dependence of switching also needs to be investigated. There are only a few techniques which can measure the piezoelectric response and the surface polarization directly, especially with nanoscale spatial resolution. Traditional electrical measurements such as the bias-dependent P-E hysteresis loop measurements or transient switching current measurements are very difficult to use for study of the switching behavior of ferroelectric capacitors at micron length scale. However a new local probing technique called Piezoresponse Force Microscopy or PFM can be applied in a straightforward way to these measurements. This is a nondestructive characterization technique with high lateral resolution and it can directly observe spatially resolved vertical displacement due to local switching behavior. By using PFM, it is also possible to contact individual capacitors of micro- and submicron-dimension and study the scaling effect of polarization switching.