Surface Potential and Temperature Analysis of Ferroelectric Materials via Advanced AFM Techniques

With shrinking dimensions, the effect of surface screening charges on the properties of ferroelectrics are no longer irrelevant. Surface electric fields and polarization switching are strongly coupled to the action of charge screening on top of the ferroelectric domains. In contrast to classical applications like actuators, which are perfectly described by the ferroelectric bulk properties, device concepts that rely on the electric field over the ferroelectric domains need understanding and control of the screening mechanisms.
Here, the surface electrical properties of Lithium Niobate and Barium Titanate under ambient conditions were investigated using advanced Atomic Force Microscopy techniques. The focus was set on controlling the charge screening by adsorbed species. Removal of those adsorbed screening charges was realized by desorption at elevated substrate temperature and by direct removal by scraping the charges off with the scanning AFM tip in contact mode, which is often referred to Charge Gradient Microscopy (CGM).
It turned out that the surfaces are often so strongly charged that methods like Kelvin Probe Force Microscopy fail, as the system voltage limits are exceeded. To compensate for the strong charging, we modified the commercial AFM system using external voltage amplifiers for extended voltage range.
Increasing the temperature, as well as CGM measurements, both result in similar trends in the surface potential. Typically, a shift from positive to negative values in the surface potential is observed. This means that the positive surface potential (at RT, without prior CGM), switches to negative values for measurements at 100°C, as well as right after CGM treatment. However, the potential values and specifically the difference between up and down domains is strongest after CGM reaching potential differences around ≈7 V. The screening effects and possible mechanisms are discussed in detail, and an attempt is made to quantify the surface charge densities using simplified electrostatic models.
Those models suggest effective surface charge densities between 10^(-4)-10^(-3) C/m^2 .
Overall, this work gives insight into the complex interaction of ferroelectrics with screening charges, as well as surface properties at ambient conditions.