Ferroelectric Domain Patterning With Electron Beams
Ferroelectric polarization in polycrystalline PZT thin film can be reoriented in both positive and negative directions using electron beams. Reorientation of the ferroelectric domains is a response to the electric field generated by an imbalance of electron emission and trapping at the surface. When an insulator surface is irradiated by electrons with energy higher than 1 keV, elastic and inelastic collisions in the crystal lead to the excitation of secondary electrons and the backscattering of incident electrons. Secondary electrons that are sufficiently close to the surface (less than 50 nm) are emitted from the surface, while the other electrons are either trapped in defect sites or self-trapped as polarons in the crystal. When the number of incident electrons is not equal to that of the emitted electrons, charge develops and an internal local electrical field is established in the film. When the field generated by the trapped charges is stronger than the coercive field of the ferroelectric compound, domain reorientation at the surface should occur.
Quantitative aspects in terms of beam dosage, energy and current density effects on polarization reorientation have been determined. A threshold of 500 mC/cm2 and a saturation of 1500 mC/cm2 were identified. Regardless of beam energy, the polarization is reoriented negatively for beam currents less than 50 pA and positively for beam currents greater than 1 nA. Due to the length scale accessible to conventional e-beam tools, this method provides the opportunity for domain engineering at the nanometer scale.
Figure shows the illustration of charging and polarization reorientation by an electron beam. Accumulation of positive charges causes the domains to point downwards (yielding the c- domains shown on left). In contrast, negative charging leads to positive domain orientation (c+ domains on right).
Figure above (a) PFM phase image showing negative domain polarization (dark area) switched by E=10 keV, I=30 pA, and dosages ranging from ~500 to 5000 mC/cm2. The exposure is increased from left to right and from bottom to top in the figure. (b) The fraction of c- domains switched perpendicularly to the surface, increases with electron dosage. (c) Model of domain switching with electric field (E) as dashed arrows. For E greater than Ecritical the domains begin switching. The fraction of switched domains increases with electron dose until the reorientation saturates at 1500 mC/cm2, as in (b).
Figure above shows the polarization reorientation dependence on beam energy and current. Dots represent experimental determination of domain polarity superimposed on a schematic representation of QT, which should follow the expected trend of the total electron emission yield s for an insulator. At high beam current (>1 nA), a net negative charge accumulates in the film, which switches the underlying domains in the positive direction (c+, bright areas in PFM phase image). It is reversed for low beam current (<50 pA). For beam currents between 50 pA and 1 nA, the sign of net charge depends on beam energy.