Polarization reorientation in ferroelectric lead zirconate titanate thin films with electron beams
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D.R. Strachan Department of Physics and Astronomy and Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104
J.H. Ferris and D.A. Bonnella) Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received 15 September 2005; accepted 5 January 2006)
Ferroelectric domain patterning with an electron beam is demonstrated. Polarization of lead zirconate titanate thin films is shown to be reoriented in both positive and negative directions using piezoresponse force and scanning surface potential microscopy. Reorientation of the ferroelectric domains is a response to the electric field generated by an imbalance of electron emission and trapping at the surface. A threshold of 500 C/cm2 and a saturation of 1500 C/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.
I. INTRODUCTION
Ferroelectric domain switching, also referred to as polarization reversal or reorientation, in thin films has traditionally attracted attention due to applications in nonvolatile storage devices.1,2 Recently, a new process for fabricating complex nanostructures based on ferroelectric domain patterning has shown promise.3,4 In these and other applications, domain reorientation and patterning are accomplished by applying an electric field with macroscopic or patterned metal electrodes or with a metallic scanning probe microscope tip.5–7 The latter is particularly effective for producing nanometer-sized features with desired polarization orientation. An alternative process for patterning small scale domains is based on e-beam induced polarization reorientation. Ferris et al. produced nanometer-sized domain patterns on polycrystalline lead zirconate titanate (PZT) thin films,8 and earlier studies produced macroscopic features on singlecrystal LiNbO39,10 by e-beam irradiation. 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. While domain reorientation by e-beam irradiation has been demonstrated for the two cases mentioned above, quantitative aspects of the mechanism are not known. For example, excitation cross sections are beam energy dependent, which implies that surface charging will also be energy depend
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