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The surface morphologies of (a) SrTiO3 (100) (STO) substrates, which are etched by a standard chemical etching process and are annealed at 950 °C; (b) SRO layers and (c) and (d) BPFO films grown for 10 and 60 min, respectively, of 45 and 289 nm thicknesses.
Structural analysis by four-axis X-ray diffractometry in a reciprocal mapping mode along asymmetric (103) peak.
Piezoelectric responses of the BPFO films. The surface morphology of films taken by AFM. The polarization directions of BPFO crystal are defined as vectors radiating out from the center of the unit cell and pointing to eight corners. The out-plane polarizations, (a) and (b), are tested in a 2 × 2 μm2 by applying positive and negative bias voltages. The in-plane polarizations are tested by applying a pulse bias. The final in-plane polarization images are shown in (c) and (d) for BPFO films grown for 10 and 60 min. Thicker BPFO films exhibit wider domain structures.
The hysteresis loop of out of plane piezoresponse as a function of DC bias voltages. The cubic lattices indicate various polarization states, and the inset represents the relative state energies of correspondent states.
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