Cooling rate controlled microstructure evolution and reduced coercivity in P(VDF–TrFE) devices for memory applications

• We tune ferroelectric properties of P(VDF–TrFE) by controlling the cooling rate.
• Fast cooling rate increases polarization and decreases coercivity.
• Microstructure changes dramatically with changes in cooling rate.
• Microscopy and X-ray analysis shows fast cooling leads to (1 1 0) oriented films.
• Infrared spectroscopy also confirms favorable alignment of C–F dipoles.

We report on the tunability of ferroelectric properties of Poly(vinylidenedifluoride–trifluoroethylene) P(VDF–TrFE) thin films by controlling the cooling rate during transformation from high temperature paraelectric α-phase to low temperature ferroelectric β-phase. A faster cooling rate of P(VDF–TrFE) thin films leads to an increased polarization by 30% and much decreased coercivity by 60%. The origin of these improvements in the ferroelectric characteristics is attributed to evolution of a favorable microstructure and crystallographic alignment leading to (1 1 0) oriented films that are cooled faster. The microstructure of the films changes from a fine fibrous structure at fast cooling rate to a flatter ripple containing structure in the slow cooled samples. This dramatic change in the microstructure is attributed to the combination of incorporation of large stresses arising from almost 50% change in the molar volume of P(VDF–TrFE) upon α → β transformation and the cooling rate assisted stress relaxation, nucleation and growth. Infrared spectroscopy further showed that the substantial improvement in the device performance of the fast cooled samples arises from a favorable alignment of C–F dipoles due to short and ordered fibers lying on the substrate plane whose orientation becomes more random as the cooling rate is decreased.

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