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

摘要

We report on the tunability of ferroelectric properties of Poly(vinylidenedifluoride-triflu-oroethylene) 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 crystallo-graphic alignment leading to (110) 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.