Quantifying the Role of Electronic Charge Trap States on Imprint Behavior in Ferroelectric Poly(vinylidene fluoride-trif
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Quantifying the Role of Electronic Charge Trap States on Imprint Behavior in Ferroelectric Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) Thin Films Connie Lew and Michael O. Thompson Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric thin films are a potentially promising material for sensors or non-volatile memories. Imprint, the time-dependent resistance to polarization reversal, is a key material property that limits applications and is poorly understood. Based on experimental time and temperature dependences, we propose and investigate the link between imprint and charge trap states. A novel fast-ramp thermally stimulated current (TSC) measurement was developed to quantify and characterize the traps in an appropriate time-frame. Thin films of P(VDF-TrFE) on oxidized Si substrates were characterized following controlled initialization, fatigue, polarization, and imprint. Trap states were thermally filled/emptied by temperature cycling between 20-100 C, using heating and cooling rates between 1 and 5 C/s. Dynamics of this fast-ramp TSC indicate the presence of not only trap states, but also reversible and non-reversible charge accumulation. The presence of electrically active traps were verified by measurements over 1-104 s imprint times. Trapped charge directly correlated with the log of the imprint time, with a rate of 0.12 µC/cm2 /decade.
INTRODUCTION The ferroelectric properties of P(VDF) arise from dipoles between the carbon and fluorine atoms in the polymer chain [1]. With the addition of 10-40 at.% TrFE to create a random copolymer, the material forms the ferroelectric β-phase, and dipoles can be readily aligned [2]. Synthesis of P(VDF-TrFE) has been optimized for such ferroelectric properties, and the material today is a promising candidate for sensors and non-volatile memories [3, 4, 5, 6]. Once polarized, ferroelectric materials become stabilized in the existing orientation. This imprint effect, a time-dependent tendency to resist polarization reversal, is a fundamental challenge for both P(VDF-TrFE) and inorganic ferroelectrics [7, 8]. Imprint is observed as a shift in the hysteresis curve to higher fields with increasing wait time in a given polarization state. It is also observed as an increase in the electric field required to achieve a given polarization switching rate. Imprint fundamentally may be related to time-dependent stabilization of a given polarization state, or to the formation of an additional internal field along the polarization direction that opposes a subsequently applied external field. In inorganic ferroelectrics, hypothesized mechanisms for imprint include oxygen vacancies forming defect dipoles [7], or charge states in interface layers at the ferroelectric/metal boundary [8]. In P(VDF-TrFE) films, the magnitude of the imprint effect increases with the log of the imprint time. To explain this behavior, we propose and investigate here a model of
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