Quantifying electronic charge trap states and the effect of imprint on ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films

Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films have been extensively studied for their ferroelectric properties and are a promising material for sensing and memory applications. Imprint, the time-dependent resistance to polarization reversal, is a key material property that limits applications and is poorly understood. We have used a fast ramp rate thermally stimulated current (FR-TSC) technique to investigate the proposed link between imprint time and filling of charge trap states, and to characterize these 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. Traps were allowed to fill during room temperature imprint for times from 1 to 10⁴ s. Remaining unfilled traps were subsequently filled and quantified by FR-TSC temperature cycling from 20 to 100 °C at 1 K/s. Results confirm that charge trap filling occurs both with time, under isothermal conditions, as well as with increasing temperature. The rate of charge accumulation induced by FR-TSC exhibits the same logarithmic dependence with time as the experimentally measured imprint voltage; this strongly suggests a direct link between these trapped charges and the imprint field. Furthermore, measurements of the switching speed in post FR-TSC films were found to be independent of the imprint time, confirming that thermal cycling brought all samples to a common final state. These results provide further understanding of the polarization dynamics and materials interactions affecting the ferroelectric properties of P(VDF-TrFE).