Negative differential resistivity in ferroelectric thin-film current-voltage relationships

The current-voltage relationship I(V) for several different ferroelectric thin films often exhibits an apparent negative differential resistivity. In the present paper we show that in lead zirconate-titanate (PZT) this is a spurious artifact arising from rapid (parametric) measurement techniques and that it disappears for d.c. measurements made with measuring times ≥300 s. However, in other ferroelectric materials (selected samples with unusually high trap densities) it appears to be a genuine effect characterized by an I = aV 3 current-voltage relationship at high current densities (≥100 nA/cm 2 ), I(V) hysteresis above a "pseudo-breakdown voltage," and distinct V 2 -to-V 3 cross-over threshold.