Photoresponse probe of the space charge distribution in ferroelectric lead zirconate titanate thin film memory capacitors

Abstract

A photoresponse study of sol-gel derived polycrystalline thin films of lead zirconate titanate (PZT) is described. Thin film ferroelectric capacitors were fabricated in a sandwich geometry with a transparent top electrode, and illuminated with ≊365 nm wavelength (PZT band gap ∼3.5 eV) light pulses. The observed photocurrent has two components: first, a transient spike coincident with the onset of the illumination pulse, and second, a steady dc photocurrent which prevails as long as the light is ‘‘ON.’’ The steady current response exhibits a weak dependence on the polarization, whereas the transient response exhibits a distinct polarization dependence. To understand the nature of this photoresponse we have studied the variation of the photoresponse as a function of the duration of the ‘‘write’’ pulse used to program the ferroelectric capacitor. The peak value of the polarization dependent transient component of the photoresponse relates to the injected space charge and the distribution of deep traps within the ferroelectric thin film. This, in turn, is modulated by the duration of the write pulse used to program the capacitor. We provide a space charge model that offers a framework for interpreting the photoresponse. The transient photoresponse thus serves as a qualitative measure of the space charge in the ferroelectric film, and to a limited extent, an indirect measure of the remanent polarization. We propose that these photoresponse effects can be applied as diagnostic probes of the space charge distribution in the ferroelectric thin film.