PHOTORESPONSE PROBE OF THE SPACE-CHARGE DISTRIBUTION IN FERROELECTRICLEAD-ZIRCONATE-TITANATE THIN-FILM MEMORY CAPACITORS

A photoresponse study of sol-gel derived polycrystalline thin films of lead zirconate titanate (PZT) is described. Thin film ferroelectric c apacitors were fabricated in a sandwich geometry with a transparent to p electrode, and illuminated with almost-equal-to 365 nm wavelength (P ZT band gap approximately 3.5 eV) light pulses. The observed photocurr ent has two components: first, a transient spike coincident with the o nset of the illumination pulse, and second, a steady dc photocurrent w hich prevails as long as the light is ''ON.'' The steady current respo nse exhibits a weak dependence on the polarization, whereas the transi ent response exhibits a distinct polarization dependence. To understan d the nature of this photoresponse we have studied the variation of th e photoresponse as a function of the duration of the ''write'' pulse u sed to program the ferroelectric capacitor. The peak value of the pola rization dependent transient component of the photoresponse relates to the injected space charge and the distribution of deep traps within t he ferroelectric thin film. This, in turn, is modulated by the duratio n of the write pulse used to program the capacitor. We provide a space charge model that offers a framework for interpreting the photorespon se. The transient photoresponse thus serves as a qualitative measure o f the space charge in the ferroelectric film, and to a limited extent, an indirect measure of the remanent polarization. We propose that the se photoresponse effects can be applied as diagnostic probes of the sp ace charge distribution in the ferroelectric thin film.