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We report on the metal-insulator transition of La0.5Sr0.5CoO3 thin films deposited by pulsed laser ablation on LaAlO3 substrates. The films were cooled in oxygen partial pressures between 760 and 10−5 Torr and electrical resistivity of the films was measured as a function of cooling oxygen pressure. La0.5Sr0.5CoO3 films changed from metallic to insulating behavior depending on their oxygen content. A defect model has been proposed to explain this transition and the change in conductivity is related to the change in the oxidation state of the cobalt ions. The model explains the relationship between oxygen partial pressure and electrical conductivity in La0.5Sr0.5CoO3, which describes the experimental dependence reasonably well. Positron annihilation studies were also done on the same set of samples and the S parameter was seen to increase by 8% from a fully oxygenated sample to a sample cooled in 10−5 Torr.

A high density ferroelectric memory process flow requires the integration of conducting barrier layers to connect the drain of the pass-gate transistor to the bottom electrode of the ferroelectric stack. We are studying the effect of crystallinity of the TiN/Pt barrier layer with Si wafers on the ferroelectric properties of La0.5Sr0.5CoO3/Pb(Nb0.04Zr0.28Ti0.68)O3/La0.5Sr0.5CoO3 (LSCO/PNZT/LSCO) capacitors. Structural studies indicate complete phase purity (i.e., fully perovskite) in both epitaxial and polycrystalline materials. The polycrystalline capacitors show lower remnant polarization and coercive voltages. However, the retention, fatigue, and imprint characteristics are similar, indicating minimal influence of crystalline quality on the ferroelectric properties.

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We report on the thermally activated hysteresis relaxation effects in (La,Sr)CoO3/ (Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 thin film ferroelectric capacitors. Films cooled in oxygen deficient ambients exhibit a marked voltage offset in the hysteresis loops. Upon the application of a dc bias voltage or unidirectional pulses of the same polarity as the offset, the loops become more symmetric. Subsequently, holding the capacitors in the original preferred polarization state leads to a relaxation of the hysteresis loop towards its original voltage offset condition. The relaxation process is described by a stretched exponential and is thermally activated.