Switching Properties of PST Ferroelectric Films for Memory Applications using Conductive Oxide LSCO Electrodes
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Switching Properties of PST Ferroelectric Films for Memory Applications using Conductive Oxide LSCO Electrodes E. Martínez1, O. Blanco2 and J.M. Siqueiros1 1
Centro de Ciencias de la Materia Condensada, UNAM, Apdo. Postal 2681, C.P. 22800, Ensenada, B.C., México. 2 Centro de Investigación en Materiales, CUCEI- U.de G., Guadalajara, México. ABSTRACT We have investigated the switching properties of ferroelectric lead strontium titanate (Pbx Sr1-x)TiO3 (PST) capacitors using epitaxial and polycrystalline La0.5Sr0.5CoO3 (LSCO) electrodes to evaluate its potential for non-volatile memory applications (FRAM´S). The electrical performance of the PST based capacitors grown on epitaxial and polycrystalline LSCO substrates were evaluated through PolarizationVoltage (P-V) and Fatigue measurements. The preferentially oriented PST ferroelectric capacitor did not show a decrease in polarization up to 108 switching cycles at an applied voltage of 4 volts and a frequency of 100 kHz. Polycrystalline PST films, on the other hand were severely fatigued. It is proposed that the good performance of the PST capacitors can be attributed to the high degree of orientation of the PST films in the direction induced by the epitaxial LSCO film. However, after 1010 switching cycles in the fatigue tests, the decay of the non-volatile polarization of these films was about 18% of its initial value. We use a simple model to describe two major microscopic scenarios for the suppression of polarization at the electrodes interfaces. This model is supported by Auger Electron Spectroscopy to determine the bulk and interfacial characteristics. The depth distribution of the elements of the PST film is nearly uniform throughout the volume of the film; however the relative concentrations of Pb and O were abnormally distributed on the surface. It is concluded that, in spite of the good performance due to the favorable conditions for the growth of the PST films provided by the textured LSCO, higher quality interfaces and better control of composition is necessary to improve on fatigue.
Keywords: PST thin films, LSCO, ferroelectric fatigue, switching properties. INTRODUCTION Ferroelectric thin film capacitors integrated on silicon devices are attractive candidates for both nonvolatile memory (NVFRAM) as well as dynamic random access memory (DRAM) applications [1]. A commercially viable NVFRAM requires high quality ferroelectric thin film capacitors with large remnant polarization values, fast switching, and excellent endurance characteristics. Degradation mechanisms such as fatigue, imprint and retention have been some of the impediments to the full commercialization of ferroelectric memories [2]. The conductive oxide La0.5Sr0.5CoO3 (LSCO) has been extensively studied recently as an electrode material for ferroelectric thin film capacitors, in particular, those involving PZT and other lead based titanates improving the device performance [3-7]. Such improvement has been mostly attributed to the better structural/chemical compatibility and the cleaner i
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