Oxygen Deficiency and Vacancy Formation in LSCO/PLZT/LSCO Capacitors
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ABSTRACT Vacancy type defects in La 0 .5 Sr 0.5 CoO 3 / Pb 0 .9La 0.1Zr 0 .2 Ti0 .8 / Lao. 5 Sr 0 .5CoO 3 capacitors were investigated by positron depth profiling. Post-growth annealing of the capacitor structure in oxygen deficient atmosphere exhibits the formation of vacancy type defects in all layers. A significant increase in open volume defects was found in the top and bottom electrode. The changes in the bottom electrode were studied more closely by etching off the top layer. INTRODUCTION The potential applications of ferroelectric thin films in the fabrication of non-volatile memory devices have drawn a lot of attention to the perovskite oxide materials. Current efforts are directed towards integration with standard silicon device technology. Long term reliability issues such as fatigue, imprint and aging are important aspects in the realization of these devices. The simplest ferroelectric device is the ferroelectric capacitor, which typically consists of two electrodes parallel to the substrate with the ferroelectric material sandwiched in between. Improved fatigue characteristics for Pbl-yLay(Zr,.xTix)O 3 thin film capacitors are achieved when conventional Pt electrodes are replaced with the conducting oxide Lao. 5SrO.5CoO 3 (LSCO) [1].
Device processing frequently exposes capacitors to oxygen reduced ambient. This can cause oxygen depletion of the capacitor and the formation of oxygen vacancies (Vo). Associated with this oxygen loss is the generation of a large voltage offset in the hysteresis loops, which can result in the failure of the device (imprint) [2, 3]. In a recent paper we were able to correlate for the first time the vacancy defect depth profile in LSCOiPLZT/LSCO capacitors with the effect of voltage shift in P-V loops [4]. Capacitors with only the top electrode oxygen-depleted show a large voltage offset in the hysteresis loop due to the existence of an internal field created by oxygen vacancies. Symmetric loops with low polarization values are observed for capacitors where all layers are deficient in oxygen. The experiments in this paper focus more directly on the buried layers. This is accomplished by removing the top electrode via etching to improve sensitivity and resolution for the ferroelectric layer and the bottom electrode. EXPERIMENTAL DETAILS A capacitor stack of LSCO(50/50)/PLZT(10/20/80)/LSCO(50/50) was deposited by pulsed laser deposition at 650 'C in an oxygen partial pressure of 100 mTorr on a single crystal LaAIO 3 (LAO) substrate. The sample was grown with a nominal thickness of 40/180/180 nm. After film t e-mail: [email protected]
393 Mat. Res. Soc. Symp. Proc. Vol. 596 ©2000 Materials Research Society
growth the oxygen partial pressure in the growth chamber was adjusted to 760 Torr to allow full oxidation of the structure. The sample was then cooled to RT at a rate of about 5 °C/min. The growth parameters and thickness of the structure are summarized in Table I. The heat treatment was performed in a vacuum furnace with the sample placed between two Mo hot plates to al
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