Pulsed laser ablation-deposition of La 0.5 Sr 0.5 CoO 3 for use as electrodes in nonvolatile ferroelectric memories
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Pulsed laser ablation-deposition of La0.5 Sr0.5 CoO3 for use as electrodes in nonvolatile ferroelectric memories R. Dat, O. Auciello,a) D. J. Lichtenwalner, and A. I. Kingon Department of Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695-7919 (Received 29 October 1994; accepted 21 February 1996)
La0.5 Sr0.5 CoO3 (LSCO) thin films have been deposited on (100) MgO substrates using pulsed laser ablation-deposition (PLAD). The crystallographic orientation of LSCO was found to be dependent on the surface treatment of (100) MgO prior to deposition. PLAD deposition parameters were optimized to yield LSCO films with an ˚ A smooth surface morphology was reproduced as RMS surface roughness of 40–50 A. long as the oxygen content of the LSCO target was preserved. Otherwise, “splashing” occurred which resulted in the transfer of condensed particles from molten spherical globules of LSCO from the target to the substrate. Splashing was subsequently eliminated and smooth surface quality was restored after annealing the LSCO target at 550 ±C in oxygen for 3 h. Optical emission spectroscopy (OES) of the LSCO’s plume identified excited atomic cobalt neutrals, excited singly ionized strontium and lanthanum, and excited molecular LaO species. Oxygen interaction with the plume produced no new species. Furthermore, the OES data suggest that the observed LaO molecules were not created by the chemical reaction between La and O2 during ablation, but were ejected directly from the target during the PLAD process.
I. INTRODUCTION
The wide range of electrical resistivities possible in thin film oxides makes these materials very attractive for various device applications. At one extreme, they can be used as superconductors, while at the other extreme their insulating properties can be employed in charge storage devices. Between these two ranges of applications, thin film oxides can also be utilized for their metallic characteristics. In nonvolatile memory applications, both the conducting and insulating properties of oxides are utilized. The ferroelectric material (insulator) is sandwiched between two conducting electrodes. Recent research has shown that conducting oxide electrodes are essential to the reliability of ferroelectric capacitors.1–7 PZT-based ferroelectric capacitors with metallic top and/or bottom electrodes show a dramatic decrease of switched polarization (fatigue) after about 106 bipolar voltage pulses. The reason for this decrease is not known; however, it is speculated that this behavior is related to the quality of the ferroelectric/electrode interface, including the presence of oxygen vacancies, as well as other charged defects in the ferroelectric layer. While oxide electrodes are essential in controlling the long-term properties of ferroelectric capacitors, their successful utilization in memory devices will ultimately a)
Also, MCNC, Electronics Technology Division, Research Triangle Park, North Carolina 27709-2889.
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