Passive Layer Formation at Ferroelectric PbTiO3/Pt Interfaces Studied by EELS
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Passive Layer Formation at Ferroelectric PbTiO3/Pt Interfaces Studied by EELS S. J. Welz1, L. F. Fu1, R. Erni1, M. Kurasawa3, P. C. McIntyre3, and N. D. Browning1,2 1
Department of Chemical Engineering and Materials Science, University of California Davis, One Shields Ave., Davis, CA 95616, USA 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, USA 3 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA
ABSTRACT
Polarization fatigue with repeated electric cycles in ferroelectric thin films is a major degradation problem in ferroelectric nonvolatile memories. However, the origin of this phenomenon is still not properly understood. The fatigue mechanism of a ferroelectric perovskite in a multilayer ferroelectric PbTiO3 thin film material has been investigated here using scanning transmission electron microscopy (STEM). Z-contrast images of the interfaces show that the ferroelectric PbTiO3 layer has partly decomposed into a single crystal PbTiO3 layer and an amorphous layer. Nanometer-sized precipitates are present near the Pt electrode. Electron energy-loss spectroscopy (EELS) analysis reveals that the amorphous layer is a Ti-rich phase between TiO2 and PbTiO3. The precipitates are determined to be a Pt-Pb rich crystalline phase. It is suggested that the formation of the structure-distorted intermediate layer and precipitates may be associated with the ferroelectric degradation process by acting as a passive layer in a ferroelectric capacitor. In addition, the formation of the Pt-Pb rich precipitates may cause an interruption of the consistent Pt electrode, which may result in failure of the device. INTRODUCTION Ferroelectric thin film materials, such as PbTiO3, have been widely studied for potential applications as nonvolatile memory since they offer significant advantages over Si-based devices - such as high dielectric constant, large polarization, and low power consumption [1,2]. However, several material-related problems, such as fatigue (reduced polarization under repeated switching cycles), severely hinder commercial applications in microelectronic devices [3]. Although numerous explanations for fatigue degradation of ferroelectric thin films have been proposed, the mechanism remains undetermined. Recently, significant attention has focused on the possibility that the growth of a non-ferroelectric passive layer near the electrodes may result in device failure [4,5]. This concept suggests that the presence of a low permittivity layer in a ferroelectric capacitor may substantially influence the depolarization field and redistribution of charge carriers [6]. Related studies are mainly based on theoretical assumptions or modeling of passive layers from ferroelectric polarization switching measurements. So far few experimental evidence of the formation of passive layers in a ferroelectric capacitor has been provided [4-7]. Therefore,
O14.3.2
a comprehensive microstructural characterization of
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