A Study of Phase Transition Behaviors of Chalcogenide Layers Using In-situ AC Impedance Spectroscopy
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A Study of Phase Transition Behaviors of Chalcogenide Layers Using In-situ AC Impedance Spectroscopy Yin-Hsien Huang, Yu-Jen Huang, Tsung-Eong Hsieh Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hseuh Road, Hsinchu, Taiwan 30010, R.O.C. ABSTRACT The electrical properties of chalcogenide thin films, both pristine Ge2Sb2Te5 (GST) and cerium-doped GST (Ce-GST), were investigated by in-situ AC impedance spectroscopy. In conjunction with the brick layer model, the contributions of both the grain and the grain boundary to the phase-transition behaviors of chalcogenide samples could be distinguished; the results illustrated the dominance of the grain boundary in the phase transition process. Moreover, impedance analysis applied to characterize the effects of doping on the phase-transition kinetics yielded results similar to those obtained by conventional methods. Therefore, in-situ AC impedance spectroscopy is a feasible tool for analyzing the phase transitions of chalcogenides. INTRODUCTION Phase-change memory based on chalcogenides [1] is a well-established research field from both the scientific and technological viewpoints. The amorphous-to-crystalline phase transitions of chalcogenides in optical memory devices have been thoroughly studied. Chalcogenides are also recognized as promising materials for next-generation nonvolatile memory, commonly called phase-change memory (PCM), because of their advantages of low power consumption, high operation speed, high recording density, and excellent scalability to nanoscale cell sizes [2]. However, insufficiency in material properties of chalcogenides constrains the electrical performance as the device size shrinks. To overcome this difficulty, chalcogenides with new formulations and alien-element doping methods are developed to modify the physical properties of chalcogenides [3-5]. Therefore, a full understanding of the origin of physical property modifications and their correlations with phase-change mechanisms are required for realizing the practical applications of chalcogenides in PCM. Impedance spectroscopy has been widely used to investigate the electrical properties of ceramics [6,7]. This work establishes an in-situ AC impedance spectroscopy to analyze the phase transition behaviors of pristine Ge2Sb2Te5 (GST) and cerium-doped GST (Ce-GST). With the aid of an equivalent circuit based on brick layer models [8,9], the contributions of the grain and the grain boundary to the electrical properties of chalcogenide layers were distinguished. X-ray diffraction (XRD) was performed to characterize the microstructures of the chalcogenide thin-film samples. Accordingly, the feasibility of in-situ AC impedance spectroscopy to study the phase-transition kinetics of chalcogenides is discussed. EXPERIMENTAL DETAILS 150-nm thick pristine GST and Ce-GST layers were deposited on silicon (Si) wafer substrates in a sputtering system at a background pressure ≤ 2×10−6 torr. The Ce doping was
achieved by the target-attachment sputtering method rep
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