Study of the Structural Transformation of Ge 2 Sb 2 Te 5 Induced by Current Pulse in Phase Change Memory
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Study of the Structural Transformation of Ge2Sb2Te5 Induced by Current Pulse in Phase Change Memory Rong Zhao1, Tow Chong Chong1, Lu Ping Shi1, Pik Kie Tan1, Hao Meng2, Xiang, Hu1, Ke Bin Li1 and An Yan Du3 1 Data Storage Institute, A*Star, Singapore 117608 2 Department of Electrical & Computer Engineering, University of Minnesota, MN 55455, USA 3 Institute of Microelectronics, A*Star, Singapore 117685 ABSTRACT The electrical induced structural transformation of Ge2Sb2Te5 thin film in phase change memory device was investigated using micro-Raman spectroscopy and transmission electronic microscopy (TEM). Selected area electron diffraction (SAD) pattern showed that the electricalinduced Ge2Sb2Te5 film was crystallized into a face-centered cubic structure. Micro-Raman spectra show that the Ge2Sb2Te5 active layer at the high resistance state exhibited two minor peaks superposed on the broad peak after several switch cycles, which is identical to those of the Ge2Sb2Te5 active layer at the low resistance state. This is most likely due to the accumulation of segregated crystallites. TEM results suggest that the existence of nano-sized nuclei clusters resulted in the reduced resistance for the Ge2Sb2Te5 active layer at the high resistance state after first several switches. The dependence of resistance on the cycle number indicates that the deterioration of the Ge2Sb2Te5 active layer is resulted from the incomplete amorphization process, which is consistent with the micro-Raman results. INTRODUCTION In the last decade, chalcogenide materials have been widely investigated. Among the most important applications are optical rewritable disk and phase change memory, which utilize chalcogenide material’s switching properties to store data. In optical rewritable disk, laser pulses cause the switching of chalcogenide materials between amorphous and crystalline states. The reflectivities for the two states are different and detected for data storage application. In phase change memory, the switches between the amorphous and crystalline states are induced by electrical pulses. Chalcogenide material at amorphous state has a higher resistivity while at crystalline state has a lower resisitivity. Among the chalcogenide materials, GeSbTe system is the most widely applied phase change alloy for both CRAM devices and optical rewritable disk, and Ge2Sb2Te5 is one of the optimal composition ratios. The laser-induced structural transformation of Ge2Sb2Te5 has been widely investigated [1-2]. However, the electrical-induced structural transformation has not been fully studied. Micro-Raman spectroscopy constitutes a powerful probing technique for structural characterization of materials in a small region. Raman analysis has been carried out for the thermal and laser induced transformation for GeSbTe thin films [3-5]. In this paper, we studied the structural transformation of Ge2Sb2Te5 thin films caused by electrical current pulses in phase change memory by micro-Raman spectroscopy and transmission electronic microscopy (TEM). The struct
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