Ce Doped-GeSbTe Thin Films Applied to Phase-change Random Access Memory Devices
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1251-H03-05
Ce Doped-GeSbTe Thin Films Applied to Phase-change Random Access Memory Devices
Yu-Jen Huang, Min-Chuan Tsai, Chiung-Hsin Wang, Tsung-Eong Hsieh Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu, Taiwan 30010, R.O.C. ABSTRACT A study on microstructure and electrical property of cerium (Ce)-doped Ge2Sb2Te5 (GST) layers for phase-change memory (PCM) application were presented. Ce doping does not suppress the resistivity of amorphous GST and the resistivity ratio of amorphous and crystalline GST remains at about 105. Further, Ce-doping escalates the recrystallization temperature (Tx) of GST from 159 to 236°C. Such a unique behavior would greatly benefit the preservation of signal contrast as well as the high-density signal storage and will not cause the increase of device writing current. X-ray diffraction (XRD) indicated that Ce doping stabilizes amorphous GST and suppresses the formation of hexagonal phase. Transmission electron microscopy (TEM) revealed Ce doping refines the grain size of GST. Kissinger’s analysis found that Tx and activation energy ( E aexo ) of phase transition for doped-GST both increase with the increase of Ce content. Isothermal experiment found the Ce doping increases temperature for 10-yr data retention from 76 and 170°C. This is attributed to the presence of Ce solutes in GST matrix that inhibits the grain growth during recrystallization. Static-mode electrical test on PCM device containing doped GST as the programming layer found that Ce incorporation indeed increases the switching threshold voltage (Vth). This confirmed that Ce doping effectively retards the crystallization of GST and improves the stability of amorphous GST. INTRODUCTION PCM device utilizes chalcogenides as the programming layer and has been recognized as one of the promising candidates for next-generation non-volatile memories due to its advantages of low power consumption, high operation speed, high recording density and excellent scalability to nanometer cell sizes. Since the chalcogenide may have a relatively large resistivity difference (∼ 105 times) in between amorphous and crystalline states, PCM is also feasible to multi-state memory [1,2]. GeSbTe (GST)-based chalcogenides have been widely implanted in PCM due to their ultrafast, reversible phase-change characteristics. However, it is found that physical properties of pristine GST cannot totally satisfy the requirements of device performance, in particular in the issues regarding of the improvements on writing current, thermal stability and overwrite cyclability with the progress of device scale-down. In order to overcome those difficulties, new device structure designs and modifying the physical properties of GST are frequently proposed. Chemical composition adjustment of GST by doping various element, e.g., nitrogen (N) [3,4], oxygen (O) [5], silicon oxide (SiOx) [6], silicon (Si) [7], molybdenum (Mo) [8], tin (Sn) [9], silver (Ag) [10], etc., have been reported. Among these, nonmetallic N e
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