Observation of Ge 2 Sb 2 Te 5 Thin Film Phase Transition Behavior According to the Number of Cycles Using Transmission E
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0961-O03-04
Observation of Ge2Sb2Te5 Thin Film Phase Transition Behavior According to the Number of Cycles Using Transmission Electron Microscope and Scanning Probe Microscope Hyunjung Kim1, Sikyung Choi1, Sukhoon Kang2, Kyuhwan Oh2, and Soonyong Kweon3 1 Dept. of Materials Science & Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea, Republic of 2 School of Material Science and Engineering, Seoul National University, San 56-1 Shillim-dong, Gwanak-gu, Seoul, 151-742, Korea, Republic of 3 Materials Science & Engineering, Chungju National University, 123, Geomdan-ri, Iryu-myeon, Chungju, 380-702, Korea, Republic of
ABSTRACT The phase transitions of a Ge2Sb2Te5 cell with a volume of 20 x 20 x 0.1 µm3 were carried out by applying a reset pulse (10 V and 50 ns) and a subsequent set pulse (5 V and 300 ns) using a homemade W heater tip fabricated by focused ion beam (FIB) lithography. The phase transformation from the crystalline state to the amorphous state was confirmed by measuring the I-V curves and the cross sectional transmission electron microscope (TEM) observation both before and after applying the reset pulse. The electron diffraction pattern obtained from the transformed area clearly showed the amorphous state. The resistance value of the transformed amorphous area was two orders higher than that of the original crystalline phase. The two order difference in the resistance value between the reset and set states was maintained for 20 reset/set pulse cycles. It is expected that this experimental set-up can be used to evaluate the fatigue behavior of Ge2Sb2Te5 cells with reset/set pulse cycles. INTRODUCTION Recently, wireless communication systems, such as mobile telephones, have grown remarkably. The systems require new memory devices with features that include low power consumption, fast read/write speeds, nonvolatile, and high densities. These requirements are a driving force to develop new nonvolatile memory devices. Phase change random access memory (PRAM) has been regarded as one of the most promising candidates [1] for wireless communication systems. The PRAM is based on the reversible phase transition between an amorphous state and a crystalline state of chalcogenide alloys. This phase transition occurs when an external electric current is applied [2]. Among the chalcogenide alloys [3], such as Ge2Sb2Te5 (GST), AgInSbTe, and so on, the GST film is considered to be the most suitable because it has excellent properties such as a fast crystallization speed, a minimal volume change during the crystallization, and stability of the amorphous phase at room temperature [2]. From the viewpoint of reliability, it is very important to understand the degradation behavior of the electrical property of GST film, i.e. a decrease in the resistance difference between the amorphous and the crystalline states with the phase transition cycles (hereafter: fatigue behavior). However, until now, there have been few studies on the fatigue behavior of GST films
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