2, Eskisehir Osmangazi University, Eskisehir, , Turkey
3, Analog Devices, Norwood, Massachusetts, United States
4, IBM Watson Research Center, Yorktown Heights, New York, United States
In phase-change memory (PCM), information is stored in a high resistive amorphous or low resistive crystalline phase of a nano-scale volume of a phase-change material. The transition times between amorphous and crystalline states (~50 ns for reset and ~100 ns for set) determine the speed of the memory operation . As crystallization can occur in nanoseconds, rapid measurements of resistance immediately after amorphization are of utmost importance. Our previously reported fast measurements (in μs timescale) of temperature dependent amorphous resistance shows metastable resistivity of amorphous Ge2Sb2Te5 (GST) – the most widely used material for PCM . Interestingly, the metastable amorphous resistivity versus temperature shows a pure exponential behavior, and the thin film molten resistivity falls on the same exponential. Considering Arrhenius behavior of conductivity, this pure exponential decrease in resistivity implies a temperature dependent carrier activation energy for metastable amorphous GST following a quadratic relation. The extracted activation energy has a peak near 450 K. The effective activation energy is also used to calculate the effective trap energy during device operation. Also, by mapping the metastable amorphous activation energy to versus activation energy for mixed phase crystalline GST , we estimate Seebeck coefficient (S) for metastable amorphous GST – which is difficult to directly measure at device level because of the requirement to maintain a small known temperature gradient during the measurement. The temperature dependent conduction activation energy and Seebeck coefficient for metastable amorphous GST extracted in this work are critical parameters for accurate modeling of PCM devices .
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