Crystallization mechanism and kinetics of Cr 2 Ge 2 Te 6 phase change material
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Research Letter
Crystallization mechanism and kinetics of Cr2Ge2Te6 phase change material S. Hatayama, Y. Sutou, D. Ando, and J. Koike, Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-11 Aoba-yama, Sendai 980-8579, Japan Address all correspondence to Y. Sutou at [email protected] (Received 30 May 2018; accepted 13 August 2018)
Abstract The crystallization mechanism and kinetics of Cr2Ge2Te6 (CrGT) films were investigated by differential scanning calorimetry. The average Avrami exponent (na) analysis indicated that CrGT exhibits a growth-dominant crystallization in the range of heating rate (β) of 10–50°C/min. In comparison, Ge2Sb2Te5 (GST) showed a nucleation-dominant crystallization. The na of CrGT was about 3, and was majorly independent of β. The na of GST decreased with an increasing β, which asymptotically approached a value of around 3. The kinetic constant of CrGT was evaluated to be almost the same with that of GST, indicating that CrGT undergoes fast crystallization.
Introduction Phase change random access memory (PCRAM) has been used as a storage class memory for filling the performance gap between dynamic random access memory and data storage systems such as solid-state drive and hard disk drive.[1] The principle of PCRAM relies on the electrical contrast of phase change material (PCM) that generally has interchangeable amorphous and crystalline phases.[2,3] The amorphous and crystalline phases have high and low resistance values, respectively. The phase transition between two phases is achieved by applying electrical pulses, causing Joule heating. Amorphization is achieved via using high-amplitude and short-duration electrical pulses, while crystallization is achieved using low-amplitude and long-duration electrical pulses.[3] Ge-Sb-Te (GST) compounds are well known as typical PCMs, and they exhibit a fast crystallization process and excellent reversibility of phase transition.[2,4] The amorphous GST crystallizes to a crystalline phase having a metastable fcc structure. By further heating, the crystal structure changes from an fcc to stable hexagonal structures.[4] GST, however, possess a low crystallization temperature of about 150°C and their amorphous phase, thus, exhibits a low thermal stability.[4] Such a low thermal stability in the amorphous phase of GST causes poor data-retention ability at a high temperature. Because thermal disturbance between cells cannot be ignored in highly scaled memory arrays, low thermal stability also prevents further scaling of memory arrays.[5] Therefore, development of alternative PCMs having a high thermal stability in their amorphous phase also exhibits a high crystallization temperature is necessary. Most recently, we found that a Cr-Ge-Te compound, Cr2Ge2Te6 (CrGT), exhibits a phase change between amorphous
and crystalline phases.[6] The amorphous CrGT crystallizes to a crystalline phase having a rhombohedral R3 symmetry. Its crystallization temperature is about 270°C, which is much higher than that of GST.[6]
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