2, Natinal Institute of Advanced Industrial Science and Technology, Tsukuba, , Japan
PCRAM is a practical next generation non-volatile memory. Currently, Ge-Sb-Te compound (GST) is widely studied for PCRAM because of its fast phase change speed and excellent reversibility of phase transition. However, GST has a low crystallization temperature of around 160°C which limits data retention at high temperature and shows a high melting point of about 600°C which causes a high amorphization energy. Moreover, with further scaling down of PCRAM cell, thermal disturbance between cells becomes a serious problem. Therefore, it is desired to develop a new PCM which shows high thermal stability in amorphous phase and enables to lower amorphization energy (e.g., low melting point PCM, high electrical resistance PCM in crystalline state etc.)
From the above background, we have studied the effect of doping element, X on the crystallization temperature, Tx in amorphous Ge-Te. Based on the total bonding enthalpy calculation of amorphous X-Ge-Te, we found that transition metal doping is effective to increase the Tx of amorphous Ge-Te. Actually, transition metal doping such as V, Cr, Ni and Cu were experimentally confirmed to increase the Tx of amorphous Ge-Te.
As mentioned above, next generation PCM is desired to show not only high Tx, but also low melting point. Among various transition metal-Ge-Te chalcogenide, we are proposing Cu-Ge-Te compound as a new PCM. In the ternary system, there is a Cu2GeTe3 (CuGT) compound with a chalcopyrite-type structure. This compound shows a low melting point of around 500°C. Actually, CuGT shows better thermal stability in the amorphous state than GST and exhibits a fast reversible phase transition with an enough resistance contrast. Also, it was confirmed that CuGT has a lower amorphization energy than GST because of its low melting point. It is noteworthy that CuGT has smaller reflectance change and simultaneously, smaller density change (2~4%) upon phase change than GST. It was found from Hard X-ray photoemission spectroscopy that Cu d electrons are expected to play an important role during the phase change process. Moreover, we also pay attention to Cr-Ge-Te ternary system. In this system, there is a Cr2Ge2Te6 (CrGT) which is known to be a semiconductor compound. Therefore, this compound shows a high electrical resistance in the crystalline state. Interestingly, this compound shows an inverse resistance change upon phase change, i.e., low resistance amorphous and high resistance crystalline states. It was confirmed that CrGT shows a high thermal stability in amorphous state and exhibits a fast reversible phase change with an enough resistance contrast. These results suggest that transition-metal based PCM possessing d-electron bonding are good candidates for future PCM with high thermal stability, large resistance contrast, low density change and fast phase change. In this presentation, we will review the phase change behaviors of TM-Ge-Te (TM: Cu and Cr) compounds in viewpoint of PCRAM application.