Mechanisms of retention loss in Ge2Sb2Te5-based Phase-Change Memory

Data retention loss from the amorphous (RESET) state over time in Phase-Change Memory cells is associated with spontaneous crystallization. In this paper, the change in the threshold voltage (V_T) of memory cells in the RESET state before and after heating is used as a probe into the nature of the retention loss mechanisms. Two mechanisms for the retention loss behavior are identified, responsible for the main distribution and the tail distribution, respectively. Experimental results suggest that (i) an optimized RESET operation produces a fully amorphized Ge₂Sb₂Te₅ (aGST) active region, with no crystalline domains inside, (ii) cells in the tail distribution fail to retain their RESET state due to spontaneous generation of crystallization nuclei and grain growth, and (iii) cells in the main distribution fail due to grain growth from the amorphous/crystalline GST boundary, instead of nucleation within the active region.