Yen-Hao Shih

Understanding amorphous states of phase-change memory using Frenkel-Poole model

A method based on Frenkel-Poole emission is proposed to model the amorphous state (high resistance state) in mushroom-type phase-change memory devices. The model provides unique insights to probe the device after amorphizing (RESET) operation. Even when the resistance appears the same under different RESET conditions, our model suggests that both the amorphous region size and the defect states are different. With this powerful new tool, detailed changes inside the amorphous GST for MLC operation and retention tests are revealed.

Mechanisms of retention loss in Ge 2 Sb 2 Te 5 -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 (VT) 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.

Analytical model for RESET operation of Phase Change Memory

We present a novel analytical model for the RESET operation of Phase Change Memory (PCM) that explicitly describes the dependency of the programming current on various cell dimensions and material parameters. This model also explains, for the first time, the fundamental physics behind the inverse relationship between dynamic resistance(Rd) and the amplitude of the programming current.

The impact of hole-induced electromigration on the cycling endurance of phase change memory

The high current density induced failure in Ge2Sb2Te5(GST)-based phase change memory (PCM) is investigated. A strong dependence of cycling endurance on the polarity of the operation current is observed and reported for the first time. The cycling endurance is reduced by 4 orders of magnitude when the current polarity is reversed. Careful TEM analysis of failed cells revealed a thin void in GST over the bottom electrode, but only in the reverse polarity samples. This phenomenon can be explained by hole-induced electromigration at the electrode/GST interface. The impact of electromigration on scaled phase change memory is discussed.

Modeling the amorphous state of phase change memory

A method based on Frenkel-Poole emission is proposed to model the I–V data of the amorphous state (high resistance state) in mushroom-type phase-change memory (PCM) devices. We found the I–V characteristics in the high resistance state are dominated by (i) the size of the amorphous GST (aGST) volume and (ii) the trap density of the amorphized GST material. Neither (i) nor (ii) cant be resolved by the conventional read scheme, which uses a predetermined voltage to measure the PCM cells. We applied the analysis to study a cell with a thin phase change layer (25nm), the phenomena during RESET quenching time, and the resistance drifting at room temperature. With this new and powerful method, detailed changes inside the PCM cells are revealed.