Modeling of Threshold Voltage Drift in Phase Change Memory (PCM) Devices

The stability of the amorphous phase in phase- change-memory (PCM) devices can be affected by temperature-activated crystallization and resistance drift due to structural relaxation (SR). While amorphous chalcogenides are relatively stable with respect to crystallization, thanks to a high activation energy of about 2.5 eV, the lower activation energy of SR can result in a change of the electronic properties of the PCM, including band-gap, resistance and threshold voltage VT for threshold switching. This work presents a physics- based model for VT drift based on physical models for electrical conduction, threshold switching and SR. We derive an analytical law relating the drift slopes of resistance and VT through the sub- threshold slope (STS) of the I-V curve. A numerical model is then presented, capable of predicting the time evolution of VT for reset and intermediate states in PCM multilevel cell (MLC).