Electronic Transport in Ge2Sb2Te5 Phase Change Memory Line Cells and Its Response to Electric-Field, Temperature, and Photoexcitation

Phase-change memory (PCM) is considered as one of the most promising non-volatile memory
technologies because of its long data retention capability, high endurance, high scalability, and fast speed making it an excellent storage-class memory solution in computer systems. The large resistivity contrast between amorphous and crystalline states of phase change materials like Ge2Sb2Te5 (GST) leads to potential implementation of multi-level cell (MLC) operation. But PCM devices suffer from resistance drift which causes the resistance in amorphous state to increase spontaneously over time, making it hard to implement MLC operation. We electrically characterized melt-quenched amorphized GST phase-change memory cells of 20 nm thickness, ~66 - 124 nm width and ~100 - 600 nm length with and without photoexcitation in 80 - 300 K temperature range. Resistance drift coefficients in the amorphized cells were calculated using constant voltage measurements starting as fast as within a second after amorphization and for about an hour duration under no photoexcitation. Drift coefficients range between ~0.02 and 0.1 with significant device-to-device variability and variations during the measurement period. At lower temperatures (higher resistance states) some devices show a complex dynamic behavior, with the resistance repeatedly increasing and decreasing significantly over periods in the order of seconds. These results point to charge trapping and de-trapping events as the cause of resistance drift. The cells also show distinctly different current-voltage (I-V) characteristics in the low-field (< ~19 MV/m), with a clear response to optical excitation, and highfield (> ~19 MV/m) regimes, with very weak response to optical excitation. The reduction in carrier activation energy with photoexcitation in the low-field regime increases from ~10 meV at 80 K to ~50 meV at 150 K (highest sensitivity) and decrease again to ~5 meV at 275 K. The heterojunctions formed at the amorphous-crystalline GST interfaces on both sides of the amorphous region lead to the formation of a potential well for holes and a potential barrier for electrons with activation energies in the order of ~0.7 eV at room temperature. The alignment of the steady state energy bands suggests formation of tunnel junctions at the interfaces for electrons and an overall electronic conduction by electrons. When photoexcited, the photo-generated holes are expected to be stored in the amorphous region, leading to positive charging of the amorphous region, reducing the barrier for electrons at the junctions and hence the device resistance in the low-field regime. Holes accumulated in the amorphous region are drained under high electric fields, hence the cells’ response to photoexcitation is significantly reduced. Also, such high-field stresses resulting in detrapping of all the trapped charges and draining out of the accumulated holes in the amorphous region leads to cell stabilization within minutes even at room temperature. These results support electronic origin of resistance drift in amorphous GST.