Subthreshold electron transport properties of ultrathin film phase change material Ge2Sb2Te5

The electron transport properties of ultra-scaled phase change material Ge2Sb2Te5 (GST) are investigated in a subthreshold bias range. We used ab-initio molecular dynamics (AIMD) and non-equilibrium Green’s function (NEGF) transport formalism based on density functional theory (DFT). We calculate the conductance and current-voltage (I-V) curve of both crystalline (c-GST) and amorphous GST (a-GST). Our purely ab-initio simulations show that the conduction mechanism of ultra-scaled a-GST is different from that of c-GST. The current-voltage (I-V) curve of a-GST shows linear and exponential behavior. Both the bias induced variation of the transmission coefficients and the enlarging of bias window is responsible for the exponential shape of the I-V curve for a-GST. Whereas the linear part of the I-V curve is a consequence of the bias window enlarging. Moreover, it is revealed that the electron transport properties of ultra-scaled c-GST are dominated by metal-induced gap states (MIGS). The measured ON/OFF ratio and I-V curves are in good agreement with the similar experimental results. The findings of this paper would be useful in designing the ultra-scaled PCM devices based on GST and the designers should consider the difference in conduction mechanism of a-GST and c-GST as a potential reason for the different behavior of their I-V and conductance curve.