A. Portavoce
Abstract Atomic-scale kinetic Monte Carlo simulations based on the GST * model were performed aiming to simulate the microstructure evolution of a 50 nm-wide phase-change memory cell based on a Ge-rich Ge–Sb–Te (GST) alloy during SET/RESET cycling. Simulations were performed considering two cases: a stoichiometric GST ternary phase and a non-stoichiometric GST ternary phase. The results considering a stoichiometric GST phase are found to be in better agreement with experiments: Ge is pushed out of the active region where the solid/liquid transition occurs during cycling. After a cycling time threshold, the active region adopts the shape of a dome, following the radial temperature gradient imposed by the bottom electrode playing the role of the heating element at the bottom of the cell. During cycling, Ge gets accumulated at the side of the active dome, while the binary compound GeTe can form on top of the dome. Concurrently, Ge and GST grains grow and coalesce in the crystalline part of the cell surrounding the active region. For long cycling times, the GST phase segregates at the border of the cell, while Ge grains tend to be confined in the center of the cell. The use of a stoichiometric GST seed layer in the top region of the cell prevents the top electrode to be in contact with Ge during cycling. Assessment of the cell resistance in the RESET and SET states show that the cell resistance is mainly dependent on the amorphous dome properties at the RESET state, while the cell resistance at the SET state is dependent on the conductivity difference between the Ge grains (Ge doping level) and the GST grains, as well as on the coalescence of these grains in the non-active region of the memory cell.