Influences of partial melting and overheating on amorphization of Ge ₂Sb ₂Te ₅ during the reset process

It is very important to observe the change in the atomic structure of melt-quenched amorphous Ge ₂Sb ₂Te ₅ (GST) to discover the failure mechanism of phase-change random access memory (PRAM) devices during the reset process. We fabricated the phase-change devices and measured the initial resistance changes with various reset pulse amplitudes. For the observation of the atomic structure changes of GST thin films, we designed the specimens for transmission electron microscopy (TEM) and annealed them below and above the melting temperature of OST (T _m = 650 °C) before quenching. The annealed-and-quenched GST at 630 °C has a hexagonal structure with a grain size of about 10 nm after the partial melting. The annealed-and-quenched GST at 750 °C also has a hexagonal structure because of the latent heat. The observed atomic structures of GST are in accordance with the resistance changes inferred from the fabricated edge-contact type of PRAM cell.