Effect of Ti diffusion on the microstructure of Ge2Sb2Te5 in phase-change memory cell.

The dependence of the microstructure of Ge2Sb2Te5 (GST) on Ti diffusion into GST by annealing in GST/Ti/TiN phase-change random access memory stack was studied by various transmission electron microscopy (TEM) techniques. The microstructure and crystal structure of GST were identified with high-resolution TEM (HRTEM) and image simulation technique, and the Ti diffusion into GST was revealed by scanning transmission electron microscope-energy-dispersive X-ray spectroscopy analysis. It was observed that Ti atoms of Ti/TiN thin layers were incorporated into GST cell through several thermal annealing steps and they could retard the phase transition from face-centered cubic (FCC) phase into hexagonal close-packed (HCP) phase partially and restrain the increase in grain size. Thus, it is concluded that Ti diffusion can affect the microstructure of GST including the type of the crystal phase and grain size of GST. It was shown that the insertion of diffusion barrier between TiN and GST could block Ti diffusion into GST and make it possible for FCC phase to completely transform into HCP phase.