Direct observation of amorphous to crystalline phase transitions in nanoparticle arrays of phase change materials

We have used time-resolved x-ray diffraction to study the amorphous-crystalline phase transition in 20-80 nm particles of the phase change materials Ge2 Sb2 Te5, nitrogen-doped Ge2 Sb2 Te5, Ge15 Sb85, Sb2 Te, and Sb2 Te doped with Ag and In. We find that all samples undergo the phase transition with crystallization temperatures close to those of similarly prepared blanket films of the same materials with the exception of Sb2 Te that shows the transition at a temperature that is about 40 °C higher than that of blanket films. Some of the nanoparticles show a difference in crystallographic texture compared to thick films. Large area arrays of these nanoparticles were fabricated using electron-beam lithography, keeping the sample temperatures well below the crystallization temperatures so as to produce particles that were entirely in the amorphous phase. The observation that particles with diameters as small as 20 nm can still undergo this phase transition indicates that phase change solid-state memory technology should scale to these dimensions. © 2007 American Institute of Physics.