Crystal morphology and nucleation in thin films of amorphous Te alloys used for phase change recording

Abstract

Ex situ transmission electron microscopy (TEM) was used to study the crystal morphology in sputtered amorphous Ge4Sb1Te5, Ge2Sb2Te5, and Ag0.055In0.065Sb0.59Te0.29 thin films used for phase change recording. Tilting of plan view samples revealed that each crystallized growth formation is a bent single crystal. Cross-sectional TEM showed that crystals only nucleate heterogeneously at the (naturally oxidized) film surface. These findings allow the determination of nucleation parameters around 150°C from earlier experiments [J. Kalb, F. Spaepen, and M. Wuttig, Appl. Phys. Lett. 84, 5240 (2004)]. The time lag for nucleation has an activation energy of (2.74±0.13)eV for Ge2Sb2Te5 and (2.33±0.18)eV for Ag0.055In0.065Sb0.59Te0.29. The activation energies for the steady-state nucleation rate were (4.09±0.20)eV for Ge4Sb1Te5 and (3.50±0.17)eV for Ge2Sb2Te5. With the activation energy for the crystal-growth velocity found in the earlier article the critical work for formation of the nucleus was found to be (1.35±0.23)eV for Ge4Sb1Te5 and (1.15±0.22)eV for Ge2Sb2Te5. These values are lower limits for homogeneous nucleation.