Influence of the additive Ag for crystallization of amorphous Ge-Sb-Te thin films

We have investigated the optical and amorphous-to-crystalline transition properties in four-types of chalcogenide thin films; Ge2Sb2Te5, Ge8Sb2Te11, Ag-Ge2Sb2Te5 and Ag-Ge8Sb2Te11. Crystallization was caused by nano-pulse illumination (λ = 658 nm) with power (P) of 1-17 mW and pulse duration (t) of 10-460 ns, and the morphologies of crystallized spots were observed by SEM and microscope. It was found that the crystallized spot nearby linearly increases in size with increasing the illuminating energy (E = P · t) and eventually ablated out by over illumination. Changes in the optical transmittance of as-deposited and annealed films were measured using a UV-vis-IR spectrophotometer. In addition, a speed of amorphous-to-crystalline transition was evaluated by detecting the reflection response signals for the nano-pulse scanning. Conclusively, the Ge8Sb2Te11 film has a faster crystallization speed than the Ge2Sb2Te5 film despite its higher crystallization temperature. The crystallization speed was largely improved by adding Ag in Ge2Sb2Te5 film but not in Ge8Sb2Te11 film. To explain these results, we considered a heat confinement by electron hopping.