Atomic force microscopy study of laser induced phase transitions in Ge2Sb2Te5

Micron- and submicron-size changes induced by local laser heating in thin films of ${\mathrm{Ge}}_{\mathrm{2}}{\mathrm{Sb}}_{\mathrm{2}}{\mathrm{Te}}_{\mathrm{5}}$ are studied by atomic force microscopy (AFM). This material is presently used for rewritable data storage that employs phase change recording. Reversible switching between the amorphous and crystalline states, which is accompanied by a considerable change in optical properties and film density, is exploited to store bits of information. The density change can be detected by AFM, which we use here to study the amorphization (writing) and recrystallization (erasure) of single bits. Both processes have been measured as a function of modification pulse power and duration. Morphology changes can be detected even if the phase change film is covered by a thin protective layer. AFM allows a precise determination of the bit size and bit depth, which characterizes the progress of the phase change in the direction of the surface normal. The present setup allows the correlation of the change in optical reflectance $\mathrm{\Delta R}$ caused by a specific laser pulse to the bit topography. This enables a detailed study of the mechanisms of bit writing and erasure.