Research On Crystallization Of The Phase Change Material Ge₂Sb₂Te₅Irradiated By Laser

Information technology is closely related with national economy. As technologyadvances, phase-change optical storage becomes increasingly more important. Despitethe phase-change optical memory develops quickly these years, phase changemechanism is still very vague, due to the ultra-rapid nanosecond transformationprocess. Nowadays in addition to the lack of experimental data, theoreticalcalculations often are not uniform with experimental data. Therefore, the phasechange mechanism of Ge2Sb2Te5(GST) and new phase-change material still need tofurther research, which not only can provide the phase change mechanism of phasechange material, but also can optimize the phase change material and promote thedevelopment of the new phase change material with high properties by enhancing theunderstanding of the phase change process. The amorphous Ge2Sb2Te5(α-GST) filmsin this paper were prepared by magnetron sputtering stoichiometric GST target. Themain research content is to show and analyze the phenomena of α-GST filmsirradiated by two different lasers. Then by adjusting film thickness and laser fluence,the prepared α-GST films were irradiated by different laser.When80and30nm α-GST films were irradiated by the KrF excimer laser with apulse width of30ns, crystallization behaviors of α-GST films with the thickness of80and30nm were investigated by using TEM integrated with SAED. Firstly, crystallinephase morphologies were shown and analyzed. Both plate grains and spherical grainswere found for80nm thick films, while only plate grains were found for30nm thickfilms. Then the relationship between the grain size and laser fluence for the80nmthick film was studied and the effects of film thickness on crystallization morphologywere analyzed. Finally a crystallization process model based on the relationship of thecrystallization starting temperature and heating temperature was constructed toelucidate how the solid-phase crystallization and melt-cooling crystallization occurredand developed. It also unified the crystallization process under static conditions andlaser inducing conditions.When α-GST films were irradiated by the Nd:YVO4laser with a pulse width of15ps, the morphology and crystalline phase characteristics of α-GST films wereinvestigated by3D surface profiler, atomic force microscopy (AFM) and transmissionelectron microscopy (TEM) integrated with selected area electron diffraction (SAED).The laser irradiated spot was divided into strong ablation area, gentle ablation area,melting area and irradiation area. By theoretical calculation, the ablation and meltingthresholds were determined to be173.05mJcm-2and99.19mJcm-2respectively. Meantime, the local fine morphologies of the ablation and melting areas were shownand analyzed. We also studied the phase-change area which belonged to theirradiation area. In the phase-change area, crystalline phase was determined to beface-centered cubic structure and different crystalline phase characteristics for filmswere discussed from the perspective of effects of the film thickness and the beamshape.Finally, the relationship of laser parameters，film thickness and microstructuremorphology was established by combining the experimental microstructuremorphologies under different conditions, and the evolution of microstructuremorphology was explained from a theoretical point of view. This is important not onlyfor theoretical significance but also for practical guidance significance.