Sub-nanosecond laser-induced structural changes in the phase change material Ge2Sb2Te5 measured by an optical pump/x-ray probe technique
Phase-change alloys are characterized by reversible switching between amorphous and crystalline phases either by laser irradiation or by an electric programming current; the resulting changes in material properties can be used for non-volatile data storage. Switching typically occurs on nanosecond or less time scales. Considering the conflicting requirements for high-speed switching, yet long term data storage integrity, a deeper understanding of the switching processes in these materials is essential for insightful application development. Although, high-speed optical pump/probe observations have been made of reflectivity changes during the Ge{sub 2}Sb{sub 2}Te{sub 5} switching process, due to the nanosecond order time scales involved little is known about the corresponding changes in structure. In addition as the amorphous phase does not diffract, its structural analysis is not amenable to analysis by high-speed diffraction techniques. We have used synchrotron-based time-resolved x-ray absorption fine structure spectroscopy (XAFS), a technique equally suitable for amorphous and crystalline phases to elaborate details in structural changes in the phase-change process. We report on two experiments using high-speed pulsed lasers that serve as optical pumps to induced material changes followed by synchrotron produced x-ray burst that serve as a time resolved structural probe. The first experiment carried out at the Advanced Photon source focuses on changes due to heating in the amorphous phase. Our experimental results indicate that the maximum temperature reached during the re-amorphization process are less than the melting point indicated in the bulk phase diagram of Ge{sub 2}Sb{sub 2}Te{sub 5} reaching a maximum temperature of 620 C and in addition, do not share the same bond length distribution of a true melt. These findings strongly suggest the possibility of non-thermal melting. In the second experiment, we have obtained near-edge x-ray absorption data for a Ge{sub 2}Sb{sub 2}Te{sub 5} film in the process of transforming from the crystalline to the amorphous phase. This was accomplished by use of a second re-crystallizing laser in addition to the amorphous state induced fast pulsed laser. In the experiment presented here a 500 ps, 532 nm high-power pump laser was focused to a spot size of 20 {micro}m via the silica substrate onto a 50 {micro}m thick Ge{sub 2}Sb{sub 2}Te{sub 5} layer with a power density of approximately 60 mJ/cm{sup 2}; the layer was capped to prevent oxidation by air. Concentric to the laser beam, a 1 x 2.5 {micro}m x-ray pulse generated from a single electron bunch in the storage ring at Spring-8 was directed to the front side side of the sample. The relative delay of the laser with respect to the x-ray pulse was varied in steps of 500 ps in the current experiment. The presence of a 30 nm thick (ZnS){sub 0.85}(SiO{sub 2}){sub 0.15} layer above the Ge{sub 2}Sb{sub 2}Te{sub 5} layer caused the Ge{sub 2}Sb{sub 2}Te{sub 5} located below the layer spot to transform from a crystalline starting phase to an amorphous high-temperature phase and then back to a crystalline phase due to the trapped heat.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1009030
- Resource Relation:
- Conference: European Symposium on Phase Change and Ovonics Science;September 1-3, 2007;Zermatt, Switzerland
- Country of Publication:
- United States
- Language:
- ENGLISH
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