Coherent phonons in amorphous and crystalline Ge/sub 2/Sb/sub 2/Te/sub 5/ films

Summary form only given. Reversible phase change materials such as compounds of the pseudo-binary GeTe-Sb/sub 2/Te/sub 3/ system are applicable as high-density optical data storage media based on the relatively high difference in the reflection coefficients of the amorphous and the crystalline states. Although this principle is already employed in commercial DVD disks, the physics of the phase transitions is not fully understood. Previous investigations on the structure of Ge/sub 2/Sb/sub 2/Te by X-ray diffraction revealed a phase transition from the amorphous to a metastable crystalline cubic phase at approximately 140/spl deg/C and a further structural transition into a hexagonal phase near 260/spl deg/C. We investigate the structural properties of Ge/sub 2/Sb/sub 2/Te, by means of coherent phonon spectroscopy. The distinct phase transitions are monitored by changes of the phonon signatures detected in femtosecond time-resolved experiments at different sample temperatures.