Laser induced crystallization of amorphous Ge2Sb2Te5 films

The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by a tomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the cry stallization mechanism, the corresponding activation barrier, and the Avram i coefficient. On the minute time scale, thin amorphous films were isotherm ally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to b e (2.0 +/-0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature depend ent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron si ze were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized. For the formation of cryst alline spots in an as deposited amorphous film a minimum time of (100 +/- 1 0) ns was found, which is identified as the minimum incubation time for the formation of critical nuclei. In contrast, the complete crystallization of melt-quenched amorphous bits in a crystalline matrix was possible in 10 ns . This is attributed to the presence of quenched-in nuclei inside the amorp hous bits. The combination of optical measurements with atomic force micros copy reveals the formation and growth of crystalline bits and shows that th e crystal growth in vertical direction strongly affects the reflectivity ch anges. (C) 2001 American Institute of Physics.