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Unformatted text preview: Crystal morphology and nucleation in thin films of amorphous Te alloys used for phase change recording J. A. Kalb, a ! C. Y. Wen, and Frans Spaepen b ! Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts H. Dieker and M. Wuttig I. Physikalisches Institut der Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen, 52056 Aachen, Germany s Received 28 April 2005; accepted 20 July 2005; published online 6 September 2005 d Ex situ transmission electron microscopy s TEM d was used to study the crystal morphology in sputtered amorphous Ge 4 Sb 1 Te 5 , Ge 2 Sb 2 Te 5 , and Ag 0.055 In 0.065 Sb 0.59 Te 0.29 thin films used for phase change recording. Tilting of plan view samples revealed that each crystallized growth formation is a bent single crystal. Cross-sectional TEM showed that crystals only nucleate heterogeneously at the s naturally oxidized d film surface. These findings allow the determination of nucleation parameters around 150 °C from earlier experiments f J. Kalb, F. Spaepen, and M. Wuttig, Appl. Phys. Lett. 84 , 5240 s 2004 dg . The time lag for nucleation has an activation energy of s 2.74±0.13 d eV for Ge 2 Sb 2 Te 5 and s 2.33±0.18 d eV for Ag 0.055 In 0.065 Sb 0.59 Te 0.29 . The activation energies for the steady-state nucleation rate were s 4.09±0.20 d eV for Ge 4 Sb 1 Te 5 and s 3.50±0.17 d eV for Ge 2 Sb 2 Te 5 . With the activation energy for the crystal-growth velocity found in the earlier article the critical work for formation of the nucleus was found to be s 1.35±0.23 d eV for Ge 4 Sb 1 Te 5 and s 1.15±0.22 d eV for Ge 2 Sb 2 Te 5 . These values are lower limits for homogeneous nucleation. © 2005 American Institute of Physics . f DOI: 10.1063/1.2034655 g I. INTRODUCTION Tellurium alloys are used for phase change recording in commercial rewritable storage devices. In rewritable s RW d compact disks s CDs d and digital video disks s DVDs d , a thin film of a Te alloy is locally and reversibly switched by laser heating between the amorphous and the crystalline state. These states can be distinguished optically by their differ- ence in reflectivity. 1 Recently, Te alloys have also shown high potential for future electronic nonvolatile data storage. 2 In these so-called phase change random access memories s RAM d , electrical power provides the heat that is necessary for transformations between amorphous and crystalline states, which can be distinguished subsequently by their pro- nounced difference in electrical conductivity. 2–5 To meet future requirements for multimedia applications of such phase change media, a higher data transfer rate is required. This can only be achieved by accelerating the time- limiting factor, which is the crystallization process in both optical and electronic media. Therefore, it is essential to un- derstand crystallization kinetics of phase change alloys. For optical data storage, depending on the composition of the alloy, two mechanisms of recrystallization have been ob-...
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