Microscopic studies of fast phase transformations in GeSbTe films
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Microscopic studies of fast phase transformations in GeSbTe films Ralf Detemple, Inés Friedrich, Walter Njoroge, Ingo Thomas, Volker Weidenhof, Han-Willem Wöltgens, Stefan Ziegler, Matthias Wuttig I. Physikalisches Institut der RWTH-Aachen, 52056 Aachen, Germany
ABSTRACT Vital requirements for the future success of phase change media are high data transfer rates, i.e. fast processes to read, write and erase bits of information. The understanding and optimization of fast transformations is a considerable challenge since the processes only occur on a submicrometer length scale in actual bits. Hence both high temporal and spatial resolution is needed to unravel the essential details of the phase transformation. We employ a combination of fast optical measurements with microscopic analyses using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM measurements exploit the fact that the phase transformation from amorphous to crystalline is accompanied by a 6% volume reduction. This enables a measurement of the vertical and lateral speed of the phase transformation. Several examples will be presented showing the information gained by this combination of techniques.
INTRODUCTION Rewritable optical data storage plays a key role for multimedia applications and could also be an interesting alternative for the mass storage of data. Phase change recording is a promising technique for the field of optical data storage since it is conceptionally compatible with present CD technologies. The principle behind phase change recording is the reversible transformation of small bits of the active layer between the stable crystalline and the metastable amorphous phase. Writing of bits corresponds to the formation of small amorphous marks in a crystalline matrix whereas the recrystallization of the amorphous areas leads to the erasure of the bit. Amorphization is achieved by locally melting the film using a focused laser beam. With rates between 109 and 1010 K/s the melt cools down and is trapped in the amorphous phase (called “quenching”). Heating the bits to a temperature between the glass transition and the melting temperature leads to its fast recrystallization. The amorphous phase can be distinguished from the crystalline phase by their optical properties. Therefore data can be read with a low power laser beam monitoring the local changes in reflectance. One of the most important factors, which limits the applications for phase change-media like Ge2Sb2Te5 [1], is a better understanding of the underlying microscopic processes, which occur during the phase transformation from amorphous to crystalline or vice versa. To unravel the processes occurring during the phase transformation, a high lateral and depth resolution is needed. We have established a combination of two techniques, atomic force microscopy and experiments with a static tester, which were used to investigate the amorphous and crystalline phases of written bits.
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EXPERIMENTAL PROCEDURES GeSbTe-alloys were deposited on glass or silicon
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