Exploring the limits of fast phase change materials
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Exploring the limits of fast phase change materials Han-Willem Wöltgens, Ralf Detemple, Inés Friedrich, Walter K. Njoroge, Ingo Thomas, Volker Weidenhof, Stefan Ziegler, Matthias Wuttig I. Physikalisches Institut der RWTH-Aachen, D-52056 Aachen, Germany Abstract In the last decade a number of chalcogenide alloys, including ternary alloys of GeSbTe and quaternary alloys of InAgSbTe, have been identified which enable fast phase change recording. In the quest for materials with improved phase change kinetics we present two different approaches. By comparing alloys with well-defined stoichiometries the mechanisms which govern the transformation kinetics are determined. Optical and electrical measurements determine the activation energy for crystallization to 2.24 ± 0.11 eV for Ge2Sb2Te5 and to 3.71 ± 0.07 eV for Ge4Sb1Te5, respectively. It is shown that for GeSbTe-alloys with different composition the activation energy increases linearly with increasing Ge content. Power-timereflectivity change diagrams recorded with a static tester reveal that Ge2Sb2Te5, in agreement with previous data, recrystallizes by the growth of sub critical nuclei, while Ge4Sb1Te5 grows from the crystalline rim surrounding the bit. To speed up the search for faster materials we employ concepts of combinatorial material synthesis by producing films with a stoichiometry gradient. Then laterally resolved secondary neutral mass spectroscopy (SNMS) combined with the static tester are used to identify the composition with superior properties for phase change applications. INTRODUCTION Materials at the pseudo-binary line (GeTe)x(Sb2Te3)1-x are known to have good properties for fast phase change applications [1,2]. Nevertheless we need to characterize and understand the transformation kinetics of these materials if we want to develop a microscopic understanding of phase change dynamics. Hence, we have investigated the phase transformation on a microscopic and macroscopic scale for two compounds along the pseudo-binary line. From the understanding of the transformation kinetics for these compounds we try to obtain insight into the behaviour expected for an arbitrary compound like AαBβCγTe. Then combinatorial concepts in conjunction with efficient strategies to measure transformation kinetics are applied to further improve our understanding of the kinetics and the properties of phase change materials. EXPERIMENTAL DETAILS The GeSbTe-compounds used for microscopic and macroscopic studies were deposited on glass or silicon substrates at room temperature by dc magnetron sputtering [3]. A static tester as described in [4] is used to measure the reflectance change upon short laser pulse irradiation. The sheet resistance was measured with a four-point probe setup following the procedure proposed by van der Pauw [5]. Further experimental details are already described elsewhere [6]. x-ray refractometry (XRR) measurements were performed to determine the thickness, roughness and the density while x-ray diffractometry (XRD) measurements were used to determine the fi
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