Ion Irradiation on Phase Change Materials
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Ion Irradiation on Phase Change Materials Emanuele Rimini1, Egidio Carria2,3, Antonio Massimiliano Mio2, Maria Miritello3, Santo Gibilisco2,3, Corrado Bongiorno1, Giuseppe D’Arrigo1, Corrado Spinella1, Francesco D’Acapito4, Maria Grazia Grimaldi2,3. 1 IMM-CNR, Catania, Italy. 2 Dipartimento di Fisica e Astronomia, Università di Catania, Italy. 3 MATIS-IMM-CNR, Catania, Italy. 4 ESRF GILDA CRG, CNR, IOM OGG, F-38043 Grenoble, France ABSTRACT Ion irradiation with 130 keV Ge+ or 120 keV Sb+ has modified, by thermal spike effect, the local atomic arrangement in the structure of as-deposited sputtered amorphous GeTe and Ge2Sb2Te5 thin films. The changes in the local order have been analyzed by Raman and EXAFS spectroscopy. In addition the crystallization kinetic, measured by “in situ” time resolved reflectivity and optical microscope analysis, is found to be faster in the irradiated samples. The nucleation rate and the grain growth velocity are enhanced by a factor of about ten with respect to the unirradiated samples in the investigated temperature range ( 120°C -170°C). INTRODUCTION Phase-Change Materials (PCMs), mainly represented by GeTe-Sb2Te3 alloys, are used for highdensity data storage in optical media and for solid-state non volatile memory [1-2]. The working principle of these devices is based on the change of the optical and electrical properties during the switch from the amorphous to the crystalline phase and vice versa. Two states are distinguishable by the pronounced difference in reflectivity (up to 30%) and resistivity (some orders of magnitude) between these two phases. Amorphous structure, of relevance for the crystallization kinetics and for the stability of the recorded data, depends on the method adopted for the preparation. Different crystallization behaviour has been found between as deposited, melt quenched and ion irradiated amorphous structures, indicating different local order arrangements [3-4-5]. Among the several procedures ion irradiation offers a unique and well reproducible method to create an amorphous structure. In addition, atomic displacements, bond breaking and bond formation caused by the collision cascade event provide information on the rearrangement in a glassy system as it is a chalcogenide alloy. In this work we report data on the crystallization kinetics of as-deposited and ion irradiated GeTe and Ge2Sb2Te5 thin films to be correlated with the local order analyzed by micro-Raman and EXAFS spectroscopy. EXPERIMENTAL DETAILS Thin GeTe and Ge2Sb2Te5 films, 50 nm thick, were deposited at room temperature by RFmagnetron sputtering over a thermally grown 550 nm thick SiO2 layer on a Si substrate. Some samples were then covered with a gold circular mask (thickness ~ 25μm and diameter ~ 3mm). Irradiation of as-deposited amorphous films was performed at RT with 130 keV Ge+ or 120 keV Sb+ at fluence in the range 1x1014 – 1x1015 ions/cm2. The samples were implanted through the mask to make more reliable the comparison between irradiated and unirradiated regions. The beam current was kep
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