Properties of Phase Change Materials Modified by Ion Implantation
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Properties of Phase Change Materials Modified by Ion Implantation Simone Raoux, Guy M. Cohen, Marinus Hopstaken, Siegfried Maurer, and Jean L. Jordan Sweet IBM T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, New York 10598, USA ABSTRACT Ion implantation of germanium and carbon ions into thin films of Ge2Sb2Te5 (GST) and GeTe was applied to modify the properties of these phase change materials. It was found that it is possible to amorphize crystalline GST and GeTe using ion implantation for optimized ion doses and energies which depend on the film thickness, ion species and capping layer. A relatively low minimum dose is required for complete amorphization as judged by the absence of diffraction peaks in x-ray diffraction (XRD) scans. It is 4-5x1013 cm-2 for germanium implantation into GST, and slightly higher (1014 cm-2) for germanium implantation into GeTe. The properties of the re-amorphized films depend on ion species, dose and energy. The re-crystallization temperature of re-amorphized GST by ion implantation is comparable or higher than asdeposited amorphous GST. Carbon implantation in particular leads to a large increase in the crystallization temperature Tx. A carbon dose of 1016 cm-2 implanted into 20 nm amorphous GST yielded a crystallization temperature of 300 ºC, much higher than the crystallization temperature of 160 ºC we recorded for as-deposited, amorphous GST. Similarly, high dose carbon implantation into amorphous GeTe leads to large increase in Tx. We recorded a shift in Tx from 195 ºC for as-deposited GeTe to 400 ºC for C-implanted GeTe. Crystalline GeTe re-amorphized by a low dose germanium ion implantation exhibits a re-crystallization temperature below Tx of as-deposited amorphous GeTe and Tx increased with the implanted Ge dose to a crystallization temperature above that of unimplanted GeTe. Ion implantation can be regarded an additional tool to create phase change materials with different and improved switching properties that cannot be achieved by conventional sputter deposition. INTRODUCTION Phase change materials exist in a metastable amorphous phase and a stable crystalline phase (and in some cases additional metastable crystalline phases) and can be switched between the phases repeatedly and in very short times on the nanosecond scale [1, 2]. The difference between the optical and electrical properties of the phases can be used to store information [3]. Phase change materials are applied in optical re-writable media [1, 4]. This technology has been very successful, 3rd generation storage media (re-writable Blu-ray discs) can now be found on the market with high storage capacity (25 GB per layer) and products with up to 100 GB can be expected soon [5]. The relatively large difference in optical reflectivity (about 30%) of the phases is the storage principle. In optical media the switching from the amorphous to the crystalline phase is achieved by a laser pulse that heats a spot on the disk above its crystallization temperature. Switching back to the amorphous phase is done
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