Manipulation of amorphous Ge 2 Sb 2 Te 5 nano-structures in isolated and crystalline environment.
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Manipulation of amorphous Ge2Sb2Te5 nano-structures in isolated and crystalline environment. A. M. Mio1, G. D’Arrigo2, E. Carria1,3, C. Bongiorno2, S. Rossini4, C. Spinella2, M. G. Grimaldi1,3, E. Rimini2 1
Dipartimento di Fisica e Astronomia, Università di Catania, 64 via S. Sofia, I-95123 Catania, Italy 2 IMM-CNR, 50 Stradale Primosole, I-95121 Catania, Italy 3 MATIS-IMM-CNR, 64 via S. Sofia, I-95123 Catania, Italy 4 Micron Semiconductor Italia S.r.l. ABSTRACT We have investigated the stability of nano-amorphous region of Ge2Sb2Te5 (GST), fabricated by Electron Beam Lithography (EBL), dry etching, and ion implantation. Nano-structures, less than 100 nm in diameter and 20 nm thick, were either embedded in a crystalline environment or just isolated. We have observed nano-structure crystallization by in situ Transmission Electron Microscopy (TEM) in the 75°C-150°C temperature range. Re-crystallization of amorphous dots embedded in a crystalline region (either in the cubic or hexagonal phase) occurs by the movement of the interface at relatively low temperature (about 90°C). Instead, in the isolated structures the transition occurs at about 145°C by nucleation and growth. These results might be of relevance for the data retention of sub-50nm devices. Indeed, the more stable amorphous phase in self-standing regions indicates the better retention properties of isolated cells with respect to the traditional mushroom cell configuration. INTRODUCTION Chalcogenide materials are used as storage media for optical memory disks and are good candidate for non-volatile Phase Change Random Access Memory (PCRAM). Studies so far have shown that Ge2Sb2Te5 (GST) has the best combination of electric and phase changing characteristics for PCRAM applications [1-3]. It is known that, in planar amorphous thin films, the phase transition of as-deposited amorphous Ge-Sb-Te alloys proceeds, during annealing, to the metastable cubic phase (130°C) and to the stable hexagonal structure (350°C) by nucleation and growth [1,4,5]. However, as the dimension of the PCRAM-based prototypal devices is of the order of several tens of nanometers [6], it is of relevant importance to study the stability of the amorphous phase in a geometrical configuration comparable to this size. Another important issue is that recrystallization of the amorphous region (SET) in the cell memory takes place in presence of a surrounding crystalline phase, usually the hcp crystal phase, that can further compromise the amorphous state. Ion implantation has been shown to be a powerful tool to produce well-reproducible amorphization in phase change materials [7], with characteristics that are very close to the meltquenched amorphous phase, distinctive of the PCM-based devices. In the present work, we show the amorphous to crystal transition of GST, by in situ Transmission Electron Microscopy (TEM), of nano structured regions either in presence of its crystalline phases or in an isolated environment.
EXPERIMENT The Ge2Sb2Te5 (GST) amorphous films, 20 nm thick, were prepared by r
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