What Makes Phase-Change Chalcogenide Alloys Materials of Choice for Optical Data Storage
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0918-H04-05
What Makes Phase-Change Chalcogenide Alloys Materials of Choice for Optical Data Storage Alexander Kolobov1,2, Paul Fons1, and Junji Tominaga1 1 CANFOR, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8562, Japan 2 LPMC CNRS UMR 5617, Université Montpellier II, Pl. E. Bataillon, Montpellier, 34095, France We discuss specific structural properties of commercially used Te-based multicomponent phase-change alloys that make them materials of choice for memory applications. In particular, we suggest that presence of longer and shorter bonds between similar kinds of atoms in the crystalline state and the (shorter) bond lengths being significantly longer that the sum of covalent radii of the corresponding atoms are crucial for efficient switching between the crystalline and amorphous state. INTRODUCTION The idea of phase-change recording dates back to the 1960s when S.R. Ovshinsky suggested [1] to use difference in electrical and optical properties between the crystalline and amorphous states of multicomponent chalcogenides for data storage. One of the latest industrial implications of phase-change recording is re-writable digital versatile discs (DVDs) first commercialized by Matsushita in the mid 1990s. The basic principle of phase-change optical recording is very simple: intense laser pulses melt the recording material that is subsequently quenched into the amorphous state. The recorded bits are thus amorphous areas against a crystalline background. To erase the recorded information, the recorded bits are heated by laser light to a temperature that is sufficient to induce crystallization. Various materials have been tried during the almost 40 years that followed the initial idea and only two groups of materials were found suitable for industrial applications, namely, Ge-Sb-Te (GST) alloys and primarily Ge2Sb2Te5 used in DVD-RAM and Ag-In-Sb-Te (AIST) alloys used in DVD-RW [2]. These materials allow for very fast switching to the amorphous state (pulses as short as hundreds picaseconds are sufficient), fast recrystallization (on the order of 30 ns), and high cyclability (over 1,000,000 cycles) [2]. It is very difficult to believe that an arbitrary material would reversibly and reproducibly melt and recrystallize so many times and one has to search for a more specific mechanism of the reversible amorphous-to-crystal phase transition. An important step forward was an observation that GST/AIST thin films crystallize into a "cubic" structure - rocksalt-like for GST [3] and A7 for AIST [4] - that is different from the stable structure (hexagonal in case of GST [5]). It was suggested that the high symmetry of the structure was the reason that the transition was fast [6].
RESULTS AND DISUSSION Crystal structure – shorter and longer bonds We have demonstrated recently that the local arrangement of atoms in Ge2Sb2Te5 is somewhat different from the rocksalt structure. While the Te atoms do indeed form a well ordered face-centered cubic (fcc) lattice, the Ge and Sb atoms are located off center and there are two kinds of G
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