Evolution of the Transrotational Structure During Crystallization of Amorphous Ge 2 Sb 2 Te 5 Thin Films
- PDF / 593,032 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 220 Views
1160-H12-08
Evolution of the transrotational structure during crystallization of amorphous Ge2Sb2Te5 thin films E. Rimini1,2, R. De Bastiani1,3, E. Carria1,3, M. G. Grimaldi1,3, G. Nicotra2, C. Bongiorno2, C. Spinella2 1
Dipartimento di Fisica ed Astronomia, Università di Catania, 64 via S. Sofia, I-95123 Catania, Italy 2 CNR-IMM, Stradale Primosole, 50, 95121 Catania, Italy 3 MATIS CNR-INFM ABSTRACT The crystallization of amorphous Ge2Sb2Te5 thin films has been studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The analysis has been performed on partially crystallized films, with a surface crystalline fraction (fS) ranging from 20% to 100%. XRD analysis indicates the presence, in the partially transformed layer, of grains with average lattice parameters higher than that of the equilibrium metastable cubic phase (from 6.06 Å at fS=20% to 6.01 Å at fS=100%). The amorphous to crystal transition, as shown by TEM analysis, occurs through the nucleation of face-centered-cubic crystal domains at the film surface. Local dimples appear in the crystallized areas, due to the higher atomic density of the crystal phase compared to the amorphous one. At the initial stage of the transformation, a fast bi-dimensional growth of such crystalline nucleus occurs by the generation of transrotational grains in which the lattice bending gives rise to an average lattice parameter significantly larger than that of the facecentered-cubic phase in good agreement with the XRD data. As the crystallized fraction increases above 80%, dimples and transrotational structures start to disappear and the lattice parameter approaches the bulk value. INTRODUCTION Chalcogenide alloys, particularly Ge-Sb-Te (GST), due to the very remarkable difference of the optical reflectivity and electronic conductivity between the amorphous and the crystalline phase, are used as rewriteable layer in Digital Versatile Disc (DVD) and Blu-Ray Disc (BD) and are considered good candidates to replace silicon for the fabrication of ultra-scaled non volatile memory devices. In these materials, the phase–change data storage concept is based on the reversible amorphous to crystal (a-c) transition induced by nanosecond laser pulse or voltage (current) pulse [1] respectively. The formation of transrotational crystals, during the initial stage of the amorphous to face centered cubic crystal has been reported and attributed to the different density of the amorphous and crystalline phases (~ 6 %) [2-4]. However, their structural evolution, from the early stages of nucleation to the complete crystallization of the GST film, has never been reported before. In this work X-ray diffraction measurements are combined with transmission electron microscopy analyses to investigate in detail the evolution of the crystalline grains during the different steps of the a-c transition.
EXPERIMENTAL Ge2Sb2Te5 amorphous films, 50 nm thick, were prepared by R.F. sputter deposition at room temperature using a stoichiometric target, over a 100 nm SiO2 layer thermally growt
Data Loading...
