Crystallization of nano amorphized regions in thin epitaxial layer of Ge2Sb2Te5

Abstract

The crystallization behavior of amorphous nano regions (20-100 nm in diameter) embedded in a textured epitaxial Ge₂Sb₂Te₅ (GST) 25 nm thick film grown on Si (111) substrate, has been investigated in situ by transmission electron microscopy (TEM) analysis. The amorphous regions were obtained by irradiation with 30 keV Ge⁺ at a fluence of 1.5x10¹⁴ ions/cm² of masked samples. The adopted configuration simulates the GST structure of a device in the RESET state, it is then of relevance for understanding their data retention characteristics. The in situ TEM analysis indicates that the amorphous to crystal transition for 20 nm dots is characterized by a growth velocity of 3.6 pms^-1 at 75°C, probably related to the external partially damaged area. At 90°C annealing crystallization is completed for 20 nm dot. In the case of 50 and 100 nm diameter amorphous dots a growth velocity of about 2.6 pms^-1 was observed at 90°C and of 170 pms^-1 at 110°C. The transition is governed only by rearrangement (nucleation is absent) of the atoms located at the boundary of the amorphous dot with the surrounding crystalline regions. In some cases a preferential growth along surfaces normal to the [111] and to the [121] directions has been found. The crystallographic characterization of the regrowth crystal indicates a good matching with the zone axis of the surrounding material although the seed, a Si (111) wafer, at the bottom interface is damaged by the implant.