Surface induced crystallization/amorphization of phase change materials.

Abstract

Surface induced crystallization/amorphization of a Germanium-antimony-tellurium (GST) nanolayer is investigated using the phase field model. A Ginzburg-Landau (GL) equation introduces an external surface layer (ESL) within which the surface energy and elastic properties are properly distributed. Next, the coupled GL and elasticity equations for the crystallization/amorphization are solved. For the initial surface crystalline nucleus, unphysical crystallization along the ESL appears for the ESL width Δ ≥ 2 while oval growth occurs for Δ ≤ 1 . The ESL results in a faster surface nucleus growth than the sharp surface model, but does not affect the crystallization rate inside the bulk. Initial homogeneous conditions cause a simultaneous crystallization in the bulk and later in the ESL. The ESL effect is studied on the amorphization to determine the ESL width more precisely. For both initial amorphous nucleus and homogenous conditions, the amorphization temperature shows a reduction from the sharp surface model to the ESL model with Δ = 0.5 and then remains almost constant for larger Δ . Combining the above results gives 0.5 ≤ Δ ≤ 1 as a proper range for the ESL width. The ratio of the effective ESL width to the interface width (∆ ⁄Δ ) and the ratio of the difference between the surface energies of transforming phases to the surface energy of the initial phase (Δ ⁄ ) are considered as crucial parameters in determining the ESL effect on the phase transformation and a linear relation as ∆ ⁄Δ ≅ 6.235 Δ ⁄ is found based on the current and previous works which can be useful for estimating the effective ESL width for any surface induced transformations. .