Surface induced crystallization/amorphization of phase change materials

Surface-induced crystallization/amorphization of a Germanium-antimony-tellurium 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 adequately 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 Delta xi >= 2nm while oval growth occurs for Delta xi <= 1nm. 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 on amorphization is studied to determine the ESL width more precisely. For both the initial amorphous nucleus and homogenous conditions, the amorphization temperature shows a reduction from the sharp surface model to the ESL model with Delta xi=0.5nm and then remains almost constant for larger Delta xi. Combining the above results gives 0.5 <=Delta xi <= 1nm as a proper range for the ESL width. The ratio of the effective ESL width to the interface width ( Delta sat/Delta eta) and the ratio of the difference between the surface energies of transforming phases to the surface energy of the initial phase ( Delta gamma/gamma in) are considered crucial parameters in determining the ESL effect on the phase transformation and a linear relation as Delta sat/Delta eta congruent to 6.235 Delta gamma/gamma in is found based on current and previous works, which can help estimate the effective ESL width for any surface-induced transformations.