Competing crystal growth in Ge-Sb phase-change films

Abstract

Analysis of crystal growth in thin films of phase-change materials can provide deeper insights in the extraordinary phase transformation kinetics of these materials excellently suited for data storage applications. In the present work crystal growth in GexSb100-x thin films with x = 6, 7, 8, 9, and 10 is studied in detail, demonstrating that the crystallization temperature increases from ∼80 °C for Ge6Sb94 to ∼200°C for Ge10Sb90 and simultaneously the activation energy for crystal growth also significantly increases from 1.7 eV to 5.5 eV. The most interesting new finding is that in the thin films containing 8, 9, and 10 at% Ge two competing growth modes occur which can have several orders of magnitude difference in growth rate at a single external temperature: an initial mode with isotropic slow growth producing circular crystals with smooth surfaces and growth fronts and a fast growth mode producing crystals with triangular shape having rough surfaces and growth fronts indicative of dendritic-like growth. The slow-growth mode becomes increasingly dominant for crystallization at low temperatures when the Ge concentration is increased from 8 to 10 at% Ge. For a certain Ge concentration, the slow growth mode becomes increasingly dominant at lower temperatures and the fast growth mode at higher temperatures. Latent heat produced during crystallization is considered a principal factor explaining the observations. The fast growth mode is associated with (eutectic) decomposition generating more latent heat and instable growth fronts and the slow growth mode is associated with thermodynamically less stable homogeneously alloyed crystals generating less latent heat, but stable growth fronts. Two competing crystal growth modes occur in thin films composed of Ge8Sb92, Ge9Sb91 and Ge 10Sb90: an initial isotropic slow growth mode is followed by a fast dendritic-like growth mode producing crystals with an overall triangular shape. The present findings are relevant for phase-change materials used in memory applications employing the fast reversible amorphous-crystalline phase change © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.