METAL-MEDIATED CRYSTALLIZATION OF AMORPHOUS-GERMANIUM IN GERMANIUM SILVER LAYERED SYSTEMS

The crystallization behaviour of amorphous Ge (a-Ge) in a-Ge/Ag/a-Ge t rilayer and a-Ge/Ag multilayer systems has been investigated using in situ transmission electron microscopy (TEM) and differential scanning calorimetry. The crystallization temperature of a-Ge in these systems is 240-270 degrees C, about 250 degrees C lower than that found in pur e a-Ge. During the reaction, in situ cross-section TEM showed that Ag grains separate the a-Ge matrix and the crystallized Ge (c-Ge) grains, and that the Ag grains migrate towards the a-Ge region, leaving the c -Ge phase behind. Our in situ atomic resolution TEM also revealed that the lattice points of the migrating Ag grains are stationary. These o bservations indicate that the Ge atoms diffuse through the Ag grains f or the growth of the c-Ge phase, whereas the migration of the Ag grain s is caused by the reverse self-diffusion of the Ag atoms. The mechani sm is essentially identical to that in other metal-mediated crystalliz ation reactions (e.g. Al-Si, Ag-Si). The observed activation energy fo r the migration of Ag grains is 1.75 +/- 0.1 eV, which is between the activation energy for Ge diffusion inside Ag and that of the Ag self-d iffusion. To account for the velocity of the migrating Ag grains, we p ropose a model in which the Ge and Ag atoms diffuse owing to the conce ntration gradient of these atoms inside the grains assisted by a net v acancy flow introduced by the volume change in the reaction.