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 trilayer 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 pure 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 observations indicate that the Ge atoms diffuse through the Ag grains for the growth of the c-Ge phase, whereas the migration of the Ag grains is caused by the reverse self-diffusion of the Ag atoms. The mechanism is essentially identical to that in other metal-mediated crystallization reactions (e.g. Al-Si, Ag-Si). The observed activation energy for 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-diffusion. To account for the velocity of the migrating Ag grains, we propose a model in which the Ge and Ag atoms diffuse owing to the concentration gradient of these atoms inside the grains assisted by a net vacancy flow introduced by the volume change in the reaction.