Crystallization of amorphous germanium in an Al/a-Ge bilayer film deposited on a SiO2 substrate

The crystallization of amorphous Ge(a-Ge) in an Al (134 nm) and a-Ge (108 nm) thin-film bilayer deposited on a SiO2 substrate has been examined by a cross section transmission electron microscope technique. When crystallization of a-Ge begins at 125 degreesC, amorphous AlGe (a-AlGe) alloy is formed in the Ge layer. Then, the a-AlGe alloy layer also appeared at the surface of the bilayer. After complete crystallization, those amorphous layers disappeared and the bilayer film has been converted to a polycrystalline film. We discussed the crystallization of a-Ge and proposed the mechanism of the diffusion of Ge atoms from the inner a-Ge layer through the outer Al layer to the topmost surface that involves the formation of the metastable a-AlGe alloy in the Ge layer, followed by the crystallization of this alloy by the pseudo-eutectic reaction, leading to the decomposition into an equilibrium Al and Ge crystal mixture and a-Ge. Then, Ge atoms is released to the Al layer for the compensation of the Al diffusion down into the Ge layer and again forms the a-AlGe alloy in the Al layer. The a-AlGe alloy in the Al layer is also crystallized by the pseudo-eutectic reaction. Consequently, decomposed a-Ge is ejected from the inside to the surface of the bilayer, resulting in the surface Ge segregation. (C) 2001 American Institute of Physics.