Gold-induced germanium crystallization

When in contact with metal, amorphous silicon (a-Si) or germanium (a-Ge) often crystallize at temperatures significantly lower than in their bulk form. In order to understand the role of the contact metal, we have studied the crystallization process of a-Ge/Au bilayers and codeposited a-${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Au}}_{\mathit{x}}$ films (x=0.04 and 0.22) by measuring the Ge and Au extended x-ray-absorption fine structure. Upon isothermal annealing, the Ge/Au bilayer crystallizes at 170 \ifmmode^\circ\else\textdegree\fi{}C, and concomitantly, a large amount of Au migrates into the Ge top layer. For codeposited films, the x=0.22 sample crystallizes at 130 \ifmmode^\circ\else\textdegree\fi{}C and the x=0.04 sample at 330 \ifmmode^\circ\else\textdegree\fi{}C. In the amorphous x=0.04 and 0.22 films, each Au atom on the average is bound with one to two Ge atoms at 2.4 \AA{}, but does not have Au neighbors. As the films crystallize, Au transforms into fcc gold in the x=0.22 sample but remains bound with Ge in the x=0.04 sample. Our results demonstrate that Au-Ge bonding is significant in the a-Ge and may play an important role in lowering the Ge crystallization temperature.