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-Ge1-xAux 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-degrees-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-degrees-C and the x=0.04 sample at 330-degrees-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 angstrom, 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.