Aluminum-induced crystallization of hydrogenated amorphous germanium thin films

Al-induced crystallization of co sputtered hydrogenated amorphous germanium films, deposited at 220 degreesC, onto crystalline silicon substrates is investigated by Raman and infrared spectroscopies as a function of the Al concentration (2x10(-6)< [Al/Ge]<2.5x10(-2)). Aluminum induces partial crystallization of the films for metal concentrations smaller than similar to1.3 at. %. A sort of explosive crystallization of the films occurs within a narrow Al concentration range (similar to1.3 < [Al/Ge]< similar to1.8 at. %). Raman spectra do not display any crystallization signal for metal concentrations above this narrow range. Data of the extended x-ray absorption fine structure of the coordination and of the local order around gallium, in Ga-doped a-Ge:H, are used to propose an overall picture of the microscopic mechanisms behind these results. A comparative analysis suggests that the crystallization seeds are fourfold-coordinated Al atoms sitting at the center of perfect tetrahedral Ge sites. (C) 2001 American Institute of Physics.