The effects of x-ray induced structural changes on the microstructure of a-Si after thermal crystallization

We have investigated the effects of intense x-ray irradiation on the local relaxation in amorphous silicon films doped with Ge(a-Si:Ge) and subsequent ly the crystallization behavior of these films. The Ge-Si bond distance in the as-grown a-Si:Ge is slightly longer (similar to 0.004 A) than for the c rystalline case (2.378 Angstrom). When the a-Si:Ge film was exposed to sync hrotron x rays, the Ge-Si bond distance increased to a value closer to the sum of the covalent radii of Si and Ge (2.39 Angstrom). This x-ray-induced bond length dilation is found to be x-ray dose dependent and is strongest i n the sample irradiated with x rays for 20 h (corresponding to a dose of si milar to 1 photon/Si atom). The x-ray-induced bond dilation in the a-Si:Ge directly affects the crystallization of the films after irradiation. We fou nd that the final grain size of the annealed Si crystal depends on the init ial Ge-Si bond length in the amorphous film. The larger the RGe-Si (due to x-ray irradiation) the larger is the Si grain size after thermal annealing. The mechanism leading to the lattice relaxation in the amorphous phase and subsequently the crystal grain size after annealing due to x-ray irradiati on will be discussed. (C) 1999 American Institute of Physics. [S0003-6951(9 9)00540-9].