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 subsequently the crystallization behavior of these films. The Ge-Si bond distance in the as-grown a-Si:Ge is slightly longer (∼0.004 Å) than for the crystalline case (2.378 Å). When the a-Si:Ge film was exposed to synchrotron 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 Å). This x-ray-induced bond length dilation is found to be x-ray dose dependent and is strongest in the sample irradiated with x rays for 20 h (corresponding to a dose of ∼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 found that the final grain size of the annealed Si crystal depends on the initial Ge-Si bond length in the amorphous film. The larger the R_Ge-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 irradiation will be discussed. © 1999 American Institute of Physics.