Laser-induced self-organization in silicon-germanium thin films

We report on the formation of self-organized structures in thin films of silicon-germanium (Si1-xGex) with 0.3<x<0.7 after exposing the films to laser irradiation. Amorphous SiGe samples that are exposed to a single laser pulse exhibit a ripple structure that changes to a hillock structure when the samples are irradiated with additional laser pulses. The topographic structure is coupled to a periodic compositional variation of the SiGe alloy. The periodicity length of the structure after a single laser pulse is in the range of 0.3-1.1 mu m, depending on Ge content, layer thickness, and laser fluence, and rapidly grows with increasing number of laser pulses. In situ conductivity measurements during solidification support the theoretical instability analysis that we have done, based on the Mullins-Sekerka theory, to elucidate the nature of this phenomenon. Moreover, as theoretically predicted, the self-organization phenomenon can be turned off by increasing the solidification velocity. (C) 2008 American Institute of Physics.