Epitaxial pulsed laser crystallization of amorphous germanium on GaAs

We have investigated the crystallization of amorphous germanium films on Ga As crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial fil ms are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a (GaAs)(1-x)Ge-2x epitaxial alloy with a graded composit ion profile at the interface with the substrate. Since Ge and GaAs are ther modynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling p rocess. Lower laser fluencies lead to polycrystalline layers with a pattern ed surface structure. The latter is attributed to the freeze-in of instabil ities in the melt during the fast solidification process. (C) 2001 American Institute of Physics.