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Published online by Cambridge University Press: 21 February 2011
Near the threshold for laser-induced melting of semiconductors like Si and Ge, the resulting surface morphology is characterized by spatial inhcaogeneities. For cw illumination, several authors have observed the formation of randan, microscopic lamellae interspersed with molten regions, for incident fluences between threshold and approximately twice threshold.We demonstrate experimentally that such lamellae also form under pulsed conditions for 20nsec, 1.06μ pulses incident on Ge.Theoretically, we consider the simpler case of cw illumination on a bulk sample: We show that, although in principle it should be possible to form uniform melt layers of arbitrarily small thickness, for melt depths less than the skin depth of the radiation in the liquid any spatial modulation of the liquid-solid interface leads to instabilities in the coupled laser-material system.These instabilities would result in runaway inhamogeneous heat flow and hence to the formation of lamellae.Three distinct types of instabilities are identified for melt-front penetrations of less than ∼l000Å in Ge.