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Femtosecond melting rates of crystalline and amorphous Column IV elements

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Abstract

Several fs time-resolved reflectivity and second harmonic studies [1] have demonstrated qualitatively that highly photoexcited crystalline semiconductors lose long-range crystalline order (’’melt”) and approach equilibrium liquid optical properties before the material becomes vibrationally excited. A quantitative tight-binding theory of lattice instability driven electronically by dense e-h plasmas, which predicts the material- and carrier density-dependence of the melting time τm, has now been formulated [2], but has not been quantitatively tested. This theory predicts, among other things, that τm scales as d02M1/2f(Neh), where M = atomic mass, d0 = lattice spacing, and f(Neh) is a nearly material-independent function of electron-hole pair density. Since d0 and f(Neh) depend only on local (e.g. tetrahedral) bonding structure, crystalline and amorphous Column IV targets of the same material should melt on the same time scale for a given peak density. The measurements presented here with systematically varied target material (C, Si, Ge), target structure (crystalline vs. amorphous), and excitation fluence F confirm the d02M1/2f(Neh) scaling law quite well for Column IV crystalline targets. However, they reveal that amorphous targets melt more slowly than crystalline targets for a given F, suggesting that peak carrier density is clamped by an ultrafast recombination mechanism unique to the amorphous state [3].

© 1996 Optical Society of America

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