Selective laser induced melting of ultrathin nanoporous silicon layers

Short laser pulses (wavelength 337 nm, duration time 0.5 ns) are used for thermal processing of ultra thin nanoporous silicon layers (UPSL) prepared electrochemically on n- and p-type Si(1 1 1) in aqueous NH₄F solution. The theoretical threshold for melting (W _m ) of UPSL is about 0.01 J/cm². This is about one order of magnitude belowW _m of crystalline silicon. A selective laser induced melting regime can be realized for which an UPSL is practically completely molten on the top of an unmolten crystalline silicon substrate. Investigations with scanning and high resolution transmission electron microscopy show the formation of crystalline silicon spheres with diameters in the range of some of ten nm in this regime.