To better understand the involved phase transformation Mechanism, we are
studying the excimer laser-induced amorphization (ELA) of ultra-thin Si
films on oxidized Si substrates. In this paper, we show that the onset of
amorphization of hydrogen-free Si films on SiO2 substrates upon
increases in the energy density is associated with the onset of complete
melting of the film. Once complete melting occurs, further increases in the
incident energy density and/or increases in the substrate temperature can
lead to incomplete amorphization of the film. Planar view TEM analysis of
nearly-amorphized Si films reveals a heterogeneous microstructure, which
consists of a mixture of densely dispersed amorphous-like annular regions
(∼20 to 40 μm−2), embedded within and typically separated by a
region containing finegrained small crystals. Such a cellular microstructure
strongly suggests that amorphization occurred not via a homogeneous but via
a heterogeneous transformation. In particular, the microstructure paints a
scenario in which amorphization proceeded via nucleation of solids, which is
then followed by interfacial amorphization. The experimental results
unambiguously reveal (1) that the previously proposed criteria of the melt
duration and the vertical temperature gradient are irrelevant in determining
amorphization of supercooled liquid Si films and (2) that the quenching
rate, not surprisingly, is the important parameter.