Laser-Induced Amorphization and Non-Equilibrium Solidification in Silicon

Following pulsed laser melting of Si, amorphous growth can occur at the liquid/solid interface if thermal conduction into the substrate creates sufficiently rapid solidification velocities (>=q15 m/s). This process of laser-induced amorphization, which represents an in-situ marker of crystal breakdown, was investigated to better understand non-equilibrium solidification behavior in Si. By examining amorphization as a function of crystallographic orientation, the role of bonding, atomic mobility, and nucleation at the interface was probed. Single crystal Si was irradiated with <=q3 nsec FWHM Nd:YAG laser pulses at 266 nm. With increasing energy density, the melt evolved from amorphous to crystalline regrowth. Optical microscopy was used to determine the surface morphology and crystallization threshold energy, E_{c}. Scanning reflectivity measurements and transmission electron microscopy followed the evolution of the amorphous Si (a-Si) thickness and microstructure with melt depth. Details near E _{c} were observed by high resolution TEM. Two regimes of solidification behavior were observed, one within ~15^ circ of (111) and the other near (100) and (011). For orientations near (111), amorphous layer thicknesses peaked at E_{c} before abruptly transforming to polycrystalline Si. However, near (100) and (011), amorphous layers decreased in thickness as E_{c} was approached and no polycrystalline Si was formed. TEM micrographs indicated a similar solidification path for all samples. Initial growth was mildly defected crystal Si, evolving into a highly faulted crystal band. An amorphous phase then formed at the moving, roughened interface. The relative thicknesses of the regions varied with orientation. These data are interpreted within a model where crystal growth extends to temperatures below the peak of the interface response function, resulting in an interface intrinsically unstable at the onset of amorphization. This model predicts an initially stable crystal growth regime, followed by unstable growth past the peak velocity, and eventually amorphization. One dimensional heat flow calculations based on this model give qualitative agreement with the observed thicknesses of the three regions and reproduces the reduced amorphous thickness near E_ {c}. Orientation effects are discussed in light of this model. Comparison of the orientation effects with new measurements of solid phase epitaxial regrowth as a function of orientation is also made.