Whereabouts of missing atoms in a laser-injected Si (Part III)

Abstract

ABSTRACT Microstructure of laser induced modified volume (LIMV) in Si was examined using transmission electron microscopy (TEM) and infra-red optical microscopy. Dominant features of LIMV are voids, dislocations, cracks and non-DS phases of Si. Nucleation of dislocations and cracks can be accounted for by dilatation caused by local temperature rise near the focus of laser. Mass of Si wafer before and after laser injection was conserved by laser injection. Therefore, those Si atoms which must have existed in pre-void, that is, that area which on laser injection is transformed to void, also stay in Si matrix. The void is thermally stable up to ∼800oC, however, it becomes unstable above ∼800oC. That is, void (or LIMV) fades above ∼800oC under infra-red optical microscope. In TEM, after annealing at 1000oC, 1∼a few μm diameter voids which had been present in the as-laser-injected Si, disappeared and very fine voids (nano-voids) appeared. In the cross-sectional TEM of LIMV sitting near the exiting surface of laser beam, a gap was observed between the exiting surface and a protective coat (W). Si atoms in pre-void become plasmas on laser irradiation. In other words, Si atom is dissociated into Si+n and n free electrons (n =1 ∼ 14). The ionized Si (Si+n) forms a void but some of them are dispersed in the adjacent matrix of DS-Si. On the other hand, free electrons gain a high energy and travel far away from the void: The ionized Si (Si+n) and free electrons do not recombine to annihilate.