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

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 ∼800^oC, however, it becomes unstable above ∼800^oC. That is, void (or LIMV) fades above ∼800^oC under infra-red optical microscope. In TEM, after annealing at 1000^oC, 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.

KEYWORDS:

• Laser
• void
• plasma
• Si

Acknowledgements

The authors thank Mr. Daisuke Kawaguchi of Hamamatsu Photonics, Co., without whose assistance this study would not have been carried out. The authors thank Dr. K. Kikunaga of AIST Kyushu for his interest. They acknowledge assistance from Mr. Yoshimi in carrying out the experiment. Preparation of TEM specimens using FIB was carried out in the scheme of the Advanced Characterization Nanotechnology Platform of Nagoya University. They thank Ms. Chieko Yukita for her assistance in preparing the TEM specimens.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan [grant number Grant-in-Aid for Scientific Research(c)20K05066].