Aug 29, 2008
Nanocrystalline silicon lights up again
It is still a daunting task to improve the applicability of light-emitting silicon micro/nanostructures for successful achievement of stimulated emission and light amplification. Optical gain in the red spectral region is reported in nanocrystalline (nc-) silicno embedded in a SiO2 matrix using pulsed laser pumping. A stimulated blue emission is reported at extremely high (100 MW cm–2) excitation density. Most recently, Raman laser is reported in a silicon on insulator (SOI) waveguide using optical gain in silica matrix.
Integrating light with silicon is one of the long-standing problems in the semiconductor technology. A higher quantum efficiency of photoluminescence (PL) intensity by one order (~10%) than that for porous silicon (~1–3%) is reported for a single nc-silicon. Formation of direct bandgap in nc-silicon, as well as widening of it due to quantum confinement of carriers are well conceived. However, the mechanism of luminescence in nc-silicon, including that of the PS, is not very well understood. According to the present perception, observed luminescence is explained in terms of electron-hole pair recombination with a role of Si-SiOx interface states. Defect states, created during the growth of nc-silicon by ion implantation in silica matrix, is also thought be responsible for the luminescence process. The role of the Si-SiOx interface in the observed PL of nc-silicon is always emphasized.
In a systematic study, room temperature PL properties are reported in Nanotechnology for uniformly sized nc-silicon grown by silicon ion implantation in both amorphous and crystalline SiO2 matrices. A super-linear power dependence of PL under continuous-wave excitation is observed in the radiative recombination process accompanied by pulse shortening. An emission process, comprised of an initial non-radiative recombination (time constant ~ 280–315 ps) of excited carriers in the defect states in SiO2 matrices to the conduction band minima of nc-silicon followed by a slower process of radiative recombination in the direct band transition for nc-silicon along with a non-radiative Auger recombination (time constant ~2.67 ns), is proposed. A possible population inversion is suggested for the emission process with a competitive Auger recombination process. The results may be encouraging, one tiny step ahead, for the practical application of silicon as an optical source.
About the author
S Dhara, K G M Nair and C David are from the Materials Science Division, Indira Gandhi Centre for Atomic Research in India where Baldev Raj is the director of the centre. S Dhara is presently affiliated as research associate fellow at the Institute of Microelectronics Engineering in the Cheng Kung University in Taiwan. Lu-Chen Yao and Chin-Pei Chen are research assistant at the Institute of Atomic and Molecular Sciences in Taiwan where Kuei-Hsien Chen is the senior research fellow. Yi-Fan Huang is a research student with the National Taipei University of Technology in Taiwan. Li-Chyong Chen is a senior research fellow in the Centre for Condensed Matter Sciences, National Taiwan University in Taiwan.