Ultrafast carrier dynamics of porous silicon and gold-silicon composites

Wu, Rihan (2021). Ultrafast carrier dynamics of porous silicon and gold-silicon composites. University of Birmingham. Ph.D.

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Abstract

The ultrafast carrier dynamics of nano- and micro-porous silicon and their underlying opto-electronic properties were investigated in this work. The femtosecond pump-probe technique was employed to characterise the excitation, relaxation and recombination processes. The results showed that the recombination rate of the nano-porous silicon is three orders of magnitude higher than that of the crystalline silicon. Auger and Shockley- Read-Hall were the two dominant recombination processes in nano-porous silicon, while the contribution of the bimolecular recombination was diminishing. Due to the fast recombination and high scattering rate of the free carriers in nano-porous silicon, the diffusion process was suppressed. By fitting the rate equation, the recombination time of Auger and Shockley-Read-Hall were retrieved from the experimental results. It was demonstrated that the Auger process was greatly enhanced by the vibrational modes of the surface adsorbates coupling to the phonons of nano-porous silicon.

A study on the micro-porous silicon was conducted to examine the suitability of the material for implementation in an all-optical modulator. A modulation contrast of 30% with a 0.55 ps response time was demonstrated. The modulator was then used to construct a high resolution Time-of-Flight set-up. A pulse broadening caused by group velocity dispersion was measured for the laser pulse traversing a 5 cm silica rod.
To introduce new characteristics to the opto-electronic properties of nano-porous silicon, a novel composite material was fabricated by embedding gold nanoparticles into its pore channels. The combination of plasmonics with semiconducting material greatly enhanced the light coupling efficiency. It was proved that the composite could be used as a SERS substrate which provides an enhancement factor of 10\(^9\), suitable for a single molecule level detection. Ultrafast dynamics investigation on the composite has also been carried out. The results showed that the free carrier absorption and third order nonlinearity was enhanced by incorporating gold clusters into np-Si.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):