Free-carrier absorption assisted photoelectrochemistry of silicon

Free-carrier absorption (FCA) in semiconductors is a process that carriers within the conduction or valence band absorb photons with energy less than bandgap to transfer themselves into a higher energy state without generating electron–hole pairs. The energy absorption activates phonon scattering and is generally considered to have an undesirable impact on the performance of electronic or optical devices. But in an electrochemical processing of silicon, the FCA has a significant impact on the formation of nanocrystals. By the effect of FCA, nanocrystals form directly on the bulk surface via a rate-limited photoelectrochemistry processing. To prove the effect, we employed a 0.9 W/cm², 1310-nm laser that can penetrate through silicon but be absorbed by free carriers. After anodizing, a brilliant nonetching spot which was irradiated by the laser appeared on the black anodized surface. Under He–Cd laser photoexcitation, we found the peak location of the photoluminescence (PL) in the spot was shifted to 540 nm. The examination of transmission electron microscope showed 2.9 nm silicon nanocrystals embedded in an amorphousness-like layer as silicon quantum dots.