The structural and optical properties of amorphous silicon (a-Si) and Ag-dispersed amorphous silicon (a-Si:Ag) thin films irradiated by femtosecond (fs) laser at various energy densities are investigated comparatively in this article. It is found that at a lower energy density of 100 mg/cm<sup>2</sup> , the film microstructure evolves from a completely amorphous phase to an intermediate one containing both amorphous and polycrystalline silicon. During laser irradiation, the formation of nanocrystals in a-Si films begins at lower energy density, but the existing Ag nanoparticles inhibits somehow the crystallization of a-Si in a-Si:Ag films at the same energy density. As the energy density is increased to a moderate value of 200 mj/cm<sup>2</sup> , the surface of a-Si:Ag films featuring a vertically aligned pillar-shaped structure is emerging. Both the crystallinity and the root mean square of surface roughness exhibit a monotonic increase with the increase of energy density. The Ag nanoparticles are dispersed uniformly in a silicon matrix, resulting in a resonant light absorption due to so-called localized surface plasmon. The localized surface plasmon resonance (LSPR) wavelengthes of the irradiated aSi:Ag films are increased significantly from 600 nm to about 820 nm, and the bandwidth of the measured absorptance is enhanced in the range of 600~1600 nm. The nanocrystallization mechanism, the formation of pillar-shaped structures and the light absorption enhancement are explained regarding the high electron density and the plasma-surface interactions.
With the maturity of germanium (Ge) growth on Si, Ge photodetectors have drawn great interests worldwide, which are potentially used in NIR/MIR light detecting, optical telecommunications, single photon detecting, biosensor applications. Lateral and vertical structured Ge-on-Si PIN photodetectors were fabricated and investigated. A dark current density of 20.4 mA/cm<sup>2</sup> was obtained, and small size devices resulted in low dark current values. The responsivity as a function of the wavelength was tested, and the highest responsivity of 0.8 A/W at the wavelength of 1310nm was obtained in vertical structured photodetectors, while the lateral structured photodetectors had the best 3dB bandwidth of 0.5 GHz, which was evaluated from the response time of 0.7 ns. The quantum efficiency was ~76%, and the reason of low 3dB bandwidth was discussed.