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/cm2 , 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/cm2 , 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.
We use metal-assist chemical etching (MCE) method to fabricate nanostructured black silicon on the surface of C-Si. In our MCE process, a chemical reduction reaction of silver cation (Ag+) will happen on the surface of silicon substrate, and at the same time the silicon atoms around Ag particles are oxidized and dissolved, generating nanopores and finally forming a layer called black silicon on the top of the substrates. The nanopores have diameter and depth of about 400 nm and 2 μm, respectively. Furthermore, these modified surfaces show higher light absorptance in near-infrared range (800 to 2500 nm) compared to that of C-Si with polished surfaces, and the maximum light absorptance increases significantly up to 95% in the wavelength region of 400 to 2500 nm. The Si-PIN photoelectronic detector based on this type of black silicon, in which the black silicon layer is directly set as the photosensitive surface, has a substantial increase in responsivity with about 80 nm red shift of peak responsivity, particularly at near-infrared wavelengths, rising to 0.57 A/W at 1060 nm and 0.37 A/W at 1100 nm, respectively. Our recent novel results clearly indicate that nanostructured black silicon made by MCE has a potential application in near-infrared photoelectronic detectors.
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