Direct fabrication of black Si with enhanced infrared transmittance using femtosecond laser irradiation

An innovative black silicon (Si) with enhanced transmittance in the infrared spectral band is directly processed by femtosecond (fs) laser irradiation in ambient air. Interestingly, this black Si can provide a low-cost, alternative substrate for infrared detector replacing the expensive CdZnTe, also offering an effective improvement of the detector performance. An enhancement in transmittance of up to 20 % is achieved in the case of black Si processed with laser fluence and scanning speed of 0.68 J•cm⁻² and 1 mm•s⁻¹, respectively. The microscopic characterization and the elemental analyses of the black Si indicate a direct link between the enhanced transmittance and the features of the processed surface. The transmittance improvement can be mainly ascribed to the surface microstructures and surface oxidation induced by the laser irradiation process. The microstructures may effectively extend the incident light path, whereas the oxidized black Si can result in a graded refractive index surface layer avoiding a sudden change of the refractive index from air to Si. The observed increase in infrared transmittance can eventually be explained by contribution of both these effects.