Femtosecond-laser micromachining of silicon for novel optoelectronic devices

Silicon is the most commonly used semiconductor in micro- and optoelectronics. However, silicon is not the best material for all applications: as an indirect band-gap material, it is a poor light emitter; silicon cannot be used to detect many important communications wavelengths; and silicon solar cells fail to convert nearly a third of the suns’ spectrum into electricity. The low cost and large manufacturing infrastructure drives research to alter the properties of silicon rather than rely on more exotic semiconductor materials. We present a novel technique that uses the intense conditions at the focus of a high-intensity, femtosecond laser pulse to alter the surface morphology, optical, and electronic properties of silicon. Following irradiation with 100-femtosecond laser pulses in the presence of sulfur containing gases, silicon surfaces are covered with a quasi-ordered array of sharp, conical microstructures. In addition, the optical properties are altered such that the microstructured surfaces have near-unity absorption from the near-ultraviolet (250 nm) to the near-infrared (2500 nm). We also present the first use of these microstructured surfaces in a functioning optoelectronic device. We produced highly sensitive silicon-based photodiodes that are sensitive to infrared wavelengths up to 1650 nm and that have greatly enhanced photoresponse to visible wavelengths. We measure responsivity levels over 100 A/W in the visible and over 100 mA/W in the near-infrared (1200-1650 nm). These represent sensivity levels 100 times larger than commercial silicon photodiodes in the visible, and four orders of magnitude higher in the near-infrared.