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Femtosecond-laser Microstructuring of Silicon
for Novel Optoelectronic Devices
James Carey
Ph.D. Thesis, Harvard University, 2004, 162 pages export citation
This dissertation comprehensively reviews the properties of femtosecond- laser microstructured silicon and reports on its first application in optoelectronic devices. Irradiation of a silicon surface with intense, short laser pulses in an atmosphere of sulfur hexafluoride leads to a dramatic change in the surface morphology and optical properties. Following irradiation, the silicon surface is covered with a quasi-ordered array of micrometer-sized, conical structures. In addition, the microstructured surface has near-unity absorptance from the near-ultraviolet (250 nm) to the near-infrared (2500 nm). This spectral range includes below-band gap wavelengths that normally pass through silicon unabsorbed. We thoroughly investigate the effect of experimental parameters on the morphology and chemical composition of microstructured silicon and propose a formation mechanism for the conical microstructures. We also investigate the effect of experimental parameters on the optical and electronic properties of microstructured silicon and speculate on the cause of below-band gap absorption. We find that sulfur incorporation into the silicon surface plays an important role in both the formation of sharp, conical microstructures and the near-unity absorptance at below-band gap wavelengths. Because of the novel optical properties, femtosecond-laser microstructured silicon has potential application in numerous optoelectronic devices. We use femtosecond-laser microstructured silicon to create silicon-based photodiodes that are one hundred times more sensitive than commercial silicon photodiodes in the visible, and five orders of magnitude more sensitive in the near-infrared. We also create femtosecond-laser microstructured silicon solar cells and field emission arrays.
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