Chun-Hao Li

Surface Detection of Strain-Relaxed Si 1– x Ge x Alloys With High Ge-Content by Optical Second-Harmonic Generation

We investigated the strain and surface structural properties of a strain-relaxed Si1-xGex alloy layer with high Ge-content using optical surface second-harmonic generation. Here, the Si1-xGex alloys are heteroepitaxial, and they are deposited onto Si substrates via ultra-high-vacuum-chemical vapor deposition. The in-plane strain and the composition x of the Si1-xGex alloys were determined using Raman spectroscopy. The SH signals generated from three Si1-xGex alloy surfaces versus the rotational angle of the substrate were measured. The SH intensities for the combination of s-input/p-output polarization show fourfold symmetry; however, for s-input/s-output, the SH signals show eightfold symmetry with the rotational angle. Residual strain would induce an enhancement of the isotropic p-output SH component in the Fourier transform coefficient. Finally, the degree of symmetry of the SH signals from these three Si1-xGex alloy surfaces versus the rotational angle was related with the surface pit defects (densities and sizes) and surface roughness.

Fabrication of Black Silicon With Thermostable Infrared Absorption by Femtosecond Laser

Annealing-insensitive black silicon with high absorption below the silicon bandgap has been achieved by femtosecond laser direct writing. Spike microstructures with sizes ranging from 4 to 25 μm are formed on the surface layer of silicon substrate, and a large amount of phosphorous impurities (1021 cm-3) is doped during the resolidification process. The infrared absorption of phosphorus-doped black silicon decreases slightly with both the annealing temperature and duration. Excitingly, the largest decrease is less than 10% at 2 μm (annealing 240 min at 873K). This thermostable infrared absorption is related to free carrier absorption. After laser irradiation, the phosphorus-doped layer maintains a relatively high crystallinity that can be improved further during thermal annealing. The density of the electrically activated impurities is approximately 1019 cm-3.

The Optical and Electrical Properties of Co-Doped Black Silicon Textured by a Femtosecond Laser and Its Application to Infrared Light Sensing

Co-doped black silicon with sulfur and nitrogen impurities was fabricated by a femtosecond laser in a mixed atmosphere of N2 and SF6. Compared with sulfur-doped black silicon, the infrared absorption of co-doped black silicon shows excellent thermal stability, indicating that the high absorption below the band-gap of silicon is insensitive to the thermal annealing process. According to the electronic nature of annealed co-doped black silicon by the Hall-effect measurement, the sheet carrier density decreases with increasing nitrogen content as a result of the neutral doping of the nitrogen pair defect. Furthermore, a near-infrared black silicon photodiode was produced based on the performance of the stable absorption for thermal annealing and its superior photo-response, in which 58 mA/W is obtained for the photodiode for a 1.31-μm detection light.