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Micro-ripple and micro-bead structures are formed on a silicon (Si) surface after irradiation with femtosecond laser pulses in nitrogen (N2) atmosphere. Simultaneously, supersaturated nitrogen (N) atoms, with a concentration above 1020 cm−3, are doped into the textured black Si layer via laser ablation. The N-doped Si exhibits strong below-bandgap infrared absorption from 1.1 to 2.5~\mu \textm , which remains nearly unchanged after annealing for 30 min at 873 K. The mechanism of this thermally stable infrared absorption is analyzed by first-principles calculations. According to the transmission electron microscopy results, multiple phases (including single crystalline, nanocrystalline, and amorphous phases) are observed in the laser-irradiated layer. Hall Effect measurements prove that N-dopants induce a low background free-carrier concentration ( \sim 1.67\times 10^16\,\,\text cm^-3 ). Finally, a Schottky-based bulk structure photodiode is made. This broadband photodiode exhibits good thermal stability and a photo-responsivity of 5.3 mA/W for 1.31~\mu \textm at a reverse bias of 10 V. [ABSTRACT FROM PUBLISHER]

Gold (Au)-doped-textured silicon (Si) material with a thermostable absorption below bandgap (>50%) is obtained by femtosecond laser irradiation. Although the concentration of Au impurity (1019 cm−3 ) in textured Si is at least four orders of magnitude greater than the solid solubility of Au in crystalline Si, the sheet carrier density (approximately 1010 cm−2) in Au-doped Si is very low due to a self-compensation effect of Au impurity in Si material. The infrared absorption of Au-doped Si is related to laser-induced-structural defects and sub-band absorption of deep energy levels of Au in Si, which is determined by temperature-dependent Hall Effect measurement. Besides supersaturated doping of Au, a gold silicide phase is formed at textured Si surface. [ABSTRACT FROM PUBLISHER]

Femtosecond (fs) laser annealing has been applied to improve the crystalline quality and absorptance below bandgap of ion implanted silicon (Si) with sulfur (S). The doping concentration of S is up to 10^20 atoms/cm3, which is at least four orders of magnitude higher than the solid solubility of S in crystalline Si. According to the Raman spectra, after fs laser irradiation with appropriate laser energy, the crystal quality is evidently better than primary ion implanted sample. Moreover, the optical absorption coefficient at wavelengths of 1100~2400 nm is up to 1.54 \times 10^4 cm ^\mathrm -1 , which is much larger than the contribution of free carriers. Excessive laser energy will induce serious ablation effect and cause the removing of doping layer. We consider that the high absorption is a combination of sub-bandgap transition of S-related localized states, free carrier absorption, ion implantation induced defect absorption and fs laser irradiation induced defect absorption. In the end, a photo-response of 8.4 mA/W is obtained for the photodiode for 1310 nm detection light. [ABSTRACT FROM PUBLISHER]

Supersaturated S-doped black silicon (Si) samples are produced by femtosecond laser fabrication in mixed atmosphere of sulfur hexafluoride and oxygen (SF6/O2). The surface morphology with cone-microstructures has been observed in black Si samples fabricated in mixed atmosphere. After thermal annealing, the absorption below bandgap of black Si is obviously reduced. Moreover, the contents of sulfur in black Si samples obtained in mixture are also decreased by the lead in of oxygen. In contrast, an obvious increase of valent state ratio of Si4+ (SiO2) to Si0 (Si) is observed after the implantation of oxygen due to the formation of SiO2 in S-doped Si. Based on the passivation effect of SiO2, we have produced near-infrared black Si photodiode devices and showed photo-response properties of the devices. A notable photo-response of 11 mA/W at −3 V (26.6 mA/W at −5 V) for near-infrared 1.31- \mu \textm detection light is obtained for the photodiode fabricated in mixture, which outweigh the performance of photodiode fabricated in pure SF6 atmosphere. [ABSTRACT FROM PUBLISHER]

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- \mu \textm detection light. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Micronanostructured silicon surface is irradiated by femtosecond laser pulses at sulfur hexafluoride atmosphere (S-doped black silicon) and photodiodes are successfully fabricated based on this material. From the scanning electronic microscope and atomic force microscope images, the black silicon layer shows micro-nanocomplex structures (microcones covered with nanoparticles). The optoelectrical properties of n+-p junction are formed between S-doped micro-nanostructured silicon-layer and substrate after thermal annealing. For the n+-p photodiode, current–voltage characteristics at different incident light powers have been investigated. The responsivity for 800-nm wavelength is 0.69 A/W at −5 V reverse bias, which is close to that of the usual commercial Si p-i-n photodiode. The present devices are stable and well reproducible. [ABSTRACT FROM PUBLISHER]

Chalcogen-doped microstructural silicon irradiated by femtosecond laser has high near-uniform absorption on a broad spectrum, but the factors leading to infrared absorption are complex and remain an open problem. To clarify the origin of infrared absorption besides hyperdoped Chalcogen atoms, microstructural silicon is fabricated by femtosecond laser under vacuum condition. The relationship between infrared absorption and as-formed new phases (amorphous silicon: $\alpha $ -Si and nanocrystal silicon) is established. It indicates that the infrared absorption is caused by defects related to Urbach states from \alpha $ -Si or nanocrystal Si, and these metastable defects disappeared after a thermal annealing process. From the absorption spectrum of microstructural silicon after etching at different times, it could be figured out that the Urbach states exist in both the surface and subsurface regions of black silicon at a depth of about 2.4 \mu \text{m} . [ABSTRACT FROM PUBLISHER]

The regular micrometer-scale triangular arrays were formed using ultrafast femtosecond laser irradiation on (111) surface of silicon wafer immersed in KOH solution (0.1 g/ml). At low laser fluence, the resulting surface is covered by triangular pits microstructures, whereas at high laser fluence, the structures are transformed to multilayer-triangular stacks-microstructures. The number of triangular stacks layer increased as the laser fluence increased. The formation of triangle microstructure arrays depends on both silicon surface crystallographic orientation and the concentration of KOH solution. Either for lower KOH solution concentration (0.02 g/ml) or other silicon crystallographic orientation, triangle arrays cannot be obtained. We attribute the formation of triangular microstructure arrays to the laser-assisted chemical etching process. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A strain-scanning second-harmonic generation technique is proposed for high-sensitivity measurement of weak strain in film surfaces or interfaces. The basic idea is the sequential application of tensile and compressive strains to a strained film sample. From the strain-dependent second-harmonic generation (SHG) intensity, the type of the strain can be easily judged from whether the SHG is enhanced or weakened, and its magnitude can be precisely calibrated by an externally applied strain that is known. Thus, the built-in strain of a SiO2/Si interface could be determined as tensile with a magnitude of 3.07\,\times 10^-4. [ABSTRACT FROM PUBLISHER]