We increased the absorptance of light by silicon to approximately 90% from the near ultraviolet (0.25 μm) to the near infrared (2.5 μm) by surface microstructuring using laser-chemical etching. The remarkable absorptance most likely comes from a high density of impurities and structural defects in the silicon lattice, enhanced by surface texturing. Microstructured avalanche photodiodes show significant enhancement of below-band-gap photocurrent generation at 1.06 and 1.31 μm, indicating promise for use in infrared photodetectors.
We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF6 produces an absorptance of 0.9 for radiation in the 1.2–2.5 μm wavelength range, higher than any of the other gases. Use of Cl2, N2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat crystalline silicon, for which the absorptance is roughly 0.05–0.2 for a 260 μm thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration.
Articles you may be interested inInfluence of ion mixing on the energy dependence of the ion-assisted chemical etch rate in reactive plasmas An ion beam etching study, designed to characterize the important kinetic and transport processes involved in the ion-assisted etching of silicon in both molecular and atomic chlorine, was performed. Monoenergetic argon ions were directed normal to a silicon wafer that was simultaneously exposed to a neutral molecular and/or atomic chlorine beam. Dissociation of the beam was induced by thermally heating the graphite tip of an effusive source via electron impact. Beam composition was characterized using a quadrupole mass spectrometer and was found to be in excellent agreement with a thermodynamic equilibrium model at the source pressure and tip temperature. Unpatterned polysilicon wafers were etched to determine the ion-induced etching yields as a function of ion energy, ion to neutral flux ratio, and neutral flux composition. A physically based kinetic model was developed to represent the yield data, incorporating chlorine adsorption, atomic to molecular chlorine surface recombination, and the ion-induced desorption of adsorbed chlorine and silicon chloride products. Feature profile etching experiments using patterned silicon wafers were also performed under ion and neutral-limited conditions of varying neutral composition. Resulting profiles were examined for aspect ratio dependent etching effects, where traditional lag was observed for features etched using an isotropically distributed background chlorine flux and inverse lag was observed for features etched with a molecular and atomic chlorine flux arriving directly from the effusive source. Microtrenching was also present in the etched features. Computer simulations of the etching process and profile development were performed using the kinetic model and a line-of-sight re-emission model for the chlorine transport. The dependence of the yield on the ion angle of incidence was also incorporated into a simulation for an isotropically distributed molecular chlorine flux and was found to have a significant impact on profile evolution as a function of the ion to neutral flux ratio. Using the simulation, atomic to molecular chlorine recombination effects were also explored as a function of the surface recombination coefficient. Predictions of the simulations were compared to experimentally derived profiles and were found to be in good agreement.
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