Optical properties of nanoscale, one-dimensional silicon grating structures

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Abstract
We report a detailed study of nanostructure fabrication and optical characterization of sub-μm-period, one-dimensional, Si grating structures. Nanoscale wall width structures were fabricated by combining laser interferometric lithography with anisotropic wet-chemical etching (KOH) and thermal oxidation. Structure wall widths were characterized by Raman scattering (RS) and scanning electron microscopy. Salient features of the RS measurements as a function of wall widths from ∼100 to 10 nm were: (a) large cross-section enhancements, ∼100×, for linewidths ∼50 nm; (b) asymmetric line shapes with tails extending to smaller Raman shifts for linewidths ∼20 nm; and (c) splitting of the bulk Raman mode, again to lower Raman shifts, for linewidths ∼10 nm. For room temperature photoluminescence (PL) measurements, the grating structures were excited at 257 nm. PL measurements are reported for oxidized and unoxidized grating structures with peaks varying between 380 and 700 nm. PL was only observed for Si structures with dimensions less than about 10 nm. PL intensities and spectral line shapes varied significantly as a result of surface modification treatments such as high temperature anneal in a N2 atmosphere, immersion in boiling H2O, and long-term exposure to ambient air. The measurements indicate a strong correlation of the visible PL with crystal size (∼5–10 nm); however, it remains unclear if the mechanism responsible is quantum confinement, passivation of the surface by Si:Hx complexes, or optically active surface states.