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Observations of efficient room temperature photoluminescence (PL) from porous Si have generated a great deal of interest in the optical properties of nm-scale Si structures. The stochastic character of porous-Si fabrication results in a distribution of crystal sizes and shapes. We report on a scalable (to large areas) and manufacturable (to high volumes) fabrication technology for uniform, nm-linewidth Si structures providing an important testbed for controlled studies of these optical properties. Large areas ( - I cm2) of extreme sub-ýtm structures (to - 5 nm) are reproducibly fabricated. Both walls (1-D confinement) and wires (2-D confinement) are reported. The fabrication process includes: interferometric lithography, highly anisotropic KOH etching, and structure dependent oxidation. For the walls, nearly perfect crystal planes form the sidewalls and very high width/depth aspect ratios (> 50) have been achieved. Raman scattering results on the walls demonstrate three regimes: 1) lineshapes and cross sections similar to bulk Si for line widths, W > 200 nm; 2) electromagnetic resonance enhancement of the cross section ( to - 100x) for W from 50-200 nm; and 3) highly asymmetric lineshapes and splittings from W < 30 nm. Photoluminescence is observed for the thinnest samples (W < 10 nm) and is as intense as that observed from porous Si with a spectral linewidth - 50 % smaller than that of porous Si. INTRODUCTION

Silicon's indirect bandgap has limited its applications to optoelectronics. A radiative transition in Si requires a 3-body (electron, hole, phonon) process, which makes it a poor luminescent material in comparison with direct bandgap semiconductors such as GaAs. Since the observations of strong room temperature PL from porous Si by Canham[l], much interest has been generated in the possibilities of Si as an optoelectronics material, potentially leading to applications fully integrated with Si electronics. Although Pickering et ai.[2,3] had previously reported on PL from porous Si, they attributed it to a complex mixture of phases, i.e., voids, a-Si:O (and/or atSi:H) complexes. The interpretation by Canham[l] and Lehman[4] that the PL may be due to quantum confinement effects has stimulated intense research efforts to understand the physical mechanisms responsible for the PL. So far, no clear picture has emerged. Many researchers have attributed porous Si PL to chemical compounds formed by hydrogen and fluorine with exposed Si atoms. Xu et al.[5] observed quenching of PL on simultaneous exposure of porous Si to 02 and visible light suggesting a role played by Si:H bonds which are replaced by 02 quenching the PL. Brandt et al.[6] investigated PL and vibrational properties of porous Si and chemically synthesized siloxene (Si60 3H6) and its derivatives. From a comparison of the respective spectra, they concluded that PL originated from siloxene derivatives in porous Si. Similar conclusions were reached by Roy et al.[ 7] from x-ray photoelectron spectroscopy of porous Si. AFM and TEM characterization of porous Si