Evidence of quantum size effect in nanocrystalline silicon by optical absorption

The optical absorption spectrum in nanocrystalline silicon (n-Si) was determined from both light transmittance and reflectance measurements. We observed that n-Si has a phonon structure in the optical absorption spectrum. This structure originates from momentum-conserving TO phonon absorption and emission, and provides direct evidence that n-Si is an indirect-band-gap semiconductor with quantum size effects. By using small-angle x-ray scattering to measure the nanocrystal size distribution, we found that the band-gap widening varies as (1/L)(1/6) with decreasing nanocrystal diameter L.