Direct transition at the fundamental gap in light-emitting nanocrystalline Si thin films

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Optical transitions at the fundamental gaps of 1.20–1.37 eV are observed at 293 K directly by electroreflectance (ER) spectroscopy in nanocrystalline Si (nc-Si) thin films, from which photoluminescence (PL) with dominant peak energies of 1.65–1.75 eV is observed. Also observed by ER are optical transitions at 2.2 and at 3.1–3.4 eV. With a decrease in the mean crystal size from ∼3 nm to below 2 nm, the transition energy of the fundamental gap is increased and the ER signal is intensified. The band-gap widening would be due to quantum confinement in Si nanocrystals, and the increased signal indicates the appearance of direct transitions. In conjunction with the ER results, the PL mechanisms have been discussed.