We show that light emission from different systems of silicon nanocrystals does behave as expected for indirect-band-gap quantum dots. Photoluminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA and TO momentumconserving phonon-assisted optical transitions. These spectra allow us to determine the ratio of nophonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energies. The ratio between these recombination channels changes by 2 orders of magnitude with increasing confinement energy. For confinement energies above 0.7 eV the radiative transitions are governed by no-phonon quasidirect processes. [S0031-9007 (98)07199-3] PACS numbers: 78.55.Ap, 78.66.Li

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Mechanism of the visible electroluminescence from metal/porous silicon/ n -Si devices

Nanometer-sized Au dots have been fabricated on p-type Si(111) substrates by field-induced transfer of tip material in a scanning tunneling microscope, thus forming a nanometer Schottky contact. Positioning the scanning tunneling microscope tip on a very tiny dot (10–15 nm in diameter), equidistant steps with a spacing of several 100 mV and regions with negative differential resistance are observed in the current–voltage characteristic. Numerical simulations confirm that the determinant capacitance between tip and Au dot falls in the 10−19 F range, implying that the origin of the monitored step structure can be interpreted to arise from single-electron tunnel effects (Coulomb staircase). As the nanometer Schottky contact forms a not well-insulating configuration, the development of negative differential resistance might be explained by a leaking of excess carriers from the dot to the immediate surroundings.

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