Stability of Se passivation layers on Si(001) surfaces characterized by time-of-flight positron annihilation inducedAuger electron spectroscopy

Silicon nanocrystals (NCs) 8-10 nm in diameter are grown on SiO 2 surfaces in an ultrahigh-vacuum chamber using hot wire chemical vapor deposition. These NCs are subjected to varying exposures of deuterated ammonia (ND 3 ). The surface chemistry of Si NCs is studied using X-ray photoelectron (XP) spectroscopy and temperature-programmed desorption (TPD). The dissociative adsorption of ND 3 on Si NCs results in the formation of ND 2 species prior to TPD, and Si 3 N (nitride) formation is observed after TPD. D 2 desorption is observed only from the monodeuteride species at 780 K. In separate experiments, a hot tungsten filament is used to predissociate ND 3 before adsorption on the NC surface. XP spectra reveal that ND 2 species form initially, and as the dose is increased, ND species dominate. After TPD, a Si x N y species is observed. D 2 desorption is observed from the mono-, di-, and trideuteride species when ND 3 is predissociated. Irrespective of the technique used for dosing ammonia, TPD spectra do not contain any ND 3 fragments, indicating that the ND 2 species are not thermally stable on the NC surface. The photoluminescence (PL) emitted from Si NCs (diameter ∼ 4.1 nm) is reported for an excitation wavelength of 405 nm. PL is observed only when the hot filament is used to predissociate ND 3 , and this can be attributed to the presence of di-and trideuteride species on the nanocrystal surface, which results in better passivation.

Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR) spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.

Silicon (Si) nanocrystals (NCs) less than 5 nm in diameter are grown on SiO 2 surfaces using hot wire chemical vapor deposition, and the dangling bonds and the reconstructed bonds at the NC surface are passivated and transformed with D and ND x by using deuterated ammonia (ND 3 ), which is predissociated over a hot filament. At low hot wire ND 3 doses, photoluminescence (PL) emission is observed from a defect state at 1000 nm corresponding to reconstructed surface bonds capped by predominantly monodeuteride and Si-ND 2 species. As the hot wire ND 3 dose is increased, di-and trideuteride species form, and intense PL is observed around 800 nm, which does not shift with NC size and is associated with defect levels resulting from ND x insertion into the strained Si-Si bonds forming Si 2 dND. A clean bandgap can be realized with fully relaxed and fully terminated NC surfaces consisting of di-and trideuterides and SiND 2 . SECTION Nanoparticles and Nanostructures

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