Excitons In Silicon Nanocrystals Research Articles

Radiation spectrum of hot excitons in Si nanocrystals

The self-trapped exciton state (STE) is very important for the dynamics of hot excitons in photoexcited silicon nanocrystals embedded in a SiO2 matrix. This fact has been recently confirmed by the experimental data obtained by the femtosecond pump– probe spectroscopy technique in Amsterdam University. In this work we have studied the energy exchange between the exciton localized in the STE state and the hot exciton in the core of silicon nanocrystals and have shown that it determines the dynamics of the energy distribution of the hot excitons. Using the Monte-Carlo we have simulated the energy distribution of excitons in the time interval 10–100 ps after excitation. Thus the model of formation of the distribution of hot excitons in silicon nanocrystals is developed and the fast formation of the wide energy distribution is demonstrated. The form of the photoluminescence spectrum almost directly corresponds to the energy distribution of excitons in a silicon nanocrystal at a given moment. In the result we have found the relaxation times of hot excitons equal to 100 ps and the inner quantum efficiency of the ultrafast photoluminescence of about 0.1%. These values are close to the experimentally observed ones.

Luminescence properties of thin nanocrystalline silicon-carbide films fabricated by direct-beam ion deposition

Nanocomposite films containing nanocrystals of silicon and silicon carbide are fabricated by direct ion-beam deposition onto silicon substrates. The films obtained are by Raman spectroscopy and the photoluminescence method. It is found that the samples under study exhibit two photoluminescence bands, in the “red” and near-infrared (IR) (600–1000 nm) and “blue” (400–550 nm) spectral ranges, which are accounted for by, respectively, the radiative recombination of excitons in silicon nanocrystals and radiative transitions between levels related to local centers (defects) on the surface of silicon nanocrystals and in the surrounding matrix. Short-duration treatment of the films in a solution based on hydrofluoric acid leads to modification of the emission spectrum and to an increase in the intensity of the exciton band due to the passivation of defects on the surface of silicon nanocrystals.

Effect of acetone vapor treatment on photoluminescence of porous nc-Si–SiOx nanostructures

The effect of treatment in saturated acetone vapors on the spectral composition and intensity of photoluminescence (PL) in porous oblique deposited SiOx films is studied. As a result of this treatment followed by high-temperature annealing at the temperature 930 °C, considerable PL intensity growth and the small blueshift of PL peak position are observed in the porous, column-like structure films containing Si nanocrystals. A more intense shortwave band (peak position - 540-560 nm) appears in the PL spectrum of these structures, in addition to the longwave band (760-780 nm). Both PL bands in treated samples are characterized by monomolecular radiative recombination, which can be attributed to annihilation of excitons in silicon nanocrystals embedded into oxide matrix (longwave band) and in carbon-enriched matrix near surface of oxide nanocolumns (shortwave band). The possibility to control the PL characteristics of the porous structures in a wide spectral range by above treatment is shown.

Air-stable full-visible-spectrum emission from silicon nanocrystals synthesized by anall-gas-phase plasma approach

A novel dual-plasma system has been developed to combine the synthesis of siliconnanocrystals (Si-NCs), the etching to controllably tailor the Si-NC size, and the surfacefunctionalization of Si-NCs into one simple all-gas-phase process. Si-NCs are synthesized inSiH4-based plasma; theythen travel through CF4-based plasma, where Si-NCs are etched and passivated by C and F. The resultingSi-NCs exhibit air-stable emission across the full visible spectrum. Structural andoptical characterization indicates that the emission in the red-to-green range isbased on the recombination of quantum-confined excitons in Si-NCs, while theblue emission originates from defect states. The quantum yields of stabilizedphotoluminescence from Si-NCs range from 16% at the red end to 1% at the blue end.

Effect of electric field on recombination of self‐trapped excitons in silicon nanocrystals

AbstractStudy of the effect of an external electric field on photoluminescence (PL) of silicon nanocrystals formed in SiO2 has been carried out. It was found that the electric field results in a red shift of PL band and a decrease of the PL intensity at any temperature in the range of 5‐290 K. A decay time of PL intensity after pulse excitation does not depend on the electric field strength. The experimental data are explained in framework of the model of recombination of self‐trapped excitons formed at nanocrystal‐matrix boundary. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface plasmon polariton mediated photoluminescence from excitons in silicon nanocrystals

Surface plasmon polaritons (SPPs) of a metal film can efficiently be excited when a light emitter is placed nearby. The excited SPPs are converted to photons by compensating for the momentum mismatch. The authors study SPP-mediated emission from excitons in Si nanocrystals (Si-nc’s) by placing an organic grating on a thin Au film placed near Si-nc’s. The dispersion relation is obtained from angle-resolved photoluminescence measurements, and all the observed modes are well explained by model calculation. The results indicate that excitons in Si-nc’s can efficiently excite SPPs in thin metal films and directed photoluminescence can be realized.

Mechanism of a remarkable enhancement in the light emission from nanocrystalline porous silicon annealed in high-pressure water vapor

To clarify the effect of surface passivation on the optical properties of nanocrystalline porous silicon (PS), the photoluminescence (PL) characteristics of PS have been investigated by employing a high-pressure water vapor annealing (HWA). PS samples with various porosities were prepared on (100)-oriented p-type (4Ωcm) single-crystalline silicon wafers by electrochemical anodization. Some samples were then electrochemically oxidized. The HWA treatment was then applied to the prepared PS samples at 0.5–3MPa and 200–300°C for 2–3h. The PL intensities, spectra, and dynamics after HWA were measured in relation to surface analyses by Fourier-transform-infrared (FTIR) spectroscopy. It is shown that the HWA treatment leads to a drastic enhancement in both the PL efficiency and stability. Under the optimum condition, the PS sample exhibits an extremely high external quantum efficiency of 23% at room temperature. According to the FTIR spectra analyses, silicon nanocrystallites in HWA-treated PS are covered with a high-quality SiO2 tissue. The PL decays are found to be longer than those of as-prepared PS, and become closer to a single-exponential behavior near the PL peak wavelength. The observed high efficiency and stability of PL emission from HWA-treated PS is attributed to (i) suppression of nonradiative surface defect density, (ii) uniform passivation by unstrained thin oxides, and (iii) strong localization of excitons in silicon nanocrystals. This low-temperature treatment is very useful for obtaining highly efficient and stable luminescent PS and devices.

Photoluminescence of Er3+ ions in layers of quasi-ordered silicon nanocrystals in a silicon dioxide matrix

The spectra and kinetics of photoluminescence from multilayered structures of quasi-ordered silicon nanocrystals in a silica matrix were studied for undoped samples and samples doped with erbium. It was shown that the optical excitation energy of silicon nanocrystals could be effectively transferred to Er3+ ions, which was followed by luminescence at a wavelength of 1.5 µm. The effectiveness of energy transfer increased as the size of silicon nanocrystals decreased and the energy of exciting light quanta increased. The excitation of erbium luminescence in the structures was explained based on dipole-dipole interaction (the Forster mechanism) between excitons in silicon nanocrystals and Er3+ ions in silica surrounding them.

Self-trapped excitons in silicon nanocrystals with sizes below 1.5 nm in Si/SiO2 multilayers

Photoluminescence from silicon nanocrystals in nc-Si/SiO2 multilayers has been investigated by time-resolved measurements. The photoluminescence (PL) decay is fitted by two exponentials with relaxation times of the order of ∼25–30 and ∼80–100 μm at room temperature which are independent of the emission wavelength. The experimental data are interpreted by considering two light emission mechanisms with closely similar wavelengths, both involving localized states and corresponding to two coupled subsystems. One involves localized states related to Si=O bonds, and the other self-trapped excitons. In the case of light emission from self-trapped exciton annihilation, PL was not limited by Auger recombination in the regime of multiple excitation of silicon nanocrystals.

Recombination of self-trapped excitons in silicon nanocrystals grown in silicon oxide

The kinetics of photoluminescence (PL) and steady-state PL from silicon nanocrystals formed in the SiO2 matrix by silicon ion implantation were studied experimentally for the first time in the temperature range from liquid-helium to room temperature. A dramatic increase in the photoluminescence decay time, accompanied by PL intensity quenching, is observed below 70 K. The results obtained indicate that the silicon nanocrystal PL arises from radiative recombination of excitons self-trapped at the silicon nanocrystal-SiO2 interface.