Two sources of room temperature visible luminescence are identified from SiO 2 films containing ion beam synthesized Si nanocrystals. From a comparison of luminescence spectra and photoluminescence decay lifetime measurements between Xe ϩ -implanted SiO 2 films and SiO 2 films containing Si nanocrystals, a luminescence feature attributable to defects in the SiO 2 matrix is unambiguously identified. Hydrogen passivation of the films selectively quenches the matrix defect luminescence, after which luminescence attributable to Si nanocrystals is evident, with a lifetime on the order of milliseconds. The peak energy of the remaining luminescence attributable to Si nanocrystals ''redshifts'' as a function of different processing parameters that might lead to increased nanocrystal size and the intensity is directly correlated to the formation of Si nanocrystals. Upon further annealing hydrogen-passivated samples at low temperatures ͑Ͻ500°C͒, the intensity of nanocrystal luminescence increases by more than a factor of 10.

Silicon nanocrystals with diameters ranging from Ϸ2 to 5.5 nm were formed by Si ion implantation into SiO 2 followed by annealing. After passivation with deuterium, the photoluminescence ͑PL͒ spectrum at 12 K peaks at 1.60 eV and has a full width at half maximum of 0.28 eV. The emission is attributed to the recombination of quantum-confined excitons in the nanocrystals. The temperature dependence of the PL intensity and decay rate at several energies between 1.4 and 1.9 eV was determined between 12 and 300 K. The temperature dependence of the radiative decay rate was determined, and is in good agreement with a model that takes into account the energy splitting between the excitonic singlet and triplet levels due to the electron-hole exchange interaction. The exchange energy splitting increases from 8.4 meV for large nanocrystals ͑Ϸ5.5 nm͒ to 16.5 meV for small nanocrystals ͑Ϸ2 nm͒. For all nanocrystal sizes, the radiative rate from the singlet state is 300-800 times larger than the radiative rate from the triplet state. © 2000 American Institute of Physics. ͓S0003-6951͑00͒04402-8͔Si nanostructures have received intense study, which is stimulated by their potential for use in Si-based optoelectronic devices.1,2 The photoluminescence ͑PL͒ from Si nanostructures that are well passivated by H or SiO 2 is attributed to the recombination of quantum-confined excitons.3-6 The nanocrystals show a higher quantum efficiency for optical emission than bulk Si, 4 and exhibit a band gap ͑emission energy͒ that can be continuously tuned over a large part of the visible spectrum and to the near infrared, by varying the size.7 One of the most controlled methods to fabricate Si nanocrystals in SiO 2 is ion beam synthesis. [8][9][10] Si nanocrystals formed by this method provide an ideal system for the study of their size-dependent optical properties since the nanocrystals ͑1͒ generally have a wide size distribution, ͑2͒ are all close to spherical in shape, and ͑3͒ are well passivated by the SiO 2 matrix. Si nanocrystal-doped SiO 2 films made by ion beam synthesis show PL that can be attributed to two distinct sources.8 One luminescence feature is due to ion irradiation-induced defects and can be fully quenched by introducing H or D into the film. The other is attributed to the recombination of quantum-confined excitons.In this letter, we present temperature-dependent measurements of the PL intensity and PL decay rate of ion beam synthesized Si nanocrystals in the size range of Ϸ2-5.5 nm. The temperature dependence of the radiative decay rate of Si nanocrystals is determined and compared to a model introduced by Calcott et al., 6 that takes into account the exchange interaction splitting of the singlet and triplet exciton energy levels. 3,6 The exchange splitting energy is determined as a function of emission energy in the spectral range 1.4-1.9 eV, and compared to data for porous Si for energies Ͼ1.8 eV. The ratio of the singlet and triplet radiative decay rates is determined for the first time.

Synthesis of Ge nanocrystals in SiO 2 films is carried out by precipitation from a supersaturated solid solution of Ge in SiO 2 made by Ge ion implantation. The films exhibit strong room-temperature visible photoluminescence. The measured photoluminescence peak energy and lifetimes show poor correlations with nanocrystal size compared to calculations involving radiative recombination of quantum-confined excitons in Ge quantum dots. In addition, the photoluminescence spectra and lifetime measurements show only a weak temperature dependence. These observations strongly suggest that the observed visible luminescence in our samples is not due to the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, observations of similar luminescence in Xe ϩ -implanted samples and reversible PL quenching by hydrogen or deuterium suggest that radiative defect centers in the SiO 2 matrix are responsible for the observed luminescence.

Si nanocrystals ͑diameter 2-5 nm͒ were formed by 35 keV Si ϩ implantation at a fluence of 6 ϫ10 16 Si/cm 2 into a 100 nm thick thermally grown SiO 2 film on Si ͑100͒, followed by thermal annealing at 1100°C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O 2 at 1000°C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO 2 film surface and later at greater depths. Upon oxidation for 30 min the photoluminescence peak wavelength blueshifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The room temperature luminescence lifetime measured at 700 nm increases from 12 s for the unoxidized film to 43 s for the film that was oxidized for 29 min. © 1998 American Institute of Physics. ͓S0003-6951͑98͒03220-3͔The recent discovery of visible light emission from nanocrystalline group IV materials has stimulated considerable experimental effort to understand its origin and utilize it to fabricate Si-based optoelectronic devices.1-4 The process of making Si nanocrystals by Si ion implantation into thermal SiO 2 films on Si, followed by precipitation, 5-7 is fully compatible with standard integrated circuit technology and the SiO 2 matrix is a robust host that provides good passivation for the Si nanocrystals. Previously, we have demonstrated that SiO 2 films containing Si nanocrystals made by ion implantation show photoluminescence in the visible and near-infrared that can be attributed to two distinct sources. One luminescence feature is related to ion irradiation induced defects in the SiO 2 matrix and can be quenched by introducing H or D into the film. 6 The other has been attributed to radiative recombination of quantum-confined excitons in the Si nanocrystals. We have also shown that the nanocrystal luminescence intensity can be increased by as much as a factor of 10 by annealing a deuterated sample at 400°C, which is attributed to the passivation of dangling bonds at the nanocrystal/SiO 2 interface.6 Previously, it has been shown that the luminescence from Si nanocrystals in SiO 2 can be continuously redshifted by thermal annealing, due to an increase in the crystallite size. In this letter, we demonstrate that the photoluminescence ͑PL͒ peak wavelength from Si nanocrystals embedded in an SiO 2 film can be blueshifted by more than 200 nm by thermal oxidation at 1000°C. Transmission electron microscopy ͑TEM͒ and Rutherford backscattering spectrometry ͑RBS͒ measurements show the oxidation of Si particles starts near the surface of the SiO 2 film and as time progresses, an oxidation front moves deeper into the film. The PL blueshift is attributed to a decrease in the average nanocrystal size as the oxidation progresses, in agreement with quantum c...

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