Tuning the emission wavelength of Si nanocrystals in SiO2 by oxidation

Abstract

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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