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...

An atomic-force microscope ͑AFM͒ is used to locally inject, detect, and quantify the amount and location of charge in SiO 2 films containing Si nanocrystals ͑size ϳ2-6 nm͒. By comparison with control samples, charge trapping is shown to be due to nanocrystals and not ion-implantation-induced defects in samples containing ion-beam-synthesized Si nanocrystals. Using an electrostatic model and AFM images of charge we have estimated the amount of charge injected in a typical experiment to be a few hundred electrons and the discharge rate to be ϳ35Ϯ15 e/min.

Conducting-tip atomic force microscopy ͑AFM͒ has been used to electronically probe silicon nanocrystals on an insulating substrate. The nanocrystal samples were produced by aerosol techniques and size classified; nanocrystal size can be controlled in the size range of 2-50 nm with a size variation of less than 10%. Using a conducting tip, the charge was injected directly into the nanocrystals, and the subsequent dissipation of the charge was monitored. Estimates of the injected charge can be made by comparison of the data with an intermittent contact mode model of the AFM response to the electrostatic force produced by the stored charge.

Two models are presented for quantitative charge imaging with an atomic-force microscope. The first is appropriate for noncontact mode and the second for intermittent contact ͑tapping͒ mode imaging. Different forms for the contact force are used to demonstrate that quantitative charge imaging is possible without precise knowledge of the contact interaction. From the models, estimates of the best charge sensitivity of an unbiased standard atomic-force microscope cantilever are found to be on the order of a few electrons.

Si nanocrystals (diameter 2 – 5 nm) were formed by 35 keV Si+ implantation at a fluence of 6×1016 Si/cm2 into a 100 nm thick thermally grown SiO2 film on Si (100), 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 O2 at 1000 °C for times up to 30 min. results in oxidation of the Si nanocrystals, first close to the SiO2 film surface and later at greater depths. Upon oxidation for 30 min. the photoluminescence peak wavelength blue-shifts 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 fabrication of a laser based on SiO2 waveguides doped with Si nanocrystals seems possible, if the nanocrystal size distribution can be narrowed down into the percent range.