Positron annihilation spectroscopy ͑PAS͒ is used to study Si-rich SiO 2 samples prepared by implantation of Si ͑160 keV͒ ions at doses in the range 3ϫ10 16 -3ϫ10 17 cm Ϫ2 and subsequent thermal annealing at high temperature ͑up to 1100°C͒. Samples implanted at doses higher than 5 ϫ10 16 cm Ϫ2 and annealed above 1000°C showed a PAS spectrum with an annihilation peak broader than the unimplanted sample. We discuss how these results are related to the process of silicon precipitation inside SiO 2

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... The study of silicon-rich SiO 2 ͑SiO x ) has been the matter of several papers. Different models have been proposed to describe this structure: a random-bonding model in which Si-Si and Si-O bonding are randomly distributed; 1 a randommixture model for which tetrahedral units are grouped together; 2 finally, a shell model in which silicon clusters are embedded in stoichiometric SiO 2 . 3 Each of these models gives a correct picture for a different degree of silicon excess. Apart from these fundamental questions, Si precipitation in Si-rich SiO 2 has been used to produce Si nanocrystals ͑Si nc ) inside a dielectric matrix which can give visible luminescence at room temperature. [4] [5] [6] In this study we examined annealing behavior of Si-rich SiO 2 using positron annihilation spectroscopy ͑PAS͒. This technique is based on the fact that positrons, when implanted at a given energy, thermalize and diffuse inside the medium, and finally annihilate with electrons. Gamma-rays emitted after the annihilation carry information about the annihilation site. In a Doppler broadening measurement this information is extracted by analyzing the broadening of the 511 keV annihilation peak, due to the non-zero momentum of electrons. 8 The availability of variable energy positron beams allow non-destructive depth-profiling of materials. 8, 9 The technique has been extensively used to study the Si- Samples were prepared by implanting 160 keV 28 Si ϩ ions into 430 nm thick SiO 2 layers thermally grown on a ͑100͒ oriented p-type Si substrate kept at room temperature during implantation. Fluences ranged from 3ϫ10 16 cm Ϫ2 to 3ϫ10 17 cm Ϫ2 , the ion current density was 0.3 A/cm 2 . For a SiO 2 layer on a silicon substrate, the Si-SiO 2 interface represents a preferential site for trapping positrons. This gives rise to a typical Doppler broadening signal with an S-parameter lower than that of Si and SiO 2 . 10 The effect is particularly important when low implantation doses are con-sidered. In order to discriminate effects related to Si ions implantation from the original Si-SiO 2 interface signal, a second set of samples, obtained by implanting in fused quartz Suprasil substrate was considered. Suprasil samples were implanted at doses from 5ϫ10 16 cm Ϫ2 to 1.5ϫ10 17 cm Ϫ2 . Positron annihilation spectroscopy measurements were performed in an ultrahigh vacuum chamber (Ͻ10 Ϫ7 Torr͒ using a variable energy positron beam. In each measurement 10 6 counts were accumulated. The Doppler broadening of the 511 keV annihilation line was measured with a HPGedetector based gamma spectroscopy system. The broadening was characterized using the line shape parameter ͑S-parameter͒, defined as the area of a fixed region (Ϸ1.59 keV wide͒ in the center of the annihilation peak divided by the total area of the peak. 8 The sharpness of the annihilation peak is related to the S-parameter, namely sharper peak produces a higher S-value. In order to compare different S-E spectra, it is customary to divide measured S-parameter by that of a reference sample measured under the same experimental conditions. A p-type high resistivity Si sample was taken as a reference. Suprasil samples were annealed in a vacuum furnace (Ϸ10 Ϫ6 Torr͒. Thermal SiO 2 layers on Si samples were annealed in situ up to 700°C with a resistively heated tantalum foil, at 10 Ϫ7 Torr. Above 700°C, annealing was performed in N 2 atmosphere. Annealing time was 30 min. Figure 1 shows the S-parameter values as a function of the positron implantation energy E ͑S vs E͒ for Suprasil samples implanted at different fluences. The upper abscissa represents the mean positron implantation depth z calculated according to the relation: zϭAE n /, where, in case of SiO 2 , ϭ2.33 g/cm 3 , nϭ1.6, and Aϭ4.0 g cm Ϫ2 keV Ϫn . 9 The S-parameter versus implantation energy curve for unimplanted sample is also reported. The low S-value observed at the surface for this sample is related to positrons implanted at non-zero energy, diffusing back to the surface and annihilating there. When the implantation energy increases, a͒ Present address: CORECOM,