The Optical Properties of ION Implanted Silica

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where Q is the volume fraction occupied by the metallic particles, no is the refractive index of the host medium, and E,and P2 are the real and imaginary parts of the frequency dependent dielectric constant of the bulk metal. Equation (1) is a Lorentzian function with a maximum value at the surface plasmon resonance frequency (op), where l(0)+2n2 = 0, (2)

351 Mat. Res. Soc. Symp. Proc. Vol. 504 © 1998 Materials Research Society

where 61= n2 - k2 with n and k being the optical constants of the bulk metal. Using the tabulated optical constants the theoretical values for the wavelength of the absorption bands for Au, Ag, Cu, and Sn are tabulated in table 1, where n. -1.46 for suprasil. Using the "characteristic" optical absorption band, the average radius of the metal spheres, is determined from the resonance optical absorption spectrum and using the Doyle theory [10] according to the equation r = V1/AO112 -, where Vf is the Fermi velocity of the metal and Aco,,1 2 is the full width at half maximum (FWHM) of the absorption band due to the plasmon resonance in small metal particles. EXPERIMENTAL PROCEDURES The silica glass used in this work is Suprasil provided by Hereaus Amersil, Inc. It contains 150 ppm OH, 0.05 ppm of Ti, Na, Ca and Al and less than 0.01 ppm other metals. Samples 10 x 10 x 0.5 mm were implanted with 3.0 MeV gold, 0.16 MeV Silver, 1.5 MeV silver, 2.0 MeV copper, or 0.35 MeV tin ions at a current density less than 2 hLA/cm 2 . Ion fluences varied from 1 x 1016 ions/cm 2 to 8 x 1017 ions/cm 2. We studied and measured the threshold fluence, above which spontaneous formation of nanoclusters was observed by optical means. The heat treatment for the samples was done in air at temperatures between 100 0 C and 1200 0 C for I hour at each heat treatment temperature. After each heat treatment, an optical absorption spectrum was measured. From these spectra we calculate the size of the metallic clusters which are responsible for the optical absorption band. The average radius of metal spheres, small compared with the wavelength of light, is then determined using Doyle theory, the measured optical photospectrometry, and the RBS results from each implanted sample. RESULTS and DISCUSSIONS The higher the atomic number and fluence of the bombarding ion, the more the damage and modification of the optical properties [8,9] of the silica. This was observed by absorption spectrometry during all of our bombardments. Except for the effects attributable to the metal clusters, these other effects did not produce any appreciable characteristic absorption band for suprasil and were reduced with increase in the heat treatment temperature. With heat treatment the near neighbor clusters coalesce and the host accommodates to the volume reduction. RBS and TEM measurements confirm that the depth profile of the metal clusters formed after heat treatment is almost identical to, or slightly narrower than, that of the atoms initially implanted by ion bombardment. Figure 1 shows the optical absorption spectra for 1.5 MeV Ag implan