DOSE-RATE DEPENDENCE OF THE OPTICAL-ABSORPTION OF CU IMPLANTED SILICA

High-purity silica (Spectrosil) samples were implanted with Cu at doses from 1.0 x 10(16) to 6.0 x 10(16) ions/cm2. Samples implanted to a dose of 6.0 x 10(16) ions/cm2 were implanted at dose rates of 7.5, 5, 2.6, and 0.7-mu-A/cm2. Some samples were implanted with smaller total doses at dose rates of 7.5 and 2.5-mu-A/cm2. Backscattering measurements determined the concentration of implanted ions as a function of dose and depth. The optical absorption, measured from 1.8 to 6.0 eV at room temperature, was observed to increase with increasing dose rate in samples implanted with a total dose greater-than-or-equal-to 3.0 x 10(16) ions/cm2. We attribute the increase in optical absorption at energies between 2 and 5 eV with dose in these samples to the formation of spherical copper colloids of various diameters. The peak at 2.2 eV that forms at dose levels greater-than-or-equal-to 3.0 x 10(16) was observed to increase nonlinearly with dose rate. We attribute this peak to the formation of prolate copper spheroids with an aspect ratio of approximately 2. We suggest that the dose rate dependence is due to localized heating which increases with increased dose rate. Samples implanted with doses < 3 x 10(16) ions/cm2 did not display this same strong dependence on dose rate. For these doses copper colloids are not a major source of absorption in the 2-5 eV range. In these samples we attribute the absorption to Cu+ and to intrinsic defects produced by radiation damage.