Dose rate dependence of the optical absorption of Cu implanted silica

Abstract

High-purity silica (Spectrosil) samples were implanted with Cu at doses from 1.0 × 10 16 to 6.0 × 10 16ions/cm 2. Samples implanted to a dose of 6.0 × 10 16ions/cm 2 were implanted at dose rates of 7.5, 5, 2.6, and 0.7 mA/cm 2. Some samples were implanted with smaller total doses at dose rates of 7.5 and 2.5 mA/cm 2. 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 ≥ 3.0 × 10 16ions/cm 2. 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 ≥ 3.0 × 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 ~ 2. We suggest that the dose rate dependence is due to localized heating which increases with increased dose rate. Samples implanted with doses < 3 × 10 16ions/cm 2 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.