F Center Formation in Sapphire Under Low Dose Low Energy Ar Irradiation

Optical spectroscopy and Rutherford Backscattering Spectrometry Channeling (RBS-C) have been used to study F center dynamics in 170 keV Ar$^+$ irradiated single crystals of sapphire ($\alpha$-Al$_2$O$_3$) at room temperature for implantation doses between $10^{13}$ Ar$^+$ cm$^{-2}$ to $5\times 10^{14}$ Ar$^+$ cm$^{-2}$ . F center density (N$_{\mathrm{F}}$) has been found to display an initial rapid linear increase with Ar$^+$ dose and then saturate to a maximum value of $4.02 \times 10^{14}$ cm$^{-3}$. Fitting experimental results with a Poisson relation suggest an estimated electron capture range of $4.24\times 10^{-10}$ m around an Oxygen vacancy. A possible explanation to this behaviour is presented.