  Inert (noble gas) elements are extremely inactive to surrounding chemical
environment and are frequently employed as protective gas in various
semiconductor fabrication processes. In this work, we surprisingly discover
that high doses of argon up to $10^{17}-10^{20} cm^{-3}$ can be measured in
silicon exposed by laser pulses even after 1300 days. First-principles
calculations and molecular dynamics identify a unique argon-locking-vacancy
(ALV) defect atomic model in silicon. The ALV defect is dynamically robust in
contrast to the frequently moving pure Si vacancy. While argon is chemically
inert, it readily modulates defect states of the occupied vacancy via steric
repulsion and rattling motions, leading to significant band splitting within
bandgap and thus strong infrared absorptions. Moreover, the repulsion between
substitutional argon and dangling bonds results in shallow donors which
explains the confusion of enhanced n-type carriers in experiments. The work
paves a way of using noble gas element to produce active infrared absorption
source for the non-heteroepitaxy photonic detectors directly on silicon wafer
at infrared communication wavelength.
