Silicon samples subjected to a CF4reactive ion etch exhibit theG‐line luminescence known from photoluminescence studies of irradiation damage in Si. We investigate the mechanism of low‐energy carbon implantation by CF4plasma and find a linear dependence of theG‐line luminescence intensity on the substitutional carbon concentration in the samples. By a standardized etching and photoluminescence procedure, we expect to determine carbon concentrations two orders of magnitude smaller than the ASTM infrared absorption method.