A detailed study of the dependence of the plasma wavelength and free-carrier absorption on the doping concentration in silicon has been made. Two approaches have been used for introducing impurities into Si to achieve high doping concentration. One was the diffusion technique, using spin-on dopants. The plasma wavelength (λp) of these doped films could be adjusted by controlling the diffusion conditions. The minimum plasma wavelength achieved was 4.8 μm. In addition, a significant amount of absorption was observed for the wavelength 2 μm and below. The second approach was doping by ion implantation followed by thermal annealing with a capped layer of doped glass. Implantation with high dosages of B and As followed by high temperature annealing (≫1000 °C) resulted in a plasma wavelength that could be controlled between 3.5 and 6 μm. The high temperature annealing (≫1000 °C) that was necessary to activate the dopant atoms and to heal the implantation damage also caused significant redistribution of the dopants. For phosphorous implanted Si, a moderate temperature (800–900 °C) was sufficient to activate most of the phosphorous and to heal the implantation damage. The position of the plasma turn-on wavelength for an implantation dose of 2×1016 cm-2 of P was at 2.9 μm. The absorption at 2 μm was less than 25% and the reflection at 10 μm was about 85%. © 1997 American Institute of Physics.