Heavily doped Si was investigated for use as spectral control filter in thermal photovoltaic (TPV) system. These filters should reflect radiation at 4 &mgr;m and above and transmit radiation at 2 &mgr;m and below. 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 (&lgr;p) of these filters could be adjusted by controlling the diffusion conditions. The minimum plasma wavelength achieved was 4.8 &mgr;m. In addition, a significant amount of absorption was observed for the wavelength 2 &mgr;m and below. The second approach was doping by ion implantation followed by thermal annealing with a capped layer of doped glass. Implantation with high dosage 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 &mgr;m. The high temperature annealing (≳1000 °C) that was necessary to activate the dopant atoms and to heal the implantation damage, also caused significant absorption at 2 &mgr;m. 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 2×1016cm−2of P was at 2.9 &mgr;m. The absorption at 2 &mgr;m was less than 20% and the reflection at 5 &mgr;m was about 70%. ©1996 American Institute of Physics.