OPTICAL-PROPERTIES OF DEGENERATELY DOPED SILICON FILMS FOR APPLICATIONS IN THERMOPHOTOVOLTAIC SYSTEMS

A detailed study of the dependence of the plasma wavelength and free-c arrier 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, us ing spin-on dopants. The plasma wavelength (lambda(p)) of these doped films could be adjusted by controlling the diffusion conditions. The m inimum plasma wavelength achieved was 4.8 mu m. In addition, a signifi cant amount of absorption was observed for the wavelength 2 mu m and b elow. The second approach was doping by ion implantation followed by t hermal annealing with a capped layer of doped glass. Implantation with high dosages of B and As followed by high temperature annealing (>100 0 degrees C) resulted in a plasma wavelength that could be controlled between 3.5 and 6 mu m. The high temperature annealing (>1000 degrees C) that was necessary to activate the dopant atoms and to heal the imp lantation damage also caused significant redistribution of the dopants . Fur phosphorous implanted Si, a moderate temperature (800-900 degree s C) was sufficient to activate most of the phosphorous and to heal th e implantation damage. The position of the plasma turn-on wavelength f or an implantation dose of 2x10(16) cm(-2) of P was at 2.9 mu m. The a bsorption at 2 mu m was less than 25% and the reflection at 10 mu m wa s about 85%. (C) 1997 American Institute of Physics.