Characteristics of degenerately doped silicon for spectral control in thermophotovoltaic systems

Abstract

Heavily doped Si was investigated for use as spectral control filter in thermal photovoltaic (TPV) system. These filters should reflect radiation at 4 μm and above and transmit radiation at 2 μm 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 (λp) of these filters 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 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 μm. 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 μm. 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 20% and the reflection at 5 μm was about 70%.