Infrared absorption in phosphorus doped silicon and the D-band

Measurements are reported of the optical absorption in phosphorus doped silicon in the wavelength range 25 mu m to 700 mu m, together with the concentration dependence of refractive index. Sample doping levels varied from N d =5*10 15 Pcm -3 , where the donors are virtually isolated, to N d =2.7*10 18 Pcm -3 where they are strongly interacting and just on the insulator side of the metal-insulator transition (critical concentration N c =3*10 18 Pcm -3 ). Specimens doped to 6*10 16 Pcm -3 exhibited two new absorption lines at 294 cm -1 and 250 cm -1 , and it is proposed that these lines lead to the formation of an absorption edge which moves towards smaller wavenumber as N d to N c . The absorption line at 294 cm -1 is identified as being the lower limit of a D 2 - band, and its position is in good agreement with Norton's measurement of a band edge using the temperature dependence of a photoexcitation technique.