The spectral dependence of the infrared absorption cross section of As in Si near 0 K has been determined from infrared transmission measurements for three As concentrations (5.3, 8.4, and 15.9 × 1017 cm−3) in the impurity band regime. The results demonstrate some features of physical interest. With increasing As concentration, the lines associated with the intra-atomic transitions broaden asymmetrically, while the integral of the total absorption cross section over photon energy is conserved as required by the oscillator strength sum rule. It thus appears that the cross section for the intra-atomic transitions is conserved as the lines hybridize with the continuum. Comparison of our results with photoionization cross-sectional data suggests that the lines contribute to the cross section for photoionization through field and thermally assisted transitions when they are near the threshold for photoionization.
Jon Geist, "Infrared absorption cross section of arsenic in silicon in the impurity band region of concentration," Appl. Opt. 28, 1193-1199 (1989)