Shallow Donor States in Semiconducting CdF₂

The origin of near infrared absorption spectra in semiconducting CdF 2 crystals is reviewed. Good agreement between theoretical calculations and experimental data for CdF 2 :Gd or Ga gives strong support to the photoionization origin of the near infrared absorption. Lack of an evident “chemical shift” for the different shallow impurities studied indicates the intrinsic nature of the shallow donors responsible for both the infrared absorption and conductivity of CdF 2 . Arguments are presented for attributing the line spectrum observed in lightly doped CdF 2 with phonon replicas of zero‐phonon ls–2p absorption masked by reststrahlen absorption of the host. It is proposed that for systems like semiconducting CdF 2 such states should be described in terms of Fano's theory of discrete states degenerate with the continuum.