抄録
The infrared absorption observed at low temperatures, where the wavelength dependence of absorption is like the free carrier absorption and also the magnitude is even larger than at room temperature, is explained on the basis of the direct transition of electrons from a shallow donor level to the lowest conduction band. It is shown that the absorption coefficient resulting from this mechanism is in good agreement with experiments for n-type AlSb, GaP and Si when a reasonable value is taken for the concentration of donor electrons.
For the excess absorption observed in the above materials, the indirect transition from a shallow donor level to the conduction bands of subsidiary minima is investigated as a possible mechanism. The calculated absorption curve is shown to be very similar to that due to the interconduction band transition investigated previously.
