Infrared measurements over the range of 2 to 38 µ have been carried out for a number of p-type silicon specimens, including boron doped single crystal specimens. At liquid helium temperature these specimens exhibit optical absorption by neutral impurities involving (a) photoionization of bound charge carriers, which is accompanied by photoconductivity, and (b) optical excitation of bound charge carriers from the ground state to higher energy states, which appears as relatively narrow bands at wave lengths beyond the photoionization limit. Three excitation bands are observed in the boron doped specimens at 28.7, 31.2 and 36.0 µ, which are attributed to ls-4p, ls-3p and ls-2p transitions, respectively. The energy levels of the boron centers calculated from these bands correspond closely to those of a hydrogen model having an ionization energy of 0.046 ev. and indicate an effective mass for holes = 0.45 m and a radius for the ground state orbit of 13.5 A• The boron ionization energy is independent of concentrations up to densities of 6 X 1017 impurity centers/cm.8 A second type of impurity center having an ionization energy of 0.07 e.v. is also observed in a polycrystalline specimen. For this center the excitation bands do not conform to those of a simple hydrogen model.

Laser calorimetric or thermal rise techniques are useful for the determination of very low absorption coefficients in solids. A number of improvements in this technique are described of which the most important is a means of separating surface and bulk absorption. These techniques have been applied to study alkali halides in the ir but are applicable where laser sources of sufficient power are available.

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