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The ability of thermoelectric power measurements to permit a description of carrier-density and phonon-drag variations caused by photoexcitation was tested in 100-Ω·cm n- and p-type silicon. At low temperatures the major effect of photexcitation is to decrease the phonon-drag contribution to the thermoelectric power by increasing the phonon density in the crystal. At higher temperatures the thermoelectric effect can be used to investigate changes in the electronic contribution due to photoexcitation. An apparently anomalous increase in thermoelectric power with photoexcitation was consistently found in p-type silicon over an intermediate temperature range.

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The dependence of trap filling on the intensity, temperature, and wavelength of the photoexcitation has been investigated for trapping centers in S–Se-annealed CdS–CdSe crystals and in crystals of CdS subjected previously to electron radiation damage and photoexcited annealing. For certain traps in both these kinds of crystals, it was found that trap filling was not only critically dependent on the temperature of excitation but also on the wavelength of excitation. In fact, for each process observed, a band of wavelengths is most effective, the maximum wavelength of this band being larger than the wavelength for maximum photoconductivity. This result makes previous attempts to explain this kind of phenomenon in terms of a Coulomb-repulsive trap less acceptable and provides additional evidence for the presence of photochemical changes at and below room temperature in CdS-like materials.

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