ИСТИНА

A new type of semiconductor local states is revealed in lead-tin telluride solid solutions doped with indium or gallium. The energy position of these states is not linked to any specific location in the semiconductor energy spectrum, but follows the quasiFermi level position, which may be tuned by photoexcitation. The binding energy of these states is less than 10 meV providing appearance of photoresponse at wavelengths exceeding 100 μm. This conclusion is based on the results of experiments that use illumination of the samples by 100 ns – long pulses of an optically pumped NH3 terahertz laser with the wavelengths of 90, 148 and 280 m corresponding to the quanta energies of 13.8, 8.4 and 4.4 meV, respectively. Positive persistent photoconductivity induced by these laser pulses was detected at low temperatures. We show that this effect is due to photoexcitation of local electron states located in energy in the nearest proximity of the quasi-Fermi level. We demonstrate that these local states appear only when the quasi-Fermi level lies on the background of an allowed band. Dependence of the density of these local states on the magnetic field applied and on the sample current is presented. The mechanisms responsible for the effects observed are discussed.