Lead telluride is a narrow gap semiconductor with an energy gap Eg = 190 meV at T = 0 K. PbTe and a wide range of its solid solutions represent a group of materials that are extensively used in different fields of opto- and microelectronics, thermoelectricity and even as gas sensors. Recent development in the synthesis using evaporation of a target source to a cooled substrate allows to obtain nanocrystalline films of PbTe(In) with variable grain size and equal orientation of single crystalline grains with respect to the substrate surface. For the In-doped lead telluride, the Fermi level is pinned at 70 meV above the conduction band edge giving rise to free electron concentration of 6 1018 cm-3 at low temperatures. For nanostructured materials, the Fermi level pinning effect provides high homogeneity of electrical properties of the grains that allows ignoring the problems of native defects and non-stoichiometry in the grains.
The films were deposited on a glass substrate. They have column-like structure with a mean grain size varied from about 60 nm to 130 nm depending on the deposition temperature. Frequency and temperature dependence of the impedance components of PbTe(In) nanocrystalline films have been investigated within the frequency range of 20 Hz-1 MHz in the temperature interval 4.2-300 K in darkness and under illumination. Analysis of the experimental data is performed in terms of an equivalent circuit approximation. The analysis revealed that conductivity of the films is determined by two mechanisms: charge transport along the inversion channels at the grain surface and activation (or tunneling) through barriers at the grain boundary. Parameters corresponding to each mechanism are estimated. The persistent photoconductivity appears in the films below T=150 K. The frequency dependence of the relative photoresponse has a pronounced maximum. It is demonstrated that the photoresponse in the AC mode may be by two orders of magnitude higher than in the DC measurements.
This work has been supported in part by the Russian Foundation for Basic Research (Grant 05-02-16657).