Impedance of photosensitive nanocrystalline PbTe(In) films

We have investigated the temperature and frequency dependences of the impedance components of PbTe(In) nanocrystalline films within the frequency range of 20 Hz-1 MHz in the temperature interval of 4.2-300 K in darkness and under illumination. The films were deposited on a glass substrate. They have a columnlike structure with a mean grain size of about 75 nm. Analysis of the experimental data is performed in terms of an equivalent circuit approximation. It is demonstrated 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 2 orders of magnitude higher than in the dc measurements.