Impedance spectra and computation of chemical diffusion coefficients in CuIn3Te5 single crystals with a massive Cu motion

Abstract

Two single crystals of the same ingot and a polycrystal with compositions close to CuIn2Te3.5, CuIn3Te5 and CuIn4Te6, respectively, have been studied using impedance spectroscopy at different temperatures in the 20–120 °C range. The negative imaginary impedance part, −Z**, as a function of the real impedance part, Z*, (Nyquist plots) can be fitted to one or two semicircles associated with the Warburg diffusion and related one or two mobile ions, depending on the slice composition. In dc current and using the current intensity decay method at 20 °C, the conductivity as a function of time permits one to compute the diffusion coefficients of one or two mobile ions observed in impedance spectroscopy. The diffusion coefficients of the Cu ion are higher than those in CuInSe2 and are associated with the number of copper vacancies, VCu, and indium–copper antisites, InCu, in the Cu sublattice. EDAX measurements in the slice profile, before and after the electrical analysis, confirm a massive motion of Cu atoms along the slice thickness. In all samples, there is a region with composition close to CuIn1.7Te3 and a new atomic distribution, different in each sample depending on the initial composition and their structure. The ionic motion permits one to understand the behaviour of chalcopyrite thin films when they are grown with a composition gradient (from a Cu-rich chalcopyrite to an In-rich one).