Lead sulfide films with low iron doping: Correlation between iron position and optical properties evolution

Abstract

Thin polycrystalline films of cubic lead sulfide doped with iron (II) PbS:Fe have been synthesized by chemical bath deposition from aqueous solutions of lead acetate, sulfidizing agent, and complexing and alkaline agents in the presence of FeCl2 salt in the concentration range from 10−5 to 7 × 10−3 M. At the iron (II) chloride concentration lower than [FeCl2] = 5 × 10−3 M, the crystal lattice parameter of PbS increases indicating the interstitial position of Fe2+ ions. At [FeCl2] = 5 × 10−3 M, the substitution of iron ions for lead ones is found. The absorption spectra show that on increasing FeCl2 concentration in the reaction bath up to 10−3 M, the band gap of the synthesized PbS:Fe films hardly changes (Eg∼0.4 eV); only the growth of the impurity bands near the absorption edge is observed. At [FeCl2] = 5 × 10−3 M, the blue shift of the absorption edge occurs, the band gap increases to 0.58 eV, and the impurity absorption band shifts toward the higher energies. Cooling the films in the magnetic field influences the intensity of the impurity absorption bands associated with iron when Fe2+ ions are located in interstitial position. The observed changes in the optical spectra are explained by the difference in the band structure and the position of the impurity levels when Fe2+ ions are at the interstices or in the Pb ions position.