Charge carrier transport in PbS films doped with iodine.

The results of the measurements of electrical and Hall resistivities on polycrystalline PbS films doped with iodine obtained through hydrochemical deposition are presented. The analysis of the temperature dependence of resistivity points out the crossover from the hopping mechanism due to thermal delocalization in the impurity band to the variable range hopping mechanism. The increase in the iodine content in the films leads to an increase in the impurity ionization energy. It has been established that the temperature dependence of resistivity over a wide temperature range obeys the inverse Arrhenius law, which is characteristic of disordered polycrystalline films with different sizes and orientations of crystallites relative to the substrate, as confirmed by AFM topography, Raman spectra and X-ray diffraction measurements. We found that the type of charge carrier changes from electrons to holes with an increase in the iodine content. Additionally, for a wide range of iodine doping, the concentration of charge carriers is low, indicating the possible occurrence of a self-compensation mechanism due to the formation of impurity defects.

The influence of iodide addition on the composition, morphology, crystal structure, and semiconductor and photoelectric properties of PbS films.

The concentration conditions for the deposition of lead sulfide and hydroxide in a citrate-ammonia reaction system by varying the pH value and the concentration of the ammonium iodide dopant are calculated. Kinetic studies of the process of conversion of lead salt into sulfide by varying the concentration of NH4I in solution within the range of 0.0-0.4 M show an inhibitory effect of ammonium iodide, which reduces the constant of the effective rate of the conversion process by almost 20 times. Thin films of lead sulfide with a thickness of 100 to 650 nm are synthesized by chemical bath deposition in the presence of NH4I on sitall and glass substrates. The introduction of NH4I into the reaction mixture decreases the size of crystallites forming the films and increases the fraction of nanoparticles to 17%. The films deposited during 90 min contain 48.5-51.7 at% of lead, 47.4-47.9 at% of sulfur, and up to 3.7 at% of iodine; the content of iodine can reach 17.1 at% in the initial period of deposition (after 20 min of deposition). The calculated fractal dimension of the surface of the iodine-doped PbS(I) films (Dc = 2.04-2.14) corresponds to the process of film formation by the cluster-cluster aggregation mechanism with small sticking probability (reaction-limited cluster aggregation model (RLCA)). According to the X-ray diffraction data, the PbS(I) films have a cubic structure of the NaCl (B1) type; an increase in the concentration of NH4I in the solution results in an increase in the lattice parameter from 0.59315(1) to 0.59442(3) nm, which is probably due to the substitution of sulfur ions for iodine ones. According to optical studies, the introduction of ammonium iodide into the reaction mixture leads to a shift of the absorption edge to the high-energy region and the appearance of an additional (impurity) band. The PbS films deposited in the presence of NH4I do not require additional photosensitization operations and show a relatively high volt-watt sensitivity to IR radiation at anomalously small values of the time constant.

A nonlinear evolution of the structure, morphology, and optical properties of PbS-CdS films with cadmium nitrate in the reaction mixture.

Herein, we describe the nonlinear processes for the formation of thin films of the PbS-CdS system using chemical bath deposition with a gradual change in the cadmium nitrate content in the reaction mixture. The morphology of films was studied via scanning electron microscopy and atomic force microscopy. The mechanism for the formation of thin-film compounds can be considered as cluster-particle aggregation (diffusion-limited aggregation). X-ray diffraction confirmed the formation of single-phase layers of substitutional B1-type CdxPb1-xS solid solutions (space group Fm3[combining macron]m) when the concentration of cadmium nitrate in the reaction bath increased up to [Cd(NO3)2] = 0.08 M. The maximum cadmium content in CdxPb1-xS solid solutions was determined to be x = 0.094. At the concentration of [Cd(NO3)2] = 0.10 M, a two-phase film was formed, where the film consisted of crystallites of cubic CdxPb1-xS with x = 0.071 (lower compared to the film obtained at [Cd(NO3)2] = 0.08 M) and fine-crystalline hexagonal B4-type Cd1-δS (space group P63mc). The texture of grains forming films was observed; where a predominant orientation with the (111) plane along substrate in PbS changed to the (200) plane in CdxPb1-xS films, the portion of (200) oriented grains increased with an increase in cadmium nitrate content up to [Cd(NO3)2] = 0.08 M, and at 0.10 M of cadmium nitrate, a radical change in the type of texture to the (111) type occurred. The concentration [Cd(NO3)2] = 0.10 M is called the critical concentration, where under this condition, the deposition process occurs due to the excess Gibbs energy. The higher cadmium content x in films determined by the energy-dispersive X-ray analysis and Auger spectroscopy compared with that estimated from the crystal lattice parameter is associated with the presence of an additional amorphous CdS phase formed as a sublayer distributed in intercrystalline spaces and island formations. Optical studies showed a nonlinear change in the band gap (Eg) of obtained films from 0.53 to 0.76 eV, whereas at the critical cadmium salt concentration (0.10 M), two crystalline phases with Eg equal to 0.73 and 2.47 eV were observed.