Preparation and characterization of optical and electrical properties of copper selenide sulfide polycrystalline thin films

Highlights: • Cu_1.8Se_1-xS_x thin films are prepared by thermal evaporation. • Cu_1.8Se_xS_1-x films are formed in a single cubic phase. • Cu_1.8Se become smoother when alloying with sulfur. • The direct bandgap can be tuned from 2.2 to 2.5 eV by adjusting sulfur content. The structural, optical and electrical properties of Cu_1.8Se_1-xS_x (0.25 ≤ x ≤ 0.75) polycrystalline thin films deposited at room temperature by vacuum evaporation were studied as a function of composition. Energy dispersive X -ray analysis was used to determine the elemental composition of different films. Results indicate the incorporation of sulfur at the expense of selenium. X-ray diffraction analysis reveal that the (220) diffraction peak of the Cu_1.8Se_1-xS_x films shifts to a higher 2θ with an increase in the sulfur content. Atomic force microscopy images of the deposited films show dense and well-defined grains. Analysis of the absorption coefficient data show the existence of two optical transition mechanisms - direct transitions with E_g-dir ranging from 2.2 to 2.5 eV, and indirect transitions with E_g-indir ranging from 1.23 to 1.52 eV as the sulfur content was increased from 0.25 to 0.75, respectively. The Hall coefficient, Hall mobility, and electrical conductivity were measured at room temperature for different film compositions. All investigated films are highly degenerated p-type semiconductors with the number of free carriers up to 10²⁰ cm⁻³. Combining the results of the optical and electrical measurements, the effective mass for the highly degenerate compositions is found to be 1.718 to 0.587 as the sulfur content varies from 0.25 to 0.75.