Structural, optical and electrical properties of thermally evaporated Ag2S thin films

Thin films of Ag₂S are prepared on glass and quartz substrates by a thermal evaporation method. The structural studies show that the films are well crystallized with an acanthite structure. The optical properties of the films are investigated using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 500-2200 nm. The refractive index, n, and the absorption index, k, of Ag₂S are determined from the absolute values of the measured transmittance and reflectance. The dispersion of refractive index in Ag₂S is analyzed using the concept of the single oscillator. Within this concept the oscillator energy, E₀, and the dispersion energy, E_d, can be determined as 5 and 32.5 eV, respectively. It is interesting to note that Ag₂S appears to fall into the ionic class. The values of the lattice dielectric constant and the ratio of the carrier concentration to the effective mass are also determined as 7.77 and 1.7×10⁴⁷ kg⁻¹ m⁻³, respectively. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals an indirect allowed transition with a band gap of 0.96 eV and associated phonons of 0.05 eV. Measurements of the dark electrical resistivity is studied as a function of film thickness and temperature. The dark electrical resistivity decreases with increasing film thickness. Graphical representation of log ρ as a function of reciprocal temperature yields two distinct linear parts indicating the existence of two activation energies ΔE₁ and ΔE₂ as 0.18 and 0.28 eV respectively. Discussion on the obtained results and their comparison with the previous published data is also given.