Investigating the effect of thickness on the structural, morphological, optical and electrical properties of AgBiSe2 thin films

Abstract

Herein, a one-step thermal evaporation was used for fabrication of silver bismuth selenide (AgBiSe2) thin films of various thickness. The influence of thickness on the structural, optical and electrical properties of AgBiSe2 thin films has been studied. The X-ray diffraction confirmed single hexagonal structure of deposited AgBiSe2 thin films. The average grain sizes D and microstrain ε of deposited AgBiSe2 films were estimated by Williamson-Hall (W-H) relation. The grain size increases from 11.52 to 29.12 nm as the thickness was increased from 150 to 550 nm, whereas strain dislocation density (ξ) and the number of crystallites N tends to decrease with thickness. The selected area electron diffraction pattern (SAED) obtained by high resolution transmission electron microscopy (HRTEM) verified the polycrystalline nature of samples and it is found to nearly agree with X-ray diffraction data. The elemental composition measurement of the as-deposited AgBiSe2 thin film showed that the as-deposited film is near stoichiometric in compound. The influence of film thickness on the optical parameters of the AgBiSe2 thin films, such as refractive index n, extinction coefficient k, real dielectric ε1, imaginary dielectric constant ε2, skin depth and the optical band gap, Eg has been studied. It is found that the values of energy gap decrease from 1.88 to 1.68 eV with increasing thickness, while the values of Urbach energy follow an opposite trend. The electrical conductivity was measured by four-probe experiment in the range from 300 to 500 K. The analysis of dc conductivity data revealed the presence of two distinct regions with two various slopes confirming the transformation from hexagonal phase to rhombohedral phase at about 460 K.