[en] Chalcogenide semiconducting glasses have been used as an important platform for understanding the structure and semiconducting properties in the disordered state of condensed matter. In metal-chalcogenide bilayer systems, Bi/As₂Se₃ bilayer films have drawn great interest due to its possible applications in thermoelectric generators and refrigerators to increase thermoelectric figure of merit. Incorporation of Bi in the As₂Se₃ matrix for example converts the p-type chalcogenide to n-type.In the present study, thermal annealing induced diffusion of bismuth at the interface of the crystalline semi-metallic Bi and semiconducting amorphous arsenic selenide layers are studied. Bilayer films of 100 nm thick Bi and 800 nm thick As₂Se₃ were prepared by thermal evaporation method using arsenic selenide powder and metallic bismuth. The films were annealed for 1/2, 1 and 2 hr in hot air oven at 165 °C. Increasing the annealing time induces completely diffusion of bismuth into the As₂Se₃ film led to the formation of Bi₃Se₂ phase as confirmed in the X-ray diffraction (XRD) study. The FESEM images reveal the smooth surface in the as deposited bilayer film and a large number of agglomerated grains of As₂Se₃ have been seen in the annealed films. Exposure of a large number of As₂Se₃ grains in the annealed film, which were otherwise hidden under the bismuth layer before annealing, could be due to the diffusion of Bi into the As₂Se₃ matrix. The variation of surface roughness with annealing as seen from the AFM study indicates grain growth stagnation or even decrease of grain size on prolonged annealing. The as deposited Bi/As₂Se₃ bilayer film exhibited strain at the interface leading to localized states in the band gap of the bottom As₂Se₃ layer with a consequent reduction of its band gap. Annealing at 165 °C from 1/2h to 2h led to strain relaxation and increase of the band gap towards that of the pristine As₂Se₃ along with the precipitation of a second phase Bi₃Se₂. The increase in optical band gap is also explained by the density of localized states in the gap. (author)