Thermal annealing induced evolution of Bi3Se2 topological phase from Bi/As2Se3 thin film: Structural, optical and morphological study

In the present study, Bi/As2Se3 bilayer thin films (100 nm Bi as top layer and 800 nm As2Se3 as bottom layer) were prepared by thermal evaporation technique and were annealed at 165 degrees C for 1/2 h, 1 h and 2 h. The structural, micro-structural and optical studies of the films were characterized by X-ray diffractometer (XRD), Field emission scanning electron microscopy (FESEM), Atomic force microscopy (AFM) and Fourier transform infrared (FTIR) Spectrophotometer. The XRD study revealed the annealing induced diffusion of Bi into As2Se3 layer and its reaction with Se leading to the formation of Bi3Se2 phase. 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. The increase in annealing time from 1/2 h-2 h led to strain relaxation and increased the band gap towards that of the pristine As2Se3 along with the precipitation of a second Bi3Se2 phase. The increase in optical band gap has been explained by the density of localized states in the gap. The change in refractive index, transmission, absorption coefficient, dielectric constant is the consequence of annealing effect. The FESEM images revealed the smooth surface for as deposited bilayer film and a large number of agglomerated grains of As2Se3 have been seen in the annealed films. The AFM study showed the grain growth stagnation or even decrease of grain size on prolonged annealing.