Evaluation of optical constants of Tl₄PbTe₃ thin films with different thicknesses

Thallium lead telluride (Tl₄PbTe₃) was prepared by direct reaction of high purity elemental thallium, lead and tellurium. Tl₄PbTe₃ thin films were prepared on well cleaned glass and silicon (100) substrates by electron beam evaporation. X-ray diffraction studies revealed that the films with low thickness were amorphous and the crystallinity increased with increasing film thickness. The thicker films were polycrystalline in nature and exhibited single phase structure. The composition of the chemical constituents present in the deposited Tl₄PbTe₃ thin films with different thicknesses was determined using energy dispersive analysis of X-ray. Spectroscopic ellipsometry was employed to determine the film thickness and optical constants. A two layer model was used to describe the experimental ellipsometric data. The refractive index and extinction coefficient were sensitive to the film thickness and increased with increasing the film thickness. The optical transmittance and reflectance spectra were dependent strongly on film thickness. The optical band gap decreased with the increase of thickness of the film. (author)