Influence of Zn~(2+) doping on the crystal structure and optical-electrical properties of CdTe thin films

摘要

The present study reports the synthesis of Cd_(1-x)Zn_xTe (x= 0,0.025,0.050,0.075 and 0.100) nanocrystalline thin film through a simple two step method. In the first step fine nanopar-ticles of Cd_(1-x)Zn_xTe was prepared by solvothermal microwave irradiation (SMI) technique and then deposited as thin film using dip-coating technique. X-ray diffraction study showed that films are polycrystalline with cubic phase, which are preferentially oriented along the (111) direction. No impurity phase was observed in the XRD pattern even after higher concentration of doping (x = 0.100) of Zn. FESEM study revealed that the films are homogeneous without cracks and pinholes. TEM micrographs revealed the particles are slightly agglomerated and lesser than 25 nm. The optical absorption study revealed that pure and doped CdTe films possess a direct band gap material with bandgap values between 2.39 and 2.63 eV (±0.02 eV). The values of optical bandgap increase with an increase in dopant (Zn) concentration from x = 0.025 to 0.10. The pure cadmium telluride (CdTe) nanocrystalline film shows a strong green emission peak centered at about 525 nm. The emission peaks of Cd_(1-x)Zn_xTe nanocrystalline films are red shifted from 525 nm to 611 nm according to the dopant (Zn~(2+)) concentration. The grains in the prepared films are uniformly distributed, which was confirmed by narrow full width at half maximum (FWHM) of the emission peaks (40-65 nm). The DC conductivity has increased by 1.25 and 4 orders as the concentration of dopant increases from x = 0.025 to 0.10 at room temperature (30 ℃) and 150 ℃ respectively. The higher conductivity value is underpinned by the smaller activation energy value and is explained by thermionic emission mechanism.