Effect of Zn-doping on the structural, optical and electrical properties of thermally vacuum evaporated CdTe thin films

Abstract

Cadmium telluride (CdTe) thin film was fabricated on a glass substrate using thermal vacuum evaporation and zinc (Zn) was doped with varying atomic ratios using stacked layer method. The structural, optical and electrical properties of the fabricated films were thoroughly investigated as a function of doping concentration. XRD patterns revealed all Zn-doped CdTe thin films as polycrystalline materials with cubic structures. All the films had relatively intense (111) diffraction peaks but the intensity of the peaks reduces with the increase of doping concentration. The optical properties, such as transmittance, absorption coefficient, bandgap, penetration depth, Urbach energy and dielectric constants were evaluated by UV-Vis-NIR spectrophotometry. When doping concentration is increased from 0 at. % to 20 at. % average optical transmittance and Urbach energy increase from 87% to 94% and 127 meV to 409 meV, respectively, while the optical bandgap goes down from 1.51 eV to 1.43 eV. On the other hand, higher doping concentration shifts the absorption edge toward a longer wavelength and increases the penetration depth significantly. Electrical characterization determined by Hall measurements using the van der Pauw method shows that 20 at. % Zn doping in pure CdTe film increases the carrier concentration from 1.95 × 1014 cm−3 to 27.52 × 1014 cm−3 and activation energy from 1.623 eV to 2.603 eV but decreases the carrier mobility from 19.34 cm2/VS to 6.23 cm2/VS. Changes in electrical transport properties caused by doping are described in terms of charge carrier scattering.