Characterization of ZnTe films electrodeposited from aqueous solution containing NaOH

Abstract

Stoichiometric films of ZnTe are electrodeposited on steel and on conducting glass substrates from an aqueous solution consisting of NaOH (1 M), ZnSO4 (50 mM), TeO2 (17 µm) and adequate H2SO4 to maintain a pH of 3.5. The films' structure, morphology, composition, optical and photoelectrical properties are studied using XRD, SEM, EDAX, optical transmittance spectroscopy and photocurrent spectroscopy. The films are composed of small crystallites (50 nm) with cubic crystal structure. Clustering of crystallites can form much larger grains (500 nm). The films are stoichiometric within the accuracy of the measurement method (EDAX). The films' absorption coefficient varies in the range 1 × 104–5.7 × 105 cm−1 for photon energies 2.20–4.20 eV and shows a steep tail for sub-bandgap energies. An optical bandgap energy of 2.25 eV and a valence band split energy of 1.05 eV are measured. A direct optical transition from the spin–orbit split valence band Γ7v to the principal conduction band Γ6c is detected. Photocurrent spectroscopy measurements reveal two indirect transitions from the principal valence band. One of these transition energies (1.05 eV) measures the position of the Fermi level in the film and the other one (1.40 eV) determines the position of a deep donor-type defect level. An energy band diagram that explains the results is proposed.