Investigation of structural, optical and electrical properties of zinc selenide thin films prepared by chemical bath deposition technique

Abstract

A Chemical Bath Deposition system is set up to deposit zinc selenide (ZnSe) thin films on glass substrate. Zinc acetate and sodium selenosulfate aqueous alkaline medium are used as zinc and selenium source, respectively. Films were deposited by varying concentration of sodium selenosulfate and for different deposition times. The surface morphology, structural and optical properties of the ZnSe thin films were investigated by using scanning electron microscopy (SEM), X-ray diffraction, UV–visible spectroscopy. The SEM observation revealed non-uniform distribution of spherical ZnSe crystallites. The elemental compositions of the films were confirmed by energy dispersive X-ray spectroscopy. The results showed that the deposition time had a strong influence on the morphology. The optical band gap also observed to vary with increasing deposition time and concentrations of the sodium selenosulfate. The thickness of the films deposited at different concentrations of Se and different time durations were 140‒225 nm and 175‒375 nm, respectively. The SEM micrographs show that ZeSe thin films prepared at different time duration are not compact but have good coverage of glass substrate. These films revealed that grains were very small in size with well defined grain boundaries. X-ray diffraction studies show that though the as‒deposited ZnSe thin films do not show crystalline nature but, annealed ZnSe thin films are polycrystalline in nature with preferential orientation along the (111), (220), and (311). The films also have ZnO and Na2SeSO3 phase. The average crystallite size is found to be 23.21±3.61 nm. The strain and dislocation density are found to be 1.62×10‒3 lines‒2 m‒4 and 2.18×1015 m‒2, respectively. The number of crystals per unit area is 2.633×1016 m‒2. All the diffraction peaks can be assigned to face centered cubic with lattice constants (a = 5.670 Å) which are in good agreement with the previous work. Various optical parameters such as absorbance, transmittance, refractive index, extinction coefficient and dielectric constant of the films have been studied for the asdeposited ZeSe thin films and are recorded in the wavelength range from 250 to 1100 nm. The films prepared for 60 min exhibit higher absorption as compared to other deposition times. The maximum transmittance is 65% for the ZnSe thin film deposited at 0.5 M concentration of Se. The direct optical band gap of the films deposited at different time duration and different molar concentration of Se source show wide band gap energies of 3.50–3.55 eV and 2.70–3.55 eV, respectively. The band gap of the ZnSe thin film deposited for 40 min of 1.0 M concentration have been increased from 3.5 to 3.75 eV after annealing effect. It is observed that the refractive index decreases, as the wavelength increase. However, the transmittance and extinction coefficient increases with the increase of wavelength. The electrical properties of the as‒deposited and annealed ZnSe thin films have been observed. The resistivity decreases with the increase of temperature and the conductivity increases with the increase of temperature which reveals the semiconducting nature of the films. The maximum resistivity of the ZnSe Films deposited for 30 min is 2.2 × 103 Ω‒m. The activation energy of the ZnSe thin film deposited at 0.3M concentration for 40 min is 0.093863 which supports the values of previous research.