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.