Abstract
Nano-crystalline CdSe thin films of different thicknesses under sub-micron range were deposited on glass substrate via thermal evaporation route. A gradual deterioration in film crystallinity confirmed by XRD line profile analysis has been accompanied by a reduction in Cd to Se molar ratio as the film thickness decreases. A coordinated microstructural and crystallographic orientation distribution analysis explicitly demonstrated that CdSe tends to grow in nano-sized columns with hexagonal c-axis parallel to its growth direction on glass substrate. A thickness dependence of structural evolution was discussed in terms of aspect ratio of the columnar structure and dispersion in orientation of hexagonal (002) basal plane. The variation in the spectra of optical constants [n(λ), k(λ)] obtained from Swanepoel envelop method was interpreted in terms of crystallographic defects arising from stoichiometric disorder which was also accounted for the observed thickness dependent shifts in band gap and valence band split energy. The bathochromic shifts in dielectric and energy loss functions, optical conductivity, skin depth and cut-off energy were discussed in detail along with the variations in their spectral shapes in connection with the dispersion in the real and imaginary parts of complex refractive index, which might shed a new light upon holistic comprehension of thickness dependent optical properties of other chalcogenide semiconducting thin films.
Highlights
Researches on chalcogenide semiconductors of group II-VI have received considerable attention worldwide for their application as nanostructured thin films in fabrication of the newly emerging optoelectronic devices
To determine the variations in crystallinity with sample thickness, structural broadening is needed to be extracted from X-ray diffraction (XRD) line profile which in turns is directly linked with crystallite size and micro-strain
M−1 increases with increase in film thickness, suggests that ε2(λ) should shift more to the low energy region. This result is very much consistent with the thickness dependent red-shift of the optical band gap that we have reported in this article
Summary
Researches on chalcogenide (sulphides, selenides and tellurides) semiconductors of group II-VI have received considerable attention worldwide for their application as nanostructured thin films in fabrication of the newly emerging optoelectronic devices. The thermal vapor deposition (TVD) technique is regarded as one of the most technically viable and convenient physical vapor deposition (PVD) process to obtain CdSe thin film with desired properties, where all the preparative conditions like the deposition rate, deposition time, composition, substrate temperature and atmospheric pressure can be readily controlled.. The thermal vapor deposition (TVD) technique is regarded as one of the most technically viable and convenient physical vapor deposition (PVD) process to obtain CdSe thin film with desired properties, where all the preparative conditions like the deposition rate, deposition time, composition, substrate temperature and atmospheric pressure can be readily controlled.19 This ease of handling renders discernible modifications in the structure and in the optoelectronic properties of the thin film with minimum effort. A sequential correlation between the above mentioned optoelectronic parameters and the observed variations in the crystallographic, microstructural and stoichiometric traits have been attempted to establish, in the light of defect induced quantum mechanical size confinement effect as proposed for other semiconductors.
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