How does increasing Se-content affect Cd 50 S 50−x Se x system thin films?
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Impact of Increasing Se-Content on Cd50S50−xSex System Thin Films
Introduction to Cd50S50−xSex Thin Films
Cd50S50−xSex thin films are a type of chalcogenide material that have garnered significant interest due to their unique optical, electrical, and structural properties. These thin films are typically produced through thermal evaporation techniques and are used in various optoelectronic applications. This article explores how increasing the selenium (Se) content in these films affects their properties.
Optical Properties and Energy Gap
Decrease in Optical Energy Gap
One of the most notable effects of increasing Se content in Cd50S50−xSex thin films is the reduction in the optical energy gap (Eg). Studies have shown that as the Se content increases from x=30 to x=50, the indirect optical energy gap decreases from 2.21 eV to 1.57 eV . This reduction is attributed to the substitution of sulfur (S) with selenium, which alters the electronic structure of the material.
Increase in Urbach Energy
Alongside the decrease in the optical energy gap, the Urbach energy (EU), which represents the width of the band tail, increases from 0.29 eV to 0.45 eV with higher Se content . This increase indicates a higher degree of disorder in the material, which is consistent with the incorporation of larger Se atoms into the lattice.
Dielectric and Opto-Electrical Properties
Changes in Dielectric Properties
The dielectric properties of Cd50S50−xSex thin films are also significantly influenced by the Se content. The high-frequency dielectric constant, as well as the real and imaginary parts of the dielectric constant, show notable variations with increasing Se content . These changes are crucial for applications in electronic and optoelectronic devices.
Opto-Electrical Parameters
Opto-electrical parameters such as optical carrier concentration, optical mobility, and optical resistivity are affected by the Se content. As Se content increases, the optical electronegativity also increases, while electronic polarizability and nonlinear optical susceptibility decrease . These changes suggest that higher Se content enhances the material's ability to interact with electromagnetic waves, thereby improving its opto-electrical performance.
Electrical Conductivity and Conduction Mechanisms
DC-Electrical Conductivity
The dc-electrical conductivity of Cd50S50−xSex thin films is highly dependent on both temperature and Se content. As Se content increases, the sheet resistance decreases, indicating improved electrical conductivity . This behavior is analyzed using Mott's variable range hopping model, which helps in understanding the conduction mechanisms in these amorphous films.
Activation Energy and Localized States
The activation energy for conduction and the density of localized states near the Fermi level are also influenced by the Se content. Higher Se content leads to a decrease in activation energy, facilitating easier charge transport through the material . This is crucial for applications requiring efficient electrical conduction.
Structural Properties
Crystallinity and Particle Size
The structural properties of Cd50S50−xSex thin films, such as crystallinity and particle size, are affected by the Se content. X-ray diffraction (XRD) studies reveal that increasing Se content leads to a decrease in crystallite size and a shift in Cd-Se peaks to lower wavelengths . This indicates a reduction in crystallinity and an increase in amorphous character with higher Se content.
Conclusion
Increasing the Se content in Cd50S50−xSex thin films significantly impacts their optical, dielectric, opto-electrical, and structural properties. The reduction in optical energy gap, increase in Urbach energy, changes in dielectric properties, and improved electrical conductivity are some of the key effects observed. These changes make Cd50S50−xSex thin films with higher Se content suitable for various advanced optoelectronic applications.
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