Investigation of Effect of Mn Concentration on Physical Properties of CdS Thin Films

Abstract

CdS thin films have been successfully deposited on commercial glass slide substrate by chemical bath deposition technique at 70±2ºC. The effect of Mn concentration (x value) on structural, morphological, optical and electrical properties have been studied. The as-deposited Cd1-xMnxS thin films were characterized using X-ray diffractometer (X-PERT PRO), SEM and UV-VIS spectrophotometer. The XRD study reveals the nano-crystalline thin films with hexagonal structure. The peak intensity of XRD peaks decreases with increase in Mn content. It is observed that as Mn content increases the average grain size varies between 189 nm and 107 nm. Morphological properties have been discussed. The value of obtained in between 2.28 eV and 2.46 eV For varying Mn content from x= 0 to 0.8 the energy bandgap and resistivity decreases apparently.

Introduction

This study investigates the structural, morphological, optical, and electrical properties of Mn-doped cadmium sulfide (Cd???Mn?S) thin films, which exhibit characteristics intermediate between CdS and MnS. Cadmium sulfide (CdS) is a well-known semiconductor with a direct band gap of 2.42 eV, widely used in photovoltaics, LEDs, and photodetectors, while manganese sulfide (MnS) is a magnetic semiconductor with a wider 3.1 eV band gap, useful in short-wavelength optoelectronic and solar applications. Doping CdS with Mn aims to enhance its optical and electronic properties for improved device performance.

Experimental Details

Thin films of Cd???Mn?S (x = 0, 0.2, 0.4, 0.6, 0.8) were synthesized on glass substrates using a modified chemical bath deposition (CBD) technique.

• Precursors: CdSO? (Cd²? source), MnSO? (Mn²? source), and thiourea (S²? source).

• Conditions: pH controlled with ammonia; temperature maintained at 72 ± 2°C; deposition times varied (10–60 min).

• Characterization: XRD for structure, SEM for morphology, UV–Vis spectroscopy for optical properties, and a four-probe method for electrical resistivity.

Results and Discussion

A. Structural Properties

XRD analysis revealed a hexagonal (wurtzite) structure for all films, confirming they are polycrystalline.

• Crystallinity decreased with increasing Mn content, indicating Mn incorporation into the CdS lattice.

• Average grain size decreased from 189 nm (x=0) to 107 nm (x=0.8).

• Lattice constants remained close to standard CdS values (a = 4.13 Å, c = 6.65 Å).

• Band gap decreased from 2.45 eV to 2.28 eV as Mn concentration increased.

B. Morphological Properties

SEM images showed smooth, uniform, crack-free, and adherent films with fibrous structures that became more pronounced with higher Mn content. The decreasing grain size with Mn addition indicates denser microstructures suitable for gas sensing and optoelectronic applications.

C. Optical Properties

Optical absorption spectra (400–1000 nm) revealed high transmittance in the visible–NIR range, making Mn-doped CdS suitable for solar window layers, coatings, and optical materials.
Using Tauc’s relation, the optical band gap was found to decrease with Mn doping, confirming successful modification of electronic structure.

D. Electrical Properties

The electrical resistivity of Cd???Mn?S films decreased with increasing Mn concentration, indicating enhanced electrical conductivity due to the Mn-induced modification of carrier concentration and lattice defects.

Conclusion

Mn doped CdS (Cd1-xMnxS) nano-crystalline thin films have been grown successfully by modified CBD technique. The effect of Mn doping on physical properties have been investigated. The multiple peaks observed in XRD study reveals the polycrystalline nature of the obtained films. The SEM image confirms the fibrous (nano-wires) nanostructure of the films which may useful in sensor applications. The Mn content affects the grain size. It is also observed that the bandgap of the as-deposited film was decreases with increase in Mn doping. Due to strong transmittance at VIS-NIR region and wider bandgap, the films were suitable for optoelectronic applications as well buffer/ window layers in solar cells.

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Copyright

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