Preferentially oriented CuCdS2 thin films and thickness effects on structural, optical and electrical properties

Abstract

Copper cadmium sulfide (CuCdS2) thin films were deposited via thermal evaporation deposition technique. Crystallinity and adhesive nature of the thin films were improved at the substrate temperature of 300 °C. CuCdS2 thin films possess nanocrystalline nature with hexagonal structure and its grain size increased from 20 to 37 nm with respect to deposition time. Linear increment in the thickness with respect to deposition time ensued preferential orientation along (002) plane. Raman spectra explained the vibrational modes of CuCdS2 nanoparticles and thin films. The intensity ratio between LO and 2LO peaks (I2LO/ILO) increased with grain size. Optical studies revealed that the transmission retained 30–60% in blue to green band region and reached maximum value in yellow–red band. Band gap values vary between 2.1 and 2.4 eV, and the expansion attributed to confinement of charges. Atomic force microscopy images showed that the films acquired nanocrystallinity with definite boundaries between the grains and grain sizes increased from 18 to 32 nm. Grains were grown as spikes during a longer deposition period and average surface roughness of the films was increased with grain size. Morphology of the surface grains tended to grow in circular shape to minimize the surface free energy. Oxidation states of the elements were found using X-ray photoelectron spectroscopy and stoichiometry of all the CuCdS2 films could be explained as 1:1:2 for Cu:Cd:S. Electrical studies revealed that all the films had p-type semiconducting nature. Resistivity values of the films are found in the order of 10−2 Ω; concomitant with the thickness it decreased to 10−3. Bulk concentration values lay in the range of 1019–1020. Photoconductivity measurements showed that the p–n junction of CuCdS2/n-Si had rectifying nature.