Characrerization, analysis and optical studies of cadmium sulfide nanostructures deposited on different substrates for optoelectroninc applications

Abstract

Recently the research on nanotechnology has become increasingly popular due to their unique physical, chemical, optical and catalytic properties compared to their bulk counterparts. Nanotechnology revolutionizes many technology and industry sectors such as energy, environmental science, safety, medical sciences, medical instrumentation and many others. An II-VI semiconductor material, CdS nanostructure with a band gap of about 2.45 eV, has attracted great attention among the researches due to the peculiar properties. The objective of this research is to synthesize CdS nanostructure thin films deposited on different type of substrates glass, quartz, n-type and p-type silicon using sol-gel spin coating technique for optoelectronic applications. The CdS nanostructure thin films was synthesized and characterized using XRD, AFM, SEM, UV-VIS, PL, FTIR, TGA, DTA, DSC and Keithley 2400 Source Meter. Silver electrodes were thermally evaporated on the surface of CdS nanostructure thin films using stainless steel shadow mask. For CdS nanostructure thin films deposited on glass substrates, the results have indicated that the CdS has hexagonal structure. The thickness of CdS nanostructure thin films as measured by AFM is found to be in the range of 150 nm and 10 nm at 1000 rpm and 5000 rpm spin coating speeds respectively. For CdS nanostructure thin films deposited onto quartz substrates and annealed at 800 ºC with different spin coating speeds 1000, 3000 and 5000 rpm, the structural, morphological and analytical studies were investigated and found that the grain size of CdS nanostructure thin films found to be in the range 1.81 nm to 4.35 nm. The band gap was measured with an indication of transmission within the visible range. It is found that the band gap changed due to small grain size of CdS nanostructure thin films. The morphology of CdS nanostructure thin films are found to be continuous, dense and well adhered. The films surface is much smoother and the particles are well distributed. For CdS nanostructure thin films deposited into n-type and p-type silicon substrates at different annealing temperatures in the range from 200 oC to 600 oC. The effects of annealing temperatures were investigated on the structural, morphological, optical and electrical properties to improve the CdS nanostructure thin films. The XRD analysis shows that the crystalline quality of CdS nanostructures can be improved by increasing the temperature to 400 oC, but further increase to 600 oC leads to degradation of the crystalline quality. The bulk modulus was calculated and showed good agreement with experimental and theoretical results for different substrates to be found in the range 27.6 to 281.3 GPa. The optical properties of absorption namely; reflection, transmission, extinction coefficient and the energy band gap were obtained by PL and UV-VIS spectroscopies. The calculated refractive index and optical dielectric constant, the results are in agreement with experimental data. The best results for CdS nanostructure thin films are found using p-type silicon substrates annealed at 400 oC. The thermal properties of CdS nanostructures also investigated and found to be evident that a good thermal treatment can largely decrease the film strain and improve its crystallinity.