Influences of growth conditions on physical, optical properties, and quantum size effects of CdS nanocluster thin films

Abstract

Cadmium sulfide (CdS) is the direct-bandgap, II–VI compound semiconductor that can be used for the fabrication of optoelectronic devices such as solar cells, laser diodes, and photoconductors. In this study, CdS thin films consisting of the quantum dots were grown by solution growth technique (SGT) on slide glass substrates, and the influences of growth conditions such as the concentration of mixed solution, ammonia concentration, growth temperature, and growth time on the structural and optical properties of CdS thin films were studied. Based on the results obtained, the quantum size effects of CdS thin films grown by SGT were investigated. The concentrations of cadmium acetate dihydrate and thiourea were changed from 1M, 2 M to 0.1 M, 0.2 M, respectively. The growth temperatures were 60 °C, 75 °C, 85 °C. The ammonia concentrations which play an important role on film thickness, grain size, and film quality were 2 M, 7 M, 14 M. The growth times varied from 1 min to 20 min. From XRD measurement results, CdS obtained in this study was found to have hexagonal structures. Depending on the various growth conditions, the film thickness ranged from 37 nm to 220 nm and the grain size ranged from 7.4 nm to 26.5 nm. The energy band gap (Eg) was calculated from absorption coefficients obtained from the optical transmittance, and Eg varied from 2.42 eV to 2.6 eV, depending upon the growth conditions. Eg increased with the decrease of concentration of mixed solution. Also, with the low ammonia concentration, Eg decreased substantially with an increase of growth temperature. This was ascribed to the large increase of film thickness and grain size in this case. These results were induced by the quantum size effects. The surface morphology was found to be strongly dependent upon the growth temperature. Roman scattering experiments were carried out for CdS quantum dot thin films grown by SGT, for the first time, and 1LO, 1TO, E2 phonon modes were observed. 1LO phonon frequencies shifted to the lower values with the decrease of grain size. This mode softening was attributed to the fine grain size of CdS in our investigation. It was also found that Roman shifts of 1TO and E2 phonon were independent of the grain size and film thickness.