Influence of low temperature annealing time on CdS thin films

In this work, a low temperature annealing process is shown to improve the photoresponse time of CdS thin films. The films were deposited on glass substrates by chemical bath deposition. The samples were annealed in air for different times (0, 2, 4 and 6 h) at 200 °C. The morphology, optical and electrical properties of the CdS thin film was studied. The films were characterized by scanning electron microscope, X-ray diffraction, spectroscopic ellipsometry, 3D optical profilometry and UV–vis spectroscopy. The CdS thin films presented a compact structure with a grain size of approximately 80–100 nm. The average thickness of the films was 200 nm. All films exhibited a cubic CdS phase with preferred orientation along the (111) direction. The average value of the crystallite size was 30 nm. The refractive index varied from 2.40 to 2.64 as the annealing time increased. The optical bandgap for the samples showed a decreasing trend as the annealing time increased from 2.36 eV to 2.24 eV while the Urbach's energy had an inverse behavior from 0.160 eV to 0.270 eV. The annealing time was studied in the photoresponse of CdS films under ultraviolet and blue light. The fastest rise and fall times were 190 ms and 90 ms respectively for the 4 h annealing under blue light. We show that a low annealing temperature is possible to improve the photoresponse by 2 orders of magnitude.