Investigation of nanoscale surface roughness, fractal growth, optical constant and dispersion parameters of rf-sputtered CdS thin films for high-performance metal–semiconductor photodetector

The fractal dimension is most significant parameters that provides a way to quantify the irregularities, complexity of surface and such studied that are very important in understanding its impact on performance of optical and designed photodetector device. Herein, we report the deposition of CdS thin films on FTO-coated glass substrates via RF magnetron sputtering. Field emission scanning electron microscopy (FESEM) images were used to calculate particle size. The fractal parameters were computed height-height correlation function (HHCF) algorithms. It was observed that fractal dimension values decreased with increasing deposition time, with samples at lower deposition times exhibiting the most irregular topographical surface (D_f = 2.21 ± 0.03). The optical study showed a decrease in the optical band gap from 2.38 to 2.36 eV. Moreover, the Wemple-DiDomenico (WDD) approach was used to extract dispersion energy parameters such as the oscillator energy (E₀) and dispersion energy (E_d) of the thin films. The dispersion energies ranged from 11.23 to 15.39 eV, while the oscillator energies of the deposited films ranged from 4.32 to 4.08 eV. In addition, we have explored the detailed photodetector characteristics of the designed detector device through current–voltage (I–V) measurement. Here, it was observed that the photodetector parameters such as, sensitivity, responsivity, and detectivity are influenced with film thickness. The designed photodetector device at 1 V bias exhibits the maximum responsivity of 0.79 mAW⁻¹, and a photo-detectivity of 8.4 × 10¹⁰ Jones under 532 nm illumination. The fabricated photodetector showed a good photo response, a fast time response, and high reproducibility with time.