Thermally Induced Enhancement of Photoresponse in Radio Frequency-Sputtered CdS Thin-Film Photodetectors

This work focuses on understanding the defect-related electronic transport in cadmium sulfide (CdS) thin films, and finds thermal treatment as an efficient tool to tailor its intrinsic defect charge carrier concentration for optimum visible-light photodetection performance. The radio frequency (RF)-sputtered CdS thin-films show a substantial decrease in measured dark-current by three orders of magnitude (μA to nA) with an increase in substrate deposition temperature (T_s) from room-temperature (RT) to a maximum of 400 °C. With increase in T_s, the current conduction behavior changes from Ohmic (at RT) to Schottky-behavior (T_s ≥ 100–400 °C). The decrease in dark-current and the crossover from Ohmic to Schottky electronic transport behavior, pointed to a decrease in defect-density charge carrier concentration, with increased T_s. Additionally, post-deposition thermal annealing of CdS thin films also is found to result in a similar decrease in dark-current (μA to nA). The photo-to-dark-current ratio of CdS thin-film visible-light photodetectors increased by two-orders of magnitude, and its dynamic response time decreased by an order of magnitude via. thermal engineering. The thermal-annealing treatment possibly reduced the defect-related trap-sites, which enables a reliable and faster photo-switching response for CdS thin-film-based visible-light photodetectors.