Modification of the junction parameters via Al doping in Ag/CdS:Al thin-film Schottky diodes for microwave sensors

In this article, it is investigated the effect of Al doping in the junction parameters of Ag/CdS:Al thin-film Schottky diodes and their electrical response to microwave irradiation. Nanocrystalline CdS:Al thin-films with thicknesses between 109 and 173 nm were prepared by chemical bath deposition and, subsequently, Ag thin-films with an average thickness of 102 nm were grown on the CdS:Al using dc sputtering. The structural, chemical, morphological and optical properties of CdS:Al and Ag films were characterized by x-ray diffraction (XRD), scanning electron microscope, atomic force microscope, energy-dispersive x-ray spectroscopy and UV-Vis spectrophotometer, respectively. Current-voltage (I − V) characteristics of Ag/CdS:Al diodes, with different Al content, were obtained at room temperature in dark conditions. XRD studies shows that CdS:Al and Ag thin-films have an hexagonal and cubic structure, respectively. Crystallite sizes decreases with Al content for CdS:Al films and were found to be in the 15–40 nm range. A decrease in the intensity of the XRD main peak of CdS:Al films is observed, caused by the inclusion of amorphous Al₂O₃ on the CdS film. It was found that band gap of CdS:Al films increases with increasing Al content, from 2.28 eV to 2.40 eV. Based on the I − V characteristics of the diodes, their barrier height ϕ ₀, ideality factor n, and series resistance R _s were calculated, and it was found that these values are modified by increasing Al content in CdS films, in the ranges: ϕ ₀: 0.7037–0.8426 eV; n: 3.485–4.213; R _s: 0.54–9.86 MΩ. Besides, it was stated that Al doping changes the average surface roughness and the energies of the charge neutrality levels of CdS:Al films. The effects of physical properties of the films on the junction parameters of the diodes were also discussed. Finally, I − V characteristics of the Ag/CdS:Al diodes were studied under X-band microwave irradiation at room temperature in dark conditions. For a specific Al doping value, the current density across the diode during irradiation was found to be lower (0.87–11.6 mA cm⁻²) than unirradiated diode (1.14–15.6 mA cm⁻²), when the bias voltage was higher than certain value (3 V), due to an increasing temperature of the diode and the presence of Al₂O₃ on the CdS:Al film. This last result could be useful in a potential X-band thin-film microwave sensor. (paper)