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

Abstract

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 f0, ideality factor n, and series resistance Rs were calculated, and it was found that these values are modified by increasing Al content in CdS films, in the ranges: f0: 0.7037-0.8426 eV; n: 3.485-4.213; Rs: 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.