Optoelectronic properties of n-Ag₂S nanotubes/p-Si heterojunction photodetector prepared by chemical bath deposition technique: An effect of deposition time

Abstract

In this paper, silver sulfide (Ag₂S) nanostructure film was prepared by chemical bath deposition (CBD) technique at different deposition times. The effect of deposition on the structural, optical and electrical properties of the Ag₂S film is studied using x-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive x-ray (EDX), transmission electron microscope (TEM), and UV-Vis spectroscopy. X-ray diffraction investigation shows that the deposited films are crystalline in nature with monoclinic structure. SEM results revealed that the film morphology depends on the deposition time. The film deposited at 10hr has mixed of agglomerated nanoparticles and nanotubes (NTs) morphologies. Transmission electron microscope (TEM) revealed that the average diameter of the nanotubes was 60nm.The optical properties show that the average optical transmission of the film decreases as the deposition time increase. The optical energy gap of the deposited films was found to vary from 1.8eV to 2.17 eV at room temperature depending on the deposition time. The figures of merit of the n-Ag₂S/p-Si heterostructure photodetector such as dark and illuminated current-voltage, linearity, responsivity, and specific detectivity have been investigated as a function of deposition time. The maximum responsivity of the photodetector was around 0.5A/W at 850nm for photodetector prepared at deposition time of 10hr.

Introduction

Silver sulfide is a n-type semiconducting material with direct energy gap in the range of (1-2.2) eV at room temperature and it belongs to I–VI compound semiconductors [1,2]. Due to its fascinating photoelectric and thermoelectric properties, Ag₂S was used extensively in many applications, for example, photosensitizer, solar cells, photoconductors, gas sensors, superionic conductor, infrared detectors, and resistive switch [3], [4], [5], [6], [7], [8]. It is reported that the morphology and composition are playing a vital role in determining the applications of Ag₂S [9]. The morphology of silver sulfide depends on the preparation method and conditions. Various techniques were used to prepare silver sulfide thin films such as chemical vapor deposition CVD, sol-gel, thermal evaporation, hydrothermal, laser ablation, spray pyrolysis, molecular beam epitaxy MBE, chemical bath deposition, etc [10], [11], [12], [13], [14]. Recently, Arulraj et al. prepared high quality stoichiometric Ag₂S film on the nickel mesh substrate using CBD technique for energy storage applications [15]. The CBD method for the deposition of thin films from an aqueous solution is a promising technique because of its simplicity, affordability, convenience for large-scale deposition and having easily-controllable parameters such as concentration, pH, deposition temperature and deposition time, etc. The preparation conditions such as solution concentration, pH value, deposition, time and bath temperature are optimized in order to get good quality of semiconductor thin films. The film properties deposited by this technique were depending on the deposition time, solution temperature, substrate type, complex agent type, and pH of the solution [18], [19], [20]. Ag₂S has been used effectively as high responsivity quantum dot (QDs) photodetectors in the UV-Vis region. Kang et al. reported on the fabrication and characterization of high photosensitivity quantum dot Ag₂S and Li-doped Ag₂S photodetectors by sonochemistry method [16]. Chen et al., study the characteristics of wide spectral range Ag₂S/ZnO core-shell nano-heterojunction photodetector prepared by chemical method [17]. Photodetectors based silicon heterojunction draw attention due their high responsivity, low concentration of recombination centers, wide spectral range, simplicity, small dark current, and low-cost fabrication. Very few studies have been reported on the fabrication of Ag₂S/Si photodetector. Tretyakov et al. [21] reported on the fabrication and characterization of Ag₂SQD/Si photodetector prepared by chemical method for the wavelengths ranged from 1 to 1.7µm with noise equivalent1.1 × 10⁻¹⁰ W/ Hz^0.5. Herein, we report the fabrication of n-Ag₂S/p-Si heterojunction photodetector by chemical bath deposition technique without using buffer layer and/or post annealing. The effect of the deposition time on the optoelectronic properties and figures of merit of the n-Ag₂S/p-Si photodetector was demonstrated.