Synthesis of a large area ReS2 thin film by CVD for in-depth investigation of resistive switching: effects of metal electrodes, channel width and noise behaviour

Abstract

The anisotropic crystal structure and layer independent electrical and optical properties of ReS₂ make it unique among other two-dimensional materials (2DMs), emphasizing a special need for its synthesis. This work discusses the synthesis and in-depth characterization of a 1 × 1 cm² large and few layered ReS₂ film. Vibrational modes and excitonic peaks observed from the Raman and photoluminescence (PL) spectra corroborated the formation of a ReS₂ film with a 1.26 eV bandgap. High resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns inferred the polycrystalline nature of the film, while cross-sectional field emission scanning electron microscopy (FESEM) indicated planar growth with ∼10 nm thickness. The chemical composition of the film analysed through X-ray photoelectron spectroscopy (XPS) indicated the formation of a ReS₂ film with a Re : S atomic ratio of 1 : 1.75, indicating a small amount of non-stoichiometric Re_xS_y. Following the basic characterization studies, the ReS₂ film was tested for resistive switching (RS) device application in which the effects of different metal electrodes (Pt/Au and Ag/Au) and different channel widths (200, 100, and 50 μm) were studied. The highest memory window equal to 10⁸ was obtained for the Ag/Au electrode while Pt/Au showed a memory window of 10². RS for the former was ascribed to the formation of a conducting filament (CF) because of the migration of Ag⁺ ions, while defect mediated charge carrier transport led to switching in the Pt/Au electrode. Furthermore, the R_HRS/R_LRS ratio achieved in this work (10⁸) is also of the highest magnitude reported thus far. Furthermore, a comparison of devices with Ag/Au electrodes but with different channel widths (50, 100 and 200 μm) gave insightful results on the existence of multiple resistance states, device endurance and retention. An inverse relationship between the retention time and the device's channel width was observed, where the device with a 50 μm channel width showed a retention time of 48 hours, and the one with a 200 μm width showed stability only up to 3000 s. Furthermore, low frequency noise measurements were performed to understand the effect of defects in the low resistance state (LRS) and the high resistance state (HRS). The HRS exhibited Lorentzian noise behaviour while the LRS exhibited Lorentzian only at low current bias which converged to 1/f noise at higher current bias.