STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF METAL OXIDE-2D MATERIAL BASED COMPOSITES FOR NON VOLATILE RESISTIVE MEMORY DEVICE

Abstract

Big data processing and storage are in high demand as human society moves into the big data era. One new gadget that has computation and memory built right in is the resistive switching memory device. As more and more conventional memory technologies, such SRAM, DRAM, and Flash, reaching the end of their useful lives, a novel idea known as "Universal Memory" emerges, fast speed (read and write), low power (access and standby), high density (low cost), non-volatility, random accessibility, and infinite durability are anticipated qualities of a universal memory. Magneto resistive memory (MRAM), phase change memory (PCM), and resistive memory (ReRAM) are a few possibilities for universal memory. The increasing number of flexible, wearable, foldable, lightweight, and transparent electronic gadgets such as displays, RFID tags, sensors, mobile phones, and watches has recently increased demand for new types of memory devices. ReRAM development has received more attention in recent years because of its straightforward design, low power consumption, and quick performance. The non-volatile memory effect is caused by a systematic resistance change between the low resistance (RLRS) and high resistance (RHRS) states by varying the applied voltage bias to the cell. In order to produce resistive switching, or to cause HRS switched to LRS (set) and LRS switched to HRS (reset) transitions, different material combinations employed in MIM cells require different kinds of electrical stresses. Therefore, the resistive switching phenomenon can be divided into the following categories: i) unipolar and ii) bipolar resistive switching, which require the application of stresses with the same or opposite polarity to initiate the set and reset processes, respectively; iii) nonpolar resistive switching, which allow the application of stresses with any polarity to achieve the set and reset transitions; and iv) threshold resistive switching, which occur when the LRS is volatile and the reset process occurs automatically when the stress is turned off. Numerous inorganic and organic materials exhibit this resistive switching effect, including solid-state electrolytes, binary transition metal oxides, perovskites, organic donor-acceptor systems, and organic charge transfer complexes. However, recent developments in material science demonstrate that the organic-inorganic nanocomposite interface device has greater advantages than its inorganic and organic equivalents. Understanding the switching mechanism in ReRAM was particularly difficult because of the several events that coexisted when the ultimate electrical stress was applied. Many methods have been proposed to explain the resistive switching of ReRAM devices, including conductive filament generation, space- vii charge-limited conduction, trap charging and discharge, Pool-Frenkel emission, and the Schottky emission. Size-dependent research and electrode material provide crucial hints for a clear grasp of the switching mechanism. Several methods, including X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), have been employed in this regard to comprehend the filament composition and, consequently, the underlying process. The major objective of the present thesis is to examine the impact of different 2D materials (rGO, MoS2, hBN) and SnO2 as an active material deposited using spin-coating technique on the resistive switching phenomenon. Bipolar resistive switching is observed for SnO2-2D material based film sandwiched between two different metal electrodes. Firstly, the effects of pure reduced graphene oxide (rGO) inclusion on the resistive switching properties of tin oxide (SnO2) based resistive memory device was studied. It has been demonstrated that the incorporation of rGO in SnO2 nanocomposite induced bipolar resistance switching with enhanced ON/OFF ratio. Here, we have fabricated flexible resistive switching memory devices of pure SnO2 and rGO-SnO2 (5, 7, and 10 wt.%) nanocomposites. The resistive switching performance of the fabricated devices were compared and observed that the memory device with 7 wt. % of rGO in SnO2 has a maximum ON/OFF ratio of  70, in comparison to  4 and  3 for 5 and 10 wt. % sample respectively. The endurance and retention tests were performed on 7 wt. % rGO-SnO2 composite film based memory device and the device shows no degradation in the memory window up to 100 cycles. After incorporating conducting rGO, semiconducting molybdenum disulfide (MoS2) was used for synthesizing tin oxide (SnO2) based nanocomposite powder as a resistive switching material, where the effect of MoS2 weight percentage (0, 5, 7, and 10 wt.%) have been evaluated in terms of the structural, optical, and electrical properties changes of resulting nanocomposite using X-ray diffraction, UV-Visible Spectroscopy, Scanning electron microscopy, and Transmission electron microscopy techniques. The nanocomposite samples were synthesized using a simple hydrothermal technique and are spin-coated on commercially available ITO-PET substrate. The top metallic electrode was deposited by thermally evaporating Aluminium through a shadow mask resulting in Al/SnO2/ITO-PET and Al/MoS2- SnO2/ITO-PET (5%, 7%, 10%) based resistive memory devices. The measured current-voltage (I-V) characteristics over the fabricated devices having different MoS2 wt. % showed the increase in IOn/IOff ratio (3, 100, and 25) with an increase in MoS2 concentration and is found viii that device with 7wt % of MoS2-SnO2 has higher IOn/IOff ratio (100) in comparison to 7 wt. % rGO-SnO2 nanocomposite-based device. Additionally, a retention and endurance test was conducted to verify the fabricated devices stability and cyclic performance. The results showed that the 7%MoS2-SnO2 device maintained the HRS and LRS states for up to 2103 s and demonstrated stable performance for up to 100 switching cycles without much degradation, respectively. It is noteworthy that the currently suggested ReRAM technology, which is based on SnO2 and MoS2, has great potential for application in the wearable device market due to its flexible and low-power properties. Further to increase the memory window, insulating hexagonal boron nitride (hBN) having wide band gap and tin oxide (SnO2) based nanocomposite powder was synthesized hydrothermally for acting as switching material. X-ray diffraction and Raman spectroscopy methods were used to characterize the synthesized nanocomposite powder. The resistive switching performance of the fabricated devices were compared and observed that the memory device with 7 wt. % of hBN in SnO2 has a maximum ON/OFF ratio of  1000, in comparison to  96 and 10 for 5 and 10 wt. % sample respectively. The endurance and retention tests were also performed on 7 wt. % hBN-SnO2 composite film based memory device and the device shows no degradation in the memory window up to 100 cycles. The present work demonstrates the systematic investigation of the effect of different ultrathin 2D layered nanomaterials and their concentration in metal oxide matrix for future flexible, reliable, and low-power non volatile memory devices.