Integration of 2D Materials in Resistive-RAM and Memtransistive Devices for Data Storage Applications

Abstract

In today's fast-growing world, information storage and processing is a significant challenge for researchers. Researchers are working hard to develop faster, stable, and reliable memristive architectures to meet the demands of the world. Two-dimensional (2D) materials have great potential in this field due to their exceptional characteristics. This study aims to explore the potential of 2D materials in building faster and more reliable memristive architectures for information storage and processing. It further involves the modification of these 2D materials to improve memristive architectures and address the challenges of information storage and processing. This may involve using different types of 2D materials or altering the properties of the materials through various techniques such as doping, functionalization, or the introduction of defects. The goal is to enhance the performance of memristive devices in terms of speed, stability, and reliability, and to improve these memristive architectures. Additionally, various new methods have been explored for fabricating and integrating 2D materials into memristive architectures to improve their performance. The study is divided into multiple chapters, each with a specific focus. In the first chapter, a detailed introduction of various 2D materials such as Graphene, MXene, and MoS2 is provided. These materials were further synthesized and used for device fabrication. The second chapter focuses on discussing important literature studies related to these 2D materials and their memory device architectures. The literature review aims to understand the ongoing trends related to these materials and the main challenges researchers are facing in the memory field. The third chapter delves into the structural, morphological, electrical, and chemical analysis of the materials. It provides details on the characterization techniques used to analyze the materials and their properties. After this discussion, the detailed 2D material synthesis and device fabrication along with device performance are described. The fourth chapter initially explains the detail synthesis of free-standing films of 2D materials (GO, rGO) along with the fabricated reduced-Graphene Oxide/Graphene Oxide/reduced-Graphene Oxide (rGO/GO/rGO) free standing memory device. The GO was synthesized using modified hummer method via graphite oxidation. Further, the GO film was laser-scribed to fabricate rGO/GO/rGO free-standing memory device under open environment. All carbon rGO/GO/rGO device showed non-volatile complementary resistive switching in a single cell framework in contrast to crossbar arrays and reduces the device fabrication complexity of these systems. Also, the device exhibited the exceptional endurance up to 2500 cycles as well as retention time of 104 s. Inside fifth chapter the free-standing single transition metal MXene (Ti3C2) film was also synthesized with the GO film. MAX phase was etched using hydrofluoric (HF) acid followed by exfoliation using TMAOH polymer base to achieve free-standing MXene films. Both GO and MXene films were assembled to form M/GO/M free-standing device under ambient environment. With the implementation of MXene (Ti3C2) in M/GO/M memory device, it was observed that the bipolar resistive switching behavior is accompanied with the capacitive effect representing a capacitive resistive switching behavior inside the non-volatile M/GO/M memory device up to >2400 cycles. This capacitive resistive switching paves a pathway towards the self-generating electronics which can empower themselves in the absence of external bias. Along with device testing X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to observe crystal structure and morphology of the material free-standing films as well as our device systems. Moreover, Fourier-transform infra-red (FTIR) spectroscopy and UV-vis spectroscopy illustrated details regarding presence of functional groups as well as the active material band gap. Optical profilometry provided the approximate thicknesses of MXene, GO and device. In chapter six the aim was to explore more about capacitive resistive switching for which there was a need to further investigate the MXene/GO interface. Hence for an extensive study, MXene (Ti3C2) metal electrodes were replaced by Double Transition-metal MXene (DTM) - Mo2TiC2 electrodes. DTM MXene is a more condensed form of MXene, semiconducting in nature and its band gap can be tuned as desired which depends on the etching as well as exfoliation processes. By careful synthesis process, the Mo2TiC2 free-standing film was obtained with no aluminum (Al) traces as supported by EDX results. The implementation of DTM-MXene instead of Ti3C2 confirmed that the MXene family on combining with graphene has an ability to produce capacitive and memristive effects at the same time inside flexible electronic systems. Further by changing thickness of active layer (GO) between metal electrode, the effect on key parameters such as retention time, endurance and the Ion/Ioff ratio of current were improved. The on/off ratio of device has been tuned up to 102 that is favorable for practical device applications. The endurance (up to 5000 cycles) and retention time (105 s) for the devices were also improved in MXene/GO/MXene devices. All the studies till chapter six were performed in open environment. Inside chapter seven we presented a comprehensive study over the growth optimization of transition metal dichalcogenide called molybdenum disulfide (MoS2) under high vacuum conditions using two broadly studied precursors (molybdenum (Mo) metal and molybdenum trioxide (MoO3)). The aim is to produce an extensive comparison of MoS2 growth using initial precursors of either molybdenum (Mo) or molybdenum trioxide (MoO3) under the same growth conditions to reduce the growth complexity and to see resultant film differences under the same growth conditions. Both the metal and metal oxide precursors were deposited on Si/SiO2 substrate using e-beam evaporation. Further, by analyzing various initial precursor thicknesses, we optimized one initial thickness to further see the effects of sulfurization temperature on the MoS2 growth. Mo and MoO3 films were evaporated and reacted with sulfur inside the CVD furnace under different temperatures to grow MoS2, based on the Mo metal precursor, as well as MoS2- xOx, in the case of the MoO3 precursor. The temperature study reveals that few to multi-layer films were produced using Mo, while films with areas of monolayer growth are achievable with MoO3, under the same growth temperature and conditions. With an optimized recipe, monolayer, bilayer and few layers sulfurized samples have been grown and verified by Raman, photoluminescence spectroscopy, XRD, XPS and AFM. After attaining atomically thin sulfurized samples, the performance as channel material and memory device were evaluated and analyzed. As a result, this comparative analysis of MoS2 growth provides an insight into achieving optimized material growth with reduced growth complexity. Furthermore, the device performances based on the resultant films aid a clearer understanding towards the relationship between material growth parameters and electrical characteristics. The optimized MoO3 grown MoS2 samples were then subjected to memtransistor and resistive random-access memory (RRAM) device characterizations. The monolayer MoS2 showed good RRAM results with on/off ratio of 104 while the few layered MoS2 grown at 750 ˚C showed memtransistive behavior with a p-type material growth and higher carrier mobility ≈ 41 cm2V -1S -1 which contrasts with typically observed n-type characteristics towards neural synaptic device applications. The work overall includes the synthesis and fabrication of flexible (graphene and MXene comprised) memory devices under ambient environment as well as mono and few layer MoS2 growth using MoO3 and Mo metal under high vacuum conditions for RRAM and FET applications. The device's performance is encouraging, and it offers an outstanding stable foundation for future industrial applications.