STUDY OF TWO-DIMENSIONAL SEMICONDUCTING MATERIAL THIN FILMS FOR DEVICE APPLICATIONS

Abstract

Over the recent decades, there has been substantial research into non-volatile memory storage devices, in this spell, the non-volatile memory storage phenomenon called resistive switching (RS) has attracted pronounced attention from scientific and industrial communities for succeeding advanced non-volatile memory devices. RS memory devices are of prime interest due to desirable merits such as simple structure, large data storage density, high ON/OFF current ratio, complementary metal-oxide-semiconductor (CMOS) compatibility, and quick process of reading/erasing the memory states. Basically, RS is the capability of periodically tuning the resistance between two or more resistance states induced by applying repeated electric stress across metal electrodes of the metal-insulator/semiconductor-metal (MIM) structure. RS behavior is governed by many mechanisms such as repeatedly forming/disrupting of metallic filament, Fowler-Nordheim tunneling, and trapping/de-trapping of charge carriers through the active layer sandwiched between two electrodes. The RS characteristics of the memory devices can be varied by modifying their intrinsic properties like numbers of vacancies, grain boundaries, phase, and crystallinity of insulating layer and electrode configurations. However, to develop the applicability of memory devices for a variety of environmental conditions, some of the external parameters including light, temperature, mechanical stress, and magnetic field are a matter of concern for upgrading the technology and multiple applicability of devices in various environments. Synergistically functioning of different external parameters on the nano-scale for characteristics tunability is a unique approach behind many advanced electronic devices such as sensors, transistors, and memories. Furthermore, MoS2 shows a strong absorption of light in the visible range, which makes it a suitable choice as an active layer in RS device with light as an external controlled parameter. Besides the two states RS devices where states switch between high resistance state and low resistance state, another kind of RS device namely the multilevel RS device has been fascinated in which resistance states switch between three or more distinguished stable states. The multilevel RS phenomenon is a versatile approach to enhance the data storage capacity significantly with an additional degree of freedom to store massive amounts of data. Furthermore, a variety of bilayer architectures with the right active materials have been chosen to improve the device's switching performance. Recently, hierarchical nanostructures such as nanowalls, nanoworms, and nanoflowers of the MoS2 layer have been utilized for gas sensing applications due to providing high interaction surface area to the gas molecules. Furthermore gas sensors based on n-p or p-n heterojunctions are preferred for their superior gas sensing performance, characterized by significant current modulation across the junction. Hereby, the main objective of this thesis is to fabricate nanostructured MoS2 and MoS2 hybrid heterojunctions or bilayer-based metal-insulator-metal (MIM) architect structures by D.C magnetron sputtering on various substrates including Pt-coated silicon, flexible kapton and ITO coated glass substrates. Thereafter, the light-triggered resistive switching properties of MoS2 and MoS2 based bilayer structure thin films are investigated for futuristic non-volatile memory applications in detail. The examination of the combined impacts of UV irradiation on a flexible Ag/NCQDs-MoS2/Ag/Kapton RS memory device is conducted to explore its potential for foldable and wearable memory technologies. Subsequently, the room temperature ammonia gas sensing characteristics of fabricated MoS2, CuO, and n-MoS2/p-CuO heterojunction thin films on ITO coated glass substrate are also examined. This thesis is arranged into six chapters. The chapter-wise summary of this work is stated below: Chapter 1: Chapter 1 starts with a historical overview and introduces two- dimensional layer materials including graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride h-BN, and molybdenum disulfide (MoS2). Further, to explore the characteristics and synthesis technique of MoS2 thin films, the incisive literature survey and reports are also included. This chapter offers crucial details for understanding non-volatile memory storage devices and gas sensors. The various concepts responsible for resistive switching and the general operation of gas sensors are also discussed in the final section. Chapter 2: This chapter describes the details of experimental methods used for the fabrication and characterization of various MoS2 thin films and hybrid heterostructures. The D.C magnetron sputtering technique was used to fabricate all thin film samples. A depth discussion had been held on a number of characterization methods, including X-ray diffraction (XRD), Raman spectroscopy, X-Ray photoelectronic spectra (XPS), Atomic force microscopy (AFM), and Field emission scanning electron microscopy (FE-SEM). Using the Keithley 4200 semiconductor characterization system (SCS), the electrical characteristics of the thin films were investigated. The resistive switching performance of the fabricated hybrid heterostructures has been studied under the irradiation of white light, and UV (376 nm) light with varying intensities. Additionally, the in-situ two-probe resistivity technique was used to examine the characteristics of ammonia gas detection with a source meter (Keithley 2400) and nanovoltmeter (Keithley 2181A). A chamber of volume 500 cm3 with a mass flow controller and PID-controlled electric heater is used for the sensing measurement to monitor the target gases. Chapter 3: White light-dependent RS functionality of MoS2 based memory device This chapter is divided into two sections. Section 3.1 explores the white light-controlled resistive switching (RS) functionality of MoS2 nanorods (NRs) by fabricating a metal-insulator-metal stack configuration. The Cu/MoS2 NRs/Pt/Si device demonstrates the reproducible two-state bipolar RS characteristics under both dark and light environments. In dark condition, the switching behavior of the NRs device could be attributed to metallic path formation/rupture between the top and bottom electrodes. While applying the white light causes a lowering of SET and RESET voltages by inducing a conducting path formation/rupture via electrons trapping/de-trapping in sulphur vacancies across the MoS2 NRs. The formation/rupture of the conducting path under dark and light illumination conditions is explained by proposing a conceptual model and analysing the resistance versus temperature measurements. It is observed that the white light acts as an external tool to modulate the RS behavior of the fabricated NRs device. The NRs structure of the MoS2 device provides good endurance of 1500 cycles and a long retention time of 103 s at room temperature under light illumination because of straight conducting path formation through NRs. These results demonstrate that the optical active MoS2 NRs-based devices have potential in next-generation tunable non-volatile resistive random access memory applications with additional functionality. Section 3.2 investigated the tunable RS functionality of Cu/MoS2/AlN/ITO bilayer nanostructured device under dark and white light illumination. The device exhibits bi-state RS behavior in dark ambient whereas, light illumination induces an extra intermediate resistance state and provides controllable tri-state RS characteristics. Interestingly, the variation of SET voltage with applied light intensity has also been demonstrated. A conceptual model is discussed and proposed to elucidate the origin of two resistance states and multiple resistance states switching behavior of the device. Under the dark ambient condition, the high resistance state (HRS) and low resistance state (LRS) in the device could be ascribed to the formation/rupture of Cu metallic filamentary path across the active layer. While the formation of an additional ionic filament via trapping/detrapping of electrons in nitride-sulphide-related vacancies along with Cu metallic filament is responsible for the tri-state switching under the light illumination. The calculated value of the temperature coefficient of resistance and temperature dependency of resistance in various resistance states confirms the existence of the proposed model. This optical activity-dependent functionality of the device provides a possibility to extend the RS-based non-volatile random access memory applications to the optical domain such as imaging sensors, photodetectors, and optoelectronic switches. Chapter 4: UV light modulated RS properties of MoS2 based flexible hybrid structure This chapter thoroughly examines the variable resistive switching (RS) behavior of a flexible Ag/NCQDs-MoS2/Ag nanostructured device under dark and UV light illumination. The device shows impressive performance, exhibiting both gradual and abrupt RS behavior with notable endurance (2200 cycles) and remarkable data retention (2500 seconds) properties without any deterioration. A conceptual model has been introduced to offer a profound and insightful understanding of the device's abrupt and gradual RS behavior. In ambient conditions, the device ability of abrupt bipolar RS with two resistance states, namely, high resistance state and low resistance state could be attributed to the creation and disruption of an Ag metallic filament between the top and bottom electrode. However, when the device was subjected to UV irradiation, an interesting abrupt as well as gradual RS behavior with distinct SET and RESET voltages was noticed. This behavior could be associated with the formation of an additional ionic filament before the formation of an Ag metallic filament. The growth of ionic filament was influenced by UV irradiation via trapping/ de-trapping of carriers in generated nitride-sulphide-related vacancies. In addition, the device's flexibility was assessed by subjecting it to various bending degrees and bending cycles to mimic real-life scenarios where the device may experience multiple forms of mechanical deformation. Therefore, this optically tuned and flexible RS device has potential to spark the advancement of futuristic Resistive Random Access Memory (ReRAM) applications in flexible optoelectronic. Chapter 5: Ammonia gas detection characteristics of MoS2-heterojunction based device This chapter describes the fabrication of the MoS2, CuO, and n-MoS2/p-CuO heterojunction nanoworms thin films on ITO coated glass substrate by D.C magnetron sputtering method. The room temperature ammonia (NH3) gas sensing properties of MoS2/CuO nanoworms bilayer sensor toward low detection range (5-500 ppm) have been discussed in detail. The as-prepared nanoworms-based gas sensor reveals a fast response/recovery time (17 sec/26 sec), high sensing response (S.R∼47 %) with remarkable reproducibility (15 cycles) during exposure to 100 ppm ammonia in the dry synthetic air at room temperature. Furthermore, the working principle for the outstanding performance of the proposed sensor towards ammonia gas was also studied in this chapter.Chapter 6: This chapter provides the summary and major conclusions of the entire work presented in individual chapters. Also, a brief report is included to extend the work in possible directions for further research.