Two Dimensional Materials Based Electronic Devices:Scalable Fabrications And Their Performances

The field effect transistor(FET)is an essential solid-state microelectronic device that is present in most technologies used in modern societies,and it has promoted the creation of multiple new services and plenty of jobs.The progress of our society is strongly linked to the continuous miniaturization of FETs(according to the Moore's law),which allowed integrating more components in the same area and promoted the development of more sophisticated computing systems.Nowadays the size of the FETs is so small(-5 nm)that it is approaching to interatomic distances,which represents a physical barrier impeding further downscaling.For this reason,new strategies to continue the performance enhancement trend need to be thoroughly investigated,including use of new materials and new device types.Two dimensional(2D)materials,are a new family of nanomaterials that has attracted the attention of many material sciencientists in the past 15 years,and it is expected that they may improve the performance of many products.However,after years of intense studies on 2D materials and electronic devices based on them,there are still multiple important limitations preventing their integration into industrial devices.Further and further analyses of the synthesis methods,material properties,device patterning,device characterization,and device simulation are essential to understand the main challenges of this pocess and to provide effective solutions.In this doctoral dissertation,we have developed several experiments in this direction,which are presented in different chapters.In chapter 1,we give an introduction of the 2D materials family,and the main methods existing to synthesize them,including mechanical exfoliation,liquid phase exfoliation,chemical vapor deposition(CVD)and physical vapor deposition.We also summarize the structure and fabrication of various 2D materials based FETs,and we will introduce the 2D materials based memristor,a novel device that might replace FETs in future information storing and processing systems.Finally,we also carefully analyze and discuss the scalability and compatibility of these fabrication methods with requirements imposed by the microelectronics industry.In Chapter 2,we present a new method to directly scalably grow hexagonal boron nitride(h-BN)on thin metal-coated wafers using a low-pressure hot-wall chemical vapor deposition furnace.Among all methods have been developed to prepare 2D materials,chemical vapor deposition is the most attractive one,because it can scalable synthesize 2D materials with reasonably high quality.By using protective cover right above the catalytic metal substrate,the prohibitive metal diffusion and de-wetting under the high temperature have been avoided,and 2D materials are successfully grown on thin-metal coated wafers.In Chapter 3,we present the transfer-free fabrication of memristors based on multilayer h-BN,and we characterized and simulated their electrical performance.Our fabrication process does not require the transferring 2D materials onto the bottom electrodes using polymer scaffolds,a process that often produces massive contaminations and cracks.This is a great advantage compared to other methods available.We also carefully analyze the structure of the h-BN stacks using focus ion beam and cross-sectional transmission microscope,and we have been able to correlate the performance of the devices with the physical structure of the h-BN.Our results indicate that this type of memristors exhibit progressive resistive switching behavior and low cycle-to-cycle variability.In Chapter 4,we present the scalable fabrication of FETs using monolayer MoS2 as 2D semiconducting channel.The fabrication of these devices has been entirely based on scalable methods,including CVD(to synthesize the 2D material),photolithography,electron beam evaporation and plasma etching(to pattern the electrodes and channel).We scalably fabricated hundreds of FETs,and their correct structure has been confirmed by scanning electron microscope and Raman mapping.In Chapter 5,our fabricated MoS2 phototransistors that exhibit strikingly low power consumption(3.25×10-9 W under illumination)and high photo detection ability(light/dark current ratios up to 170).These performances are related to the small domain size of the polycrystalline monolayer MoS2 sheets(164 ± 54 nm in diameter)used in the devices.We also successfully minimized the hysteresis by introducing a thermal annealing step.Finally,we propose the different parameters to be selected during the chemical vapor deposition growth process to offer good control over the properties of these devices.This doctoral dissertation presents pathways for the integration of 2D materials into solid-state Micro-and Nano-electronic deivces,specially FETs and memristors,and therefore it may become a useful guide for scientist in both academia and industry.