Authors: Jian Guo
Source: eScholarship (California Digital Library) (article, 2019-01-01)
Field: Materials Science — Materials Chemistry

Two-dimensional semiconductor and heterostructure for novel electronics and optoelectronics The success of carbon nanotubes and graphene drives people to continuously search for novel low dimensional materials. Among the various discovered two-dimensional (2D) materials, 2D semiconductors (2DSCs) has been most extensively studied due to their unique electronic, optoelectronic, and mechanical properties. In terms of fundamental semiconducting performance, the ultra-thin body of 2DSCs enable strong electrostatic gate control, which is favored for high output current and ultra-low leakage current for field effect transistors. Meanwhile, the atomic thin film of 2DSCs is free of dangling bonds due to its layered structure, which contributes to low interfacial density of state and sharp turn on of density of states at the band edges. Those electronic properties are specially desired for short channel field effect transistor and tunneling transistor. Besides, the layer dependent band structure offers tremendous options for opto-excitation detection and heterostructures. However, the research of 2DSCs is limited on transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2. Experimental studies of 2DSCs on other group compounds has little been reported. Besides, detailed studies of charge transport in 2DSCs transistor in both lateral and vertical direction requires further understanding; studies on metal/semiconductor contact and interface engineering in 2DSCs heterostructure potentially enable fascinating electronic functions. In this thesis, I will present my effort on uncovering a new group of 2DSCs, charge transport in 2DSCs transistor and their heterostructure with either other 2DSCs or metals. I will include the work from two of my first author publication. I will first present my research studies on characterizing a new group of 2DSCs from IV-V group compound, 2D GeAs for the first time. By examining the semiconducting properties of few-layer GeAs transistors, we achieve high ON-OFF ratio of 105 and maximum hole field-effect mobility of 99 cm2 V-1s-1at room temperature. Due to highly anisotropic crystal structure, highly angle dependent optical and electronic transport is also observed. To reveal the intrinsic carrier transport, we carried out systematic studies on Schottky barrier and contact resistance. The intrinsic mobility proves charge impurity scattering limited transport of GeAs transistor. At last, we demonstrate mid-infrared photodetector by utilizing the band gap of GeAs crystal. Then I will show our investigation of charge transport in multilayer MoS2 transistor with optimized van der Waals contact. For the first time we demonstrate a vertical built-in potential in 2DSCs that induces negative transconductance (NTC) behavior. By varying measurement temperature, body thickness and terminal numbers, we reveal that the NTC originates from the vertical barrier due to distinct doping type and carrier distribution. We further demonstrate a frequency doublers and phase shift keying circuits based on single transistor by utilizing the NTC phenomena. Furthermore, I will present a study on tunneling transistor based on 2DSC SnSe/GeAs heterostructure. By stacking two flexible n-type and p-type semiconductor, we acquire a hetero-pn junction. The cryo-temperature study of the heterostructure proves a type III band alignment and Esaki type diode, which indicates intrinsic tunneling current at low bias. The subthreshold swing and gate coupling efficiency has also been discussed. For the final part, we will discuss about a novel approach to enable memristive switching behavior on metal and 2DSCs interface. With alignment transfer technique, we can integrate active metal with fragile 2DSC without introducing any destruction on 2DSC. Due to gentle interdiffusion of active metal and native oxide on 2DSC surface, a resistive switching layer forms and enable switching with ON/OFF ratio up to three orders. Resistive random-access memory and synaptic modulation has been realized with our metal/oxide/2DSC memristor. Moreover, we examine the working principle of our memristor and propose the formation and dissolution of metal oxide as the working mechanism. These findings and engineer could provide exciting opportunities for 2DSCs for novel electronics and optoelectronics. Heterojunction ; Transistor ; Semiconductor ; Optoelectronics ; Materials science ; Quantum tunnelling ; Field-effect transistor ; Nanotechnology ; Electronics ; Graphene ; Dangling bond ; Electrical engineering ; Voltage ; Engineering ; Silicon 2D Materials and Applications