Novel Two-Dimensional Materials for Transistor, Photodetector, and Light Emitting Device Applications

The exotic quantum properties of two-dimensional (2D) layered materials are used for various potential applications, such as nanoscale electronics, optoelectronics for ultrasensitive sensors, tissue engineering, catalysis, and energy storage. Pursuing the 2D materials is a high priority in the discovery of suitable and promising candidates for fabricating the devices. In this study, we report the growth of novel 2D materials, such as indium selenide (InSe) and germanium sulfide (GeS) high-quality single crystals. The electrical and optical properties of both these single crystals were investigated. The main focus of this thesis is the careful fabrication and demonstration of multi-layered as well as few-layered InSe- or GeS-based field-effect transistors (FETs). The multi-layered InSe-FET shows a mobility of >1000 cm2/Vs at room temperature, which is higher than that of recently reported FETs made of widely studied 2D transition metal dichalcogenides. In this work, various substrates, such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multilayer InSe-FET devices. Through back gating and Hall measurement in four-probe configuration, the device’s field-effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the material’s intrinsic transport behavior and the effect of the dielectric substrate. The sample’s field-effect and Hall mobilities over the range of 20−300 K fall in the range of 0.1−2.0 × 103 cm2/Vs. The multi-layered GeS-FET photodetector exhibits a high photoresponsivity about 206 AW-1. The observed photocurrent is higher than the known GeSe and SnS2 photodetectors. Moreover, the multi-layered GeS photodetector exhibits high external quantum efficiency (EQE ~ 4.0 × 104 %) and specific detectivity (D* ~ 2.35 × 1013 Jones). The measured D* is comparable to those of the advanced commercial Si- and InGaAs-based photodiodes. The GeS photodetector also shows an excellent long-term photo switching stability over a long period of operation (>1 h). Few-layered InSe photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of conducting broadband photodetection from the visible to near-infrared region (450−785 nm) with high photoresponsivity of up to 12.3 AW−1 at 450 nm (on SiO2/Si) and 3.9 AW−1 at 633 nm (on PET). These InSe devices can also operate on a flexible substrate with or without bending and reveal comparable performance to those devices on SiO2/Si. In the final part, we propose a simple structure adopting single-gate electrode and different work function metals as contacts to achieve ambipolar behavior. A series of experiments were conducted to demonstrate that the designated metal-InSe junctions can lead the carrier behavior of FETs. The results indicate that the polarity of InSe-FET can be controlled by different metal work functions. Furthermore, we adopt asymmetric metal with different work functions as source-drain electrode to reduce Schottky barriers for electron-hole recombination. We also conducted experiments to demonstrate 2D materials-based FETs with carefully selected metal electrode contacts can achieve ambipolar behavior. These extraordinary properties of InSe and GeS show that the materials are highly qualified candidates for the future high performance nanoelectronics devices. With these excellent optoelectronic merits, we envision that the nanoscale InSe and GeS layers will not only find applications in optoelectronics and flexible devices but also act as an active component to configure versatile 2D heterostructures devices.