| Since the invention of the semiconductor transistor,the rapid miniaturization of siliconbased field-effect transistors(FETs)has significantly transformed the field of information storage and processing,enabling increased speed and integration of devices.However,the continued shrinking of silicon transistors has led to the emergence of short-channel effects that have increasingly impacted the performance of silicon-based transistors,pushing them closer to the physical limitations.This presents a significant challenge in sustaining the momentum of ’Moore’s Law.’ To address this challenge,researchers have turned to van der Waals layered two-dimensional(2D)materials like MoS2,WS2,and WSe2.These materials boast atomically flat surfaces that offer excellent electrical properties,even when they are in their thinnest states,which make them highly promising as FET channel materials.They are expected to replace silicon crystal materials and facilitate the continuation of Moore’s Law.However,in order for these materials to be integrated into the most advanced transistors and simple integrated circuits,they must have large dimensions,high crystal quality,and other critical requirements.Therefore,it is crucial to prepare and use high-quality 2D materials with large thickness uniform.Moreover,the contact resistance at the metal electrode/semiconductor interface remains an important but unsolved scientific problem in the semiconductor industry.When layered 2D materials are thinned to the single layer,the ultra-high contact resistance at the metal electrode in contact with the 2D semiconductor interface severely impedes their scaling and performance in the electronic devices.In order to overcome this challenge and enable the large-scale application of 2D materials in the semiconductor industry,it is essential to achieve the controlled preparation of wafer-scale 2D materials and to design effective metal electrode/2D semiconductor interface contacts.The first chapter provides a literature review that covers the growth techniques used for wafer-scale transition metal dichalcogenides(TMDCs)and recent advancements in enhancing metal/2D semiconductor interface contacts while reducing contact resistance.In the second chapter,the methods used to grow single-crystal films of Fe-doped TMDCs and prepare field-effect transistors(FETs)are described.In Chapter 3,a high throughput one-step chemical vapour deposition(CVD)method was developed for the successful synthesis of 4-inch long monolayer Fe-doped TMDCs single crystal films in a controlled manner.This was achieved using commercial c plane sapphire without specific miscut angles as a substrate,with the introduction of FeCl2 during the CVD growth process that facilitating the reduction of the formation barrier of atomiclevel parallel steps on the surface of the substrate.At 900℃,atomically parallel steps were formed on the c-surface sapphire,enabling the unidirectional alignment of TMDCs domains.This was achieved through altering the van der Waals epitaxy of TMDCs on c-plane sapphire,aligning the Fe-doped TMDCs crystal domain edges and step edges in parallel,thus breaking the energy simplicity of the anti-parallel domains in TMDCs.As a result,a 4-inch long single layer of Fe-doped TMDCs with seamlessly stitched unidirectionally aligned domains was produced.The electronic doping behaviour of TMDCs was demonstrated by XRD,Raman,and DFT theoretical calculations,with the introduction of Fe changing the band structure of TMDCs and modulating their electronic properties.This material is cost-effective and straightforward to prepare,and the use of commercially available substrates may provide the feasibility of large-scale growth and industrial application of high-quality wafer-scale TMDCs.In Chapter 4,we address the issue of ultra-high contact resistance at the metal electrode2D semiconductor interface and enhance the performance of 2D semiconductor electronic devices by precisely and controllably doping TMDCs single-crystal films.We achieve an ultra-low contact resistance(～820 Ωμm)and perfect ohmic contact in the single-layer Fedoped MoS2 FET,resulting in an ultra-high electron mobility(～54 cm2 V-1 s-1)and excellent on/off current ratio(108).In-depth structural analysis and theoretical calculations reveal that the ultra-low effective mass of electrons and appropriate doping position greatly suppress impurity scattering and improve the metal electrode/2D semiconductor interface contact.The Schottky barrier height is further reduced by selecting a half-metal Bi with a suitable work function as the electrode and suppressing the Fermi energy level pining effect.By using a 4-inch-long Fe-MoS2 single-crystal FET,we achieve an ultra-low contact resistance(-489Ω μm),an ultra-high electron mobility(～146 cm2 V-1 s-1),and an excellent on/off current ratio(～109).We also construct a new liquid-gate FET with a very small threshold voltage,and achieve the highest recorded electron mobility(～231 cm2 V-1 s-1)and excellent subthreshold swing(～95 mV dec-1),which close to the theoretical limit for FETs constructed from single-layer TMDCs.This work demonstrates the great potential of high-performance monolayer MoS2 as transistor channel materials,which will greatly facilitate further scaling and performance improvements in the "post-Moore’s Law era".In Chapter 5,we offer a comprehensive summary of our research and provide an outlook for future studies on 2D semiconductors and high-performance devices. |
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