https://absimage.aps.org/image/MAR14/MWS_MAR14-2014-020819.pdf

Van der Waals heterostructures

Low-resistivity metal-semiconductor (M-S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kohm*um and 0.5 kohm*um, respectively. The significant reduction of the Rc is attributed to the achieved high electron doping density thus significant reduction of Schottky barrier width. As a proof-ofconcept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 uA/um, an on/off ratio of 4*106, and a peak field-effect mobility of 60 cm2/V*s. This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nano-electronic devices.

Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2

An overview of recent developments in controlled vapor-phase growth of 2D transition metal dichalcogenide (2D TMD) films is presented. Investigations of thin-film formation mechanisms and strategies for realizing 2D TMD films with less-defective large domains are of central importance because single-crystal-like 2D TMDs exhibit the most beneficial electronic and optoelectronic properties. The focus is on the role of the various growth parameters, including strategies for efficiently delivering the precursors, the selection and preparation of the substrate surface as a growth assistant, and the introduction of growth promoters (e.g., organic molecules and alkali metal halides) to facilitate the layered growth of (Mo, W)(S, Se, Te)2 atomic crystals on inert substrates. Critical factors governing the thermodynamic and kinetic factors related to chemical reaction pathways and the growth mechanism are reviewed. With modification of classical nucleation theory, strategies for designing and growing various vertical/lateral TMD-based heterostructures are discussed. Then, several pioneering techniques for facile observation of structural defects in TMDs, which substantially degrade the properties of macroscale TMDs, are introduced. Technical challenges to be overcome and future research directions in the vapor-phase growth of 2D TMDs for heterojunction devices are discussed in light of recent advances in the field.

Recent Developments in Controlled Vapor-Phase Growth of 2D Group 6 Transition Metal Dichalcogenides

Two-dimensional (2D) materials hold great promise for future nanoelectronics as conventional semiconductor technologies face serious limitations in performance and power dissipation for future technology nodes. The atomic thinness of 2D materials enables highly scaled field-effect transistors (FETs) with reduced short-channel effects while maintaining high carrier mobility, essential for high-performance, low-voltage device operations. The richness of their electronic band structure opens up the possibility of using these materials in novel electronic and optoelectronic devices. These applications are strongly dependent on the electrical properties of 2D materials-based FETs. Thus, accurate characterization of important properties such as conductivity, carrier density, mobility, contact resistance, interface trap density, etc is vital for progress in the field. However, electrical characterization methods for 2D devices, particularly FET-related measurement techniques, must be revisited since conventional characterization methods for bulk semiconductor materials often fail in the limit of ultrathin 2D materials. In this paper, we review the common electrical characterization techniques for 2D FETs and the related issues arising from adapting the techniques for use on 2D materials.

https://iopscience.iop.org/article/10.1088/2053-1583/abc187/pdf

Electrical characterization of 2D materials-based field-effect transistors

IEEE International Electron Devices Meeting (IEDM)

Because the surface-to-volume ratio of quasi-two-dimensional materials is extremely high, understanding their surface characteristics is crucial for practically controlling their intrinsic properties and fabricating p-type and n-type layered semiconductors. Van der Waals crystals are expected to have an inert surface because of the absence of dangling bonds. However, here we show that the surface of high-quality synthesized molybdenum disulfide (MoS2) is a major n-doping source. The surface electron concentration of MoS2 is nearly four orders of magnitude higher than that of its inner bulk. Substantial thickness-dependent conductivity in MoS2 nanoflakes was observed. The transfer length method suggested the current transport in MoS2 following a two-dimensional behavior rather than the conventional three-dimensional mode. Scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements confirmed the presence of surface electron accumulation in this layered material. Notably, the in situ-cleaved surface exhibited a nearly intrinsic state without electron accumulation.

W. C. Shen

Papers

Two-dimensional electronic transport and surface electron accumulation in MoS2.