Synthesis, characterization, and electrical transport in 2-D transition metal dichalcogenides grown by chemical vapor deposition

Transition metal dichalcogenides (TMDs), possessing a multitude of interesting properties, have emerged as an interesting choice for various types of electronic, optoelectronic and beyond CMOS device applications. Chemical vapor deposition (CVD) has been used extensively as an efficient, fast, reliable, and scalable route to grow uniform, high quality, large area TMDs. In this work, we report atmospheric pressure CVD (APCVD) and metal-organic CVD (MOCVD) growth of TMDs and study the effects of growth temperature, metal/chalcogen flux, reaction environment, etc. in modulating the shape, size, crystal structure, and uniformity of the grown film. To control the morphology more efficiently, we established a process for transition from compact two-dimensional (2D) domain to branched domain morphologies by varying the growth temperature and transition metal flux. Two different types of branched domains, fractals and dendrites, are observed which follow different growth mechanisms. In addition to the experimental investigations, we used a phase field simulation method for a better understanding of the dependence of the domain morphologies on the growth parameters. To control the 2D/3D growth mode, crucial role of chalcogen flux is investigated. While multilayer islands form in a chalcogen-deficient condition, a chalcogen-rich condition promotes lateral growth by restricting transition metal-rich nuclei formation. Study of APCVD growth with different carrier gases show that a reducing environment under hydrogen gas is more favorable to achieve uniform 2D growth. Based on the experimental observations, we propose an optimized CVD growth condition to achieve large-area high quality 2D TMD domains. Beside the APCVD growth of TMDs, an alternative approach via MOCVD growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure has also been explored. This two-step process can substantially heal chalcogen vacancies, suppress carbon/oxygen contamination, and produce more homogeneously distributed triangular monolayer domains with the electrical performance comparable to APCVD-grown domains.Electrical and Computer Engineerin

Solid Electrolytic Substrates for High Performance Transistors and Circuits

Ionic liquids/gels have been used to realize field-effect-transistors (FETs) with two dimensional (2D) transition metal dichalcogenides (TMDs) [1]. Although near ideal gating has been reported with this biasing scheme, it suffers from several issues such as, liquid nature of the electrolyte, its humidity dependency and freezing at low temperatures [2]. Recently, air-stable solid electrolytes have been developed, thanks to the advancement in battery technology [3]. Although insulator-to-metal transition has been reported, the realization of 2D TMD FETs on solid electrolytic substrate has not been reported so far to the best of our knowledge [4]. In this work, we demonstrate a lithium ion (Liion) solid electrolytic substrate based TMD transistor and a CMOS amplifier, with near ideal gating efficiency reaching 60 mV/dec subthreshold swing, and amplifier gain ~34, the highest among comparable inverte

Lithium-ion electrolytic substrates for sub-1V high-performance transition metal dichalcogenide transistors and amplifiers

Electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), leading to the accumulation of high surface charge carrier densities, has been often exploited in 2D devices. However, the intrinsic liquid nature of ILs, their sensitivity to humidity, and the stress induced in frozen liquids inhibit ILs from constituting an ideal platform for electrostatic gating. Here we report a lithium-ion solid electrolyte substrate, demonstrating its application in high-performance back-gated n-type MoS2 and p-type WSe2 transistors with sub-threshold values approaching the ideal limit of 60 mV/dec and complementary inverter amplifier gain of 34, the highest among comparable amplifiers. Remarkably, these outstanding values were obtained under 1 V power supply. Microscopic studies of the transistor channel using microwave impedance microscopy reveal a homogeneous channel formation, indicative of a smooth interface between the TMD and underlying electrolytic substrate. These results establish lithium-ion substrates as a promising alternative to ILs for advanced thin-film devices

Direct growth of MoS 2 on electrolytic substrate and realization of high-mobility transistors

Although electrostatic gating with liquid electrolytes has been thoroughly investigated to enhance electrical transport in two-dimensional (2D) materials, solid electrolyte alternatives are now actively being researched to overcome the limitations of liquid dielectrics. Here, we report direct growth of few-layer (3-4 L) molybdenum disulfide (MoS2), a prototypical 2D transition metal dichalcogenide (TMD), on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD), and demonstrate a transfer-free device fabrication method. The growth resulted in 5-10 µm sized triangular MoS2 single-crystals as confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Field-effect transistors (FETs) fabricated on the as-grown few-layer crystals show near-ideal gating performance with room temperature subthreshold swings around 65 mV/dec while maintaining an ON/OFF ratio around 10 5. Field-effect mobility in the range of 42-49 cm2V-1s-1 and current densities as high as 120 µA/µm with 0.5 µm channel length has been achieved, back-gated by the solid electrolyte. This is the highest reported mobility among comparable FETs on as-grown single/few-layer CVD MoS2. This growth and transfer-free device fabrication method on solid electrolyte substrates can be applied to other 2D TMDs for studying advanced thin-film transistors, interesting physics, and is amenable to diverse surface science experiments, otherwise difficult to realize with liquid electrolytes.D.A. acknowledges the PECASE award from the Army Research Office (ARO) grant #W911NF-16-1-0277, and the National Science Foundation (NSF) MRSEC Center (DMR- 1720595). S.K.B. acknowledges support from ARO grant #W911NF-17-1-0312 (MURI), and the NSF NASCENT ERC. The work was partly done at the Texas Nanofabrication Facility supported by NSF grant #NNCI-1542159.Center for Dynamics and Control of Material

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight