Synthesis And Device Applications Of Monolayer WSe₂ And Its Alloy

Recently two-dimensional layered semiconductors with promising electronic and optical properties have opened up a new way for applications in atomically thin electronics and optoelectronics. Monolayer transition metal dichalcogenides (TMDs), such as MoS2, WS2, MoSe2, and WSe2, have attracted considerable interest due to their promising electronic and optical properties that are complementary to yet distinct from graphene and boron nitride. There are few reports on synthesis of large area monolayer WSe2 compared to MoS2, which a representative material in TMDs. Flexible, nearly transparent characters with a direct band gap of 1.6 eV enable monolayer WSe2 to fulfill the most basic requirement for electronic and optoelectronic devices. Developing methods for synthesizing large-area and uniform layers is an important step for applications such as large-scale fabrication of electronic devices and flexible, transparent optoelectronics. To explore the full potential of WSe2 requires a precise control of their band gap and electronic properties, which represents a significant challenge in 2D material systems.In this paper, we investigate the synthesis of monolayer WSe2 by CVD method. The atomic analysis of the as-grown WSe2 was conducted by spherical-aberration-corrected HRSTEM and AFM. The band gap was confirmed by photoluminescence, Elemental composition and bonding were examined by X-ray photoelectron spectroscopy (XPS). Back-gated FET was fabricated to investigate its transport property. To get monolayer WSe2(1-x)S2x alloy (x ranging from 0.07 to 0.85) with a tunable band gap, we varied the content of sulfur and then confirmed its electronic properties. A higher carrier mobility was obtained in the top-gated FET devices based on the monolayer WS2(1-x)Se2x (x=0.28).The main results are follows:(1)We first report a large area synthesis of monolayer WSe2 directly on SiO2Si substrate at atmosphere pressure by CVD method. A strong photoluminescence peak at a wavelength of 770 nm (1.61 eV) was observed at room temperature in the as-grown monolayer WSe2, indicating the sample was direct band gap semiconductor. The defect-free structure of the monolayer WSe2 at the atomic level was demonstrated by HRSTEM. P-type behavior of as-grown monolayer WSe2 with mobility of-0.2 cm2 V-1 s"1 and photo-responsivity Rλ of 20 mAW-1, which is compatible to the reported value of other monolayer 2D materials.(2)We report large area synthesis of monolayer WSe2(1-x)S2x alloy (x ranging from 0.07 to 0.85) with a tunable band gap controlled by varying the content of sulfur. Gated field effect transistor performance confirmed the p-type transport behavior in monolayer WSe2(1-x)S2x with a high on/off ratio (>104). After exposed to air for three months, the device performance manifested the excellent stability with no source-drain current drop observed. A mobility of～46.5 cm2V-1s-1 was obtained in the top-gated FET, two orders higher than that of the back-gated device.