Effects of post-annealing on MoS2 thin films synthesized by multi-step chemical vapor deposition

Abstract

Multi-step chemical vapor deposition (CVD) is a synthesis method which is capable of producing a uniform, large area, and high-quality thin films. In this work, we report the effect of post-annealing on the structural and optical properties of few-layers (FL) MoS2 thin films synthesized by multi-step CVD. Based on atomic force microscopic image, the thickness of the MoS2 thin film is ∼3 nm, which is equivalent to five layers. After annealing at 900°C for 17 min, intensity of the A1g and E  2 g 1 Raman modes increased by ∼3 times while the full-width-at-half-maximum (FWHM)* reduced from ∼10 cm−1 to ∼7.5 cm−1 for A1g and from ∼13.6 cm−1 to ∼7.5 cm−1 for E  2 g 1 . Both of the as-grown and annealed samples showed X-ray (002) diffraction peak at 14.2° but the intensity was more prominent for the annealed sample. It was found that the annealed sample showed clear and distinct absorbance peaks at 666, 615, 448, 401, and 278 nm which correspond to the A, B, C, D, and E excitons, respectively. The results indicate that annealing significantly improved the optical and structural quality of the MoS2 film. Field-effect transistor based on annealed MoS2 thin film was fabricated and showed electron mobility of 0.21 cm2V−1s−1, on/off ratio of 1.3 × 102 and a threshold voltage of 0.72 V. Our work highlights the importance of high-temperature annealing in multi-step CVD to obtain a uniform and high-quality FL MoS2 thin films.