Abstract
In this work, we have successfully demonstrated the fabrication of a few WSe2 monolayers on the 8-inch Si/SiO2 wafer by the two-step CVD methods. Compared to conventional CVD ways, the two-step CVD method used in this work, including metal oxide deposition and chalcogenization, can achieve a large-area synthesis with precise thickness control by tuning the thickness of the as-deposited metal oxide. In addition, the thinnest thickness of the WSe2 film obtained by our method approaches to a single monolayer with some mixed bilayer areas and yet displaying photoluminescence due to the transition to direct band gap with the potential for optoelectronic applications. For the synthesis of WSe2, different parameters were optimized including oxide deposition and quality, substrate temperature, selenization time, forming gas ratio. For the optimization, the films were characterized under several material analysis, including Raman, XPS and TEM. In Raman spectrum, the relative intensity ratio and the full width at half maxima (FWHM) of characteristic peaks served as a criteria to judge the crystallinity of as-grown WSe2. XPS analysis was used to determine the WSe2 transformation rate. Besides, we made a use of fast Fourier transformation(FFT) to judge the grain size of as-grown WSe2 based on the TEM plane-view image. Then, after optimization we demonstrate large area growth with controlled thickness of high quality WSe2 judged from the material characterization. In the literature review of TMDCs material, the largest size of TMDCs fabrication is at the scale of 4-inch wafer so far. Here we demonstrate the formations of WSe2 with good uniformity in 8-inch wafer size. Besides, only sulfide based TMDCs such MoS2 has been synthesized at temperature closer to 500℃. However, TMDCs based on selenides are usually growth at higher temperatures. We successfully fabricated the high-quality thin layer WSe2 at temperatures as low as 500℃. We also expected to constantly lower the fabrication temperature of TMDCs in the future, especially for some flexible electronic applications.