A wide-angle X-ray scattering laboratory setup for tracking phase changes of thin films in a chemical vapor...

The few-layer transition metal dichalcogenides (TMD) are an attractive class of materials due to their unique and tunable electronic, optical, and chemical properties, controlled by the layer number, crystal orientation, grain size, and morphology. One of the most commonly used methods for synthesizing the few-layer TMD materials is the chemical vapor deposition (CVD) technique. Therefore, it is crucial to develop in situ inspection techniques to observe the growth of the few-layer TMD materials directly in the CVD chamber environment. We demonstrate such an in situ observation on the growth of the vertically aligned few-layer MoS₂ in a one-zone CVD chamber using a laboratory table-top grazing-incidence wide-angle X-ray scattering (GIWAXS) setup. The advantages of using a microfocus X-ray source with focusing Montel optics and a single-photon counting 2D X-ray detector are discussed. Due to the position-sensitive 2D X-ray detector, the orientation of MoS₂ layers can be easily distinguished. The performance of the GIWAXS setup is further improved by suppressing the background scattering using a guarding slit, an appropriately placed beamstop, and He gas in the CVD reactor. The layer growth can be monitored by tracking the width of the MoS₂ diffraction peak in real time. The temporal evolution of the crystallization kinetics can be satisfactorily described by the Avrami model, employing the normalized diffraction peak area. In this way, the activation energy of the particular chemical reaction occurring in the CVD chamber can be determined.