Two-dimensional transition metal dichalcogenide (TMDC) alloys and devices

Two-dimensional transition metal dichalcogenide (2-D, TMDC) and their alloys have attracted significant attention due to their extraordinary optical and electronic properties such as high mobility, high photosensitivity, direct bandgap that does not exist in their bulk forms. Molybdenum sulphide (MoS2), molybdenum sulphoselenide (MoS2(1-x)-Se2x (x=0-1)), molybdenum selenide (MoSe2), and their alloys are candidates for new devices in optoelectronics, semiconductor electronics, photodetectors, photovoltaics and energy harvesting. In this thesis, we present repeatable MoS2, MoS2(1-x)-Se2x, MoSe2 monolayer formation with a systematical investigation of chemical vapor deposition (CVD) growth parameters including gas flow rates, substrate temperature and precursor concentration (S/Se ratio). We have achieved a complete control of x value (x=0-1) allowing for bandgap change in between 1.82 eV (MoS2) and 1.56 eV (MoSe2). In general, the sizes of the obtained monolayer flakes have been as large as 150 μm on SiO2 (300 nm)/Si substrates. The reaction temperature and S/Se ratio have been found to be the most critical parameters to control the composition (x= 0-1) of the monolayer alloy TMDCs. The growth of Se rich alloys at a higher growth temperature is explained with the chalcogen exchange mechanism (CEM) in relation with substitution of S atoms in as grown alloy host lattice site with Se atoms present in the Se rich vapor. Additionally, NaCl salt involvement in the CVD process is investigated to enable large area MoSe2 monolayer flake growth with full substrate coverage, which is proven successful. We performed different characterizations, which include photoluminescence (PL), Raman spectroscopy, and optical and dark field image, atomic force microscopy (AFM), and scanning electron microcopy (SEM) measurements. We also fabricated field effect transistor (FET) devices using our 2D MoSe2 flakes and performed device performance measurements.