Recent findings on transition metal dichalcogenides (TMDCs) having exotic and attractive electrical prop-erties have aroused lots of attentions on the classes of 2D materials that could eventually replace all the conven-tional semiconducting, insulating, and conducting materials to constitute atomically thin, flexible, transparent circuits and devices. However, scientists are consistently reporting many limitations associated with chemically labile character of the material itself, mechanically fragile character, and the absence of appropriate large-area growth techniques. Most importantly, electrical transport characteristics are not sufficient for device applications in reality since the large-area growth techniques based on vapor phase synthesis usually accompany small grain sizes ranging from nanometer (nm) to few micrometer ($\mu$m). Such low crystallinity is an origin of low mobility as the grain boundaries play a role as charge scattering centers, resulting bottleneck in the charge transport. Also, a recent theoretical study shows that the samples with larger grain sizes have significantly reduced sulfur-asso-ciated vacancy density, implying the importance of crystal size engineering. Therefore, there should be some predictable approaches on the growth techniques to improve the quality of crystals.
In this thesis, conventional system widely adopted for the chemical vapor deposition (CVD) of $MoS_2$ was interpreted systematically. The growth system was analyzed in terms of reaction kinetics, momentum and mass transport, and chemical potential to explain and predict the observed growth behaviors. From the analyses, the diffusion was found to be dominant in the mass transport inside the reactor, and the proposed atomic scale reaction mechanisms with the experimental results suggest that sulfur-rich (or $MoO_x$-deficient) conditions should be met to enlarge the size of crystals. From that result, strategies to minimize the diffusion of MoOx was proposed, and found out to be extremely useful in growing extra-large crystals of $MoS_2$ with ~1mm of grain size, which is ~5 times larger than the largest $MoS_2$ grains ever reported and ~50 times larger than conventional CVD-grown TMDCs.
Recent findings on transition metal dichalcogenides (TMDCs) having exotic and attractive electrical prop-erties have aroused lots of attentions on the classes of 2D materials that could eventually replace all the conven-tional semiconducting, insulating, and conducting materials to constitute atomically thin, flexible, transparent circuits and devices. However, scientists are consistently reporting many limitations associated with chemically labile character of the material itself, mechanically fragile character, and the absence of appropriate large-area growth techniques. Most importantly, electrical transport characteristics are not sufficient for device applications in reality since the large-area growth techniques based on vapor phase synthesis usually accompany small grain sizes ranging from nanometer (nm) to few micrometer ($\mu$m). Such low crystallinity is an origin of low mobility as the grain boundaries play a role as charge scattering centers, resulting bottleneck in the charge transport. Also, a recent theoretical study shows that the samples with larger grain sizes have significantly reduced sulfur-asso-ciated vacancy density, implying the importance of crystal size engineering. Therefore, there should be some predictable approaches on the growth techniques to improve the quality of crystals.
In this thesis, conventional system widely adopted for the chemical vapor deposition (CVD) of $MoS_2$ was interpreted systematically. The growth system was analyzed in terms of reaction kinetics, momentum and mass transport, and chemical potential to explain and predict the observed growth behaviors. From the analyses, the diffusion was found to be dominant in the mass transport inside the reactor, and the proposed atomic scale reaction mechanisms with the experimental results suggest that sulfur-rich (or $MoO_x$-deficient) conditions should be met to enlarge the size of crystals. From that result, strategies to minimize the diffusion of MoOx was proposed, and found out to be extremely useful in growing extra-large crystals of $MoS_2$ with ~1mm of grain size, which is ~5 times larger than the largest $MoS_2$ grains ever reported and ~50 times larger than conventional CVD-grown TMDCs.
