  Metal organic chemical vapor deposition (MOCVD) is one of the main
methodologies used for thin film fabrication in the semiconductor industry
today and is considered one of the most promising routes to achieve large-scale
and high-quality 2D transition metal dichalcogenides (TMDCs). However, if not
taken special measures, MOCVD suffers from some serious drawbacks, such as
small domain size and carbon contamination, resulting in poor optical and
crystal quality, which may inhibit its implementation for the large-scale
fabrication of atomic-thin semiconductors. Here we present a Growth-Etch MOCVD
(GE-MOCVD) methodology, in which a small amount of water vapor is introduced
during the growth, while the precursors are delivered in pulses. The evolution
of the growth as a function of the amount of water vapor, the number and type
of cycles and the gas composition is described. We show a significant domain
size increase is achieved relative to our conventional process. The improved
crystal quality of WS2 (and WSe2) domains was demonstrated by means of Raman
spectroscopy, photoluminescence (PL) spectroscopy and HRTEM studies. Moreover,
time-resolved PL studies show very long exciton lifetimes, comparable to those
observed in mechanically exfoliated flakes. Thus, the GE-MOCVD approach
presented here may facilitate their integration into a wide range of
applications.
