Thin
films of MoS2 with vertically aligned layers have
been less investigated than their horizontal counterparts although
they find important applications in energy storage, catalysis, and
solar energy harvesting. Many applications require precise control
of the film thickness and homogeneity. In this context, spectroscopic
ellipsometry is a powerful technique that allows precise determination
of the film thickness and evaluation of the film homogeneity and crystallinity
from the features of the dielectric function spectrum. Homogeneous
films of MoS2 were grown by chemical vapor deposition by
sulfurizing Mo foils in sulfur vapor at atmospheric pressure in the
800–1000°C temperature range. The surface topography obtained
by scanning electron microscopy images was characterized by vertical
structures with a platelet-like morphology. Fractured cross sections
showed compact MoS2 films with uniform thickness and a
columnar morphology compatible with a vertical orientation of the
layers. The vertical alignment was confirmed by Raman spectroscopy
which showed an E1
2g to A1g peak
ratio of around 0.3. The ellipsometric spectra could be adequately
modeled with two-phase (MoS2/air) or three-phase (Mo/MoS2/air) models depending on the MoS2 film thickness.
The polycrystalline structure of the MoS2 film was evidenced
in the small amplitude of excitonic features in the complex dielectric
function. The film thickness rapidly increases with formation temperature
at high temperatures. Whereas at 650 °C, the film has a thickness
of 40 nm, it increases from 200 to 1000 nm in the 800–1000°C
temperature range after a growth time of 10 min.