Ellipsometric Investigation of Thick Vertically Oriented MoS2Films Grown on Mo Foil at High Temperatures

Abstract

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 E12g 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.