Combination of characterization techniques for atomic layer deposition MoO3 coatings: From the amorphous to the orthorhombic α-MoO3 crystalline phase

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Abstract
Thin films of MoO3 deposited on Si(111) and Al2O3(001) substrates by atomic layer deposition have been investigated by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Raman spectroscopy for detailed characterization of composition and morphology. Comparison of angle resolved x-ray photoelectron spectroscopy (ARXPS) and XPS depth profiles based on Ar+ sputtering is reported. Sputtering induces a reduction of molybdenum in MoO3 from +IV to metallic Mo as the interface toward Si is approached, whereas ARXPS on a 10 nm thin film shows that Mo(VI) remains outside the interface toward Si where lower valent molybdenum compounds are formed. Upon annealing, the as-deposited amorphous thin films of MoO3 crystallize into β- or α-MoO3 as identified by x-ray diffraction. The current study provides a convenient route toward formation of metastable β-MoO3 and a full crystallization pathway from amorphous to crystalline α-MoO3. Combined AFM and Raman analysis have been performed on thin films of α-MoO3 deposited on Al2O3(001) and prove that the crystallization proceeds via island growth at 600 °C. The Raman intensity ratios between different bands depend strongly on morphology and size of crystalites.

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