Madeleine Diskus

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.

Thin films of cobalt oxide are obtained by atomic layer deposition (ALD) using cobaltocene (CoCp₂) and ozone as precursors. A window for ALD growth exists in the temperature range 137 to 331 °C. Self‐limiting growth is verified at a deposition temperature of 167 °C. X‐ray diffraction (XRD) analysis indicates that the films consist of polycrystalline Co₃O₄ up to a deposition temperature of some 285 °C with CoO being identified as a secondary phase at 331 °C. Atomic force microscopy (AFM) proves increased roughness and grain size with increasing deposition temperature, apart from films deposited at 235 °C where the roughness is very low due to a change in preferred orientation of the growth. Scanning electron microscopy (SEM) and energy dispersive X‐ray (EDX) mapping measurements on coated anodiscs show a homogeneous deposition of cobalt oxide on the 5 µm upper part of the pores structure. The content of cobalt decreases inside the pores, however, still proving the existence of cobalt inside all pores.

Thin films of MoO₃ have been obtained by the atomic layer deposition (ALD) technique using molybdenum hexacarbonyl (Mo(CO)₆), ozone, and water as precursors. A window for ALD growth was found in the temperature range 152 to 172 °C. Self-limiting growth was verified at a deposition temperature of 163 °C. The upper temperature range is determined by the thermal stability of the Mo(CO)₆ precursor. The growth dynamics was further investigated by quartz crystal microbalance to determine the effect of ozone and water on the deposition process. Growth using only water as oxygen source is hardly detectable. The growth rate increases to 0.75 Å per cycle when ozone is introduced. X-ray diffraction analysis indicates that the films are amorphous as deposited, but crystallise into the α- and β-MoO₃ phases during annealing in air at 500 °C and to phase-pure, highly oriented α-MoO₃ at 600 °C. Analysis by X-ray photoelectron spectroscopy shows that both as-deposited and annealed films contain Mo(VI). Atomic force microscopy proves a very low surface roughness for as-deposited films, which becomes rather rough for annealed films. This investigation has clearly proven the capability of carbonyls as useful precursors for ALD growth of oxides.