Plasma-enhanced atomic layer deposition of molybdenum oxides using molybdenum hexacarbonyl as the precursor

Molybdenum oxide (MoO_x) thin films are fabricated by plasma-enhanced atomic layer deposition (PEALD) using molybdenum hexacarbonyl Mo(CO)₆ as the precursor and oxygen as the reactant. The effect of plasma power for oxygen has been investigated on the evolution of film structure. Molybdenum oxide shows the orthorhombic α-MoO₃ phase at low powers and gradually changes to the metastable monoclinic β-MoO₃ phase as the power continually increases, which is confirmed by Raman spectroscopy. α-MoO₃ is two-dimensional (2-D) phase with Pbnm space group, while β-MoO₃ film belongs to three-dimensional phase with P2₁/c. From the observation of scanning electron microscopy (SEM), a rod-like structure has been detected, whereas a bulk-like grain corresponds to a high ratio of β-MoO₃ to α-MoO₃ phase at higher power of 150 W. Due to better coverage of 2-D layer preventing surface from oxidation, the interfacial layer of α-MoO₃ checked from transmission electron microscopy (TEM) shows thinner at low powers when contacting with silicon substrate, which is consistent with the performance of leakage current. The changes in binding energies of Mo 3d and O 1s orbits at different milling depths are compared by different powers. Though the significant amount of Mo⁴⁺ oxidation state is shown from x-ray photoelectron spectroscopy (XPS), only α-MoO₃ and β-MoO₃ crystallites with +6 state of Mo are observed from x-ray diffraction (XRD) patterns. Above phenomenon is due to the structural change caused by the loss of oxygen atoms. Thus, according to the charge transformation and compensation, the defect reaction model can well explain that oxygen vacancies inside films are more prevalent in orthorhombic phase at low plasma power for oxygen of our reduced MoO_3-x.