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

Abstract

Molybdenum oxide (MoOx) 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 MoO3-x.