Field-induced reversible insulator-to-metal transition and the onset of ferroelectricity in molybdenum trioxide films

This paper presents the experimental evidence of reversible insulator–metal transition (IMT) in thin-film amorphous molybdenum trioxide (MoO₃) induced by electric fields of just a few volts. The presence of oxygen vacancies in MoO₃ is considered to play a significant role in the reported reversible IMT. The oxygen vacancies not only impact MoO₃ stoichiometry but also the optical bandgap. The subthreshold slope for IMT in 10 nm-thick MoO₃-based devices is 48.3 mV/decade, which represents a transition from an insulator to a metallic state, and the electric field threshold for such a transition was found to be equal to 0.034 V/Å. Following the IMT in MoO₃, there are six orders of magnitude differences between the resistivity of the insulator state (27.5 M Ω at −9 V) and the metallic state (80 Ω between +5 and +9 V). In addition, we reported stabilization of a nanocrystalline hexagonal MoO₃ (h-MoO₃) phase in thicker MoO₃ (150 nm-thick) in the presence of oxygen vacancies that behave as a wide bandgap (3.1 eV) ferroelectric semiconductor with a coercive field of about 50 kV/cm, a saturation polarization of about 30 μC/cm², and a remanent polarization of about 10 μC/cm². This ferroelectricity in nanocrystalline h-MoO₃ (150 nm-thick) remains stable even after 8 months of storage of the sample in ambient conditions, with remanent polarization increasing up to 20 μC/cm². These are unexpected results from MoO₃.