Tuning of electronic state of correlated materials is key to their eventual use in advanced electronics and photonics. The prototypical correlated oxide VO 2 is insulating at room temperature and transforms to a metallic state when heated up to 67 °C (340 K). We report the emergence of a metallic state that is preserved down to 1.8 K by annealing thin films of VO 2 at an ultra-low oxygen partial pressure (P O2~1 0-24 atm). The films can be reverted back to their original state by annealing in oxygen, and this process can be iterated multiple times. The metallic phase created by oxygen deficiency has a tetragonal rutile structure and contains a large number of oxygen vacancies far beyond the solubility at equilibrium (greater than ~50X). The oxygen starvation reduces the oxidation state of vanadium from V 4+ to V 3+ and leads to the metallization. The extent of resistance reduction (concurrent with tuning of optical properties) can be controlled by the time-temperature envelope of the annealing conditions since the process is diffusionally driven. This experimental platform which can extensively tune oxygen vacancies in correlated oxides provides a new approach to study emergent phases and defect-mediated adaptive electronic and structural phase boundary crossovers.

In this study, a plasma-modified process was developed to control the electrical properties of atomic layer deposition (ALD)-grown vanadium dioxide (VO 2 ), which is potentially useful for applications such as resistive switching devices, bolometers, and plasmonic metamaterials. By inserting a plasma pulse with varying H 2 gas flow into each ALD cycle, the insulator-to-metal transition (IMT) temperature of postdeposition-annealed crystalline VO 2 films was adjusted from 63 to 78 °C. Film analyses indicate that the tunability may arise from changes in grain boundaries, morphology, and compositional variation despite hydrogen not remaining in the annealed VO 2 films. This growth method, which enables a systematic variation of the electronic behavior of VO 2 , provides capabilities beyond those of the conventional thermal ALD and plasma-enhanced ALD.

Niobium oxide (NbO x ) materials of various compositions are of interest for neuromorphic systems that rely on memristive device behavior. In this study, we vary the composition of NbO x thin films deposited via atomic layer deposition (ALD) by incorporating one or more in situ hydrogen plasma exposure steps during the ALD supercycle. Films with compositions ranging from Nb2O5 to NbO2 were deposited, with film composition dependent on the duration of the plasma exposure step, the number of plasma exposure steps per ALD supercycle, and the hydrogen content of the plasma. The chemical and optical properties of the ALD NbO x films were probed using spectral ellipsometry, X-ray photoelectron spectroscopy, and optical transmission spectroscopy. Two-terminal electrical devices fabricated from ALD Nb2O5 and NbO2 thin films exhibited memristive switching behavior, with switching in the NbO2 devices achieved without a high-field electroforming step. The ability to controllably tune the composition of ALD-grown NbO x films opens new opportunities for realizing a variety of device structures relevant for neuromorphic computing and other emerging electronic and optoelectronic applications.