Radical-Enhanced Atomic Layer Deposition of a Tungsten Oxide Film with the Tunable Oxygen Vacancy Concentration

This work reports a radical-enhanced atomic layer deposition (REALD) process using WH₂(Cp)₂–O*–H* reaction cycles (Cp = cyclopentadienyl group) to grow WO₃–ₓ films with a wide range of tunable oxygen vacancy (VO) concentrations where O* and H* represent oxygen and hydrogen radicals, respectively. The fundamental WH₂(Cp)₂–O* ALD process was characterized by saturation behavior for the W-precursor/O* dose, high deposition uniformity, and a short incubation period. The VO concentration could be limitedly controlled up to ∼0.7 at. % when the O* dose was appropriately decreased within the ALD saturation range. However, a further increase in the VO concentration could hardly be achieved by simply decreasing the O* dose, which accompanied the carbon-related impurities due to incomplete ligand release from the growing film. The addition of the H* pulse step in each ALD cycle rendered it possible to achieve a much higher VO concentration up to ∼5 at. % without disturbing the ALD saturation conditions and involving any carbon-containing impurities. Ab initio calculations of the valence band (VB) spectra, assuming oxygen-deficient WO₃–ₓ, showed good agreement with the experimental VB X-ray photoelectron spectroscopy (XPS) data, which corroborated the estimated VO concentrations based on the core-level XPS data. The increase in the VO concentration obtained in the new reaction cycle process was accompanied by a significant film resistivity decrease and a noticeable change in the crystalline structure. The VO concentration-controlled WO₃–ₓ films could be a viable material for diverse electronic applications, including resistive random-access memory devices.