Radical-Enhanced Atomic Layer Deposition of a Tungsten Oxide Film with the Tunable Oxygen Vacancy Concentration
Р. И. Романов, Maxim G. Kozodaev, Yu. Yu. Lebedinskiǐ, Timofey V. Perevalov, Aleksandr S. Slavich, Cheol Seong Hwang, Andrey M. Markeev
Abstract
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.