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

Abstract

This work reports a radical-enhanced atomic layer deposition (REALD) process using WH 2 (Cp) 2 −O*−H* reaction cycles (Cp = cyclopentadienyl group) to grow WO 3−x films with a wide range of tunable oxygen vacancy (V O ) concentrations where O* and H* represent oxygen and hydrogen radicals, respectively. The fundamental WH 2 (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 V O 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 V O 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 V O concentration up to ∼5 at. % without disturbing the ALD saturation conditions and involving any carboncontaining impurities. Ab initio calculations of the valence band (VB) spectra, assuming oxygen-deficient WO 3−x , showed good agreement with the experimental VB X-ray photoelectron spectroscopy (XPS) data, which corroborated the estimated V O concentrations based on the core-level XPS data. The increase in the V O 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 V O concentrationcontrolled WO 3−x films could be a viable material for diverse electronic applications, including resistive random-access memory devices.

Atomic Layer Deposition of Ultrathin Tungsten Oxide Films from WH2(Cp)2 and Ozone

“…According to Figure , the relative intensity of the W 5+ doublet decreases with an increasing O 3 feeding time. A similar tendency is also fair for the visible peak near BE of 0.9 eV on the related VB spectra, which is unequivocally related to the V o presence according to ab initio simulations . According to the proposed methodology, the V o concentration could be estimated as W 5+ /(6 × (W 5+ + W 6+ )) × 100% from the W 4f spectra, where W 5+ and W 6+ denote the corresponding doublet intensities.…”

“…Also, no visible film thickness variation was observed by SEM on the trench structure with an aspect ratio ≈5. This observation favorably distinguishes the novel ALD process from the previously reported radical-assisted one, which could not possess the film growth on the wafer backside due to the O* radical flux absence . Therefore, we can conclude that the suggested process satisfies the basic ALD rules.…”

“…Next, the chemical state of the grown films was investigated by XPS. According to our previous experiment with oxygen radicals, WO 3 films may contain a significant number of oxygen vacancies ( V o ), which strongly impacts their properties . Similarly, the reactant feeding time may also influence the residual impurities level.…”

“…Song et al reported an ALD process using WH 2 (iPrCp) 2 (Cp = cyclopentadienyl group) and oxygen plasma, which resulted in an even higher growth rate (0.9 Å/cycle) but unusual stoichiometry (O/W = 2.4) related to the presence of a multiple W oxidation states . Later it was also shown that WH 2 (Cp) 2 utilization with oxygen radicals allows the oxygen deficiency level tuning in WO x films within the saturated conditions range …”

Preparation of Monodispersed Cs0.33WO3 Nanocrystals by Mist Chemical Vapor Deposition for Near-Infrared Shielding Application

“…The peak with lower binding energy (528.2 eV) and a FWHM of 1.27 eV corresponds to lattice oxygen (LO) in the stoichiometric WO 3 phase (O atoms binding with W 6+ ). The second peak, at 530.8 eV (FWHM ~1.95 eV), may originate from surface O-H states or non-lattice oxygen (NLO) in the nonstoichiometric phase (O atoms binding with W 5+ ), which is in agreement with W4f XPS peaks [ 27 , 28 ].…”