Tungsten Oxide Film and Nanorods Grown by Aerosol-Assisted Chemical Vapor Deposition Using κ2-β-Diketonate and β-Ketoesterate Tungsten (VI) Oxo-Alkoxide Precursors

• Published in: ECS journal of solid state science and technology Vol. 5; no. 11; pp. Q3095 - Q3105
• Main Authors: Kim, Hankook, Bonsu, Richard O., Bock, Duane C., Ou, Nathan C., Korotkov, Roman Y., McElwee-White, Lisa, Anderson, Tim
• Format: Journal Article
• Language: English
• Published: The Electrochemical Society 01.01.2016
• ISSN: 2162-8769
• DOI: 10.1149/2.0171611jss

Deposition of continuous, dense WOx films and nanorods was accomplished by aerosol-assisted chemical vapor deposition (AACVD) using the recently synthesized precursors WO(OCH3)3(acac) (1), WO(OCH2C(CH3)3)3(tbac) (2), WO(OCH2C(CH3)3)3(dpm) (3), WO(OC(CH3)3)3(tbac) (4), and WO(OCH2C(CH3)3)3(tbpa) (5). This works seeks to define the deposition conditions and precursors that yield C-free tungsten oxide and the potential to control the stoichiometry and phase of deposited WOx. In addition, the systematic variation of the ligand chemistry provides insight into precursor design. Variation of the precursor and growth temperature during deposition revealed a window where C-free WOx was deposited using 3, 4, and 5. The surface morphology of the WOx varied from amorphous thin film to crystalline nanorods to dendrites as temperature was increased. Films grown between 150 and 350°C in pure N2 atmosphere are sub-stoichiometric, amorphous and contaminated with carbide species (3-9 at.%). As the deposition temperature increased (400-550°C), the tungsten became more oxidized, the sub-stoichiometric crystalline W18O49 monoclinic phase formed, and increased surface bound C was detected. Material was also deposited under oxidizing conditions (1-2% O2 in N2) as well as annealing in air at the deposition temperature. The material grown in N2/O2 carrier gas at low temperatures (200 and 300°C) is amorphous WOx, similar to that grown in N2. At higher temperature (350 and 550°C), however, GIXRD results reveal WOx corresponding to a sub-stoichiometric tetragonal phase transitioning to the monoclinic WO3 phase for samples grown at 550°C. This demonstrates that crystalline structure of WOx is affected by the growth temperature and introducing O2 in the carrier gas. Air annealing samples grown in pure N2 also produced structural and compositional changes, but not identical to those grown in a N2/O2 carrier gas. Notably, annealing samples grown at 250 and 350°C at the same temperature, produced C-free material with unchanged amorphous morphology. The measured stoichiometry and crystallinity showed a dependence on the precursor structure. The growth rate of deposited material was measured as a function of temperature and activation energies were estimated for growth of amorphous and nanostructured material. The systematic variation in activation energies is consistent with initial dissociation of the alkoxide C-O bonds and modifications of the steric bulk of the β-diketonate ligand.