Atomic Layer Deposition of Zinc Oxide: Understanding the Reactions of Ozone with Diethylzinc

Warner, E.; Cramer, C. J.; Campbell, S. A.; Gladfelter, W. L.
J. Vac. Sci. Technol. A 2013, 31, 041504 (doi:10.1116/1.4806800).

To understand the reactions involved in the atomic layer deposition (ALD) of zinc oxide films using ozone as the oxygen source, two model systems were examined at the M06-L and M06 levels of density functional theory. The first model involved a two-coordinate zinc complex, HO-Zn-Et, and the second, [(HO)₇Zn₄(Et)], a cluster having a cubane-like geometry in which each of the zinc ions are four-coordinate. In both cases, the ozone reaction requires two distinct steps to generate a new hydroxyl ligand, which is required for the second phase of the ALD process (reaction with Et₂Zn). In step 1, an exothermic insertion of O₃ into the Zn-C bond produces an ethyltrioxide (EtOOO^–) ligand as an intermediate. Subsequently, a mildly exothermic elimination of singlet oxygen produces an ethoxide complex. In step 2, a second equivalent of ozone abstracts a methylene hydrogen from the ethoxide ligand resulting in the elimination of acetaldehyde and the formation of a hydrotrioxide (HOOO^–) ligand that ultimately eliminates O₂ and leaves a hydroxide group bound to the zinc. To simulate one complete ALD cycle, Et₂Zn was subsequently reacted with the hydroxyl terminated products from step 1, i.e., Zn(OH)₂ or Zn₄(OH)₈. In the cubane-like model, the geometric availability of additional OH groups opens a 1,4 ethane elimination pathway with an activation energy 7.1 kcal/mol lower than that for 1,2-elimination. A series of experimental ZnO depositions using Et₂Zn and O₃ were run in a reactor that was modified to allow collection of condensable organic products of the reaction. Acetaldehyde was detected, and quantitative NMR established a linear correlation between the amount of acetaldehyde and the number of ALD cycles, consistent with the mechanism inferred on the basis of the computational models.