Atomic layer deposition of zinc oxide: Understanding the reactions of ozone with diethylzinc

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 is 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 Et ₂Zn). 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 nuclear magnetic resonance 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.