Atomic Layer Deposition of Zinc Oxide: Study on the Water Pulse Reactions from First-Principles

Atomic layer deposition
(ALD) of zinc oxide thin films has been
under intense research in the past few years. The most common precursors
used in this process are diethyl zinc (DEZ) and water. The surface
chemistry related to the growth of a zinc oxide thin film via atomic
layer deposition is not entirely clear, and the ideal model of the
process has been contradicted by experimental data, e.g., the incomplete
elimination of the ethyl ligands from the surface and the non-negative
mass change during the water pulse. In this work we investigate the
surface reactions of water during the atomic layer deposition of zinc
oxide. The adsorption and ligand-exchange reactions of water are studied
on ethyl-saturated surface structures to grasp the relevant surface
chemistry contributing to the deposition process. The complex ethyl-saturated
surface structures are adopted from a previous publication on the
DEZ/H2O-process, and different configurations are sampled
using ab initio molecular dynamics in order to find
a suitable minimum structure. Water molecules are found to adsorb
exothermically onto the ethyl-covered surface at all the ethyl concentrations
considered. We do not observe an adsorption barrier for water at 0
K; however, the adsorption energy for any additional water molecules
decreases rapidly at high ethyl concentrations. Ligand-exchange reactions
are studied at various surface ethyl coverages. The water pulse ligand-exchange
reactions have overall larger activation energies than surface reactions
for diethyl zinc pulse. For some of the configurations considered,
the reaction barriers may be inaccessible at the process conditions,
suggesting that some ligands may be inert toward ligand-exchange with
water. The activation energies for the surface reactions show only
a weak dependence on the surface ethyl concentration. The sensitivity
of the adsorption of water at high ethyl coverages suggests that at
high ligand-coverages the kinetics may be somewhat hindered due to
steric effects. Calculations on the ethyl-covered surfaces are compared
to a simple model containing a single monoethyl zinc group. The calculated
activation energy for this model is in line with calculations done
on the complex model, but the adsorption of water is poorly described.
The weak adsorption bond onto a single monoethyl zinc is probably
due to a cooperative effect between the surface zinc atoms. A cooperative
effect between water molecules is also observed; however, the effect
on the activation energies is not as significant as has been reported
for other ALD processes.