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

Abstract

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.

In Situ Ellipsometry Study of the Early Stage of ZnO Atomic Layer Deposition on In0.53Ga0.47As

“…ZnO growth on either InGaAs or ZnO surface is very different . ZnO ALD on ZnO has been the subject of experimental works and more recently of density functional theory and Monte Carlo simulations . In addition, ZnO surface reactivity, which depends on surface polarity and reconstruction, has been studied .…”

Atomic Layer Deposition for Polypropylene Film Engineering—A Review

“…[ 33 ] The ZnO deposition and subsequent growth (36 and 67 nm in thickness) assisted by such a buffer layer, were fulfilled on a 58 µm thick BOPP film with excellent uniformity. [ 34 ]…”

“…[ 36b,38 ] Regardless of the substrate, the ALD reactions using DEZ and water can be mostly summarized as (CH 3 CH 2 ) 2 Zn + 2H 2 O → ZnO + 2C 2 H 6 . [ 34 ]…”

Kinetic Monte Carlo Study of the Atomic Layer Deposition of Zinc Oxide

“…The activation energy for removing the ligand from a monoethylzinc surface fragment is significantly higher. In DFT calculations on surface reactions during the subsequent water pulse, 11 the ligands were found to be non-reactive towards ligand elimination with water molecules. Only a weak ligandcoverage dependency on the reaction barriers was observed.…”

“…In the DFT calculations, 11 water was found to adsorb mainly to 2-or 3-coordinated zinc atoms and to monoethylzinc groups coordinated with two oxygens. Thus, water adsorption is defined in kMC to occur for zinc sites with cn 2 and 3 and ZnX sites with cn 3 (cn = 2 for oxygens and 1 for ligand).…”