Water molecule is able to approach the surface with no barrier. The optimal adsorption site is the zinc atom of a monoethyl zinc group, where the water is able to form a hydrogen bond with a surface hydroxyl group right. In our calculated trajectory, the water molecule has a metastable intermediate state 0.
We find no barrier of significance for adsorption. Figure 3. Figure 4. The reaction mechanism is the same as for CMEZ-1H 2 O with additional water forming a hydrogen bond to the reacting hydroxyl group. The example mechanisms shown roughly correspond to the given activation energy for which the reaction rate coefficients have been calculated.
The adsorption rates range depending on the pressure of the precursor from 2 to Pa. The reaction rates have been calculated using the Eyring equation and the adsorption rate from the particle flux from kinetic gas theory. Figure 5. Growth-per-cycle GPC as a function of temperature adopted from Yousfi et al. Different symbols empty squares, empty circles, black circles correspond to different experimental runs.
Case 1 and case 2 are the low and high temperature ethyl-saturated structures, respectively. Description of xyz-format files PDF. Reaction mechanisms of calculations done on the ethyl-saturated surface and presented in Table 1 as trajectories in xyz-format ZIP. The authors thank the Center for Scientific Computing for the use of their computational resources. Funding by the Academy of Finland Project Nos. We also thank Simon Elliott from the Tyndall National Institute, University College Cork, for very insightful and helpful discussions on interpreting the data in this article. This article references 27 other publications.
C 14 More by Timo Weckman. More by Kari Laasonen. Cite this: J. Article Views Altmetric -. Citations 1. PDF 2 MB. Abstract High Resolution Image.
Thin film growth using atomic layer deposition ALD has become an important manufacturing process in nanotechnology within the recent decade, driven by the decreasing dimensions of modern technology. ALD is based on sequential, self-limiting gas—solid reactions. In ALD, the reactants, precursors, are introduced into the reactor chamber separately, thus avoiding any gas-phase reactions. All the chemical reactions are restricted to take place on the surface as the reactor chamber is purged with inert gas between alternating reactant pulses.
The zinc oxide thin films have been used in various applications, for example, in transistors, solar cells, and sensors. The interest in deposition of zinc oxide thin films using ALD has increased as the dimensions of microscopic devices have decreased. The thin film growth of zinc oxide using these precursors is the focus of our present study. The as-deposited thin films are polycrystalline with various lattice orientations present, and these orientations are sensitive to temperature.
The monoethyl zinc may also react further on the surface to produce a bare zinc on the surface: 1b The mechanism in eq 1b is usually omitted in experimental publications, and the dominant end-product from the DEZ pulse is assumed to be MEZ. After the surface has been saturated with DEZ, it is assumed that all the surface ethyl ligands are removed during the subsequent water pulse resulting in a hydroxylated surface at the end of the ALD cycle. However, the mass change measured using a quartz-crystal mass-balance QCM 8 is slightly positive during the water pulse.
One proposed explanation for this is the presence of bare zinc atoms on the surface in addition to the monoethyl zinc groups. These bare zinc sites could adsorb more water onto the surface and balance out the mass change from the ligand-exchange reaction. Ferguson et al. The commonly made assumption that all the surface ethyl ligands are removed from the surface after the water pulse has been shown to be incorrect by Mackus et al.
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The authors conclude that there is a high kinetic barrier for removing some of the ligands from the surface during the water pulse. There have previously been only a few computational papers on the growth of zinc oxide with ALD.
Ren 13 used a similar cluster model to investigate the heterodeposition of ZnO on Si substrate. On the Si substrate, the removal of the first ligand from DEZ had a barrier ranging from 0. The barrier for a reaction between a monoethyl zinc and a water molecule was calculated to be 1.
We denote this first ligand-exchange reaction as LE1. This barrier was observed to be higher in calculations done on a stepped surface where the barrier rose to 0.
A proposed pyrolysis mechanism of diethyl zinc was also investigated, but based on our calculations the pyrolysis of ethyl ligands to form butane has a large barrier of 1. The pyrolysis is therefore unlikely to contribute to the growth of the thin film.
Homoepitaxial branching: an unusual polymorph of zinc oxide derived from seeded solution growth.
The activation energy for the removal of the final ligand from the monoethyl zinc was considerably high. We denote this mechanism as LE2. On the planar surface the barrier for the reaction with a monoethyl zinc and a hydroxyl group ranged from 1. However, a lower barrier for this reaction was found on a surface step where the monoethyl zinc was three-coordinated.
In this case the barrier for the reaction was only 0. Instead of the pyrolysis mechanism, the second ligand-exchange reaction may produce bare zinc atoms onto the surface, and this can partially explain the positive mass change during the water pulse as more water can adsorb onto the surface zinc atoms resulting in a stoichiometric film.
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Two different ethyl-saturated surface structures were constructed. For the high temperature estimate we assumed that the second ligand-exchange can also occur and the amount of hydroxyl groups on the surface becomes the limiting factor. This leads to a combination of both monoethyl zinc and bare zinc atoms on the surface case 2 structure.
To understand the overall growth process, one must study both precursor pulses. In a common computational study on an ALD-process, the follow-up calculations for the second precursor pulse usually omit the complexity of the surface structure after saturation by the previous precursor. In this work we investigate surface reactions relevant for the water pulse using the ethyl-saturated structures from our previous publication and address the discrepancies between the ideal model and experimental data from an atomic-scale perspective.
The structure of the ethyl-covered surface is not as well-defined as that of an ideal hydroxylated oxide surface, because of more degrees of freedom in finding a suitable minimum energy structure. To overcome this enormous complexity, we propose that a sequence of reactions on complex surface environments can produce a reasonable range of possible surface processes that contribute to the growth of the thin film.
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The reaction pathways were studied using density functional theory as implemented in GPAW. The PBE functional has been previously used in studies on the zinc oxide surface chemistry and has been shown to produce reasonable results.
All geometry optimizations were carried out to gradients smaller than 0. The surface slab had six stoichiometric oxide layers as in our previous study. Some short Born—Oppenheimer molecular dynamics simulations were carried out to find a suitable minimum on the potential energy surface PES.
While the surface structure of a hydroxylated zinc oxide surface is rather well-defined with only few possible configurations to explore, the minimum energy structure of the ethyl-saturated surface of zinc oxide is not unambiguous because of the tremendous increase in the degrees of freedom. Therefore, two approaches were taken to investigate the surface reactions during the water pulse.
First, we studied the adsorption and ligand-exchange reaction of a single water molecule on an isolated monoethyl zinc site on the planar zinc oxide surface. Using a single reactive site to represent the surface reaction is an approach commonly used in ab initio surface studies. Second, we studied water molecules on the two different ethyl-saturated surface structures representing the zinc oxide surface after the DEZ pulse. All the adsorption structures for water on the saturated structures presented below were produced by first inserting a single molecule onto some suitable adsorption site i.
From this trajectory, few snapshots at fixed intervals were taken and optimized, and the lowest energy configuration was chosen as the staring point for a ligand-exchange reaction. The transition state was located using the nudged elastic band NEB method. After the reaction, the newly formed ethane molecule was removed, and another short AIMD simulation was run as described above.
This allows the system to relax after the ligand had been removed.