Al Interaction with ZnO Surfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Al Interaction with ZnO Surfaces Yuzhi Gao, Mathilde Iachella, Eric C. Mattson, Antonio T. Lucero, Jiyoung Kim, Mehdi Djafari Rouhani, Yves J. Chabal, Carole Rossi, and Alain Esteve J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04952 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The Journal of Physical Chemistry

Al Interaction with ZnO Surfaces Yuzhi Gao,2 Mathilde Iachella,1 Eric Mattson,2 Antonio T. Lucero,2 Jiyoung Kim,2 Mehdi Djafari Rouhani,1 Yves Chabal,2 Carole Rossi,1 Alain Estève*,1 1

University of Toulouse, LAAS-CNRS, 7 avenue du colonel Roche, 31031 Toulouse, France 2

Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United Sates

ABSTRACT

Deposition of Al onto ZnO surfaces is important for metal/insulator contacts in microelectronics and photovoltaic devices, and also for nano-energetic materials; yet there have not been fundamental studies of these interfaces, in particular those involving the polar faces of ZnO. Density Functional calculations and Low Energy Ion Scattering (LEIS) studies are combined to unravel the chemistry of Al interaction on polar ZnO surfaces, revealing that Al atoms quasi spontaneously replace surface Zn atoms on both O-and Zn-terminated ZnO surfaces. In this process, aluminum atoms attract oxygen atoms, releasing zinc atoms through electrostatic repulsion within the growing alumina film. Kinetics and thermodynamics calculations indicate that zinc atoms accumulate on the surface rather than migrating into ZnO bulk at room temperature, due to high bulk diffusion barriers. Upon annealing to moderate temperatures, LEIS

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studies indicate that surface Zn atoms desorb at ~ 140-150 °C, which is consistent with the calculated 1.31 eV activation barrier.

Introduction ZnO, a prototypical II-VI group semiconductor, has been attractive for a number of applications, such as in energy converters and generators, and sensors for detecting gases and chemicals.1-4 Due to its low toxicity and high biocompatibility, ZnO is also a promising material for the biomedical field.5-8 ZnO-based materials are currently used as mainstream catalysts for methanol synthesis.9-11 Furthermore, the photo-sensitivity of ZnO makes it a unique candidate for applications in photovoltaics and photo-catalysis.12-13 This is due to several exceptional properties: n-type semiconductor, direct band gap (3.37 eV) in the near-UV spectral region, large free-exciton binding energy (60 meV) at room temperature and good oxidation capability. ZnO is also a versatile material that can be engineered in many different ways, with controlled fabrication at the nanoscale,14 which is valuable for the microelectronics and MEMs technologies that require challenging miniaturization. Aluminum is commonly used with ZnO. For instance, it is deposited directly on ZnO to make low-resistance ohmic contacts in electronic, optoelectronic and MEMS devices15-16 or electrode in ultrasonic microsensors.17-18 The quality of the Al films is often critical, as its roughness can negatively affect electrical properties and even optical properties in some photonic applications.19 In all these cases, the interface between ZnO and the metal layer controls the device performances. Besides interfacial interaction, the incorporation of Al inside ZnO is also important, as it can be used to tailor the free-carrier concentration (i.e. resistivity) of Al-doped zinc oxides (AZO). Typically, AZO thin films are prepared by mixing or depositing a very small amount of Al or Al2O3 directly inside or on ZnO materials,20-22 as demonstrated by deposition


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and annealing of a thin Al film.22 Al-ZnO nanorods have recently been synthesized to improve the photoresponse of UV detectors.23 For nanoenergetic systems that motivate our work, Atomic Layer Deposition (ALD) has been used to deposit ultrathin ZnO layers at the interface between aluminum and copper oxide films to improve the onset temperature and energy release of conventional Al/CuO thermites.24 In all these cases, an atomic scale understanding of the inhomogeneity and composition of interfacial regions is necessary to control the metal/ZnO interface and continue to scale down and improve devices. In fact, despite clear technological needs, metal (e.g. Al) interaction with ZnO surfaces is sparsely documented. Only very few studies have investigated the first stage of interface formation and potential intermixing upon temperature activation.19,

22, 25

This lack of knowledge of detailed atomic-level interface

composition and formation mechanisms makes it difficult to correlate thermal, optical and electrical properties with interfacial chemistry between Al and ZnO. We have recently explored experimentally the surface chemistry of Al interaction and interface formation with ZnO using infrared spectroscopy, X-ray photoelectron spectroscopy (XPS) and Low energy ion scattering (LEIS). The findings made it possible to propose a complex scenario involving a multi-step mechanism:25 initial oxidation of Al through surface reduction of the ZnO, resulting in an ultrathin alumina layer and reduced Zn atoms; subsequent desorption of Zn atoms at ~150°C; and ultimate oxidation of all Al atoms between 300 to 600 °C. Throughout this work, it became clear that the surface was chemically highly inhomogeneous with patches of Al2O3 and Zn rich areas. Overall, the reaction mechanisms remain unresolved and the exact location of the various species as a function of temperatures is still uncertain. For instance, how is ZnO reduced by the first incoming aluminum atoms and where do reduced Zn atoms go? Why are Zn atoms detectable even after several nm of Al deposition? Where do the surface Zn atoms go at 150 oC?


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To address these fundamental questions about Al and ZnO interaction, theoretical calculations and some complementary experiments on model systems have been performed. The DFT calculations show that, during the initial interaction of Al with ZnO surfaces, the substitution of surface Zn atoms by Al is quasi-spontaneous on both O- and Zn-terminated surfaces. The substituted Zn is found to segregate away from newly formed Al-O species, forming metallic Zn clusters on the surface, on all surfaces considered. The metallic Zn atoms are loosely bound and can evaporate at 150 oC, as confirmed by LEIS measurements. In addition, calculations show that additional Al atoms are more likely to adsorb on metallic Al domains than on metallic Zn domains. Therefore, the segregation of Zn further fosters an inhomogeneous growth of Al. The Zn domains are easily formed because i) Zn can migrate quasi-freely on Al surfaces and ii) the Zn clusters can stabilize on both flat aluminum and ZnO surfaces at room temperature.

Materials and Methods Rectangular silicon (Si) samples, cut from wafers with a ~ 6 nm thermally grown oxide on the surface, are used as the substrates for all the experiments. They are first sonicated sequentially in dichloromethane, acetone and methanol for 5 minutes each. Then, the samples are cleaned and hydroxylated by immersion in a piranha solution (a mixture of hydrogen peroxide and sulfuric acid with a ratio of 1:3) at 80 oC for 30 min. To study Al interaction with ZnO, a 20-nm thick ZnO film is deposited on the cleaned Si substrate at 200 oC in a Savannah-100 ALD reactor (Cambridge NanoTech), using diethylzinc (DEZ) and water vapor as precursors. The ZnO-coated sample is then introduced into an ultrahigh vacuum chamber to deposit 11-nm Al using a Mantis e-beam evaporator. A Quartz crystal microbalance (QCM), previously calibrated, provides a quantitative measure of the Al film


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thickness, with a deposition rate of 5 Å per minute. Three stacks are used to study the surface composition evolution of Zn on different surfaces (ZnO, Al2O3, Al): a) Zn-ZnO, b) Zn-Al2O3 and c) Zn-Al. The ZnO film for stack a) is identical to the ZnO film used for Al interaction study. The 2-nm Al2O3 film for stack b) is deposited on the cleaned Si substrate by ALD at 130 oC, using trimethylaluminium (TMA) and water vapor as precursors. The 10-nm thick Al film for stack c) is deposited on the Si substrate by e-beam evaporation in an ultra-high vacuum environment. For all three substrates, a 5 nm-thick Zn layer is deposited using a Mantis e-beam evaporator in an ultra-high vacuum chamber, in which an Ion-TOF LEIS system (using 3 KeV He+ ions) and a PHI 5600 XPS (with a monochromated Al Kα line of 1486.5 eV) can monitor insitu the evolution with temperature of the chemical composition of both surface and near surface regions. The growth rate is calculated to be 10 Å per minute for Zn. To ensure Al is not oxidized, the Zn layer is deposited on the Al film in the same UHV chamber.

Computational details All DFT calculations are carried out using the CP2K-2.7 package,26 and GGA level within PBE functional.27 Nuclei and core electrons are modeled with GTH pseudo-potentials28 and the valence electrons are modeled by molecularly optimized DZVP basis sets.29-30 The energy cutoff is set to be 500 Ry, the relative energy cutoff is set to be 50 Ry and five grid levels. We also use a wavelet Poisson solver with 2D periodic boundary conditions.31 The Climbing-Image Nudge Elastic-Band (CI-NEB) method is employed to calculate activation barriers for selected reactions, like atomic migrations on a surface or substitution between adsorbate and a surface atom. To treat ZnO polar surfaces, we use a dipole correction scheme in all calculations; in


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addition, we use an algorithm with 2D periodicity instead of 3D as is typically done, which prevents the infinite replication of any residual dipole in the axis normal to the surface. The model surfaces are taken as the most stable crystal planes, i.e. (111) for fcc Al and (0001) for other hcp materials, all with a hexagonal structure. In order to end up with almost square surfaces that improve the convergence in DFT calculations, we use body centered rectangular elementary cells with a shape factor 1:√3. For the ZnO slab, two wurtzite crystalline structures are selected, one with both O-termination ZnO (0001ത) and the other with Zn-termination ZnO (0001). The corresponding slab is made of 12 layers of alternating Zn and O. The surface is a 6x3 repetition of the elementary rectangle (19.78 Å × 17.13 Å) and contains 36 atoms (Zn or O). The ZnO (0001) is chosen as it corresponds to the growth orientation determined by XRD measurements of our ALD ZnO.25 Under ultrahigh vacuum and upon annealing, ZnO is known to compensate for its overcharged surface by generating surface defects: pits of different nature (truncated pits, presence of adatoms), such as different-size pits on Zn-terminated ZnO and oxygen missing rows on the O-terminated ZnO.32-35 However, these reconstructions may be not satisfactory in realistic environments. For instance, Onsten et al. have demonstrated that the pit defects disappear upon exposure to gas phase water molecules, resulting in flat terraces.36 In our case, the motivation for choosing flat model surfaces is twofold: 1) large portions of flat surfaces are experimentally observed32-33,

36

and preliminary calculations of their interactions with

aluminum indicate that the local chemical reaction largely dominates reconstruction effects; and 2) flat surfaces make it possible to establish a clear reactivity hierarchy upon Al exposure based on surface chemical composition rather than defects. The Zn surface is a slab of 6 layers thick, with a 6x3 repetition of the elementary rectangle (16.17 Å × 13.98 Å), containing 36 Zn atoms. The Al substrate is modeled as a slab of 9 layers


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thick, with a 7x4 repetition of the elementary rectangle (20.00 Å × 19.79 Å) containing 56 Al atoms per layer. In all the above slabs, the two bottommost layers are kept frozen in the bulk coordinates and all remaining topmost layers are allowed to relax. The Al2O3 slab uses the α-Al2O3 hexagonal close-packed structure with (0001) orientation, which is more complex than the other substrates: two very closely separated Al (0001) planes alternate with two closely separated O (0001) planes. In order to construct the alumina slab, we have cleaved α-Al2O3 between two adjacent Al planes. The slab top and bottom planes are therefore Al planes, with 4 Al planes and 6 O planes sandwiched in between. The model surface is a 2x2 repetition of the elementary body centered rectangular cell. The slab (16.49 Å × 9.52 Å) contains a total of 96 Al and 144 oxygens. In this alumina slab, the bottommost layers of Al and O are kept frozen in the bulk coordinates and the 10 topmost layers are allowed to relax. The binding energy is calculated by subtracting the energies of the initial states from those of the final states: ‫ܧ‬௕ = ‫ܧ‬௔ௗ௦௢௥௕௔௧௘@௦௟௔௕ − (‫ܧ‬௔ௗ௦௢௥௕௔௧௘ + ‫ܧ‬௦௟௔௕ ) Where ‫ܧ‬௔ௗ௦௢௥௕௔௧௘@௦௟௔௕ is the total energy of the metal atom adsorbed on a surface, ‫ܧ‬௔௕௦௢௥௕௔௧௘ is the total energy of one adsorbate metal atom in the gas phase (Zn or Al, for instance), ‫ܧ‬௦௟௔௕ is the total energy of the slab (ZnO, Al2O3, Zn or Al). All possible adsorption sites were considered for Al and Zn adsorption on the different model surfaces, but only the most stable are reported below.

Results and discussion Al interaction with ZnO: surface and subsurface composition


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The surface composition after 11 nm Al deposition on ZnO is first analyzed with LEIS (Figure 1). The as-deposited sample is characterized by a strong Al peak, and a weaker but clear Zn peak, with no evidence for O. This composition remains stable after 100°C annealing, but the Zn peak completely disappears after 150°C annealing. The absence of O at the surface suggests that, for such a thick Al film deposition, metallic aluminum covers the surface even though O atoms are initially reacted with Al to form a thin interfacial oxide below metallic Al. The presence of Zn indicates that some surface regions are covered by metallic Zn, even after several nm of Al deposition. This surface Zn disappears when the system is annealed up to 150 oC, either by

evaporation or penetration into the bulk. Figure 1. LEIS spectra of 11 nm Al deposition on ZnO and annealing up to 150 oC.


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Examination of the Zn core level using XPS confirms that metallic Zn is formed after Al deposition (see Figure 2), and that this metallic Zn is removed after 150 oC anneal. Since XPS is also sensitive to the near surface region, the disappearance of the metallic Zn component is most

likely due to Zn desorption rather than Zn incorporation into the substrate. Figure 2. XPS spectra of the Zn region, after 11 nm Al deposition on ZnO and annealing up to 150 oC. The interpretation of the spectroscopic data in Figure 1 and Figure 2, points to several potential scenarios to decipher Al deposition on ZnO surfaces, other than oxidation of Al and reduction of ZnO, and which remained unanswered in the previous work:25 the migration path of metallic Zn atoms to the surface above the Al film or in between Al islands; in the latter case, is


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surface diffusion important and on which part of the surface (ZnO, Al, Al2O3, Zn)? To get some quantitative insights into the nature of the surface and migration of the Zn atoms, we have performed DFT calculations for several systems, starting with the identification of initial interaction mechanisms of Al with polar ZnO model surfaces, either oxygen-terminated surfaces (O-ZnO) or zinc-terminated ZnO (Zn-ZnO), and examining the stability and mobility of Zn atoms.

Al reaction with ZnO: adsorption and barrierless mechanisms We first consider the interaction of a single Al atom with two model polar ZnO surfaces, namely O-ZnO and Zn-ZnO. On these faces, three types of sites are considered: on top, bridge (sharing bonds with two surface atoms) and hollow sites; for the last case, the Al atom is

surrounded by three surface atoms, either oxygen or zinc atoms for O or Zn-terminated surfaces, respectively. Figure 3. Top and side views : (a) and (b) show the most stable configuration on O-terminated ZnO for one Al atom adsorption; (c) and (d) show the most stable configuration on Zn-


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terminated ZnO for one Al atom adsorption. Zn atoms are depicted in grey, O atoms in red and Al atoms in cyan. Figure 3 illustrates the most stable end configurations, i.e. after total energy minimization, obtained for both O-ZnO and Zn-ZnO, respectively. The corresponding binding energies are reported in Table 1. Table 1. Calculated binding energies (eV) and the best adsorption sites for one Al on Oterminated ZnO (000-1) and Zn-terminated ZnO (0001). The structures are shown in Figure 1. Al on Zn-ZnO

Al on O-ZnO Initial Al adsorption configurations

Adsorption site Eb (eV)

After minimization configurations

-10.66 Bridging Zn-O

Adsorption site Eb (eV)

After minimization configurations

Barrierless Zn substitution

-2.92

Top O

Top Zn

-10.61

Barrierless Zn substitution

-2.81

Hollow

Bridging O

-9.27

Hollow

-2.82

Hollow

Bridging Zn

-9.25

Hollow

-2.91

Top O

Hollow

-9.25

Hollow

-2.81

Hollow

Top O

-5.19

Top O

-2.91

Top O

On the O-ZnO surface, Al is strongly bonded to the surface with 5 to 10.6 eV adsorption energies. The two highest adsorption values are associated with barrierless substitution mechanisms between one surface Zn atom and the incoming Al atom. Figure 3 (a) and (b) illustrates the most energetic scenario, with a gain of 10.66 eV. In this substitution process, the Zn atom is pushed down to an interstitial hollow position in the first sublayer. Note that the


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substituted Zn atom is fully coordinated before energy minimization, underlining the strong reactivity of the Al atom. In addition, this substitution reaction leads to a local roughening of the surface on top of the Zn interstitial: after total energy minimization, two Zn atoms are pushed out of the subsurface and are positioned slightly above the oxygen layer covering the surface. Figure 4 shows the isosurface of charge densities in the case of Al substitution with Zn on the O-ZnO

surface. On the one hand, one can clearly see that the interstitial Zn atom is linked to two surface and one subsurface oxygen atoms. On the other hand, one can see the electron density isosurface slightly deformed towards Al atoms, which indicates a weakening of surrounding Zn-O bonds.

Figure 4. (a) Most stable configuration after Zn substitution by Al adsorbate in O-terminated ZnO surface. (b) Representation of charge density isosurfaces within the same structure (0.01 e/Å3). Anisotropy of surface oxygen density are represented with black arrows make them more visible. Zn atoms are depicted in grey, O atoms in red and Al atoms in cyan. Among the other adsorption sites, the hollow position, the third most energetic state identified for adsorption, can be seen as a local minimum along a pathway leading to Al substitution. This


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intermediate adsorption site greatly increases the possible pathways for Al reaction, making ZnO reduction through Zn substitution highly probable. On the Zn-ZnO surface, all tested adsorption sites lead to configurations that do not noticeably modify the surface topography. The metallic character of the Zn-terminated surface, together with the saturation of subsurface oxygen atoms, make the surface less reactive to incoming Al atoms, compared to the unsaturated oxygen atoms of the O-ZnO surface. As a result, all adsorption sites are characterized with relatively low energies in the range -2.8 to -2.9 eV. Among these positions, the on-top oxygen configuration (see Figure 3 (c) and (d)) is slightly more energetic than the hollow configuration due to a better electronic exchange between both aluminum and oxygen atoms, which results in an Al-O bond length of 2.51 Å. Yet, in the hollow configuration, distances between the Al and the three surface O atoms range from 3.22 to 3.27 Å.

Al reaction with ZnO: activated processes (substitution, Zn diffusion) The results presented above raise several questions. On the O-ZnO surface, there is a direct substitution leading to the formation of a Zn interstitial on the subsurface. It is not clear from the data if Zn can further migrate from this position and whether is it thermodynamically more favorable to reside on the outer surface or to diffuse into the bulk of ZnO. On the Zn-ZnO surface, the situation is more complex because only metastable states are obtained for Al adsorption. It is therefore important to quantify potential substitution and also Zn migration. We start by considering a number of equilibrium states from which mechanistic reaction scenarios can be derived. All results are reported in Table 2 for the main configurations obtained for O and Zn-terminated surfaces. On the O-ZnO surface, four main configurations are considered. The first two configurations involve the stabilization of the zinc atoms onto the surface after Al substitution,


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preferentially in hollow positions, with energies of -10.88 and -10.96 eV (configuration shown in Figure 5 (a) and (b)). Note that the zinc atom prefers not to be in the direct vicinity of the Al substitutional atom as a gain of almost 0.1 eV is obtained when the zinc atom is positioned slightly further from Al, indicating a repulsive interaction between surface Al and Zn. In contrast, for an interstitial positioning of the Zn atom immediately below in the subsurface layer, there is a loss of 0.3 eV in binding energy from the interstitial located just underneath the surface. Pushing further the Zn atom to the second subsurface layer further diminishes its binding energy (by 0.62 eV compared to the most stable substitution where the Zn atom is located on the surface (see Table 2). We therefore conclude that the migration of Zn atoms towards the bulk is not a favorable process. Table 2. Calculated binding energies (eV) for all substitution configurations of Al on O-ZnO (000-1) and Zn- ZnO (0001). Selected structures are shown in Figure 3 and Figure 4 and 5 (all other configurations are shown in the Supplementary information file Figure S1 and S2). Al on O-ZnO Energy (eV)

Al on Zn-ZnO Energy (eV)

Al position

Zn position

Top layer

Surface adatom

substitutional

Hollow, first neighbor

Top layer

Surface adatom

Top layer

-10.88

-4.99

Zn position Surface adatom

substitutional

Close hollow

to

Top layer

Surface adatom

substitutional

Top-O

-5.04

-10.96 Substitutional Hollow, farther from Al Top layer

Top layer

Interstitial -5.62

-10.66 Substitutional Below the surface layer Top layer -10.34

Al position

Interstitial

Substitutional In

Surface adatom

Substitutional slightly Bridging Zn below the surface * Top layer

Surface adatom

substitutional

Bridging

unstable

the

second


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subsurface layer

* coordinate with a fourth O atom belonging to the second sublayer of ZnO.


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On the Zn-ZnO surface, Al adsorption is neither accompanied by barrierless insertion nor Zn substitution observed on the O-terminated ZnO. Table 2 reports four configurations where Al atoms are forced to occupy a surface substitutional position, relative to Al on Zn-ZnO. We considered four positions of the zinc atom in the Al vicinity, in the surface or in the subsurface. We first note that all energies (~5 eV) are almost twice more favorable than Al adsorption configurations discussed in the previous subsection (Table 1), indicating the metastable nature of adsorbed states. The major contrast seen, compared to the O-ZnO surface, is that Zn cannot be stabilized into the subsurface layer in the vicinity of the Al substitutional position (Figure 5 (c) and (d)). Wherever the interstitial Zn is released, it moves spontaneously upwards above the surface. When the Zn atom is positioned on the surface, no major change is found between hollow or top positions. However, a slight change in the Al position towards the inner layer allows gaining a noticeable amount of energy (gain of 0.6 eV). This is attributed to the fact that the Al atom can now coordinate with four O atoms, including three O atoms of the top layer and one new O atom of the second subsurface layer. In order to investigate the kinetics of Al


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substitution, we calculate the energy barrier necessary to substitute one surface Zn atom by Al. The overall pathway is shown in Figure 6. With a 0.11 eV barrier for this Al to Zn exchange mechanism, we can assume a quasi-spontaneous substitution to occur at room temperature.

Figure 5. (a) and (b) show the top and side views of the most stable configuration on Oterminated ZnO for surface Zn substitution. (c) and (d) show the same views of the end configuration starting from the substituted Zn atom in a subsurface position of Zn-terminated ZnO.


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Figure 6. Substitution energy profile for interstitial Zn generated upon Al-induced ZnO reduction on Zn-terminated ZnO. Zn atoms are depicted in grey, O atoms in red and Al atoms in cyan. Table 2 shows that, on both O-ZnO and Zn-ZnO surfaces, it is thermodynamic very favorable to reduce ZnO through a Zn/Al exchange mechanism, accompanied with the preferential transport of Zinc atoms to the surface. To complete these thermodynamic considerations for the O-terminated surface, we calculate the energy barrier for the zinc interstitial atom located in its initial subsurface position (see Table 1) to migrate either to a surface position or deeper into the subsurface layers. Migration pathways are presented in Figure 7. The energy barriers are 0.36 eV and 1.82 eV for upward and downward migrations, respectively. Upward migration is therefore much easier and almost spontaneous at room temperature, since only two Zn-O bonds are broken in this process. Note that the migration is an indirect process: the interstitial Zn atom, by moving up towards the surface, kicks out another surface Zn atom above the surface. Downward migration, which requires a net activation of 1.82 eV, necessitates temperatures above 400 oC. We conclude that, on both ZnO surfaces, Al substitution and Zn segregation on the film surface occur quasi spontaneously, with a tendency for Zn atoms to sit on the surface and to move apart from the oxidized Al atoms.


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Figure 7. Interstitial Zn migration mechanism upon Al-induced ZnO reduction on O-terminated ZnO: (a) upwards diffusion pathway, (b) downwards diffusion pathway and (c) energy profiles for both migration processes. Zn atoms are depicted in grey, O atoms in red and Al atoms in cyan.

Discussion Al/Zn segregation during ZnO reduction and evaporation: theoretical section In the previous subsections, we have described the initial chemistry of Al/oxidation, characterized by substitution mechanisms leading to oxidation of aluminum and ZnO reduction. We demonstrated that reduced Zn atoms prefer to sit onto the top ZnO surface rather than to intermix within the remaining bulk ZnO. These findings agree well with the findings of our preceding experiment25 showing the initial growth of an aluminum oxide at the early stage of Al deposition on ZnO, and the presence of metallic zinc on the top surface, even after 11 nm Al deposition unless the sample is annealed to 150 °C. To get further insight into the behavior of those Zn atoms when more Al atoms are deposited on the system, we have systematically calculated the potential interaction of aluminum and zinc on top of all possible model surfaces


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used in this study. We consider four surfaces: 1) ZnO surfaces, representing the initial substrate or potentially portions of the surface when Al does not cover the entire surface; 2) alumina surface since incoming Al atoms are initially oxidized; 3) zinc surface that can be formed after zinc reduction, segregation and agglomeration; and 4) aluminum surface that forms after more deposition of Al atoms. All results are reported in Table 3. Table 3. Most stable calculated binding energies for one Al atom or one Zn atom adsorbed on different model surfaces. Al

Zn

Energy (eV)

Al position

Energy (eV)

Zn position

-10.96

Al substitute one top layer Zn. Zn becomes surface adatom

-5.90

Hollow

Znterminated ZnO

-5.62

Al substitute one top layer Zn. Zn becomes surface adatom

-3.21

Bridging Zn-Zn

Al2O3

-3.42

Hollow above Al

-5.81

Bridging Al-O

Al(111)

-2.82

Hollow-hcp

-2.96

Hollow-fcc

-0.27

Hollow

Zn(0001)

-1.96

Hollow-hcp -1.31*

Surface atom

Oterminated ZnO

*This value corresponds to the calculated formation energy of a surface vacancy The binding energies of both Al and Zn are higher on oxide surfaces than on metallic surfaces, reflecting the large oxidation heats compared to metal cohesive energies. Note that the highest values are obtained with aluminum interacting with the oxygen-terminated ZnO surface. This is consistent with the substitution mechanism that is operated, all other surfaces exhibiting smoother chemical adsorption processes and energies ranging from roughly 3 to 6 eV. On top of alumina, the Al and Zn adsorption energies are -3.42 eV and -5.81 eV, respectively. We observe


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that zinc atoms have a better affinity with the alumina surfaces than to aluminum itself. Al adatoms have roughly the same adsorption energy on Al or Zn surfaces (~-2.82 and -2.96 eV respectively). In contrast, Zn adatoms have a very low adsorption energy (-0.27 eV), which indicates that at very low pressure, the Zn surface should be flat. Note that Al sticks on the Zn surface with a reasonably high adsorption energy of -1.96 eV. We now focus on the behavior Zn atoms. We saw that at the earliest stage of Al deposition, Zn atoms are ejected on top of ZnO surfaces. Table 3 does not point to preferred end locations of Zn as its adsorption is favorable on all model surfaces. We first study the migration of zinc on the ZnO surface and its ability to agglomerate. The barrier on the O-terminated ZnO surface is characterized by a 0.67 eV barrier, leading to a relatively slow diffusion at room temperature. In addition, examining Table 4, we see that if Zn-Zn exhibits repulsive interaction on the Oterminated ZnO (+16 eV), the interaction becomes exothermic (-0.24 eV) when going to the Znterminated surface. The latter surface is a more reasonable surface as the Al deposition produces more Zn. The migration barrier, plus the ability of Zn to agglomerate should favor pure Zn nucleation on ZnO portions at early deposition stages. Further on during deposition, Table 3 also indicates that Zn could be located on top of aluminum. We have determined the migration barrier of Zn on top of aluminum to be as low as a 0.07 eV, indicating that Zn should move quasi freely at room temperature on top of Al. With such a low activation barrier, Zn would tend to form Zn islands. The energy profile for Zn atom migration on Al and O-terminated ZnO surfaces are shown in the Supplementary information file Figure S3 and Figure S4, respectively. Finally, note that from Table 3, only one value is consistent with the evaporation of Zn at a 150 °C temperature, which is the formation energy of the zinc vacancy (-1.31 eV). This shows


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that nucleation of evaporation takes place on flat and stable Zn terraces, with the creation of vacancies.

Table 4. Single and double adsorption of zinc atoms within a single unit cell, on both O and Znterminated ZnO surfaces Zn on O-ZnO

Zn on Zn-ZnO

Number of Zn atoms

Zn configuration

Energy (eV)

Zn configuration

Energy (eV) ion

1

Hollow

-5.90

Bridging zinc

-2.97

2

Hollow-neighbor

-5.74

Bridging zinc neighbor

-3.21

2

Hollow far

-5.85

Bridging zinc far

-2.93

Al/Zn segregation during ZnO reduction and evaporation: complement from experiment To further analyze why Zn should be present on the surface during Al deposition, and its subsequent behavior when increasing the temperature, we complement our DFT calculations, and particularly the Table 3, by selected experiments and LEIS analysis in which we deposit Zn on model surfaces, namely ZnO, Al2O3 and Al surfaces. Figure 8 gives the LEIS spectra of a 5 nm thick layer of Zn on top of metallic Al and Al2O3 surfaces. Concerning Zn/Al, the zinc peak is clearly visible, as well as a small Al contribution. After annealing at 150 °C, the Zn peak disappears almost completely while the Al peak appears again. Looking at Table 3, we can see that Zn adatoms on Al, bonded with -1.96 eV, cannot evaporate at this annealing temperature. Our conclusion is that Zn atoms on Al (111) surface will tend to form islands with increasing the temperature, which we will decrease in size as evaporation of Zn will take place. In the


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supporting information Figure S5, we show that no Zn is detected by XPS after the annealing step, indicating that no Zn diffuses into the system. This probably results from the high migration rate of Zn/Al (111). In addition, this islanding mechanism, and the subsequent 3D growth, is very probably responsible for the presence of Al on as deposited layers. A different behavior is observed for Zn/Al2O3. First, the surface of the as deposited layers still contains a non-negligible amount of Al and oxygen, showing clearly the 3D growth of Zn. After annealing at 150 °C, the presence of Zn on the surface attests of the partial oxidation of Zn atoms, corroborated by the very high adsorption energy of Zn/ZnO (5.90 eV).

Figure 8. LEIS spectra after Zn deposition on Al (left) and Al2O3 (right). Black and red curves show respectively spectra from as deposited and annealed at 150oC samples. We now investigate the case of Zn deposited on the ZnO surface. From Figure 9 (a), after 5 nm Zn deposition, there is almost no O detected on the surface, while a clear Zn peak is observed. Figures 9 (b) and (c) show the surface composition after several annealing temperatures. Figure 9 (d) represents the Zn and O ratio, as a function of annealing temperature. When the sample is annealed at 50 oC, the Zn film fully covers the surface and the intensity of


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Zn peak reaches its maximum value. We used this Zn peak as a reference to calculate the Zn amount detected on the surface after each anneal. We notice that the Zn surface is quite stable until the stacks are annealed at 130 and 140 oC. By giving enough time, surface Zn amount decreases to 73% at 130 oC and further to 12 % at 140 oC. This drastic decrease of the surface Zn, together with an increase of the O amount on the surface is consistent with our previous observation that pure Zn will evaporate in UHV at temperatures close to 150 oC. Yet, the surface Zn and O ratio reaches an equilibrium upon 140 oC (see Figure 9 (d)). Both these results agree with the adsorption energy of Zn on Zn-ZnO that amounts to 3.21 eV, allowing a sub-monolayer of Zn absorption on ZnO surface.


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Figure 9. LEIS spectra after Zn deposition on ZnO, a) bare surface and as deposited Zn layer, b) and c) after annealing at different temperatures. The surface Zn:O ratio detected by LEIS is plotted against the annealing temperature in d). Conclusion In this paper, we shed light on the multiple and complex processes arising from the deposition of aluminum on polar ZnO surface. We first show quasi-spontaneous substitution of surface Zn by incoming Al atoms on both O and Zn terminated surfaces, even though surface Zn atoms are fully coordinated before energy minimization. In contrast to the barrierless substitution observed on the O-terminated surface, we show that a low activation (only 0.11 eV activation barrier) is necessary to obtain substitution in a largely exothermic pathway (2.13 eV gain) on the Zn-


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terminated surface. The metallic character of Zn-terminated surface explains this slight difference in the kinetics of substitution compared to O-ZnO surface. We further give new insights into the segregation of zinc and its evaporation at 150 °C, based on DFT calculations of adsorption, agglomeration and migration of main species, that we completed by appropriate LEIS experiments. Calculations provide clear indication of islanding and stabilization of zinc films on both Al and ZnO surfaces. Increasing Al exposure, when there are only metals on the surface, the deposited Al prone to attaching on Al rather than Zn. Further increasing aluminum coverage, we calculate that Zn can migrate quasi-freely on the Al surface. At this point, we examine the case of Zn adatom on Zn (0001). The very low adsorption energy of -0.27 eV leads to a quasi-spontaneous desorption of the adatom, even at room temperature. We conclude that Zn islands, resulting from the segregation of Zn during Al deposition, are composed of flat terraces. Massive desorption of Zn atoms from flat terraces, observed experimentally, requires the creation of Zn vacancy defects on the surface. With this intermediate energy barrier, evaporation is actually prohibited at room temperature and activated only above 130 °C. This scenario is compatible with LEIS results after annealing of the film (ZnO, Al and Al2O3) at 150 °C.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript


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Supporting Information. Energetics of Zn adsorption on O-terminated ZnO. Atomistic views of substitution configurations of Al on both O- and Zn-terminated ZnO surfaces. Energy barrier for migration of Zn on the Al surface and on the O-terminated ZnO surface. XPS spectra after Zn deposition onto Al and annealed at 150°C. Acknowledgment We acknowledge the support of CALMIP for supercomputer resources. This work has been

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Al Interaction with ZnO Surfaces

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