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Basic Mechanisms of Al Interaction with the ZnO Surface Yuzhi Gao, Lorena Marín, Eric Mattson, Jeremy Cure, Charith E Nanayakkara, JeanFrancois Veyan, Antonio Lucero, Jiyoung Kim, Carole Rossi, Alain Esteve, and Yves J. Chabal J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

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The Journal of Physical Chemistry

Basic Mechanisms of Al Interaction with the ZnO Surface Yuzhi Gao†, Lorena Marín‡, Eric C. Mattson†, Jeremy Cure†, Charith E. Nanayakkara†, JeanFrancois Veyan†, Antonio T. Lucero†, Jiyoung Kim†, Carole Rossi‡, Alain Estève‡, and Yves J. Chabal*† † Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States ‡ LAAS-CNRS, Université de Toulouse, 31400 Toulouse, France

KEYWORDS: nanoenergetic materials, ZnO, Al, intermixing, interface characterization

ABSTRACT

Deposition of Al on ZnO is used for a number of electronic and catalytic devices as well as for nano-energetic materials. The interface structure and chemical composition often controls the performance of devices. In this study, in situ infrared spectroscopy, X-ray photoemission spectroscopy and low energy ion scattering are combined to investigate the initial stage of interface formation between Al and ZnO. We find that a) the interface is highly inhomogeneous

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with discontinuous Al patches, leaving ~10% of the ZnO surface uncovered even after deposition of an equivalent of 11 nm-thick Al film; b) upon Al deposition, Al reduces ZnO by forming Al2O3 and releasing Zn to the surface, and this process continues as more Al is deposited; c) the reduced surface Zn atoms readily desorb at 150oC; and d) at higher temperature (> 600oC) all Al is oxidized as a result of mass transport. Deposition of a thin Al2O3 layer on ZnO prior to Al deposition effectively prevents Al penetration and Zn release, requiring higher temperatures to oxidize Al.

TEXT Introduction The interfaces between metals and metal oxides are critical for a number of technologies in materials sciences supporting applications in electronics, catalysis, and energy among others.

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As devices are scaled down to the nanoscale, the materials properties such as surface chemical reactivity and electronic properties (e.g. band alignment and transport) are affected due to complex chemical changes at the interfaces. These changes can potentially lead to new and tailored materials functionalities,6 which can only be harvested if a fundamental understanding of the interfaces is developed. One of the most technologically relevant oxide is zinc oxide, characterized by a wide bandgap (3.37 eV) and high exciton bond energy (60 meV). Thanks to these properties, ZnO is used as a photoelectronic material for sensors and solar cells.7-10 It also exhibits photocatalytic activity with UV light.11,12 More recently, the combination of Al and ZnO has been shown to be powerful for constructing nano-energetic materials,

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or even for optoelectronics and catalytic devices.


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For instance, transparent conductive Al-doped zinc oxides (AZO) have been prepared by

depositing alternative ZnO and Al2O3 layers (AZO for short). In this case, the resistivity of the AZO films is controlled by varying the ZnO to Al2O3 sublayer ratio, which can be done accurately with ALD. This ratio is also shown to selectively impact the temperature of crystallization of the multilayers during high temperature annealing.17 Similarly, the deposition of Pt, Al and other metal contacts on ZnO films has achieved faster switching speed and lower turn-on voltages for ZnO-based Schottky diodes used in high-performance electronic devices,18 For these and in particular photonic applications, the interface and optical properties are influenced by the thickness and the roughness of the metal layer.19 A fundamental issue common to many of these research efforts is the lack of knowledge of the detailed atomic-level mechanism of the metal (e.g. Al) interaction with ZnO, i.e. the first stage of interface formation. Very few studies have characterized the interface composition between Al and ZnO

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and the observed intermixing at high temperature.21 Efforts to correlate thermal,

optical and electrical properties of Al/ZnO films to interfacial chemistry have led to exploration of various deposition techniques and layer thicknesses, without clear correlation for lack of in situ characterization.

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In this paper, we combine in situ molecular level techniques, such as

Fourier transform infrared (FTIR) spectroscopy, X-ray photoemission (XPS), and low energy ion scattering (LEIS), to investigate the early stages of interface formation upon Al deposition on thin ZnO films, and ex situ X-ray diffraction and atomic force microscopy to determine their structure. We have selected atomic layer deposition (ALD) to grow ZnO oxide in order to obtain the smoothest films, 22 which are preferred for atomic-level investigations. After ZnO deposition in a separate ALD reactor, all characterization and Al deposition are performed in a home-made


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ultra-high vacuum (UHV) cluster tool that combines FTIR, XPS and LEIS.

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The base pressure

of ~2×10-10 Torr is important to prevent oxidation of the Al layer during and after Al deposition on ZnO, in particular during transport to the XPS and LEIS modules, where the chemical changes of elements (e.g. oxidation state) and the surface composition are determined. FTIR spectroscopy is performed during Al deposition without moving the sample position to monitor the formation of chemical bonds, e.g. of oxygen with Zn and Al, and also changes in surface conductivity affecting the broadband absorption spectrum. All three techniques can monitor the thermal evolution of the system through in situ annealing. We find that Al deposition disrupts the ZnO film, driven by oxygen scavenging by Al that leads to Zn reduction. The Zn(0) preferentially resides at the surface, even after thick (~11 nm) Al film deposition, possibly due to Zn migration or the formation of a highly inhomogeneous Al film. However, surface Zn atoms are readily desorbed upon annealing to 150oC. The upper Al layer is oxidized gradually with increasing temperature. A thin Al2O3 layer deposited on ZnO by ALD prior to Al deposition effectively prevents reduction of ZnO and Zn migration, and is also helpful to retard the diffusion of O atoms, and as a result, to postpone the oxidation of Al layer to a higher temperature. A higher temperature anneal (~ 600 oC) is needed to fully oxidize the whole Al layer. Experimental Methods Rectangular (3.8×1.5 cm2) silicon (Si) samples are cut from a Si wafer with a ~6 nm thermally grown SiO2 layer. They are first degreased by sonicating sequentially in dichloromethane, acetone and methanol for 5 minutes. The samples are then soaked in a piranha solution (a mixture of concentrated sulfuric acid and hydrogen peroxide with a ratio of 3:1) at 80oC for 30 minutes to fully hydroxylate the clean oxide surface. A 20 nm-thick ZnO film is deposited on the


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substrate by ALD in a Savannah-100 ALD reactor (Cambridge NanoTech) at 200°C, using diethylzinc (DEZ) and water vapor as the zinc and oxygen precursors, respectively.

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The

process pressure is ~ 50 mTorr and the N2 purging gas flow rate is 20 sccm, with respective pulse times of 0.03 s for diethylzinc, 0.1 s for water, and 20 s N2 purge for a total of 150 cycles. The sample is then introduced into the UHV cluster for Al deposition using a Mantis e-beam evaporator. The equivalent thickness of the Al layer is calibrated by a quartz crystal microbalance (QCM) before the deposition (the QCM is used to measure the total Al thickness deposited in 10 min and the average growth rate is calculated to be 5 Å per minute. More details are given in the SI section). In situ characterization is carried out by a Thermo-Nicolet FTIR, a PHI 5600 XPS (with a monochromated Al Kα line of 1486.5 eV) and an Ion-TOF LEIS (using 3 KeV He+ ions). To obtain a depth profile of the composition of the bilayer material, 8 keV Ar+ ions are used to sputter the surface for different times and LEIS spectra are collected to characterize the surface composition after each sputter cycle. Also, the sample is annealed resistively for 5 min in a UHV environment and the composition evolution with temperature is studied. To test whether a well-formed Al2O3 layer can change the reaction between the oxide and Al, an ALD 2 nm-thick Al2O3 layer is deposited on ZnO in a Savannah-100 ALD reactor (Cambridge NanoTech) at 120°C, using trimethylaluminum (TMA) and water vapor as aluminum and oxygen precursors, respectively. The process pressure is ~ 50 mTorr and the N2 purging gas flow rate is 20 sccm, with respective pulse times of 0.015 s for TMA, 0.015 s for water, and 20 s N2 purge for a total of 20 cycles. Characterization of the initial ZnO film ZnO films are notoriously rough, characterized by columnar growth. It is therefore important to examine the morphology and composition of our ALD-grown ZnO films. We have done that


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by combining atomic force microscopy (AFM), X-ray diffraction (XRD), infrared (IR) spectroscopy, and X-ray photoemission spectroscopy (XPS). From AFM maps obtained after deposition (see image in Figure S1 of the SI section), the RMS of the surface roughness in a 1 µm x 1 µm region is found to be 0.797 ± 0.051 nm. This roughness is substantially larger than that of the initial SiO2 surface (RMS ~ 0.3 nm), indicating that even with ALD the growth of ZnO film is columnar in nature, hence the difficulty to monitor mass transport phenomena and the impact on Al film homogeneity discussed later. The XRD spectrum of the as-deposited ZnO film, measured with a 0.2 mm × 0.2 mm X-ray beam, is shown in Figure 1 a). All detectable diffraction planes of the ZnO film are labeled. The preferred orientation is clearly the (0 0 2) plane. In addition, there is broad band centered at 56.40° that is assigned to a Zn2SiO4 phase, suggesting that there is some interaction between the Zn precursor and the underlying SiO2 film during the initial stage of the process leading to an ultra-thin composite interface layer. The large width is consistent with a very thin film with small crystallites. On the other hand, there is a sharper contribution at 51.49o from the underlying Si substrate as expected. An IR absorption spectrum is recorded in transmission, at an incidence angle close to the Si Brewster angle (~74o). In this geometry, the spectrum shown in Figure 1 b) is dominated by the LO phonon of ZnO that centered at 580 cm-1, characteristic of good quality ZnO.

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This

conclusion is supported by XPS measurements of the binding energies of the Zn 2p and O 1s core levels at 1021.6 eV and 530.2 eV ( Figure 1 c and 1 d), respectively, which are both expected for ZnO. 26 Altogether, the above measurements indicate that the ZnO films are of quality similar to films previously reported in the literature. 27,28


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Results I. Al deposition on ZnO at room temperature The initial investigation of Al deposition on ZnO is first performed with IR spectroscopy because it can be done during deposition as a function of the average thickness of deposited Al, over the range 0.6 nm to 11 nm. Figure 2 shows that there are positive contributions (bond formation) in the 600-900 cm-1 region. This part of the spectral region is related to the stretching vibrations of Al-O bonds, indicating that Al atoms scavenge oxygen from ZnO to form aluminum oxide. These AlOx contributions are plotted as a function of average thickness in the inset, suggesting that this AlOx interface region continues to develop as Al is deposited at least up to 11 nm of effective thickness. This hypothesis is supported by the appearance of a negative contribution in the 500-600 cm-1 region, corresponding to a loss of Zn-O bonds (ZnO phonon) originally present in the reference IR spectrum. Upon closer examination of the AlOx stretch spectral region, two contributions are identified: the lower frequency part of the Al-O vibration region (600-700 cm-1) can be assigned to the stretching modes of Al3+ ions in octahedral sites of AlO6 groups. The higher frequency part (700800 cm-1) can be assigned to Al3+ ions in mixed sites with in octahedral AlO6 and tetrahedral AlO4 positions.29,30 Thus, the FTIR spectra provide evidence for the formation of a range of Al-O bonding configurations typical of an inhomogeneous structure. All contributions increase with Al deposition, as summarized in the inset, well above the minimum average Al thickness for obtaining a continuous film, suggesting that the ZnO film must be structurally very rough, a point that will be addressed in the discussion. IR spectroscopy can also be used to determine the degree of electronic conductivity by monitoring the broadband absorption (400-4000 cm-1). When a thin metallic film becomes


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continuous, its absorption becomes dominated by a Drude contribution that is characterized by an increase in absorption with decreasing energy (wavenumber).

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Such a Drude tail is

essentially negligible in this case, pointing to an inhomogeneous and discontinuous film at the nanoscale. To complement the bonding information derived from IR absorption spectroscopy, XPS is used to monitor the oxidation state of each element after Al deposition. At the lowest Al coverage, only one peak is observed at 75.0 eV binding energy in the Al2p core level region, which corresponds to fully oxidized aluminum. At higher coverages, another peak appears at lower binding energy, 72.4 eV, corresponding to metallic Al. With increasing Al coverage, the oxidized Al peak at 75.0 eV remains present and constant in intensity, indicating that a finite amount of ZnO is reduced at the initial stage of deposition. Note that the minor decrease of the intensity of this oxidized Al peak in going from 9 nm to 11 nm Al film deposition on ZnO is due to the screening by the upper Al metal layer. The metallic Al peak increases with increasing Al coverage indicating that pure Al is deposited after surface reduction by the first incoming Al atoms on the ZnO surface. The oxidation of Al atoms at the interface shown by XPS (Figure 3) is consistent with Al-O bond formation inferred from FTIR spectra during Al deposition. In order to obtain composition information within the surface region (top 0.5 nm), LEIS is used after each Al deposition at room temperature, and the results are summarized in Figure 4. First, there is a notable increase of surface Zn with the Al film thickness even when the total average thickness is larger than 10 nm. This observation suggests that more Zn atoms are reduced as more Al is deposited on the surface. While this result agrees with the FTIR spectra showing more Al-O bonds are formed and more ZnO is reduced as more Al is deposited, the interpretation is not straightforward. If the films were continuous with a homogeneous thickness,


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then reduced Zn atoms would have to diffuse through the Al film to reach the surface. We already have some indications that the Al layer is highly inhomogeneous, so that a complete coverage of the surface is probably not achieved, even when the average Al thickness reaches 11 nm. The hypothesis that a fraction of the surface is not covered by ZnO could be checked by examining the O surface peak. Unfortunately, the sensitivity to O with LEIS is poor, and the signal-to-noise ratio (SNR) in the He+/O binary collision region is such that an appreciable surface coverage of ZnO could yield an O peak that is within the noise of the measurement. We therefore estimate an upper bound on the coverage of unreduced ZnO by using a ZnO reference surface with known coverage. Based on the intensity of the O peak in measurements of asgrown ZnO films subjected to an analogous 200°C annealing step, we estimate that the coverage corresponding to the limit at which the O signal may be detected is ≈10%. We can therefore conclude that the absence of detectable O can only indicate that the portion of ZnO that is uncovered is ~10% or less of the surface. Note that for low Al deposition both the Zn and O signals are still relatively low due to contamination of the original ZnO surface by adventitious hydrocarbon and carbonates (See Figure S2 in supplementary information), which cannot be eliminated by low level Al deposition (i.e. hydrocarbons remain at the surface with the potential to migrate). Carbonates and hydrocarbons are likely impurities at the ZnO surfaces after handling in air (i.e. prior to loading into the UHV system), as previously pointed out by Thornton and Campbell32,33 and confirmed by examination of the C 1s core level (see Figure S3 in supplementary information). Part of this contamination is removed upon annealing in UHV, leading to an increase of the Zn signal (and also O from ZnO) in the LEIS spectra. Despite all these issues, the lack of O signal in the LEIS spectra after ~ 2 nm Al is deposited indicates that, in contrast to Zn, the O atoms do not migrate upwards. Rather, the Al atoms


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migrate towards O, releasing Zn atoms. This last hypothesis is consistent with both XPS and IR data, as detailed above. In summary, the combination of IR, XPS and LEIS measurements clearly show that Al migrates towards ZnO, reduces Zn atoms that tend to migrate to the surface of the bare ZnO islands, but also along the surface of Al domains. It is unlikely that Zn atoms migrate through pure Al regions as the solubility of Zn in Al at room temperature is lower than 2%. We therefore hypothesize that the accumulation of reduced Zn atoms at the surface originates from uncovered regions and propagates through surface diffusion. II. Thermal evolution of Al films on ZnO To study the surface composition changes with temperature, samples are annealed stepwise to higher temperatures and examined with LEIS. The LEIS spectra of a nominally 11nm-thick Al film are shown in Figure 5 as a function of annealing temperature (maintaining the sample at each temperature for 5 min). The Zn signal, arising from Zn transport to the surface, remains clearly visible up to 100oC. However, it completely vanishes after 150oC annealing, and does not re-appear until the temperature reaches 400oC. On the other hand, O can be detected after 300oC anneal and its intensity then increases with annealing temperature. Note that the intensity of the Al peak decreases after each annealing, suggesting that there can be some diffusion of oxygen above 300oC, with subsequent oxidation of the Al surface. We tentatively attribute the appearance of Zn at high temperature to an alloy formation.34,35 The evolution of the whole film is best characterized with XPS. Figure 6 shows the changes of the Al 2p core level (top panel) and Zn 2p core level (middle panel) as a function of annealing temperature. The as-deposited films (red spectra) are characterized by a main peak at 72.4 eV and a small shoulder at 75.0 eV. This indicates that the Al layer is initially mainly composed of


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metallic Al except at the interface where Al is oxidized because of Al reaction with O atoms. Concurrently, the Zn 2p core level has a lower binding energy component (broader on the low energy side) that is consistent with some of the surface ZnO being reduced. The estimates of the integrated areas of both metallic and oxidized Zn atoms, as described in the supplementary information (Figure S4) are reported in Table 1. In bare ZnO films, there is no obvious contribution of metallic Zn. After Al deposition, metallic Zn is detected and this concentration increases upon annealing at 100°C. However, the concentration of reduced Zn atoms drops to zero after annealing at 150oC, as the Zn atoms on the surface desorb and cannot be detected any more, leaving only the Zn in the underlying ZnO substrate. The latter is detectable because the Al film is inhomogeneous (leaving open ZnO areas) and because a portion of the photoelectrons from Zn can still be detected (the escape length is larger than the thickness of Al patches in some Al-covered areas). XPS spectra also indicate that, while Al gets completely oxidized at 600oC (peak at 75.0 eV), the progression is slow, starting above 300oC and really increasing above 500oC. This observation is consistent with the work of Tong et al,21 who reported partial oxidation of Al layers slightly below 400oC and complete oxidation at 600oC. Finally, we note a slight shift (0.2 eV) of the Zn core level towards the higher binding energy and a corresponding opposite shift (-0.3 eV) of the Al 2p core level after the sample annealed at 600 °C (dark red curve in Figure 6). These shifts are consistent with the formation of an alloy composed of Zn, Al and O.36 III. Elemental distribution in films in the low temperature regime (30-150oC) In order to further understand the distribution of different elements in the interface layer, LEIS spectra are collected before and after annealing at 150oC as a function of Ar+ sputtering time, as


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summarized in Figure 7(a-e). These spectra provide a depth profile of the near surface composition of the “bilayer” material. Without any sputtering (a), Zn atoms are detected after Al deposition (top black spectrum) as discussed above. However, the Zn signal completely disappears after annealing at 150oC. This indicates that reduced surface Zn atoms evaporate at 150oC. This conclusion is consistent with the fact that the vapor pressure of Zn increases from 4.28×10-14 Torr at RT to 9.33×10-8 Torr at 150oC, and is much higher than that of Al (4.02×10-50 Torr at RT and 3.66×10-34 at 150oC). Importantly, the vapor pressure of Zn at 150oC is higher than the UHV chamber pressure of ~2×10ିଵ଴ Torr. It is therefore reasonable that elemental Zn can desorb from the surface at 150oC during the 5 minute anneal. Note that no oxygen is detectable, yet the Al signal decreases slightly after 150oC anneal. This is possibly related to the adsorption of a small amount of carbon or hydrogen to the surface during annealing as very little is needed to shadow signals in LEIS. Such small amount of carbon or hydrogen contamination likely comes from sample holder degassing and from the background H2 gas in the UHV chamber (pumped with a turbomolecular pump).37 Upon sputtering, more Zn atoms are detected at room temperature. Since in principle surface Zn atoms should be sputtered off, there are three possible reasons for this increase: 1) the surface is inhomogeneous and mild sputtering can uncover ZnO regions, and 2) preferential sputtering will allow more Zn to remain on the surface than Al and O atoms, and 3) surface Zn atoms are intermixed with the Al bulk arising from energetic Ar+ ion bombardment. Although all mechanisms could be expected, support for mechanism 1) is found from the spectrum obtained after 60 s sputtering with the appearance of an O signal, consistent with the uncovering of ZnO regions. This eliminates the possibility that the relative increase of the Zn atoms originates from preferential sputtering of Al atoms.

Furthermore, while an intermixing effect as described in


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mechanism 3) coupled with preferential sputtering could be expected to give rise to a relative increase in the Zn peak relative to the Al peak, such effects could not give rise to an absolute increase in the Zn LEIS signal, which directly indicates a higher concentration of surface Zn atoms. We can therefore conclude that the primary contribution to the increased Zn signal in the spectra from the sputtered surface is the exposure of previously Al-covered regions of the ZnO surface.

A quantification of the area not initially covered by Al will be done in the next section

(discussion).

Discussion The results presented above show that upon arrival on the ZnO surface, Al atoms scavenge the oxygen from ZnO to form Al-O bonds, a mechanism that has been pointed out by Campbell on energetic grounds,38 thus reducing the Zn atoms that are subsequently expelled to the surface. However, the interface is highly inhomogeneous, with regions without Al coverage. Only when the average thickness of Al reaches 0.8 nm, as determined by quartz-crystal calibration of the Al flux (i.e. equivalent thickness of metallic Al only), can some metallic Al atom coexist, as detailed in supplementary information (Figure S5). Since the interface is inhomogeneous, these metallic Al atoms are probably sitting on Al2O3 regions that are much thicker than 2 nm, underscoring the strong driving force to scavenge oxygen. At all coverages studied (i.e. up to 11 nm average thickness), there is Zn at the surface and the amount increases with average Al thickness. The fundamental issue associated with the above picture is the mechanism that drives the surface inhomogeneity. To address this point, we need to quantify the surface composition and structure.


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I. Interface formation between ZnO and Al and low temperature anneal Upon Al deposition, the IR absorption spectra are characterized by very broad absorption bands in the Al-O stretch region, pointing to a highly inhomogeneous environment, which can include a variety of phases such as Al2O3, ZnAl2O4 and other possible compositions. In general, the formation of a uniform crystalline layer (e.g. ZnAl2O4) requires higher temperature, so we believe that all these chemical phases do not have long-range structures or chemical stoichiometry. This lack of homogeneity may be related to the rough nature of the ZnO surface, due to a preferentially columnar growth even when ALD is used. Even for thin films (~200 Å thick), our measured RMS roughness is ~8 Å, which implies that the peak-to-peak variations are > 1.5 nm. Since these variations occur on the nm scale, this structure will influence the uniformity of deposited Al because the angle of incidence of the Al flux is around 30 degrees with respect to the sample surface normal. To illustrate this point, we present in Figure 8 a possible scenario that, while not proven, is at least consistent with the observations: schematically, Al adsorbs in sub-nm patches on the exposed side of nanostructures without forming a continuous film, even after ~11 nm average thickness is deposited. Recall that, upon reaction with ZnO, the Zn atoms that are reduced most likely migrate to the edges of the Al2O3/Al nanopatches, as schematically drawn in the right inset of the second panel. Some Zn atoms may also migrate on top of the Al nanopatches. The last panel emphasizes that, even after larger Al deposition, the Al patches may still not coalesce, even though their individual thickness is more than 10 nm. As more Al is deposited, more ZnO gets reduced as evidenced by a loss of ZnO phonon absorption at ~580 cm1

and stronger TO and LO Al2O3 phonon absorption in the IR spectra.


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When the surface is sputtered by Ar+ ions, more Zn from the ZnO regions is exposed and detected by LEIS because the edge of Al islands is thinner and easier to be sputtered by the ions, thus exposing the ZnO substrate. This is the reason why it is impossible to remove the Zn on the surface completely by Ar+ ion sputtering (Figure 7). This conclusion is supported by the fact that O becomes detectable as well in the LEIS spectra. When this system is annealed to 150°C, the remaining reduced Zn atoms desorb from the surface and the adjacent Al atoms migrate to bond with exposed oxygen atoms, resulting in a more complete coverage of the surface, although Al cannot fully cover the surface. II. Impact of an alumina barrier layer Figure 9 suggests that Zn reduction could be prevented by first covering the ZnO surface with a thin Al2O3 film, which can easily be deposited by ALD using trimethylaluminum (TMA) and water (H2O) prior to Al deposition. We have performed such a test by depositing 2 nm Al2O3 (20 cycles of TMA and H2O), and collecting LEIS spectra after Al deposition and after a step-wise anneal of the sample. Figure 9 shows that, in contrast to the results presented above, no Zn is detected in the LEIS spectra collected after 9 nm Al deposition on this 2 nm Al2O3 buffer layer. This shows that a thin ALD Al2O3 layer prevents any reduction of ZnO and therefore the ejection of Zn towards the surface. Figure 10 shows the LEIS recorded as a function of annealing to 600oC. In this case, the oxidation of the metallic Al layer is not detected until a 400oC anneal; the LEIS spectra show clearly that O atoms cannot be detected until the sample is annealed to 400oC. This result is consistent with previous studies stating that a well-formed ALD Al2O3 layer between the oxide and the fuel increases the thermal stability of the system and increase the reaction temperature.39 Conclusion


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The interface between ZnO after Al deposition has been characterized with a combination of IR absorption, XPS and LEIS measurements in a UHV cluster tool. The main finding is that this interface is highly inhomogeneous with discontinuous Al patches, leaving ~10% of the ZnO surface uncovered even after deposition of an equivalent of ~11 nm Al. Upon deposition, Al reduces ZnO, releasing Zn to the surface, and this process continues as more Al is deposited. The reduced surface Zn atoms readily desorb at 150oC. The as-deposited natural interface is shown to be thermally stable to 300oC, a temperature at which Al is further oxidized. Above 600oC, an AlZnO alloy forms. These findings make it possible devise ways to prevent interdiffusion, such as the deposition of a thin Al2O3 layer on ZnO prior to Al deposition, providing routes to monitor interfacial layering and properties for a wide range of applications. This work is particularly relevant for the community of energetic materials because it provides a means to engineer the interfacial barrier layers in nanothermites.

FIGURES Figure1. a) XRD pattern; (b) IR absorption spectrum referenced to the initial oxidized Si substrate; (c) Zn 2p core level spectrum; and (d) O 1s core level spectrum of a 20 nm-thick ALD ZnO film.


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a)

b)

c)

d)


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Figure 2. Differential FTIR spectra of different thicknesses Al layers deposition on ZnO, referenced to the initial ZnO film. The inset shows the amounts of Al-O in FTIR spectra and they are calculated using the integrate area of the Al-O peaks.

Figure 3. Al 2p core level spectra from different thicknesses Al layer deposition on ZnO. The inset shows the concentration of metallic Al and oxidized Al as a function of Al thickness.


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Figure 4. LEIS spectra of different thicknesses Al deposited on ZnO.

Figure 5. LEIS spectra of 11 nm Al deposition on ZnO and stepwise annealing to 600oC.


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Figure 6. XPS spectra of Al 2p core level (top) and Zn 2p core level (middle) of 11 nm Al deposition on ZnO and stepwise annealing to 600oC.The bottom figure shows the integrated areas of Al 2p core level and Zn 2p core level peaks as a function of annealing temperature.


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Figure 7. LEIS spectra of as deposited 9 nm Al on ZnO as a function of Ar+ ion sputtering (5s, 15s, 30s, 60s, and 180s) shown black; and after 150°C for 5 min shown in red.


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Figure 8. Schematic mechanism of Al deposited on ZnO. Step 1 indicates the ZnO sample and the position of the Al e-beam source. Step 2 illustrates the situation when a small amount of Al is deposited on ZnO. In step 3, the Al nanopatches grow with more Al deposition on ZnO.


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Figure 9. LEIS spectra of 11 nm Al deposition on ZnO (black) and 9 nm Al on Al2O3(2 nm)ZnO (red). (Different thicknesses are used to maintain the total Al amount on ZnO).

Figure 10. LEIS spectra of 9 nm Al deposition on Al2O3 (2 nm)-ZnO and stepwise annealing to 600oC.


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Table 1. Integrated areas of Zn 2p peak of metallic and oxidized Zn before and after Al deposition and after low temperatures anneal.

Integrated area Integrated area of metallic Zn of oxidized Zn ZnO

0

77266.3

Al on 1742.6 ZnO

13705.7

100°C

3173.4

15761.0

150°C

0

15413.9

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org and form the authors. QCM calibration details. Initial characterization of ZnO by AFM (Figure S1). Surface atoms information of initial stage of Al deposition on ZnO by LEIS (Figure S2). XPS studies on C 1s measured on the starting ALD-grown ZnO film (Figure S3). Thermal evolution of ZnO in the low temperature regime shown by XPS (Figure S4). XPS studies on Al 2p during the initial stage of Al deposition (Figure S5). AUTHOR INFORMATION


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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 Funding Sources This work was initially supported by the ANR grant (CIREN 411531) and NSF-DMR (1312525), under the umbrella of the CNRS associated laboratory (LIA-ATLAB), and later by the chaire d'attractivité MUSE" from the Université Fédérale de Toulouse.

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