Evolution of Crystal Structure During the Initial ... - ACS Publications

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Evolution of Crystal Structure During the Initial Stages of ZnO Atomic Layer Deposition R. Boichot,† L. Tian,‡ M.-I. Richard,§,⊥ A. Crisci,† A. Chaker,‡ V. Cantelli,‡,# S. Coindeau,† S. Lay,† T. Ouled,§ C. Guichet,§ M. H. Chu,¶ N. Aubert,¶ G. Ciatto,¶ E. Blanquet,† O. Thomas,§ J.-L. Deschanvres,‡ D. D. Fong,*,∥ and H. Renevier*,‡ †

Univ. Grenoble Alpes, SIMAP, F-38000 Grenoble, France, and CNRS, SIMAP, F-38000 Grenoble, France Univ. Grenoble Alpes, LMGP, F-38000 Grenoble, France, and CNRS, LMGP, F-38000 Grenoble, France § Aix-Marseille Université, CNRS, Université de Toulon, IM2NP UMR 7334, F-13397 Marseille, France ⊥ ID01, European Synchrotron Radiation Facility, F-38043 Grenoble, France ¶ Synchrotron SOLEILBeamline SIRIUS, L’Orme des Merisiers, Saint-Aubin, F-91192, Gif sur Yvette, France ∥ Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: A complementary suite of in situ synchrotron X-ray techniques is used to investigate both structural and chemical evolution during ZnO growth by atomic layer deposition. Focusing on the first 10 cycles of growth, we observe that the structure formed during the coalescence stage largely determines the overall microstructure of the film. Furthermore, by comparing ZnO growth on silicon with a native oxide with that on Al2O3(001), we find that even with lattice-mismatched substrates and low deposition temperatures, the crystalline texture of the films is dependent strongly on the nature of the interfacial bonds.

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the film.7 However, these properties can be very difficult to predict a priori, because they are dependent on a variety of growth parameters.8 This has led to several recent studies in which researchers employ new in situ tools able to penetrate into the reactive ALD environment and directly probe growth behavior.9−16 Here, we report a detailed study on the evolution of crystalline texture and strain during the initial growth of a binary oxide by ALD, utilizing a custom-built chamber that permits use of a variety of in situ synchrotron X-ray techniques. ZnO, a semiconducting oxide with a wide array of important optical, electronic, and electromechanical properties,17−19 was chosen as a model system.20 We compare its growth behavior on two different types of substrates, Si(001) with its native oxide (a-SiO2), and Al2O3(001) (c-Al2O3), focusing our studies on the first 10 cycles of deposition. Furthermore, we investigate the use of N2O rather than H2O as the oxygen source, as it is much less studied21 and can provide more reactive atomic oxygen at elevated temperatures, readily decomposing to N2 and O.22

INTRODUCTION The great utility of atomic layer deposition (ALD) lies in both its high-precision thickness control and the ability to uniformly coat structures of arbitrary geometry, even those with nanoscale features. This has made it useful for a variety of applications, including high permittivity dielectrics for semiconductors,1 heterogeneous catalysis,2 and solid oxide fuel cells.3 The subnanometer thickness control comes from the cyclical nature of the growth process, in which different precursors are sequentially pulsed into a deposition chamber, where they saturate and chemisorb onto the surface before being purged from the system. The substrate temperatures are kept low enough to inhibit decomposition of the precursor but high enough to prevent uncontrolled condensation. A common model of the ALD process is monolayer-bymonolayer growth. However, in the initial stages, ALD films do not typically grow in a monolayer-by-monolayer fashion, but rather with the nucleation and growth of nanoscale islands.4 Depending on the application, this initial stage can be of considerable importance in that the seed layer can determine the overall properties of the resulting film, particularly if the final thickness is on the nanometer scale. For example, in ZnO thin films, a particular crystallographic texture may be beneficial for gaining optimal piezoelectric behavior5 or enhanced velocities in surface acoustic wave devices,6 and the photoluminescence properties can be determined by the strain within © XXXX American Chemical Society

Received: October 31, 2015 Revised: January 4, 2016

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DOI: 10.1021/acs.chemmater.5b04223 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

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Automated Crystal Orientation Mapping (ACOM) was conducted using a step size of 1 nm.28

EXPERIMENTAL SECTION

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The ZnO films were grown in an ALD reactor custom built by STIGMA23 and equipped with Kapton and polyether ether ketone (PEEK) or beryllium windows for in situ synchrotron X-ray measurements (Figure 1). The reactor was mounted onto the tower

RESULTS X-ray Fluorescence. Figures 2a and 2b show the Zn Kα fluorescence signal during the first 10 cycles of growth for the a-

Figure 1. Schematic representation of the ALD reactor built for in situ synchrotron X-ray studies. As shown, the reactor has X-ray transparent windows for X-ray scattering and spectroscopy with the sample in horizontal scattering geometry. The chamber maintains several ports for ALD (and potentially metal−organic chemical vapor deposition) gas injection and is mounted on a counter-rotating flange to permit sample rotation but prevent rotation of the reactor body.

of a heavy-duty diffractometer (Newport) located at the SIRIUS beamline of the SOLEIL synchrotron, using a counter-rotating flange to allow sample rotation but prevent rotation of the reactor body. Detectors included an X-ray pixel area detector (XPAD) to collect the scattered X-ray signal and a four-element silicon drift detector (SDD) for X-ray fluorescence mounted 30° above the surface plane. The zinc precursor was diethylzinc (DEZn), with argon as the carrier gas, and N2O was the oxidant. The pulse times for DEZn and N2O were 1 and 10 s, respectively, and nitrogen was used to purge the chamber for 15 s between pulses. For this study, X-ray measurements were conducted in nitrogen after completion of the N2O pulse; measurement times were ∼1 h. The chamber was maintained at a pressure of 10 mbar, and growths were performed at a substrate temperature of 250 °C, which was previously determined to be within the ALD growth window. Both the silicon (Sil’Tronix ST24) and Al2O3(001) (Kyocera25) substrates were received as 50.8 mm wafers with epi-polished surfaces. They were cleaned prior to deposition in a 1:4 mixture of 98% H2SO4:33% H2O2 (piranha solution) for 15 min. After this treatment, the substrate surfaces are expected to be fully hydroxylated and stable in air and primary vacuum to temperatures above 250 °C.26,27 The substrates were annealed at 250 °C for 1 h in argon prior to deposition. After each ALD cycle from 1 to 10, a series of X-ray measurements were conducted with different photon energies. X-ray scattering and fluorescence measurements were performed at 9.4 and 10 keV, respectively; X-ray absorption near-edge structure (XANES) spectroscopy was performed across the Zn K-edge at 9.66 keV. After the first 10 cycles were completed, another 190 cycles of growth were performed, and the resulting films were characterized by a variety of in situ and ex situ techniques. In particular, X-ray diffraction, reflectivity, and pole figures were measured on a Rigaku SmartLab, and crosssectional transmission electron microscopy (TEM) was performed on a JEOL Model 2100F instrument equipped with an ASTAR system.

Figure 2. X-ray fluorescence spectra measured during ZnO growth by atomic layer deposition: Zn fluorescence yields for films grown on (a) a-SiO2 and (b) c-Al2O3 are shown. The blue-shaded regions indicate the total (DEZn + N2O) injection times for each cycle. (c) The change in Zn fluorescence intensity as a function of cycle number for both the a-SiO2 (open blue circles) and c-Al2O3 (solid red circles) substrates.

SiO2 and c-Al2O3 substrates, respectively, with the ∼1 h time intervals used for X-ray characterization omitted. The fluorescence signal measured before and after these intervals were almost identical, indicating negligible desorption of any zinc compounds at the growth temperature. The change in fluorescence yield as a function of cycle number is shown in Figure 2c. As seen, the change in fluorescence yield reaches a plateau for both substrates at the second cycle, indicating that the islands nucleate during the first cycle and coalesce during the second cycle.4 These results are in good agreement with the B


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Chemistry of Materials finding that high nucleation densities are favored by high hydroxyl coverages29,30 and relatively long injection times.31 Grazing Incidence X-ray Diffraction. We performed inplane X-ray diffraction (XRD) measurements after each cycle of growth. Figure 3 displays radial in-plane q∥ scans measured at

Figure 4. Schematic diagram of the qz ≈ 0 plane in reciprocal space for ZnO/c-Al2O3. The radial scans shown in Figure 3b and azimuthal ωscans shown later in Figure 6b are indicated by the dotted arrow and arc, respectively. Radii for q∥ = 2.23, 2.54, and 3.86 Å−1 are also displayed, which correspond to 2π/d100, 2π/d101, and 2π/d110, respectively, with the latter two indicated by dashed arcs. Reflections for the Al2O3 substrate and the different textures of ZnO are colorcoded, according to the legend in the upper left.

to no grain growth occurring during the first 10 cycles. This is in agreement with Figure 5c, which shows the in-plane domain size, as determined by the Scherrer equation, D(hkl) =

Kλ β cos θ

(1)

where λ is the X-ray wavelength, β the width of the hkl reflection measured at half its maximum intensity, and θ the Bragg angle. A value of 0.89 was used for the Scherrer constant (K).32 After the second cycle, the in-plane domain size, approximated as the lateral grain size (and as confirmed by cross-section TEM), remains fixed at ∼13 nm. From X-ray reflectivity measurements, each cycle adds 0.2−0.3 nm in thickness. Interestingly, the first cycle of ZnO on a-SiO2 did not lead to any diffraction intensity from the wurtzite structure. This may stem from the formation of Zn−O−Si bonds at the interface and the distribution of Si within the amorphous aSiO2 surface. The in-plane strain (ϵ) of the growing ZnO layer can be determined from Bragg’s law:

Figure 3. In situ grazing incidence X-ray diffraction (GIXRD) radial scans run after the completion of each ZnO layer for cycles 1−10: results for (a) a-SiO2 and (b) c-Al2O3 are shown.

an incident angle of 0.5° for both the a-SiO2 substrates (Figure 3a) and c-Al2O3 substrates (Figure 3b). In the latter case, the scan was performed along the [21̅0] direction of the Al2O3 substrate; this can be seen as the dotted arrow along the radial direction in Figure 4. Unlike a previous in situ study conducted at a lower deposition temperature and using H2O as the oxidizer,11 the ZnO films grown here were crystalline in the asdeposited state. The recorded diffraction peaks match with the wurtzite crystal structure and correspond to the 100, 002, and 101 reflections. The 110 in-plane reflection was also measured in a separate scan. The intensities of the three strongest reflections, the 100, 101, and the 110, are shown as a function of cycle number in Figures 5a and 5b. The ZnO/a-SiO2 exhibits (001) fiber texture; a scan along the specular rod shows only a single reflection from the ZnO, the 002 (Figure S1 in the Supporting Information). This texture can also be observed in Figure 5a by the strong 100 in-plane reflection, which rises monotonically as a function of cycle. The intensities of other reflections, 101 and 110, stem from nontextured grains but also increase monotonically with cycle number. Since the fluorescence data in Figure 2c show saturation of the surface after the second cycle of growth, the monotonic increases in the intensities indicate that the initial nuclei serve as templates for the next layer, with little

ϵ=

d − d0 sin θ0 = −1 d0 sin θ

(2)

where d refers to the interplanar spacing. The parameters d0 and θ0 come from a stress-free reference sample, and we employ the bulk ZnO lattice constants at 250 °C.33 The Bragg positions were determined by fitting the peaks to a Voigt line shape. As shown in Figure 5e, the in-plane strain for ZnO/aSiO2 reaches an almost-constant level of biaxial tension after the second cycle. This tensile strain results from grain coalescence,16,34 where the lateral surfaces of the initially unattached islands merge together to form grain boundaries. Since the initial deposit serves to template the growth of the next monolayer, the grain boundaries remain largely perpendicular C


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Figure 5. X-ray diffraction (XRD) results from in-plane q∥ scans run after each ALD cycle. (a, b) Evolution of the integrated intensity of the 100, 101, and 110 ZnO reflections, as a function of cycle number. Since no in-plane texture was observed in the case of a-SiO2, the 100, 101, and 110 intensities have been divided by their corresponding multiplicity factors (6, 12, and 6, respectively). (c, d) In-plane domain sizes, as determined using the Scherrer equation, for the 100, 101, and 110 ZnO reflections. (e, f) In-plane strains for the 100, 101, and 110 ZnO reflections referenced to bulk ZnO at 250 °C.

to the plane of the film (confirmed by cross-section TEM below), and the strain remains flat for the first 10 cycles. This constant level of strain suggests that the amount of in-plane stress within the ZnO increases linearly with thickness, which is in agreement with initial wafer curvature measurements for ZnO growth on a-SiO2. Furthermore, the in situ X-ray absorption near-edge structure (XANES) data discussed below and post-growth ex situ XRD are compatible with ZnO films of good stoichiometry. Others have found that significant amounts of oxygen deficiency would lead to smaller lattice constants.35 After 200 cycles of growth, both in situ and ex situ diffraction scans indicate that the biaxial strain is mostly relaxed. The XRD results for ZnO/c-Al2O3 are provided in Figures 5b, 5d, and 5f. As seen, the results are nominally similar to those of ZnO/a-SiO2. However, from the pole figures discussed below, ZnO does not form randomly oriented in-plane grains on c-Al2O3; rather, the grains exhibit three distinct preferred orientations, which we label T1, T2, and T3. The intensities in Figure 5b show the texture evolution; however, although the inplane 100 intensity implies grains with (001)-oriented surfaces, as may be expected for growth on Al2O3(001), we will show that the intensity originates from a different preferred orientation. A scan along the specular rod (Figure S2 in the Supporting Information) exhibits two reflections, the 002 and the 101, with the former representative of T1 and the latter

representative of T2. Crystallites with the T3 orientation do not exhibit reflections that appear along the specular rod. These orientations and their in-plane epitaxial relationships will be discussed further below. Because of these preferred orientations, the 100 and 101 reflections in Figures 5b, 5d, and 5f show the evolution of grains with the T3 orientation, while the 110 reflection depicts the evolution of T1 and T2 (see Figure 4). Figure 5d shows that the lateral grain sizes are somewhat smaller than those for a-SiO2, with a plateau at ∼12 nm, starting from an initial grain size of ∼10 nm or less. Furthermore, the in-plane strains, shown in Figure 5f, are smaller than those for ZnO/a-SiO2 and relax even further after the fourth cycle, reaching full relaxation after 200 cycles. For bulk materials, lattice constant a of ZnO is 31.7% smaller than that of Al2O3, and its c lattice constant is ∼60.0% smaller, suggesting that ZnO should begin to relax almost immediately upon growth, as is observed here. Texture. During growth, in-plane ω-scans through the ZnO 100 in-plane reflection were measured for both the ZnO/a-SiO2 and ZnO/c-Al2O3 samples. The results are shown in Figure 6a and 6b for the a-SiO2 and c-Al2O3 substrates, respectively. The dotted arc in Figure 4 shown at a radius of 2.23 Å−1 indicates the path of the ω-scan for ZnO/c-Al2O3. The specular rod for ZnO/a-SiO2 shows the 002 reflection (Figure S1 in the Supporting Information), but there are no D


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Figure 6. In-plane ω-scans through the 100 reflection for cycle numbers 1−10 and cycle number 200 for ZnO on (a) a-SiO2 and (b) c-Al2O3. In panel (b), the dashed vertical line at ω = 0° indicates the position of the 100 ZnO peak measured during the in-plane radial scans shown in Figure 5b; another dashed vertical line is drawn at ω = −30°. The intensities for cycle number 200 have been rescaled to fit onto the plot.

preferred in-plane orientations from cycles 1 to 10, as seen by the absence of peaks in Figure 6a. The overall intensity rise in the ω-scans comes from the increasing number of in-plane scatterers with each cycle of growth. Figure 6b shows that there are strong in-plane orientation relationships for ZnO/c-Al2O3. Furthermore, the intensity profile observed at the end of cycle number 200 is similar to that for cycle number 2. However, the behavior for cycle number 1 is distinct from the others: only one peak is observed, positioned at ω ≈ −20°. As will be discussed below for ZnO/c-Al2O3, the 100 reflection at ω = 0° originates from the T3 orientation, whereas the peak at ω ≈ −30° is only from T1, as shown in Figure 4. The smaller intensities at ω ≈ −17° and −47° come from a minority component with (001) surface orientation but rotated ±15° with respect to T1. Beginning with cycle number 7 or 8, the relative populations of these different orientations appear to be determined. To better understand the preferred orientations in the ZnO films, we measured (100), (002), and (101) pole figures for both the a-SiO2 substrates (Figures 7a−c) and c-Al2O3 substrates (Figures 7d−f). These measurements were conducted ex situ after 200 cycles of growth. The ZnO film grown on a-SiO2 exhibits (001)-fiber texture. This orientation is favored by the low energy of the (001) surface when screened.36,37 In the (100), (002), and (101) pole figures, continuous rings are observed at along the outer edges at ψ = 90°, demonstrating that no in-plane texture appears for the ZnO film on a-SiO2, in agreement with the ω-scans measured during growth (Figure 6a). Since the surface of aSiO2 is amorphous, no epitaxial relationship is expected between the substrate and the film. In the case of ZnO/c-Al2O3, Figures 7d−f show well-defined features indicating a textured ZnO layer. The black squares correspond to the 104 reflections of the Al2O3 substrate. As stated above, three different preferred orientations, T1, T2 and T3, were needed to simulate the pole figures. Projections of these three orientations onto the Al2O3(001) surface are shown in Figure 8; the [100] and [010] directions of the Al2O3 substrate are indicated. Texture T1 refers to (001)-oriented

Figure 7. (100), (002), and (101) pole figures of ZnO layers grown by ALD at 250 °C on (a−c) a-SiO2 and (d, e) c-Al2O3 (see text). For ZnO/c-Al2O3, the peaks are indexed as follows: the open black squares correspond to the 104 Al2O3 substrate peaks while the open white squares (□), crosses (×), and circles (○) correspond to reflections originating from the three different types of texture occurring in the ZnO layer.

grains (i.e., with the c-axis of ZnO parallel to the c-axis of the substrate), leading to the peaks marked by the white squares in Figures 7d−f. These ZnO grains fulfill the following epitaxial relationship: (001) ZnO∥(001) Al2O3 and [100] ZnO∥[100] Al2O3 (Figure 8a). While others have also found (001) domains with an in-plane rotation of 30°, with respect to these,38−43 a significant population of such domains were not observed here, since these would lead to additional (unobserved) reflections in the (101) pole figure. Texture T2 refers to a family of (101)-oriented grains with three distinct rotational domains related by the 3-fold symmetry of the sapphire surface. The out-of-plane relationship is (101) ZnO∥(001) Al2O3, thus producing the strong 101 reflection along the specular rod (Figure S2 in the Supporting Information), and the in-plane relationships are [010] ZnO∥[010] Al2O3, [010] ZnO∥[1̅1̅0] Al2O3, and [010] ZnO∥[100] Al2O3, as shown in Figure 8b. These grains lead to the peaks marked with white crosses (×) in Figures 7d−f. A third family of grains are oriented such that the [111] direction, on average, lies along the surface normal: this texture appears to not have been previously reported. The corresponding set of planes perpendicular to the [111] are near the ZnO(115) but are slightly misoriented such that there are no reflections along the specular rod for T3. However, three wellE


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Figure 9. In situ XANES measured at the Zn K-edge for (a) ZnO/aSiO2 and (b) ZnO/c-Al2O3 after cycle numbers 1−10 and cycle number 200. The derivative spectra for panels (a) and (b) are shown in panels (c) and (d), respectively. The derivative spectrum for a Zn reference foil is shown at the bottom of panels (c) and (d).

Figure 8. Moiré diagrams showing a top-down view of the three distinct textures observed for the ZnO layers grown on c-Al2O3 (lattice in black): (a) T1 (ZnO lattice in red); (b) T2 (ZnO lattice in green), with three rotational domains; and (c) T3 (ZnO lattice in blue), with three rotational domains. The unit cell for the different coincidence superstructures are indicated.

The spectral differences for the first cycles are particularly evident for ZnO/a-SiO2, which exhibits a broader white line and a singular peak (instead of two) at an energy of 9727 eV. Furthermore, the derivative spectrum shows a much smaller peak at 9670 eV. As for the first cycle of ZnO on c-Al2O3, similar differences are observed, but they are less pronounced. Preliminary ab initio simulations of the XANES of the first cycle for ZnO/a-SiO2 (not shown) suggest that these modifications may be related to the film,not having fully developed local order in the second atomic shell around Zn. This may underlie the lack of wurtzite diffraction peaks for the first cycle in Figure 5a. TEM. Ex situ cross-sectional TEM images of the ZnO layers after 200 cycles of growth are shown in Figure 10a and Figures 10b and 10c for ZnO/a-SiO2 and ZnO/c-Al2O3, respectively. For ZnO on a-SiO2, the image confirms the presence of a continuous amorphous silica layer ∼1 nm thick at the surface of the substrate. The film consists of grains ranging from 15 nm to 25 nm in size, along with a rough surface. The ZnO layer deposited on c-Al2O3 shows similarly sized grains (∼15−20 nm in size), but with a smoother surface. Orientation mapping, performed on ∼50 ZnO crystallites, confirms that most grains adopt an epitaxial relationship with the c-Al2O3 substrate, some belonging to the T1 and T2 families described above. The images also confirm columnar microstructures for both ZnO films.

defined in-plane orientations exist for this texture: [11̅0] ZnO∥[1̅20] Al2O3, [11̅0] ZnO∥[1̅1̅0] Al2O3, and [11̅0] ZnO∥[21̅0] Al2O3, as depicted in Figure 8c. These grains produce the reflections indicated by the open white circles in Figures 7d−f. As seen, however, these reflections are seen to occur as doublets with roughly half the reflections misoriented by approximately 4° and the other half misoriented by approximately −4°, although orientations between these values are also observed. Most of the misorientation is in-plane (i.e., rotational domains), but a small amount of out-of-plane tilt is also present. Either way, the T3 domains follow the 3-fold symmetry of the Al2O3 surface. XANES. Figure 9 shows the Zn K-edge XANES spectra taken at the end of each of the first 10 cycles and after 200 cycles for epilayers grown on both a-SiO2 (Figure 9a) and cAl2O3 (Figure 9b). The measurements were conducted in fluorescence detection mode at an incidence angle of 1.3°. Since the surface was almost horizontal, the X-ray polarization was such that the XANES measurements were more sensitive to the orbitals oriented parallel to the surface plane. The derivatives of the absorption spectra are shown in Figures 9c for the a-SiO2 samples and Figure 9d for the c-Al2O3 samples. The derivative spectrum for a Zn reference foil is shown at the bottom of these panels. From an examination of the derivative spectra, there is an ∼2 eV shift between the K-edge of the reference foil and all of the ZnO layers. This 2 eV shift corresponds well with a +2 oxidation state,44,45 suggesting that all of the layers, including the first, consist of stoichiometric ZnO. The XANES spectra for the first layers are different from the rest, as discussed below. However, from cycle number 2 onward, all of the spectra are similar and match the shape expected for bulk ZnO. This suggests that, starting from the second cycle, the epilayers are already locally ordered with well-defined orbital distributions, in agreement with the wurtzite structure.

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DISCUSSION For well-hydroxylated surfaces and relatively long pulse durations,31 the first cycle of ALD leads to high nucleation densities on both a-SiO2 and c-Al2O3. The ZnO islands on cAl2O3 already exhibit the wurtzite structure and show in-plane epitaxial alignment (Figures 3b and 6b). The islands coalesce during the second cycle for both substrates, as evidenced by the fluorescence spectra in Figure 2c. During coalescence, the ZnO grains on a-SiO2 form the wurtzite structure, with preferred (001)-oriented surfaces. Furthermore, island coalescence leads to in-plane biaxial tension (Figures 5e and 5f). F


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Figure 11. Schematic diagram showing side views of the three dominant textures observed for ZnO/c-Al2O3 for (a) T1, (b) T2, and (c) T3. The high index surface for T3 is pictured with (001) facets for illustrative purposes.

grains exhibit random in-plane orientations, while, for Al2O3, only one in-plane orientation (or domain) is preferred for T1. This domain aligns the [100] directions for both ZnO and Al2O3 and has been observed in other ALD studies.43 For ZnO deposition at higher temperatures, other rotational domains have been observed, mainly one rotated around the [001] axis by ±30°, with respect to this one.38−42 The coincidence superstructure for T1 shown in Figure 8a exhibits 6-fold symmetry. Indeed, the open white squares in the (100) and (101)-pole figures (Figures 7d and 7f) are consistent with this symmetry. From these pole figures, we can reconstruct the inplane reciprocal space diagram, as was shown in Figure 4 with the T1 Bragg reflections shown as solid red circles. A scan along the specular rod for ZnO/c-Al2O3 shows both the 002 and 101 reflections (Figure S2), the first from T1 and the second from T2. The (101) surface is also polar, as shown from the side view of T2 in Figure 11. While this orientation has been observed by others,37,43,49 it appears to disappear with annealing at higher temperatures. Nevertheless, this texture is stable at 250 °C and also exhibits in-plane alignment with the Al2O3 surface, with three rotational domains, as shown in Figure 8b. These results stem from the (100), (002), and (101) pole figures in Figures 7d−f, where the 3-fold symmetry of the domains are made evident by the white crosses (×); the streaks through the crosses show deviations from perfect in-plane alignment. The corresponding in-plane reflections are shown by the open green circles in Figure 4, where the solid, dashed, and dotted circles distinguish between the three different rotational domains. The third texture, T3, has not been observed previously, and is presumably unstable with a high-index surface close to the (115). The side view of this surface is shown in Figure 11c, and as shown, it may be partially stabilized by (001) facets, although more work is required to confirm this. The direction along the surface normal is the ZnO [111], and the projection along this direction onto the Al2O3(001) surface is shown in Figure 8c. As seen, there are, again, three rotation domains, related to each other by an in-plane rotation of ±120°. The open white circles in the (100), (002), and (101) pole figures in Figures 7d−f confirm the 3-fold symmetry; furthermore, the open blue circles in Figure 4 illustrate the in-plane Bragg reflections from

Figure 10. Cross-sectional TEM images of the ZnO films deposited on (a) a-SiO2 and (b) c-Al2O3 after 200 cycles of growth. (c) Crosssectional orientation map of ZnO/c-Al2O3, with the orientations of the wurtzite unit cell indicated for each grain, as determined by the ACOM software. The step size for the map was 1 nm.

After coalescence, ZnO on a-SiO2 loses interaction with the substrate surface, and each subsequent layer templates its growth, strain, and crystal orientation from the layers below it, leading to a columnar grain structure (Figure 5c and Figure 10a) and a fixed amount of strain. The grains exhibit dominant (001)-fiber texture, a “self-texture” that is due to its low surface energy.36,37 The (001) surface is polar and terminated with either O or Zn (schematically shown with O-termination in Figure 11a), requiring screening by an electronic reconstruction46,47 and/or leftover ligands or adsorbates in the ALD environment.48 In the case of ZnO/c-Al2O3, both surface and interface energies play important roles. Prior to coalescence, the ZnO is crystalline but the in-plane island orientations differ from those post-coalescence, as shown in Figure 6b, suggesting that coalescence leads to a reduced rotational degree of freedom. Furthermore, the grains exhibit only small strains, relative to bulk ZnO, indicating that, although the grains respond to the crystal symmetry of the Al2O3 surface, the ZnO/Al2O3 interface is largely incoherent, with little evidence of any coherency strain. The average grain size, strain, and texture of the resulting ZnO film are largely determined by the layer grown during the coalesence cycle. Although other grain orientations do exist (giving rise to the weak 002 reflection in Figure 3b or the minor peaks in Figure 6b, for example), three main textures appear after the second cycle, each of which exhibits epitaxial alignment with the Al2O3 surface. As stated earlier, these are labeled T1, T2, and T3, and the projections of these crystal orientations onto the Al2O3 surface are shown in Figure 8. Our results for the ZnO/a-SiO2 film, grown under the same conditions as ZnO/c-Al2O3, indicates that ZnO grains favor surfaces with the low-energy (001) orientation; however, those G


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Chemistry of Materials the three rotational domains (solid, dashed, and dotted blue circles). Absent from this schematic but observable in the pole figures is the fact that the grains are not in perfect in-plane and out-of-plane alignment, but rather are tilted by approximately ±4°, producing doublets in the pole figures. In general, crystal orientations are dependent on many factors: surface energy, interface energy, strain, substrate termination, and the kinetics of deposition, among others.21 Although the kinetics are relatively slow at 250 °C, the textures observed for ZnO/c-Al2O3 show that there is sufficient mobility in the initial stages of growth to achieve long-range ordering and epitaxial alignments, demonstrating the importance of reducing interface energy despite the large lattice misfit. The Al2O3(001) surface has two possible terminations: a 3-fold symmetric aluminum plane and a 6-fold symmetric oxygen plane.38,40 The diffraction results for ZnO/c-Al2O3 show that the domains for the different textures follow the 3-fold symmetry of the aluminum plane. Our substrate treatment hydroxylates the surface, leading to Al−OH termination.50,51 Most likely, the Zn-(C2H5)2 reacts with the terminating OH group, leaving Al−O−Zn−C2H5 and emitting C2H6.52 The strong Al−O bond along this chain helps to lower the interface energy and has been used to explain the alignment of the T1 domains.39,43 The Moiré patterns in Figure 8 suggest that a high areal density of such bonds can be formed, but theoretical calculations will be necessary to determine the relative interfacial energies for each orientation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D. D. Fong). *E-mail: [email protected] (H. Renevier). Present Address #

Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, European Synchrotron Radiation Facility, F-38043 Grenoble, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work by ANR Moon (No. ANR-11NANO-0014) is gratefully acknowledged. V.C. was supported by the Nanosciences Foundation, and D.D.F. was supported by both an award from the Nanosciences Foundation and the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. We thank Synchrotron SOLEIL for general facilities placed at our disposal, and, in particular, the ECA group for reliable synchronization of the reactor body counter-rotation. The experiment at the SIRIUS beamline benefited from the SOLEIL beam time allocation (No. 20131343).

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CONCLUSIONS We have performed the first detailed study on the evolution of crystalline texture and strain during the initial stages of ZnO growth by atomic layer deposition. Using in situ synchrotron Xray scattering, fluorescence, and absorption spectroscopy, we observed the development of tensile strains during coalescence and found that the microstructure developed at this stage, which is sensitively dependent on the structure and chemistry of the substrate surface, determines the overall structure of the resulting film for ZnO on a-SiO2 and c-Al2O3. ZnO exhibits similar strains and grain sizes for both substrates, as neither provide good lattice match to the wurtzite structure. However, the different symmetries of the substrate surfaces lead to distinct preferred orientations. ZnO/a-SiO2 exhibits self-texture with a preference for (001) surfaces. On the other hand, ZnO/ c-Al2O3 exhibits three dominant textures with in-plane rotational domains. Even with limited surface kinetics, the domains follow the 3-fold symmetry of the underlying substrate, and the aluminum plane in particular. This work demonstrates the unique power of in situ synchrotron methods for elucidating the atomistic processes taking place during the initial stages of ALD. As shown here, a complementary suite of tools is often necessary to gain a deep understanding of growth behavior and may be essential for the investigation of more-complex systems, such as the many varieties of ZnO nanostructures53 or hierarchical layers.54 We expect that such in situ studies will lead to much improved control over the ALD process, and ultimately the ability to tailor the properties of nanostructured materials atomically.

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Results from the out-of-plane specular rod scans for the ZnO/a-SiO2 and ZnO/c-Al2O3 are provided in the Supporting Information (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04223. H


Article

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