Realization of Spatially Addressable Library by a Novel Combinatorial

Research Article Cite This: ACS Comb. Sci. XXXX, XXX, XXX−XXX

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Realization of Spatially Addressable Library by a Novel Combinatorial Approach on Atomic Layer Deposition: A Case Study of Zinc Oxide Harrison Sejoon Kim,† Joy S. Lee,† Si Joon Kim,‡ Jaebeom Lee,† Antonio T. Lucero,†,⊥ Myung Mo Sung,§ and Jiyoung Kim*,† †

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Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States ‡ Department of Electrical and Electronics Engineering, Kangwon National University, 1 Gangwondaehakgil, Chuncheon, Gangwon-do 24341, Republic of Korea § Department of Chemistry, Hanyang University, Seoul 04763, Republic of Korea S Supporting Information *

ABSTRACT: Though the synthesis of libraries of multicomponent metal oxide systems is prevalent using the combinatorial approach, the combinatorial approach has been rarely realized in studying simple metal oxides, especially applied to the atomic layer deposition (ALD) technique. In this literature, a novel combinatorial approach technique is utilized within an ALD grown simple metal oxide to synthesize a “spatially addressable combinatorial library”. The two key factors in gradients were defined during the ALD process: (1) the process temperature and (2) a nonuniform flow of pulsed gases inside a cross-flow reactor. To validate the feasibility of our novel combinatorial approach, a case study of zinc oxide (ZnO), a simple metal oxide whose properties are well-known, is performed. Because of the induced gradient, the ZnO (002) crystallite size was found to gradually vary across a 100 mm wafer (∼10−20 nm) with a corresponding increase in the normalized Raman E2/A1 peak intensity ratio. The findings agree well with the visible grain size observed from scanning electron microscope. The novel combinatorial approach provides a means of systematical interpretation of the combined effect of the two gradients, especially in the analysis of the microstructure of ZnO crystals. Moreover, the combinatorial library reveals that the process temperature, rather than the crystal size, plays the most significant role in determining the electrical conductivity of ZnO. KEYWORDS: spatially addressable library, combinatorial materials science, temperature gradient, atomic layer deposition (ALD), zinc oxide (ZnO), crystallinity

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laser deposition (PLD), 16−22 molecular beam epitaxy (MBE),23−28 and sputtering systems29−33 have extensively adopted the combinatorial approach and implemented such combinatorial processing capabilities in commercialized tools (e.g., PVD Products, Inc.). Achieving multicomponent composition spread by introducing sample without rotation, displacement of a physical patterned mask, or a combination of these methods in sputtering,34 PLD,35 and MBE36 systems provides a method of synthesizing combinatorial libraries in PVD systems.1−3,37 Furthermore, a continuous-compositionalspread (CCS) technique, based upon cosputtering from offaxis sources in a sputtering system29,38−44 or coablating from

INTRODUCTION

The combinatorial approach is invaluable because of its ability to synthesize libraries of compositions, crystalline phases, electrical properties, etc., within a single experiment.1−5 The combinatorial approach largely aids researchers in investigating a vast amount of material properties, even when characterizing inorganic thin films.6−9 Exploring the effect of selected variables in greater detail, while all other processing parameters are kept constant by design, is a tremendous advantage of the “combinatorial approach” to conventional “single-sample approaches” as individual “single-sample” processes can involve unintentional process variability. Recently, the combinatorial approach for physical vapor deposition (PVD) grown materials garnered attention because of its expeditious characterization through the synthesis of “libraries” of a novel material’s properties.10−15 Notably, pulsed © XXXX American Chemical Society

Received: January 7, 2019 Revised: May 5, 2019 Published: May 7, 2019 A

DOI: 10.1021/acscombsci.9b00007 ACS Comb. Sci. XXXX, XXX, XXX−XXX

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ACS Combinatorial Science

Figure 1. (a) Schematic drawing of cross-flow reactor system (Savannah 100 ALD reactor) used in this study. (b) Schematic diagram of two gradients given into the ALD reactor without any thickness deviation: (1) process temperature and (2) distribution of flow of pulsed gases inside a cross-flow reactor. (c) Combinatorial library that generates various information about the material.

two different targets in a PLD system,7,16,19,45−47 was also introduced to synthesize combinatorial libraries. The combinatorial approaches mentioned above, however, prevalently discuss the synthesis of pseudobinary oxides (e.g., BaxSr1−xTiO3 from SrTiO3 and BaTiO3) or pseudoternary oxides (e.g., thin film constituted from In0.95Sn0.05Ox, zinc oxide, and yttrium-stabilized zirconium oxide) phase diagrams.46,48 A combinatorial approach on simple metal oxides, rather than on pseudobinary or pseudoternary species, has not yet been widely explored within PVD systems. Similar approaches can be found in case of combinatorial approaches in chemical vapor deposition (CVD) systems; studies are limited to studying the compositional spread of pseudobinary or pseudoternary systems by employing the two distinct configurations. Combinatorial CVD systems are configured to (1) provide gradients in precursor impinging rate that produces pseudobinary systems49,50 or (2) physically separate the source precursor inlet line to create spatially addressable libraries of pseudoternary systems.51−53 The use of the combinatorial approach to investigate properties, other than compositional spreads, has yet to be explored, particularly for ALD. ALD has been frequently considered an ideal selflimiting surface process for thin films for processes within typical ALD conditions, that is, grown within the ALD window. For such films, chemical and material properties have been assumed to remain nearly constant. However, we have found that significant variations in the material properties, namely, in the film microstructure, can be found even for processes lying within the ALD window. Such findings have motivated our investigation into the material properties of films grown within the ALD window. To do so, we provide insight into utilizing the novel combinatorial approach in ALD. A process temperature gradient and an accompanying gradient in the pulsed precursor distribution are the critical variables in synthesizing this combinatorial library. Along with clarifying the chemical process effects (i.e., by gradually varying process temperatures), studying the effect of the distribution of precursor gas is also significant from a physical process point of view.54−57 To validate this novel system, zinc oxide (ZnO) is adopted as a prototype material as its various material properties are already well-known as a function of a number of parameters within ALD systems, thus, allowing systematic comparison.58−66 We have found our combinatorial approach effectively provides access to a spatially addressable combinatorial library whose microstructural differences can be investigated with relation to its electrical conductivity. In addition to synthesizing combinatorial libraries, it is also important to characterize “libraries” rapidly with high spatial

resolution. Hence, we demonstrate the reliability of Raman spectroscopy as a high-throughput characterization tool to efficiently estimate the microstructure of ZnO.67−70 Lastly, the effect of ALD process temperature and crystallinity of the ZnO on correlating electrical conductivity have been studied.

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EXPERIMENTAL DETAILS Synthesis of Combinatorial Library Using Atomic Layer Deposition System. A combinatorial library of ALDgrown ZnO-containing material properties dependent on both process temperature and the non-even flow of the travelingwave of precursors is built using a Cambridge NanoTech Savannah 100 ALD system. The Savannah 100 ALD reactor is a cross-flow reactor system in which gases are introduced by convective gas flow.54,55,71 The reactor pathway consists of a precursor cylinder, pneumatic valves, precursor/carrier gas manifold, chamber, outlet manifold, and a vacuum pump. The 100 mm susceptor, where the substrate is placed, is embedded in a 150 mm reactor chamber. The susceptor can be heated from room temperature to 400 °C, and the ZnO samples used in this study were deposited at temperature of 250 °C for the heating module of the susceptor and an outer wall temperature of 200 °C. The height, from susceptor to the lid of the reactor, is designed to be ∼5 mm as to maximize the rate of flow of gases, within a sub-100 ms range, with help from convective gas flow.71 Within the cross-flow reactor chamber, a plate of glass (3 mm, Fisher Scientific) was placed beneath one edge of the wafer substrate to create an angle of the substrate to the susceptor. This allows for a reproducible process temperature gradient across the wafer with the high temperature end near the substrate heater (225 °C) and a low temperature near the hot wall of the reactor (200 °C). The configuration is schematically shown in Figure 1a. A 100 mm thermocouple wafer (Thermo Electric) was used to calibrate the actual wafer temperature over different 9 points (Figure S1). Gas flow is driven parallel to the surface of the angled substrate via an inlet and exhaust hole. By pairing gradients of both the process temperature and distribution of the traveling-wave of precursors, we were able to achieve combinatorial libraries that contain information on not only the heavy dependency on the thermal and chemical processes of surface conditions but also the fluid flow and local molar fraction of the pulsed precursors that makes gradually varying distributions inside the reactor under nonsteady state.54,55,72 Figure 1b schematically shows that such two gradients given during the ALD process have been successfully achieved, while total film thickness is maintained constant since the process temperature range lies within the ZnO ALD window (ALD window for ZnO lies B


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Figure 2. Raman Effect in ALD grown wurtzite ZnO. (a) Raman spectrum of Wurtzite ZnO. ZnO is grown by ALD on top of each substrate (SiO2/Si vs Raman substrate) to compare the Raman background signal. (b) Series studies of process temperature shows that intensities of the E2 (high) peak in the Raman spectra are process temperature dependent. (c) Raman spectrum from five different points over 100 mm wafer. Red solid triangles indicate ZnO peaks while green solid circles refer to the Si substrate peaks.

taken at a laser power of ∼0.1 mW using a 25.00 s exposure with 20 accumulations. Initial Raman spectra of ALD ZnO, grown at 210 °C, using SiO2 on Si as substrate (navy dashed line in Figure 2a) were overwhelmed by an intense background signal from the single crystalline Si that hindered investigation of the ALD thin film properties,60,75,76 and the issue was unavoidable even with glass substrate, the most commonly used substrate for characterizing material using Raman spectroscopy (Figure S3). To enable Raman spectroscopy, we prepared an Al2O3/Ni/SiO2/Si substrate (so-called Raman substrate in this literature) for wafer scale combinatorial Raman spectroscopic studies.76 The Raman substrate was created by depositing 300 nm of thermally grown SiO2 on a Si (100) wafer, followed by deposition of 300 nm Ni by e-beam evaporation at room temperature and growth of 50 nm ALD Al2O3 at 300 °C using TMA/H2O. Amorphous Al2O3 is grown on polycrystalline Ni not only because it makes the surface smooth74 but also because it acts an an optical insulator with charge gap exceeding the energy of the visible light photon (∼9 eV, wide band gap), resulting in negligible background signal for Raman. Figure 2a shows that the Raman substrate (orange line) does not display any strong background between 400 and 1200 cm−1 and shows that the Raman substrate does not mask any excitation that comes from the ZnO film when compared to the Si substrate (navy dashed line). Following deposition of the ALD ZnO film, the crystallinity of the film was characterized by XRD (Rigaku SmartLab) analysis. The Rigaku SmartLab was equipped with a Cu Kα (1.541 Å) X-ray source, and the θ/2θ spectra was taken in the 2θ range of 20− 90°. The X-ray diffractometer was configured with a 5.0° Soller slit for both incident and receiving parts. The measurement speed was fixed at 1 deg/min. Visual analysis of the ALD ZnO crystallinity was confirmed using field-emission SEM (Zeiss Supra-40) with an electron beam power of 10 keV. Electrical analysis of the ALD ZnO was conducted through 4-point probe (Alessi manual 4-point probe) measurements performed at R.T. The probe employs a Cascade C4S probe head with 1 mm spacing between tungsten carbide probe tips. The metering was provided through a QuadTech LR2000 digital milliohm meter. The 100 mm combinatorial library was then cleaved into 8 sections of 10 mm × 10 mm. A van der Pauw pattern with four symmetrical electrodes was defined on each

between 170 and 220 °C).73 The ALD window is the temperature regime in which an ALD process does not suffer from any kind of condensation, incomplete surface reactions because of low reactivity or decomposition, or desorption issues due to high thermal energies.74 A uniform thickness of the ALD ZnO (42 nm ±1 nm) was achieved across the 100 mm wafer (Figure S2). Thus, analysis of the combinatorial library can be conducted without regard to any thickness effect within the given gradient system. The resulting sample processed through these two gradients produces a spatially addressable combinatorial library with high spatial resolution from which material properties of ALD ZnO varying according to influences from the gradients can be investigated as shown in Figure 1c. With the substrate in place, ALD growth of the ZnO film is conducted under the following conditions: Diethylzinc (DEZ, ≥ 52 wt % Zn basis, Sigma-Aldrich) and deionized water (H2O) were used as Zn precursors and oxidant, respectively. Argon (Ar, 99.9999%, Airgas) served both as a carrier and a purging gas at a rate of 20 sccm. Ar gas was flown continuously through the process reactor for all ALD runs. DEZ and H2O were evaporated at RT. A rotary vane vacuum pump was connected to the outlet pipe to maintain the rough vacuum (∼10−2 Torr). The process pressure of the reactor was maintained at ∼250 mTorr. A single full ZnO ALD cycle consisted of 0.03 s/10 s/0.03 s/10 s (pulse/purge/pulse/ purge) exposures of DEZ/Ar/H2O/Ar, respectively. Characterization of Material Properties from Combinatorial Library. The combinatorial library with highly indexed spatial resolution was characterized through the following tools: spectroscopic ellipsometry (SE) to measure thickness, Raman and X-ray diffraction (XRD) for determining the crystallinity, scanning electron microscope (SEM) to provide visual evidence of crystallinity of the material, and a four-point probe measurement, along with Hall measurements, to examine the electrical properties. SE (J.A. Woollam, M2000DI) is used to determine the film thickness. Spectroscopic fitting was based on J.A. Woollam “Cauchy film” model for the “ZnO”. Raman (Renishaw inVia confocal spectroscope) was conducted at RT using 532 nm laser excitation with 2400 line/ mm grating, NA = 0.85 (laser spot size of ∼0.5−0.7 μm), and a spectral resolution of 1.1 cm−1. Spectral acquisitions were C


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Figure 3. Crystallite size information from combinatorial library. (a) Color-coded XRD patterns from 9 different points within a 100 mm combinatorial library. (b) Representative XRD patterns from two extremesP5 (225 °C) and P3 (200 °C)show that integral breadth clearly varies with changes in the crystallography as a function of temperature. (c) 100 mm wafer-scale mapping of the (002) crystallite sizes calculated based upon the Scherrer equation from XRD patterns.

attributed to several different aspects to date,79,83,84 another of which will be raised by the end of the discussion. Not only should the relative intensity of the E2 (high) peak be a significant feature to consider,79 but also the line shape (e.g., the formation of a tail toward low frequencies or peak broadening) and peak shifts are important features to consider. Raman spectra in Figure 2(b) show an asymmetric tailoring of the E2 (high) mode at all process temperatures investigated. M. Yoshikawa et al., claims that tailoring toward low frequencies of E2 (high) peak seen in wurtzite ZnO can be attributed to a decrease in crystallite size, calculated from Scherrer’s equation from ∼60 to ∼11 nm.85 Considering the typical crystallite size of ALD grown wurtzite ZnO should be in the range of ∼10− 20 nm,86,87 tailoring features seen at Raman spectra of this report may be attributed to the inherently small crystallite size of ALD ZnO. Figure 2c is the collection of Raman spectra to show a 100 mm scale feasibility of Raman substrate. For this single process temperature, which of the film was grown at 300 °C, Raman spectra spanning the 100 mm wafer demonstrate that the E2 (high) and A1 (LO) peak ratio remains nearly constant (E2/A1 ∼ 2.5). Note that the value of E2/A1 peak ratio reported in this literature is used to make comparative analysis, not to report a definitive value. Characterizing Combinatorial Library. Depending on the process temperatures, the wurtzite-structured ALD ZnO orients differently. When ZnO is grown by DEZ/H2O-based ALD processes, a general trend forms as follows: at low process temperature (200 °C) strongly orients the lattice along the (002) direction.59,61,86,88 To discuss the 100 mm scaled combinatorial library result, XRD patterns of ALD ZnO are first collected from 9 different points across the 100 mm scaled combinatorial library, as shown in Figure 3a. At the high end of the temperature gradient, the XRD spectra indicate that the ALD ZnO gradually start to crystallize preferentially along the (002) orientation but begin to form along the (100) orientation as the process temperatures decreases to 200 °C (downward in Figure 3a). Moreover, a trend in the integral breadth (β) is observed from the representative XRD pattern from two end temperatures (P5 vs P3) (Figure 3b). At higher process temperatures (P5, 225 °C), the β is narrow, but gradually broadens by approximately 190% at a lower temperature

section by placing a shadow mask on top of the synthesized combinatorial library and depositing e-beam evaporated Ti/Au metal stacks (20 nm/80 nm) (Temescal 1800 e-beam evaporator). Details of the sample preparation and layout is shown in Figure S4. The Hall measurement (Lake Shore 8400 Series) was performed at R.T to characterize the electrical properties of combinatorial library. Prior to the Hall measurement, Ohmic check correlation was done to ensure that the worst case Ohmic check correlation is larger than 0.9990. Details of how the Ohmic check correlation was carried out are described in Figure S4.

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RESULTS AND DISCUSSION Raman Effect in Atomic Layer Deposited Zinc Oxide without Temperature Gradient. At atmospheric pressure and room temperature, ZnO crystallizes in a hexagonal wurtzite structure, the most thermodynamically stable phase.77 Since wurtzite ZnO has C46v (P63mc) symmetry, it is a Raman-active material.67 Near the center of the Brillouin zone, symmetries of an A1 branch, a doubly degenerate E1 branch, two doubly degenerate E2 branches, and two B branches are predicted by Group theory. The A1 and E1 branches are both Raman- and infrared-active, and the E2 branches are Raman-active only, while the B branches are inactive.68,78−80 For highly oriented ZnO films, only the A1 (LO) and E2 modes are observed when the incident light is exactly perpendicular to the surface, representing our measurement system, while other modes are prohibited according to the Raman selection rule (Figure 2a).58,75,81 Note that the A1 (LO) (574 cm−1) mode is a polar (out-of-plane oxide vibration), and the E2 (high) (438 cm−1) mode is a nonpolar mode (in-plane oxide vibrations).82 Both modes are schematically shown in the inset of Figure 2a. Following tests of a series of Raman studies of ALD ZnO films, prepared over a range of process temperatures (120, 150, 180, and 210 °C, samples for individual Raman without any temperature gradient, Figure 2b), was conducted to explore the process temperature effect on the wurtzite ZnO Raman peaks. Figure 2b shows that the intensities of the E2 (high) phonon mode, which were initially low at the lowest process temperature (120 °C), increase gradually with increasing processing temperature (up to 210 °C). Similarly, the intensity ratio of the E2 (high) mode to the A1 (LO) mode steadily increases (i.e., E2/A1: from 0 to 0.9) as the process temperature increases (from 120 to 210 °C). The cause of changes of intensities of Raman peaks has been D


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Figure 4. Raman mode information from combinatorial library. (a) Color-coded Raman spectra from 23 different points within a 100 mm combinatorial library. (b) The representative Raman spectra of two extreme temperatures are shown. One from point a (225 °C) and the other from point w (200 °C). (c) 100 mm wafer-scale mapping of the E2/A1 peak ratios derived from Raman spectroscopy.

regime (P3, 200 °C). Since crystals smaller than 100 nm result in broadening of the Debye rings, the degree of broadening (∝β) is inversely proportional to the crystallite size.89 The β value, obtained by dividing the integrated intensity by the maximum intensity, correlates to the width of a rectangle having an identical area and height as the observed line pattern. Using the β value from each of the collected XRD patterns, in combination with the Scherrer equation (crystallite size = Kλ/ β cos θ, where λ is the X-ray wavelength used, β in radians, θ is the Bragg angle, and K is a correction factor),90 the 100 mm combinatorial library of the (002) crystallite size can be mapped as in Figure 3c. Thus, gradually varying (002) crystallite size along the process temperature gradient is observed; the higher the process temperature, the larger the (002) crystallite size formed (225 °C, ∼20 nm vs 200 °C, ∼9 nm). This is a straightforward finding as higher ALD process temperatures typically demonstrate an increase in the crystallite size of ZnO.60,86,88 The interesting feature to note here is that (002) crystallite size of ZnO varies diagonally across the temperature gradient line. This might be attributed to the coupled effect of the two gradients: (1) the process temperature and (2) an uneven distribution of pulsed gases. The mechanism must be further validated, though we suggest that the difference in the partial pressure, between the near-inlet area and the near-outlet area, could result in the differences in (002) crystallite size. The partial pressure can be varying as a function of flow field based on the findings done by D. Pan et al.54−57 The authors suggest several numerical approaches in combination with experimental verification, demonstrating that ALD parameters do not solely rely on surface chemisorption (whose factor is mainly determined by the process temperature), but also depend on the physical thermal-fluid process (that is, nonuniform distribution of concentration of gases at the moment of pulsing) within the cross-flow reactor system. The author asserts that, along the flow field, the inlet area encounters the precursor flow first, meeting higher concentration (i.e., higher partial pressure of the precursor), which in turn increases the collision probability, thereby increasing rate of adsorption (rads) based on principles of the Langmuir isotherm. Therefore, we suggest here that the increased rads found in the near-inlet area would encourage crystallization of ALD ZnO in the most energetically favorable direction, (002). However, in areas in which rads is not sufficiently high, the poisoning effect, that is, the effect caused by either captured polar water molecules after a H2O pulse or the anionic production of byproducts after a

DEZ pulse (either dissociated or as in unexchanged ligands, for example, CH3CH2δ−) near the polar wurtzite ZnO surface,88,91−93 could drive the crystals to grow along the (100) direction.61 This poisoning effect dominates the near-outlet region, resulting in an overall ALD ZnO crystallinity difference along the in-plane direction within the wafer. It should be noted that poisoned (002) growth does not mean that ALD ZnO does not physically grow on one side of the library; instead, it refers to preferential growth in either the (100) or (101) or amorphous deposition, as indicated by XRD patterns in Figure 3a. Thus, no distinct differences in thickness can be found over the 100 mm combinatorial library (Figure S2). Overall, differences in rads, as a function of flow field may result in the diagonal variation of crystallinity of ALD ZnO; the nearinlet region shows preferential crystallization along (002), despite the fact that process temperature alone would dictate a crystallization along (100) (e.g., P2 versus P4). Indeed, without a temperature gradient introduced into the system, Raman mapping of the ALD ZnO grown on the 100 mm Raman substrate shows the supporting evidence of the claim. The E2/A1 ratio varies along with the flow field, having higher E2/A1 ratio near inlet region and a lower ratio near outlet region (Figure S5). The diagonal behavior of the ZnO crystallinity is complemented by a study of the Raman modes of wurtzite ZnO contained in the combinatorial library. Twenty-three different Raman spectra, collected across the combinatorial library of ALD ZnO, are shown in Figure 4a. The trend can be clearly seen from the representative Raman spectrum from two end temperatures (point a, 225 °C vs point w, 200 °C) (Figure 4b). As can be seen from Figure 4b, it is shown that the trend in Raman mode is clear along the temperature gradient line. Under a higher temperature regime (point a), the E2/A1 peak ratio is large (>1), while at a lower temperature regime (point w), the E2/A1 peak ratio becomes smaller (∼0.3). Ratios of Raman mode, E2/A1 intensity ratio, are mapped as in Figure 4c. A diagonally varying E2/A1 peak ratio was observed, a similar feature seen in the XRD studies, and assumed to be attributed by the coupled effect of the two gradients within the system. Therefore, on the basis of the findings from combinatorial library, it can be concluded that the (002) XRD peak broadening and relative peak ratio of E2/A1 Raman modes of wurtzite ZnO are tightly associated. Since the E2 mode is related to in-plane oxygen vibrations,82 if the crystallite size of (002) plane is larger, then, Raman resonance due to E


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become dominant features on the surface (length ∼40 nm, width ∼30 nm, blue arrows). In this temperature regime, circular grains are hardly present, and their size is reduced to approximately 10 nm or less. The appearance of the substructures across the surface is attributed to incomplete thermal decomposition of the precursors.61,95 A similar trend in the morphologies of ALD grown ZnO with similar process temperatures has been cited in other literature as well.61,62,88,95,96 Intriguingly, however, a uniformly covered nanosized circular grain surface, approximately 20 nm, is also observed at the region of #B and #C. Though the process temperature contour line ties region #C similarly with region #E and #G (213−216 °C), their surface morphologies differ greatly. Further investigation into the surface morphology of regions #C, #E, and #G reveals that the substructures are more easily observed in region #G, compared to in region #E, while uniformly circular grains are found mixed in region #E and exclusively in region #C. In relation to the gas pathway within the reactor, region #C sits nearest the inlet, region #E rests in the center of the wafer, and region #G rests nearest the outlet. In addition to the correlating XRD and Raman mapping data (Figures 3 and 4, respectively), visual evidence seems to confirm that the precursor distribution and gas flow also play a role in determining the physical properties of ALD ZnO as previously discussed. Aside from the elongated substructures, SEM analysis concluded that the visible circular grain sizes in all regions agree well with the crystallite sizes determined from XRD analysis. Moreover, for Raman of ALD ZnO films with E2/A1 > 1, uniform grains with excellent crystallite morphology is observed. Lastly, the electrical properties of ALD ZnO was investigated using the combinatorial library. The correlation between process temperature, crystallinity, and electrical properties of ALD ZnO has been studied. Figure 7 shows the 100 mm wafer mapping of calibrated temperatures and electrical properties of ALD ZnO (e.g., electrical conductivity and carrier concentration). Under the temperature gradient, a calibrated wafer temperature is mapped as in Figure 7a. In terms of exploring the electrical properties of ALD ZnO, a 4-

symmetricity is largely enhanced,94 making E2 intensity higher. Thus, the correlation between the E2 (high) mode and (002) crystallite size can be illustrated as in Figure 5.

Figure 5. Schematics of how XRD determined (002) crystallite size can be correlated with E2 (high) peak in Raman spectra.

Plan view SEM images were taken asymmetrically from 9 points (#A−#I, as in Figure 6), giving a top view of the ZnO morphology. The SEM images clearly show the effect of both process gradients on the surface morphology of the ALD ZnO. At high temperature (#A, 225 °C), approximately 20 nm nanosized circular grains (highlighted in red arrows) uniformly covered the surface of the ALD ZnO. As the process temperature cools (#I, 200 °C), the morphology of the ALD ZnO also changes (from highest temperature to lowest temperature, #A vs #E vs #I). For the #E region, the surface is still mostly covered by circular grains, however, the average size of the grains spans approximately 15 nm. In this temperature regime, the substructures appear as a few grains without any preferential direction (length ∼40 nm, width ∼25 nm, highlighted in blue arrows). At the lowest temperature regions (#I, 200 °C), the substructures widen slightly and

Figure 6. Surface morphology information from combinatorial library. Top view SEM micrographs, at asymmetrically 9 different locations, were taken. Scale bar is 100 nm. F


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Figure 7. Electrical property information from combinatorial library. (a) Calibrated temperature of 100 mm combinatorial library is mapped. Electrical conductivity mapping data measured by (b) 4-point probe over 23 different points and (c) Hall measurement from 8 different devices are shown. (d) Carrier concentration, measured from Hall devices, is also mapped. The white dashed hexagon in panel b is the region where the Hall conductivity and carrier concentration mapping data are regionally matched.

measurements taken of the Hall carrier concentration demonstrated only a temperature dependence on the electron carriers. The carrier concentration was determined to be n-type for all Hall devices with a range of carrier concentrations from 7.1 × 1020 (#2, ∼225 °C) to 4.0 × 1020 cm−3 (#8, ∼200 °C). The detailed corresponding values are tabulated in Figure S7. The dependence of electrical conductivity to process temperature of ALD ZnO is reaffirmed here, as other literature suggests.60,63,65,66,92,97−100 Though understanding of the clear cause of the dominant donor in ALD ZnO is still under debate,60,101−105 it has been long assumed that the conductive nature can be attributed to a native defect as dominant donor, either by oxygen vacancies (VO) or by zinc interstitial atoms (ZnI) incorporated during growth. Fundamentally, this study demonstrates the negligible effect of crystallite size in determining the electrical conductivity of ALD ZnO. The Hall mobilities, though highly dependent on the grain boundary blocking effect of polycrystals,106 do not play a significant role in determining the electrical conductivity of ALD ZnO. Across the 100 mm combinatorial library, Hall mobility variation due to crystallite size has been nearly negligible (from 5 to 6 cm2 V−1 s−1, Figure S8). In conclusion, it is found that the ALD process temperature is solely responsible for the electrical conductivity of ALD ZnO.

point-probe measurement over 23 different points (from point a to point w, Figure 7b) within the 100 mm combinatorial library was employed to generate information about the ALD ZnO electrical conductivity. It is clearly seen that the electrical conductivity increases with increasing process temperatures. Electrical conductivities vary directly along with the temperature gradient; the lowest electrical conductivity observed is 130 ± 11 Ω−1 cm−1 (point v, 200 °C), while the highest observed is 827 ± 9 Ω−1 cm−1 (point c, 225 °C). The detailed conductivity values are tabulated in Figure S6. It is found that the distribution of the traveling-wave of pulsed gases does not significantly impact the electrical conductivities of ALD ZnO. To support this claim, the electrical conductivity measured by 4-point-probe was complemented by Hall measurements over 8 different points (from 1 to 8, Figure 7c and 7d). The white hexagonal area in the 4-point-probe mapping data in Figure 7b signifies the region where the Hall measurement mapping data is regionally overlapped. Owing to a different measurement configuration, there are slight differences in the Hall conductivities to those measured from 4-point-probe (Figure 7b vs 7c) though the difference lies within a ∼15% threshold. The qualitative trend gathered from the Hall measurements agrees well with that from the 4-point-probe mapping data. The highest electrical conductivity measured from the Hall measurement is 680 Ω−1 cm−1 (#1, ∼225 °C), while the lowest conductivity was 343 Ω−1 cm−1 (#8, ∼200 °C). Likewise, G



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CONCLUSIONS In this research, we have successfully developed a novel methodology to synthesize a “spatially addressable combinatorial library” through the ALD system to witness the physical properties and electrical effects of simple metal oxides. To validate the results of our novel combinatorial approach, a case study of ZnO was performed. The coupled effect of the gradual change in process temperature and nonuniform distributed flow of the pulsed precursors is a proposed reason that produced a diagonal variation of the (002) crystallite size. As determined by the Scherrer equation from corresponding XRD studies, the crystallite sizes of (002) ZnO ranged from ∼10 nm to ∼20 nm, with larger crystallite size formed at higher process temperature. Correlations of the (002) crystallite size from normalized Raman intensities of the E2/A1 peak ratio complement the XRD analysis, yielding a high-throughput characterization technique with greater spatial resolution than XRD for determining the film microstructure. To provide visual evidence of the microstructure, SEM analysis was performed to trace the corresponding crystal particles and demonstrate that the results agree well with the XRDdetermined crystallite size. However, in terms of the electrical properties, the combinatorial library demonstrates that the process temperature, rather than crystal size, plays a major role in determining the electrical conductivity of ALD ZnO. Due to these findings, significant variations of the microstructure have been evidenced for a crystallized metal oxide thin film grown within the ALD window.

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ACKNOWLEDGMENTS This work was supported by the Creative Materials Discovery Program on Creative Multilevel Research Center (2015M3D1A1068061) through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning.

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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/acscombsci.9b00007.

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Research Article

Wafer mapping of calibrated temperature under temperature gradient, wafer mapping of thickness of ZnO grown on the combinatorial library, substrate effects on Raman spectroscopy, process flow of characterizing combinatorial library, additional Raman mapping without temperature gradient, tabulated values of electrical conductivities measured by 4-point probe, summarized values of Hall measurements, and wafer mapping of Hall mobilities (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Harrison Sejoon Kim: 0000-0002-6488-5915 Si Joon Kim: 0000-0001-9191-9079 Myung Mo Sung: 0000-0002-2291-5274 Jiyoung Kim: 0000-0003-2781-5149 Present Address ⊥

A.T.L.: Qorvo Inc., 500 West Renner Road, Richardson, Texas 75080, United States.

Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. H


Research Article


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Realization of Spatially Addressable Library by a Novel Combinatorial

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