Hydrophobicity of Rare Earth Oxides Grown by Atomic Layer

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Hydrophobicity of Rare Earth Oxides Grown by Atomic Layer Deposition Il-Kwon Oh,† Kangsik Kim,‡ Zonghoon Lee,‡ Kyung Yong Ko,† Chang-Wan Lee,† Su Jeong Lee,# Jae Min Myung,# Clement Lansalot-Matras,∥ Wontae Noh,∥ Christian Dussarrat,⊥ Hyungjun Kim,*,† and Han-Bo-Ram Lee*,§ †

School of Electrical and Electronic Engineering, Yonsei University, Seoul, 120-749, Korea School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea # Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea ∥ Air Liquide Laboratories Korea, Yonsei Engineering Research Park, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea ⊥ Air Liquide Research & Development c/o University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan § Department of Materials Science and Engineering, Incheon National University, Incheon 406-772, Korea ‡

S Supporting Information *

ABSTRACT: Rare earth oxide (REO) atomic layer deposition (ALD) processes are investigated for hydrophobic coatings. Thermal and plasmaenhanced ALD (PE-ALD) Er2O3 and Dy2O3 are developed using the newly synthesized Er and Dy precursors bis-methylcyclopentadienyl-diisopropylacetamidinate-erbium and bis-isopropylcyclopentadienyl-diisopropyl-acetamidinate-dysprosium, with H2O and O2 plasma counter oxidants. The Er and Dy precursors show typical ALD growth characteristics with no nucleation incubation, indicating that they are suitable ALD precursors. The hydrophobicities of ALD-grown Er2O3 and Dy2O3 are investigated, together with those of ALD-grown Y2O3, La2O3, and CeO2 that were previously developed for high-k applications. All the ALD-grown REOs show high hydrophobicity, with water contact angles as high as 90°. After annealing at 500 °C in air for 2 h, hydrophobicity is degraded depending on the kind of material; this degradation is related to the hygroscopy of REOs. In addition, we demonstrate the fabrication of a superhydrophobic surface by depositing highly conformal ALD REO films on 3D Si nanowire nanostructures. The Si NWs are conformally coated with ALD Y2O3, yielding a surface with a water contact angle of about 158°. The ALD REOs reported herein should find widespread applicability in the fabrication of robust hydrophobic coatings.

1. INTRODUCTION Hydrophobic coatings are widely used in various applications from industrial components to housewares, such as in condenser parts for generators,1 gas turbines,2 automobile parts,3 oil/water separators in oil purifiers,4 cooking wares,5 optical windows,6 and eyeglasses.7 In many applications, organic material coatings such as fluorosilane8,9 and polypropylene10 have been used for hydrophobic coating due to their low material cost, simple coating process, and chemical stability.11 However, organic coatings have several disadvantages in practical applications, chiefly their inherently poor mechanical durability and thermal stability. For example, repetitive strain on an organic coating layer can generate cracks that provide paths for water penetration, resulting in coating failure.12 Additionally, the hydrophobicity of some organic materials is not sustained in high-temperature environments. For instance, the hydrophobicity of polydimethylsiloxane was degraded above 270 °C due to the cross-linking and scission of the polymer network.13 © 2014 American Chemical Society

Hydrophobic properties have been explored in a few inorganic metal oxide systems as well, such as the ZnO14,15 and TiO216 systems. Generally, inorganic metal oxides have better mechanical durability than organic materials.16 However, the hydrophobicity of ZnO14,15 and TiO216 was not retained after high-temperature annealing or UV exposure due to the generation of surface hydroxyl groups, eventually becoming hydrophilic during these processes. In 2013, Azimi et al. reported that inorganic REO materials show thermally stable hydrophobicity up to 1000 °C due to their unique electronic structure: because the unfilled inner 4f orbitals of the rare earth metal atoms, which contribute to reactions, are shielded by the filled 5s2p6 outer shell, REOs have a lesser tendency to interact with water molecules, resulting in hydrophobicity.17 Moreover, Azimi et al. demonstrated that the Received: October 5, 2014 Revised: December 4, 2014 Published: December 19, 2014 148

dx.doi.org/10.1021/cm503659d | Chem. Mater. 2015, 27, 148−156

Chemistry of Materials

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were deposited on Si nanostructures for the fabrication of superhydrophobic surfaces.

wear rate of REOs was significantly lower than that of aluminum, even similar to that of stainless steel, enabling durable hydrophobicity coatings. Although the work of Azimi et al. introduced a well-established theoretical study of REOs, there are still many challenges in the fabrication of REOs. Azimi et al. only utilized bulk REOs fabricated by sintering disk pellets from REO powders,17 rather than thin REO films. In the following study, Zenkin et al. prepared oxides of rare earth metals such as Nd, Y, and La by sputtering and demonstrated their hydrophobicity; they also presented that the surfaces of REOs dominantly have the nonpolar component which does not electrostatically interact with polar water molecule, so REOs show the hydrophobicity.18 Generally, thin film deposition processes have many advantages over bulk shape for practical applications that require complex micro- and nanoscale structures. For instance, the fabrication of hydrophobic surfaces is one of the important processes in microelectromechanical systems devices, allowing control over the flow of fluids and reducing frictions, and thinfilm hydrophobic coating is essential for this purpose.19 In addition, thin film processing of REOs can be applied to the fabrication of superhydrophobic surfaces, which has been applied in many applications such as self-cleaning surfaces and low-friction pipes.20 It is well-known that the superhydrophobic surfaces can be realized by surface roughening of hydrophobic surfaces because roughening contributes to superhydrophobicity by trapping of air below water droplets.21 Thin-film deposition of REOs on nanostructured surfaces is known to be an easy route to fabricate superhydrophobic surfaces. Therefore, a nanoscale thin film process of REOs with high conformality and uniformity is required for robust hydrophobic coating for various applications. Although one report on REO thin film fabrication by sputtering has been published so far,18 sputtering is not suitable for coating on 3D nanostructures due to its inherently poor step coverage.22 ALD is suitable for this purpose because it provides excellent conformality and precise thickness control at the atomic scale. Because of the advantages of ALD, it has been adopted for the fabrication of nanoscale semiconductor device components such as diffusion barriers, electrodes and insulators of metal oxide semiconductor field effect transistors, and dynamic random access memory.23 Most REOs have high dielectric constants around 1524 and large energy band gaps25 and accordingly have been studied for use as high-k materials. Y2O3,26 La2O3,27 CeO2,28 Dy2O3,29 Gd2O3,29 Sm2O3,30 and Lu2O331 films fabricated using ALD have been reported for gate insulator applications, and the results showed precise thickness control, excellent electrical properties, and high purity. Additionally, the high thermal stability of REOs on silicon substrates is advantageous for the Si device fabrication process, which includes several high-temperature processes.32 Although REOs fabricated by ALD have been reported several times, there has been no attempt to use ALD REOs for hydrophobic and superhydrophobic nanoscale coatings. In the work described herein, we developed ALD processes to fabricate Dy2O3 and Er2O3 based on the use of newly synthesized metal organic Dy and Er precursors and explored their hydrophobic properties. In addition, we investigated the hydrophobicity of ALD Y2O3, La2O3, and CeO2 films which have been previously developed by our group.26−28 The thermal stability of hydrophobic ALD REO films was studied by annealing them at 500 °C. Highly conformal ALD REOs

2. EXPERIMENTAL SECTION 2.1. REO ALD Process. We used a commercial ALD chamber (NCD Co., Lucida M100-PL) with a double showerhead for good uniformity.26 Bis-methylcyclopentadienyl-diisopropylacetamidinateerbium (Er(MeCp)2(N-iPr-amd)) and bis-isopropylcyclopentadienyldiisopropyl-acetamidinate-dysprosium (Dy(iPrCp)2(N-iPr-amd)) provided by Air Liquide were evaporated at 120 and 135 °C, respectively, in stainless-steel bubblers to obtain sufficient vapor pressure. Vaporized precursors were transported into the reaction chamber by argon carrier gas, the flow rate of which was controlled by a mass flow controller. Argon gas held at the same flow rate was also used to purge excess gas molecules and byproducts between each precursor and oxidant exposure step. Two types of counter oxidants were used: H2O for thermal ALD and O2 plasma for PE-ALD. The flow rates of water vapor and O2 were controlled by a leak valve and a mass flow controller, respectively. O2 plasma was generated between the substrate heater and the showerhead connected to a radio frequency plasma generator. Thermal ALD and PE-ALD Er2O3, PE-ALD Dy2O3, and PE-ALD Y2O3 processes are developed in this work, and thermal ALD Y2O3, thermal ALD and PE-ALD La2O3, and PE-ALD CeO2 were employed based on the process conditions developed in our previous reports.26−28 Y(iPrCp)2(N-iPr-amd),26 La(iPrCp)3,27 and Ce(iPrCp)328 were used for Y2O3, La2O3, and CeO2 ALD, respectively. 2.2. Substrate Preparation for ALD REO. Si(100) was used as a substrate. The substrate was cleaned in RCA solution (1:1:5 NH4OH/ H2O2/H2O by volume) at 70 °C for 10 min, followed by dipping in buffered oxide enchant solution for 30 s to remove native oxide. Growth characteristics of ALD REOs on Si substrates were systematically investigated by changing precursor exposure time (ts), oxidant exposure time (tr), purging time (tp) for precursor and oxidant, cycle number, and growth temperature (Ts) to find optimized conditions for ALD growth. The purging times for precursor and oxidant were kept at 5 s in all the experiments since growths per cycle (GPCs) of all REOs were saturated over 5 s of purging time (see Figure S2a and S2b in the Supporting Information). Additionally, Si nanowires (NWs) synthesized by using metal-assisted chemical etching were also used for the nanostructured substrate to fabricate superhydrophobic surfaces. For the metal-assisted chemical etching process, a Si(100) wafer was dipped in an etchant solution of 100 mL of hydrofluoric acid (10 wt % HF) and 0.1 g of silver nitrate (AgNO3). After 30 min etching at 70 °C, vertically aligned Si NWs with 10 μm length and 300 nm diameter were obtained. The Si NWs were cleaned in nitric acid (HNO3) and rinsed with deionized water to eliminate residues from the etching process. Detailed processes on the fabrication of NWs can be found in our previous report.33 2.3. Wetting Property and Thermal Stability of ALD REO Thin Films. Hydrophobic properties of ALD REOs were evaluated by the sessile drop technique using a contact angle analyzer (Phoenix-300 Plus, SEO) with deionized water. Contact angle images were acquired by a charge-coupled device video camera and an image analysis system (Image XP ver. 5.9, SEO). The volume of each deionized water droplet used was 4 μL. For the thermal stability experiments, ALD REO films were annealed in a furnace at 500 °C for 2 h in air, and the contact angles of ALD REOs were measured. 2.4. Analysis of ALD REO Thin Films. The thickness and refractive index of ALD REOs were measured by using an ellipsometer (Elli-SE-F, Ellipso Technology). The chemical compositions of ALD REOs were analyzed by X-ray photoelectron spectroscopy (XPS; KAlpha model, Thermo Scientific Co.) with a 1486.6 eV Al Kα monochromatic source. Surface cleaning was performed by using Ar sputtering for 20 s to remove surface contaminants prior to XPS analysis. To analyze adsorbed species on REO thin films, Fourier transform infrared (FT-IR) spectroscopy (Vertex 70, Vertex Co.) was used. The conformality of ALD REOs on Si NWs was investigated by using field emission scanning electron microscopy (FE-SEM; JEOL JSM-7001F, JEOL Ltd.) and transmission electron microscopy (TEM; 149



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Figure 1. Thermal ALD and PE-ALD growth characteristics of Er2O3 at Ts = 180 °C. (a) GPC versus ts for tr = 1 s and tp = 5 s. (b) GPC versus tr for ts = 3 s for thermal ALD and 2 s for PE-ALD, and tp = 5 s. (c) Film thickness versus number of ALD cycles and (d) GPC versus Ts under the saturation conditions ts = 3 s, tr = 1 s, and tp = 5 s for thermal ALD and ts = 2 s, tr = 1 s, and tp = 5 s for PE-ALD Er2O3.

and from 160 to 250 °C for PE-ALD; this is typical for ALD growth due to the insensitivity of surface adsorption sites to temperature, and the temperature region is usually called the ALD window. For temperatures below the minimum of the ALD window, the GPCs of both ALD techniques increase with increasing Ts because there is not enough thermal energy for surface adsorptions and reactions.35 Contrastingly, in the hightemperature region over 250 °C, the GPCs decreased with increasing Ts. In this region, precursor molecules adsorbed on the surface are desorbed again due to the high thermal energy of the substrate, reducing the GPC relative to that in the ALD window region.36 The GPCs of PE-ALD films were higher than those of thermal ALD films at all temperatures studied. Similar results were observed in our previous reports. The GPCs of La2O3, ZrO2, and TiO2 deposited by PE-ALD using O2 plasma oxidant were higher than those deposited by thermal ALD using H2O oxidant because the reactivity of oxygen radicals in O2 plasma is higher than that of H2O.23,27,37 So, the higher GPC of PE-ALD Er2O3 can be attributed to the difference in oxidant reactivity. In addition, the ALD window region of PE-ALD Er2O3 is wider than that of thermal ALD. Highly reactive radicals facilitate surface reactions even when there is less thermal energy, extending the ALD window to lower temperatures. In contrast to Er(MeCp)2(N-iPr-amd), which could be used to deposit Er2O3 films using either H2O or O2 plasma oxidants, Dy2O3 film deposition from the Dy(iPrCp)2(N-iPr-amd) precursor occurred when PE-ALD using O2 plasma was applied but not when thermal ALD using H2O was applied. This is probably due to effects of the counter oxidant. As mentioned above, the O2 plasma oxidant is more reactive than the H2O oxidant. Compared to H2O, the O2 plasma more strongly influences the reactions of the Dy precursor on the surface in two ways: one is the change of the initial surface properties to promote Dy precursor adsorption,38 and the other is strong

JEOL JEM-2100F). The transmission electron microscope was operated at 200 kV. Surface morphology and chemical composition of the Y2O3-coated Si NW was investigated using TEM, high-angle annular dark field scanning TEM (HAADF-STEM), and energydispersive X-ray spectroscopy (EDS) of STEM. The Si NW substrate coated with thermal ALD Y2O3 was dipped into isopropyl alcohol contained in a vial and sonicated for 20 s to detach Si NWs from the substrate. Drops of the isopropyl alcohol solution containing Si NWs were gently placed on a TEM grid by using a fountain-pen filler.

3. RESULTS AND DISCUSSION 3.1. ALD of Er2O3, Dy2O3, and Y2O3. Figure 1a shows the GPCs of thermal ALD and PE-ALD Er2O3 films as a function of ts at the Ts of 180 °C. For thermal ALD, saturation of thickness was observed when exposure times were increased over 3 s. Saturation of thickness is a growth characteristic typical of ALD,34 indicating that the Er(MeCp)2(N-iPr-amd) precursor is suitable for the ALD process. The saturated GPC of thermal ALD is about 0.5 Å/cycle. For PE-ALD of Er2O3, a saturated curve was also observed for ts over 2 s, and the saturated GPC was 0.7 Å/cycle. The saturation behaviors of GPC were also observed in fixed-ts conditions for both thermal ALD and PE-ALD; the GPCs of both plateaued for tr greater than 1 s (Figure 1b). The saturated GPCs in the fixed-ts condition were consistently identical to those in the fixed-tr condition. The film thickness was measured as a function of growth cycle (Figure 1c); for both thermal ALD and PE-ALD, the plots of thickness versus ALD cycle number were fit well by a linear fitting model passing through the origin. This indicated that there was no nucleation incubation, an advantageous property of ALD precursors. Temperature dependence curves of thermal ALD and PEALD were similarly shaped, although PE-ALD yielded greater GPC than thermal ALD at all temperatures studied (Figure 1d). A temperature region in which the GPC was almost unchanged was observed from 180 to 250 °C for thermal ALD 150



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Figure 2. PE-ALD growth characteristics of Dy2O3 at Ts = 180 °C. a) GPC versus ts for tr = 1 s and tp = 5 s. (b) GPC versus tr for ts = 2 s and tp = 5 s. (c) Film thickness versus number of PE-ALD cycles and (d) GPC versus Ts under the saturation conditions ts = 2 s, tr = 1 s, and tp = 5 s for PE-ALD Dy2O3.

Figure 3. PE-ALD growth characteristics of Y2O3 at Ts = 180 °C. (a) GPC versus ts for tr = 1 s and tp = 5 s. (b) GPC versus tr for ts = 1.5 s and tp = 5 s. (c) Film thickness versus number of ALD cycles and (d) GPC versus Ts under the saturation conditions ts = 1.5 s, tr = 1 s, and tp = 5 s for PE-ALD Y2O3.

cycle (Figure 2a). Similar growth saturation was also observed for tr over 1 s in a fixed-ts condition of 2 s (Figure 2b). Film thicknesses increased linearly with increasing growth cycle number (Figure 2c). The results shown in Figure 2a−c are typical of the growth behaviors of ALD, indicating that Dy(iPrCp)2(N-iPr-amd) is a good precursor for the PE-ALD

oxidation of the adsorbed Dy precursor, leading to generation of fresh adsorption sites for the following exposure step.39 When O2 plasma was employed as the oxidant, deposition of Dy2O3 was observed. Plotting the GPCs of PE-ALD Dy2O3 films as a function of ts at 180 °C showed that GPC saturation occurs at ts ≥ 2 s and that the saturated GPC is about 0.3 Å/ 151



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Table 1. Summary of ALD Process Parameters of REOs Including Er2O3, Dy2O3, Y2O3, La2O3, and CeO2 REOs

precursor

ALD method

oxidant

GPC [Å/cycle]

ts/tr until saturation

GPC saturated region [°C]

ref

Er2O3

Er(MeCp)2(N-iPr-amd) Y(iPrCp)2(N-iPr-amd)

La2O3

La(iPrCp)3

Dy2O3 CeO2

Dy(iPrCp)2(N-iPr-amd) Ce(iPrCp)3

H2O O2 plasma H2O O2 plasma H2O O2 plasma O2 plasma O2 plasma

∼0.5 ∼0.7 ∼0.4 ∼0.6 ∼0.7 ∼1.5 ∼0.3 ∼0.35

3 2 4 1.5 1 1 4 0.83

180−250 160−250 180−200 180−230 300−350 300−350 140−230 200−300

this work

Y2O3

thermal PE thermal PE thermal PE PE PE

26 this work 27 this work 28

to tr support this explanation. The ratios for PE-ALD are smaller than those for thermal ALD, including the Er2O3, Y2O3 and La2O3 systems, meaning that O2 plasma saturates the surface more quickly than H2O.41 Thus, the difference of the ratios is attributed to the higher reactivity of O2 plasma than H2O. Interestingly, although the Dy precursor and the Ce precursor have the same ligands as those in the Y precursor and the La precursor, respectively, the GPCs of PE-ALD Dy2O3 and CeO2 were much less than those of the PE-ALD Y2O3 and La2O3. As discussed in the previous section, the inability of thermal ALD to deposit Dy2O3 can be attributed to the low reactivity of H2O. In addition, this can be interpreted to result from the low reactivity of the Dy precursor with oxidants. So, the number of Dy precursors adsorbed on the surface during the precursor exposure time is small compared to the Y precursor, leading to generation of fewer oxidized Dy precursors during the following oxidant exposure time; this is the same for the Ce and the La precursors. In addition, because the sizes of each precursor are almost the same, the effects of steric hindrance on the different GPCs can be neglected. 3.2. Hydrophobicity of ALD REOs. Effects of film thickness on hydrophobicity were studied to find optimized thickness. Figure 4a shows water contact angle values and pictures of ALD Y2O3 with different thicknesses on Si substrate. The 30- and 50 nm-thick ALD Y2O3 films show hydrophobic property over 100 of water contact angle. However, the water contact angle of the 10 nm-thick sample is approximately 70°. Similar thickness dependencies were observed from other REOs, and hydrophobic property over 100° was obtained over 50 nm. Previous reports showed that surface energies of nanosize materials, such as nanoparticle, are larger than that of bulk size materials due to different force balance between surface atoms and interior atoms.45,46 In the thin film system, the increase in surface energy was observed with decreasing film thickness.47 So, in our experiment, the interaction between ALD Y2O3 and water becomes stronger due to the increase of the surface energy at smaller thickness, resulting in smaller contact angle value. Additional investigations on effects of thickness and underneath substrate on hydrophobicity are being carried out. The wetting properties of as-deposited 50 nm-thick REOs on Si substrate were compared; all the water contact angles of ALD REOs including Y2O3, Er2O3, La2O3, Dy2O3, and CeO2 were higher than 90°, indicating hydrophobic surface properties (Figure 4b). ALD Er2O3, Y2O3, La2O3, Dy2O3, and CeO2 respectively showed the contact angles of 100°, 102°, 106°, 108°, and 97°, which were comparable with previous results for films prepared by sintering17 and sputtering.18 To evaluate the thermal stability of REO hydrophobicity, we conducted high-temperature annealing of ALD REOs at 500 °C

process, similar to Er(MeCp)2(N-iPr-amd). The temperature dependence of the film GPCs was studied under saturation conditions; similar to Er2O3, an ALD window region was observed from 145 to 230 °C. Outside the ALD window, however, PE-ALD Dy2O3 showed different behavior from PEALD Er2O3. The GPCs increased with increasing Ts at temperatures higher than the ALD window. The increase in GPCs can be explained by the decomposition of the precursor molecules on the surface. Because of the high thermal energy, the surface self-saturated reaction does not occur, similar to the reactions in CVD process, resulting in higher GPC than that in the ALD window.36,40,41 We previously investigated the growth characteristics and film properties of thermal ALD Y2O3 films deposited using a Y(iPrCp)2(N-iPr-amd) precursor and H2O oxidant.26 Thermal ALD Y2O3 showed growth that increased linearly with the increasing ALD cycle number, without nucleation incubation; the GPC of 0.4 Å/cycle was obtained for ts over 2 s and tr over 1 s. In the present study, we investigated PE-ALD Y2O3 using the same yttrium precursor and O2 plasma oxidant. PE-ALD Y2O3 also showed saturation behavior with varying ts and tr (Figure 3a and b). GPC was saturated at about 0.6 Å/cycle for ts over 1.5 s and tr over 1 s. The GPC of PE-ALD Y2O3 was higher than that of thermal ALD Y2O3, similar to the trends for thermal ALD and PE-ALD Er2O3 shown in Figure 1. In addition, similar to Er2O3 and Dy2O3, the film thickness of PEALD Y2O3 increased linearly as a function of growth cycle (Figure 3c). An ALD window was observed in the temperature region from 180 to 230 °C (Figure 3d). GPC decreased with increasing Ts over the ALD window region, showing a similar tendency to the temperature dependence of Er2O3 shown in Figure 1d. Below the ALD window, however, the GPC increased, different from the trends for Er2O3 and Y2O3, for which GPC decreased with increasing growth temperature. The Y(iPrCp)2(N-iPr-amd) precursor molecules were condensed on the surface instead of chemisorbing with self-saturation, resulting in the higher GPC than that in the ALD window. Similar results have been reported many times for other ALD systems, including our reports on thermal ALD and PE-ALD La2O3,27 thermal ALD HfO2,42 and thermal ALD Ta2O5.43 Table 1 summarizes the ALD process parameters of REOs including Er2O3, Dy2O3, Y2O3, La2O3, and CeO2. Er2O3, Y2O3, and La2O3 films can be formed by both thermal ALD and PEALD, whereas Dy2O3 and CeO2 can be deposited only by the PE-ALD process. In addition, PE-ALD yields greater GPCs than thermal ALD in ALD Er2O3, Y2O3, and La2O3. Since O2 plasma oxidant is more reactive than H2O oxidant, during a single cycle the O2 plasma radicals oxidize a larger number of precursors adsorbed on the surface and generate fresher adsorption sites for the following precursor exposure than the H2O oxidant does, thereby increasing the GPC. The ratios of ts 152



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O 1s core level XPS spectra of as-dep and annealed Y2O3, respectively. The main peak was composed of two deconvoluted peaks corresponding to Y−OH at 531.2 eV and Y−O at 529.9 eV.50 After high temperature annealing, the amount of OH bonding in the films is increased about 11% while the total amount of oxygen is almost unchanged as shown in Figure 5c. Figure 5d shows FT-IR spectra before and after annealing. After annealing, a significant increase of the transmittance signals around 3700 cm−1 was observed, which is caused by the surface hy-droxyl group on the surface of Y2O3 films.51 The hygroscopy of REOs was previously reported to be inversely proportional to the electronegativity of their rare earth elements.52 That is to say, if a rare earth element has low electronegativity, its oxide is highly hygroscopic, and consequently its surface is easily changed to become more hydrophilic. In addition, the high-temperature environment used for the annealing process promotes the interaction between REOs and water. In fact, the contact angle reductions of the REOs were inversely proportional to the electronegativity of their rare earth elements (see Figure S5 in the Supporting Information). Particularly, La was the least electronegative of the rare earth elements used in our experiments, and the annealed La 2 O 3 was the most hygroscopic, with the greatest reduction in contact angle among the REOs studied; Er was the most electronegative, and the Er2O3 had the least reduction in contact angle, 21%, after annealing. Other reports have consistently noted large amounts of hydroxyl groups detected in La2O3 films,53 and the amount of hydroxyl groups has been increased by exposure to hightemperature environments because the interactions between La2O3 and absorbed water are kinetically accelerated by greater temperatures.54 Therefore, the correlation between hygroscopy and hydrophobicity is one way to explain the observed differences in REO thermal stability. Also, it is believed that there are other effects that change the hydrophobicity, because the contact angle reductions of Dy2O3 and Y2O3 were different even though Dy and Y have the same electronegativity. Hydrophobic ALD REOs are suitable for the fabrication of superhydrophobic surfaces. Si NWs 10 μm long were employed as a roughened surface (see SEM images in Figure S6a and b in the Supporting Information). After the deposition of 50 nm of Y2O3 on Si NWs, TEM analysis was performed to investigate the conformality of ALD at the atomic scale. As shown in Figure 6a, a long single Si NW coated with ALD Y2O3 was imaged by TEM; its length was about 10 μm. Based on the contrast difference between Y2O3 and Si, a conformal coating of Y2O3 over the whole Si NW was observed (Figure 6a−e). Figure 6f shows a STEM HAADF TEM image obtained from the bottom-middle part of the NW and Si; O and Y EDS elemental mapping obtained from Figure 6f showed uniform signals of Y and O over the entire Si NW region (Figure 6g−i). Compositional profiles of Si, Y, and O elemental signals were collected perpendicular to the Si NW axis; the Y and O signals were uniform along the profile direction inside the Si NW region and were higher immediately outside the Si NW region, where the e-beam passed laterally through the Y2O3 coating. Thus, the TEM images and EDS spectra indicated that Si NWs were conformally coated with ALD Y2O3. Si NWs without ALD REOs exhibited hydrophilic property as shown in Figure 6m. After Y2O3 ALD on Si NWs, however, the hydrophobicity was dramatically increased. The water contact angle of a surface of ALD Y2O3 deposited on Si NWs

Figure 4. Water contact angles of (a) as-deposited 50-, 30-, and 10 nm-thick Y2O3/Si surfaces, (b) as-deposited 50 nm-thick ALD REO/ Si surfaces, and (c) 50 nm-thick ALD REO/Si surfaces after annealing at 500 °C for 2 h in air.

for 2 h in air. Water contact angles of annealed ALD REOs were less than those of as-deposited samples; however, the extent of this reduction differed for each of the ALD REOs (Figure 4c). The percentage reductions of contact angles relative to the as-deposited values were 19% for Y2O3, 30% for Dy2O3, 80% for La2O3, 38% for CeO2, and 21% for Er2O3. Because ALD REOs were annealed in the same conditions, the differences in the wetting property changes arose solely from the material properties of the REOs. A material’s hygroscopy is correlated with its hydrophobicity.48 Materials with high hygroscopy have a strong tendency to absorb moisture, leading to a high probability of subsequent reactions between those materials and water. It results in the production of a large amount of hydrogen-bonded oxygen (O−H) on surfaces, which changes the properties of those surfaces to be more hydrophilic.49 Figure 5a and 5b show 153



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Figure 5. XPS O 1s core-level spectra of 10 nm-thick Y2O3 films (a) before and (b) after high-temperature exposure. (c) Normalized amounts of Y− OH and Y−O bonding in as-dep and annealed Y2O3 films. (d) FT-IR spectra of 10 nm-thick Y2O3 films before and after high-temperature exposure.

was about 158 ± 4°, which was higher than the contact angles of ALD REOs on a planar surface (Figure 6n). This contact angle is comparable with other reported results for which organic materials were used.55,56 The reported water contact angle of a poly(styrene-b-dimethylsiloxane)55 fiber mat fabricated by electrospinning was 163° and that of dodecyltrichlorosilane-coated Si NW arrays was 158°.56 As we expected, the increase of contact angle in the Si NW structure relative to the Si planar surface was due to the effects of roughness. Due to the excellent conformality of ALD REOs, the current processing scheme is not limited to Si NWs but can be used with numerous other nanotemplates, including NWs, nanoparticles, nanomeshes, and nanohole structures.

4. CONCLUSIONS In summary, we have reported ALD REO processes to fabricate hydrophobic coatings. We developed Er2O3 and Dy2O3 ALD by using newly synthesized Er and Dy precursors and investigated the hydrophobic properties of ALD Er2O3 and Dy2O3 together with other REOs, including ALD Y2O3, La2O3, and CeO2 materials that we reported previously. The Er precursor was usable for the growth of Er2O3 by means of both thermal ALD and PE-ALD, using H2O and O2 plasma oxidants, respectively, whereas the Dy precursor was usable for the growth of Dy2O3 films by means of only PE-ALD, using O2 plasma as the oxidant. ALD films 50 nm thick of Er2O3, Y2O3, La2O3, Dy2O3, and CeO2 exhibited hydrophobicity, with water contact angles as high as 90°. After annealing at 500 °C in air ambient for 2 h, the hydrophobicity of the ALD REOs was degraded to various extents, which depended partially on their hygroscopy. We demonstrated that the high conformality of the ALD REOs process enables the fabrication of superhydrophobic surfaces. The water contact angle of Y2O3-coated Si NWs was about 158°. It should be noted that the ALD REOs process can be extended to many applications requiring uniform hydrophobic surfaces.

Figure 6. (a) TEM image of one whole single Si NW coated with ALD Y2O3, 15 TEM images taken at the same magnification were merged to show a long single Si NW. (b−e) Magnified TEM images at top (b), top-middle (c), bottom-middle (d), and bottom (e) of Si NW indicated as color boxes in (a); contrast differences in the TEM images. (f) HAADF-STEM image of the NW. (g−i) EDS elemental maps of (g) Si, (h) O, and (i) Y. (j−l) Elemental profiles acquired from EDS mapping across the NW; (j) Si, (k) O, and (l) Y. Contact angle picture of a surface of (m) Si NWs without ALD REOs coating and (n) 50 nm-thick ALD Y2O3 on Si NWs.

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ASSOCIATED CONTENT

S Supporting Information *

Evaporation characteristics of is of the Er(MeCp)2(N-iPr-amd) and Dy(iPrCp)2(N-iPr-amd) precursors by thermogravimetric 154



Article

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(TG) measurement, ALD growth of Er2O3 and Dy2O3 with various precursor and oxidant purging time, chemical compositions of ALD ErO2 and Dy2O3 films, the effect of films crystallinity of REOs to the hydrophobicity, the relationship between electronegativity and hydrophobicity after high temperature annealing, and images of Si NWs and ALD Y2O3 coated Si NWs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.K.). *E-mail: [email protected] (H.-B.-R.L.). Author Contributions

I.-K. Oh developed the REO ALD processes and characterized the hydrophobicity of REO films. K. Kim and Z. Lee investigated the conformality of ALD REOs on Si NWs. K. Y. Ko fabricated Si NWs. C. W. Lee contributed the XPS analysis. S. J. Lee and J. M. Myung measured the wetting property of the ALD REOs films. C. Lansalot-Matras, W. Noh, and C. Dussarrat synthesized Dy and Er precursors for ALD Dy2O3 and Er2O3. H. Kim and H.-B.-R. Lee advised and oversaw the research. All authors contributed to the writing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (10041926, development of high-density plasma technologies for thin-film deposition of nanoscale semiconductor and flexible display processing), funded by the Ministry of Knowledge Economy (MKE, Korea).

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