Biepitaxial Growth of High-Quality Semiconducting NiO Thin Films on

Apr 4, 2012 - We report on the effects of substrate temperature on microstructural characteristics and electrical properties of p-type semiconducting ...
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Biepitaxial Growth of High-Quality Semiconducting NiO Thin Films on (0001) Al2O3 Substrates: Microstructural Characterization and Electrical Properties Ju Ho Lee,†,∥ Yong Hun Kwon,‡,∥ Bo Hyun Kong,§ Jeong Yong Lee,*,† and Hyung Koun Cho*,‡ †

Department of Materials Science and Engineering, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746, Korea § R&D Institute, Samsung LED, Yongin-si 446-711, Korea ‡

ABSTRACT: This paper reports the effects of substrate temperature on the microstructural characteristics and electrical properties of p-type semiconducting NiO thin films grown on (0001) Al2O3 substrates. NiO thin films were biepitaxially grown on (0001) Al2O3 substrates by radiofrequency magnetron sputtering, and they showed specific crystallographic orientation relationships: [1̅1̅0]NiO∥[011̅0]Al2O3, [1̅12̅]NiO∥[21̅1̅0]Al2O3 (in-plane), and [1̅11]NiO∥[0001̅]Al2O3 (out-of-plane). Thus, a low lattice mismatch of 7.52% was obtained between the NiO thin films and the (0001) Al2O3 substrates. The film grown at 600 °C consisted of cubic and rhombohedral NiO grains, while the NiO thin films grown at substrate temperature below 400 °C only consisted of cubic NiO grains. Atoms at the grain boundaries between the cubic and the rhombohedral NiO grains perfectly coincided with each other because of the same atomic stacking sequences along [11̅1̅]c‑NiO and [0003]r‑NiO and with equal interatomic distances. Further, the paper discussed the observations of the perfectly coinciding nickel and oxygen atoms at the grain boundaries between the cubic and the rhombohedral NiO grains using high-resolution transmission electron microscopy (HRTEM) along with atomic modeling on the atomic scale. In addition, the dependence of the electrical properties of the NiO thin films on the substrate temperature and crystallinity is presented in this paper.

1. INTRODUCTION In recent years, ZnO-based semiconductors have been extensively investigated because of their superior optical and electrical properties and considered to overcome the limitation of the current Si-based electronics. A wide direct band gap of 3.37 eV with high transparency (∼80%) in the visible region is appropriate for optical devices such as light-emitting diodes (LEDs) and optical sensors in the ultraviolet region.1−4 For next-generation electronics, development of p-type oxide-based materials is vitally important to realize various applications such as p−n junction diodes, LEDs, and complementary metal oxide semiconductors. However, fabrication of stable p-type oxidebased materials, especially p-type ZnO thin films by doping of group V elements in the periodic table, has not been successful because of their self-compensation caused by a large background n-type carrier concentration.5,6 Therefore, stable p-type oxide-based materials such as Cu2O, SnO, and NiO were studied as novel p-type oxides with high stability and reproducibility. © 2012 American Chemical Society

Among the aforementioned p-type oxide-based materials, great attention has been paid to NiO thin films because of their excellent electrical, optical, and magnetic properties. NiO single crystals have chemical stability, a wide direct band gap of 3.7 eV, and high transparency in the visible region.7 It has been theoretically and experimentally proven that the stable p-type characteristics of NiO can be induced by nickel vacancies and/ or oxygen interstitials. These unique properties of NiO allow it to be used in versatile applications such as formation of p−n junctions with n-type ZnO as well as transparent conducting films (TCOs), electrochromic devices, spin-valve giant magnetoresistance sensors, gas sensors, and cathodes in alkaline batteries.7−11 Generally, NiO thin films are synthesized as randomly oriented polycrystallines, and this polycrystallinity explains their isotropic properties. However, polycrystalline NiO includes Received: January 26, 2012 Revised: March 28, 2012 Published: April 4, 2012 2495

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After film deposition, the morphologies of the synthesized NiO thin films were observed by performing field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). A Hall measurement system (Ecopia, HMS-3000) was used at room temperature to investigate the electrical properties of the NiO thin films. Microstructural and crystallographic characterization of the synthesized NiO thin films was carried out using XRD (Bruker D8 DISCOVER) with Cu Kα radiation (λ = 1.54 Å) and no monochromator and TEM. The plan-view TEM specimens were prethinned by mechanically polishing the backside of the substrate using a tripod polisher, and the specimens were finally thinned using an ion miller for electron transparency. Cross-sectional TEM specimens were prepared by mechanical polishing, and they were ion milled at a low-current Ar+ ion dose to prevent deformation during ion milling. The prepared TEM specimens were examined using a JEOL JEM-3010 microscope with a LaB6 electron source operated at 300 kV.

numerous defects such as grain boundaries and dislocations that act as trap sites for carriers, thus significantly reducing carrier mobility. Therefore, from the viewpoint of the material properties, epitaxially grown crystalline materials with high crystallinity are expected to have a great advantage because of their reduced structural defects, even though they show anisotropic crystallographic orientation characteristics because of differences related to the stacking sequence of the atoms and the interatomic distance. To obtain epitaxial NiO thin films with anisotropic properties, various deposition methods such as pulsed laser deposition, e-beam evaporation, atomic layer deposition, and low-pressure metal−organic chemical vapor deposition have been used.12−15 However, commercially available physical vapor deposition (PVD) techniques such as sputtering methods are widely used to study the thermal and electrical insulating properties of the dielectric layer for phasechange random access memory applications. Surprisingly, these PVD techniques are very limitedly used to synthesize p-type semiconducting thin films.16 In addition, previous studies that reported on the synthesis of p-type semiconducting thin films have shown randomly oriented polycrystalline films with isotropic properties. Furthermore, there are a very limited number of reports on the growth and microstructural characterizations of epitaxially grown NiO thin films. This paper reports the biepitaxial growth of high-quality ptype semiconducting NiO thin films on (0001) Al 2 O 3 substrates by radiofrequency (RF) magnetron sputtering. Insulating crystalline substrates such as TiO, SrTiO3, and Al2O3 are commonly used to grow epitaxial crystalline films. Among the previously mentioned insulating crystalline substrates, the Al2O3 substrate is widely used because its insulating property prevents possible current shunts that can occur at metal substrates and minimizes the possibility of chemical reactions between the thin film and the substrate.17 In addition, crystalline thin films having specific crystallographic orientation relationships (CORs) with respect to the substrates are comparatively easily obtainable on the Al2O3 substrates. In addition, the substrate temperature in the PVD technique is well known to be the most critical parameter for the microstructural characteristics of crystalline materials. Therefore, we synthesized NiO thin films on (0001) Al2O3 substrates with high surface quality under various substrate temperatures and investigated the effects of the substrate temperature on the microstructural characteristics of the NiO thin films on an atomic scale by X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, we studied changes in the electrical properties of the NiO thin films according to the substrate temperature.

3. RESULTS AND DISCUSSION To evaluate the effects of the substrate temperature on the microstructural characteristics and electrical properties of the NiO thin films, the substrate temperature was varied from room temperature to 600 °C in steps of 200 °C. Figure 1 shows

Figure 1. Tilted-view SEM images of NiO thin films grown at substrate temperatures of (a) room temperature, (b) 200 °C, (c) 400 °C, and (d) 600 °C. Scale bars indicate 100 nm.

2. EXPERIMENTAL SECTION tilted-view SEM images of the synthesized NiO thin films on the (0001) Al2O3 substrates by RF magnetron sputtering. As shown in Figure 1a−c, the NiO thin films that were grown below 400 °C show well-defined column structures. In contrast, the NiO thin film grown at 600 °C has a coalesced structure with a large grain size, as shown in Figure 1d. Therefore, to avoid formation of the coalesced structures and obtain a column structure instead, a substrate temperature less than 600 °C should be chosen. As the substrate temperature increased, the film thickness decreased slightly because of the desorption of adatoms on the film surface; this result was also well confirmed by cross-sectional TEM observations (Figure 4).

High-quality NiO thin films with a film thickness of ∼200 nm were grown on (0001) Al2O3 substrates by RF magnetron sputtering. To determine the effects of the substrate temperature on the microstructural characteristics and electrical properties of the NiO thin films, the substrate temperature was varied from room temperature to 600 °C in steps of 200 °C. Before NiO thin films were grown on the substrates, the (0001) Al2O3 substrates were degreased in an ultrasonic bath using acetone, ethanol, and deionized water for 10 min each and then dried in an oven for 10 min. A commercial NiO target (purity 99.99%) was used as a single source, and the working pressure was fixed at 15 mTorr using high-purity oxygen gas (O2, 99.9999%) at a flow of 30 sccm. The substrate-to-target distance was approximately 12 cm. Sputtering power was maintained at 150 W during film deposition. 2496

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alignment on the Al2O3 substrate, because the three intense and sharp peaks in the rhombohedral Al2O3 (11̅02) φ scan exist at 30° intervals to the NiO (111) peaks in the φ scan. To further understand the CORs between the NiO thin films and the (0001) Al2O3 substrates as well as the microstructural characteristics of the NiO thin films, we carried out TEM observations. Figure 3a−d shows plan-view bright-field TEM images with the corresponding selected area diffraction patterns (SADPs) of

To obtain information about the microstructural characteristics of the NiO thin films, XRD measurements were performed. Figure 2a shows the XRD patterns of the NiO

Figure 2. (a) XRD patterns obtained from NiO thin films grown under different substrate temperatures. All XRD patterns were normalized with the Al2O3 (0006) plane at 41.72°. (b) XRD φ scans at 2θ = 25.58° and ψ = 57.61° for the Al2O3 {11̅02} reflections and at 2θ = 43.28° and ψ = 54.37° for the NiO {111} reflections.

thin films grown under different substrate temperatures. As shown in Figure 2a, only the (111) NiO peaks were observed in all synthesized films. These XRD results indicate the existence of NiO (111) planes oriented horizontally with respect to the substrate surface. It is worth noting that as the substrate temperature increased from room temperature to 600 °C the (111) NiO peaks in the XRD patterns shifted from 36.22° to 37.15° and the peak position of 37.15° corresponding to the NiO (111) planes grown at a substrate temperature of 600 °C was almost similar to that of bulk NiO (PDF #47-1049). This means that compressive stress was applied to the perpendicular direction of the (111) planes of the NiO films fabricated at a relatively low substrate temperature (≤400 °C), and this stress was almost relieved under a higher substrate temperature (i.e., 600 °C). This result indicates that stress-free NiO thin films with strong diffraction intensity could be obtained at a high substrate temperature of 600 °C. In order to define the CORs between the NiO thin films and (0001) Al2O3 substrates, we chose the film grown at 600 °C, which had almost no stress, and performed an XRD φ scan. In the XRD φ scan, the specimen was tilted 57.61° and 54.73° with respect to the Al2O3 (0001) planes to detect the {11̅02} Al2O3 and {100} NiO planes, respectively. As shown in Figure 2b, the six peaks in the NiO (111) φ scan with rotational intervals of exactly 60° clearly demonstrate that the NiO thin film grown at 600 °C was deposited with a specific in-plane

Figure 3. Plan-view bright-field TEM images with SADPs obtained from NiO thin films grown at (a) room temperature, (b) 200 °C, (c) 400 °C, and (d) 600 °C. Scale bars indicate 100 nm. SADPs were taken using an aperture diameter of 1.4 μm. (e) Schematic diagram of the atom positions of (111) NiO grown on (0001) Al2O3. Dots mark the position of oxygen atoms, and dashed lines show the unit cells of Al2O3. Squares mark the position of nickel atoms, and solid lines show the unit cells of (111) NiO.

the NiO thin films grown at various substrate temperatures. As shown in Figure 3a−d, the average grain size of the NiO thin film measured from the plan-view TEM images increased with increasing substrate temperature. Furthermore, the intensity of the SADPs from the NiO thin films also increased with increasing substrate temperature. These phenomena indicate that the degree of the preferred orientation for the (111) planes of the NiO thin films was enhanced by elevating the substrate temperature. The SADPs obtained with the beam direction of [1̅11]NiO for the NiO thin films (insets of Figure 3a−d) are regular hexagons in shape. From a crystallographic point of view, it is noteworthy that the (111) planes of NiO with cubic structures show 3-fold symmetry. Therefore, the six peaks with the 60° intervals in the NiO (111) φ scan (Figure 2b) and the 2497

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other hand, the structure of the NiO thin film grown at 600 °C is different from those grown below 400 °C. This difference is attributed to the coalescence of the adjacent columns of the films grown below 400 °C as a result of the high adatom mobility under the relatively high substrate temperature of 600 °C. As a result, the column structure collapsed, as shown in Figures 1d and 4d. In addition, the surface roughness and average column width of the NiO thin films increased up to the substrate temperature of 400 °C. Meanwhile, the NiO thin film grown at 600 °C had a relatively flat film surface compared to those grown below 400 °C. This phenomenon is also related to the high adatom mobility under the higher substrate temperature.18,19 As shown in the SADP images in Figure 4, all SADPs obtained not only from the NiO/Al2O3 interface but also from the NiO thin films show the same diffraction patterns (DPs) regardless of the substrate temperature. This means that biepitaxial NiO thin films can be obtained even at room temperature. In addition, all SADPs obtained from the films exhibit an arc shape. However, the angle of the arced diffraction spots decreased and converted to more distinct single spots with increasing substrate temperature. These results indicate that as the substrate temperature increased the tendency of the NiO thin film for the preferred orientation increased. The zone axes of the NiO thin films and the (0001) Al2O3 substrate for the SADPs obtained from cross-sectional TEM specimens were [1̅1̅0]NiO and [01̅10]Al2O3, respectively, and these CORs are in good agreement with the CORs that were confirmed by the plan-view TEM observations (Figure 3e). To examine the microstructural characteristics of the NiO thin films in detail we chose the NiO thin film grown at 600 °C, which showed a relatively high degree of preferred orientation compared to those grown below 400 °C. Figure 5a shows a high-resolution TEM (HRTEM) image obtained from the

regular hexagon shape of the SADPs in the plan-view TEM images (insets of Figure 3a−d) indicate that the NiO thin films were biepitaxially grown on the (0001) Al2O3 substrates with two types of domains with a 60° in-plane rotation with respect to the [11̅ 1]NiO zone axis. Therefore, on the basis of XRD analysis and plan-view TEM observations, it can be concluded that the CORs between the NiO thin films and (0001) Al2O3 substrates are as follows: (i) [1̅ 1̅ 0 ] NiO ∥[011̅ 0 ] Al2O3 , [11̅ 2]̅ NiO∥[211̅ 0̅ ]Al2O3 (in-plane) and (ii) [11̅ 1]NiO∥[0001]̅ Al2O3 (out-of-plane). The origin of these CORs between the NiO thin films and (0001) Al2O3 substrates is related to the degree of lattice mismatch. When the CORs are [1̅1̅0]NiO∥[1̅21̅0]Al2O3 and [1̅12̅]NiO∥[101̅0]Al2O3, the lattice mismatch can be calculated as follows 2 aNiO − aAl2O3 × 100 = 24.15% aAl2O3

where the lattice parameters of aNiO and aAl2O3 are 4.177 and 4.758 nm, respectively. However, the lattice mismatch of 24.15% can be reduced to 7.52% by introducing domain matching epitaxy (DME). This 7.52% lattice mismatch can be calculated as follows ( 2 aNiO × sin 60◦) × 2 − (aAl2O3 × 2) × 100 = 7.52% (aAl2O3 × 2)

where each lattice parameter is multiplied by 2 to introduce DME. In short, the NiO lattice was aligned along the oxygen sublattice of Al2O3 to reduce the strains and defect density in the NiO thin films, as shown in Figure 3e. Figure 4 shows cross-sectional bright-field TEM images with the corresponding SADPs of the NiO thin films grown at various substrate temperatures. The NiO thin films grown below 400 °C are comprised of many columns, and the corresponding cross-sectional TEM images (Figure 4a−c) are consistent with the SEM observation results (Figure 1). On the

Figure 5. (a) Cross-sectional HRTEM image along the [1̅1̅0]NiO zone axis of NiO/Al2O3. (b, c, and d) Fast Fourier transform (FFT) images obtained from each grain, denoted as I, II, and III, respectively. (e) Cross-sectional HRTEM image obtained from the rectangular region marked by the dotted line in Figure 4d. c-NiO and r-NiO indicate cubic and rhombohedral NiO, respectively. (f) Schematic atomic modeling for the interface between the c-NiO and the r-NiO grains. (g) Cross-sectional HRTEM image obtained from the rectangular region marked by the solid line in Figure 4d. (Inset) FFT image obtained from the grain boundary between two c-NiO grains.

Figure 4. Cross-sectional bright-field TEM images with SADPs obtained from NiO thin films grown at (a) room temperature, (b) 200 °C, (c) 400 °C, and (d) 600 °C. Left column of SADP images was obtained from the NiO/Al2O3 interface using an aperture size of 1.4 μm. Right column of SADP images was obtained from the NiO thin films. 2498

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interface between the NiO thin film and the (0001) Al2O3 substrate. Two different types of grains were observed, as shown in Figure 5a, and these grains were identified as cubic NiO (henceforth referred to as “c-NiO”, grains I and III in Figure 5a) and rhombohedral NiO (henceforth referred to as “r-NiO”, grain II in Figure 5a) by indexing the lattice parameters of each grain and the DP images obtained by a fast Fourier transform (FFT). In addition, the r-NiO grains were observed not only at the NiO thin film/Al2O3 substrate interface but also at the middle and top parts of the film, as shown in Figure 5e. Interestingly, r-NiO grains were not observed in the NiO thin films grown below 400 °C (not shown here). The c-NiO (PDF #47-1049) with a space group of Fm3m (225) and the r-NiO (PDF #44-1159) with a space group of R3̅m (166) have completely different crystal systems and lattice parameters. However, there is almost no difference between the angles of c-NiO, which satisfy Bragg’s diffraction law in the XRD measurements, and that of r-NiO, even though the Miller index [i.e., (hkl)] of a c-NiO plane is different from that of an r-NiO plane. As a consequence, no XRD peaks related to r-NiO were observed in the XRD measurements, and a TEM study is needed for further detailed microstructural characterization. The r-NiO grains existed between two c-NiO grains, as can be seen in Figure 5a and 5e. In spite of the different crystal systems, the c-NiO and r-NiO grains have an epitaxial relationship of (11̅1̅)c‑NiO∥(0001)r‑NiO, [11̅1̅]c‑NiO∥[0001]r‑NiO, and [110]c‑NiO∥[1̅100]r‑NiO, and the d spacings of the (11̅1̅) planes of c-NiO and (0003) planes of r-NiO are almost the same at 0.241 and 0.2409 nm, respectively. In addition, the interatomic distances of c-NiO and r-NiO and the stacking sequences along [11̅1̅]c‑NiO and [0001]r‑NiO are also the same. Therefore, the atoms of the c-NiO and r-NiO grains perfectly coincide with each other at the grain boundaries between cNiO and r-NiO. An atomic-scale schematic diagram showing the perfect atomic matching at the grain boundaries is shown in Figure 5f. Figure 5g shows an HRTEM image obtained from the inclined bright contrast line marked with the solid box in Figure 4d. The line from the substrate to the film surface is identified as an inclined grain boundary (IGB), which was formed by collision of two adjacent c-NiO grains with slightly tilted [11̅1̅]NiO with respect to the (0001) planes of the substrate. The atoms of each grain were well matched at the IGB, as shown in Figure 5e. However, the atomic bonding strength at the IGB was weaker than inside the grains. Therefore, this IGB was preferentially ion milled because of a weaker atomic bonding strength during preparation of the TEM specimen, resulting in the bright contrast. In order to evaluate the effects of the substrate temperature on the electrical properties of the NiO thin films, Hall measurements were conducted at room temperature. Figure 6 shows the electrical properties of the NiO thin films grown under various substrate temperatures. The mobility and resistivity of the NiO thin films grown below 400 °C are ∼10 cm2/(V s) and ∼10 Ω cm, respectively. These mobility values are considerably higher than those of polycrystalline NiO thin films grown on amorphous substrates (∼10−1 cm2/(V s)), as reported elsewhere.20 The high mobility of our NiO thin films can be attributed to the reduced defects, which work as trap sites for carriers. However, the mobility and resistivity rapidly changed as the substrate temperature increased from 400 to 600 °C with enhancing crystallinity. The mobility and resistivity of the NiO thin film grown at 600 °C were 7.352

Figure 6. Electrical properties of NiO thin films as a function of substrate temperature.

cm2/(V s) and 521.4 Ω cm, respectively, even though it had fewer defects than the NiO thin films grown below 400 °C. On the basis of the experimental facts, we can conclude that the relatively low mobility and high resistivity of the NiO thin film grown at 600 °C originated from the relatively high crystallinity and perfectly coinciding grain boundaries between the c-NiO and the r-NiO grains. According to Oka et al., the electrical properties of epitaxially grown NiO thin films were strongly affected by band edge tailing, which was caused by crystallinity deterioration. Imperfections and/or defects in thin films not only work as trap sites but cause band tailing, and these bands merge with the nearest parent bands.21,22 Thus, the drastic increase in the resistivity of the NiO thin film grown at 600 °C was closely related to the enhanced crystallinity, which reduced the extended states. Therefore, the resistivity increased as a result of the reduced extended states with decreasing imperfections. On the other hand, if the energy states were fully localized and the direct current conductivity was zero at a temperature of 0 K, conduction would take place only by a hopping mechanism,23 that is, the localized states of the NiO thin film grown at 600 °C predominantly affected the electrical properties of the thin film. In addition, conduction highly depended on the hopping mechanism, which reduced the mobility and increased the resistivity. It has already been proved experimentally that the XRD peak for the NiO thin film grown at 600 °C was the most intense peak among the NiO thin films synthesized under various substrate temperatures, and the (111) NiO peak shifted to the same peak position as the (111) planes of bulk NiO, as shown in Figure 2a, which has fewer extended states and insulating characteristics. Biepitaxial growth of semiconducting NiO thin films was successfully achieved using commercially available Al2O3 substrates and RF magnetron sputtering for optoelectronic devices. In addition, the microstructural characterization of NiO thin films consisting of cubic and rhombohedral NiO and their atomic modeling was successfully studied using HRTEM. The electrical properties of these NiO thin films could be controlled by defect density and doping, and a study on the development of the p-type NiO films with a high carrier density at a high growth temperature using Li doping is underway.

4. CONCLUSIONS We reported the microstructural characteristics and electrical properties of NiO thin films grown on (0001) Al2O3 substrates under various substrate temperatures by RF magnetron sputtering. XRD and TEM results revealed that the NiO thin films were biepitaxially grown on the Al2O3 substrates and had two types of domains with a 60° in-plane rotation with respect to the [1̅11]NiO zone axis. The CORs between the NiO thin films and Al2 O3 substrates were [1̅1̅0 ]NiO ∥[011̅0 ]Al2O3 , [1̅12̅]NiO∥[21̅1̅0]Al2O3 (in-plane), and [1̅11]NiO∥[0001̅]Al2O3 2499

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(15) Sun, J. W.; Kim, H. S.; Ji, B. K.; Park, H. W.; Hong, G. W.; Jung, C. H.; Park, S. D.; Jun, B. H.; Kim, C. J. IEEE Trans. Appl. Supercond. 2003, 13, 2. (16) Loke, D.; Shi, L.; Wang, W.; Zhao, R.; Ng, L.-T.; Lim, K-.G.; Yang, H.; Chong, T.-C.; Yeo, Y-.C. Appl. Phys. Lett. 2010, 97, 243508. (17) Hildner, M. L.; Minvielle, T. J.; Wilson, R. J. Surf. Sci. 1998, 396, 16. (18) Shin., J. W.; Lee, J. Y.; No, Y. S.; Kim, T. W.; Cho, W. K. J. Appl. Phys. 2006, 100, 013526. (19) Lee, J. H.; Ahn, C. H.; Hwang, S.; Woo, C. H.; Park, J.-S.; Cho, H. K.; Lee, J. Y. Thin Solid Films 2011, 519, 6801. (20) Chen, H.-L.; Lu, Y.-M.; Hwang, W.-S. Surf. Coat. Technol. 2005, 198, 138. (21) Oka, K.; Yanagida, T.; Nagashima, K.; Tanaka, H.; Kawai, T. J. Appl. Phys. 2008, 104, 013711. (22) Dimitriadis, C. A.; Tassis, D. H.; Economou, N. A.; Giakoumakis, G. Appl. Phys. Lett. 1994, 64, 2709. (23) Elliott, E. R. Physics of amorphous materials; Longman: London, 1990; p 194.

(out-of-plane) and showed a reduced lattice mismatch of 7.52%. The NiO thin film grown at 600 °C consisted of cubic and rhombohedral NiO. In spite of the different crystal systems, a specific epitaxial relationship between c-NiO and r-NiO existed: (11̅1̅)c‑NiO∥(0001)r‑NiO, [11̅1̅]c‑NiO∥[0001]r‑NiO, and [110]c‑NiO∥[1̅100]r‑NiO. The mobility and resistivity of the NiO thin films were affected by the defect density in the film, and the carrier concentration as a p-type oxide layer for a p−n junction should be enhanced.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Y.L.); [email protected] (H.K.C.). Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work at KAIST was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0029714).This work was also supported by the Center for Inorganic Photovoltaic Materials (No. 20120001170) grant funded by the Korea government (MEST). The research at SKKU was also financially supported by the IT R&D program of MKE/KEIT (KI002182) and the Energy International Collaboration Research & Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the MKE (2011-8520010050). This work was also supported by Mid-career Researcher Program through NRF grant funded by the MEST (Grant No. 2011-0009494).

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REFERENCES

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