Wafer-scale and selective-area growth of high-quality hexagonal boron nitride on Ni(111) by metal-organic chemical vapor deposition

We demonstrate wafer-scale growth of high-quality hexagonal boron nitride (h-BN) film on Ni(111) template using metal-organic chemical vapor deposition (MOCVD). Compared with inert sapphire substrate, the catalytic Ni(111) template facilitates a fast growth of high-quality h-BN film at the relatively low temperature of 1000 °C. Wafer-scale growth of a high-quality h-BN film with Raman E2g peak full width at half maximum (FWHM) of 18~24 cm−1 is achieved, which is to the extent of our knowledge the best reported for MOCVD. Systematic investigation of the microstructural and chemical characteristics of the MOCVD-grown h-BN films reveals a substantial difference in catalytic capability between the Ni(111) and sapphire surfaces that enables the selective-area growth of h-BN at pre-defined locations over a whole 2-inch wafer. These achievement and findings have advanced our understanding of the growth mechanism of h-BN by MOCVD and will contribute an important step toward scalable and controllable production of high-quality h-BN films for practical integrated two-dimensional materials-based systems and devices.

www.nature.com/scientificreports www.nature.com/scientificreports/ metal substrates such as Ni 25 can facilitate the growth of h-BN at relatively low temperatures while maintaining the advantages offered by MOCVD including multi-wafer scale growth and very precise control over the combination of complex growth parameters. However, there has been little study on the wafer-scale growth of h-BN layers on metal substrates using MOCVD. In addition, the understanding and utilization of the catalytic effect of transition metal substrates during MOCVD growth can enable a site-controlled, i.e., selective-area growth of h-BN that can realize a fundamental micro-scale "h-BN building block" for integrated 2D materials-based devices and systems. Compared with many attempts for scale-up synthesis and spatially-controlled growth of other 2D materials including graphene 26,27 , little efforts have been made for a large-and-selective area growth of high-quality h-BN by MOCVD.
In this study, we report the wafer-scale growth of high-quality few-layer h-BN film on Ni(111) template at the relatively low temperature of 1000 °C using a commercial multi-wafer MOCVD reactor. Various characterization tools including Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to compare the microstructural and chemical characteristics of the MOCVD-grown h-BN films on the Ni(111) template and sapphire substrates. It was suggested that the adsorption and decomposition of NH 3 and its radicals is catalytically facilitated on the Ni(111) surface, whereas the NH 3 precursor is incompletely decomposed on the sapphire, resulting in a substantial difference in the growth kinetics and quality of the h-BN films. Based on the experimental results, we propose a promising route for the micro-scale selective-area growth of h-BN at desired locations over a whole 2-inch wafer using conventional lithography techniques. We believe this can be a significant progress toward the multi-wafer scale production of high-quality h-BN building blocks for integrated 2D materials-based devices and systems.

Results and Discussion
The growth of h-BN on a Ni(111) template on sapphire was carried out using a commercial multi-wafer MOCVD system. Prior to the growth, the Ni(111) template was prepared by depositing a 600 nm thick Ni thin film on a 2-inch c-plane sapphire substrate by using a sputtering system. Then, the as-deposited Ni template was annealed in the MOCVD reactor at 1050 °C and 150 mbar for 20 min under H 2 ambient to improve surface flatness and promote the growth of grains with (111) crystallographic orientation in the Ni film 28 . In fcc metals, the surface/ interface energy of (111) planes is the lowest among all crystallographic orientations 29 , therefore, the growth of Ni grains with (111) preferred orientation becomes dominant to minimize the energy between Ni and underlying substrate. The crystallinity and surface quality of the Ni(111) template obtains an epi-ready condition after the thermal annealing as revealed by X-ray diffraction (XRD) patterns, electron backscatter diffraction (EBSD) image ( Fig. 1), scanning electron microscopy (SEM) images, and atomic force microscopy (AFM) ( Supplementary  Fig. S7). As shown in Fig. 1a, the Ni (111) peak becomes larger and sharper while the Ni (200) peak disappears after the annealing, indicating the growth of Ni grains with (111) preferred orientation. The EBSD map of the Ni(111) template after the annealing clearly shows that the individual grains have a uniform (111) orientation with the average grain size of approximately 75 μm (Fig. 1b). The surface of the Ni(111) template after the annealing is very smooth with a root-mean-square (RMS) roughness of 0.605 nm, while the as-deposited Ni template has a much rougher surface topography (see Supplementary Fig. S7).
After the thermal annealing, a h-BN film was grown at 1000 °C and 30 mbar for 50 pulsed injection periods of triethylborane (TEB) and NH 3 as B and N precursors, respectively. The pulsed-mode growth whereby TEB and NH 3 are alternatively injected into the reactor with an interruption time was applied to suppress www.nature.com/scientificreports www.nature.com/scientificreports/ parasitic reactions between B and N gas-phase precursors 30,31 . Figure 2a Figure 2b shows the flat region within the grain, revealing a sub-nanometer smooth surface with a RMS roughness of 0.470 nm (see additional AFM image in Supplementary Fig. S8). In addition, no wrinkle was observed on the whole surface through SEM and AFM measurements. The wrinkle-free feature may result from the strong interfacial interaction between h-BN and Ni(111) 32,33 . To confirm the uniform existence of the h-BN film and its quality, spectroscopic characterizations were carried out. Figure 2c shows the Raman spectra measured at the five positions as depicted in the inset of Fig. 2c. There are distinct peaks with similar intensities at about 1370 cm −1 , which correspond to the E 2g vibration mode of h-BN 34,35 , observed at all the positions over the 2-inch wafer area. The full width at half maximum (FWHM) values of the E 2g peaks are approximately 18~24 cm −1 , comparable to those of monolayer h-BN films exfoliated from sintered crystals 35 , indicating a high-quality h-BN is grown on the Ni(111). Optical absorption spectroscopy was performed for the h-BN film after transfer onto a double-side polished sapphire substrate. The absorbance spectrum exhibits a significant peak at around 202 nm. As shown in the inset of Fig. 2d, the optical bandgap (E g ) derived from the spectrum is approximately 5.9 eV in good agreement with the previously reported values 12,36 . The material characterization results demonstrate that a high-quality, wafer-scale h-BN film can be grown on a Ni(111) template at the relatively low growth temperature of 1000 °C by using a MOCVD system. It is a distinguishable result from previously reported h-BN growth on sapphire in which harsh growth conditions (>1400 °C) were required owing to the lack of catalytic reaction pathways [22][23][24] . To investigate the catalytic effect of the substrate on MOCVD growth of h-BN films, we compared the difference between h-BN growth on the Ni(111) and sapphire. On bare sapphire substrate, h-BN film was grown under identical growth conditions except for the higher growth temperature of 1050 °C and prolonged growth time of 200 source injection periods owing to much slower growth rate on sapphire than on the Ni(111) (see Supplementary Note 2). We confirmed the growth of h-BN on sapphire by measuring Raman and optical absorption spectroscopy ( Supplementary Fig. S9). However, the FWHM value of the Raman peak is 43.2 cm −1 , much larger than those of h-BN on the Ni(111), indicating much lower crystallinity of the h-BN film on sapphire. The remarkable differences in both growth rate and material quality indicate that the catalytic effect of the substrate is a critical factor for MOCVD growth of h-BN.
Microstructural and chemical properties of the h-BN films grown on the Ni(111) and on sapphire were characterized by HR-TEM, NEXAFS spectroscopy, and XPS. Figure 3 shows plan-view and cross-sectional HR-TEM www.nature.com/scientificreports www.nature.com/scientificreports/ micrographs of the h-BN films grown on the Ni(111) and on sapphire. The fast Fourier transform (FFT) pattern of the h-BN film grown on the Ni(111) (inset of Fig. 3a) exhibits a clear set of 6-fold symmetric spots, indicating that the h-BN domains constituting the film have crystallographic homogeneity. Cross-sectional HR-TEM image of the h-BN film on the Ni(111) shows a clearly layered structure of 6-7 layers (Fig. 3b). On the other hand, this shows a large contrast to the h-BN film grown on sapphire which shows a more defective layered structure (Fig. 3c,d). The FFT pattern of the h-BN film grown on sapphire (inset of Fig. 3c) exhibits a 6-fold symmetry with diffused spots as a result of grain misalignment and crystalline imperfections in the h-BN film. See additional cross-sectional HR-TEM images in Supplementary Fig. S10.
The structural properties of h-BN were further investigated by NEXAFS analyses. Figure 4a,b shows B K-edge and N K-edge NEXAFS spectra acquired from the h-BN film grown on the Ni(111), respectively, for different X-ray incident angle θ between the sample plane and the X-ray propagating direction. The NEXAFS peaks at 192.0 eV in the B K-edge spectra (Fig. 4a) and at 401.0 eV in the N K-edge spectra (Fig. 4b) originate from the transitions of core-level electrons into the π* states of h-BN [37][38][39] . On the other hand, the high energy features of both B K-edge and N K-edge NEXAFS spectra come from the transitions of core-level electrons into the σ* states  www.nature.com/scientificreports www.nature.com/scientificreports/ of h-BN [37][38][39] . These spectra reveal that B and N atoms are sp 2 -hybridized, implying the hexagonal structure of the grown BN film. In addition, as the incident angle θ increases from 30° to 70°, the intensity of π* peaks decrease while the σ* peaks increase in both NEXAFS spectra, indicating that each h-BN atomic layer on the Ni(111) is nearly parallel to each other and to the surface of the substrate 37 . Similarly, the h-BN film on sapphire shows spectral features reflecting electronic transitions into π* and σ* states, which indicates the sp 2 -hybridized structure 37 (see Supplementary Fig. S11). Figure 4c compares the normalized B K-edge spectra from the h-BN films grown on the Ni(111) and on sapphire, taken at the X-ray incident angle of 30°. The spectra are almost identical, except for distinctive shoulders of the π* peak appearing in the h-BN film on sapphire (the inset of Fig. 4c). The low-energy shoulder A 1 at around 191.0 eV represents metallic-like B-B bonds from boron clusters 40 . On the other hand, the high-energy shoulders A 2 near 192.7 eV and A 3 at around 193.3 eV represent B bonding with two N and one O atoms, and B bonding with one N and two O atoms, respectively 40 . In other words, nitrogen vacancies are generated during the growth, leaving behind boron dangling bonds. The energetically unstable boron dangling bonds are readily saturated by oxygen atoms when exposed to air. This indicates that the h-BN film grown on sapphire contains a much larger amount of atomic disorders such as boron clusters and nitrogen vacancies compared to the h-BN film on the Ni(111). In addition, XPS chemical analyses reveal the h-BN film on sapphire has a higher B/N ratio and an additional bonding state that is attributed to the BN x O y component, implying the more defective nature of the h-BN film on sapphire (see Supplementary Note 3).
With the experimental observations, we turn to discuss the mechanism involved in the growth of h-BN relying on the catalytic influence of the underlying substrates. Because the growth temperature is far higher than the decomposition temperature of TEB 41 , dehydrogenated TEB is adsorbed on the surface as B atoms or B-containing radicals regardless of the substrate. Therefore, the adsorption and decomposition of NH 3 on the surface of the substrate is very likely to be the key steps determining the growth behavior of h-BN. A high thermal dissociation energy is required for hydrogen dissociation of NH 3 (NH 3 → NH 2 + H, 4.8 eV) 42  On the other hand, in the absence of catalytic activity, most NH 3 molecules remain intact at the substrate surface owing to the very low degree of NH 3 decomposition at typical MOCVD growth temperatures 45 , which causes a significant reduction in activated V/III ratio. The incomplete decomposition of NH 3 precursor can be the bottleneck hampering the formation of h-BN layers on sapphire due to the lack of available N adatoms. As a result, the h-BN film on sapphire grown at the relatively low temperature of 1050 °C has substantial atomic disorders including boron clusters, nitrogen vacancies, and disordered bonding states as shown in the Raman spectroscopy, HR-TEM, NEXAFS spectroscopy, and XPS studies.
Based on the experimental results, selective-area growth of h-BN at desired locations with micro-scale, and even nano-scale, patterned geometries can be enabled by taking advantage of different catalytic activities of underlying substrates. Figure 5 demonstrates the micro-scale selective-area growth of h-BN using a pre-patterned Ni(111) template on sapphire. A cubic array of circles with a diameter of 17 μm and pitch of 48 μm was defined on the 2-inch Ni(111) template by conventional photolithography, followed by wet etching of Ni, forming the array of Ni pillars on sapphire. The selective-area growth of h-BN was performed on the Ni patterns covering the whole 2-inch sapphire with the same condition used for the growth on the Ni(111) template. Figure 5a is a top-view SEM image of as-grown h-BN on the pre-patterned Ni(111) template and Fig. 5b is a corresponding optical image after transferring the grown h-BN layers onto a SiO 2 /Si substrate. Due to the predominant catalytic effect of the Ni(111) patterned area in comparison with the Ni-etched sapphire, a large difference in both growth rate and crystal quality results in the high quality h-BN layers being selectively grown on the Ni(111) patterns. To ensure that the h-BN growth is spatially confined on the Ni(111) patterned area, Raman spectroscopy and scanning photoelectron microscopy (SPEM) were carried out. As shown in Fig. 5c, there is a characteristic Raman peak measured from the transferred h-BN layers on the Ni(111) patterned area, while no such a peak is observed at the Ni-etched sapphire surface, indicating the localized growth of h-BN on Ni(111). The local chemical states of the selective-area grown h-BN layers were investigated using SPEM. Figure 5d,e shows spatially resolved XPS spectra and SPEM images acquired for the B 1 s and the N 1 s core-level energies. The characteristic B 1 s and N 1 s photoelectron spectra are only observed on the h-BN domains, which is consistent with the XPS results of h-BN on the Ni(111) template (see Supplementary Fig. S3 and Supplementary Table S1). The slight blue-shift and the broadening of the peaks are attributed to the surface charge buildup on the spatially-isolated and insulating h-BN 46 . Additional SPEM images for B 1 s and N 1 s photoelectrons acquired at various energy windows are shown in Supplementary Fig. S12. The SPEM measurement confirms the spatially confined growth of h-BN on the Ni(111) patterned area. Figure 5f is a photograph of wafer-scale selective-area grown h-BN after transfer onto a 4-inch SiO 2 /Si substrate. This technique for the selective-area growth of h-BN on a wafer level can significantly contribute to the capabilities of h-BN as a fundamental building block for large-scale 2D materials-based device fabrication and integration.

Conclusions
In summary, we have demonstrated the wafer-scale growth of high-quality few-layer h-BN film on Ni(111) template at the relatively low temperature of 1000 °C using a MOCVD system. It is shown that the catalytic effect of the substrate is a critical factor determining the growth kinetics as well as the crystal quality of h-BN. The Ni(111) template can facilitate the adsorption and decomposition of NH 3 and its radicals, leading to the formation of high-quality h-BN film. In contrast, the NH 3 precursor is incompletely decomposed on the inert sapphire, resulting in the formation of low-crystalline h-BN film at a slower growth rate. In addition, we have demonstrated selective-area growth of h-BN at desired locations with micro-scale geometries by using a photo-lithographically pre-patterned Ni(111) template on sapphire. We believe that these findings have advanced our understanding of the growth mechanism of h-BN by MOCVD and will contribute an important step toward scalable and controllable production of high-quality h-BN building blocks for a variety of practical applications.

Methods
Preparation of Ni(111) template on sapphire. A Ni film (~600 nm thickness) was deposited on a c-plane sapphire substrate via sputtering of a polycrystalline Ni target. The sputtering chamber was vacuumed to a based pressure of 7 × 10 −7 Torr, then filled with Ar gas to a working pressure of 7.0 mTorr. The Ni film was deposited with a sputtering power of 50 W. The as-deposited Ni template was then loaded to a multi-wafer (11 × 2-inch wafers) MOCVD reactor (AIXTRON, AIX 2400 G3-HT, Supplementary Fig. S13) and annealed in H 2 atmosphere at 1050 °C, 150 mbar for 20 min to obtain clean and smooth Ni(111) template. Note that the thickness optimization of the Ni film is discussed in Supplementary Note 4.

MOCVD growth of h-BN films.
After the annealing process, an h-BN film was grown on the Ni(111) template at 1000 °C, 30 mbar in the MOCVD reactor. A pulsed-mode growth with alternating supplies of triethylborane (TEB) and ammonia (NH 3 ) using H 2 carrier gas was applied to restrain gas phase pre-reactions. The procedure of the pulsed-mode growth was composed of 4 steps per period: (1) 5 seconds of TEB injection, (2) 2 seconds of H 2 interruption, (3) 4 seconds of NH 3 injection, and (4) 2 seconds of H 2 interruption. The flow rates of TEB and NH 3 were maintained at 10 sccm and 8000 sccm, respectively, corresponding to the V/III ratio of around 23500 with consideration for the individual source injection times. A total of 50 source injection periods were applied for the growth of h-BN films on Ni(111). As a control experiment, the h-BN film was also grown on a bare c-plane sapphire substrate at 1050 °C, 30 mbar for 200 periods due to the slower growth rate.
Selective-area growth of h-BN. Conventional photolithography was used to fabricate a pre-patterned Ni(111) template on sapphire for selective-area growth of h-BN. First, a photoresist was spin-coated on the Ni template on sapphire, followed by a 100 °C baking on a hot plate. After pattern transfer and development, FeCl 3 reagent was used to etch the patterned Ni template. The sample was then cleaned using solvents such as acetone, isopropyl alcohol (IPA), and distilled water, leaving a well-defined array of Ni pillars. Afterwards, the pre-patterned Ni template was loaded into the MOCVD reactor and annealed in H 2 atmosphere at 1050 °C, 150 mbar for 20 min. Finally, h-BN was grown at 1000 °C, 30 mbar for 50 source injection periods.