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[NiO_Ga2O3.docx](https://mdr.nims.go.jp/filesets/7ea8ef08-0da7-442d-ab6e-1d8caa7238ee/download)

## Creator

[Takayoshi Oshima](https://orcid.org/0000-0001-8550-9735), Shinji Nakagomi

## Rights

© 2023 The Japan Society of Applied Physics<br>
This is an author-created, un-copyedited version of an article accepted for publication /published in Japanese Journal of Applied Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript orany version derived from it. The Version of Record is available online at https://doi.org/10.35848/1347-4065/ad0ac9.[Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/)

## Other metadata

[Epitaxial relationship of NiO on (-102) β-Ga2O3](https://mdr.nims.go.jp/datasets/0918752a-1e3f-466e-b058-313716ccf3cd)

## Fulltext

Template for JJAP Rapid Communications and Brief Notes (Mar. 2022)Epitaxial relationship of NiO on (02) β-Ga2O3Takayoshi Oshima1*, Shinji Nakagomi21Research Center for Electronic and Optical Materials, National Institute for Materials Science, Namiki, Tsukuba 305-0044, Japan2Faculty of Science and Engineering, Ishinomaki Senshu University, Minamisakai, Ishinomaki 986-8580, JapanE-mail: OSHIMA.Takayoshi@nims.go.jpWe investigated the epitaxial relationship of an electron-beam-evaporated NiO film on a custom-ordered (02) β-Ga2O3 substrate with a surface orientation rotated by 13.8° around [010] axis relative to the commonly-used (001) substrate. X-Ray diffraction and TEM measurements confirmed that the film was monocrystalline with out-of-plane and in-plane alignments of (110)NiO||(02)β-Ga2O3 and NiO||[010]β-Ga2O3, respectively, indicating that the film grew on the substrate while maintaining the orientation of the ccp oxygen sublattice across the interface. The use of the low-index (110) epitaxial plane within the ccp lattice may be preferred for pn-heterojunctions over the higher-index (331) plane on the (001) β-Ga2O3 substrate.β-Ga2O3 has attracted considerable attention among wide-bandgap semiconductors due to its exceptionally large bandgap of nearly 5 eV, wide effective donor doping range from 1015 to 1020 cm−3, and availability of melt-grown scalable wafers, positioning it as one of the primary power semiconductors for the development of low-cost power devices with efficient power conversion.1) Although the lack of p-type conductivity in β-Ga2O3 still remains a critical drawback, p-type NiO with a bandgap of 4.3 eV2) can be used as an alternative. This approach using NiO as a p-type counterpart in β-Ga2O3-based devices has shown considerable success, as demonstrated by pn-heterojunction diodes,3)4)5)6) junction barrier diodes,7) Schottky barrier diodes with field-limiting rings8) and junction termination extensions9), and superjunction transistors10) with promising device performance. In particular, vertical pn-heterojunction diodes have demonstrated remarkable withstand voltage and on-resistance trade-offs that exceed the unipolar limits of SiC and GaN.4)5)6) This breakthrough has generated increased interest in the NiO/β-Ga2O3 heterojunction within the β-Ga2O3 research community.To effectively utilize the NiO/β-Ga2O3 heterojunction, it is essential to understand its crystal orientation relationship. In general, the basic structural framework of most metal oxides is formed by an oxygen sublattice, due to the larger radius of anions compared to cations.11) This principle holds true for β-Ga2O3 with the monoclinic β-gallia structure12) and NiO with the cubic rock salt structure,13) whose frameworks are both formed by cubic-close-packed (ccp) oxygen ions. As a result, epitaxial NiO films on β-Ga2O3 substrates maintain structural coherence within the ccp oxygen lattice, as evidenced by NiO films oriented along (100), (111), (110), and near (331) grown on commercially available (100), (01), (010), and (001) β-Ga2O3 substrates, respectively.14) Among the reported heterojunctions, only the (331) NiO/(001) β-Ga2O3 configuration aligns with the higher-index (331) oxygen plane in the ccp lattice, causing the low-index (110) of NiO to be tilted by 13.8° from the surface normal around the [010] axis of β-Ga2O3.14) Therefore, the (001) β-Ga2O3 substrate may not be the optimal choice when considering the heterojunction from the perspective of the ccp oxygen framework. While, other orientations of (100), (01), and (010) for β-Ga2O3 substrates should not be preferred due to the crystal defects on the β-Ga2O3 side. Twin domains are formed during homoepitaxy on the (100) and (01) substrates,15)16) while many dislocations and voids exist along [010] to appear on the (010) surface.17) These domain boundaries, dislocations, and voids can act as leakage paths and hinder the application of vertical power devices. Therefore, it is still worthwhile to explore substrate orientation in order to create a pn heterojunction with an interface consisting of a low-index ccp plane, while addressing the problems associated with defects in β-Ga2O3. We propose the use of (02) orientation substrate for the fabrication of a desirable NiO/β-Ga2O3 heterojunction. The (02) orientation is rotated by 13.8° around the [010] axis relative to the surface normal of the (001) β-Ga2O3, making (110) the expected epitaxial plane of NiO. The appearance of the (110) epitaxial plane is also expected from the fact that (02) β-Ga2O3 films grow epitaxially on (110) MgO substrates,18) which have the same crystal structure as NiO. Furthermore, single-domain homoepitaxial growth can be achieved on the (02) β-Ga2O3 substrates,19) and most of the dislocations and voids along [010] in the β-Ga2O3 crystal do not appear on the surface because (02) is parallel to [010]. Therefore, in this study, we performed epitaxial growth of NiO on a (02) β-Ga2O3 substrate and investigated the resulting epitaxial relationship.A 150-nm thick NiO film was grown by electron beam evaporation using NiO grains of 99.9% purity as the target source on a custom-ordered (02) β-Ga2O3 substrate (Novel Crystal Technology Inc.) whose mechanochemically-polished substrate surface was atomically flat with a root mean square roughness of 0.20 nm. The evaporated NiO was deposited on the substrate on a holder heated at 600°C under a chamber pressure of ~2 mPa at a deposition rate of ~3.8 nm/min. The epitaxial orientation of the film was characterized by X-ray diffraction (XRD) measurement using a monochromatic Cu Kα1 radiation. The epitaxial structure near the film/substrate interface was examined by a cross-sectional transmission electron microscopy (TEM) using an acceleration voltage of 200 kV.First, the crystal orientation of the film was characterized by XRD measurement, as shown in Fig. 1. In the θ-2θ scan patterns, only a single 220 NiO peak was observed apart from the substrate peak [Figs. 1(a) and 1(b)], confirming that the film was grown with a single (110) orientation on the (02) substrate, as expected. In the ϕ scan patterns, the 020 and 002 NiO peaks, both of which are crystallographically equivalent, appeared at 180° apart and ±90° away from the 01 β-Ga2O3 peak [Fig. 1(c)]. Considering the in-plane twofold symmetry of the (110)-oriented cubic crystal structure of NiO, the ϕ scan result indicates the single crystallinity of the NiO film. Furthermore, the in-plane epitaxial relationship was determined to be NiO || [010] β-Ga2O3. Consequently, the out-of-plane and in-plane epitaxial relations were summarized as (110) NiO || (02) β-Ga2O3 and NiO || [010] β-Ga2O3, respectively. In addition, the crystal mosaicity of the grown film was evaluated by the full-width at half maximums (FWHMs) of the ω-rocking curves of the diffraction peaks found in the θ-2θ and ϕ scans [Fig. 1(d)]. The FWHMs of the 220 and 200 NiO peaks (0.51 and 0.56°, respectively) were more than one order of magnitude larger than those of the  and  β-Ga2O3 peaks (0.026 and 0.026°, respectively), indicating that the film exhibited some degree of tilt and twist misorientations due to the lattice-mismatched heteroepitaxy.A cross-sectional TEM was then performed to observe the epitaxial structure at the interface, as shown in Fig. 2. In the low magnification image, the entire region of the NiO film was observed [Fig. 2(a)]. The film showed uniform thickness due to the electron beam evaporation method. However, some grain boundaries and/or dislocations were present in the film, consistent with the relatively large FWHM values of the XRD ω-rocking curves. In the high-resolution TEM image in the vicinity of the interface, well-aligned crystal lattices of the NiO film and the β-Ga2O3 substrate were clearly visible [Fig. 2(b)]. However, the interface was not abrupt, but had a thin intermediate layer (1–2 nm thick) with unclear lattice structures, suggesting the existence of an interfacial layer composed of NiGa2O4.14) NiGa2O4 has a cubic spinel structure with a lattice constant of 8.25895 Å,20) almost twice as long as a = 4.1684 Å of NiO,13) resulting in both NiO and NiGa2O4 sharing almost identical structural framework. A closer look at the NiO lattice revealed a periodic arrangement reflecting the left-right symmetry of the (110)-oriented NiO crystal structure when viewed from [10], in which horizontal (110), vertical (001), and diagonal (111) and (11) lattice planes were visible. On the other hand, vertical (100) and inclined (001) lattice planes were found in the β-Ga2O3 lattice. Interestingly, the lowest surface energy density planes of (001) NiO21) and (100) β-Ga2O322) were vertically aligned in this epitaxial configuration. These observable NiO and β-Ga2O3 lattice planes and the epitaxial relationship between them were further confirmed by the selected-area diffraction patterns of the NiO film and the β-Ga2O3 substrate [Fig. 2(c) and 2(d), respectively].Finally, the epitaxial relationship was elucidated using schematics of β-Ga2O3 and NiO structures, as shown in Fig. 3. Here, we illustrate the (02) β-Ga2O3 substrate surface and the (110) NiO epitaxial plane, which are indicated by dashed lines in the cross-sectional views [Fig. 3(a) and 3(b), respectively], to understand their in-plane ionic arrangements [Fig. 3(c) and 3(d)]. Note that there may be a very thin NiGa2O4 layer at the interface, but for simplicity we discuss the epitaxial structure without considering the layer. In these structure models, the O2− positions are nearly identical and form ccp networks, whereas the Ga3+ and Ni2+ occupancy positions are different. Therefore, the observed out-of-plane and in-plane epitaxial relationships should result from the matching the alignment of their oxygen arrangements on the interfacial planes, as discussed above. In addition, the lattice mismatches in the orthogonal in-plane directions were extracted by comparing the (average) distances between the neighboring oxygen ions on the (02) β-Ga2O3 and (110) NiO, which can be calculated using the labeled lattice parameters. The obtained mismatches along [010] and [201] of β-Ga2O3 (or [10] and [001] of NiO) were −2.95 and 5.44%, respectively. In summary, we have reconsidered the different structures of β-Ga2O3 and NiO in the same ccp oxygen framework, and proposed the use of (02) β-Ga2O3 substrate to achieve the low-index (110) epitaxial plane in the NiO/β-Ga2O3 heterojunction. Since homoepitaxial growth is possible on (02) β-Ga2O3 substrate, and that dislocations/voids in the β-Ga2O3 crystal do not appear on the (02) surface, it may be a favorable choice for fabricating vertical pn-heterojunction power devices.AcknowledgmentsThe authors would like to thank T. Harada of the National Institute for Materials Science (NIMS) for helping the XRD measurement. TEM observations were performed at the Electron Microscopy Unit at NIMS (Project No. 23NM5073). This research was partially supported by the TEPCO Memorial Foundation.References1) A.J. Green, J. Speck, G. Xing, P. Moens, F. Allerstam, K. Gumaelius, T. Neyer, A. Arias-Purdue, V. Mehrotra, A. Kuramata, K. Sasaki, S. Watanabe, K. Koshi, J. Blevins, O. Bierwagen, S. 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Tsusaka, K. Sasaki, A. Kuramata, Y. Sugawara, and Y. Ishikawa, Appl. Phys. Lett. 121, 012105 (2022).18) S. Nakagomi and Y. Kokubun, J. Cryst. Growth 479, 67 (2017).19) Y. Oshima and T. Oshima, Semicond. Sci. Technol. 38, 105003 (2023).20) A. Pajaczkowska, O. De Melo, F. Leccabue, C. Pelosi, D. Fiorani, A.M. Testa, and E. Paparazzo, J. Cryst. Growth 104, 498 (1990).21) P.W. Tasker and D.M. Duffy, Surf. Sci. 137, 91 (1984).22) S. Mu, M. Wang, H. Peelaers, and C.G. Van de Walle, APL Mater. 8, 091105 (2020).Fig. 1 Summary of X-ray diffraction characterization using CuKα1 radiation. Symmetric θ-2θ scan patterns of the NiO film on the (02) β-Ga2O3 substrate taken in (a) wide and (b) narrow 2θ ranges. (c) Asymmetric ϕ scan patterns of the film and the substrate. (d) ω-rocking curves of the corresponding symmetric and asymmetric peaks in (a)–(c).Fig. 2 Summary of transmission electron microscopy (TEM) images of the NiO film on the (02) β-Ga2O3 substrate observed along [010] of the substrate. (a) Low-magnification bright-filed (BF)-TEM image. (b) High-resolution TEM (HRTEM) image near the heterointerface. Selected area electron diffraction (SAED) patterns of the (c) NiO film and (d) β-Ga2O3 substrate.Fig. 3 Schematics of the β-Ga2O3 and NiO crystal structures are shown in (a) and (b), respectively, along with their atomic positions on the epitaxial planes shown in (c) and (d), respectively. Open circles represent oxygen ions, while filled circles represent Ga/Ni ions. Solid lines outline the unit cell boundaries, and dashed lines denote the epitaxial planes. In (a) and (b), the directional relationships are consistent with those shown in Fig. 2(b). In (c) and (d), the labels of d indicate the (average) distance between neighboring oxygen ions on the epitaxial planes, which can be calculated from the lattice parameters. The subscripts in the d and the a, b, and β lattice parameters indicate the corresponding directions and compounds. 8image3.emfd[110],NiO= aNiO/d[001],NiO= aNiO[ 10][001][110](a)  β-Ga2O3structure viewed along [010] (d)  NiO atomic positions on (110)Ga3+O2-( 02)[001][201][010][100]Ni2+O2-(110)(b)  NiO structure viewed along [ 10] [110][ 10][001]6 d[201] ave,Ga2O3= 2 aGa2O3sin(βGa2O3) d[010],Ga2O3= bGa2O3[010][201][001](c)  β-Ga2O3atomic positions on ( 02) Ga3+O2-Ni2+O2-image1.emf(b) θ-2θ scan (narrow range)(c) ϕ scan(d) ω scan(a) θ-2θ scan (wide range)image2.emf(a) BF-TEM  (low magnification) 100 nm( 02) β-Ga2O3substrateNiO film(d) SAED β-Ga2O3000 2004002 nm-100 000010020000(c) SAED NiO5 nm-1000220002000111111[110][001][ 10](b) HRTEM (high magnification)5 nmNiO filminterface layercarbon( 02) β-Ga2O3substrate[001][201][010][100][001][201][010][100][110][001][ 10](001)(110)(100)