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

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[Step-and-terrace surface formation on (−112) β-Ga                    <sub>2</sub>                    O                    <sub>3</sub>                    via thermal annealing](https://mdr.nims.go.jp/datasets/c6a19f22-e9aa-425d-9f06-7d4783fa3814)

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Japanese Journal of Applied Physics 65, 178001 (2026) BRIEF NOTEhttps://doi.org/10.35848/1347-4065/aea1b2Step-and-terrace surface formation on (−112) β-Ga2O3 via thermal annealingTakayoshi Oshima1*1Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba Ibaraki, 305-0044, Japan*E-mail: OSHIMA.Takayoshi@nims.go.jpReceived August 7, 2026; revised August 31, 2026; accepted September 1, 2026; published online September 11, 2026Step-and-terrace formation on a (−112) β-Ga2O3 substrate subjected to chemical mechanical polishing was investigated by thermal annealingand subsequent wet-chemical treatments. Atomic force microscopy revealed that annealing at approximately 1000 ◦C produced a well-definedstepped morphology with predominantly monolayer steps about 0.2 nm high, consistent with the 0.210 nm spacing between adjacent (−112)planes. In contrast, treatments with hydrofluoric acid, a sulfuric acid–hydrogen peroxide mixture, and tetramethylammonium hydroxide degradedthe thermally formed stepped morphology. These results demonstrate that thermal annealing is effective for preparing ordered (−112) β-Ga2O3surfaces and provide guidance for future pre- and post-treatment processes. © 2026 The Author(s). Published on behalf of The Japan Society ofApplied Physics by IOP Publishing Ltdβ-Ga2O3 has attracted considerable attention as an ultrawide-bandgap semiconductor because of its wide bandgap, highcritical electric field, and availability of large bulk singlecrystals.1) Various epitaxial growth and device-processingtechniques have therefore been developed.1) However, β-Ga2O3 has a monoclinic crystal structure, and its growth,etching, and surface properties strongly depend on the crys-tallographic orientation.2–5) The selection and control of thesubstrate surface are therefore important for establishing reli-able material-processing methods.Surface morphology is particularly important becauseatomically smooth surfaces with ordered steps and ter-races are desirable for epitaxial growth and subsequentdevice fabrication. Detailed surface structures, includingsteps and facets, can influence surface reaction kinetics,adatom incorporation, and impurity incorporation. In addi-tion, the actual surface after epitaxial growth and materialprocessing may consist of crystallographic facets differentfrom the nominal substrate plane.6–8) Surface preparationhas therefore been investigated particularly for the com-monly used (100), (010), (001), and (201) β-Ga2O3 sub-strates. Thermal annealing at approximately 1000 ◦C hasbeen reported to produce a distinct step-and-terrace surfacemorphology on the (100) substrate.9) In contrast, annealingof the (010), (001), and (201) substrates has been reportedto result in the development of facets other than the nom-inal substrate planes.1,10,11) Wet-chemical treatments havebeen reported to produce well-defined step-and-terrace mor-phologies on the (001) and (201) surfaces,10,12) whereasfacets other than the nominal plane develop on the (010)surface.3)Recently, the (112) plane has been proposed as a funda-mental orientation of β-Ga2O3 based on its pseudo-cubicoxygen sublattice framework.13,14) In this framework, the(112) and (100) planes belong to the same {100} family of thepseudo-cubic cell,13) suggesting that thermal annealing mayalso produce an ordered step-and-terrace morphology on the(112) surface, similar to that observed on the (100) surfaceas described above. Furthermore, our previous study demon-strated twin-free homoepitaxy on a (112) substrate, producingan atomically smooth step-and-terrace surface, indicating thepotential of this orientation for epitaxial growth and subse-quent device processing.14)However, suitable surface-preparation methods for (112)β-Ga2O3 substrates have not yet been established. In par-ticular, the effects of thermal annealing and commonly usedwet-chemical treatments on the surface morphology remainunclear. Therefore, in this study, we investigated step-and-terrace formation on a (112) β-Ga2O3 substrate by ther-mal annealing and evaluated the effects of subsequent wet-chemical treatments.We used a custom-made Sn-doped (112) β-Ga2O3 sub-strate with a nominal carrier concentration of 5× 1018 cm−3,supplied by Novel Crystal Technology. Note that, after chem-ical mechanical polishing (CMP), the substrate was cleanedwith organic solvents (acetone, isopropanol, and methanol),hydrofluoric acid (HF), and a sulfuric acid–hydrogen peroxidemixture (SPM).The as-received substrate was thermally annealed in acustom-built horizontal quartz furnace under flowing N2(>99.99995%) at a flow rate of 2.00 slm and atmosphericpressure for 1 h at 800, 900, or 1000 ◦C. Details of the furnace,including a schematic illustration, were reported in our previ-ous study.10) Wet-chemical treatments were also performedon the stepped surface obtained after annealing at 1000 ◦C.The substrate was cut into three pieces along the (100) cleav-age plane. The three pieces were treated separately for 1 hwith 46–48 wt% HF at room temperature, SPM consisting of96 wt% H2SO4 and 30 wt% H2O2 mixed at a volume ratioof 3:1 without external heating, or 25 wt% tetramethylammo-nium hydroxide (TMAH) at room temperature. The surfacemorphologies of the as-received, annealed, and chemicallytreated substrates were examined by tapping-mode atomicforce microscopy (AFM; Jupiter XR, Oxford Instruments).We first investigated the annealing-temperature depen-dence of the (112) surface morphology. Figure 1 showsthe evolution of the surface morphology of the (112) sub-strate from the as-received state through sequential anneal-ing at progressively increasing temperatures of 800, 900,and 1000 ◦C for 1 h at each temperature. The as-receivedsubstrate exhibited a flat surface without discernible steps orterraces [Fig. 1(a)]. The root-mean-square (RMS) roughnessContent from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution ofthis work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.178001-1© 2026 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing Ltdhttps://doi.org/10.35848/1347-4065/aea1b2https://crossmark.crossref.org/dialog/?doi=10.35848/1347-4065/aea1b2&domain=pdf&date_stamp=2026-9-11https://orcid.org/0000-0001-8550-9735mailto:OSHIMA.Takayoshi@nims.go.jphttps://Creative%20Commons%20Attribution%204.0%20licenceJpn. J. Appl. Phys. 65, 178001 (2026) T. OshimaFig. 1. AFM images of a (112) β-Ga2O3 substrate showing the evolution of the surface morphology in (a) the as-received state and after thermal annealingat (b) 800 ◦C, (c) 900 ◦C, and (d) 1000 ◦C. (e) AFM image of the surface in (d) after terrace-by-terrace flattening. The arrow indicates the miscut directionof the substrate surface. (f) Height profile taken along the arrow in (e). Note that hkl* denotes the direction normal to the (hkl) plane.was 0.11 nm. It should be noted that this surface morphologywas essentially the result of the wet-cleaning process per-formed after CMP. The initially featureless surface graduallyevolved into a distinct stepped morphology as the anneal-ing temperature increased from 800 to 1000 ◦C. At 800 ◦C,steps and terraces appeared on the surface; however, the stepedges were poorly defined, and numerous pits were present onthe terraces [Fig. 1(b)]. These surface irregularities becameless pronounced at 900 ◦C [Fig. 1(c)], and a well-definedstep-and-terrace structure was obtained at 1000 ◦C [Fig.1(d)]. The corresponding RMS roughness values were assmall as 0.10, 0.09, and 0.10 nm at 800, 900, and 1000 ◦C,respectively.The results indicate that the (112) plane of β-Ga2O3 candevelop an ordered step-and-terrace morphology upon ther-mal annealing at approximately 1000 ◦C. As noted above,such a highly ordered arrangement of surface steps formedby thermal annealing has previously been observed only for178001-2© 2026 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdJpn. J. Appl. Phys. 65, 178001 (2026) T. OshimaFig. 2. Crystal structure of β-Ga2O3 projected along the [132] direction.the (100) plane among the conventionally used substrateorientations. This similarity may arise from the commonarrangements of oxygen atoms on the (112) and (100) sur-faces.13) Notably, homoepitaxial growth by metal-organicchemical vapor deposition (MOCVD) and halide vapor phaseepitaxy,15–17) dopant-activation annealing for homoepitaxiallayers grown by MOCVD,18) and post-implantation recov-ery annealing19,20) are generally performed at temperaturesaround 1000 ◦C, suggesting that step-and-terrace formationcan be achieved concurrently with these processes.We then characterized the stepped surface structure usingthe sample annealed at 1000 ◦C [Fig. 1(d)]. Most step edgescontained straight segments oriented parallel to the [132]direction. This direction is parallel to the (310) and (712)planes, along which the oxygen atoms form well-ordered sub-lattice planes,13) as shown in Fig. 2. Therefore, step edgesoriented along this crystallographic direction may have alower density of dangling bonds and, consequently, a lowerstep energy. Notably, the (310) plane appears as a facet dur-ing HCl gas etching of (001)−, (010)−, and (102)-orientedsubstrates,21–23) which suggests that this plane has a relativelylow surface energy.To measure the step heights, terrace-by-terrace flatteningwas applied to the topographic data [Fig. 1(e)]. The observedstep heights corresponded predominantly to one monolayer,with some steps having a height of two monolayers. Fromthe height profile, the single-monolayer step height was deter-mined to be approximately 0.2 nm [Fig. 1(f)], correspondingto the spacing between adjacent (112) planes (S = 0.210 nm)[Fig. 2], calculated using lattice parameters of β-Ga2O3.24)In addition, the miscut angle θ was estimated to be≈0.4◦using the simple trigonometric relation: tan θ = NS/L, whereL is the length of the arrow along the miscut direction(L = 172 nm) and N is the number of monolayer stepscounted along the arrow (N = 5) [Fig. 1(e)]. This miscutangle was not intentionally introduced but originated from thecustom substrate fabrication process. A smaller miscut anglewould result in wider terraces and might also suppress stepbunching.Finally, we evaluated the effects of wet-chemical treat-ments using HF, SPM, and TMAH. HF is commonly usedto remove colloidal silica originating from polishing slurriesand siloxane-based contaminants deposited upon exposureto ambient air.25,26) SPM is widely used for the oxida-tive removal of organic contaminants, including photore-sists and polymer residues, from semiconductor surfaces.27)TMAH is often used to mitigate dry-etch-induced damageFig. 3. AFM images of (112) β-Ga2O3 substrates after subsequentwet-chemical treatments with (a) HF, (b) SPM, and (c) TMAH.and reduce surface roughness3,28,29) and is suitable for form-ing a distinct step-and-terrace structure on (001) and (201)surfaces as described above.10,12) Fig. 3 compares the surface178001-3© 2026 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdJpn. J. Appl. Phys. 65, 178001 (2026) T. Oshimamorphologies after the three wet-chemical treatments. Alltreatments degraded the thermally formed step-and-terracemorphology, although the degree of degradation dependedstrongly on the chemical used. After HF and TMAH treat-ment, the original stepped structure was completely lost, andno distinct terraces remained [Figs. 3(a) and 3(c)]. In con-trast, the SPM-treated surface retained a partially preservedstepped morphology, although the step edges became indis-tinct and pits formed on the terraces [Fig. 3(b)]. The relativelysmall influence of SPM on the surface can be attributed toits negligible etch rate, consistent with previous wet-etchingexperiments on (100) substrates.30) These results indicate thatthermal annealing is effective for developing an ordered (112)surface, whereas wet-chemical treatments examined here dis-rupted the step-and-terrace morphology. Therefore, wet treat-ments should be used with caution when the stepped surfaceneeds to be preserved. For example, the treatment time shouldbe kept to the minimum necessary. When organic contami-nants need to be removed, ozone treatment may be consideredas an alternative.In conclusion, we demonstrated the formation of a well-defined step-and-terrace surface morphology on a CMP-processed (112) β-Ga2O3 substrate via thermal annealingat approximately 1000 ◦C. The observed step height was ingood agreement with the spacing between adjacent (112)planes (0.210 nm). We also confirmed that commonly usedwet-chemical treatments using HF, SPM, and TMAH did notpreserve the stepped surface morphology. These findings pro-vide useful guidance for pre- and post-treatment processes of(112) β-Ga2O3, an orientation that has only recently begun toattract research interest.Acknowledgment All experiments were conducted at theNanofabrication Unit of NIMS as part of the Advanced ResearchInfrastructure for Materials and Nanotechnology (ARIM) programsupported by the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Japan (No. JPMXP1225NM5079). This work wasalso supported by a Grant-in-Aid for Scientific Research (B) from theJapan Society for the Promotion of Science (JSPS), MEXT, Japan (No.JP24K01368) and by a project commissioned by the New Energy andIndustrial Technology Development Organization (NEDO), Ministry ofEconomy, Trade and Industry (METI), Japan (No. JPNP22007).ORCID iD Takayoshi Oshima  https://orcid.org/0000-0001-8550-97351) K. Sasaki, Appl. Phys. Express 17, 090101 (2024).2) K. Goto, H. Murakami, A. Kuramata, S. Yamakoshi, M. Higashiwaki, andY. Kumagai, Appl. Phys. Lett. 120, 102102 (2022).3) T. Oshima, AIP Adv. 16, 025145 (2026).4) T. Liu, Z. Li, J. Zhao, X. Fei, J. Feng, Y. Zuo, M. Hua, Y. Guo, S. Liu, andZ. Zhang, Acta Mater. 300, 121484 (2025).5) M. Lee, T. Chou, S. B. Anooz, Z. Galazka, A. Popp, and R. L. Peterson,ACS Nano 16, 11988 (2022).6) P. Mazzolini, P. Vogt, R. Schewski, C. Wouters, M. Albrecht, andO. Bierwagen, APL Mater. 7, 022511 (2019).7) C.-H. Lin, K. Ema, S. Masuya, Q. T. Thieu, R. 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Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing Ltdhttps://orcid.org/0000-0001-8550-9735https://orcid.org/0000-0001-8550-9735https://orcid.org/0000-0001-8550-9735https://doi.org/10.35848/1882-0786/ad6b73https://doi.org/10.1063/5.0087609https://doi.org/10.1063/5.0305647https://doi.org/10.1016/j.actamat.2025.121484https://doi.org/10.1021/acsnano.2c01957https://doi.org/10.1063/1.5054386https://doi.org/10.35848/1347-4065/acb4fbhttps://doi.org/10.35848/1347-4065/ae83f5https://doi.org/10.1016/j.apsusc.2008.02.184https://doi.org/10.1016/j.apsusc.2008.02.184https://doi.org/10.35848/1347-4065/adf380https://doi.org/10.1016/j.apsusc.2021.151651https://doi.org/10.1016/j.apsusc.2021.151651https://doi.org/10.1080/14686996.2026.2666988https://doi.org/10.35848/1347-4065/ae42achttps://doi.org/10.1080/14686996.2026.2680969https://doi.org/10.35848/1882-0786/acf8aehttps://doi.org/10.7567/APEX.8.015503https://doi.org/10.1080/14686996.2025.2585551https://doi.org/10.35848/1882-0786/adcfeehttps://doi.org/10.7567/APEX.6.086502https://doi.org/10.1063/1.5050040https://doi.org/10.1063/1.5050040https://doi.org/10.1063/5.0138736https://doi.org/10.35848/1882-0786/acdbb7https://doi.org/10.1063/5.0186319https://doi.org/10.1107/S0108270195016404https://doi.org/10.1107/S0108270195016404https://doi.org/10.1039/C7RA11570Ahttps://doi.org/10.1063/5.0191280https://doi.org/10.1063/5.0191280https://doi.org/10.1149/1.2086825https://doi.org/10.1016/j.apsusc.2024.161569https://doi.org/10.1116/6.0004289https://doi.org/10.1002/pssc.200779223 Step-and-terrace surface formation on (-112) β-Ga2O3 via thermal annealing