# Fileset

[Oshima_2025_Appl._Phys._Express_18_116501-1.pdf](https://mdr.nims.go.jp/filesets/3667f576-30cf-4fd1-8941-fde4a035aa7e/download)

## Creator

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

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Fabrication of                    <i>β</i>                    -Ga                    <sub>2</sub>                    O                    <sub>3</sub>                    /air-gap structures on (010)                    <i>β</i>                    -Ga                    <sub>2</sub>                    O                    <sub>3</sub>                    by wet etching in tetramethylammonium hydroxide (TMAH)](https://mdr.nims.go.jp/datasets/7d2f8b58-a978-4d52-9171-cffbb7da7fdd)

## Fulltext

aaaFabrication of β-Ga2O3/air-gap structures on (010) β-Ga2O3 by wet etching intetramethylammonium hydroxide (TMAH)Takayoshi Oshima*Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan*E-mail: OSHIMA.Takayoshi@nims.go.jpReceived October 23, 2025; revised November 5, 2025; accepted November 10, 2025; published online November 25, 2025We demonstrated the fabrication of β-Ga2O3/air-gap structures on (010) β-Ga2O3 substrates through sequential dry etching and crystallographicwet etching. Wet etching in a 25 wt% tetramethylammonium hydroxide solution at 90 °C yielded a lateral-to-vertical etch-rate ratio ofapproximately 11 when the lateral direction was aligned with [001]. This pronounced lateral etching enabled the undercutting of dry-etchedβ-Ga2O3 mesas to form cantilevers and air bridges extending along [201], which is perpendicular to [001]. This etch-only process using standarddevice-fabrication equipment offers a straightforward route for fabricating β-Ga2O3/air-gap structures that are promising for microelectromecha-nical systems. © 2025 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltdβ -Ga2O3 is an emerging compound semiconductor thathas attracted considerable attention owing to its widerange of applications. With a wide bandgap of4.4–4.9 eV and a large critical electric field of approximately8MV cm−1,1–4) β-Ga2O3 holds great promise for powerelectronic and deep-ultraviolet optical applications.5)Moreover, its electrical conductivity is sensitive to bothreducing and oxidizing gases, paving the way for gas-sensingapplications.6,7) It also exhibits high tolerance to elevatedtemperatures and radiation, making the devices suitable foroperation in harsh environments.8–12) Furthermore, it pos-sesses mechanical properties comparable to those of Si,suggesting strong potential for future applications in micro-electromechanical systems (MEMS).13) In particular,β-Ga2O3 MEMS devices are expected to exhibit uniquefunctionalities beyond those achievable with Si, such asharsh-environment actuators and resonant sensors, as well asresonance-based components (e.g. filters, mixers, and fre-quency references for on-chip oscillators) that can comple-ment β-Ga2O3 power and RF devices.13) To date, severalβ-Ga2O3-based MEMS devices, including resonators,14–16)and resonant deep-ultraviolet detectors,17,18) pressuresensors,19) and transistors20) have been demonstrated, under-scoring its potential for MEMS technologies.However, previously demonstrated β-Ga2O3-based MEMShave faced a critical challenge in the fabrication ofβ-Ga2O3/air-gap structures, such as diaphragms14,16,17) and airbridges.15,16,18–20) These structures were fabricated by manuallytransferring β-Ga2O3 flakes—either low-pressure chemicalvapor deposition (LPCVD)-grown14,17) or exfoliated from bulksingle crystals15,16,18–20)—onto prepatterned foreign substrates,resulting in poor reproducibility. For practical applications,β-Ga2O3/air-gap structures should be fabricated through highlyreproducible and controllable processes. Considering the currentSi MEMS fabrication technology, crystallographic etchingshould be a promising approach for this purpose.21)Crystallographic etching of β-Ga2O3 has recently attractedsignificant research interest. Both gas-phase etching using HClgas,22–29) forming gas,30,31) Ga flux,32–35) and triethylgalliumgas31,36,37) and wet chemical etching with H3PO431,38–40) and Ptmetal and a mixture of HF and K2S2O8 (MacEtch)41,42) havebeen demonstrated on β-Ga2O3 substrates with various substrateorientations, including (100),28,38) (010),24,31,32,35–37,40–42)(001),22,23,27,29,31–33,36,37,39) (2 01),31,34) (1 02),25,30) and(011),26) primarily for fabricating fins and trenches withplasma-damage-free sidewalls to improve electronic deviceperformances.33,41,42) Furthermore, even β-Ga2O3/air-gap struc-tures—such as cantilevers and air bridges, which are funda-mental MEMS components—have been achieved using HCl gasetching on (100) and (001) β-Ga2O3 substrates by exploiting thestrong lateral etching capability along the 〈010〉 directionsrelative to the vertical ones.28,29) These demonstrations highlightthe potential of crystallographic etching of β-Ga2O3 even forMEMS applications.In this study, we explore the feasibility of fabricatingβ-Ga2O3/air-gap structures through crystallographic wetetching. Crystallographic gas etching demands specializedsystems such as halide vapor-phase epitaxy,22–29) metal-organic chemical vapor deposition,31,36,37) molecular beamepitaxy,32,33) LPCVD,34,35) a gas-flow annealing furnace.30)In contrast, wet etching can be performed using a simplechemical bath, thereby substantially reducing the etchingcost. However, air-gap formation by wet etching has not yetbeen realized. Therefore, this study aims to verify whethercrystallographic wet etching can achieve sufficient aniso-tropy for air-gap formation in β-Ga2O3. For the experiments,(010)-oriented substrates were employed because this sur-face orientation exhibits high etch resistance to wetetchants,40) with the expectation of faster lateral than verticaletching, suitable for air-gap formation.28) In addition, weselected tetramethylammonium hydroxide (TMAH) as thewet etchant. Although TMAH has not previously beenapplied to the crystallographic etching of β-Ga2O3, it hasdemonstrated effectiveness in forming step-and-terracesurfaces43) and in removing dry-etch-induced roughness anddamage,44–47) indicating its suitability as a wet etchant.We conducted systematic experiments similar to the air-gapformation demonstrated in our previous HCl gas etchingstudy.29) Two Sn-doped, conductive (010) β-Ga2O3 substrateswith a nominal carrier density of 2.8× 1018 cm−3 (Samples Aand B) were used. Sample A was employed to investigate thein-plane anisotropy of side etching, and Sample B to demon-strate the fabrication of β-Ga2O3/air-gap structures. The processsequences for the two samples are shown in Fig. 1.Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution ofthis work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.116501-1© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdApplied Physics Express 18, 116501 (2025) LETTERhttps://doi.org/10.35848/1882-0786/ae1e59https://crossmark.crossref.org/dialog/?doi=10.35848/1882-0786/ae1e59&domain=pdf&date_stamp=2025-11-25https://orcid.org/0000-0001-8550-9735mailto:OSHIMA.Takayoshi@nims.go.jphttps://creativecommons.org/licenses/by/4.0/https://doi.org/10.35848/1882-0786/ae1e59For Sample A [Fig. 1(a)], a 0.14 μm thick SiO2 maskinglayer was deposited by plasma-enhanced chemical vapordeposition (Samco, PD-220NL). Subsequently, the samplewas annealed at 900 °C in an N2 atmosphere under atmo-spheric pressure for 1 h to densify the layer and improve etchresistance.48) Wagon-wheel-shaped etching windows—com-prising 72 linear openings arranged at 5° intervals from the[102] direction—were then prepared by laser lithography(Heidelberg Instruments, DWL66+), followed by bufferedhydrofluoric acid (BHF) etching. The photoresist wasremoved by organic cleaning in N-methyl-2-pyrrolidoneand isopropyl alcohol, followed by oxygen plasma ashing.The resulting linear windows were 50 μm in length and1.5 μm in width. Finally, wet etching was performed in a25 wt% TMAH solution at 90 °C for 1.5 h using a laboratorymade polytetrafluoroethylene container, which was heatedon a hot-plate stirrer.It should be noted that 90 °C is the practical temperaturelimit of the 25 wt% TMAH solution,44–46) as its boiling pointis 103 °C. Since the wet etch rate was assumed to varyexponentially according to the Arrhenius relation,38–40) theetching temperature was precisely monitored and controlledusing a perfluoroalkoxy-coated thermocouple immersed inthe solution, which was continuously stirred to ensuretemperature uniformity. In addition, to maintain a constantTMAH concentration, the etching container was equippedwith a sealable lid, allowing vapor to condense and refluxback into the solution.For Sample B [Fig. 1(b)], mesa structures were fabricatedby defining the mesa regions via laser lithography, followedby inductively coupled plasma reactive ion etching (ICP-RIE, Samco RIE-101iPH) using BCl3 and Ar. Thephotoresist was then removed using the same proceduredescribed above, and the redeposited materials were subse-quently removed by HNO3 etching at room temperature. Theresulting mesas, approximately 30.6 μm in length, 2.5 μm inwidth, and 1.03 μm in height were aligned along the [201]direction; the reason for selecting this orientation will bediscussed later. A 0.12 μm thick SiO2 masking layer wassubsequently deposited and annealed under the same condi-tions as Sample A. Etching windows were then formed bysequential laser lithography, BHF etching, and photoresistremoval, leaving the mesa tops and sidewalls covered withthe mask. Finally, wet etching in a 25 wt% TMAH solutionat 90 °C for 6 h produced air gaps, after which the mask wasremoved by BHF etching.In this process, the geometry of the β-Ga2O3/air-gapstructures reflected that of the mesa patterns. The mesaheight was defined by the dry-etch depth, which is controlledwithin 5% accuracy (specification of RIE-101iPH). Thelateral width was defined by the photoresist mask patternedby laser lithography, with a critical dimension uniformity of0.06 μm (specification of DWL66+), corresponding to 3%accuracy for a 2.5 μm wide mesa. Because the resonantfrequency of cantilevers and air bridges is proportional to thestructure height according to the well-known Euler–Bernoulli beam model, the process equipment used in thisstudy may cause variations in the resonant frequency of up toapproximately 5%. Such a deviation is acceptable forresearch and prototyping purposes; however, for practicalimplementation, higher process precision within 1% wouldbe required, necessitating a dry-etching system with im-proved depth control accuracy.The resulting etched structures were characterized usingdifferential interference contrast (DIC) microscopy, scanningelectron microscopy (SEM), and atomic force microscopy(AFM). Focused ion beam milling (FIB) was also employedfor the cross-sectional SEM observation.We first investigated the in-plane side-etching behavior.Figure 2 summarizes the results obtained from the etchedtrenches formed beneath the wagon-wheel windows onSample A.Figure 2(a) shows a DIC image of the upper half of thewagon-wheel-patterned trenches. Owing to the twofoldrotational symmetry around the [010] axis on the (010)plane,49) the lower half is omitted. The mask was notremoved; therefore, the side-etched regions appear in adifferent color from the substrate surface and the linearwindows. The side etching was clearly anisotropic. Trenchesoriented near the [201] direction exhibited pronounced sideetching, as shown in the enlarged image [Fig. 2(b)],indicating that the lateral etching was most enhanced indirections close to 〈001〉. Figure 2(c) presents a polar plot ofthe side-etch rate, constructed from the side-etch lengthsmeasured using DIC microscopy and SEM. The plot exhibitstwofold rotational symmetry, reflecting the crystallographicsymmetry on the (010) plane.49) The maximum side-etchrates were 0.76 μm h−1 in directions approximately along〈001〉. In contrast, the rates along the 〈201〉 direction, whichis perpendicular to 〈001〉, were as low as 0.05 μm h−1. Sucha large difference in etch rates between orthogonal in-planedirections is advantageous for forming cantilevers and airbridges, as the supporting regions remain largely intactFig. 1. Schematic illustrations of the process sequences for (a) investi-gating in-plane side-etching behavior (Sample A) and (b) fabricatingβ-Ga2O3/air-gap structures (Sample B).116501-2© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 18, 116501 (2025) T. Oshimaduring side etching. Therefore, cantilevers and air bridgeswere designed along the [201] direction.Figure 2(d) shows a DIC image of an etched trench formedbeneath a linear window aligned along the [201] direction. Theshapes of the side-etched regions on both sides of the lineartrench were almost identical, except at both ends, where theetching fronts were parallel to the [102] and [100] directions,suggesting the development of (201) and (001) facets, respec-tively. Figure 2(e) presents a 54°-tilted cross-sectional SEMimage of the [201]-oriented etched trench. The FIB-exposedplane corresponds to (100), which is perpendicular to [201].The etched depth within the window region was as small as0.10 μm, whereas the side-etched length reached 1.13 μm. Sucha high lateral-to-vertical etch-rate ratio of approximately 11enables the formation of air gaps. Notably, the bottom surfacein the side-etched regions was not parallel to the (010) plane butslightly curved upward toward etch fronts.We then carried out air-gap formation to achieveβ-Ga2O3/air-gap structures. Figure 3 summarizes the resultsof the fabricated cantilever and air bridge on Sample B.Figures 3(a) and 3(b) present oblique-view SEM imagesof the cantilever and air bridge extending along the [201]direction, respectively. Dark shadows were observed beneathboth structures, confirming the formation of air gapsresulting from the pronounced lateral etching along the〈001〉 directions, which are perpendicular to [201]. Incontrast, no shadows appeared on the etched sidewallsparallel to the [001] direction, consistent with the weaklateral etching along the 〈201〉 directions. The grooved,rough sidewall morphology of both structures originatedfrom ICP-RIE, which could be smoothed by optimizing thedry-etching conditions. Figure 3(c) shows a 54°-tilted cross-sectional image taken at the midpoint of the air bridge, wherethe exposed plane corresponds to (100). A distinct air gapwas clearly visible. The air-gap spacing was smaller at thecenter (0.26 μm) than at the ends (0.52 μm), consistent withthe lateral etching characteristics along the 〈001〉 directionsdescribed previously in Fig. 2(e). The reverse side of the airbridge also exhibited inclinations in opposite directions tothe bottom surface, owing to the mirror symmetry withrespect to the (010) plane.49) The bridge thickness was 0.63–0.85 μm, smaller than the initial mesa height of 1.03 μm,because wet etching slowly proceeded from the reverse sideas well. Although the gap spacing was small for MEMSFig. 2. Summary of the in-plane side-etching characteristics on a (010) β-Ga2O3 substrate (Sample A). (a) DIC image showing side-etched structuresformed beneath a wagon-wheel window pattern. (b) Enlarged view of the region with the larger side etching in (a). (c) Polar plot of side-etch ratesperpendicular to the wagon-wheel windows shown in (a). (d) DIC image of the etched structure formed beneath a linear window aligned with the [201]direction. (e) 54°-tilted SEM image of the cross section of the etched structure shown in (d).116501-3© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 18, 116501 (2025) T. Oshimaapplications, it could be increased by performing Cl-basedICP-RIE of both SiO2 and β-Ga2O3, instead of BHF etchingof SiO2 alone, prior to TMAH etching.Aside from the β-Ga2O3/air-gap structures, the TMAH-etched (010) bottom surface exhibited a smooth yet nonflatmorphology [Figs. 3(a), 3(b), and 3(d)]. Other crystal facetsextending along the [001] direction preferentially developedin place of the (010) plane. Therefore, the high etchresistance observed on the (010) surface was actuallyattributed not to the (010) plane itself, but to these crystal-lographic planes. The root-mean-square roughness of thesurface was 4.4 nm, as revealed by the AFM image shown inFig. 3(d). Considering the mirror symmetry, a similar surfaceroughness is expected on the reverse side of the cantileverand air bridge. Such sub-10 nm roughness is sufficient toachieve quality factors on the order of 104–105 in MEMSresonators.50)In conclusion, we demonstrated the fabrication ofβ-Ga2O3/air-gap structures on a (010)-oriented substrate.The key processing step was crystallographic wet etching inheated TMAH, where the lateral etch rates along the 〈001〉directions were sufficiently higher than the vertical etch rateon the (010) plane, enabling the formation of air gaps forfabricating cantilevers and air bridges. These results high-light the strong potential of crystallographic wet etching ofβ-Ga2O3 and are expected to stimulate future research onβ-Ga2O3-based MEMS.Acknowledgments This study was supported by the Materials FormingUnit and the Nanofabrication Microscopy Unit at the NIMS within the frame-work of the Advanced Research Infrastructure for Materials and Nanotechnology(ARIM), supported by the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Japan (No. JPMXP1225NM5079). This work was finan-cially supported by a Grant-in-Aid for Scientific Research (B) from the JapanSociety for the Promotion of Science (JSPS), MEXT, Japan (No. JP24K01368).ORCID iDs Takayoshi Oshima https://orcid.org/0000-0001-8550-97351) T. Onuma, S. Saito, K. Sasaki, T. Masui, T. Yamaguchi, T. Honda, andM. Higashiwaki, Jpn. J. Appl. Phys. 54, 112601 (2015).2) M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi,Appl. Phys. Lett. 100, 013504 (2012).3) T. Oshima, M. Hashikawa, S. Tomizawa, K. Miki, T. Oishi, K. Sasaki, andA. Kuramata, Appl. Phys. Express 11, 112202 (2018).4) Z. Xia et al., Appl. Phys. Lett. 115, 252104 (2019).5) S. J. Pearton, F. Ren, A. Y. Polyakov, A. Haque, M. Labed, and Y. S. Rim,Appl. Phys. Rev. 12, 031336 (2025).6) M. Bartic, Y. Toyoda, C.-I. Baban, and M. Ogita, Jpn. J. Appl. Phys. 45,5186 (2006).7) S. Jang, S. Jung, J. Kim, F. Ren, S. J. Pearton, and K. H. Baik, ECS J. SolidState Sci. Technol. 7, Q3180 (2018).8) C. Hou, K. R. York, R. A. Makin, S. M. Durbin, R. M. Gazoni, R. J. Reeves,and M. W. Allen, Appl. Phys. Lett. 117, 203502 (2020).9) A. E. Islam et al., Appl. Phys. Lett. 121, 243501 (2022).10) W. A. Callahan, K. Egbo, C.-W. Lee, D. Ginley, R. O’Hayre, andA. Zakutayev, Appl. Phys. Lett. 124 (2024)153504.11) M. H. Wong, A. Takeyama, T. Makino, T. Ohshima, K. Sasaki,A. Kuramata, S. Yamakoshi, and M. Higashiwaki, Appl. Phys. Lett. 112,023503 (2018).12) S. A. Khan, S. Saha, U. Singisetti, and A. F. M. A. U. Bhuiyan, J. Appl.Phys. 136, 225701 (2024).13) X.-Q. Zheng, H. Zhao, and P. X. L. Feng, Appl. Phys. Lett. 120, 040502(2022).14) X.-Q. Zheng, J. Lee, S. Rafique, L. Han, C. A. Zorman, H. Zhao, and P. X.L. Feng, ACS Appl. Mater. Interfaces 9, 43090 (2017).15) M. Peres et al., ECS J. Solid State Sci. Technol. 8, Q3235 (2019).16) X.-Q. Zheng, H. Zhao, Z. Jia, X. Tao, and P. X. L. Feng, Appl. Phys. Lett.119, 013505 (2021).17) X.-Q. Zheng, J. Lee, S. Rafique, M. R. Karim, L. Han, H. Zhao, C.A. Zorman, and P. X. L. Feng, IEEE Electron Device Lett. 39, 1230 (2018).18) X.-Q. Zheng, Y. Xie, J. Lee, Z. Jia, X. Tao, and P. X. L. Feng, APL Mater.7, 022523 (2019).19) W. Sui, S. M. Enamul Hoque Yousuf, Y. Liu, S. J. Pearton, and P. X.L. Feng, Adv. Mater. Technol. 9, 2301356 (2024).20) X.-Q. Zheng, T. Kaisar, and P. X.-L. Feng, Appl. Phys. Lett. 117, 243504(2020).21) W. Lang, Mater. Sci. Eng. R 17, 1 (1996).22) T. Oshima and Y. Oshima, Appl. Phys. Lett. 122, 162102 (2023).23) Y. Oshima and T. Oshima, Jpn. J. Appl. Phys. 62, 080901 (2023).24) T. Oshima and Y. Oshima, Appl. Phys. Express 16, 066501 (2023).25) T. Oshima and Y. Oshima, Appl. Phys. Lett. 124, 042110 (2024).Fig. 3. 54°-tilted SEM images of the fabricated (a) cantilever and (b) air bridge on a (010) β-Ga2O3 substrate (Sample B). (c) 54°-tilted cross-sectionalSEM image corresponding to the air bridge shown in (b). (d) AFM image of the TMAH-etched bottom surface.116501-4© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 18, 116501 (2025) T. Oshimahttps://orcid.org/0000-0001-8550-9735https://orcid.org/0000-0001-8550-9735https://doi.org/10.7567/JJAP.54.112601https://doi.org/10.1063/1.3674287https://doi.org/10.7567/APEX.11.112202https://doi.org/10.1063/1.5130669https://doi.org/10.1063/5.0285075https://doi.org/10.1143/JJAP.45.5186https://doi.org/10.1143/JJAP.45.5186https://doi.org/10.1149/2.0261807jsshttps://doi.org/10.1149/2.0261807jsshttps://doi.org/10.1063/5.0026345https://doi.org/10.1063/5.0113744https://doi.org/10.1063/5.0185566https://doi.org/10.1063/1.5017810https://doi.org/10.1063/1.5017810https://doi.org/10.1063/5.0233995https://doi.org/10.1063/5.0233995https://doi.org/10.1063/5.0073005https://doi.org/10.1063/5.0073005https://doi.org/10.1021/acsami.7b13930https://doi.org/10.1149/2.0441907jsshttps://doi.org/10.1063/5.0050421https://doi.org/10.1063/5.0050421https://doi.org/10.1109/LED.2018.2850776https://doi.org/10.1063/1.5054625https://doi.org/10.1063/1.5054625https://doi.org/10.1002/admt.202301356https://doi.org/10.1063/5.0031503https://doi.org/10.1063/5.0031503https://doi.org/10.1016/0927-796X(96)00190-8https://doi.org/10.1063/5.0138736https://doi.org/10.35848/1347-4065/acee3bhttps://doi.org/10.35848/1882-0786/acdbb7https://doi.org/10.1063/5.018631926) T. Oshima and Y. Oshima, Jpn. J. Appl. Phys. 64, 018003 (2025).27) Y. Oshima and T. Oshima, Sci. Technol. Adv. Mater. 26, 2546285 (2025).28) T. Oshima and Y. Oshima, AIP Adv. 15, 055207 (2025).29) T. Oshima and Y. Oshima, Sci. Technol. Adv. Mater. Methods 5, 2554046(2025).30) T. Oshima, R. Togashi, and Y. Oshima, Sci. Technol. Adv. Mater. 25,2378683 (2024).31) L. Meng, V. G. Thirupakuzi Vangipuram, D. S. Yu, C. Hu, and H. Zhao, J.Vac. Sci. Technol. B 43, 052201 (2025).32) N. K. Kalarickal et al., Appl. Phys. Lett. 119, 123503 (2021).33) S. Dhara, N. K. Kalarickal, A. Dheenan, S. I. Rahman, C. Joishi, andS. Rajan, Appl. Phys. Lett. 123, 023503 (2023).34) D. Kumar and A. Verma, J. Vac. Sci. Technol. A 43, 053405 (2025).35) S. A. Khan, A. Ibreljic, and A. F. M. A. U. Bhuiyan, Appl. Phys. Lett. 127,102105 (2025).36) A. Katta, F. Alema, W. Brand, A. Gilankar, A. Osinsky, and N.K. Kalarickal, J. Appl. Phys. 135, 075705 (2024).37) N. Das, F. Alema, W. Brand, A. Katta, A. Gilankar, A. Osinsky, and N.K. Kalarickal, J. Appl. Phys. 138, 065703 (2025).38) T. Oshima, T. Okuno, N. Arai, Y. Kobayashi, and S. Fujita, Jpn. J. Appl.Phys. 48, 040208 (2009).39) S. Rebollo, T. Itoh, S. Krishnamoorthy, and J. S. Speck, Appl. Phys. Lett.125, 012102 (2024).40) Y. Zhang, A. Mauze, and J. S. Speck, Appl. Phys. Lett. 115, 013501 (2019).41) H. Huang, M. Kim, X. Zhan, K. Chabak, J. D. Kim, A. Kvit, D. Liu, Z. Ma,J.-M. Zuo, and X. Li, ACS Nano 13, 8784 (2019).42) H.-C. Huang et al., Appl. Phys. Lett. 121, 052102 (2022).43) T. Oshima, Jpn. J. Appl. Phys. 64, 088001 (2025).44) H.-K. Lee, H.-J. Yun, K.-H. Shim, H.-G. Park, T.-H. Jang, S.-N. Lee, andC.-J. Choi, Appl. Surf. Sci. 506, 144673 (2020).45) X. Lu, T. Xu, Y. Deng, C. Liao, H. Luo, Y. Pei, Z. Chen, Y. Lv, andG. Wang, Appl. Surf. Sci. 597, 153587 (2022).46) F. Zhang, X. F. Zheng, Y. H. Li, Z. J. Yuan, S. Z. Yue, X. C. Wang, Y.L. He, X. L. Lu, X. H. Ma, and Y. Hao, Appl. Surf. Sci. 684, 161569(2025).47) A. R. Gutierrez et al., J. Vac. Sci. Technol. A 43, 033210 (2025).48) F. S. Becker, D. Pawlik, H. Anzinger, and A. Spitzer, J. Vac. Sci. Technol.B 5, 1555 (1987).49) J. Åhman, G. Svensson, and J. Albertsson, Acta Crystallogr., Sect. C 52,1336 (1996).50) Z. Zhang, G. Chen, K. Gu, S. Koizumi, and M. Liao, Funct. Diam. 3,2221280 (2023).116501-5© 2025 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 18, 116501 (2025) T. Oshimahttps://doi.org/10.35848/1347-4065/ada706https://doi.org/10.1080/14686996.2025.2546285https://doi.org/10.1063/5.0260753https://doi.org/10.1080/27660400.2025.2554046https://doi.org/10.1080/27660400.2025.2554046https://doi.org/10.1080/14686996.2024.2378683https://doi.org/10.1080/14686996.2024.2378683https://doi.org/10.1116/6.0004620https://doi.org/10.1116/6.0004620https://doi.org/10.1063/5.0057203https://doi.org/10.1063/5.0151808https://doi.org/10.1116/6.0004596https://doi.org/10.1063/5.0277909https://doi.org/10.1063/5.0277909https://doi.org/10.1063/5.0195361https://doi.org/10.1063/5.0274126https://doi.org/10.1143/JJAP.48.040208https://doi.org/10.1143/JJAP.48.040208https://doi.org/10.1063/5.0209222https://doi.org/10.1063/5.0209222https://doi.org/10.1063/1.5093188https://doi.org/10.1021/acsnano.9b01709https://doi.org/10.1063/5.0096490https://doi.org/10.35848/1347-4065/ada706https://doi.org/10.1016/j.apsusc.2019.144673https://doi.org/10.1016/j.apsusc.2022.153587https://doi.org/10.1016/j.apsusc.2024.161569https://doi.org/10.1016/j.apsusc.2024.161569https://doi.org/10.1116/6.0004289https://doi.org/10.1116/1.583673https://doi.org/10.1116/1.583673https://doi.org/10.1107/S0108270195016404https://doi.org/10.1107/S0108270195016404https://doi.org/10.1080/26941112.2023.2221280https://doi.org/10.1080/26941112.2023.2221280 Acknowledgments A2