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[Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), [Yoshihiro Irokawa](https://orcid.org/0000-0002-6531-4356), [Toshihide Nabatame](https://orcid.org/0000-0002-5973-0230), [Tadakatsu Ohkubo](https://orcid.org/0000-0003-3548-1951), Yasuo Koide

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[Observation of Atomic-Scale Structural Changes in Al<sub>2</sub>O<sub>3</sub>/GaN Interfacial Layers Prepared with a Dummy-SiO<sub>2</sub> Process](https://mdr.nims.go.jp/datasets/90aabd3a-3338-4ea5-be54-354a0b0cdd43)

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Observation of Atomic-Scale Structural Changes in Al2O3/GaN Interfacial Layers Prepared with a Dummy-SiO2 ProcessObservation of Atomic-Scale Structural Changes in Al2O3/GaNInterfacial Layers Prepared with a Dummy-SiO2 ProcessJun Uzuhashi,1,z Yoshihiro Irokawa,1,z Toshihide Nabatame,1 Tadakatsu Ohkubo,1 andYasuo Koide1,21National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047, Japan2Meijo University, Nagoya, Aichi, 468-8502, JapanWe previously reported that a dummy-SiO2 process improved the dielectric/GaN interface properties [Y. Irokawa et al., ECS J.Solid State Sci. Technol. 13, 085003 (2024)]; however, the improvement mechanism has remained unclear. In this study, theatomic-scale structural changes at Al2O3/GaN interfaces prepared with the dummy-SiO2 process are investigated throughaberration-corrected scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy. The results reveal thatdisordered GaN(O) polarity in the interfacial layer in a sample prepared by the standard process was restored to some extent afterthe dummy-SiO2 process, which likely led to the improved interface electrical properties.© 2025 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open accessarticle distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI:10.1149/2162-8777/adf3e3]Manuscript received July 2, 2025. Published August 5, 2025.To lower energy losses in electrical systems such as invertersand converters, researchers have investigated various semicon-ductor materials other than Si and have found that GaN is one ofthe most attractive materials because of its wide bandgap (3.4 eV)and well-developed device fabrication processes.1,2 Among suchprocesses, the formation of a dielectric layer on GaN is importantbecause metal-oxide-semiconductor (MOS) structures play a keyrole in conventional electrical systems. We previously reportedthat a dummy-SiO2 process improved the properties of dielectric/GaN interfaces.3 The process consists of three steps: First, a SiO2layer is deposited on GaN, followed by post-deposition annealing(PDA) at 800 °C under N2. Second, the SiO2 layer is removedusing a buffered HF solution. Third, a new dielectric layer isdeposited again to fabricate the MOS device. In this process, theinitially deposited SiO2 functions as a sacrificial layer. Theenhanced crystallinity of GaOx interfacial layers formed on GaNusing this process is presumed to be responsible for improvedinterface electrical properties such as excellent positive-biasstress test results. Thus far, however, the improvement me-chanism has remained unclear because the GaOx interfacial layer,which is less than a few nanometers thick, is so thin that observingits atomic-scale structure is difficult. However, recent advances inanalytical electron microscopy may enable us to observe such aminute structure.4In the present study, the atomic-scale structural changes atAl2O3/GaN interfaces prepared using the dummy-SiO2 process areinvestigated through aberration-corrected scanning transmissionelectron microscopy (STEM) with energy-dispersive X-ray spec-troscopy (EDS) and the results are compared with those for asample prepared by the standard process. The results reveal threeissues: First, the GaOx layer contains some amount of N; wetherefore refer to this layer as GaN(O) hereafter. Second, thepolarity of the GaN(O) layers is not uniform; it is disordered,especially in areas proximate to the Al2O3/GaN interface (less than∼1.0 nm from the Al2O3) prepared with the standard process.Here, we define “disordered polarity” as the state where O and Natoms are disorderly arranged, with Ga atoms occupying the samepositions as in the conventional wurtzite GaN structure (Fig 2i).Third, the disordered GaN(O) polarity in the interfacial layer of asample prepared by the standard process can be restored to someextent after the dummy-SiO2 process, which likely led to theimproved Al2O3/GaN interface properties we previously reported.3ExperimentalIn the present study, we investigated two types of samples:Al2O3/GaN structures fabricated with the standard and dummy-SiO2processes. The details of the sample preparation have been reportedelsewhere.3 Atomic-scale cross-sectional high-angle annular darkfield (HAADF) and annular bright field (ABF)-STEM analysis withEDS was performed at 200 kV using a Spectra Ultra S/TEM(Thermo Fisher Scientific) for these samples. To ensure the qualityof comparisons of STEM results for the two types of samples, all ofthe STEM lamellae were prepared to have the same thickness of30 nm via a thickness-controllable technique using a focused-ion-beam (FIB)-scanning electron microscopy (SEM) dual-beamHelios5UX equipped with AutoScript (Thermo Fisher Scientific)while minimizing FIB-induced damage.5,6Results and DiscussionFigure 1 shows HAADF-STEM images of the Al2O3/GaNstructure fabricated with the (a) standard and (b) dummy-SiO2processes. EDS compositional line profiles for Ga, Al, and O at theinterfaces as a function of depth from the Al2O3 layer toward thebulk GaN region of the (c) standard-processed and (d)dummy-SiO2-processed samples are also shown. These data wereobtained to confirm the GaN(O) layer thickness and oxygenconcentration in the layer. In Figs. 1a and 1b, the first to fifth Galayers are indicated; these layer numbers correspond to those in theEDS profile for Ga in Figs. 1c and 1d. Note that each oscillation ofthe Ga concentration in Figs. 1c and 1d corresponds to each Gahorizontal atomic line observed in Figs. 1a and 1b, respectively. Asshown in Figs. 1c and 1d, the Ga concentration saturates near theseventh layer for both processes; therefore, the thickness of the GaN(O) is determined to be 1.5–2.0 nm for both samples. In addition, wefound that the oxygen concentration as a function of depth is similarfor the two samples. Notably, the detection limit of EDS is on theorder of a few percent. Oxygen diffusion from SiO2 toward GaNduring PDA has been previously confirmed;7 however, we did notobserve a difference in the oxygen concentration between thesamples, likely because of the oxygen detection limit of EDS.We next investigated the area proximate to the Al2O3/GaNinterface. First, we observed a bulk GaN region of the Al2O3/GaNstructure to confirm that the crystal orientation was in perfectalignment and to enable a comparison of the data for the bulkregion with those for the Al2O3/GaN interface layers. Figures 2a and2b show an ABF-STEM image and EDS elemental map, respec-tively, of the GaN bulk region of the Al2O3/GaN structure fabricatedwith the standard process (depth of approximately 20 nm from thezE-mail: UZUHASHI.Jun@nims.go.jp; IROKAWA.Yoshihiro@nims.go.jpECS Journal of Solid State Science and Technology, 2025 14 085001https://orcid.org/0000-0003-2023-8158https://orcid.org/0000-0002-6531-4356https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1149/2162-8777/adf3e3mailto:UZUHASHI.Jun@nims.go.jpmailto:IROKAWA.Yoshihiro@nims.go.jphttps://crossmark.crossref.org/dialog/?doi=10.1149/2162-8777/adf3e3&domain=pdf&date_stamp=2025-08-05interface). Larger spherical areas of dark contrast in the ABF-STEMimage in Fig. 2a represent Ga atoms, which are colored purple inFig. 2b. However, ABF-STEM imaging also enabled the visualiza-tion of light elements; the positions of the N atoms were observed assmaller spherical regions of dark contrast in the ABF-STEM image(Fig. 2a), and the N atoms are represented as yellow spheres in theEDS elemental map (Fig. 2b). For easier understanding, some of theatomic arrangements of Ga and N atoms are schematicallyFigure 1. HAADF-STEM images of the Al2O3/GaN structure fabricated with the (a) standard and (b) dummy-SiO2 processes, and the EDS compositional lineprofiles for Ga, Al, and O at the interfaces as a function of depth from Al2O3 layers toward GaN bulk regions of the (c) standard-processed and (d)dummy-SiO2-processed samples.Figure 2. (a) ABF-STEM image and (b) EDS elemental map of the GaN bulk region of a Al2O3/GaN structure fabricated with the standard process. ABF-STEMimages of Al2O3/GaN interfaces for (c) standard-processed and (d) dummy-SiO2-processed samples. EDS elemental maps of Ga and N for (e) the standard-processed and (f) dummy-SiO2-processed samples. EDS elemental maps of Ga and O for (g) the standard-processed and (h) dummy-SiO2-processed samples. (i)Schematic of the polarities.ECS Journal of Solid State Science and Technology, 2025 14 085001represented by purple and yellow spheres in Fig. 2a. Figures 2a and2b show the Ga-polar GaN crystal is oriented toward the surface (theupward direction in the present paper).ABF-STEM imaging and EDS elemental mapping were appliedto the Al2O3/GaN interfaces of the standard-processed anddummy-SiO2-processed samples. The results are shown in Figs. 2c–2h. Specifically, Figs. 2c and 2d show ABF-STEM images ofAl2O3/GaN interfaces for the standard-processed and dummy-SiO2-processed samples, respectively. Figures 2e and 2f showEDS elemental maps of Ga and N for the standard-processed anddummy-SiO2-processed samples, respectively. Figures 2g and 2hshow the EDS elemental maps of Ga and O for the standard-processed and dummy-SiO2-processed samples, respectively. Inthese figures, the first to fifth Ga layers from Al2O3 are indicated,as in Fig. 1. As shown in Figs. 2c and 2d, Ga atoms are observed aslarger spheres at the same position as in the bulk GaN region andsmaller dark spheres are also apparently observed at the sameposition as in the bulk GaN region. (Some Ga atoms are schema-tically represented by purple spheres in Figs. 2c and 2d, as inFig. 2a). Notably, however, the smaller dark spheres in Figs. 2c and2d appear cloudy compared with those corresponding to the bulkGaN region (Fig. 2a), suggesting that these atomic arrangements in afew atomic layers near the GaN surface vary from the atomicarrangement in the original Ga-polar crystal structure. In addition,the images of the smaller dark spheres shown in Fig. 2c are muchblurrier than those shown in Fig. 2d, indicating that the polarity ofthe standard-processed sample is less uniform than that of thedummy-SiO2-processed sample. To investigate the local polarity, wecarried out EDS elemental mapping.First, the EDS elemental maps in Figs. 2e–2h show that theinterface layers are composed of GaN(O) instead of pure GaOx. Inthe interfacial layers, the O content gradually decreases toward thebulk GaN region, whereas the N content steadily increases withincreasing distance from the surface, as described later in thediscussion of Fig. 3. The atomic composition ratio between Ox andNy, i.e., the x/y value, changed from 5.0 to 0.3 toward the GaN bulkin the GaN(O) regions shown in Fig. 2, indicating that O diffusionprogresses along the depth direction. Note that this value could beconsidered a slight overestimation because both sides of the thinSTEM lamellae were naturally oxidized during the transfer of thesample for observation. The thicknesses of the GaN(O) layers—thatis, N–O intermixing or O diffusion into GaN—are 1.5–2.0 nm forboth of the samples (Fig. 1). The presence of N in GaOx layers hasbeen reported in a study of the early stages of thermal oxidation ofGaN.8Second, the EDS elemental maps in Figs. 2e–2h reveal theminute atomic arrangements of Ga, N, and O atoms in the GaN(O)layers. Although no differences are observed in the GaN(O)thicknesses between the two processes, the N and O atomicarrangements are substantially changed via the dummy-SiO2 pro-cess. In Figs. 2e and 2f, Ga atoms are purple and N atoms are orange.The white arrows in Figs. 2e and 2f represent irregular N atomarrangements with respect to the Ga-polar GaN structure. Thenumber of irregular N atom arrangements dramatically decreasedwhen the dummy-SiO2 process was used. However, for O atoms, thesituation is different. In Figs. 2g and 2h, Ga atoms are purple and Oatoms are blue. As shown in Fig. 2g, observing a particular O atomsite is difficult because the blue region is too vague for the specificatomic positions to be identified, suggesting that most of the O atomsare not located in the ordered specific atomic sites. Conversely,observing O atoms in Fig. 2h is easier because the O atomsrepresented by yellow arrows appear relatively well ordered at theatomic scale. In particular, the arrows with a star indicate thepositions where O atoms have replaced N atoms in the Ga-polarGaN structure. The previously reported native oxide structure ofGaN was similar to that of β-Ga2O3 with its O atoms uniformlyarranged in an inversion of polarity.9 However, the presentlyobserved structure for the standard-processed sample is not asuniformly polarized as the previously reported one;9 rather, thepolarity is disordered. Figure 2i shows a schematic of the polaritiesdiscussed here.Figures 3a and 3b show normalized EDS signal intensity lineprofiles for Ga, O, and N at the interfaces as a function of depth fromthe Al2O3 layer toward the GaN bulk region of the standard-Figure 3. Normalized EDS signal intensity line profiles for Ga, O, and N at the interfaces as a function of depth from Al2O3 layers toward the GaN bulk regionof (a) the standard-processed and (b) dummy-SiO2-processed samples.ECS Journal of Solid State Science and Technology, 2025 14 085001processed and dummy-SiO2-processed samples, respectively. Thethickness of the GaN(O) layer is found to be 1.5–2.0 nm for bothsamples, consistent with the data in Fig. 1. In Fig. 3, the numbers onthe Ga EDS signal intensity profiles indicate the number of Ga layerscounted from the surface; the first to fifth layers correspond to thosein Figs. 2c–2h. As shown in Fig. 3, the ratios of O and N graduallyvary through the interface layers and are not fixed values. Thus, theGaN(O) layers at the interface do not comprise a single homogenouscrystal structure, as previously mentioned. At depths greater than∼1.0 nm, the periodicities and positional relationship of the EDSsignal intensity for Ga and N exhibited proximate Ga-polarity forboth the standard-processed and dummy-SiO2-processed samples(Fig. 3). In addition, O atoms are observed at similar positions as Natoms, implying that oxidation progresses with O atoms replacing Natoms while the GaN crystal maintains the same crystal structure andthat O atoms might behave as a donor, as we have previouslydiscussed.10 By contrast, at depths shallower than ∼1.0 nm, thesignal oscillation for O and N is less clear, meaning that theseelements do not occupy ordered positions; this situation is vividlyshown in Figs. 2e–2h. However, with careful observation, anindication of a reversed-polarity structure can also be observed inthese regions for both samples. A major difference between Figs. 3aand 3b is the distribution of the O atoms. The periodicity of the Ointensity, as indicated by the blue arrows, is observed more clearly inFig. 3b than in Fig. 3a, suggesting that O atoms occupy more orderedpositions after the dummy process, consistent with the data shown inFigs. 2g and 2h. Figure 3 also shows that the Ga bond length isapparently greater near Al2O3, which may be related to strain at theAl2O3/GaN interfaces.The data obtained in this study demonstrate that the dummyprocess improves the disordered polarity in the GaN(O) layer tosome extent. At this point, however, the relationship between theimprovement of the disordered polarity in the GaN(O) layer andthe improvement of the interfacial electrical property is unclear.Previously, thin Ga-oxide interlayers were reported to improvethe electrical properties in SiO2/GaN MOS interfaces;11 theminute structural information about the interface layers wouldbe important for elucidating GaN MOS interface properties. Inaddition, the reason why the dummy process improves thedisordered polarity in the GaN(O) layers is also unclear; evenwith the dummy process, total restoration of the disorderedpolarity was not achieved. Therefore, if we develop a moresophisticated polarity control method, it could lead to therealization of interfaces with better electrical properties. Studiesof other crystal plane orientations such as m-face might providehints to a solution.12ConclusionsWe investigated the atomic-scale structural changes atAl2O3/GaN interfaces prepared with the dummy-SiO2 process. Theresults revealed that the disordered GaN(O) polarity in interfaciallayers of a sample prepared by the standard process was restored tosome extent after the dummy-SiO2 process. The polarity disorder inthe interfacial layer might influence the GaN MOS interfacialelectrical properties in a manner similar to the reported effect ofgrain boundaries on the electrical properties of polycrystallinesemiconductors.13AcknowledgmentsThis research was supported by the Ministry of Education, Culture,Sports, Science and Technology, Japan (MEXT), through its “Creationof Innovative Core Technology for Power Electronics” program underGrant No. JPJ009777 and ARIM (JPMXP1223NM5088). Part of thiswork was supported by the Electron Microscopy Unit, NationalInstitute for Materials Science (NIMS) and JSPS KAKENHI GrantNumbers 23K03949. The authors thank Kyoko Suzuki for hertechnical support in preparing STEM lamellae.ORCIDJun Uzuhashi https://orcid.org/0000-0003-2023-8158Yoshihiro Irokawa https://orcid.org/0000-0002-6531-4356References1. S. J. Pearton, F. Ren, A. P. Zhang, and K. P. Lee, Mater. Sci. Eng. R Rep., 30, 55(2000).2. T. Kachi, Jpn. J. Appl. Phys., 53, 100210 (2014).3. Y. 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