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[Takahiro Nagata](https://orcid.org/0000-0002-8591-2943), [Asahiko Matsuda](https://orcid.org/0000-0001-5989-027X), [Takashi Teramoto](https://orcid.org/0000-0002-9368-1284), [Dominic Gerlach](https://orcid.org/0000-0003-1859-0750), [Peng Shen](https://orcid.org/0000-0002-1971-5490), [Shigenori Ueda](https://orcid.org/0000-0001-9425-0614), [Takako Kimura](https://orcid.org/0009-0007-0109-4482), [Christian Dussarrat](https://orcid.org/0009-0000-1063-6473), [Toyohiro Chikyow](https://orcid.org/0000-0003-3860-4806)

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[Effects of nitrosyl fluoride based gas treatment on fluorination and redox reaction at GaN surface and Pt/GaN interface](https://mdr.nims.go.jp/datasets/22b08252-5687-4f2e-b3de-c6ab08231b54)

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Effects of nitrosyl fluoride based gas treatment on fluorination and redox reaction at GaN surface and Pt/GaN interfaceViewOnlineExportCitationRESEARCH ARTICLE |  MARCH 04 2025Effects of nitrosyl fluoride based gas treatment onfluorination and redox reaction at GaN surface and Pt/GaNinterfaceTakahiro Nagata   ; Asahiko Matsuda  ; Takashi Teramoto  ; Dominic Gerlach  ; Peng Shen  ;Shigenori Ueda  ; Takako Kimura  ; Christian Dussarrat  ; Toyohiro Chikyow J. Appl. Phys. 137, 095304 (2025)https://doi.org/10.1063/5.0224068Articles You May Be Interested InGrowth of cobalt films at room temperature using sequential exposures of cobalt tricarbonyl nitrosyl andlow energy electronsJ. Vac. Sci. Technol. A (October 2019)Analytical gradients of complete active space self-consistent field energies using Cholesky decomposition:Geometry optimization and spin-state energetics of a ruthenium nitrosyl complexJ. Chem. Phys. (May 2014)Vertical Al2O3/GaN MOS capacitors with PEALD-GaOx interlayer passivationAppl. Phys. Lett. (February 2025) 18 March 2025 08:16:54https://pubs.aip.org/aip/jap/article/137/9/095304/3338311/Effects-of-nitrosyl-fluoride-based-gas-treatmenthttps://pubs.aip.org/aip/jap/article/137/9/095304/3338311/Effects-of-nitrosyl-fluoride-based-gas-treatment?pdfCoverIconEvent=citejavascript:;https://orcid.org/0000-0002-8591-2943javascript:;https://orcid.org/0000-0001-5989-027Xjavascript:;https://orcid.org/0000-0002-9368-1284javascript:;https://orcid.org/0000-0003-1859-0750javascript:;https://orcid.org/0000-0002-1971-5490javascript:;https://orcid.org/0000-0001-9425-0614javascript:;https://orcid.org/0009-0007-0109-4482javascript:;https://orcid.org/0009-0000-1063-6473javascript:;https://orcid.org/0000-0003-3860-4806https://crossmark.crossref.org/dialog/?doi=10.1063/5.0224068&domain=pdf&date_stamp=2025-03-04https://doi.org/10.1063/5.0224068https://pubs.aip.org/avs/jva/article/37/6/060906/246933/Growth-of-cobalt-films-at-room-temperature-usinghttps://pubs.aip.org/aip/jcp/article/140/17/174103/316941/Analytical-gradients-of-complete-active-space-selfhttps://pubs.aip.org/aip/apl/article/126/8/081603/3337411/Vertical-Al2O3-GaN-MOS-capacitors-with-PEALD-GaOxhttps://e-11492.adzerk.net/r?e=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&s=CTgie5JWgzJR564QZLaYCtwcTcUEffects of nitrosyl fluoride based gas treatmenton fluorination and redox reaction at GaN surfaceand Pt/GaN interfaceCite as: J. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068View Online Export Citation CrossMarkSubmitted: 19 June 2024 · Accepted: 6 February 2025 ·Published Online: 4 March 2025Takahiro Nagata,1,a) Asahiko Matsuda,2 Takashi Teramoto,3 Dominic Gerlach,1,b) Peng Shen,3Shigenori Ueda,1,4 Takako Kimura,3 Christian Dussarrat,3 and Toyohiro Chikyow5AFFILIATIONS1Research Center for Electronic and Optical Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan2Materials Data Platform, NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan3K.K. Air Liquide Laboratories, 2-2 Hikarinooka, Yokosuka, Kanagawa 239-0847, Japan4Synchrotron X-ray Station at SPring-8, NIMS, Sayo, Hyogo 679-5148, Japan5Center for Basic Research on Materials, NIMS, Tsukuba, Ibaraki 305-0047, Japana)Author to whom correspondence should be addressed: NAGATA.Takahiro@nims.go.jpb)Present address: Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen,The Netherlands.ABSTRACTThe effects of nitrosyl fluoride (FNO) gas treatment on the surface of GaN(0001) and its interface with sputtered Pt were investigated byhard x-ray photoelectron spectroscopy (HAXPES). Annealing GaN and Pt/GaN samples in an FNO gas atmosphere resulted in the appear-ance of prominent F 1s peaks in the HAXPES spectra, indicating the efficient formation of Ga–Fx bonding states not only in bare-GaN butalso in Pt/GaN, even when the FNO gas treatment was performed after Pt deposition. In addition, the chemical shifts of the Ga 2p3/2 and N1s peaks corresponded to a Fermi level shift toward the valence band. The FNO gas treatment induced greater oxidation of the GaN surfacethan the Pt/GaN interface. By contrast, at the Pt/GaN interface, the unintentionally formed oxide GaOx was reduced, resulting in animprovement of the electrical properties. The results of this study suggest that FNO gas treatment is an effective post-processing method forthe fluorination of GaN-based systems after metal deposition.© 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/). https://doi.org/10.1063/5.0224068I. INTRODUCTIONNitride semiconductor devices are becoming increasing practi-cal, and demand exists for even higher functionality, such as high-speed operation and higher breakdown voltages. In GaN deviceapplications, the high density of vacancies associated with danglingbonds is one unresolved issue.1 In Si devices, defect passivation isgenerally achieved by a hydrogen-based process that terminates thedangling bonds with H and removes the vacancy states.2 However,this method is ineffective for defect passivation in GaN devices.3–5As an alternative to H, F has been proposed as a candidate elementthat can terminate the dangling bonds. F termination has beeninvestigated with Si and Ge.6,7 F has a high electronegativity, and ittends to form highly stable bonds. It is also expected to be usefulfor post-process defect passivation because of its small atomicradius and high diffusivity. The introduction of F into GaN andAlGaN/GaN devices has been studied for passivating defects, modi-fying defects, and altering device properties such as the thresholdvoltage (Vth).8,9 F incorporation is typically accomplished by ionimplantation,10,11 CF4 plasma exposure,8,10,12 or, less commonly,annealing under an NF3 atmosphere.13 These methods are used inconventional Si device process technology; however, the kineticenergy in the process is high and recovery from damage requiresJournal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068 137, 095304-1© Author(s) 2025 18 March 2025 08:16:54https://doi.org/10.1063/5.0224068https://doi.org/10.1063/5.0224068https://pubs.aip.org/action/showCitFormats?type=show&doi=10.1063/5.0224068http://crossmark.crossref.org/dialog/?doi=10.1063/5.0224068&domain=pdf&date_stamp=2025-03-04https://orcid.org/0000-0002-8591-2943https://orcid.org/0000-0001-5989-027Xhttps://orcid.org/0000-0002-9368-1284https://orcid.org/0000-0003-1859-0750https://orcid.org/0000-0002-1971-5490https://orcid.org/0000-0001-9425-0614https://orcid.org/0009-0007-0109-4482https://orcid.org/0009-0000-1063-6473https://orcid.org/0000-0003-3860-4806mailto:NAGATA.Takahiro@nims.go.jphttps://creativecommons.org/licenses/by-nc/4.0/https://creativecommons.org/licenses/by-nc/4.0/https://doi.org/10.1063/5.0224068https://pubs.aip.org/aip/japhigh-temperature heat treatment and other processes. Atomic layeretching of GaN surfaces by RF processing has also been reportedrecently.14,15 Fluorination of the GaN surface and Pt/GaN interfacewith N2 gas containing NF3 has also been demonstrated;16however, these methods introduce additional defects into the bulkregion. In the present study, to suppress the introduction of suchdefects and to promote weak fluorination, we used nitrosyl fluoride(FNO)-based gases.FNO is a highly reactive fluorinating agent that converts manymetals to fluorides and releases nitric oxide (NO) in the process.As a rudimentary consideration of the reaction thermodynamics,the standard heat of formation (−ΔHf°) for FNO, GaN, and fullyfluorinated GaF3 is 66, 110, and 1163 kJ/mol, respectively.17,18Thus, the ΔHf° values indicate that the fluorination of FNO is thepreferred reaction. We hypothesized that FNO decompositionwould also be effective compared with NF2–F bond breaking(−ΔHf° = 239 kJ/mol).19 The dissociation energy of 192 kJ/mol forthe N–F bond of nitrile fluoride (FNO2) is ∼75 kJ/mol lower thanthat of a typical N–F single bond.In addition, to verify the feasibility of FNO gas treatment asa post-process dangling-bond-termination method, we fabricatedPt/GaN contacts and subjected them to post-annealing under an FNOatmosphere. To nondestructively characterize the formed interface,hard x-ray photoelectron spectroscopy (HAXPES, hν = 5953.4 eV),which has a probe depth of Pt greater than 10 nm, was employed.20,21The deep detection depth of HAXPES enabled direct observation ofthe Pt/GaN interface. We found that the detection depth could bevaried by using HAXPES in conjunction with conventional soft x-rayPES (SXPES) using an Al-Kα light source (hν = 1486.6 eV).II. EXPERIMENTAL PROCEDURESFor HAXPES electrical measurements, unintentionally dopedGaN (0001) films (thickness: 4–6 μm) grown by epitaxial MOCVDon c-sapphire substrates (POWDEC) were used, with a full width athalf maximum (FWHM) value of 252 arcsec in the x-ray diffractionomega-rocking curve of (0002) reflection (XRD-RC). For electricalmeasurements, n-type Si-doped GaN bulk crystal (Shinyo, thickness:362 μm, resistivity <0.05Ω cm) were used, with an FWHM value of72 arcsec in the XRD-RC. Notably, in the HAXPES experiments, thesame epitaxial GaN film used in our previous study16 on GaNsurface fluorination with NF3 and F2 was used so that the resultscould be compared. By contrast, for electrical characteristics, a verti-cal device structure with top and bottom electrodes formed on GaNbulk crystals was used to simplify the sample fabrication process andsuppress the effect of surface modification by an additional wetprocess. In both cases, the GaN surface was a Ga-terminated (0001)plane (+c plane).The film and bulk GaN samples were rinsed sequentially withacetone and ethanol. One group of samples had no Pt layer (hereafter,“Bare-GaN”). The other samples had a layer of Pt deposited before thegas annealing process (hereafter, “Pt-GaN”). For HAXPES measure-ments, a 5 or 10 nm-thick Pt layer was uniformly deposited over theentire GaN surface. For electrical measurements, after wet cleaning, a12 nm-thick Ti/100 nm-thick Pt stacked ohmic electrode was formedon the backside by DC sputtering, followed by annealing at 350 °C for30min under N2 atmosphere. Then, a Pt top electrode with a diameterof 110 μm and a thickness of 250 nm was formed by DC sputteringwith a metal mask. Some samples were annealed at 250 °C for 120 sunder 30 kPa with the FNO gas diluted to 2% with N2 gas (hereafter,“FNO-treated”). In the case of the sample for electrical measurement,the back electrode surface was in contact with the susceptor surface ofthe heat treatment system during the FNO gas treatment. Current–voltage (I–V) measurements were performed using a semiconductorparameter analyzer (Keysight Technologies, B1500A).PES was used to study the chemical bonding in the samples.The layers were nondestructively characterized from the surface tothe interface using SXPES (Thermo Scientific Sigma Probe) andHAXPES. HAXPES measurements were performed at the BL15XUundulator beamline of SPring-8 using a high-resolution hemispher-ical electron analyzer (VG Scienta R4000). Details of the HAXPESexperimental setup are described elsewhere.22,23 The total energyresolutions of the SXPES and HAXPES measurements were set to700 and 240 meV, respectively. The binding energies were cali-brated from the Au 4f7/2 photoelectron peak (84.0 eV) for an Aufilm placed at the same electrical ground level as the sample.The KolXPD program24 was used for curve fitting and analysis withthe Voigt function (KolXPD’s implementation of a pseudo-Voigtprofile21,25) after Shirley-type background subtraction.26 For thePt-deposited samples, charge correction was also performed byfitting the Pt 4f7/2 peak with a Doniach–Šunjić function27 after sub-tracting the Shirley-type background and adjusting its peak bindingenergy to 71.2 eV.28,29 The surface morphology was observed byatomic force microscopy (AFM; Hitachi High-Technologies,AFM5000II).III. RESULTS AND DISCUSSIONA. Bare-GaNFigure 1 shows the surface morphologies of FNO-treated anduntreated Bare-GaN. After the FNO-treatment, the atomic-scalestep and terrace structures were maintained, but an increase insurface roughness was observed. In the 2 μm scan area of the AFMimages, the root mean square (RMS) roughness value increasedfrom 0.20 nm (Bare-GaN) to 0.32 nm (FNO-treated GaN). TheFNO-treated Bare-GaN showed mainly small particles on the edgesof terrace structures, which might indicate reactions such asetching by F-based gas treatment.30Figure 2 compares the HAXPES F 1s, Ga 2p3/2, N 1s, and O 1sspectra of the FNO-treated and untreated Bare-GaN samples.The spectrum of the FNO-treated Bare-GaN shows GaFx and GaF3bonding states [Fig. 2(a)], similar to the results in our previousreport on the treatment of GaN with NF3 gas.16 Bermudez investi-gated the bonding states in the reaction of XeF2 with GaN sur-faces17 and described the influence of band bending and thechemical shift of the Ga 3d peak due to GaFx (x = 1, 2, 3) forma-tion. To our knowledge, the literature on fluorinated GaN is insuffi-cient to determine the degree of fluorination based on chemicalshifts. The two F 1s peaks observed in the spectra collected in thepresent study could indicate either of species represented as GaF,GaF2, or GaF3; the energy positions of the GaF and GaF2 peaks aresimilar, complicating attempts to distinguish between these speciesor determine whether one of the species is absent due to instabilityor desorption. A comparison of the results presented in the presentJournal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068 137, 095304-2© Author(s) 2025 18 March 2025 08:16:54https://pubs.aip.org/aip/japstudy with previously reported results suggests that the high-binding energy side corresponds to GaF3 and the low-bindingenergy side corresponds primarily to GaF. For the Ga 2p3/2 peak inFig. 2(b), the corresponding bonding state of Ga–F and/or GaOcan also be confirmed on the high-binding energy side, whichshows a small intensity change after the FNO treatment and islikely due to band bending; the effect of band bending on theshape of N 1s peak is not clear [Fig. 2(c) and Fig. S1(a) in thesupplementary material]. GaFx is expected to include a smallnumber of oxidized structures. Residual oxygen is present in thesubstitution reaction involving FNO. The O 1s spectrum [Fig. 2(d)]confirms the presence of three O bonding states. The influence ofthe surface adsorption layer (–OH) remains in the tail state at∼533.5 eV; the intensity of this peak was reduced after the FNOtreatment, whereas that at the low-binding energy side tended toincrease. The middle peak corresponds to the GaOx bonding state.The lowest binding energy side corresponds to the binding states ofthe oxynitride layer, which might contain F, although its exactcomposition is difficult to determine. The N peak obscures theother peaks. These changes have been confirmed by both SXPESand HAXPES at depths greater than a few nanometers from thesurface (Figs. S1 and S2 in the supplementary material). Basically, anatural oxide film is formed on the GaN surface. This oxide film hasbeen reported to be an ultra-thin GaOx layer;31 however, it is respon-sible for parasitic resistance at the electrode interface and Schottkyjunction, leading to a reduction of the ON current because ofincreased series resistance during ON operation, which adverselyaffects the ON–OFF ratio and reduces the operating voltage.In addition, in Fig. 2, the Ga 2p3/2 and N 1s peaks in thespectrum of the FNO-treated Bare-GaN sample are shifted tolower binding energy. On the basis of the above assignment, theseshifts indicate a shift of the valence band toward the Fermi level.To discuss the valence-band structures for the FNO-treated anduntreated Bare-GaN samples, we present the acquired valence-band spectra and a band diagram of the region near the Fermilevel, EF, in Fig. 3. A shift of the valence-band maximum (EVBM)toward the EF direction is observed [Fig. 3(b)], indicating electrondepletion at the surface; in addition, changes are observed inthe gap region and the spectral shapes of the valence band.Comparing the edges of the bands reveals that the tail extendingtoward EF is more pronounced in the spectrum of the FNO-treatedsample than in the spectrum of the untreated sample [Fig. 3(b)],which was pronounced at the surface and confirmed by SXPES.These data suggest an increase in the density of electronic states inthe gap. Changes in the spectral shape of the valence band suggest achange in the surface termination. GaN has a hexagonal structure,asymmetric in the 〈0001〉 direction, and the termination plane has aGa or N face. These GaN surfaces strongly affects the valence-bandspectral shape.32–34 The relative intensities of the structures located atthe middle and lower binding energies in the valence-band spectrumof the GaN thin film [labeled as P1 (∼5.0 eV), P2 (∼7.8 eV), and P3(∼10.0 eV) in Fig. 3(a)] are sensitive to the GaN surfaces. Ohsawaet al. reported that the P1 peak for Ga-terminated GaN is moreintense than the P2 peak in HAXPES spectra acquired at hv = 6 keV,whereas the P1 peak for N-terminated GaN is weaker than the P2peak, which is consistent with the results calculated by using abinitio calculations based on density functional theory (DFT).34 Theyalso reported that the intensity of peak P3 decreases relative to thatof peak P2 in the spectrum of N-terminated GaN.30 Typically,Ga-terminated surfaces are formed by metal–organic chemical vapordeposition (MOCVD) of GaN onto a sapphire substrate. However,since the XPS results include information several atomic layersdeeper than the top surface, they may not correctly reflect the struc-ture of the top surface in atomic layers due to defects. On the otherhand, we have previously compared results of time-of-flight low-energy atom scattering spectroscopy (TOFLAS), which are more sen-sitive to the topmost atomic layer, with XPS results; in which wefound that the surface structure was modified by defect structures,but the tendency for the P1 to be stronger is consistent due to the Gasurface.35 In the present study, the intensity of the P1 peak is reducedto the same level as that of the P2 peak in the FNO-treated samples.This decrease in peak intensity is inconsistent with a Ga-terminatedstructure and suggests a change in surface termination.Collectively, the core-level spectra indicate that fluorina-tion in GaN by the FNO treatment could proceed as follows:The Ga-terminated structure ideally has dangling bonds at thesurface. The FNO treatment acts upon the bonds, terminatingthem with F and resulting in a GaF3 structure. Other bonding statesFIG. 1. AFM images of FNO-treated and untreated Bare-GaN. The insets are enlarged images.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068 137, 095304-3© Author(s) 2025 18 March 2025 08:16:54https://doi.org/10.60893/figshare.jap.c.7662113https://doi.org/10.60893/figshare.jap.c.7662113https://pubs.aip.org/aip/japcan exist, such F interstitials caused by defects in GaN or by surfaceoxidization. Possible defects include through-dislocations, as observedby AFM, and irregular structures resulting from inadvertent surfaceoxidation. However, these results show that the incorporation of Fslightly increased the density of gap states rather than passivating andoxidizing them. We reported in the present study that the Pt/GaNstacking structure reduces the effect of strong fluorination andinduces additional reactions. To identify the effect of strongfluorination in the FNO-treated Pt/GaN samples, we investigatedtheir Pt/GaN stacking structure.B. Pt-GaNFigure 4 compares F 1s, Ga 2p3/2, O 1s, and Pt 4f spectraobtained by HAXPES analysis of the FNO-treated and untreatedPt-GaN samples. The F 1s peak appears in the FNO-treated5 nm-thick Pt-GaN sample. The GaF3-to-GaFx intensity ratioincreased compared with that of FNO-treated Bare-GaN. By contrast,in the FNO-treated 10 nm-thick Pt-GaN sample, in the current mea-surement setup, the target peak seems to be present, but no clearintensity was obtained for the background noise of the measurement.There is a possible explanation for these changes of peaks owingto the probing depth of our HAXPES measurement. For Pt, theinelastic mean free path (IMFP20) of Pt 4f7/2 for HAXPES is4.68 nm, and the probing depth is approximately 15 nm (probingdepth: 3× IMFP21) below the surface in our experimental setup.However, since the detected signal intensity decreases exponen-tially, it decreased by more than two orders of magnitude at 5 and10 nm, and it is considered that it could not be detected at 10 nmunder the present measurement conditions because it was at 5 nmand required a long integration time. Given that the intensityof the standardized peak for N in the Pt-GaN sample increasedcompared with that for N in the Bare-GaN sample and that theFIG. 2. (a) F 1s, (b) Ga 2p3/2, (c) N 1s, and (d) O 1s core-level spectra forFNO-treated and untreated Bare-GaN. The solid lines are the experimentaldata, the dashed lines are the Voigt profiles, and the red dots are their sum.FIG. 3. (a) Valence-region spectra for FNO-treated and untreated Bare-GaN, asobtained by SXPES and HAXPES. The binding energy of 0 eV is the Fermilevel (EF). (b) Magnified spectra corresponding to the Fermi level. (c)Schematics of the Fermi level position in FNO-treated and untreated Bare-GaN.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068 137, 095304-4© Author(s) 2025 18 March 2025 08:16:54https://pubs.aip.org/aip/japdetection depth in Bare-GaN was slightly larger at the GaNsurface than that in Pt-GaN, and F is present in a very thin regionnear the interface between the GaN surface and the Pt layer.In the Ga 2p3/2 spectrum corresponding to the GaN side after Ptlayer formation, the peak shifts by ∼0.5 eV to higher bindingenergy compared with the corresponding peak in the Ga 2p3/2spectrum of the untreated Bare-GaN, irrespective of the gas treat-ment. This trend is similar to that observed in the N 1s spectra[Fig. S3(a) in the supplementary material], indicating that the for-mation of the Pt junction shifts the Fermi level to the in-gapregion and suggesting that depletion occurs at the interface side,accompanied by the formation of a Schottky barrier. In addition,the full width at half-maximum (FWHM) of the Ga 2p3/2 peak at∼1118 eV is reduced by 0.1 eV [Fig. S3(b) in the supplementarymaterial], suggesting that oxide layer reduction and compensationof defective layers are occurring near the interface. In the Ga spec-trum of Bare-GaN, the FWHM of the main peak does notincrease because of the change in GaON; by contrast, in the spec-trum of Pt, the FWHM changes. In this regard, we focused on theO 1s spectra.Figure 4(c) shows the results of peak fitting of the O 1sspectrum after the Pt 3p and background data were subtractedfrom the acquired data [original data are shown in the Fig. S4(a) inthe supplementary material]. The formation of Pt–O is also con-firmed by the Pt 4f spectra, as shown in Fig. 4(d) [fitted data areshown in Fig. S4(b) in the supplementary material]. In the spectraof FNO-treated Bare-GaN, core-level peaks related to Ga–O, OH,and GaO–F or GaO–N bonding states are observed in the O 1sregion [Fig. 2(d)]; by contrast, for FNO-treated Pt/GaN, an addi-tional peak observed at ∼532 eV in the spectra is attributed to a core-level peak derived from Pt–O.36 In some cases, a small amount ofPt–O is formed at the Pt/oxide interface and deformation is observedon the high-binding energy side of the Pt 4f spectrum, where peaksassociated with metallic Pt 4f7/2 and 4f5/2 overlap, indicating Pt–Oformation. However, evidence of the formation of Pt–O was notobserved in the spectrum of untreated Pt/GaN. The formation ofPtOx is considered to be more closely related to the redox process atthe GaN interface than to the uptake of oxygen from FNO gasduring the post-treatment process. In the O 1s region, the peakintensity ratio clearly changes, confirming the phenomenon observedfor the Ga–O peak and the increase in the Pt–O peak intensity. Anincrease in peak intensity at low binding energy is also observed;however, it is attributable to residual elements from fluorination, aswas the case with Bare-GaN. At the Pt/GaOx interface, a catalyticeffect is expected, and NO desorption is induced.37,38 In addition,the formation of PtO2 is promoted,39 albeit at a high temperature rel-ative to that used in the present experiment. This catalytic effect isspeculated to have led to decomposition of FNO, reduction of GaOxnear the interface, and fluorination of the resultant reduced Ga. Onthe basis of the above discussion, fluorination during the FNO gastreatment might be effectively promoted by a combination of metalsexpected to have a catalytic effect. Note that this Pt 4f change wasalso observed in the FNO-treated 10 nm-thick Pt-GaN sample; fur-thermore, our previous results indicate that due to the columnarstructure of Pt, fluorine-based gas (NF3 gas) can penetrate into thePt layer.16 In contrast to the NF3 gas, the fluorination ability of FNOgas is weaker than that of NF3 gas, so the fluorination of Pt andGaN (F 1s signal) in the FNO-treated 10 nm-thick Pt-GaN samplewas not clearly observed. The HAXPES results are simply summa-rized in Table S1 in the supplementary material.C. Electrical propertiesThe FNO gas treatment is expected to improve the Pt/GaNinterface by reducing the remaining GaOx interfacial bonds.Although the Pt layer is partly oxidized, the spectral changes forPt before and after treatment suggest that the Pt layer containingoxidized Pt retains its electrode function. Because the reductioneffect occurring at the GaN interface can enhance the Schottkycontact properties, we fabricated and evaluated Schottky junc-tions. Figure 5(a) shows I–V curves at the voltage from −5 to 5 Vfor the untreated and FNO-treated Pt/GaN samples. The increasein current with forward voltage is approximately three orders ofmagnitude higher for the FNO-treated sample than for theuntreated sample. Although the effect of the FNO gas treatmenton back contact should also be considered, it is also thought that thedecrease in the number of Ga–O bonds at the interface, as confirmedFIG. 4. (a) F 1s, (b) Ga 2p3/2, (c) O 1s, and Pt 4f core-level spectra forFNO-treated and untreated Pt-GaN. The solid lines are the experimental data,the dashed lines are the Voigt profiles, and the red dots are their sum.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068 137, 095304-5© Author(s) 2025 18 March 2025 08:16:54https://doi.org/10.60893/figshare.jap.c.7662113https://doi.org/10.60893/figshare.jap.c.7662113https://doi.org/10.60893/figshare.jap.c.7662113https://doi.org/10.60893/figshare.jap.c.7662113https://doi.org/10.60893/figshare.jap.c.7662113https://doi.org/10.60893/figshare.jap.c.7662113https://pubs.aip.org/aip/japby HAXPES analysis, is also having an effect. The ON–OFF ratioincreased because of the higher forward current.To investigate the Schottky barrier height in detail, weanalyzed the I–V characteristics using thermal electron emissiontheoretical Eqs. (1) and (2),40I ¼ IS expqVnkT� �� 1� �, (1)IS ¼ SA**T2exp � qΦBkT� �, (2)where S is the contact area, q is the electronic charge, n is the ideal-ity factor, k is the Boltzmann constant, T is the temperature, ΦB isthe Schottky barrier height, and A** is the effective Richardsonconstant. The theoretical value of A** = 32 A/(cm2 K2) was used inthe present study.41 The values of ΦB for the FNO-treated anduntreated samples were obtained as 1.06 and 0.97 eV, which are aver-aged values obtained from five measurement points on the sample,and their ideality factors were also estimated as 1.11 and 1.28, respec-tively. The change in barrier height may also be due to the effect ofFermi level pinning. Isobe et al. reported that the Fermi level pinningeffect is reduced by changes in the oxidation state of the interface,and that this improves the barrier height.42 The reduction in the oxi-dation layer at the interface is shown in the XPS results and is inagreement with the experimental results. With respect to the leakagecurrent in the reverse voltage direction up to −100 V, as shown inFig. 5(b), the FNO-treated samples showed an increase in leakagecurrent of a few digits in the range from −10 to −20 V but showedstable characteristics up to the high voltage range at several measure-ment points. On the other hand, in the untreated sample, there weremeasurement points where the leakage current characteristics werelower than those of the FNO-treated sample in the high voltagerange, but there was variation in the breakdown voltage. It is thoughtthat the uniformity of the interface is improved by the reductionreaction of the FNO gas treatment, but in the high voltage range, it isbelieved that the improvement of the interface characteristics by opti-mizing the processing time, etc., is necessary to suppress leakage.These results suggest that FNO gas treatment has the potential to bean effective post-processing method for improving the interface ofPt/GaN systems.IV. CONCLUSIONWe investigated the effects of FNO gas treatment of GaN andfound that FNO is effective in fluorinating its surface, giving rise toa prominent F peak and chemically shifted Ga peaks in the corre-sponding HAXPES spectra, indicating the formation of GaFx(x = 1, 2, and 3) species. The effects of the FNO treatment wereobserved not only in Bare-GaN samples but also in Pt/GaNsamples treated with FNO after Pt deposition. The Pt/GaN struc-ture showed enhanced redox reactions because of the catalyticeffect induced by Pt/GaO; this enhancement was not observed inthe case of Bare-GaN. The FNO treatment oxidized the Pt side butreduced the oxide layer on the GaN side, improving its electricalproperties. The findings of the present study suggest that FNOtreatment is an effective post-processing method for fluorinatingGaN-based systems after metal deposition. Moreover, comparedwith other fluorination methods such as exposure to CF4 plasma,the proposed method fluorinates the GaN or its metal interfacewithout subjecting the surface to high-energy incident ions andcan, therefore, be considered a viable low-damage process.SUPPLEMENTARY MATERIALFigure S1(a) shows the N 1s spectra for Bare-GaN. Figure S2shows F 1s, O 1s, and Ga 2p3/2 core-level spectra for Bare-GaN.Figure S3(a) shows N 1s spectra of the 5 nm-thick Pt-GaN samples.Figure S4 shows Pt 4p3/2, O 1s, and Pt 4f core-level spectra of the5 nm-thick Pt-GaN samples.ACKNOWLEDGMENTSWe are grateful to HiSOR, Hiroshima University, andJAEA/SPring-8 for the development of HAXPES at BL15XU ofSPring-8. The HAXPES measurements were performed underapproval of the NIMS Synchrotron X-ray Station (Proposal Nos.2016A4600, 2016B4601, 2016B4602, 2017A4604, and 2020A4602).This work was supported in part by JSPS KAKENHI Grant No.20H02188, “Advanced Research Infrastructure for Materials andNanotechnology in Japan (ARIM)” of the Ministry of Education,FIG. 5. I–V properties (a) from −5 to 5 V and (b) on the reverse bias side of 0to −100 V of FNO-treated and untreated Pt-GaN.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 137, 095304 (2025); doi: 10.1063/5.0224068 137, 095304-6© Author(s) 2025 18 March 2025 08:16:54https://pubs.aip.org/aip/japCulture, Sports, Science and Technology (MEXT), Proposal No.JPMXP1223NM5168.AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsTakahiro Nagata: Data curation (lead); Formal analysis (lead);Investigation (lead); Methodology (equal); Project administration(supporting); Writing – original draft (lead); Writing – review &editing (equal). Asahiko Matsuda: Data curation (equal); Formalanalysis (equal); Investigation (equal); Writing – review & editing(equal). Takashi Teramoto: Formal analysis (supporting); Fundingacquisition (lead); Investigation (supporting); Methodology (equal);Project administration (supporting); Supervision (equal). DominicGerlach: Conceptualization (equal); Formal analysis (equal);Investigation (supporting); Methodology (equal); Project adminis-tration (lead). Peng Shen: Conceptualization (equal); Formalanalysis (equal); Investigation (supporting); Methodology (equal);Project administration (lead). Shigenori Ueda: Formal analysis(equal); Investigation (equal); Methodology (equal); Writing –review & editing (supporting). Takako Kimura: Conceptualization(lead); Formal analysis (equal); Funding acquisition (lead);Investigation (supporting); Methodology (supporting). ChristianDussarrat: Conceptualization (lead); Funding acquisition (lead);Investigation (supporting); Methodology (supporting); Supervision(equal). Toyohiro Chikyow: Conceptualization (lead); Formalanalysis (supporting); Funding acquisition (lead); Investigation(supporting); Methodology (equal); Project administration(supporting); Supervision (equal).DATA AVAILABILITYThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.REFERENCES1S. J. Pearton, J. C. Zolper, R. J. Shul, and F. 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