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[Fumio Kawamura](https://orcid.org/0000-0003-0724-1475), Takeyoshi Onuma, [Kazutaka Mitsuishi](https://orcid.org/0000-0002-9361-4057)

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[An Advanced Epitaxial Strategy Enabling Vertical GaN Devices on Silicon Wafers](https://mdr.nims.go.jp/datasets/adf5343d-378d-44fe-8e43-b30904f876dc)

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Advanced Physics Research www.advphysicsres.comRESEARCH ARTICLEAn Advanced Epitaxial Strategy Enabling Vertical GaN Devices on Silicon Wafers Fumio Kawamura1 Takeyoshi Onuma2 Kazutaka Mitsuishi3 1 Research Center for Electronic and Optical Materials, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan 2 Department of Applied Physics, School of Advanced Engineering and Department of Electrical Engineering and Electronics, Graduate School of Engineering, Kogakuin University, Hachioji, Tokyo, Japan 3 Center for Basic Research on Materials, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan Correspondence: Fumio Kawamura ( kawamura.fumio@nims.go.jp) Kazutaka Mitsuishi ( mitsuishi.kazutaka@nims.go.jp) Received: 25 February 2026 Revised: 22 March 2026 Accepted: 2 May 2026 Keywords: buffer layers | gallium nitride | heteroepitaxy | silicon wafers | vertical power devices ABSTRACT While vertical gallium nitride (GaN)-on-silicon architectures promise a transformative leap in cost-effective power electronics and high-resolution micro-LEDs, their deployment remains bottlenecked by the high electrical resistance of conventional epitaxial buffer layers. Here, a universal and straightforward sputtering-based strategy is presented to realize high-quality GaN epitaxial films on Si(111) substrates characterized by exceptionally low vertical resistance, ohmic behavior, and robust thermal stability. This technique centers on the in situ formation of a sub-nanometer ( ∼ 0.5 nm) silicide-based template via rapid thermal annealing—a method demonstrating unprecedented versatility across 25 different metallic species. Scanning transmission electron microscopy (STEM) reveals that a unique amorphous-like interlayer (AL-IL) effectively accommodates lattice mismatch and relaxes epitaxial strain. These AL-IL templates further serve as high-performance platforms for metalorganic chemical vapor deposition (MOCVD) overgrowth, successfully bridging the gap between scalable, low-cost fabrication and device-grade vertical performance. 1T  a  g  d  rI  o  t  b  b  d  m  m                  To©Ah Introduction he development of vertical Gallium Nitride (GaN) devices isnticipated to dramatically enhance the performance of next-eneration electronics, from power transistors to light-emittingiodes (LEDs), and is currently a subject of intensive globalesearch [ 1–10 ]. n power electronics, vertical GaN transistors are poised toffer superior efficiency and higher power-handling capabili-ies, surpassing even the widely adopted Silicon Carbide (SiC)ased counterparts. Similarly, in the burgeoning field of GaN-ased micro-LEDs for high-resolution, low-electric consumptionisplays, a vertical architecture enabling backside electrode for-ation is projected to slash fabrication costs, facilitate furtheriniaturization, and improve brightness uniformity. However,his is an open access article under the terms of the Creative Commons Attribution Licenriginal work is properly cited. 2026 The Author(s). Advanced Physics Research published by Wiley-VCH GmbH dvanced Physics Research , 2026; 5:e70143 ttps://doi.org/10.1002/apxr.70143a critical barrier to the widespread adoption of these verticaldevices is the prohibitive cost of native GaN single-crystalsubstrates. While growth methods such as hydride vapor phaseepitaxy (HVPE) [ 11–15 ], the ammonothermal method [ 11, 16–20 ],and the Na-flux method [ 21–28 ] can produce such substrates,their high expense remains a fundamental obstacle. Moreover,the intrinsic cost of the raw material, gallium, places a fun-damental limit on cost reduction. Conversely, realizing verticalGaN devices on silicon wafers offers a compelling path towarddrastic cost reduction. Recent advances, such as the insertion ofan AlGaN/GaN superlattice, have enabled the commercializationof cost-effective GaN-on-Si lateral transistors [ 29–38 ]. The highelectrical resistance of this superlattice buffer, however, precludesvertical current flow, confining device architectures to lateralconfigurations [ 39 ]. Therefore, the development of a buffer layerthat facilitates the epitaxial growth of high-quality GaN onse, which permits use, distribution and reproduction in any medium, provided the 1 of 9http://www.advphysicsres.comhttps://doi.org/10.1002/apxr.70143https://orcid.org/0000-0002-9361-4057mailto:kawamura.fumio@nims.go.jpmailto:mitsuishi.kazutaka@nims.go.jphttp://creativecommons.org/licenses/by/4.0/https://doi.org/10.1002/apxr.70143http://crossmark.crossref.org/dialog/?doi=10.1002%2Fapxr.70143&domain=pdf&date_stamp=2026-05-29FIGURE 1 Crystallographic quality of sputtered GaN films as a function of the pre-sputtered element. (a) The data are ordered from best to worst epitaxial alignment. The identity of the pre-sputtered element for each film is noted in the upper left. In cases where six-fold symmetric peaks are observed, the full width at half-maximum (FWHM) of the strongest peak is denoted in each profile, quantifying the degree of in-plane orientation. As an exception to the case of Ga deposition, a buffer layer was formed by rapidly heating the GaN film grown on Si(111) up to 1200 ◦C to thermally decompose the GaN and induce a reaction between the resulting Ga and the Si substrate. (b) Based on the results in Figure 1a , samples exhibiting sixfold symmetric GaN diffraction peaks were classified as ’effective’, while others were categorized as ’ineffective’. s  c  G22W  e  o          2 27511200, 2026, 7, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/apxr.70143 by National Institute For, Wiley Online Library on [15/09/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creailicon while maintaining low vertical resistance represents theritical breakthrough needed to unlock the potential of verticalaN-on-Si devices [ 40–45 ].  Results and Discussion .1 Versatile Ultrathin Buffer Strategy e have developed a novel, low-resistance buffer layer thatnables the heteroepitaxial growth of high-crystallinity GaN filmsn silicon substrates. of 9tiFigure 1a demonstrates the pivotal role of an ultrathin bufferlayer in enabling the epitaxial growth of sputtered GaN films onSi(111) substrates. Note that all experiments were conducted on3◦-offcut Si(111) substrates. The epitaxial quality of GaN grownusing such ultrathin buffer layers is sensitive to factors includingsubstrate off-cut angle, annealing temperature, and impuritylevels (Figures S1–S4 ). To identify an optimal buffer material, we conducted a systematicscreening of 43 different elements. Each element was depositedas a 0.5 nm-thick interlayer, annealed at 725◦C, and subsequentlycapped with a 400 nm-thick GaN film. The success of GaNAdvanced Physics Research, 2026ve Commons LicenseFIGURE 1 (Continued) Advanced Physics Research, 2026 3 of 9 27511200, 2026, 7, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/apxr.70143 by National Institute For, Wiley Online Library on [15/09/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseFIGURE 2 Effect of annealing temperature on GaN epitaxy. XRD φ- scans of GaN films grown by sputter deposition on Si(111) substrates. A 0.5- nm Co layer was deposited on the substrate and subsequently annealed at the indicated temperatures to form a template prior to GaN growth. e  i  i  r  t  p  w  m  e  a  o  u  o  r  i  o  s  o  c  e2IT  e  aF  t  p  s  m                                                    4 27511200, 2026, 7, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/apxr.70143 by National Institute For, Wiley Online Library on [15/09/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creatipitaxy, defined by the observation of 6 fold rotational symmetryn its diffraction pattern (Figure 1b ), reveals a distinct trend. Thenitial growth mode of GaN persists even as the film thicknesseaches 1 µm, indicating a stable epitaxial process throughouthe deposition (Figure S5 ). We find that elements ineffective inromoting epitaxy fall into two categories: 1) refractory metalsith melting points above 1900◦C, and 2) highly oxophilic ele-ents. Conversely, all other tested elements successfully enabledpitaxial growth. This indicates that a 0.5 nm-thick film of nearlyny element with a melting point below 1900◦C and moderatexidation resistance can form a viable template for GaN epitaxypon high-temperature annealing. We propose that the failuresf the epitaxial growth are due to two distinct mechanisms: theefractory metals do not react with silicon to form a silicidenterlayer, while the highly oxophilic elements are prematurelyxidized by residual gases in the chamber, which preventsilicidation. These results strongly suggest that the formationf an ultrathin silicide interlayer—irrespective of its specificomposition—is the key factor enabling the subsequent GaNpitaxy. .2 Mechanism of Epitaxy via Amorphous-Like nterlayer o validate our hypothesis that the silicide interlayer is the keynabler, we investigated the effect of annealing temperature using 0.5 nm cobalt film. igure 2 presents the XRD φ-scan results, which clearly revealhat GaN epitaxy is critically dependent on the annealing tem-erature. For samples annealed at or below 600◦C, no 6 foldymmetry was observed. This demonstrates that the as-depositedetal film itself is inert; rather, it is the cobalt silicide layer [ 46–of 950 ], formed during high-temperature annealing, that facilitatesthe epitaxial growth of GaN. We investigated the GaN/buffer/Si(111) interfacial structure usingscanning transmission electron microscopy (STEM). Figure 3presents cross-sectional STEM images for representative tem-plates formed with Co, Au, and Zr, along with their correspondingenergy-dispersive X-ray spectroscopy (EDS) elemental maps.Intriguingly, a common feature is observed across all samples:the formation of a distinct amorphous-like interlayer (AL-IL)directly beneath the GaN film. However, the composition ofthis AL-IL varies with the initial metal used: for Co and Autemplates, it is primarily composed of Si and N, whereas forthe Zr template, it consists of Si, Zr, and N. Beneath thisAL-IL, the underlying structure reflects the specific metal-Siinteraction; the Co-template exhibits a distinct crystalline CoSi2 phase, whereas the Au-template shows a non-stoichiometric Au-Si layer, consistent with the fact that Au reacts with Si withoutforming stable stoichiometric silicide phases [ 51 ]. Similarly, forthe Zr template, although Zr itself forms silicides or nitridesdepending on phase stability [ 52–54 ], the incorporation of Siand N into the Zr-containing layer promotes amorphizationby interrupting the crystalline lattice formation. This resultsin a compositionally and functionally similar amorphous-likestructure to those formed with Co and Au. Despite these variedinitial interfacial reactions, the consistent formation of a Si-and N-rich AL-IL at the immediate GaN interface emerges as aunifying feature. XPS analysis provides definitive proof of the formation mecha-nism. As shown in Figure 4 , exposing a 0.5 nm Co/Si(111) sampleto a nitrogen plasma for just 10 s at 725◦C induces a clear shiftin the Si 2s peak, indicative of Si ─N bond formation [ 55 ], whilethe Co 2p peak remains completely unaffected [ 56 ]. This confirmsthat the formation of the Si–N-rich interlayer is an extremelyrapid process that occurs via direct nitridation of the silicon (orsilicide surface) as soon as GaN growth begins [ 57 ]. Specifically,it is inferred that the mechanism behind the formation of theamorphous-like interlayer (AL-IL) involves the simultaneousnitridation and amorphization of the topmost ultrathin silicidelayer upon exposure to nitrogen plasma at the very early stagesof GaN growth. In this process, the resulting surface becomesthermodynamically stable as either a Si–N phase (in the cases ofCo and Au) or a Metal–Si–N phase (in the case of Zr). Figure 5a–c presents the XRD φ-scan results for a sputteredGaN film on Si(111) formed with an AL-IL, and for a 2- µm-thick GaN film subsequently overgrown on this template byMOCVD. The data reveal a significant improvement in the in-plane crystalline alignment after MOCVD overgrowth, with a fullwidth at half maximum (FWHM) of 0.47◦. Photoluminescencespectroscopy (Figure 5c ) confirms the film’s good crystallinequality, as evidenced by a strong near-band-edge (NBE) emission[ 58–63 ]. Figure 6 presents the vertical current–voltage ( I-V ) characteristicsof epitaxially grown GaN films on Si(111) by sputtering. Notably,the sample prepared with an ultrathin Co layer exhibits excellentohmic behaviour. Furthermore, the resistance under verticalbias progressively decreases as the sample is annealed at highertemperatures. The achievement of ohmic contact through ourAdvanced Physics Research, 2026ve Commons LicenseFIGURE 3 STEM image and EDS mapping analysis of the epitaxial GaN/buffer/Si(111) interface. We analyzed the interfaces of sputtered GaN films. The underlying templates were prepared by depositing (a) 7 nm Co, (b) 0.5 nm Au, and (c) 0.5 nm Zr, which were then annealed in vacuum at 725◦C for 10 min prior to GaN growth. A  m  a  r  f  t  l  m  o  t  i  a  H  o  o  d  d  s               A 27511200, 2026, 7, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/apxr.70143 by National Institute For, Wiley Online Library on [15/09/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable CreaL-IL technique, coupled with the reduction in resistance whileaintaining ohmic characteristics even after high-temperaturennealing, represents a highly promising step toward theealization of vertical GaN-on-Si power devices. Moreover, theact that a low-resistance, ohmic interface is formed despitehe AL-IL being an intrinsically high-resistivity Si-N-basedayer suggests that tunneling current is the dominant transportechanism. We attribute this behavior to the in situ Si dopingf the GaN layer; specifically, the Si-rich AL-IL in contact withhe GaN likely promotes the diffusion of Si atoms into the GaNnterface, significantly increasing the local carrier concentrationnd narrowing the depletion region to facilitate tunneling.owever, to further validate this mechanism and rule outther potential conduction paths, such as nanoscale pinholesr compositional inhomogeneities within the interlayer, moreetailed electrical characterization—including temperature-ependent I–V measurements—will be conducted in futuretudies. dvanced Physics Research, 20263 Conclusion Here we establish a novel technique, which we term theamorphous-like interlayer (AL-IL) approach, for the direct epitax-ial growth of GaN on Si(111) substrates by sputtering. Remarkably,GaN films grown using this method are not only epitaxial butalso demonstrate exceptionally low vertical resistance and clearohmic behaviour. Our findings reveal that the AL-IL formationis driven by a unique process, wherein the pre-deposition of anultrathin metal layer onto the Si(111) surface triggers the in situformation of a Si-N-rich amorphous-like layer at the onset of high-temperature GaN growth. Strikingly, this effect is not limited to aspecific metal; we discovered that the formation of the AL-IL andthe resulting GaN epitaxy can be achieved with the pre-depositionof any elemental metal having a melting point below 1900◦C.These findings firmly establish the AL-IL approach as a powerfuland versatile new platform, poised to accelerate the realization ofvertical GaN-on-Si devices. 5 of 9tive Commons LicenseFIGURE 4 XPS spectra obtained after annealing and nitrogen plasma irradiation of an ultrathin Co film deposited on Si(111). A 0.5 nm-thick Co layer was deposited on a 3◦ off-axis Si(111) wafer, followed by nitrogen plasma irradiation during annealing at 725 ◦C under vacuum. The method of nitrogen plasma irradiation is described in Ref. [ 57 ]. FIGURE 5 MOCVD overgrowth of GaN on a sputtered GaN-on-Si(111) template. XRD φ-scans and corresponding schematic architectures for (a) a sputtered GaN-on-Si(111) template and (b) an MOCVD-overgrown GaN film. The template was formed via a 0.5-nm Co layer, which creates an amorphous-like interlayer (AL-IL). (c) Room-temperature photoluminescence (PL) spectrum for the sample shown in (b). 44G  r  m  a  p      6 27511200, 2026, 7, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/apxr.70143 by National Institute For, Wiley Online Library on [15/09/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable C Experimental Section .1 Sputter-Deposition of GaN Films aN films were deposited to a thickness of 400 nm on ultra-lowesistivity n-type Si(111) wafers with a 3◦ off-cut angle using an RFagnetron sputtering system. A 1-inch GaN target was sputteredt an RF power of 20 W in a mixed Ar/N2 (1:1 flow ratio) at a totalressure of 0.8 Pa. of 94.2 Deposition of Ultrathin Metal Layers and AL-IL Formation Prior to GaN deposition, ultrathin metal layers were depositedusing their respective metal targets. The deposition rate for eachmetal was pre-calibrated to achieve a nominal thickness of 0.5nm. The AL-IL was formed in situ through a specific thermalsequence. The substrate with the metal layer was rapidly heatedto the target temperature within 2 min and held for 10 min.Advanced Physics Research, 2026reative Commons LicenseFIGURE 6 Vertical current–voltage characteristics of GaN on Si(111) grown with an amorphous-like interlayer. The plot shows vertical I–V curves as a function of post-growth annealing temperature. The GaN film was grown at 675◦C on a Si(111) substrate, which was pre-deposited with and without a Co layer. G  I  w4F  b  T  t  (4T  t  C  J  u  uAF  X  a  cAK  sFT  I  w  P  T  o  M                        A 27511200, 2026, 7, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/apxr.70143 by National Institute For, Wiley Online Library on [15/09/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable CreatiaN deposition was subsequently initiated, during which the AL-L formed automatically. Nitrogen atoms constituting the AL-ILere supplied only at the onset of GaN deposition. .3 MOCVD Growth or MOCVD growth, a 60 nm-thick low-temperature (LT) GaNuffer layer was first deposited on the sputtered GaN template.he temperature was then increased to 1170◦C to grow a 2- µm-hick GaN layer using trimethylgallium (TMGa) and ammoniaNH3 ). .4 Characterization he in-plane alignment was evaluated by XRD φ-scans usinghe asymmetric (10-11) reflection (PANalytical X’Pert Pro MRD).ross-sectional STEM and EDS were performed using a JEOLEM-ARM200F operated at 200 kV. XPS analysis was performedsing a ULVAC-PHI Quantera-SXM system. PL measurementssed a He-Cd laser (325 nm). uthor Contributions .K. conceived the project, synthesized the thin films, and performedPS, XRD, and electrical characterization. K.M. conducted STEM analysisnd the associated EDS elemental analysis. T.O. performed photolumines-ence measurements. cknowledgments .M would like to thank Ms. M. Taketomi and Ms. Y. Nakayama for theirupport with TEM sample preparation. unding his study was supported by the Innovative Science and Technologynitiative for Security (Grant Number JPJ004596), ATLA, Japan. Thisork was supported by the Adaptable and Seamless Technology Transferrogram through Target-driven R&D (A-STEP) from Japan Science andechnology Agency (JST), Japan, Grant Number JPMJTR25T5. A partf this work was supported by “Advanced Research Infrastructure foraterials and Nanotechnology in Japan (ARIM)” of the Ministry ofdvanced Physics Research, 2026Education, Culture, Sports, Science and Technology (MEXT). ProposalNumber JPMXP1225NM5364. Conflicts of Interest The authors declare no conflicts of interest. Data Availability Statement The data that support the findings of this study are available from thecorresponding author upon reasonable request. References 1 . H. Amano, Y. Baines, E. 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J.hields, “Radiative Recombination Mechanisms in Polar and Non-polarnGaN/GaN Quantum Well LED Structures,” Applied Physics Letters 1092016): 151110. upporting Information dditional supporting information can be found online in the Supportingnformation section. upporting File: apxr70143-sup-0001-SuppMat.docx. dvanced Physics Research, 2026 9 of 9 articles are governed by the applicable Creative Commons License An Advanced Epitaxial Strategy Enabling Vertical GaN Devices on Silicon Wafers 1 | Introduction 2 | Results and Discussion 2.1 | Versatile Ultrathin Buffer Strategy 2.2 | Mechanism of Epitaxy via Amorphous-Like Interlayer 3 | Conclusion 4 | Experimental Section 4.1 | Sputter-Deposition of GaN Films 4.2 | Deposition of Ultrathin Metal Layers and AL-IL Formation 4.3 | MOCVD Growth 4.4 | Characterization Author Contributions Acknowledgments Funding Conflicts of Interest Data Availability Statement References Supporting Information