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[Takayoshi Oshima](https://orcid.org/0000-0001-8550-9735), [Masataka Imura](https://orcid.org/0000-0002-4236-9549), [Yuichi Oshima](https://orcid.org/0000-0001-8293-4891)

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[Formation of GaN mesas with reverse-tapered edge structures on a lattice-matched AlInN layer for a positive beveled edge termination](https://mdr.nims.go.jp/datasets/30d56e77-05bd-41b2-9cf7-f2c18f504039)

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Formation of GaN mesas with reverse-tapered edge structures on a lattice-matched AlInN layer for a positive beveled edge terminationFormation of GaN mesas with reverse-tapered edge structures on a lattice-matched AlInN layer for a positive beveled edge terminationTakayoshi Oshima* , Masataka Imura , and Yuichi OshimaResearch Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan*E-mail: OSHIMA.Takayoshi@nims.go.jpReceived June 26, 2024; revised July 15, 2024; accepted July 17, 2024; published online July 31, 2024GaN mesas were fabricated by sequential dry and wet etching of a +c-oriented GaN layer onto a lattice-matched AlInN layer for future applicationsof positive beveled edge termination, which is desirable for preventing premature breakdown of power devices. The dry etching producedhexagonal AlInN/GaN mesas surrounded by m-plane sidewalls with six protrusions at the vertices. The subsequent hot phosphoric acid etchingselectively etched the AlInN layer to expose and etch the chemically unstable −c surface of the GaN layer, which formed reverse-tapered {1012¯ ¯ }facets. The protrusions were sacrificed during the wet etching to prevent undesirable positive tapering at the vertices.© 2024 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing LtdIn the field of power electronics, GaN is a crucial materialowing to its superior electrical properties such as a widebandgap, high electron mobility, and high breakdownvoltage,1) which empower GaN devices to operate at elevatedvoltages, frequencies, and temperatures compared to Sicounterparts. Thus, GaN power devices are well-suited fordiverse applications such as power converters, radiofre-quency amplifiers, and electric vehicle inverters.2–4)A major design consideration for GaN power devices isedge termination. Managing the electric field distribution atthe edges of a device is necessary to prevent prematurebreakdown and ensure reliable operation.5) Thus far, termina-tion structures such as field plates, field-limiting rings, andjunction termination extensions have been employed.6–9)However, sharp electric field gradients persist at the edgesof the dielectric plate and near the p–n junction, andthese termination structures require deposition, implantation,and patterning steps that increase manufacturing costs.Consequently, an edge termination that addresses thesedrawbacks is potentially necessary.A positive beveled edge termination is a viable solution tothe problems faced by current termination structures because itgradually decreases the electric field strength toward the edgeof the device, which prevents localized high electricfields.10,11) Furthermore, a beveled edge termination is cost-effective because it can be fabricated by etching only.Basically, there are two types of beveled edge terminations:positive and negative beveled edge terminations. In thepositive beveled edge termination, more material is removedfrom the edge when progressing from the heavily doped sideto the lightly doped side of the junction, or the junction area islinearly decreasing when going from the heavily-doped side tothe lightly-doped side. While, in the negative beveled edgetermination, more material is removed from the edge whenprogressing from the lightly doped side to the heavily dopedside of the junction, or the junction area is linearly decreasingwhen going from the lightly-doped side to the heavily-dopedside.10,11) Theoretically, the positive beveled edge terminationdemonstrates a superior ability to reduce electric field at theedge compared to the negative beveled edge termination.10)However, fabricating a positive beveled edge termination ischallenging because most vertical power devices, such asSchottky barrier diodes, PN junction diodes, have a voltage-sustaining junction near the surface, where a lightly dopedlayer is on the lower side.5) Hence, the implementation ofpositive beveled edge terminations for these devices requiresthe establishment of a reverse tapering process.In the case of GaN, a reverse-tapered structure can befabricated by utilizing its crystallographic nature. Stocker et al.achieved reverse tapering of c-oriented GaN on an AlN under-lying layer by combining dry etching and material-/facet-selective wet etching.12) The dry etching exposes the m-planesidewalls of the GaN and AlN layers, and the subsequent wetetching in H3PO4 or KOH selectively etches the sides of the AlNlayer while simultaneously undercutting the GaN layer to exposethe etching-sensitive −c side to create reverse-tapered sidewallsof etching-resistant 1012{ } or 1011{ } facets, respectively.Several studies have validated the efficacy of this techniquefor optical applications.13–15) However, the AlN layer compro-mises the performance of power devices because of its latticemismatch with GaN, which degrades the crystallinity of GaN. Inaddition, the low-doping capability of the AlN layer increases theseries resistance of the device. For practical application in powerdevices, AlN should be replaced by a semiconductor with aweaker wet-etching resistance than GaN, the same in-planelattice constant as GaN, and a higher doping capability than AlN.The above criteria can be satisfied by using an AlInN layerinstead of the AlN layer. AlInN achieves a lattice match withGaN at an In composition of ∼18%.16) Moreover, lattice-matched AlInN layers sandwiched with GaN layers can beetched selectively via electrochemical oxidation and/or hotnitric acid etching to create lateral air gaps between GaNlayers to realize diffuse Bragg reflectors.17–20) Furthermore,the carrier concentration of AlInN can be increased up to∼1019 cm−3 by Si and Ge doping.21,22) The specific contactresistance at the AlInN/GaN interface is as low as 1.5 ×10−7 Ω cm2,21) and the vertical direction resistivity of a300 nm thick AlInN layer is as low as 5.8 × 10−4 Ωcm2.22)These attributes make AlInN a desirable candidate as a side-etching layer material to initiate the formation of reversetapered edge of GaN for power devices applications.In this study, we used the above etching technique tofabricate hexagonal GaN mesas with reverse-tapered edges ona lattice-matched AlInN layer. Although some ingenuity wasContent from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution of thiswork must maintain attribution to the author(s) and the title of the work, journal citation and DOI.086501-1© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdApplied Physics Express 17, 086501 (2024) LETTERhttps://doi.org/10.35848/1882-0786/ad64bahttps://crossmark.crossref.org/dialog/?doi=10.35848/1882-0786/ad64ba&domain=pdf&date_stamp=2024-07-31https://orcid.org/0000-0001-8550-9735https://orcid.org/0000-0001-8550-9735https://orcid.org/0000-0002-4236-9549https://orcid.org/0000-0002-4236-9549https://orcid.org/0000-0001-8293-4891https://orcid.org/0000-0001-8293-4891mailto:OSHIMA.Takayoshi@nims.go.jphttps://creativecommons.org/licenses/by/4.0/https://doi.org/10.35848/1882-0786/ad64banecessary at the vertices, we successfully achieved reversetapering around the entire circumference of the hexagonal mesa.Figure 1 shows the steps of the entire process, whichcomprise GaN/AlInN/GaN epitaxy on a sapphire wafer[Fig. 1(a)], formation of a Ni mask for dry etching[Fig. 1(b)], dry etching using Cl2 and BCl3 plasma[Fig. 1(c)], removal of the Ni mask [Fig. 1(d)], and under-cutting by wet etching using hot H3PO4 acid [Figs. 1(e) and1(e’)].A GaN/AlInN/GaN stack was prepared on a 2-in c-planesapphire wafer by metalorganic vapor phase epitaxy usingstandard growth recipes [Fig. 1(a)]. The thicknesses of thebottom GaN, AlInN, and top GaN layers were 2.83, 0.19, and2.83 μm, respectively, which were measured by focused ionbeam milling (FIB) and scanning electron microscopy(SEM). The epitaxial structures of the GaN/AlInN/GaNlayers were investigated by X-ray diffraction using a mono-chromatic CuKα1 radiation. In the θ–2θ wide scan (notshown), only (0001) oriented peaks of GaN and AlInN layersappeared except for those of the substrate, and no otherorientation or phase peaks were detected. Laue fringe peaksof the AlInN layer were clearly observed [Fig. 2(a)], whichindicated its high crystallinity and abrupt AlInN/GaN and(a) (b)(c) (d)(e) (e’)Fig. 1. Schematic for the process flow of forming reverse-tapered GaNsidewalls: (a) GaN/AlInN/GaN epitaxy, (b) Ni mask formation, (c) Cl2/BCl3dry etching, (d) Ni mask removal, and (e) initial and (e’) final stages ofundercut wet etching in hot H3PO4 acid.(a)(b)Fig. 2. X-ray diffraction: (a) θ–2θ pattern of 0002 peaks and (b) reciprocalspace map in the vicinity of the 1015¯ spots of the GaN/AlInN/GaNmultilayers.(a)(b)(c)Fig. 3. Cross-sectional scanning electron microscopy images showingundercut etching of +c-oriented GaN on a lattice-matched AlInN layer. Theimages were taken (a) before etching and after (b) 20 min and (c) 55 min ofwet etching in H3PO4 acid at 135 °C. Sectioning was done at the middle ofone side of a hexagon using focused ion beam milling. The observed Ni andcarbon layers are conductive and protective layers for the milling. Theelectron beam incidence was tilted by 54° with respect to the surface normal.086501-2© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 17, 086501 (2024) T. Oshima et al.GaN/AlInN interfaces. Based on the fringe peak angles, thethickness of the AlInN layer was calculated as 195 nm. Thereciprocal space map in the vicinity of 1015¯ spots revealedthat the AlInN and GaN spots had the same QX values[Fig. 2(b)], which means that the AlInN layer was fullystrained to the GaN layers. Under the assumption of in-planelattice matching, the In composition of the AlInN layer wasestimated as 14.6% based on the main peak angle in the θ–2θpattern, which is as described in the literature.23)On the surface of the top GaN layer, a patterned Ni maskwith a thickness of 200 nm was prepared by standard laserlithography, electron beam evaporation, and liftoff processes[Fig. 1(b)]. The Ni mask was in the shape of a regular hexagonwith six protrusions at the vertices. All sides of the mask shapewere aligned with the a directions (i.e. [112̄0], [12̄10], and[2̄110]). Each side of the hexagon had a length of 50 μm whilethe protrusions had a length and width of 30 and 2 μm,respectively. The necessity of the protrusions will be discussedlater. Dry etching was performed with the patterned Ni maskto access the m-plane sidewalls of the top GaN and AlInNlayers [Fig. 1(c)]. The etching was conducted in Cl2 and BCl3plasma24) at gas flow rates of 45.0 and 5.0 sccm, respectively,and a chamber pressure of 0.5 Pa. The inductively coupledplasma and bias power inputs were 300 and 200W, respec-tively. Under these conditions, the typical etching rate for+c-GaN was 0.27 μmmin−1. The etching reached the top partof the bottom GaN layer, which exposed the m-plane surfacesof the top GaN and AlInN layers. The Ni mask was removed(a) (a’)(b) (b’)(c) (c’)Fig. 4. Bird’s-eye view scanning electron microscopy images showing the formation of hexagonal GaN mesas with reverse-tapered sidewalls. The imageswere taken (a) before etching and after (b) 20 min and (c) 55 min of wet etching in H3PO4 acid at 135 °C. (a’), (b’), and (c’) are images corresponding to thevertices in (a), (b), and (c), respectively.086501-3© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 17, 086501 (2024) T. Oshima et al.by immersing the sample in nitric acid at room temperature[Fig. 1(d)]. The dry-etched structures remained intact in thisprocess as evidenced by tilted-view cross-sectional and surfaceSEM images [Figs. 3(a), 4(a) and 4(a’)].Finally, undercutting was conducted to create reverse-tapered GaN structures [Figs. 1(d) and 1(e)] by wet etching inhot phosphoric acid, which was stirred and heated at 135 °Cin a glass beaker using a hot plate stirrer. The processtemperature was monitored and controlled by using a Teflon-coated thermocouple immersed in the acid. Etching times of20 and 55 min were used for different samples.The etching predominantly progressed at the AlInN/GaNinterface. Figures 3(b) and 3(c) show cross-sectional SEMimages after wet etching for 20 and 55min, respectively.When the top part of the AlInN was side-etched, the exposedback side of the top GaN layer was immediately etched to formreverse-tapered facets. The second GaN etching process wasmuch faster than that of the AlInN layer. Accordingly, the facetstructure was maintained for the etched GaN surface throughoutthe etching process, and the lateral position of the facet wasconstrained to the inner end for the undercut etching. The faceangle between the top surface and reverse-tapered facet was∼48°, which is closer to the calculated value of 43.2° for the1012{ } facet than 62.0° for the 1011{ } facet using the latticeparameters of GaN.25) Therefore, the observed facet was likely tobe 1012 .{ } The slight angle difference of ∼5° can be attributedto deviations in the holder normal direction and Ga beam andelectron beam directions of the used FIB-SEM system. Themechanism of the undercut etching was almost the same as thatof the etching process at the AlN/GaN interface.12) However,most of the AlInN layer remained at the inner end with undercutetching because it has a higher chemical stability than AlN.Thus, the +c plane of AlInN exhibited a higher etchingresistance than the −c plane of GaN, and a very thin AlInNlayer was sufficient for this etching technique, which would helpdecrease the series resistance of power devices.Reverse-tapered edges were obtained along the entirecircumference of the hexagonal mesa. Figures 4(b) and4(b’) and Figs. 4(c) and 4(c’) show bird’s-eye view SEMimages of the GaN mesas after wet etching for 20 and 55 min,respectively. Undercut etching was clearly observed at thetop-GaN/AlInN interface on the sidewalls. The protrusions atthe vertices of the hexagons were also subjected to etchingand were removed after 55 min to leave behind a GaNhexagonal mesa without protrusions. Some inverted pyra-midal structures resembling stalactites appeared on thereverse-tapered facets, which have also been observed onwet-etched −c-face GaN surfaces.26–28) A plausible reasonfor their formation is dislocations acting as preferential sitesfor the pyramidal etching process.28) These undesirableinverse pyramids should be decreased by using low-disloca-tion-density GaN substrates grown in the liquid phase.29)The protrusions at the vertices were intentionally includedin the patterned Ni mask to maintain the reverse-taperedstructure at the vertices. Figure 5 shows SEM images of avertex without a protrusion before [Fig. 5(a)] and after 20 and55 min of wet etching [Figs. 5(b) and 5(c), respectively].Because the sidewall of the vertex included an etching-vulnerable a-plane facet, the vertex was removed during theetching to form a positively tapered structure. Thus, asacrificial structure (e.g. protrusion) directed in the a direc-tion was necessary to avoid positive tapering of the vertex.In conclusion, we demonstrated a state-of-the-art processfor reverse tapering GaN on a lattice-matched AlInN layer.We successfully fabricated hexagonal GaN mesas with areverse taper along their entire circumference. This structurecan be applied to realizing positive beveled edge terminationsfor GaN power devices, which has not yet been demonstrateddespite its effectiveness. We believe this fabrication processwill greatly contribute to the development of high-perfor-mance GaN power devices.Acknowledgments This work was supported by the TIA Kakehashi(TK23-001), and the “Advanced Research Infrastructure for Materials andNanotechnology in Japan (ARIM)” initiative of the Ministry of Education,Culture, Sports, Science and Technology (MEXT), (JPMXP1224NM5062).ORCID iDs Takayoshi Oshima https://orcid.org/0000-0001-8550-9735 Masataka Imura https://orcid.org/0000-0002-4236-9549 Yuichi Oshima https://orcid.org/0000-0001-8293-48911) T. J. Flack, B. N. Pushpakaran, and S. B. Bayne, J. Electron. Mater. 45, 2673(2016).2) B. N. Pushpakaran, A. S. Subburaj, and S. B. Bayne, J. Electron. Mater. 49,6247 (2020).3) J. He, W. Cheng, Q. Wang, K. Cheng, H. Yu, and Y. Chai, Adv. Electron.Mater. 7, 2001045 (2021).4) R. T. Yadlapalli, A. Kotapati, R. Kandipati, S. R. Balusu, and C. S. Koritala,Int. J. Energy Res. 45, 12638 (2021).(a)(b)(c)Fig. 5. Bird’s-eye view scanning electron microscopy images showingerosion at vertices of hexagonal GaN mesas fabricated without protrusions.The images were taken (a) before etching and after (b) 20 min and (c) 55 minof wet etching in H3PO4 acid at 135 °C.086501-4© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 17, 086501 (2024) T. 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