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

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[Plasma-free dry etching of (001) β-Ga2O3 substrates by HCl gas](https://mdr.nims.go.jp/datasets/e8a4248c-403b-41c2-9589-01f887f9b826)

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Plasma-free dry etching of (001) β-Ga2O3 substratesby HCl gasViewOnlineExportCitationRESEARCH ARTICLE |  APRIL 17 2023Plasma-free dry etching of (001) β-Ga2O3 substrates by HClgas Takayoshi Oshima   ; Yuichi Oshima Appl. Phys. Lett. 122, 162102 (2023)https://doi.org/10.1063/5.0138736 28 May 2024 09:29:40https://pubs.aip.org/aip/apl/article/122/16/162102/2882852/Plasma-free-dry-etching-of-001-Ga2O3-substrates-byhttps://pubs.aip.org/aip/apl/article/122/16/162102/2882852/Plasma-free-dry-etching-of-001-Ga2O3-substrates-by?pdfCoverIconEvent=citejavascript:;https://orcid.org/0000-0001-8550-9735javascript:;https://orcid.org/0000-0001-8293-4891https://crossmark.crossref.org/dialog/?doi=10.1063/5.0138736&domain=pdf&date_stamp=2023-04-17https://doi.org/10.1063/5.0138736https://servedbyadbutler.com/redirect.spark?MID=176720&plid=2382966&setID=592934&channelID=0&CID=876065&banID=521854339&PID=0&textadID=0&tc=1&scheduleID=2301543&adSize=1640x440&data_keys=%7B%22%22%3A%22%22%7D&matches=%5B%22inurl%3A%5C%2Fapl%22%5D&mt=1716888580138328&spr=1&referrer=http%3A%2F%2Fpubs.aip.org%2Faip%2Fapl%2Farticle-pdf%2Fdoi%2F10.1063%2F5.0138736%2F16821408%2F162102_1_5.0138736.pdf&hc=275e48669d6e14174d71181886824deb45dc17a1&location=Plasma-free dry etching of (001) b-Ga2O3substrates by HCl gasCite as: Appl. Phys. Lett. 122, 162102 (2023); doi: 10.1063/5.0138736Submitted: 14 December 2022 . Accepted: 5 April 2023 .Published Online: 17 April 2023Takayoshi Oshimaa) and Yuichi OshimaAFFILIATIONSResearch Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japana)Author to whom correspondence should be addressed: OSHIMA.Takayoshi@nims.go.jpABSTRACTIn this study, we dry etched SiO2-masked (001) b-Ga2O3 substrates in HCl gas flow at a high temperature without plasma excitation. Theetching was done selectively in window areas to form holes or trenches with inner sidewalls of (100) and/or {310} facets, which are thesmallest surface-energy-density plane and oxygen-close-packed slip planes, respectively. In particular, (100) faceted sidewalls were flat andrelatively close to the substrate surface normal. Therefore, this simple dry etching method is promising for fabricating plasma-damage-freetrenches and fins used for b-Ga2O3-based power devices.Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0138736Monoclinic-structured b-Ga2O3 is a promising ultra-widebandgap semiconductor for power electronics.1 b-Ga2O3 has a highestimated critical electric field of �8 MV cm�1 (Refs. 2 and 3) and adecent electron mobility of �200 cm2 V�1 s�1,4–8 resulting in a highBaliga’s figure of merit that outperforms that of conventional powersemiconductors, such as SiC and GaN. Furthermore, the fabricationcost of b-Ga2O3 single crystals via melt growth9–12 is lower than thatof SiC and GaN bulk crystals grown from vapor phases. Because ofthese two advantages over the competing semiconductors, b-Ga2O3has emerged as a next-generation power semiconductor, therebyattracting attention among semiconductor research communities.Although the unipolarity of b-Ga2O3 can be an obstacle for deviceapplications, promising device prototypes, such as Schottky barrierdiodes (SBDs),13–17 metal-oxide-semiconductor field-effect transistors(MOSFETs),18–22 and modulation-doped field-effect transistors(MODFETs),23–27 have been demonstrated. Particularly, recently dem-onstrated fin FETs (FinFETs)28–33 and trench MOS-type SBDs(MOSSBDs)34–37 have exhibited excellent device performances, inwhich the fins and trenches aid in achieving normally off opera-tion28,29,31–33 and high reverse breakdown voltages,34,36,37 respectively,by regulating the current flow in the confined regions without usingp–n junctions.Plasma-based anisotropic dry etching has been widely used tocreate such sophisticated device structures. Almost all b-Ga2O3devices with fins/trenches reported to date have been fabricatedusing reactive ion etching (RIE) with chlorine-based chemistry,which is now virtually the de facto standard etching technique inthe b-Ga2O3 community.38 However, the RIE process causesplasma damage on the processed surfaces, resulting in interfacetraps that degrade device performances due to limited effectivechannel mobility31 and a large hysteresis loop28 in FinFETs andincreased on-resistance in MOSSBDs.35 To restore device perform-ances, the plasma damage should be removed through wet treat-ments in acid or alkaline solutions,39–42 annealing,41,43 or self-reaction etching with gallium flux.42However, plasma-free anisotropic etching approaches have beenexplored to produce plasma-damage-free high-aspect-ratio structures.So far, various etching techniques, such as hot phosphoric acid etch-ing,44,45 metal-assisted chemical etching (MacEtch),46,47 atomic gal-lium flux etching in an ultra-high vacuum environment,48 andhydrogen environment anisotropic thermal etching (HEATE),49 havebeen reported, where the strong anisotropic nature of the b-Ga2O3crystal structure is more or less reflected in the etched structures. Interms of MacEtch, damage-free multiple fin channels were produced,and nearly zero-hysteresis operation of the FinFET was demon-strated.50 However, the practical application of MacEtch is difficultbecause of its complicated etching system, in which etching proceedsvery slowly (�100nm/h) in a hydrofluoric acid solution under deep-UV illumination using a patterned Pt layer as a catalytic mask.46Additionally, the sidewall profiles of the MacEtch-formed fins on(010) substrates are positively tapered even when the fins are designedalong vertical cleavage planes such as (100), indicating its weak anisot-ropy.47 In contrast, HEATE is the most promising method among allnon-plasma-based etching methods in terms of simplicity andAppl. Phys. Lett. 122, 162102 (2023); doi: 10.1063/5.0138736 122, 162102-1Published under an exclusive license by AIP PublishingApplied Physics Letters ARTICLE scitation.org/journal/apl 28 May 2024 09:29:40https://doi.org/10.1063/5.0138736https://doi.org/10.1063/5.0138736https://www.scitation.org/action/showCitFormats?type=show&doi=10.1063/5.0138736http://crossmark.crossref.org/dialog/?doi=10.1063/5.0138736&domain=pdf&date_stamp=2023-04-17https://orcid.org/0000-0001-8550-9735https://orcid.org/0000-0001-8293-4891mailto:OSHIMA.Takayoshi@nims.go.jphttps://doi.org/10.1063/5.0138736https://scitation.org/journal/aplresulting etched profiles. HEATE is based on hydrogen-assisted ther-mal decomposition51 and has recently been reported by Kikuchi et al.for SiO2-masked (010) b-Ga2O3 substrates.49 They investigated the in-plane anisotropic etching behavior to find that (100) and {10�1} facetshad the slowest and second-slowest lateral etching rates, respectively.Moreover, they succeeded in fabricating very high-aspect-ratio finswith (100)-faceted perfectly flat and vertical sidewalls without plasmadamage that can be applied to distributed Bragg reflectors and nano-fluidic channels. However, HEATE or the equivalent plasma-free dryetching method has not been applied to (001)-oriented b-Ga2O3 sub-strates, although most b-Ga2O3-based vertical power devices, includ-ing FinFETs and MOSSBDs, have now been fabricated onthem.15–17,22,31–37 Therefore, it is essential to investigate a plasma-freedry etching on (001) substrates to fabricate fins/trenches for verticallystructured b-Ga2O3-based power devices.In our previous study, we demonstrated selective area growthusing HCl-based halide vapor phase epitaxy (HVPE) on (010) and(001) b-Ga2O3 substrates to fabricate plasma-damage-free fins/trenches as a bottom-up growth method.52 In this growth system, theintroduction of HCl etching gas in addition to the growth precursorswas required for the suppression of parasitic gas-phase reaction andundesirable nucleation on the mask to achieve perfect selectivity.Furthermore, the excessive HCl gas supply led to selective area etchingof the substrate in the window areas, which could be used as a plasma-free dry etching technique.In this study, we investigated selective area HCl gas etching of(001) b-Ga2O3 substrates. Scanning electron microscopy (SEM) of theetched depressions revealed that the structures were dominated by(100) and {310} facets. In particular, (100) facets were smooth and freeof plasma damage, although they were slightly inclined from the sub-strate normal. Therefore, gas-etched fins/trenches with faceted side-walls can be applied to sophisticated power devices, including FinFETsand MOSSBDs.We performed the HCl gas etching of b-Ga2O3 as follows. Acircular-, radial-line-, stripe-, and square (100� 100 lm2)-patternedSiO2 masks (0.1-lm thickness) were prepared on (001) b-Ga2O3 sub-strates. The masks were fabricated via conventional photolithography.The details of the process are found in our previous study.52 The gas-phase etching was performed using a laboratory-made HCl-basedHVPE system under atmospheric pressure. This system can directlysupply a gas mixture of HCl (>99.999% pure) and N2 (dew point <�110 �C) to the substrate.53 In this study, an HCl/N2 gas mixture withan HCl partial pressure of 63Pa was supplied to the heated SiO2-masked b-Ga2O3 substrate. Here, the substrate was vertically held at thecenter of the rotating holder in a horizontal quartz tube reactor, withthe substrate surface perpendicular to the horizontal gas flow direction.Because the etching rate may strongly depend on the temperature, asverified in the H2 etching experiments,51 we first examined etching ratesof the (001) surface at the different reactor temperatures of 521, 750,863, and 1038 �C by measuring the depths in the square windows(100� 100 lm2) using a stylus profiler. The extracted etching ratemonotonically increased with the temperature (Fig. S1), indicating thatthe rate can be controlled by the temperature. In this paper, we focusedon the sample etched with the highest etching rate at 1038 �C and inves-tigated its etching behaviors. We used SEM to characterize the etchedstructure’s shape. Ga-focused ion milling was used to expose the crosssection after depositing of a carbon surface protective layer. Atomicforce microscopy was used to observe the surface morphology of theetched (001) surface.The HCl gas etching of b-Ga2O3 proceeded not only in the win-dow area but also under the mask. When the acceleration voltage(Vacc) is high enough to allow primary electrons to pass through themask, the trace of the under-etching can be observed using SEM fromthe surface side without removing the mask. Figures 1(a) and 1(b)compare SEM images of the same circular pattern observed at differentVaccs of 1 and 10 kV. When Vacc¼ 1 kV, only a part of the etchedregion was visible through the circular window [Fig. 1(a)]. However,when Vacc¼ 10 kV, an etched region under the mask was observed[Fig. 1(b)]. This high-Vacc condition allowed us to observe the outlineof the etched depression and the relative positional relationshipbetween the window edge and depression simultaneously, allowing usto measure the under-etching length [Fig. 1(c)]. Thus, to understandthe etching behaviors, including under-etching features, we did notremove the mask and set Vacc to 10 kV when observing the etchedstructures from the surface side. Note that the etched structures with-out the mask were also observed with SEM after removing the maskFIG. 1. Top-view scanning electron microscopy images of the etched trench on thesame circular-patterned mask (1.7 lm in window diameter) recorded at (a)Vacc¼ 1 kV and (b) 10 kV. (c) Schematic cross section of the etched depression inthe vicinity of the window edge, which illustrates the definition of under-etchinglength.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 122, 162102 (2023); doi: 10.1063/5.0138736 122, 162102-2Published under an exclusive license by AIP Publishing 28 May 2024 09:29:40https://scitation.org/journal/apl(Figs. S2–S5). Figure 1(b) also shows the in-plane anisotropic etchingbehavior. Although the window was circular, the surface edges of theetched depression were parallel to the crystallographic directions of[010], [�130], and [130] to form an elongated hexagon. This shapetransformation from circular to elongated hexagon could be due to themonoclinic b-Ga2O3 crystal structure.54We investigated the in-plane anisotropic etching behavior inmore detail using two radial-line window patterns. The designs of thetwo patterns are as follows: For one pattern, window lines were placedevery 10� (36 lines), one of which was along [010] [Fig. 2(a)]. For theother pattern, window lines were placed along crystallographic orien-tations parallel to possible oxygen sublattice planes (16 lines) [Fig.2(b)]. Note that [hk0] and [h�k0] are crystallographically equivalent.For these radial lines, under-etching lengths perpendicular to the win-dow lines were measured using SEM and summarized in a polar plot[Fig. 2(c)]. The notation of n[hk0] indicates the direction rotated coun-terclockwise by 90� from [hk0] on the substrate surface. Note thatn½hk0� and n½hk0� are crystallographically equivalent.The polar plot shows the in-plane orientation dependence of theunder-etching length in detail. The under-etching was the fastest alongthe vicinity of n½190�, n½190�, and their equivalent directions to make thelargest peaks in the plot. The second fastest orientation was [010].However, under-etching was the slowest along [�100] and [100] tomake sharp and deep dips because of the emergence of (100) facetsthat have the smallest surface energy density.55 The under-etching wasalso suppressed along n½130�, n½130�, and their equivalent directions toform small dips, which is attributed to the formation of {310} facetsthat are slip planes, consisting of close-packing planes of the oxygensublattice.56 Generally, such planes with low surface-energy-densityexhibit chemical resistance to form faceted structures during etching,which is consistent with our present results. Note that the polar plot ofthe under-etching length is not perfectly symmetric. There is a smallbut significant difference between the under-etching lengths of theupper half ([�100] side) and the lower half ([100] side) of the polarplot. For example, under-etching lengths along [�100] and [100] were�0.1 and �0.6lm, respectively, although the corresponding (�100)and (100) facets are crystallographically equivalent and should havethe same surface energy density. How the difference arose based onthe results of cross-sectional SEM observation is described later.We focused on the stripe arrays along [010] to further investigatethe etched structures with (100) facets. We used two stripe patternswith different widths of mask/window (hereinafter patterns A and B).The widths of the mask/window were 2.8/1.2lm for pattern A and1.0/5.5lm for pattern B. Under-etching should be minimized in apractical etching process to allow fine patterning. In this case, [010] isthe most favorable window direction. Figure 3 shows top- and tilted-view SEM images of the trenches formed along [010] by the etchingthrough the striped windows of pattern A [Figs. 3(a)–3(c)] and patternB [Figs. 3(d)–3(f)]. In both cases, HCl etching formed trenches underthe windows with small under-etching lengths of less than 0.7lm.Furthermore, flat and smooth (100)-faceted sidewalls were observedon the [�100] side (see also Fig. S4). In addition to the (100)-facetedsidewalls, relatively rough and inclined facets appeared at the bottomcorners of the [100] side, as indicated by the (h0l) label in Fig. 3(f).This facet is discussed later. Moreover, in the case of pattern B, narrowfins with a width of approximately 0.3lm were formed betweenthe trenches because of the narrow mask width [Figs. 3(d)–3(f)],FIG. 2. Top-view scanning electron microscopy images of the etched trenches onthe radial-line-patterned masks (1.2lm in window width), where the lines were pat-terned (a) every 10� and (b) along crystallographic orientations parallel to the possi-ble oxygen sublattice planes. (c) Polar plot of under-etching length measured frompattern (a) (filled circles) and pattern (b) (open circles). See the text about the nota-tion of n[hk0].Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 122, 162102 (2023); doi: 10.1063/5.0138736 122, 162102-3Published under an exclusive license by AIP Publishing 28 May 2024 09:29:40https://scitation.org/journal/aplindicating that gas etching can fabricate not only trenches but alsofins. Aside from these sidewalls, the surface morphology of the etched(001) bottom surface was measured using AFM, as shown in Fig. S6.The surface was relatively rough with the RMS roughness of 7.7 nmdue to the presence of macro steps along [010] and byproducts depos-ited on the surface. Similar macro steps along [010] were observed forHVPE growth on (001) substrates,52 and, thus, the suppression of theformation of these steps should be a tough challenge. However, thedeposited byproducts could be removed by optimizing the etchingconditions, which should be our future work.Cross-sectional observation of the trenches along [010] revealedmore detailed information about the etched structures with (100)-faceted sidewalls. SEM images of the cross-sectional structures of thetrenches corresponding to patterns A and B are shown in Figs. 4(a)and 4(b), respectively. Schematic cross-sectional structures for bothpatterns are also shown in Fig. 4(c). Both sidewalls of the trencheshave inclined (100) facets. The face angles between the (100) facetsand surface (001) were measured to be 103�–105�, which agrees withthe lattice angle b¼ 103.7�.54 The inclination of the (100) facets couldhave caused uneven under-etching behavior between [�100] and [100].The (100) facet should be formed almost immediately on the [�100]side because the plane was positively tapered. Therefore, the under-etching length on the [�100] side was very short because the etchingrate was minimized soon after the (100) facet was formed. However,the negatively tapered (100) facet should take longer to form on the[100] side because more crystal volume must be removed. The etchingFIG. 3. Top- and tilted-view scanning electron microscopy images (0� and 50� from the surface normal, respectively) of etched trenches produced using the [010]-orientedstripe masks of (a)–(c) pattern A and (d)–(f) pattern B.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 122, 162102 (2023); doi: 10.1063/5.0138736 122, 162102-4Published under an exclusive license by AIP Publishing 28 May 2024 09:29:40https://scitation.org/journal/aplwould proceed with planes other than (100) during the transitionalperiod, and the etching rate should be faster than that of (100).Therefore, fast etching should occur for a longer time to increase theunder-etching length on [100] side longer. The inclined planes at thebottom corners of the [100] side were identified as (�101) facets usingcross-sectional SEM images. The measured face angles between the(�101) and the (100) facets were 100�–102�, whose measured valueswere close to the calculated face angle of 103.7�. The emergence of the(�101) facet agrees with the fact that (�101) exhibited the next slowestetching rate to (100) among the planes belonging to the [010] zonebased on the HEATE method.49 Interestingly, the trench bottom wassolely formed by the (�101) facet when the window width was small(pattern A) [Fig. 4(a)].Using the geometry described in Fig. 4(c), the etching depth (Dd)can be calculated using the following relationship:Dd ¼ cot ðb� 90�ÞDa � 4:1Da;where Da is the projected length of the inclined (100) facet on (001),which can be measured using top-view SEM. For instance, Da is mea-sured to be 0.47lm from the top-view SEM image for the narrow win-dow pattern A [Fig. 3(a)], and Dd is extracted to be 1.9lm. This valueagrees with the observed depth of Dd¼ 1.8lm obtained from the cor-responding cross-sectional SEM image [Fig. 4(a)]. Given that suchnarrow trenches cannot be probed with a stylus or cantilever, thedepth estimation method described above is very useful.We also characterized the cross-sectional structure of an etchedtrench with {310}-faceted sidewalls. As discussed above, {310} facetshad the second slowest lateral etching rate for forming faceted trenchsidewalls. Thus, the cross-sectional structure of the trench with {310}-faceted sidewalls is worth investigating. Figure 5(a) shows a cross-sectional SEM image of the trench formed by the etching through aline-shaped window along [�130], which is one of the radial linesshown in Fig. 2(b). A schematic of the cross section is shown in Fig.5(b). The trench profile was defined solely by inclined (310)-facetedsidewalls and a (001)-faceted bottom. The cross-sectional profile issimpler than that of the [010]-oriented trench made using the patternB, which has (100)-faceted sidewalls and an additional (�101) facet atthe bottom corner (compare Figs. 4 and 5). The measured face anglesof the (310) facets from the surface were 98�–99�, which were veryclose to the calculated angle of 98.4�. However, the (310) sidewallswere relatively rough due to the presence of macro steps, as evidencedby the SEM image recorded after removing the mask (Fig. S5). Thegeneration of macro steps could be caused by the emergence of (100)FIG. 5. (a) Tilted-view SEM images (54� from the surface normal) of a cross-sectional structure of the etched trench produced using the line-patterned mask(1.2lm in window width) along [�130], which are one of the radial lines in Fig. 2(b).(b) Schematic cross section corresponding to (a).FIG. 4. (a) and (b) Tilted-view scanning electron microscopy images (54� from thesurface normal) of cross-sectional structures of the trenches corresponding to Figs.3(a)–3(c) (pattern A) and 3(d)–3(f) (pattern B). The carbon depositions near thecross sections are surface protective layers. Note that different scales should beused to measure vertical and lateral lengths on the cross sections. (c) Schematiccross sections for both patterns.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 122, 162102 (2023); doi: 10.1063/5.0138736 122, 162102-5Published under an exclusive license by AIP Publishing 28 May 2024 09:29:40https://scitation.org/journal/aplfacets. Therefore, the (310) faceted sidewalls are unfavorable for practi-cal fin/trench applications.In conclusion, we used plasma-free HCl gas etching on a (001) b-Ga2O3 substrate to investigate its potential as a fin/trench fabricationprocess. After systematic characterization of the etched structures, weconcluded that the direction of stripe windows should be along [010]to fabricate fins and trenches with faceted sidewalls of (100). Although(100) facets are inclined from the substrate’s normal by 13.7�, theirsmooth and plasma-damage-free sidewall surfaces can improve theperformances of fin/trench devices on (001) substrates. We considersuch facet-formation-based fin/trench fabrication methods by plasma-free dry etching, and our previously proposed selective area growthwill contribute to the development of the fabrication process of b-Ga2O3 devices.See the supplementary material for the etching rate of (001) sur-face as a function of reactor temperature (Fig. S1), etched structuresafter removing the SiO2 mask (Figs. S2–S5), and surface morphologyof the etched (001) surface characterized by AFM (Fig. S6).The preparation of the SiO2 patterned mask and thecharacterization of the etched structures were performed at theNamiki Foundry and the Nanofabrication Facility (Project No.22NM5110) in the National Institute for Materials Science.AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsTakayoshi Oshima: Conceptualization (equal); Methodology (equal);Writing – original draft (lead). Yuichi Oshima: Conceptualization(equal); Methodology (equal); Writing – review & editing (lead).DATA AVAILABILITYThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.REFERENCES1A. J. Green, J. Speck, G. Xing, P. Moens, F. Allerstam, K. Gumaelius, T. Neyer,A. Arias-Purdue, V. Mehrotra, A. Kuramata, K. Sasaki, S. Watanabe, K. Koshi,J. Blevins, O. Bierwagen, S. Krishnamoorthy, K. Leedy, A. R. Arehart, A. T.Neal, S. 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