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[Mugove Maruzane](https://orcid.org/0009-0004-9207-0424), [Yuichi Oshima](https://orcid.org/0000-0001-8293-4891), [Olha Makydonska](https://orcid.org/0009-0008-5562-2137), [Paul R Edwards](https://orcid.org/0000-0001-7671-7698), [Robert W Martin](https://orcid.org/0000-0002-6119-764X), [Fabien C-P Massabuau](https://orcid.org/0000-0003-1008-1652)

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[Luminescence properties of dislocations in α-Ga<sub>2</sub>O<sub>3</sub>](https://mdr.nims.go.jp/datasets/2835669e-8564-4834-8646-84c1a0de1291)

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Luminescence properties of dislocations in α-Ga2O3Journal of Physics D: AppliedPhysics     LETTER • OPEN ACCESSLuminescence properties of dislocations in α-Ga2O3To cite this article: Mugove Maruzane et al 2025 J. Phys. D: Appl. Phys. 58 03LT02 View the article online for updates and enhancements.You may also likeInfluence of external AC transversemagnetic field on air arc oscillationpatterns and its application in DC forcedcurrent zero techniquesYushi Zhang, Yi Wu, Hao Sun et al.-Effects of heating rate and sinteringtemperature on the tensile properties ofsintered -Ti/Al nanoparticle chainsHuadian Zhang, Qingrui Jiang, Yiwei Hanet al.-VOCs conversion in He/H2O plasmaproduced in a micro-capillary tube atatmospheric pressureG Bauville, M Heninger, J Lemaire et al.-This content was downloaded from IP address 144.213.253.16 on 05/11/2024 at 05:04https://doi.org/10.1088/1361-6463/ad8894/article/10.1088/1361-6463/ad8893/article/10.1088/1361-6463/ad8893/article/10.1088/1361-6463/ad8893/article/10.1088/1361-6463/ad8893/article/10.1088/1361-6463/ad8892/article/10.1088/1361-6463/ad8892/article/10.1088/1361-6463/ad8892/article/10.1088/1361-6463/ad8892/article/10.1088/1361-6463/ad8892/article/10.1088/1361-6463/ad8006/article/10.1088/1361-6463/ad8006/article/10.1088/1361-6463/ad8006/article/10.1088/1361-6463/ad8006https://pagead2.googlesyndication.com/pcs/click?xai=AKAOjssS_cV6DIPwzh7Vq7_E3QR6QEu9gFjAgE6IN4XbjAgqpcJYUwCzbEz9rc9UFePZAvo4QtrwHcAUKi0Fyd6eFTKca_zOh7xMEexeJ-rFs_gjkVwsRCdar5odL_BtrVZAdu0HrE6zkUOI4EYbrWRNZVihLvaHGfKLKcULUYRmHVxbPweGgzal6RQF0W-YM-tFZRRCgG8IBH9IMqN6gKCVl5rSd5bgb9UCBPniWZnlUxKppNWPq65hgDuhB-z8dQnhpD5mgUxGtahW1K9c_M7Qum5tuwRwV_t7GJEKFtlEsf7_EdmMKsUA_tj4gE643DtV0rT9FDqXxCWdUyX9Ub_lDUncow0VqX4tv62HnzmP7oF1&sig=Cg0ArKJSzBNXa9lFm0FI&fbs_aeid=%5Bgw_fbsaeid%5D&adurl=https://www.electrochem.org/247/%3Futm_source%3DIOP%26utm_medium%3Dbanner%26utm_campaign%3DIOP_247_abstract_submission%26utm_id%3DIOP%2B247%2BAbstract%2BSubmissionJournal of Physics D: Applied PhysicsJ. Phys. D: Appl. Phys. 58 (2025) 03LT02 (7pp) https://doi.org/10.1088/1361-6463/ad8894LetterLuminescence properties ofdislocations in α-Ga2O3Mugove Maruzane1,∗, Yuichi Oshima2, Olha Makydonska1, Paul R Edwards1,Robert W Martin1 and Fabien C-P Massabuau11 Department of Physics, SUPA, University of Strathclyde, Glasgow, United Kingdom2 National Institute for Materials Science, Tsukuba, JapanE-mail: mugove.maruzane@strath.ac.ukReceived 19 August 2024, revised 12 September 2024Accepted for publication 18 October 2024Published 28 October 2024AbstractDislocations in epitaxial lateral overgrown α-Ga2O3 are investigated using hyperspectralcathodoluminescence spectroscopy. The dislocations are associated with a reduction ofself-trapped hole-related luminescence (ca. 3.6 eV line) which can be ascribed to their actions asnon-radiative recombination sites for free electrons, to a reduction in free electron density due toFermi level pinning or to electron trapping at donor states. An increase in the intensity of the ca.2.8 eV and 3.2 eV lines are observed at the dislocations, suggesting an increase indonor–acceptor pair transitions and providing strong evidence that point defects segregate atdislocations.Supplementary material for this article is available onlineKeywords: gallium oxide, dislocation, cathodoluminescence1. IntroductionWide bandgap semiconductors offer exciting perspectives forthe fabrication of high-power and high-frequency electronicdevices, as illustrated by their high Baliga and Johnson figuresof merit [1, 2]. One such wide bandgap semiconductor isGa2O3 which can form five polymorphs, labelled α, β, γ,κ and δ [3, 4]. Monoclinic β-Ga2O3 is the most studiedphase of Ga2O3 due to its thermal stability, but corundum α-Ga2O3 has been gaining popularity recently due to its widerbandgap in the range of 5.1–5.3 eV [5, 6] and its promise of∗Author to whom any correspondence should be addressed.Original Content from this work may be used under theterms of the Creative Commons Attribution 4.0 licence. Anyfurther distribution of this work must maintain attribution to the author(s) andthe title of the work, journal citation and DOI.bandgap engineering through alloying with other corundumsesquioxides [7–12]. However, unlike β-Ga2O3 which can begrown from the melt, the metastable nature of α-Ga2O3 meansthat it can only be produced by epitaxial methods such as hal-ide vapour phase epitaxy [13–15], molecular beam epitaxy[16–19], metal-organic chemical vapour deposition [17, 20],mist chemical vapour deposition [5, 21, 22] and atomic layerdeposition [6, 23, 24]. α-Ga2O3 is usually grown on iso-morphic sapphire (α-Al2O3) substrates; however, the 4.7%and 3.3% lattice mismatch between the film and the substratein the a and c directions, respectively, results in a high densityof threading dislocations of ca. 10 10cm−2 [5, 14].The effects of dislocations on the properties of any poly-morph of Ga2O3 are poorly understood. Kasu et al [25] repor-ted that dislocations resulted in higher reverse leakage currentsin β-Ga2O3 [25], and Yang et al [26] stated that crystal defects(including dislocations) can explain the reason why the theor-etical rectifying limit for β-Ga2O3 has not yet been attained[26]. In α-Ga2O3, dislocations have been linked to reduced1 © 2024 The Author(s). Published by IOP Publishing Ltdhttps://doi.org/10.1088/1361-6463/ad8894https://orcid.org/0009-0004-9207-0424https://orcid.org/0000-0001-8293-4891https://orcid.org/0009-0008-5562-2137https://orcid.org/0000-0001-7671-7698https://orcid.org/0000-0002-6119-764Xhttps://orcid.org/0000-0003-1008-1652mailto:mugove.maruzane@strath.ac.ukhttp://crossmark.crossref.org/dialog/?doi=10.1088/1361-6463/ad8894&domain=pdf&date_stamp=2024-10-28https://doi.org/10.1088/1361-6463/ad8894https://creativecommons.org/licenses/by/4.0/J. Phys. D: Appl. Phys. 58 (2025) 03LT02electron mobility [27, 28]. The anticipated negative impact ofdislocations has triggered research into the design of meth-ods to mitigate their occurrence. A promising method cur-rently employed for α-Ga2O3 is epitaxial lateral overgrowth(ELOG) [29–33] which allows the reduction of threading dis-location density from ca. 10 10cm−2 to ca. 10 6cm−2 [29,31–33]. Despite these efforts to reduce dislocation densities,it is also important to understand their properties to predicttheir impact on future device performance and design effect-ive strategies to mitigate their effects.Cathodoluminescence (CL) is a powerful technique used toprobe the optical properties of semiconductors with nanoscaleresolution and correlate thesewith the presence of defects [34].The method has been successfully used to reveal the opticalproperties of dislocations in other wide bandgap semiconduct-ors like III-nitrides [35–37] but has not yet been employed forstudying dislocations in α-Ga2O3.The luminescence spectrum of α-Ga2O3 (as well as β-Ga2O3) is characteristically broad [38–42]. Luminescencespectra generally do not contain a near-band edge contribu-tion close to 5 eV [38, 40–43], and instead, the luminescencespans from ca. 2.0–3.8 eV [38, 41]. Nicol et al [41] reportedH-related luminescence in α-Ga2O3 at 3.8 eV [41]. UV lumin-escence near ca. 3.2–3.6 eV has been ascribed to the recombin-ation of self-trapped holes with free electrons [38–40, 42, 44–46]. Blue luminescence in the range of ca. 2.8–3.0 eV, has beenascribed to donor–acceptor pair transitions between shallowdonors and acceptors involving gallium or oxygen vacancies[38, 46–50]. Green luminescence observed in the range 2.0–2.7 eV has been ascribed to donor–acceptor pair transitionsbetween deep donors and acceptors, usually formed from com-plexes of oxygen vacancies, gallium vacancies, gallium inter-stitials and oxygen interstitials [45, 51, 52]. Finally, red lumin-escence at ca. 1.7–1.9 eV, has been observed in β-Ga2O3 buthas yet to be observed in α-Ga2O3. In β-Ga2O3, red lumines-cence has been linked to the presence of dopants such as nitro-gen, chromium and silicon [45, 53–56]. The effect of extendeddefects on the luminescence properties in Ga2O3 polymorphshas not been widely discussed. Cooke et al [57] assessed theeffect of extended defects on the photoluminescence spectra ofβ-Ga2O3 films grown on different substrates, which resultedin a variation of strain and, by extension, a variation of exten-ded defect density. In that study, extended defects were linkedto a decrease in intensity and a redshift of the luminescence[57]. In the present report, we investigate how threading dis-locations affect the CL spectrum of ELOG α-Ga2O3 with ananoscale resolution.2. Experimental methodsWe investigated the luminescence properties of dislocations inan ELOG α-Ga2O3 sample grown using halide vapour phaseepitaxy (HVPE) on c-plane sapphire following the ELOGrecipe detailed by Oshima et al [29]. After the growth of aninitial α-Ga2O3 seed layer, a TiO2 mask consisting of 5µmwide stripes separated by 1µm wide windows was depositedwith stripes oriented parallel to [11̄00]. This was followed bythe deposition of a ca. 21µm layer of α-Ga2O3. Finally, thesample surface was etched using HCl gas to reveal pits indicat-ing where threading dislocations, in particular edge-type, ter-minate at the sample surface [33].Secondary electron (SE) and hyperspectral CL imagingwere conducted using a JEOL JXA-8530F field emission elec-tron probemicro-analyser (EPMA). Themicroscopewas oper-ated with an acceleration voltage of 8 kV and a beam currentof 5 nA, resulting in an interaction volume size of ca. 240 nmbased onMonte Carlo simulations [58]. The luminescence wascollected at room temperature using a reflecting objective andwas directly coupled to a cooled CCD spectrograph, allowinga CL spectrum to be recorded for each pixel in the scan. Theresulting CL spectra were corrected for system response usingthe transition radiation of pure aluminium [59].3. Results and discussionsFigure 1(a) shows an SE image of the sample surface. Thewin-dow (1µm wide) and mask (5µm wide) regions can be easilyidentified as regions with high and low densities of etch pits,respectively. At the centre of the mask region, where the α-Ga2O3 coalesces, is a thin line of lightly packed etch pits thatalso runs parallel to the window regions (i.e. [11̄00]) as repor-ted by Oshima et al [33]. Figure 1(b) shows the correspondingpanchromatic CL image (photon energies in the range of 1.5–5 eV) showing the integrated CL intensity recorded at eachpixel. These maps show that the positions of the etch pits in theSE image seem to correlate with reduced CL luminescence.More importantly, the map also reveals additional dark con-trast lines oriented along the ⟨11̄00⟩ directions which propag-ate from the window region to the etch pits. Kawara et al[31] previously reported, in dot-patterned ELOG α-Ga2O3,that dislocations could bend from the window region intothe masked region following the ⟨11̄00⟩ direction [31]. Thesestudies also observed that the dislocations which bend deepwithin the film would terminate at the coalescence boundary[29, 31]. Such dislocations would therefore not cause an etchpit at the sample surface, and their bent section would occurtoo deep in the sample for the electron beam to probe. Thisstrongly suggests that the dark contrast lines in our panchro-matic CL map relate to dislocations bending sufficiently closethe sample surface (i.e. within the 240 nm depth probed by theelectron beam) from thewindow region and terminating at etchpits in the mask region.To start assessing how dislocations affect the luminescenceproperties of α-Ga2O3, we first look at CL spectra taken awayfrom a dislocation line (figure 1(c)) and at a dislocation line(figure 1(d)). Both spectra alignwith the literature onα- and β-Ga2O3 luminescence, and can be well fitted using 3 Gaussianpeaks in agreement with other examples of luminescence spec-tra from the literature [40, 41, 45]. We observe a first peak(labelled ‘Peak 1’) centred at ca. 3.6 eV and attributed toself-trapped holes, as well as a second and third peak (‘Peak2J. Phys. D: Appl. Phys. 58 (2025) 03LT02Figure 1. (a) SE and (b) panchromatic CL maps of the same area of the sample. Example CL spectra taken on a pixel (c) away fromdislocations, and (d) at a dislocation. Bandpass-integrated CL maps for emission energies (e) above 3.25 eV and (f) below 3.25 eV. To guidethe eye, the positions of etch pits identified from the SE image were manually marked by dots in (e) and (f). An example of an etch pitlinked (not linked) to a dislocation line is highlighted by a square (circle) box.2’ and ‘Peak 3’) centred at ca. 3.2 eV and 2.8 eV, respect-ively, attributed to donor–acceptor pair transitions [38, 40, 42].We can already observe differences depending on the posi-tion relative to the dislocation, where the CL spectrum takenat the dislocation exhibits an overall weaker and redshiftedluminescence compared to that of the region away from thedislocation—we analyse these variations in more detail in thenext section. Figures 1(e) and (f) show bandpass-integratedCL maps of the sample extracted from the hyperspectral CLmap, discriminating emission of photons >3.25 eV (i.e. dom-inant contribution from Peak 1, figure 1(e)) from emission ofphotons <3.25 eV (i.e. dominant contribution from Peaks 2and 3, figure 1(f)). To guide the eye, red-bordered dots are usedto indicate the locations of the etch pits. These two maps showa contrast inversion, where the dislocation lines appear darkin the map for emission >3.25 eV (figure 1(e)) and bright inthe map for emission <3.25 eV (figure 1(f)), in line with ourobservation of individual CL spectra (figures 1(c) and (d)).Further investigation identifies two different categories ofetch pits. The first category corresponds to pits connected to adislocation line in the CLmap (an example of which is markedby a square in figure 1), while the second category correspondsto pits that are not linked to a dislocation line in the CLmap (anexample of which is marked by a circle in figure 1). We calcu-lated a density of etch pits of ca. 1.5× 106 cm−2 in the maskregion, which is in line with the value reported by Oshima et al[33], and we estimate that ca. 85% of etch pits fall into the firstcategory of pits, with the remainder ca. 15% falling into thesecond category.To better understand the luminescence properties of the dis-locations, we acquired a linescan to investigate the evolution ofthe luminescence spectrum as the electron probe scans acrossdislocation lines, shown in figure 2. While we note that thedislocation line and the etch pit terminating it exhibit the sameluminescence, analysing the dislocation line, which is a sub-surface feature, allows us to rule out the potential impact ofthe HCl etching on the CL data. As discussed previously, eachCL spectra was fitted using 3 Gaussian peaks, and the evol-ution of the integrated intensity and energy of each Gaussianpeak was monitored across the linescan (figures 2(d) and (e)).The deconvolution of each individual spectrum from the lin-escan is shown in the supplementary information (figure S1).The first peak, centred at ca. 3.6 eV can be seen to vary sig-nificantly across the linescan.We observe that compared to theregion away from the dislocation, the intensity decreases by afactor ca. 2 at the dislocation and redshifts by ca. 0.04 eV. Thisluminescence line is commonly ascribed to radiative recom-bination of free electrons with self-trapped holes [38, 40, 42,44–46, 60]. Given that self-trapped holes are strongly local-ised at O sites [45], it is reasonable to interpret this decrease inintensity as a reduction of the density of free electrons, whichwould therefore be involved in competing recombination path-ways near the dislocation. One interpretation could be thatthese electrons get more efficiently trapped at donor states. Ifthis was the case, we should expect to see an increase in donor–acceptor pair transitions at dislocations, which we observeto some extent with variations of Peaks 2 and 3. However,that increase in donor–acceptor pair transition does not com-pensate for the reduction of Peak 1 luminescence, suggestingan additional recombination pathway is at play. The reductionof free electron recombination with self-trapped holes couldalso be caused by an increase in non-radiative recombina-tion at the dislocation, as has been widely observed in othersemiconductors [35, 61, 62]. One last interpretation could bethat the electrons drift away from the dislocation region due toFermi level pinning at the dislocation, for example, because ofa charged dislocation core [63]. A comprehensive assessmentof the correct interpretation necessitates knowledge of the3J. Phys. D: Appl. Phys. 58 (2025) 03LT02Figure 2. Analysis of a linescan taken across 2 dislocation lines. (a) CL (above 3.25 eV) and (b) SE showing the position of the linescan.Evolution of the (c) CL spectra, (d) integrated intensity, and (e) centre energies of the different Gaussian peaks across the lines scan.atomic structure of the dislocation core (which could dependon the dislocation type, growth method conditions, etc.) sup-ported by theoretical modelling of its electronic structure. Weattribute the redshifts of the luminescence to strain variationsin the vicinity of the dislocation, as the redshift is consist-ent with the predicted bandgap energy reduction induced bya small percentage of strain [64].Peaks 2 and 3, centred at ca. 3.2 eV and 2.8 eV, respect-ively, are also observed to vary across the linescan. Both peaksfollow a similar trend, whereby their intensity approximatelydoubles at the dislocation (consistent with the bright contrastlines seen in figure 1(d)). Since these luminescence lines arenormally assigned to donor–acceptor pair transitions [38, 46–50], the increased intensity of Peaks 2 and 3 is a strong signthat point defects segregate at the dislocation. Given the highdensity of point defects present in current Ga2O3 samples, itis not surprising to see an accumulation of point defects neardislocations as a mechanism for partial strain release. As men-tioned above, the greater density of donor and acceptor statesinduced by this point defect segregation can provide a com-peting recombination pathway for the free electrons in theconduction band, partially explaining the decreased intensityof Peak 1 at the dislocation. Looking at the centre energy ofPeaks 2 and 3, we observe that both peaks redshift by ca.0.08 eV and 0.1 eV at the dislocation, respectively. It is diffi-cult to interpret this energy shift as it combines several effects,including a reduction of donor–acceptor separation and a vari-ation of strain. While a greater density of donors and acceptorsshould result in a blueshift of the luminescence, the impact ofstrain is impossible to predict at present. Donor–acceptor pairluminescence energy is determined by the bandgap energy aswell as the donor and acceptor ionisation energies, which areall strain-dependent. However, the strain dependence of theseionization energies is currently unknown, especially when thedonor–acceptor pair luminescence is not assigned to one par-ticular defect but rather to a library of defects.Figure 3 shows a linescan performed on an etch pit notconnected to a dislocation line, i.e. the second category ofetch pits. In figure 3(c), we can see that there is no clearvariation of the luminescence spectra across the etch pit,which is confirmed in figures 3(d) and (e) with the plotsof the integrated intensities and the centre energies of thepeak deconvolution—each individual deconvoluted spectrumfrom the linescan is shown in the supplementary informa-tion (figure S2). This further supports the negligible effectof etching on the luminescence. The lack of variation of theintegrated intensity and energy of Peaks 1, 2, and 3 indicatesthat this second category of etch pits must have a differentnature than the etch pits connected to dislocation lines (firstcategory of etch pits) analysed in figure 2. Instead of termin-ating a dislocation that bends from the window region, thissecond category of etch pit could relate to a different cat-egory of dislocation, such as dislocations that nucleate fromthe coalescence boundary of the ELOG windows [31]—thushaving a different atomic environment than a dislocation thatbends from the window region. It could also be that this typeof etch pit does not relate to dislocations at all, as Oshimaet al [33] reported that 1.5% of etch pits were not associ-ated with dislocations [33]. This value does not match the15% proportion of etch pits we associated with the secondcategory, but we must bear in mind that the pit categorisa-tion was based on the presence of (or lack of) a dark line con-necting them in the panchromatic CL image. It is possible thatsome etch pits were wrongfully associated with the second cat-egory because the dislocation line they relate to was bendingslightly deeper in the sample than what the interaction volumewould probe, thus appearing in the CL images as not connectedto any line.4J. Phys. D: Appl. Phys. 58 (2025) 03LT02Figure 3. Analysis of a linescan taken across an etch pit not connected to a dislocation line. (a) CL (above 3.25 eV) and (b) SE showing theposition of the linescan. Evolution of the (c) CL spectra, (d) integrated intensity, and (e) centre energies of the different Gaussian peaksacross the lines scan.4. ConclusionIn conclusion, we conducted the first investigation of theluminescence properties of dislocations in ELOG α-Ga2O3using CL. The luminescence spectrum at dislocations devi-ates from that of the regions away from dislocations. Weobserve a reduction of self-trapped hole-related luminescence(ca. 3.6 eV line), which we ascribe to dislocations acting asnon-radiative recombination sites for free electrons, to a reduc-tion in free electron density due to Fermi level pinning or tocarrier trapping at donor states. An increase in the intensity ofthe ca. 3.2 eV and 2.8 eV lines is also observed, which sug-gests an increase in the rate of donor–acceptor pair transitionsand stands as strong evidence that point defects segregate atdislocations.Data availability statementThe data that support the findings of this study are openlyavailable from the University of Strathclyde KnowledgeBaseat (https://doi.org/10.15129/8bf33f6a-66d1-4a37-9330-422ad03aa382).AcknowledgmentsThe authors acknowledge support from the Engineering andPhysical Sciences Research Council (EPSRC Grant No.EP/K011952/1). OMwould like to acknowledge support fromthe EPSRC Vacation Internship.ORCID iDsMugove Maruzane https://orcid.org/0009-0004-9207-0424Yuichi Oshima https://orcid.org/0000-0001-8293-4891Olha Makydonska https://orcid.org/0009-0008-5562-2137Paul R Edwards https://orcid.org/0000-0001-7671-7698Robert W Martin https://orcid.org/0000-0002-6119-764XFabien C-P Massabuau https://orcid.org/0000-0003-1008-1652References[1] Johnson E 1965 Physical limitations on frequency andpower parameters of transistors IRE Int. Conven. Rec.13 27–34[2] Baliga B J 1989 Power semiconductor device figure of meritfor high-frequency applications IEEE Electron Device Lett.10 455–7[3] Roy R, Hill V and Osborn E 1952 Polymorphism of Ga2O3and the system Ga2O3—H2O J. Am. Chem. 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Introduction 2. Experimental methods 3. Results and discussions 4. Conclusion References