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[Lewis T. Penman](https://orcid.org/0000-0002-0147-9995), Zak M. Johnston, [Paul R. Edwards](https://orcid.org/0000-0001-7671-7698), [Yuichi Oshima](https://orcid.org/0000-0001-8293-4891), Clifford McAleese, [Piero Mazzolini](https://orcid.org/0000-0003-2092-5265), [Matteo Bosi](https://orcid.org/0000-0001-8992-0249), [Luca Seravalli](https://orcid.org/0000-0003-2784-1785), Roberto Fornari, [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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[Comparative Study of the Optical Properties of α‐, β‐, and κ‐Ga<sub>2</sub>O<sub>3</sub>](https://mdr.nims.go.jp/datasets/3e74aeda-98d0-424c-8d64-4a1e1fe60592)

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Comparative Study of the Optical Properties of α‐, β‐, and κ‐Ga2O3Comparative Study of the Optical Properties of α-, β-, andκ-Ga2O3Lewis T. Penman,* Zak M. Johnston, Paul R. Edwards, Yuichi Oshima,Clifford McAleese, Piero Mazzolini, Matteo Bosi, Luca Seravalli, Roberto Fornari,Robert W. Martin, and Fabien C.-P. Massabuau1. IntroductionGallium oxide (Ga2O3) is one of the leadingwide bandgap semiconductor candidatesfor applications in high-power electronicsand UV optoelectronics.[1] Ga2O3 is a highlypolymorphic compound, with the mainphases labeled β-Ga2O3 (monoclinic),α-Ga2O3 (rhombohedral), and κ-Ga2O3(orthorhombic) which is also known asorthorhombic ε-Ga2O3[2,3] as illustrated inFigure 1. The β-phase has attracted mostof the research effort as it is the thermody-namically stable phase.[4] Meanwhile, themetastable α- and κ-phases have recentlysparked renewed interest due to the widerbandgap of 5.3 eV for the α-phase[5] andthe intrinsic polarization of the κ-phase,[6,7]which open new perspectives for highpower devices such as high electronmobility transistors (HEMT).[8,9] Additionally, α-Ga2O3 can beused for wide-bandgap engineering with other rhombohedralsequioxides (e.g., Al2O3, In2O3, Fe2O3, Ti2O3), which isdesirable due to the low lattice mismatch and wide range ofachievable bandgaps.[10] However, little is known about the opti-cal properties of the α- and κ-phases, which are often assumed byanalogy to be similar to those of β-Ga2O3. A consistent compara-tive study of the optical properties of α-, β-, and κ-Ga2O3 isneeded, as each phase’s unique structure implies distinct prop-erties. Accurate knowledge of these properties would be anessential aid in the design and fabrication of Ga2O3-basedoptoelectronic devices for applications such as solar-blindphotodetectors.[11,12]2. Background2.1. β-Ga2O3β-Ga2O3 has a monoclinic structure, which contains two Ga andthree O nonequivalent sites forming one tetrahedral and oneoctahedral structure around the Ga sites.[1,16] When deposited onc-plane sapphire, β-Ga2O3 grows with the (�201) orientation.[17]The structure exhibits six in-plane rotational domains, rotated by60° around the [�201] direction, caused by twofold symmetry inβ-Ga2O3 on top of the threefold symmetry substrate.[17–19]Current literature suggests that β-Ga2O3 has a bandgap of aroundL. T. Penman, Z. M. Johnston, P. R. Edwards, R. W. Martin,F. C. P. MassabuauDepartment of PhysicsSUPA, University of Strathclyde107 Rottenrow East, Glasgow G4 0NG, UKE-mail: lewis.penman@strath.ac.ukY. OshimaResearch Center for Electronic and Optical MaterialsNational Institute for Materials Science1-1 Namiki, Tsukuba 305-0044, JapanC. McAleeseAixtron Ltd.Anderson Road, Swavesey, Cambridge CB24 4FQ, UKP. Mazzolini, R. FornariDepartment of Mathematical, Physical and Computer SciencesUniversity of ParmaArea delle Scienze 7/A, 43124 Parma, ItalyP. Mazzolini, M. Bosi, L. Seravalli, R. FornariIMEM-CNRArea delle Scienze 37/A, 43124 Parma, ItalyThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/pssb.202400615.© 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbH. This is an open access article under theterms of the Creative Commons Attribution License, which permits use,distribution and reproduction in any medium, provided the original work isproperly cited.DOI: 10.1002/pssb.202400615A systematic investigation of the optical properties of β-, α-, and κ-phase galliumoxide (Ga2O3) polymorphs is conducted by UV–vis spectrophotometry throughthe Swanepoel method and temperature-dependent photoluminescence. Usingthe same approach and apparatus allows similarities and differences betweenthese three phases to be directly established. Differences between polymorphs areobserved, including refractive indices of 1.89 (β), 2.00 (α), and 1.85 (κ) and opticalbandgaps of 4.99 eV (β), 5.32 eV (α), and 4.87 eV (κ). In the luminescence studies,four emission peaks in each polymorph are revealed, located at different energiesin the UV (3.1–3.9 eV), blue (2.7–3.0 eV), and green (2.2–2.6 eV) regions, withcauses attributed to self-trapped holes, donor–acceptor pair transitions involvingGa and O vacancies (VGa, VO), Ga─O divacancies (VGaþ VO), O interstitials (Oi),and H impurities (VGa–nH, Hi, Ho). In this systematic study, unique opticalproperties of the different Ga2O3 polymorphs are highlighted and it is warned thatthe commonly practiced analogy to β-Ga2O3 can lead to misinterpretations.RESEARCH ARTICLEwww.pss-b.comPhys. Status Solidi B 2025, 262, 2400615 2400615 (1 of 8) © 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbHmailto:lewis.penman@strath.ac.ukhttps://doi.org/10.1002/pssb.202400615http://creativecommons.org/licenses/by/4.0/http://www.pss-b.comhttp://crossmark.crossref.org/dialog/?doi=10.1002%2Fpssb.202400615&domain=pdf&date_stamp=2025-02-124.8–5.0 eV[5,20–22] and a refractive index of 1.89.[23,24] Comparedto the other polymorphs, β-Ga2O3 currently possesses the mostcomprehensive literature on luminescence properties. The mate-rial exhibits broad luminescence over the UV and visiblerange, with components in the UV (3.2–3.6 eV),[25–27] blue(2.7–3.0 eV),[25,26,28–31] green (2.3–2.5 eV),[30] and red (1.7–1.9 eV)regions.[31–33] It is currently thought that the UV componentsresult from the recombination of free electrons with self-trappedholes (STHs) located at O sites.[25,27,34] Luminescence in the blueregion is generally attributed to shallow donor–acceptor pair(DAP) transitions with associated defects being Ga vacancies(VGa), Ga─O divacancies (VGaþVO), or interstitial Ga(Gai).[25,28,30] Deep DAP transitions are the currently acceptedcause of green luminescence, where Ga and O vacancies (VO,VGa) and interstitial O (Oi) have all been proposed as the defectsinvolved.[30] Lastly, red luminescence has been attributed to levelsresulting from N, Fe, and Cr impurities.[31–33] Additionally,Onuma et al.[25] and Cho et al.[26] conducted temperature-dependent luminescence studies of undoped β-Ga2O3 and foundthat the UV and blue emission peaks follow the conventionalrelation between emission and temperature first outlined byVarshni.[35] Also, Onuma et al. reported how the addition ofdopants (Si, Mg) modify the luminesce of β-Ga2O3 creatingtransitions that cause a visible reduction in overall luminesceas temperature decreases compared to undoped samples, as wellas having an impact on crystal orientation.[25]2.2. α-Ga2O3α-Ga2O3 is a metastable phase of Ga2O3 which has gainedinterest for its wider bandgap and isomorphism with othersesquioxides.[36] It has a rhombohedral (corundum) structurecontaining one Ga and one O nonequivalent site creating a struc-ture comprising of only octahedral sites.[1,14] When depositedon c-plane sapphire, α-Ga2O3 exhibits (0001) orientation.[35]This phase exhibits the widest bandgap among all polymorphs,around 5.1–5.6 eV,[5,21,22,38–40] and a refractive index of1.74–1.95.[24,38,41] Compared with β-Ga2O3, less research efforthas focused on α-Ga2O3, but points for comparison do exist.Similar to β-Ga2O3, α-Ga2O3 features a broad luminescencespectrum covering a similar region, with peaks in the UV(3.2–3.8 eV),[26,42–46] blue (2.7–3.0 eV),[26,42–45,47] and green(2.5–2.6 eV) regions.[26,42] A higher energy UV emission at3.8 eV has been assigned to DAP caused by H impurities combin-ing with VGa to form hydrogenated gallium vacancies (VGa–nHwhere n is the number of H atoms occupying the VGa).[42]The lower energy UV emission has been attributed toSTH.[43,45,47,48] Shallow DAP transitions caused by VGa andVGaþ VO have been attributed to the blue luminescence.[42,48,49]There has been tentative assignment of green emission to deepDAP transitions from VGa and VO centers.[42] However, through-out all the literature reviewed, it is very common to note that theseassignments were drawn by analogy to β-Ga2O3 luminescence,which could lead to misinterpretation. Literature reports varyingdegrees of temperature dependence of the luminescence spec-trum. Cho et al. reported strong temperature dependencein the lower UV peak at 3.47 eV with a reduction in intensityby a factor of 10 in addition to a redshift of ≈0.09 eV.[26]Meanwhile, Moriya et al. reported a varying peak intensity withSn dopant concentration, with higher concentrations causing anincrease in blue emission of ≈2.9 eV and a reduction in UV emis-sion ≈3.6 eV.[47] The UV peak exhibited a temperature depen-dence in the form of a reduction of intensity from 100 to300 K of a factor of 3, while the blue peak seems to exhibit nochange with temperature.[47] Nicol et al. reported that the3.8 eV luminescence line became dominant at temperatures<100 K, while the other lines remained unchanged.[42] Janzenconducted a temperature and orientation-dependent study usingp-polarized light from a dual monochromated Xe lamp source, ontrigonal α-Ga2O3 deposited on m-plane sapphire.[43] Janzen iden-tified two luminescence peaks, the higher energy 3.6 eV peakdecreased in intensity as temperature increased. At 300 K, thepeak was only 30% as intense compared to the peak at 5 K alongthe a-direction; additionally, it only became predominant attemperatures <40 K during excitation. When excited along thec-direction, the temperature dependence was more pronounced,with the UV peak disappearing entirely at temperatures >100 K.Furthermore, Janzen identified a blue peak at ≈2.78 eV, whichshowed an intensity increase from 5 to 100 K, followed by adecrease from 100 to 300 K. These properties were observed inboth excitation orientations.[43]2.3. κ-Ga2O3κ-Ga2O3 is also a metastable phase of Ga2O3 which has gainedinterest for its spontaneous polarization, which may be exploitedFigure 1. Unit cells of a) β-Ga2O3, b) α-Ga2O3, and c) κ-Ga2O3 represented using VESTA3[13] with structural information taken from refs. [3,14,15].www.advancedsciencenews.com www.pss-b.comPhys. Status Solidi B 2025, 262, 2400615 2400615 (2 of 8) © 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbH 15213951, 2025, 8, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssb.202400615 by National Institute For, Wiley Online Library on [08/08/2025]. 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 Licensehttp://www.advancedsciencenews.comhttp://www.pss-b.comfor preparing heterostructures with a 2D electron gas at theirinterface.[6,7] It has an orthorhombic structure with four Gaand six O nonequivalent sites, the planes of O atoms have apurely hexagonal symmetry while the Ga planes in betweenare made of either only (two) Ga octahedral sites or Ga octahedraland Ga tetrahedral sites so that a 4H stacking forms along the(001) direction when deposited on c-sapphire.[2,3] To maintainthe correct Ga:O stoichiometry of 2:3, one-third of the Ga sitesare empty in both planes. The filled octahedra and tetrahedrasites are not randomly distributed in their planes but arranged inregular ribbons, which in the case of heteroepitaxy on c-planesapphire leads to formation of 120° rotational domains separatedby (110) twin planes, which ultimately give rise to a pseudo-hexagonal structure consisting of three separate crystalorientations.[2,3] Current literature suggests a bandgap of around4.7–5.0 eV,[21,50–55] with refractive index of 1.96,[55] as estimatedfrom ellipsometry measurements of orthorhombic ε-Ga2O3 epi-layers. Like the other polymorphs, κ-Ga2O3 exhibits a broad lumi-nescence spectrum, exhibiting UV (3.1–3.5 eV),[43,56,57] blue(2.6–3.0 eV),[43,45,56–60] green (2.3–2.4 eV),[43,56,59] and red lumi-nescence (1.6–1.8 eV).[60] STH have been assigned to be thecause of UV emission.[43,56] VGa shallow DAP transitions andVO deep DAP transitions are presumed to be responsible for blueand green emission,[43,56–59] respectively, but work by Janzenindicates that STH can cause blue emission.[43] Montedoroet al. identified peaks at 2.4, 2.75, 3.0, and 3.15 eV, and foundthat the 3.0 and 3.15 eV peaks showed a steady decrease in inten-sity when temperature was increased, both approximately halv-ing in intensity from 80 to 280 K.[56] The 2.75 eV peak sees amuch stronger temperature dependence, with an intensitydecrease over the same temperature range of a factor of 4.Finally, the 2.4 eV peak shows a slightly weaker temperaturedependence, with intensity decreasing by a factor of 3 overthe 80–280 K range. Janzen’s investigation identified a polariza-tion dependence for a single domain κ-Ga2O3 deposited on anε-GaFeO3 substrate,[61] using the same apparatus as in theα-Ga2O3 investigation. Excitation along the a-direction producedpeaks at 2.41, 2.70, 2.99, and 3.16 eV. Peaks at 2.70 and2.99 eV were emitted during excitation along the b-direction.[43]During excitation along the a-axis, the temperature dependenceof intensity was comparable to Montedoro et al. with onenotable exception: a sharp increase at 250 K followed by a sharpdecrease at 300 K, combined with a redshift throughout. In con-trast, the b-axis study showed minimal dependence, with a slightincrease at 250 K, an abrupt redshift at 300 K, and an increase inintensity.[43]3. Experimental Section3.1. SamplesFive unintentionally doped Ga2O3 films were investigated in thisstudy: one sample in the β-phase, two samples in the α-phase,and two samples in the κ-phase. The sample names, phases,and key features are summarized in Table 1.The β-phase sample (β) was grown by metal-organic chemicalvapor deposition (MOCVD) on single-side polished c-plane sap-phire, in an Aixtron 3� 2 00 closed coupled showerhead reactorutilizing trimethylgallium (TMG) and O2 precursors and N2 car-rier gas. The growth process began with the deposition of a thinnucleation layer at 700 °C, followed by the formation of an≈ 750 nm thick Ga2O3 layer at 1050 °C and constant pressureof 100mbar. The distance between the showerhead and the sub-strate could be adjusted dynamically during growth to minimizethe pre-reaction of precursors, here a constant 9 mm gap wasmaintained throughout the growth. The thickness of the samplewas obtained by usage of a known growth rate.The α-phase samples (α1 and α2) were grown on single-side pol-ished c-plane sapphire by halide vapor-phase epitaxy (HVPE) in ahorizontal quartz reactor at 520 °C under atmospheric pressure.O2 and GaCl were used as the precursors. The GaCl was synthe-sized by a chemical reaction of metal Ga and HCl gas upstream inthe reactor at 570 °C with N2 was used as the carrier gas.[37] Thethicknesses of both α-phase samples were estimated using agrowth rate obtained by analyzing sample cross sections.The κ-phase sample κ1 was grown by HVPE on single-side pol-ished c-plane sapphire with a TiO2 interlayer, at a temperature of550 °C in the same HVPE reactor used for the growth of samplesα1 and α2.[50] Finally, sample κ2 was grown by MOCVD on single-side polished c-plane sapphire in a proprietary horizontal reactor,using TMG and ultrapure H2O, with He as carrier gas at atemperature of 650 °C and pressure of 100mbar.[62] This samplefeatures a notable thickness gradient across its surface, with esti-mated thickness of 1100 nm in the gas inlet side and 600 nm at thegas outlet side, spaced about 2 cm. The thickness of κ1 wasobtained using the same method as the α-phase samples, whilethe thickness of κ2 was obtained by optical reflectometry.We noted that lattice strain could affect the optical proper-ties,[63] but given that all our samples consisted of 100 s nm thickfilms grown by heteroepitaxy, it could be assumed that they werefully relaxed.Samples α1 and κ2 were used for the Swanepeol methods dueto their well discernable fringes in transmittance spectra, whileTable 1. Summary of the of the samples investigated here and comparison with literature.Sample name Phase Growth method Orientation Thickness d [nm] Bandgap Eg [eV] Refractive Index [n]Target Measured Literature This work Literature This workβ β MOCVD (�201) 750 755 (3) 4.8–5.0[5,20–22] 4.99 (4) 1.89–1.91[23,24,55] 1.89 (1)α1 α HVPE (0001) 350 352 (3) 5.1–5.6[5,21,22,38–40] 5.32 (9) 1.74–1.95[24,38,41] 2.00 (1)α2 1500 – – –κ1 κ HVPE {001} 1200 – 4.7–5.0[21,50–55] – 1.96[53] –κ2 MOCVD 600–1100 743 (5) 4.87 (4) 1.85 (1)www.advancedsciencenews.com www.pss-b.comPhys. Status Solidi B 2025, 262, 2400615 2400615 (3 of 8) © 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbH 15213951, 2025, 8, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssb.202400615 by National Institute For, Wiley Online Library on [08/08/2025]. 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 Licensehttp://www.advancedsciencenews.comhttp://www.pss-b.comα2 and κ1 were chosen for photoluminescence (PL) due to theirhigher luminescence. Sample β was suitable for both techniques.3.2. SpectrophotometryUV–vis spectrophotometry was conducted using a ShimadzuUV-2600 spectrophotometer with ISR-2600Plus integratingsphere attachment. The typical illuminated area in the systemwas ≈2� 5mm, and a bespoke sample holder reducing the illu-minated area to a circle with diameter of ≈1.8 mm was fabricatedto analyze sample κ2 to avoid the thickness gradient affecting thetransmittance. We estimated that the transmittance had an abso-lute 0.5% uncertainty, and the wavelength had a 0.3 nm absoluteuncertainty. The transmittance was used to obtain the opticalbandgap Eg, refractive index n, and thickness d of the films.The films, being hundreds of nanometers thick, exhibited fringesin the high transmittance region, which we analyzed using theSwanepoel method.[64] This approach utilized the maxima andminima of fringes to fit two lines creating an envelope withthe upper line known as TM and the lower as Tm to the transmit-tance spectra. The refractive index n was calculated as a functionof TM, Tm, and the refractive index of the substrate. Then with nand λ, the sample thickness d could then be determined where nand λ were adjacent maxima or minima of the fringes; for an in-depth example of the Swanpoel method in practice, we directedthe readers to the work of Sánchez-González et al.[65]The Cauchy equation, Equation (1)[66]n ¼ Aþ Bλ2(1)was used to express a fit of refractive index n against wavelength λwith A and B being constants. The optical bandgap was extractedfrom the linear interpolation of a α2 versus hν plot, as is custom-ary for direct bandgap semiconductors.[67] We noted that Ga2O3was not a direct bandgap semiconductor but its flat valence bandallowed it to be treated as a direct bandgap for this purpose.[68]The absorption coefficient α was obtained from the transmit-tance T only and thickness d of the film using Equation (2).α ¼ 1d� ln1T� �(2)3.3. PLA custom-built PL setup employing a Photon Systems HeAg224.8 nm pulsed laser at a frequency of 20 Hz with a 100 μs pulselength was utilized for this experiment. The sample wasmounted on a cold stage which incorporated a He-based closedloop cryostat covering a temperature range of 20–300 K. Theemitted light from the sample was directed to an Oriel 1/8 mspectrograph with a 400 lines per mm ruled diffraction gratingblazed at 325 nm, and a cooled Andor CCD camera. A 280 nmlong-pass filter was used at the opening aperture of the spectrom-eter to filter out reflected laser light. The spectrum was correctedfor system response using a DH3 CAL Ocean Optics radiomet-rically calibrated UV–vis–(NIR) near-infrared light source. Theacquired spectra were decomposed into Gaussians using theFitYK software package.[69]In both UV–vis and PL, samples were subjected to incidentlight and measured along the growth direction.4. Results and DiscussionThe results from the UV–vis spectrophotometry investigation ofsamples α1, β, and κ2 are summarized in Table 1 and Figure 2.Fringes are clearly visible for all samples in the high transmit-tance region, i.e., for wavelengths greater than ≈300 nm, withthe fringe spacing inversely related to the film thickness. Thepeak transmittance is lower than expected,[19,38,51] this is dueto the usage of single-side polished substrates. As can be seenin Table 1, the film thickness values obtained from theSwanepoel analysis are in excellent agreement with the samplespecifications. In the case of sample κ2, which exhibits a thick-ness gradient, the extracted thickness fits within the range ofFigure 2. a) Transmittance plots, b) α2 versus hν plot, and c) refractiveindex with Cauchy fit for the samples β, α1, and κ2.www.advancedsciencenews.com www.pss-b.comPhys. Status Solidi B 2025, 262, 2400615 2400615 (4 of 8) © 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbH 15213951, 2025, 8, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssb.202400615 by National Institute For, Wiley Online Library on [08/08/2025]. 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 Licensehttp://www.advancedsciencenews.comhttp://www.pss-b.comexpected thicknesses. These results demonstrate the strength ofusing transmittance methods for providing prompt feedback togrowth.Figure 2b shows the α2 vs hν plots for each of the samples,allowing us to obtain the optical bandgap of each of theGa2O3 polymorphs; these values are summarized in Table 1.For β-Ga2O3 with (�201) orientation, we obtain an Eg of4.99� 0.04 eV, which is in agreement with the literature.[5,20–22]Our data confirm that (0001) oriented α-Ga2O3 exhibits the wid-est bandgap among all three studied polymorphs,[5,21,22,38–40]with a value of 5.32� 0.09 eV, which is in the upper regionof reported values.[40] Finally, we obtain an Eg value of4.87� 0.04 eV for {001} oriented κ-Ga2O3, which is within therange of values reported in the literature.[21,50–55] With respectto the literature our data does agree that α-Ga2O3 possessesthe largest Eg value, though we note that our study finds theEg of β-Ga2O3, to be greater than κ-Ga2O3 which is not alwaysthe case. The varying reports of Eg of the β- and κ-phase illustratethe anisotropy of the crystal properties, thus affecting how the Egvalues exists with relation to each other.[5,20–22,43,50–55,63,70,71]Like the majority of papers that have been reviewed, we haveused transmission measurements to obtain the bandgaps,although various analysis methods were employed within thatbody of studies, including α2 versus hν, (αhν)2 versus hν, or directreading from transmittance. Meanwhile, Gucmann et al. haveestimated the Eg by observing the near-band-edge cathodolumi-nescence, and Segura et al. used both ellipsometry and absorp-tion with a Elliott–Toyozawa model; both methods foundagreement on the final result of 5.6 eV, the upper boundary ofreported values.[5,40]The Swanepoel method also allows the refractive index disper-sion to be extracted for all three phases, as illustrated inFigure 2c. All the data points can be well fitted using theCauchy equation. For β-phase, we obtain a refractive index of1.89� 0.01, which is in excellent agreement with the findingsof Rebien et al. and Zolnai et al. both using ellipsometry withphoton energies ranging from 0.74� 1.5 eV.[23,55] We observea higher refractive index for α-Ga2O3 than β-Ga2O3, as predictedtheoretically by He et al.[24] The refractive index for α-Ga2O3 is2.00� 0.01, close to the value from ellipsometry measurementswhich were taken over a range of 1.46–2.88 eV.[38] Finally, therefractive index of κ-phase is the lowest of all investigated phases,with a value of 1.85� 0.01. However, this deviates from the valueof 1.96 obtained Zolnai et al.[55] with a 0.09 discrepancy at a com-parable photon energy of 1.5 eV where nκ2≈ 1.87. While the closematch of the other polymorphs with published reports gives con-fidence in our value, the discrepancy is notable. Perhaps this dif-ference is due to the fact that we employ a different techniquecompared to Zolnai et al. who used ellipsometry, to obtain adielectric value ε which can be converted to a refractive indexn using the relation (Re(ε)≈pn). Examining reports ondielectric values across polymorphs, these values are contradic-tory with reported refractive index results, in that they indicatethe refractive index of κ-Ga2O3 is lower than that of β-Ga2O3,which supports our findings with dielectric values of 9.85 forκ-Ga2O3 and 10.2–12.4 for β-Ga2O3.[72,73]The luminescence properties of samples α2, β, and κ1 werethen investigated by means of temperature-dependent PL overthe 23–300 K range, as illustrated in Figure 3. Since we useabove-bandgap excitation (224.8 nm, i.e., 5.5 eV) and the thickestsamples (minimum thickness ≈750 nm), we can rule out anycontribution from the substrate to the luminescence. To identifythe contributions to the spectrum and enable comparison withthe literature, the low-temperature spectrum of each polymorphwas decomposed into several Gaussian lines—a common prac-tice in existing literature[26,27,30,42,43,46,48,56,74]—and shown in thebottom plot of Figure 3. We have chosen to use four peaks todescribe the fit which is consistent with literature.[30,48,56] Bothβ- and κ-Ga2O3 exhibit peak shift, this has been highlighted witha black dot in Figure 3a,e.For (�201)-oriented β-Ga2O3, in Figure 3a,b, we observe lumi-nescence lines (at 23 K) centered at 2.5, 2.9, 3.3, and 3.6 eV—theincrease of intensity below 2 eV comes from second order diffrac-tion in the spectrometer. Based on the extensive literature on theluminescence of this polymorph, we attribute the peaks at 3.6and 3.3 eV to STH.[27,70,75] The 2.9 and 2.5 eV peaks correspondFigure 3. Temperature-dependent PL spectra of a) β-Ga2O3 (sample β), c) α-Ga2O3 (sample α2), and e) κ-Ga2O3 (sample κ1). Decompositions of the 23 KPL spectrum of b) β-Ga2O3, d) α-Ga2O3, and f ) κ-Ga2O3. For visibility, the peak position of each spectrum is marked with a black dot.www.advancedsciencenews.com www.pss-b.comPhys. Status Solidi B 2025, 262, 2400615 2400615 (5 of 8) © 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbH 15213951, 2025, 8, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssb.202400615 by National Institute For, Wiley Online Library on [08/08/2025]. 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 Licensehttp://www.advancedsciencenews.comhttp://www.pss-b.comto DAP recombination between VO acting as donors[29] and VGaor (VGaþ VO) as acceptors.[75] We observe a steady decrease in PLintensity by a factor of 10 as the temperature increase from 23 to300 K. The PL spectrum intensity weakens with little change inthe spectral shape as the temperature increases and exhibits asmall peak redshift of ≈0.12 eV across the 23–300 K range.In Figure 3c,d, α-Ga2O3 with (0001) orientation exhibits peaks(at 23 K) at 2.6, 3.1, 3.6, and 3.9 eV—the PL signal below 2 eV isan artefact coming from second-order diffraction in the spec-trometer. We can see that as the temperature increases from23 to 300 K, the 3.9 eV line intensity rapidly decreases by a factorof around 100 and the 3.6 eV line decreases by a factor of 10 whilethe other lines remain unchanged. This behavior is in strong con-trast to what we observed for β-Ga2O3. Similar spectra and tem-perature dependence were observed by Nicol et al.[49] and Modaket al.[44] but they differ from observation from Janzen[43]where two distinct peaks can be observed in excitation alongthe a-direction and the maximum intensity being at a lowerenergy in excitation along the c-direction—we note however thatJanzen was investigating the impact of polarization which couldexplain the discrepancies. The 3.9 eV peak is likely a DAP transi-tion line involving Hi as shallow donor and VGa–nH as acceptor,where H impurities are brought from the growth precursors.[42]The second UV peak at 3.6 eV is likely related to STH.[26,45] Thepeak at 3.1 eV could be assigned to shallow DAP transitionsinvolving VO donors and (VGaþVO) acceptors and 2.6 eV to deepDAP transitions involving VO donors and VGa or Oi acceptors.While these attributions are derived by analogy to the2.9 and 2.5 eV lines of β-Ga2O3,[30,75] respectively, we note thatMaruzane et al. conducted cathodoluminescence mapping ofα-Ga2O3 which revealed that these luminescence lines increasein intensity in the vicinity of dislocations (as a result of point defectsegregation), thus supporting a DAP transition interpretation.[46]Finally, the luminescence of {001}-orientated κ-Ga2O3 exhibitspeaks at 2.2, 2.7, 3.2, and 3.7 eV (Figure 3e,f ). We observe that, incomparison to β- and α-Ga2O3, the lower energy componentsexhibit a greater contribution to the luminescence, as has beenobserved in the literature.[56,76] The PL spectrum of κ-Ga2O3gradually weakens with temperature, the 2.7 eV peak decreasesintensity by a factor of 6 and both the 2.2 and 3.2 eV peaks onlydecrease intensity by a factor of 4 while the 3.7 eV peaks exhibitnegligible change. There also appears to be a redshift of the spec-trum of ≈ 0.25 eV as the temperature increases from 23 to 300 K,in agreement with literature.[43,53,55,76] The 3.7 eV peak can betentatively attributed to H-related defects (e.g., VGa–nH deepacceptors which facilitate recombination with electrons fromeither the conduction band or VO–H, a hydrogenated oxygenvacancy) as shown by Mazzolini et al.[53] Since samples α2 andκ1 were both grown by HVPE, i.e., employing HCl gas, it is per-haps not surprising if both samples exhibit a H-related lumines-cence line. The 3.2 eV peak has been ascribed to STH,[56]additionally the 2.7 eV peak agrees with that seen in literaturecaused by hydrogenated deep acceptors (VGa–2H).[53] In κ-Ga2O3studies both the 3.7 and 2.7 eV peaks have been assigned byanalogy to β-Ga2O3, but this attribution has been backed upby theoretical calculations.[53] Finally, the 2.2 eV peak has notbeen reported in literature, but it has been observed in otherpolymorphs.[75]5. ConclusionThe optical properties of β-, α-, and κ-Ga2O3 polymorphs havebeen investigated. A systematic approach allowed us to highlightphase-specific optical properties. Analysis of the transmittancespectra revealed refractive indices of 1.89� 0.01 (β),2.00� 0.01 (α), and 1.85� 0.01 (κ), and optical bandgaps of4.99� 0.04 eV (β), 5.32� 0.09 eV (α), and 4.87� 0.04 eV (κ).Temperature-dependent PL revealed that the emission spectrumof the polymorphs and their dependence with temperature differsignificantly from each other with peaks at 2.5, 2.9, 3.3, and3.6 eV (β-Ga2O3); 2.6, 3.1, 3.6, and 3.9 eV (α-Ga2O3); and 2.2,2.7, 3.2, and 3.7 eV (κ-Ga2O3). Our findings emphasize thatdeducing the optical properties of Ga2O3 polymorphs by analogyto the well-documented β-Ga2O3 could lead to misinterpretationssuch as an inaccurate defect characterization being made andthen propagated in the literature and underline the necessityfor detailed phase-specific investigations.AcknowledgementsThe authors acknowledge support from the Engineering and PhysicalSciences Research Council (grant nos. EP/W524670/1 and EP/V034995/1)and from the Rank Prize Undergrad Vacation scholarship. MB and RFacknowledge the financial support from PNRR-M4C2-I1.1—Funded bythe European Union-NextGenerationEU; Italian Ministry of Universityand Research Call for proposals n.104 of 02-02-2022-PRIN2022-ERC sectorPE3 Project title: UV-C Sensors based on GalliumOxide (USE GAO)-ProjectCode 2022A4AN2F-CUP CodeD53D23002180006 (grant decree no.2022A4AN2F), dated 28/09/2023. The authors would also like to thankIndraneel Sanyal, Andrew Pakes, and Stuart Semple for their contributionsto this work.Conflict of InterestThe authors declare no conflicts of interest.Data Availability StatementThe data that support the findings of this study are openly available fromthe University of Strathclyde KnowledgeBase at https://doi.org/10.15129/61b028bc-643b-42ea-b55c-4f86b5ba56ca.Keywordsgallium oxides, photoluminescences, polymorphs, semiconductors,transmittances, wide-bandgapsReceived: November 15, 2024Revised: January 24, 2025Published online: February 12, 2025[1] S. J. Pearton, J. Yang, P. H. Cary IV, F. Ren, J. Kim, M. J. Tadjer,M. A. Mastro, Appl. Phys. Rev. 2018, 5, 011301.[2] Y. Oshima, K. Kawara, T. Oshima, T. Shinohe, Jpn. J. Appl. Phys. 2020,59, 115501.[3] I. Cora, F. Mezzadri, F. Boschi, M. Bosi, M. Čaplovičová, G. Calestani,I. Dódony, B. Pécz, R. Fornari, CrystEngComm 2017, 19, 1509.www.advancedsciencenews.com www.pss-b.comPhys. 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Shimamura, E. G. E. G. Víllora, T. Ujiie, K. Aoki, Appl. Phys. Lett.2008, 92, 201914.[75] Q. D. Ho, T. Frauenheim, P. Deák, Phys. Rev. B 2018, 97,115163.[76] T. Hidouri, A. Parisini, S. Dadgostar, J. Jimenez, R. Fornari, J. Phys. D:Appl. Phys. 2022, 55, 295103.www.advancedsciencenews.com www.pss-b.comPhys. Status Solidi B 2025, 262, 2400615 2400615 (8 of 8) © 2025 The Author(s). physica status solidi (b) basic solid state physicspublished by Wiley-VCH GmbH 15213951, 2025, 8, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssb.202400615 by National Institute For, Wiley Online Library on [08/08/2025]. 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 Licensehttp://www.advancedsciencenews.comhttp://www.pss-b.com Comparative Study of the Optical Properties of &alpha;-, &beta;-, and &kappa;-Ga2O3 1. Introduction 2. Background 2.1. &beta;-Ga2O3 2.2. &alpha;-Ga2O3 2.3. &kappa;-Ga2O3 3. Experimental Section 3.1. Samples 3.2. Spectrophotometry 3.3. PL 4. Results and Discussion 5. Conclusion