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Phys. 118, 085301 (2015) and may be found at https://doi.org/10.1063/1.4929417.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [Epitaxial growth of phase-pure e-Ga2O3 by halide vapor phase epitaxy](https://mdr.nims.go.jp/datasets/872cc017-ad58-4159-b762-21f77d8f0488) ## Fulltext Epitaxial growth of phase-pure ε-Ga2O3 by halide vapor phase epitaxyViewOnlineExportCitationCrossMarkRESEARCH ARTICLE | AUGUST 24 2015Epitaxial growth of phase-pure ε-Ga2O3 by halide vaporphase epitaxy Yuichi Oshima; Encarnación G. Víllora; Yoshitaka Matsushita; Satoshi Yamamoto; Kiyoshi ShimamuraJ. Appl. Phys. 118, 085301 (2015)https://doi.org/10.1063/1.4929417 30 January 2024 06:28:16https://pubs.aip.org/aip/jap/article/118/8/085301/141798/Epitaxial-growth-of-phase-pure-Ga2O3-by-halidehttps://pubs.aip.org/aip/jap/article/118/8/085301/141798/Epitaxial-growth-of-phase-pure-Ga2O3-by-halide?pdfCoverIconEvent=citehttps://pubs.aip.org/aip/jap/article/118/8/085301/141798/Epitaxial-growth-of-phase-pure-Ga2O3-by-halide?pdfCoverIconEvent=crossmarkjavascript:;javascript:;javascript:;javascript:;javascript:;javascript:;https://doi.org/10.1063/1.4929417https://servedbyadbutler.com/redirect.spark?MID=176720&plid=2288747&setID=592934&channelID=0&CID=840257&banID=521619172&PID=0&textadID=0&tc=1&scheduleID=2208988&adSize=1640x440&data_keys=%7B%22%22%3A%22%22%7D&matches=%5B%22inurl%3A%5C%2Fjap%22%5D&mt=1706596096011293&spr=1&referrer=http%3A%2F%2Fpubs.aip.org%2Faip%2Fjap%2Farticle-pdf%2Fdoi%2F10.1063%2F1.4929417%2F13023421%2F085301_1_online.pdf&hc=cb0a25e816d535a5af8aa392f72551cbe723ff23&location=Epitaxial growth of phase-pure e-Ga2O3 by halide vapor phase epitaxyYuichi Oshima,1,a) Encarnaci�on G. V�ıllora,1 Yoshitaka Matsushita,2 Satoshi Yamamoto,3and Kiyoshi Shimamura11Optical Single Crystals Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan2Materials Analysis Station, National Institute for Materials Science, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan3Optoelectronic Materials Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan(Received 18 July 2015; accepted 11 August 2015; published online 24 August 2015)Epitaxial growth of e-Ga2O3 is demonstrated for the first time. The e-Ga2O3 films are grown onGaN (0001), AlN (0001), and b-Ga2O3 (�201) by halide vapor phase epitaxy at 550 �C usinggallium chloride and O2 as precursors. X-ray x-2h and pole figure measurements prove that phase-pure e-Ga2O3 (0001) films are epitaxially grown on the three kinds of substrates, although someminor misoriented domains are observed. High temperature X-ray diffraction measurements revealthat the e-Ga2O3 is thermally stable up to approximately 700 �C. The optical bandgap of e-Ga2O3 isdetermined for the first time to be 4.9 eV. VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4929417]I. INTRODUCTIONGa2O3 has been reported to possess five different crystalstructures, namely, a-, b-, d-, e-, and c-phase.1 Among them,b-Ga2O3 crystallizes in the monoclinic structure (spacegroup C2/m) and is believed to be thermodynamically themost stable under atmospheric pressure. b-Ga2O3 has abandgap energy as large as 4.7–4.9 eV, and therefore, itexhibits a wide transparency even in the UV region.2–4 Inaddition, b-Ga2O3 is a semiconductor, with Si and Sn as themost efficient donors.5,6 Furthermore, high-quality singlecrystalline wafers can be produced through melt growthtechniques.7–9 These unique features make this material asuitable and promising wide bandgap semiconductor fordiverse applications such as conductive transparent sub-strates for GaN-based high-performance LEDs,10,11 andsolar-blind UV sensors.12 Above and beyond these, b-Ga2O3is also attracting a remarkable attention due to its greatpotential to realize power devices with higher breakdownvoltages and lower energy losses than its counterparts GaNand SiC.13–15In contrast to b-Ga2O3, there are only a limited numberof reports on e-Ga2O3, the target material of this work.e-Ga2O3 is one of the meta-stable phases of Ga2O3, whichwas first synthesized by Roy et al. through the thermaldecomposition of Ga(NO3)3.1 Although they indicated a highthermal stability of their e-Ga2O3 up to 870 �C, the full struc-tural characterization was not made, and the crystal structureremained unclear for over half a century. Recently, Playfordet al. carried out the structural analysis of their powder mate-rial synthesized through the thermal decomposition ofGa(NO3)3�9H2O, and finally identified the crystal structureof e-Ga2O3, belonging to the high symmetry hexagonal sys-tem with space group P63mc (PDF# 01–082-3196),16 likehexagonal GaN. In any case, the samples prepared by bothauthors suffered from low phase-purity. Even the best mate-rial of Playford et al. was still a mixture of e-Ga2O3 (57%)and b-Ga2O3 (43%).As described above, the synthesis of e-Ga2O3 has so farbeen limited to powdered materials with a lack of phase-purity, so that, till date, even very fundamental properties,such as the bandgap energy, are still unknown. However, thee-phase is expected to have a sufficient thermal stability forpractical device applications, and therefore, e-Ga2O3 couldbe a wide bandgap semiconductor as promising as b-Ga2O3.In order to clarify the material properties, first of all, asynthesis technique which enables the effective growth ofe-Ga2O3 with high phase-purity and low impurity concentra-tions has to be established. On top of that, it is necessary todevelop an epitaxial growth technique to deposit this mate-rial for semiconductor device applications.In this work, we employed halide vapor phase epitaxy(HVPE) as the growth method of e-Ga2O3. HVPE is a type ofchemical vapor deposition (CVD), which has been widelyused to grow high-quality epilayers with large growth rates inthe III-V compound semiconductor industry.17,18 Recently,HVPE has been successfully applied for the growth ofb-Ga2O319–21 and even a-Ga2O3.22 The present work is thefirst report on the epitaxial growth of e-Ga2O3 layers.II. EXPERIMENTAL METHODThe HVPE growth was carried out in a horizontalatmospheric reactor at 550 �C using gallium chloride and O2(>99.99995% pure) as precursors. The gallium chloride wassynthesized in situ upstream in the reactor through the chem-ical reaction between Ga (>99.99999% pure) and HCl(>99.999% pure). The HCl and O2 were supplied with par-tial pressures of 0.25 kPa and 1.0 kPa, respectively. N2(>99.9999% pure) was flown together with the precursors asa)Electronic mail: OSHIMA.Yuichi@nims.go.jp0021-8979/2015/118(8)/085301/5/$30.00 VC 2015 AIP Publishing LLC118, 085301-1JOURNAL OF APPLIED PHYSICS 118, 085301 (2015) 30 January 2024 06:28:16http://dx.doi.org/10.1063/1.4929417http://dx.doi.org/10.1063/1.4929417http://dx.doi.org/10.1063/1.4929417http://dx.doi.org/10.1063/1.4929417http://dx.doi.org/10.1063/1.4929417mailto:OSHIMA.Yuichi@nims.go.jphttp://crossmark.crossref.org/dialog/?doi=10.1063/1.4929417&domain=pdf&date_stamp=2015-08-24the carrier gas. The growth times were 2 min or 7 min. Threedifferent substrates were tried, namely, c-plane GaN sub-strates, c-plane AlN(13-lm-thick)/SiC templates, and (�201)b-Ga2O3 substrates.The surface and the cross-section of the films wereobserved by scanning electron microscopy (SEM). The crys-tal structure and the orientation were investigated by X-raydiffraction (XRD) x-2h scan and pole-figure measurements.The structural quality was estimated by X-ray rocking curve(XRC) measurements. To clarify the thermal stability of thee-Ga2O3, high-temperature X-ray diffraction (HT-XRD)studies were carried out. The sample was set on a Pt holderand its temperature was elevated from room temperature upto 1300 �C stepwise, with 30 min intervals for temperaturestabilization. The XRD patterns were recorded under air inthe standard x-2h scan mode. Impurity concentrations wereevaluated by secondary mass spectrometry (SIMS). The opti-cal bandgap was determined by means of a transmittancemeasurement.III. RESULTS AND DISCUSSIONSEM images of the surface and the cross-section of thefilms grown on GaN (0001) for 2 min and 7 min are shown inFigs. 1(a)–1(d), respectively. Smooth compact layers weresuccessfully grown, although some three-dimensional (3D)grains can be observed on the surface. The growth rate of thefilm was estimated to be approximately 20 lm/h from Fig.1(d). The crystal structure and the orientation of the 3D-grains are unclear at present. As can be seen in Fig. 1(d),these are originated at the interface with the substrate; thus,their surface density was independent of the growth time.Furthermore, as the 3D-grains grew laterally and verticallyfaster than the epilayer, the grain-size increased continu-ously, becoming more protruding with the growth time.Therefore, it is necessary to improve the growth conditionsat the early growth stage, during the nucleation, in order tosuppress the formation of the 3D-grains.Figures 2 and 3 show top-view SEM images of the filmsdeposited on AlN (0001) and b-Ga2O3 (�201), respectively.The morphologies were similar to that of the film grown onGaN (0001); however, the 3D-grain density on b-Ga2O3(�201) was significantly higher. In the following, only theresults on the 7-min-grown samples (approximately 2.3 lmthick) are described, unless otherwise specified.Figure 4 shows the XRD x-2h scan profiles of theHVPE-grown films on the three kinds of substrates. In all ofthese cases, apart from the diffraction peaks correspondingto the substrates, only the diffraction peaks from e-Ga2O3(0001) appeared, and no other polymorphs of Ga2O3 weredetected.The pole figures of the e-Ga2O3 film and the correspond-ing GaN substrate are shown in Figs. 5(a) and 5(b), respec-tively. Diffraction spots of e-Ga2O3 10�14 appeared only atthe positions expected for single crystalline e-Ga2O3. The3D-grains observed by SEM in Figs. 1(a) and 1(c) may bemisoriented domains of e-Ga2O3, but no corresponding peakswere detected probably due to the small volume fraction.FIG. 1. SEM images of e-Ga2O3 layers grown on GaN (0001) with differentgrowth times: (a) and (b) 2 min; and (c) and (d) 7 min.FIG. 2. SEM images of e-Ga2O3 layers grown on AlN (0001) with differentgrowth times: (a) 2 min and (b) 7 min.FIG. 3. SEM images of e-Ga2O3 layers grown on b-Ga2O3 (�201) with differ-ent growth times: (a) 2 min and (b) 7 min.FIG. 4. XRD x-2h scan profiles of e-Ga2O3 layers grown on (a) GaN(0001), (b) AlN (0001), and (c) b-Ga2O3 (�201).085301-2 Oshima et al. J. Appl. Phys. 118, 085301 (2015) 30 January 2024 06:28:16The comparison of the peak positions in Figs. 5(a) and 5(b)revealed the following epitaxial relationships: e-Ga2O3(0001)jjGaN (0001) and e-Ga2O3 [10�10]jjGaN [10�10].Analogously, the pole figures of the e-Ga2O3 film and thecorresponding AlN (0001) template were similar (Figs. 6(a)and 6(b)), and the epitaxial relationships were elucidated tobe equally e-Ga2O3 (0001)jjAlN (0001) and e-Ga2O3[10�10]jjAlN [10�10]. The in-plane lattice mismatches are 8.8and 6.6% with GaN (0001) and AlN(0001), respectively.Figures 7(a) and 7(b) show the pole figures of thee-Ga2O3 film and the corresponding b-Ga2O3 (�201) sub-strate, respectively. In contrast with the case on the GaN andAlN substrates, additional very small diffraction peaks ofe-Ga2O3 were detected probably due to the larger volumefraction of the 3D-grains. The epitaxial relationshipsbetween the c-plane e-Ga2O3 and the substrate were deter-mined to be e-Ga2O3 (0001)jjb-Ga2O3 (�201) and e-Ga2O3[10�10]jjb-Ga2O3 [102]. This result is in good accordancewith the observed growth of hexagonal GaN on the samesubstrate.23 Further, the in-plane atomic arrangement of Gaor O in both e-Ga2O3 (0001) and b-Ga2O3 (�201) are virtuallytriangular lattices, and the mismatch is as small as 1.1%.It is worth mentioning that the crystal structure ofGa2O3 has been reported to suit that of the substrate flexibly.For example, corundum-structured a-Ga2O3 was grown on c-plane sapphire under the same growth conditions as in thepresent work.22 It is also reported that cubic c-Ga2O3 wasgrown on spinel (100) (the both materials belong to the samespace group Fd�3m) by mist-CVD, while a-Ga2O3 was grownon c-plane sapphire under the similar growth condition.24Therefore, it seems natural that e-Ga2O3 can grow on GaNand AlN, since all belong to the same space group P63mc.However, at first sight, it is surprising that e-Ga2O3 grows“heteroepitaxially” on b-Ga2O3, which is believed to be themost stable polymorph of Ga2O3. It should be noted that theuse of similar growth conditions, except for the highergrowth temperatures around 1050 �C, leads to the growth ofthe b-phase.21 Further investigations including the surfacestructure of b-Ga2O3 (�201) at the atomic level in the growthatmosphere of HVPE will be needed to clarify the mecha-nism of this unusual phenomenon.The XRC profiles of the e-Ga2O3 0004 and 10�11 diffrac-tions, which were measured in symmetric and skew-symmetric geometries, respectively, are shown in Fig. 8. TheFWHMs of 0004 and 10�11 diffractions reflect the tilting ofc-plane and the twisting around the c-axis, respectively. TheFWHMs of e-Ga2O3 grown on GaN and AlN were similar,FIG. 5. X-ray pole figures (log-scale) of (a) e-Ga2O3 10�14 and (b) GaN10�12.FIG. 6. X-ray pole figures (log-scale) of (a) e-Ga2O3 10�14 and (b) AlN10�13.FIG. 7. X-ray pole figures (log-scale) of (a) e-Ga2O3 10�14 and (b) b-Ga2O3002. Note that the b-Ga2O3�202 peak also appears because the (001) and(�101) planes have almost the same spacing and thus the same Bragg angle.FIG. 8. XRCs of e-Ga2O3 layers grown on (a) GaN (0001), (b) AlN (0001),and (c) b-Ga2O3 (�201).085301-3 Oshima et al. J. Appl. Phys. 118, 085301 (2015) 30 January 2024 06:28:16while those of e-Ga2O3 deposited on b-Ga2O3 were muchsmaller. This tendency agrees well with that of the in-planelattice mismatch. Note that the 0004 XRC profile of thee-Ga2O3 grown on b-Ga2O3 (�201) was measured around therocking axis of b-Ga2O3 [010], and the FWHM was not sig-nificantly different from that measured around the perpendic-ular rocking axis of b-Ga2O3 [102].Figure 9 shows the result of the HT-XRD measurementsof the e-Ga2O3 grown on GaN. Apart from the substrate andPt holder diffractions, the spectra are dominated by the dif-fraction peak of e-Ga2O3 up to 700 �C. With the furtherincrease of temperature, the b-Ga2O3 401 peak appears andits intensity increases with the temperature up to around800 �C. Inversely, the intensity of the e-Ga2O3 0004 diffrac-tion peak starts to decrease above 700 �C, and virtually dis-appears around 800 �C. Thus, the HVPE-grown e-Ga2O3 isfound to be thermally stable up to around 700 �C, and ittransforms into b-Ga2O3 at higher temperatures. Thereported transition temperature from e-Ga2O3 to b-Ga2O3varies depending on the authors. Roy et al. and Playfordet al. reported the temperature to be 870 �C (Ref. 1) andabove 500 �C,16 respectively. Their samples were synthe-sized by the thermal decomposition of gallium nitrate, andtherefore, the impurities and their concentrations are prob-ably different from those in HVPE-grown e-Ga2O3. Such dif-ference could be the origin of the different transitiontemperatures, since impurities sometimes affect the crystalstructure of Ga2O3. For example, Hayashi et al. demon-strated the growth of Mn-doped c-Ga2O3 (7 at. %) on c-planesapphire by pulsed laser deposition, while their un-dopedsample crystalized in the b-Ga2O3 phase.25Table I summarizes the impurity concentrations in e-Ga2O3 grown on GaN measured by SIMS. [C], [N], [Si],[Al], [Cr], [Fe], and [Ni] were below the detection limits.[H] and [Cl] were relatively higher than those in a-Ga2O3grown under the same growth conditions in the same HVPEreactor ([H]< 4� 1017, [Cl]¼ 7 � 1016).22 Although theinfluence of H or Cl impurities on the electrical properties ofe-Ga2O3 has not clarified yet, it is worth mentioning thatMurakami et al. has also reported the incorporation of Cl im-purity in HVPE-grown b-Ga2O3, and that this did not act as adonor.20The transmittance spectrum of e-Ga2O3 grown onAlN(0001)/SiC template for 2 min is shown in Fig. 10. Priorto the measurement, the SiC substrate was removed by lap-ping and polishing in order to avoid the absorption cutoff ofthis. Although the transition type of e-Ga2O3 is stillunknown, we estimated the bandgap energy to be 4.9 eVfrom the (h�a)2–h� plot (inset of Fig. 10), in which the linerfitting is much better than that in (h�a)1/2–h� plot (notshown). This result indicates that the bandgap energy is simi-lar to that of b-Ga2O3, and therefore, opto-electrical proper-ties in general might be comparable for both phases.IV. SUMMARYIn conclusion, the present investigation demonstrates forthe first time the successful epitaxial growth of phase-puree-Ga2O3. The epilayers were deposited by the HVPE method,achieving a growth rate as high as 20 lm/h. It was found thatthe use of c-plane GaN and AlN substrates, whose spacegroup is the same as that of e-Ga2O3, is one of the key pointsto deposit e-Ga2O3 epitaxial layers. The second critical param-eter was the deposition temperature, so that e-Ga2O3 could begrown heteroepitaxially even on b-Ga2O3 (�201) substrates. ItFIG. 9. HT-XRD of e-Ga2O3 grown on GaN (0001).TABLE I. Impurity concentrations in e-Ga2O3 measured by SIMS.ElementPossibleoriginDetection limit(D. L.) (cm�3)Concentration(cm�3)H HCl 4� 1017 1� 1018C SiC, graphite 6� 1016