Growth of Ga2o3 Nanowires Via Cu-As-Ga Ternary Phase Diagram

crystals

Article Growth of Ga2O3 Nanowires via Cu-As-Ga Ternary Phase Diagram

Hang Wang 1,2,†, Ying Wang 2,3,†, Shuyan Gong 2, Xinyuan Zhou 2, Zaixing Yang 4, Jun Yang 1,5,*, Ning Han 2,3,* and Yunfa Chen 2,3,*

1 School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China; [email protected] 2 State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; [email protected] (Y.W.); [email protected] (S.G.); [email protected] (X.Z.) 3 Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China 4 Center of Nanoelectronics and School of Microelectronics, Shandong University, Jinan 250100, China; [email protected] 5 Metallurgical Engineering and Technology Research Center of Shaanxi Province, Xi’an 710055, China * Correspondence: [email protected] (J.Y.); [email protected] (N.H.); [email protected] (Y.C.); Tel.: +86-13152420820 (J.Y.); +86-010-6255-8356 (N.H.) † These authors contributed equally to this work.

Received: 13 February 2019; Accepted: 12 March 2019; Published: 15 March 2019

Abstract: Currently, it is challenging to develop new catalysts for semiconductor nanowires (NWs) growth in a complementary-metal-oxide-semiconductor (CMOS) compatible manner via a vapor-liquid-solid (VLS) mechanism. In this study, chemically synthesized Cu2O nano cubes are adopted as the catalyst for single crystalline β-Ga2O3 NWs growth in chemical vapor deposition. The growth temperature is optimized to be 750 to 800 ◦C. The NW diameter is controlled by tuning the sizes of Cu2O cubes in the 20 to 100 nm range with a bandgap of ~4.85 eV as measured by ultraviolet-visible absorption spectroscopy. More importantly, the catalyst tip is found to be Cu5As2, which is distinguished from those Au-catalyzed Au-Ga alloys. After a comprehensive phase diagram investigation, the β-Ga2O3 NWs are proposed to be grown by the ternary phase of Cu-As-Ga diffusing Ga into the growth frontier of the NW, where Ga react with residual oxygen to form the NWs. Afterward, Ga diminishes after growth since Ga would be the smallest component in the ternary alloy. All these results show the importance of the catalyst choice for CMOS compatible NW growth and also the potency of the ternary phase catalyst growth mode in other semiconductor NWs synthesis.

Keywords: Ga2O3 nanowires; Cu2O cubes; Cu5As2; Cu-As-Ga ternary phase diagram; chemical vapor deposition

1. Introduction

In recent years, Ga2O3 nanowires (NWs) with a wide bandage of ~4.9 eV have attracted greater attention because of the unique structural, electronic, mechanical, and optical properties, such as having a Debye length comparable to the small size and tuned band gap based on size restriction, which is applicable to various electronic and optical nano-devices [1–8]. Previously, numerous methods were employed to grow Ga2O3 NWs including pulsed laser deposition (PLD), molecular beam epitaxy (MBE), and thermal evaporation, via the well-established vapor-liquid-solid (VLS) mechanism [7,9–18]. Typically, Au nanoparticles are widely adopted as the catalyst for the NWs

Crystals 2019, 9, 155; doi:10.3390/cryst9030155 www.mdpi.com/journal/crystals Crystals 2019, 9, 155 2 of 11 growth, which behaves as a precursor pyrolysis catalyst and a nucleation center in numerous NWs growth including Ga2O3, GaAs, Si, and other semiconductors [17,19–24]. However, Au might diffuse into the silicon substrate, forming foreign impurities and deep level dopants and, thus, degenerate the electronic properties [25,26]. Therefore, it is necessary to adopt substitute metal catalysts for the complementary-metal-oxide-semiconductor (CMOS) compatibility [21,27,28]. In past research studies, to solve this problem, self-catalyzed growth methods and CMOS compatible metal catalyst induced growth technologies that have been developed for the semiconductor NWs growth [15,16,28]. However, complicated substrate treatment would be desired in order to obtain a diameter uniform NW growth because NW diameters would vary greatly if there is no foreign catalyst in the self-catalyzed growth [15]. In addition, many scholars have tried new catalysts such as Ag and Co materials to obtain Ga2O3 NWs [29,30]. However, thermally annealing the metal ﬁlms, which are physically deposited by thermal and/or e-beam evaporation that mainly form the catalyst particles. In the meantime, Cu is a good CMOS compatible metal, and has been adopted for the Si NW growth [31–34]. However, whether Cu or potential cuprous oxide plays the role in the catalytic growth process is deliberated. For instance, Vincent et al. fabricated Si NWs using copper-based catalysts and ﬁnding the original seed layer that is absolutely oxidized to Cu2O at the optimum oxygen pressure by comparing different oxygen pressures for producing straight NWs [27]. Thus, progress in designing more appropriate Ga2O3 NWs agreeing with CMOS requires a clear understanding of the nature of the Cu catalyst and a controllable tunability of the catalyst. In this study, chemically-synthesized Cu2O nanoparticles are adopted as catalysts for β-Ga2O3 NWs growth on Si/SiO2 substrates by the chemical vapor deposition method using GaAs powders as the source. The optimum growth temperature is 750–800 ◦C and the NW diameter is successfully tuned by regulating different Cu2O particle sizes in the range of 20 to 100 nm. The synthesized β-Ga2O3 NWs are single crystalline with a bandgap of ~4.85 eV (256 nm) as obtained by ultraviolet-visible absorption spectroscopy. The catalyst tip is found to be Cu5As2 alloy, without any Ga component within the resolution of the energy dispersive spectroscopy. This is different from the Au-Ga alloy catalyst in Au-catalyzed growth, which is further studied by analyzing the Cu-As-Ga ternary phase diagram to explore the growth mechanism.

2. Materials and Methods

In this study, the Ga2O3 NWs are synthesized in a solid-source chemical vapor deposition (SSCVD) system utilizing GaAs powders as the source material (0.5 g, 99.999% purity), and chemically synthesized Cu2O as the catalyst. Cu2O was fabricated with a controlled cube morphology and diameter between 25 to 177 nm, as shown in the supporting information (Figure S1). Then, Cu2O nano cube catalysts were dispersed on the surface of the <100>-oriented Si/SiO2 (50 nm thick thermally grown oxide) substrates. A horizontal tube furnace was used as the reactor with the GaAs source placed in the middle of the heating zone and the catalyst in the downstream (3 cm away from the source zone) of the zone with a tilt angle of about 20◦. Prior to the growth of NWs, the substrates were cleaned with ultrasonic bath in deionized water, ultrapure water, and ethanol for 5 min, and dried by the nitrogen blow. In the process of growth, the reacting system was evacuated to about 10 mtorr and purged by Ar (99.999% pure) gas at 100 standard cubic centimeter per min (sccm) for 10 min. The substrate and the source were heated to the preset growth temperature (700 ◦C to 900 ◦C) synchronously. The growth time was 90 min and a constant ﬂow rate of Ar gas at 100 sccm was maintained in the quartz tube during the growth process. Lastly, the system was cooled down to room temperature and the white Ga2O3 NWs were harvested. The surface morphologies of the grown Ga2O3 NWs were examined clearly with a scanning electron microscope (SEM, JEOL JSM-6700F, Japan, 15 kV, 10 mA). High-resolution transmission electron microscopy (HRTEM) images and corresponding energy dispersive spectroscopy (EDS) of NWs were obtained on a JEOL JEM-2100F microscope with an accelerating voltage of 200 kV. The crystallinity of grown Ga2O3 NWs was investigated by employing an X-ray diffractometer with Crystals 2019, 9, 155 3 of 11

Cu Kα radiation (1.5406 Å) operated at 40 kV and 40 mA in the diffraction angle (2θ) range from 20◦ to 70◦. The ultraviolet-visible absorption spectrum was recorded on a LAMBDA 750 spectrophotometer (Perkin Elmer, Waltham, MA, USA) at room temperature.

3. ResultsCrystals and2019, Discussion9, x FOR PEER REVIEW 3 of 11

In70°. order The ultraviolet-visible to study the effect absorption of temperature spectrum was on therecorded growth on a of LAMBDA Ga2O3 NWs750 spectrophotometer catalyzed by Cu 2O, (Perkin Elmer, Waltham, MA, USA) at room temperature. 25 nm Cu2O cubes were adopted as catalysts to find the optimal growth conditions. Figure1a–e show the SEM images of Ga2O3 NWs grown at various temperatures and insets are corresponding 3. Results and Discussion cross-sectional SEM images illustrating the length of the NWs. To further quantify the influence of the temperatureIn order onto study the NW the growtheffect of temperature rate, corresponding on the growth NW of length Ga2O3 and NWs growth catalyzed rate by are Cu2 plottedO, 25 in Figurenm1f. Cu Through2O cubes comparativewere adopted observations,as catalysts to find we canthe optimal see NWs growth grow conditions. sparsely atFigure 700 1a–e◦C with show a low the SEM images of Ga2O3 NWs grown at various temperatures and insets are corresponding cross- growth rate. This can be interpreted by limited gallium source vaporized and deposited because of low sectional SEM images illustrating the length of the NWs. To further quantify the influence of the temperature, which leads to insufficient super-saturation of the catalyst droplets on the downstream temperature on the NW growth rate, corresponding NW length and growth rate are plotted in Figure ◦ substrates.1f. Through From comparative growth speed observations, of NWs and we corresponding can see NWs grow illustrations, sparsely at we 700 conclude °C with thata low 750 growth to 800 C is therate. promising This can temperature be interpreted for NWby limited growth, galliu whichm source possesses vaporized a good and NW deposited uniformity because and straightness. of low ◦ Manytemperature, twists and which bends leads occur to insufficient in NWs at super-satura 850 C in Figuretion of the1d andcatalyst NWs droplets become on the shorter downstream and more disorderedsubstrates. as the From temperature growth speed keeps of NWs rising and untilcorrespondi 900 ◦C,ng whichillustrations, is shown we conclude in Figure that1e. 750 Ultimately, to 800 original°C is morphology the promising disappears temperature when for replacedNW growth, by thewhich nano-cone possesses structure a good NW when uniformity the temperature and reachesstraightness. 900 ◦C. This Many is becausetwists and the bend super-saturations occur in NWs ofat droplets850 °C in decreasesFigure 1d and with NWs the become increase shorter of growth temperature,and more which disordered slows as down the temperature the nucleation keeps and rising reduces until the900 growth°C, which rate, is shown according in Figure to the 1e. Gibbs Ultimately, original morphology disappears when replaced by the nano-cone structure when the Thomson model in Equation (1) [28,35]. temperature reaches 900 °C. This is because the super-saturation of droplets decreases with the increase of growth temperature, √ which! slows2 do√wn the nucleation√ and!2 reduces the growth rate, b∆µ b∆µ b4Ωa according to the Gibbs Thomsonv = model in Equation= (1) [28,36].0 − vs (1) kT kT kTd √𝑏∆𝜇 √𝑏∆𝜇 √𝑏4𝛺𝑎 v (1) 𝑘𝑇 𝑘𝑇 𝑘𝑇𝑑 where k is Boltzmann’s constant, b is a kinetic coefficient of crystallization, ∆µ0 is the super-saturation in thewhere planar k is limit, Boltzmann’sΩ is the constant, atomic b volumeis a kinetic of coefficien the growtht of crystallization, species, avs is ∆𝜇 the is average the super-saturation surface energy densityin the ofthe planar NW limit, surface 𝛺 is facets, the atomic and Tvolume is the temperature.of the growth Thespecies, given 𝑎 explanation is the average for surface the phenomenon energy coulddensity be that of the the growth NW surface rate is inverselyfacets, and proportional T is the temperature. to growth temperatureThe given explanation affecting thefor lengththe of NWsphenomenon (L = vt). Furthermore, could be that the the rolegrowth of sourcerate is inversely gas pressure, proportional though to usually growth temperature not considered affecting a pivotal factor,the is actuallylength of crucial NWs ( inLvt determining). Furthermore, NWs synthesisthe role of [ 36source]. However, gas pressure, the density though of usually the NWs not is not considered a pivotal factor, is actually crucial in determining NWs synthesis [37]. However, the perfect. It would be decided by the Cu2O catalyst nanoparticles used since they are grown by the VLS density of the NWs is not perfect. It would be decided by the Cu2O catalyst nanoparticles used since mechanism, which might also be tuned by the catalyst density. A low growth rate might be due to the they are grown by the VLS mechanism, which might also be tuned by the catalyst density. A low new ternarygrowth rate phase might diagram be due andto the also new the ternary low growth phase diagram temperature. and also the low growth temperature.

◦ ◦ FigureFigure 1. SEM 1. SEM surface surface images images of of Ga Ga22OO33 NWs grown grown at atvarious various temperatures. temperatures. (a) 700 (a °C,) 700 (b) 750C, (°C,b) 750(c) C, (c) 800800◦C, °C, (d ()d 850) 850◦ C,°C,( e(e)) 900900 ◦°C,C, and ( ff)) relationship relationship between between the the length length and and growth growth rate rateof NWs of NWs and and growthgrowth temperature. temperature. Inset Inset of (ofa– e(a)– aree) are magniﬁed magnified cross-sectional cross-sectional SEMSEM images illustrating illustrating the the length length of the NWs.of the From NWs. NWsFrom growthNWs grow speed,th speed, 750 to750 800 to 800◦C °C is aispromising a promising temperature. temperature.

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2 FiveFive sizessizes of of Cu Cu2OO particlesparticles werewere synthesized synthesized byby adjusting adjusting NaOH NaOH concentration concentration (supporting (supporting 2 information,information, Table Table S1) S1) [ 37[38].]. Based Based on on the the TEM TEM images, images, the the mean mean lengths lengths of of Cu Cu2OO cube cube edges edges and and the the NWNW average average diameters diameters can can be be determined determined by by the the statistics statistics of of 50 50 individual individual catalyst catalyst cubes cubes and and NWs NWs 2 synthesizedsynthesized by by different different sizes sizes of of Cu Cu2OO as as aa catalystcatalyst (supporting(supporting information, information, Table Table S2). S2). Figure Figure2 2shows shows 2 3 2 thethe relationship relationship between between the the Ga Ga2OO3 NWNW diameterdiameter andand thethe CuCu2OO cubecube sizesize obtainedobtained atat the the optimum optimum growthgrowth temperature temperature ofof 750 750◦ °C.C. ItIt is is observed observed that that the the diameters diameters of of NWs NWs strongly strongly depend depend upon upon the the 2 sizesize of of the the Cu Cu2OO catalystcatalyst withinwithin 2020 toto 100100 nm.nm. WhenWhen thethe sizesize ofof thethe catalystcatalyst isis overover 100100 nm, nm, a a huge huge diversitydiversity emergesemerges betweenbetween thethe diameterdiameter ofof the the catalyst catalyst and and synthesized synthesized NWs. NWs. Specifically,Specifically,a a large large fluctuationfluctuation in in catalyst catalyst size size led led to to the the growth growth of NWsof NWs with with low low uniformity, uniformity, while while a relatively a relatively good good size uniformitysize uniformity of the of catalyst the catalyst resulted resulted in the in production the production of highly of highly uniform uniform NWs, NWs, which which was was proven proven by statisticsby statistics of standard of standard deviations deviations of NWs of NWs diameter diamet presenteder presented in the redin the cross. red This cross. phenomenon This phenomenon can be explainedcan be explained by the critical by the diameter critical diameter of the VLS of mechanism the VLS mechanism because a highbecause supersaturation a high supersaturation is necessary is innecessary the VLS mechanism,in the VLS mechanism, as shown in as Equation shown in (1), Equation which is(1), inversely which is related inversely with related a catalyst with diameter.a catalyst Indiameter. fact, a larger In fact, size a means larger smallersize means supersaturation smaller supersaturation (Gibbs Thomson (Gibbs effect) Thomson for effective effect) NWs for effective growth byNWs VLS, growth which by can VLS, be exhibited which can by Equationbe exhibited (1). by This Equation can also (1). be inferredThis can from also Figurebe inferred1b that from thin Figure NWs can1b that grow thin longer NWs than can thickergrow longer ones. Inthan addition, thicker whenones. In the addition, diameter when is larger the diameter than its critical is larger diameter, than its NWscritical would diameter, possibly NWs grow would via possibly the vapor grow solid via (VS) the mechanism. vapor solid For(VS) example, mechanism. Choi For et al.example, found thatChoi theet al. growth found of that NWs the follows growth the of VS NWs mechanism follows the when VS the mechanism catalyst size when is larger the catalyst than 65 size nm is catalyzed larger than by sputtered65 nm catalyzed thin films by [sputtered38–42]. thin films [39–43].

Figure 2. Relationship between the NW diameter grown at 750 ◦C and average size of the catalyst. Figure 2. Relationship between the NW diameter grown at 750 °C and average size of the catalyst.

For the further structural and chemical characterization, the Ga2O3 NWs were then investigated For the further structural and chemical characterization, the Ga2O3 NWs were then investigated using TEM, as shown in Figure3a, where a typical Ga 2O3 NW is observed with a smooth surface andusing a hemispherical TEM, as shown tip. in InsetFigure is 3a, corresponding where a typical EDS Ga mapping2O3 NW ofis theobserved NW, from withwhich a smooth we cansurface see and Ga anda hemispherical O are uniformly tip. Inset distributed is corresponding in the NWs EDS body, ma andpping As of and the Cu NW, are from concentrated which we in can the see catalytic Ga and head.O are Theuniformly typical distributed elemental in composition the NWs body, and and the typicalAs and spectraCu are concentrated of the tip and in bodythe catalytic is analyzed head. byThe EDS, typical which elemental are illustrated composition in Figure and 3theb,c. typical More spectra speciﬁcally, of the the tip element and body line is scananalyzed also clearlyby EDS, showswhich theare distributionillustrated in ofFigure the four 3b and elements Figure in3c. Figure More 3specifically,d. These results the elemen evidentlyt line scan show also that clearly the shows the distribution of the four elements in Figure 3d. These results evidently show that the obtained NWs are Ga2O3 grown via the VLS mechanism with the possible catalytic tips of the Cu-As alloy.obtained It should NWs are also Ga be2O emphasized3 grown via the that VLS no mechanism As-related signalwith the is obtainedpossible catalytic in the NW tips body of the within Cu-As alloy. It should also be emphasized that no As-related signal is obtained in the NW body within the the detection limit of EDS performed in this study. This would be attributed to the fact that Ga2O3 hasdetection a far lower limit of Gibbs EDS freeperformed energy in (approximately this study. This− 998.3would kJ/mol) be attributed thanGaAs to the (fact−67.8 that kJ/mol) Ga2O3 has [43]. a Therefore,far lower itGibbs is more free preferentially energy (approximately grown from −998.3 the thermal kJ/mol) dynamics than GaAs point (−67.8 of view.kJ/mol) In [44]. the meantime, Therefore, GaAsit is more decomposes preferentially at a higher grown temperature from the therma than 650l dynamics◦C and, thus, point As of would view. be In hard the meantime, to bind with GaAs Ga. Indecomposes addition, the at introductiona higher temperature of As requires than a 650 high-energy °C and, thus, ion implantation As would be method. hard to The bind experimental with Ga. In conditionsaddition, the are introduction extremely harsh of As and requires it is impossible a high-energy to introduce ion implantation As doping method. by the conventional The experimental CVD methodconditions [44, 45are]. extremely In addition, harsh it is notedand it thatis impossible GaAs particles to introduce are adopted As doping as the sourceby the insteadconventional of metallic CVD gallium,method since[45,46]. GaAs In particlesaddition, canit is evaporate noted that Ga GaAs to give particles the precursor are adopted in a relatively as the lowsource temperature instead of ofmetallic 750 to 800gallium,◦C. On since the GaAs contrary, particles if metallic can evaporate Ga is used, Ga it willto give form the a liquidprecursor metal in anda relatively a very high low temperaturetemperature (e.g., of 750 1000 to 800◦C, °C. [46 ]),On which the contrary, is needed if metallic to evaporate Ga is enough used, it Ga will precursors. form a liquid metal and a very high temperature (e.g., 1000 °C, [47]), which is needed to evaporate enough Ga precursors.

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FigureFigure 3.3.Elemental Elemental distributiondistribution ofof thethe NWs.NWs. ((aa)) TEM TEM image image of of a a typical typical Ga Ga2O2O33 NWNW andand insetinset isis EDSEDS mappingmapping ofof thethe NW,NW, ( b(b)) EDS EDS spectrum spectrum of of the the tip,tip, ((cc)) EDSEDS spectrumspectrum ofof thethe NWNW body,body, and and ( (dd)) EDSEDS lineline scanscan alongalong thisthis NW.NW. The The curves curves indicate indicate the the distribution distribution of of Cu, Cu, As, As, Ga, Ga, and andO O elementselements alongalong thethe axialaxial direction of the NW. It can be clearly seen that the wire is Ga O and the catalyst tip is the Cu-As alloy. direction of the NW. It can be clearly seen that the wire is Ga22O33 and the catalyst tip is the Cu-As alloy.

To investigate the crystal structure of the NWs, XRD patterns are obtained, as shown in Figure4a, To investigate the crystal structure of the NWs, XRD patterns are obtained, as shown in Figure where a structure and phase purity of β-Ga2O3 is conﬁrmed with the diffraction pattern in good 4a, where a structure and phase purity of β-Ga2O3 is confirmed with the diffraction pattern in good agreement with the monoclinic β-Ga2O3 structure. Figure4b shows the HRTEM and corresponding agreement with the monoclinic β-Ga2O3 structure. Figure 4b shows the HRTEM and corresponding fast Fourier transformation (FFT) images of the NWs, which indicates a single crystalline structure. fast Fourier transformation (FFT) images of the NWs, which indicates a single crystalline structure. LatticeLattice fringesfringes ofof (1(11111),), ( (221111),), and and ( 3(302)02) planes planes are are marked marked in in the the images,images, whichwhich correspondcorrespond wellwell withwith the β-Ga2O3 phase. In the meantime, the growth direction of the NW can be indexed to be [111] by the β-Ga2O3 phase. In the meantime, the growth direction of the NW can be indexed to be [111] by thethe FFTFFT inin thethe inset.inset. OnOn thethe otherother hand,hand, thoughthough similarsimilar fringesfringes ofof (111),(111), ((102),02), andand ((22111)1) planesplanes areare indexed in Figure4c, which veriﬁes the β-Ga2O3 phase, the growth orientation is different from [102], indexed in Figure 4c, which verifies the β-Ga2O3 phase, the growth orientation is different from [102], asas indexedindexed byby thethe selectedselected areaarea electronelectron diffractiondiffraction(SAED) (SAED)pattern. pattern. Non-uniformNon-uniform growthgrowth directionsdirections underunder oneone growthgrowth condition are also also reported reported in in the the literature. literature. For For exam example,ple, [111], [111], [001], [001], [130] [130] and and so soon on [13,48–50], [13,47–49 ],which which might might be be a aresult result of of the the th thermodynamicsermodynamics favoring favoring the the lowest lowest surface energy. TheThe HRTEMHRTEM imagesimages ofof thethe toptop regionregion isis shownshown inin FigureFigure4 d4d and and a a spacing spacing of of about about 0.289 0.289 nm nm is is measured, which corresponds to the distance between (010) planes of the Cu5As2 phase (PDF No. measured, which corresponds to the distance between (010) planes of the Cu5As2 phase (PDF No. 00- 00-013-0581).013-0581). In Inaddition, addition, the the FFT FFT of of the the catalyst catalyst tip tip in in the the inset inset image image also also show show similar similar (010) and (121)(121) planes,planes, whichwhich infersinfers the possible catalytic catalytic tip tip of of the the Cu-As Cu-As alloy. alloy. We We analyzed analyzed the the crystal crystal structure structure of ofseveral several other other obtained obtained catalyst catalyst tips tips and and found found that that their their crystal crystal plane plane spacings spacings were were also about also about 0.289 0.289nm, as nm, shown as shown in Figure in Figure S2a–c. S2a–c. By consulting By consulting the alloys the alloysformed formed by Cu byand Cu As, and we As,found we that, found among that, among the possible phases, including Cu5As2, Cu3As, Cu6As, Cu9As, CuAs2 and Cu2As, the 0.289 nm the possible phases, including Cu5As2, Cu3As, Cu6As, Cu9As, CuAs2 and Cu2As, the 0.289 nm crystal crystal plane spacing only exists in orthorhombic Cu5As2. This is also in agreement with the atomic plane spacing only exists in orthorhombic Cu5As2. This is also in agreement with the atomic ratio of ratioCu: As of Cu:of 2.3 As (69.43%:30.57%) of 2.3 (69.43%:30.57%) in the EDS in the data EDS in dataFigure in S2d. Figure S2d.

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Figure 4. Crystal structure characterizations of the NWs. (a) XRD patterns of the NWs and the Si/SiO2 substrate, (b) HRTEM image of NW grown along the ⟨111⟩ direction, according to FFT images in the Figureinset 4.forCrystal one of structure a representative characterizations Ga2O3 NWs, of the ( NWs.c) another (a) XRD NW patterns grown ofalong the NWsthe ⟨1 and02⟩ the direction Si/SiO 2as 5 2 substrate,identifiedFigure ( b4. by) Crystal HRTEM the SAED structure image pattern of characterizations NWin the grown inset, and along of (d the the) HRTEM NWs. <111> ( imagea direction,) XRD and patterns FFT according (inset) of the to ofNWs FFTthe andCu imagesAs the alloySi/SiO in 2 thetips. insetsubstrate, for one (b) ofHRTEM a representative image of NW Ga2 Ogrown3 NWs, along (c) anotherthe ⟨111 NW⟩ direction, grown according along the to <1 FFT02> images direction in the

as identiﬁedinset for byone the ofSAED a representative pattern in theGa inset,2O3 NWs, and ((dc) HRTEManother NW image grown and FFT along (inset) the of⟨102 the⟩ Cudirection5As2 as To explore the bandgap information of the Ga2O3 NWs, ultraviolet-visible absorption spectrum alloyidentified tips. by the SAED pattern in the inset, and (d) HRTEM image and FFT (inset) of the Cu5As2 alloy is obtained,tips. which is shown in Figure 5. Tauc’s equation was normally adopted, and the bandgap is calculatedTo explore from the the bandgap plot of information (αhv)2 vs of hv. the It Ga clearly2O3 NWs, displays ultraviolet-visible that the band absorption gaps edge spectrum lies at is obtained, which is shown in Figure5. Tauc’s equation was normally adopted, and the bandgap is approximatelyTo explore 4.85 the eV bandgap (256 nm) information of the β-Ga 2ofO 3the NWs Ga grown2O3 NWs, at 750ultraviolet-visible °C, according to absorption extrapolated spectrum line 2 calculatedcutsis obtained,of the from abscissa thewhich plot axis. is of shown (Thisαhv) is in vsin Figure hv.good It clearly agreement5. Tauc’s displays equation with that research thewas band normally studies gaps [51]. edgeadopted, lies at and approximately the bandgap is β ◦ 4.85calculated eV (256 nm) from of thethe plot-Ga 2Oof3 NWs(αhv)2 grown vs hv. at It 750 clearlyC, according displays tothat extrapolated the band linegaps cuts edge of thelies at abscissa axis. This is in good agreement with research studies [50]. approximately 4.85 eV (256 nm) of the β-Ga2O3 NWs grown at 750 °C, according to extrapolated line cuts of the abscissa axis. This is in good agreement with research studies [51].

Figure 5. Ultraviolet-visible absorption spectrum of the β-Ga2O3 NWs. Figure 5. Ultraviolet-visible absorption spectrum of the β-Ga2O3 NWs. Lastly, we analyzed the formation mechanism of β-Ga2O3 NWs catalyzed by Cu2O, as schematically shown in Figure6. During the experiment, we quickly increased the furnace ◦ temperature to 750 CFigure in 20 min, 5. Ultraviolet-visible when GaAs starts absorption to evaporate spectrum to provideof the β-Ga the2O Ga3 NWs. and As precursors

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CrystalsLastly,2019, 9 ,we 155 analyzed the formation mechanism of β-Ga2O3 NWs catalyzed by Cu2O,7 of as 11 schematically shown in Figure 6. During the experiment, we quickly increased the furnace temperature to 750 °C in 20 min, when GaAs starts to evaporate to provide the Ga and As precursors to the the growth growth substrate. In In addition, addition, the the Cu Cu2OO cubic cubic nanoparticles nanoparticles start start to to decompose decompose into into Cu Cu droplets droplets and then react with Ga and As to formform CuCuxxAsyGaGazz ternaryternary alloy, alloy, according according to to the the VLS VLS mechanism. mechanism. The saturation of the alloy droplets continues to increase, and then Ga choose the most favorable direction to start reacting with residual oxygen to initiate NW growth and generate an interface with a clear boundary asas thethe NWNW growth growth front. front. In In a a stable stable growth growth environment environment of of 90 90 min, min, more more Ga Ga and and As Asvolatilize volatilize and and transfer transfer to beto absorbedbe absorbed on on the the catalyst catalyst drop drop by by maintaining maintaining a concentration a concentration gradient gradient of ofthe the component component of the of liquid the liquid alloy, whichalloy, results which in re asults small in super-saturation a small super-saturation leading the incorporation leading the incorporationof new mater atof thenew drop-NW mater at interfacethe drop-NW to further interface extend to further the NWs extend [51–53 the]. NWs [52–54].

Figure 6. Schematic illustration of thethe ββ-Ga-Ga22O3 NWsNWs growth process. The Cu 2OO nano cube catalysts firstﬁrst decomposes to form Cu droplets, whichwhich thenthen absorbabsorb GaGa andand AsAs precursorsprecursors totoform forma aCu CuxxAsAsyyGazz catalyst alloy. TheThe GaGa precursorsprecursors thenthen diffuse diffuse through through the the alloy alloy and and then then react react with with residual residual oxygen oxygen to toform form the the NWs. NWs. Above Above the the picture picture is the is the Cu-As-Ga Cu-As-Ga ternary ternary alloy alloy phase phase diagram. diagram. The The red, red, green, green, and blueand bluecurves curves represent represent the alloythe alloy wire wi lines,re lines, and and the the dotted dotted lines lines are are the the temperature temperature curves. curves. TheThe eutectic

point can be clearly seen in the diagram to form the Cu5As2 alloyalloy phase. phase.

In order to explain the cause of formation of the catalyst head and the mechanism of phase transformation, liquidusliquidus projection projection of of the the Cu–As–Ga Cu–As– systemGa system is shown is shown in Figure in Figure6[ 54–56 6]. [55–57]. Three vertices Three verticesof a triangle of a triangle represent represent three pure three components pure componen of Cu,ts of As, Cu, Ga, As, respectively, Ga, respectively, and threeand three sides sides of the of theequilateral equilateral triangle, triangle, which which indicate indicate the alloysthe allo formedys formed by theby correspondingthe corresponding two two elements. elements. The The red, red,green, green, and blueand linesblue lines are Ga-Cu, are Ga-Cu, Cu5As Cu2, and5As2, Cu and3As Cu binary3As binary alloy equilibriumalloy equilibrium lines. Fromlines. theFrom phase the ◦ phasediagram, diagram, ternary ternary eutectic eutectic point point temperature temperature is under is under 700 700C and, °C and, at this at this ternary ternary eutectic eutectic point point ‘o’, ‘o’,the the contents contents of three of three components components are Cu:are 73%,Cu: 73%, As: 25%,As: 25%, and Ga:and 2%,Ga: which2%, which is in is good in good agreement agreement with withour experiment our experiment results results of Cu: 69.43%of Cu: and69.43% As: and 30.57% As: (atomic 30.57% ratio). (atomic The ratio). incorporation The incorporation and deposition and depositionof Ga would of followGa would the redfollow line the in thered phaseline in diagram, the phase and diagram, the content and the of Gacontent is decreasing of Ga is decreasing from ‘a’ to ‘o.’from Lastly, ‘a’ to at‘o.’ the Lastly, end ofat thethe growthend of the process, growth Ga proc speciesess, Ga in the species catalyst in the alloy catalyst droplets alloy were droplets consumed were consumedin the formation in the offormation Ga2O3 NWs, of Ga while2O3 NWs, the remaining while the Curemaining and all theCu Asand species all the were As species solidiﬁed were to solidifiedthe Cu5As 2tophase. the Cu To5As conclude,2 phase. continuous To conclude, Ga atoms continuous ﬂow into Ga the atoms CuxAs flowyGa zintosophisticated the CuxAs phaseyGaz sophisticatedand increase thephase super-saturation and increase ofthe the super-saturation alloy. Therefore, of it the pushes alloy. NWs Therefore, interfacial it layerpushes growth, NWs interfacialwhich proceeds layer untilgrowth, the temperaturewhich proceeds is naturally until cooledthe temperature to room temperature. is naturally The cooled reaction to canroom be temperature.described as EquationsThe reaction (2)–(4) can be [19 described]. as Equations (2)–(4) [19].

Ga(l) + O2(g) → Ga2O3(s) (2) Ga(l) + O2(g) → Ga2O3(s) (2)

Ga(l) + Ga2O3(s) → Ga2O(g) (3) Ga(l) + Ga2O3(s) → Ga2O(g) (3)

Crystals 2019, 9, 155 8 of 11

Ga2O(g) + O2(g) → Ga2O3(s) (4)

Therefore, VLS growth of Ga2O3 NWs is actually dominated by the phase equilibrium of the Cu-As-Ga alloy catalyst. In addition, all those results show the importance and potency of the catalyst choice for semiconductor NWs growth through a ternary phase diagram.

4. Conclusions

Single crystalline β-Ga2O3 NWs are grown by the solid-source chemical vapor deposition method ◦ at 750 to 800 C based on chemically synthesized Cu2O catalytic nanoparticles scattered on the Si/SiO2 substrate. The NWs diameter is dependent on the Cu2O nanotube size in the range of 20 to 100 nm. The NWs growth orientation is varied to be a mixture of [111] and [102] and the NWs bandgap is ~4.85 eV. The catalyst tip after growth is found to be orthorhombic Cu5As2, which infers a VLS growth mode of the NWs where Ga is diffused from Cu-As-Ga ternary alloy to the growth front and then is reacted with the residual oxygen. All these results indicate that the successful synthesis of a large-bandgap Ga2O3 NWs material based on chemically assisted incubation of CMOS compatible catalysts, which is also promising for other semiconductor NWs synthesis.

Supplementary Materials: TEM and diameter distribution of Cu2O nanoparticle catalyst and corresponding NW. This material is available free of charge via the Internet at http://www.mdpi.com/2073-4352/9/3/155/s1. Figure S1: The broad views of several different sizes Cu2O cubes synthesized by chemical methods. Table S1: Synthesis conditions of Cu2O using ascorbic acid as a reductive agent. Table S2: Catalyst size, NW diameter, and corresponding standard deviation. Figure S2a–c: HRTEM of catalyst heads of different Ga2O3 NWs and d is the TEM image and elemental atomic ratio of the tip. Author Contributions: The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Conceptualization, H.W. Data curation, H.W., Y.W., and X.Z. Formal analysis, H.W. Investigation, H.W. and Y.W. Methodology, H.W., Y.W., and S.G. Project administration, N.H. and Y.C. Resources, H.W., Y.W., and Z.Y. Software, S.G. and X.Z. Validation, J.Y. Visualization, J.Y. Writing—original draft, H.W. and Y.W. Writing—review & editing, J.Y. and N.H. Funding: The National Natural Science Foundation of China (51602314 and 61504151), National Key R&D Program of China (2017YFA0305500), and the CAS-CSIRO project of the Bureau of International Co-operation of Chinese Academy of Sciences (122111KYSB20150064) financially supported this research. Conflicts of Interest: The authors declare no conflict of interest.

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