Characterization of homoepitaxial β-Ga2O3 films preparedby metal–organic chemical vapor deposition

Xuejian Du a, Wei Mi a, Caina Luan a, Zhao Li a, Changtai Xia b, Jin Ma a,n

a School of Physics, Shandong University, Jinan 250100, PR Chinab Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

a r t i c l e i n f o

Article history:Received 5 May 2014Accepted 6 July 2014Communicated by: D.P. NortonAvailable online 12 July 2014

Keywords:A1. X-ray diffractionA3. Metal–organic chemical vapordepositionB1. OxidesB2. Semiconducting materials

a b s t r a c t

β-Ga2O3 films have been homoepitaxially deposited on β-Ga2O3 (1 0 0) substrates by metal organicchemical vapor deposition (MOCVD) method. The influences of different growth temperatures on thestructure, Raman and optical properties of the homoepitaxial films have been studied. The structure ofthe obtained films is monoclinic β phase gallium oxide and the film deposited at 650 1C exhibits the bestcrystalline quality. The average transmittance of the samples in the visible and UV wavelength range isabout 80%. The optical band gap of the films deposited at 600, 650 and 700 1C are about 4.72, 4.73 and4.68 eV, respectively.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Wide band gap transparent oxide semiconductors (TOSs) suchas In2O3, SnO2 and ZnO are gaining much attention due to theirwide applications like light-emitting diodes, laser diodes, ultra-violet detectors, transparent thin film transistors and solar cells[1–5]. With recent technological development, the trend of optoe-lectronics research has turned to shorter wavelength, and nowpeople focus on the deep ultraviolet (DUV) region [6]. However,the conventional materials can hardly meet the requirement.Therefore, new materials need to be investigated.

β-Ga2O3 is a promising candidate material due to its largedirect band gap (Eg�4.9 eV) [7] compared with the other materialsmentioned above. β-Ga2O3 material has monoclinic structure withthe lattice parameters of a¼12.23 Å, b¼3.04 Å, c¼5.80 Å, α¼901,β¼103.71 and γ¼901(JCPDS 43-1012). β-Ga2O3 is highly transpar-ent in the visible and deep-UV region [8], and especially thetransmittance in UV region is about 80%. Besides, there are muchmore outstanding properties like high mechanical strength, highthermal and chemical stability, etc.

β-Ga2O3 thin films could be prepared by numerous methodssuch as physical evaporation [9], arc-discharge [10], carbothermalreduction [11], RF magnetron sputtering [12] and so on.However, almost all the β-Ga2O3 thin films were hetero growth.The structure and properties of β-Ga2O3 thin films were always

inferior because of large lattice mismatch between the films andsubstrates by these deposition processes. Few studies abouthomoepitaxial β-Ga2O3 films were reported, especially by meansof MOCVD. In this paper, the fabrication and characterizations ofhomoepitaxial β-Ga2O3 films deposited on β-Ga2O3 (1 0 0) sub-strate at different substrate temperatures by MOCVD were inves-tigated in detail.

2. Experimental details

The homoepitaxial Ga2O3 films were grown on β-Ga2O3 (1 0 0)substrates (double-face polishing, thickness: 0.5 mm) by using ahigh vacuum MOCVD system. Trimethylgallium (TMGa) and O2

(purity, 5 N) were used as precursors. High purity N2 (purity, 9 N)was used as the carrier gas. The TMGa was stored in a stainlesssteel bubbler maintained at �14.5 1C and was transported into thereactor by N2 with a flow rate of 2 sccm. O2 was injected into thereactor as the oxidant through a separated delivery line with aflow rate of 50 sccm. During the deposition, the growth pressurewas kept at 20 Torr and the growth temperatures were fixed at600, 650 and 700 1C, respectively.

The structural properties of the films were determined by X-raydiffraction (XRD, Bruker D8 Advance X-ray diffractometer with CuKα1 radiation) and Raman spectroscopy (LabRAM HR800).In-plane Φ-scans were performed by using a Bede D1 HR-XRD.The chemical composition of the prepared films was measured bythe X-ray photoelectron spectroscopy (XPS) on the ESCALAB MK IIMulti-Technique Electron Spectrometer. A SUPRATM 55 scanning

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Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.07.0110022-0248/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ86 531 88361057; fax: þ86 531 88564886.E-mail address: sdqddxj12@126.com (J. Ma).

Journal of Crystal Growth 404 (2014) 75–79

electron microscope (SEM) was used to obtain the thickness of theprepared films. In order to observe the surface morphologies, thenanoscope A-multimode atomic force microscope (AFM) was used.The optical transmittance spectra were measured by means ofShimadzu TV-1900 double-beam UV–vis–NIR spectrophotometerin the wavelength ranging from 200 to 800 nm.

3. Results and discussion

Fig. 1 shows the XRD θ–2θ scans of the Ga2O3 substrate and thehomoepitaxial films deposited at different temperatures. It can beseen that, the peaks corresponding to Ga2O3 (4 0 0) and (6 0 0)planes appear in the substrate and the epitaxial films deposited atdifferent substrate temperatures. This means all the films weregrowing with the orientation of the β-Ga2O3 substrate (1 0 0)plane. The full width at half maximum (FWHM) of the Ga2O3

(4 0 0) peak corresponding to the substrate and the epitaxial filmsdeposited at 600, 650 and 700 1C are 0.12, 0.15, 0.12 and 0.151,respectively. The results indicate that the structure of the epitaxialfilms is affected by the growth temperature and the film preparedat 650 1C exhibits the best crystalline quality.

Fig. 2 displays the XRD off-specular Ф-scans of the Ga2O3

(7 1 0) planes reflection (ψ¼291) for the substrate and theepitaxial film prepared at 650 1C. It should be known that theslant angle (ψ) is the angle between the reflection plane andsubstrate surface plane. Only one peak corresponding to Ga2O3

(7 1 0) plane was detected both in curve (a) and (b) in the wholerange of 0–3601, which means the films were grown homoepi-taxially without twinning structures. The FWHM of the substrateand sample are 0.311 and 0.561, respectively, demonstrating thatthe crystalline quality of the film is a little inferior to the substrate.

A cross-sectional SEM image of the homoepitaxial β-Ga2O3

films deposited at 650 1C is shown in Fig. 3(a). The thickness of theprepared film is estimated about 160 nm from the SEM image.The surface AFM images of the Ga2O3 substrate and the filmdeposited at 650 1C are shown in Fig. 4(b) and (c). In Fig. 4(b), thesurface of the substrate is very smooth and the RMS roughness isonly about 0.58 nm. Fig. 4(c) clearly shows a surface with uniform

grains and well-defined boundaries which indicates superiorcrystallization. The RMS roughness of the epitaxial Ga2O3 filmsgrowth at 650 1C is about 4.87 nm, significantly greater than thatof substrate, which confirms that the epitaxial films have beensuccessfully grown on the Ga2O3 substrate.

Fig. 4 shows the Raman spectra of the Ga2O3 films deposited at600–700 1C and the substrate as comparison. β-Ga2O3 has amonoclinic structure and belongs to the C2h space group. 15Raman and 12 infrared (IR) active modes should be existing inthe samples based on the group theory [13]. Ten sharp peakslocated at 147, 172, 202, 323, 349, 419, 478, 633, 662, 770 cm�1

were observed which is in good agreement with the resultsreported previously [10]. The strongest peak in low-frequencylocated at 202 cm�1 is the characteristic vibration and translationmode of Ga–O chains [14]. It can be seen that, the intensity of thispeak in curve (b) is the strongest and its FWHM is the narrowestcompared with curve (c) and (d), which indicates the best crystal-line quality. Besides, the Raman peaks of the sample deposited at650 1C in mid-frequency and high-frequency match with those ofthe substrate better than other samples. In a word, the β-Ga2O3

films prepared at 650 1C exhibit the best crystalline quality andshow broad potential application prospects.

To identify the composition of the films, the X-ray photoelec-tron spectroscopy was performed. Fig. 5 shows the XPS spectra ofthe sample prepared at 650 1C. Fig. 5(a) displays a full scan in theenergy ranging from 0 to 1200 eV. The peaks corresponding toO1s, Ga2p and Ga3d are observed. In Fig. 5(b), the gallium corelevels Ga(2p1/2) and Ga(2p3/2) observed at 1145 eV and 1118 eV,respectively, with a peak-to-peak separation of 27 eV confirm theformation of Ga2O3 [15]. Fig. 5(c) presents the O1s peak located at530.9 eV, which can be divided into two separate peak located at530.8 and 532.2 eV by using the Gaussian Fitting method. Thebinding energy of O1s core level is about 531 eV [16]. The key peaklocated at 530.8 eV can be assigned to the Ga–O bonding and theother weak peak at 532.2 eV owing to the C/O or OH� adsorbedspecies on the surface [17,18]. From the XPS spectra, the ratio of Gaand O is estimated to be about 1:1.52; the concentration of O is alittle higher than normal chemical composition possibly due to theexistence of surface adsorbed oxygen. Therefore, the XPS resultsconfirm that the film grown on the substrate is the expectingGa2O3 layer.

Fig. 1. XRD θ–2θ scans of Ga2O3 substrate and the epitaxial films deposited atdifferent substrate temperatures.

Fig. 2. XRD off-specular Φ-scans of the Ga2O3 substrate and the epitaxial filmprepared at 650 1C.

X. Du et al. / Journal of Crystal Growth 404 (2014) 75–7976

The transmittance spectra as a function of wavelength in therange of 200–800 nm are shown in Fig. 6. The average transmit-tances in the visible range are about 80%, 77%, 75% and 77%

corresponding to the samples deposited at 600, 650, 700 1C andthe Ga2O3 substrate, respectively. For the direct transition semi-conductors, the absorption coefficient (α) and optical band gap(Eg) are related by αhν¼A(hν�Eg)1/2 [19]. A is a constant related tothe material and hν is the energy of the incident photon. The insetshows the plot of (αhν)2 as a function of photon energy hν. Theoptical band gap can be obtained by extrapolating the tangent lineof this plot to the energy axis. The Eg of the samples deposited at600, 650, 700 1C and the Ga2O3 substrate are 4.72 eV, 4.73 eV,4.68 eV, 4.74 eV, respectively. The transmittance spectrum of thesample deposited at 650 1C reveals a cliffy absorption edge and itis similar to that of the substrate, which indicates the bestcrystalline quality of the homoepitaxial β-Ga2O3 films. Theseresults demonstrate that the obtained Ga2O3 films have greatcrystalline quality and excellent transparence in the visible and UVregion.

4. Conclusion

Homoepitaxial β-Ga2O3 films have been deposited on β-Ga2O3

(1 0 0) substrate by MOCVD. The Ga2O3 epitaxial film grown at650 1C exhibits the best crystallinity with the monoclinic struc-ture. The epitaxial orientation of the obtained films is alongβ-Ga2O3 (1 0 0) direction and is consistent with the direction ofthe substrate. No twinning structures existed inside the film. Theaverage transmittance of the Ga2O3 films in the visible and UVrange is about 80%. The optical band gap of the samples depositedat 600, 650 and 700 1C were about 4.72, 4.73 and 4.68 eV,respectively. Growth of high-quality single crystalline β-Ga2O3

Fig. 3. Cross-sectional SEM image of the sample deposited at 650 1C (a), AFM micrographs (5 μm�5 μm) for the Ga2O3 substrate (b) and the sample deposited at 650 1C (c).

Fig. 4. Raman spectra for the Ga2O3 substrate (a) and films deposited at (b)600 1C,(c)650 1C and (d)700 1C.

X. Du et al. / Journal of Crystal Growth 404 (2014) 75–79 77

epitaxial films will lay a good foundation for the doped-Ga2O3

films and it can be widely applied in many fields such as deep UVphoto detectors, short wavelength light-emitting devices andtransparent optoelectronic devices.

Acknowledgments

This work is financially supported by the National NaturalScience Foundation of China (Grant no. 51072102) and the Scienceand Technology Commission of Shanghai Municipality (no.13111103700).

References

[1] V. Scherer, C. Janowitz, A. Krapf, et al., Transport and angular resolvedphotoemission measurements of the electronic properties of In2O3 bulk singlecrystals, Appl. Phys. Lett. 100 (21) (2012) 212108.

[2] C. Li, D. Zhang, S. Han, et al., Diameter-controlled growth of single-crystallineIn2O3 nanowires and their electronic properties, Adv. Mater. 15 (2) (2003)143–146.

[3] Z. Liu, D. Zhang, S. Han, et al., Laser ablation synthesis and electron transportstudies of tin oxide nanowires, Adv. Mater. 15 (20) (2003) 1754–1757.

[4] M.H. Huang, S. Mao, H. Feick, et al., Room-temperature ultraviolet nanowirenanolasers, Science 292 (5523) (2001) 1897–1899.

[5] A. Tsukazaki, A. Ohtomo, T. Onuma, et al., Repeated temperature modulationepitaxy for p-type doping and light-emitting diode based on ZnO, Nat. Mater.4 (1) (2005) 42–46.

[6] N. Ueda, H. Hosono, R. Waseda, et al., Synthesis and control of conductivity ofultraviolet transmitting β-Ga2O3 single crystals, Appl. Phys. Lett. 70 (26) (1997)3561–3563.

[7] K. Matsuzaki, H. Hiramatsu, K. Nomura, et al., Growth, structure and carriertransport properties of Ga2O3 epitaxial film examined for transparent field-effect transistor, Thin Solid Films 496 (1) (2006) 37–41.

[8] N. Suzuki, S. Ohira, M. Tanaka, et al., Fabrication and characterization oftransparent conductive Sn-doped β-Ga2O3 single crystal, Phys. Status Solidi C4 (7) (2007) 2310–2313.

Fig. 5. XPS spectra obtained from the sample grown at 650 1C, full scan (a) and precision scan for Ga2p (b) and O1s (c).

Fig. 6. Optical transmittance spectra of the homoepitaxial Ga2O3 samples andsubstrate, with the plot of (αhν)2 vs. hν for the films in the inset.

X. Du et al. / Journal of Crystal Growth 404 (2014) 75–7978

[9] H.Z. Zhang, Y.C. Kong, Y.Z. Wang, et al., Ga2O3 nanowires prepared by physicalevaporation, Solid State Commun. 109 (11) (1999) 677–682.

[10] Y.C. Choi, W.S. Kim, Y.S. Park, et al., Catalytic growth of β-Ga2O3 nanowires byarc discharge, Adv. Mater. 12 (10) (2000) 746–750.

[11] X.C. Wu, W.H. Song, W.D. Huang, et al., Crystalline gallium oxide nanowires:intensive blue light emitters, Chem. Phys. Lett. 328 (1) (2000) 5–9.

[12] K. Takakura, D. Koga, H. Ohyama, et al., Evaluation of the crystalline quality ofβ-Ga2O3 films by optical absorption measurements, Physica B 404 (2009)4854–4857.

[13] R. Rao, A.M. Rao, B. Xu, et al., Blueshifted Raman scattering and its correlationwith the [1 1 0] growth direction in gallium oxide nanowires, J. Appl. Phys. 98(9) (2005) 094312–094312-5.

[14] D. Dohy, G. Lucazeau, A. Revcolevschi, Raman spectra and valence force field ofsingle-crystalline β-Ga2O3, J. Solid State Chem. 45 (2) (1982) 180–192.

[15] Q. Xu, S. Zhang, Fabrication and photoluminescence of β-Ga2O3 nanorods,Superlattices Microstruct. 44 (6) (2008) 715–720.

[16] L. Fu, Y. Liu, P. Hu, et al., Ga2O3 nanoribbons: synthesis, characterization, andelectronic properties, Chem. Mater. 15 (22) (2003) 4287–4291.

[17] J.L. Hueso, J.P. Espinós, A. Caballero, et al., XPS investigation of the reaction ofcarbon with NO, O2, N2 and H2O plasmas, Carbon 45 (1) (2007) 89–96.

[18] J. Domaradzki, D. Kaczmarek, A. Borkowska, et al., Influence of annealing onthe structure and stoichiometry of europium-doped titanium dioxide thinfilms, Vacuum 82 (10) (2008) 1007–1012.

[19] H. Gomez, A. Maldonado, M. Olvera, et al., Gallium-doped ZnO thin filmsdeposited by chemical spray, Sol. Energy Mater. Sol. Cells 87 (1) (2005)107–116.

X. Du et al. / Journal of Crystal Growth 404 (2014) 75–79 79