# Fileset

[alpha-Ga2O3-ver4.0.pdf](https://mdr.nims.go.jp/filesets/5f218d02-dad4-4295-9a3b-38ee95387940/download)

## Creator

[Yuichi Oshima](https://orcid.org/0000-0001-8293-4891), Encarnación G. Víllora, [Kiyoshi Shimamura](https://orcid.org/0000-0001-6502-8731)

## Rights

This is an author-created, un-copyedited version of an article accepted for publication/published in Applied Physics Express. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at https://doi.org/10.7567/APEX.8.055501.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Halide Vapor Phase Epitaxy of twin-free a-Ga2O3 on sapphire (0001) substrates](https://mdr.nims.go.jp/datasets/ff3d0997-b819-452f-974b-c0f3b4fd8aae)

## Fulltext

Microsoft Word - alpha-Ga2O3-ver4.0.docx  Template for APEX (Jan. 2014) 1 Halide vapor phase epitaxy of twin-free αααα-Ga2O3 on sapphire (0001) substrates Yuichi Oshima*, Encarnaciόn G. Vίllora, and Kiyoshi Shimamura Optical Single Crystals Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan  E-mail: OSHIMA.Yuichi@nims.go.jp  The halide vapor phase epitaxy of α-Ga2O3 is demonstrated for the first time. The films are twin-free, heteroepitaxially grown on sapphire (0001) substrates using gallium chloride and oxygen as precursors. X-ray ω-2θ and pole figure measurements reveal that the film is single-crystalline (0001) α-Ga2O3 with no detectable formation of β-Ga2O3. The optical bandgap is determined from the transmittance spectrum to be 5.16 eV. The growth rate monotonically increased with the partial pressures of the raw material gases, reaching approximately 150 µm/h, which is over two orders of magnitude larger than those of conventional vapor phase epitaxial growth techniques, such as mist-CVD or MBE.        Template for APEX (Jan. 2014) 2  Ga2O3 is known to possess five different crystal structures, i.e., α-, β-, δ-, ε-, and γ-phase. Among them, β-Ga2O3 is thermodynamically the most stable. 1) The bandgap of β-Ga2O3 is reported to be 4.7 eV, 2) therefore it is very transparent even in the UV region. In addition, the electrical conductivity can be controlled by the doping with Si or Sn. 3, 4) Furthermore, single crystal substrates produced from melt are available, 5-7) and high-quality homoepitaxial layer can be grown on those substrates. 8-11) Accordingly, β-Ga2O3 is attracting remarkable attention as a promising wide bandgap semiconductor for various applications such as substrates for GaN-based LEDs, 12, 13) UV detectors 14), and power devices such as SBDs, 10) MESFETs, 15) and MOSFETs. 16)  On the other hand, α-Ga2O3, which is the target material of this work, is a meta-stable phase, and the number of publications is limited compared to those of β-Ga2O3. However, α-Ga2O3 has also been reported to have a large bandgap energy (Eg = 5.3 eV), 17) and its electrical conductivity can be controlled as well. 18, 19) Furthermore, this compound has been proposed to realize novel functional materials through the formation of solid-solutions with other corundum-structured oxides, such as α-Al2O3. 20) α-Ga2O3 is thus a very promising wide bandgap semiconductor.  Unfortunately, single crystal substrates of α-Ga2O3 cannot be produced from melt, since the material is meta-stable. Accordingly, single crystalline α-Ga2O3 films need to be produced by heteroepitaxy. Sapphire is one of the most promising substrate materials, because it possesses the same corundum structure, and large-area high-quality substrates are available at a reasonable price. The lattice mismatches between α-Ga2O3 and sapphire along a- and c-axis are ~4.5% and ~3.3%, respectively. So far, the vapor phase growth of α-Ga2O3 has been demonstrated only by mist-CVD17-20) and MBE 21). Kumaran et al. reported the MBE of single crystalline Nd-doped α-Ga2O3 on sapphire (112�0) 21). The α-Ga2O3 films grown by mist-CVD on sapphire (0001) exhibited a narrow FWHM (~60 arcsec) of the X-ray rocking curve (XRC) in out-of-plane diffraction, while no data is reported about the in-plane mosaicity. 17-20) The domain-matching epitaxy mechanism has been proposed as the reason for the narrow tilt angle.22) However, in-plane twinning, with a small fraction of 180o rotational domains, was observed. 17) The maximum reported growth rate is approximately 1 µm/h. 18) High concentrations of impurities have   Template for APEX (Jan. 2014) 3 been reported for nominally un-doped films ([H] = 3 × 1019 cm-3, [C] = 1 × 1019 cm-3, [Si] = 9 × 1018 cm-3). 19) In this work, we have employed the halide vapor phase epitaxy (HVPE) 23) to grow α-Ga2O3 layers. HVPE is a kind of CVD technique, which is characterized by a large growth rate and a high-purity of the resulting crystals. HVPE is nowadays widely used in III-V semiconductors industry. 24) Regarding the HVPE of Ga2O3, there are some publications about β-Ga2O3. 11, 25-27) However, there is no report about the HVPE of α-Ga2O3. In this work, we demonstrate the successful high-speed growth of high-purity α-Ga2O3 layers by HVPE.  The HVPE growth was conducted in an atmospheric horizontal reactor using gallium chloride and O2 (＞ 99.99995% pure) as the precursors. The gallium chloride was synthesized through the chemical reaction between Ga metal (> 99.99999% pure) and HCl gas (> 99.999% pure) upstream in the reactor. The partial pressures of the HCl and O2 (P(HCl) and P(O2)) were 0.1 - 0.75 kPa and 0.5 - 6 kPa, respectively. N2 (> 99.9999% pure) was used as the carrier gas. The deposition temperature was fixed within the range 525-650°C. We used sapphire (0001) substrates.  The surface morphology of the films was observed by scanning electron microscopy (SEM). The growth rate was determined by cross-sectional SEM. The crystal structure and orientation were investigated by X-ray diffraction (XRD) ω-2θ scan and pole figure measurements. The structural quality was estimated by XRC measurement. The impurities concentrations were evaluated by secondary ion mass spectrometry (SIMS). Optical transmittance spectrum was utilized to determine the optical bandgap. The baseline was measured with a sapphire blank substrate. Firstly, we investigated the influence of the growth temperature under the fixed partial pressures of P(HCl) = 0.25 kPa and P(O2) = 1.0 kPa. The whole area of each film grown below 575°C was specular for human eyes. SEM images of a film grown at 550°C are shown in Fig.1. However, some random surface areas of the films grown at 575°C or higher temperatures exhibited a rough morphology, which appears mat white for human eyes. The area of such translucent regions increased with the temperature. X-ray ω-2θ scan profiles (not shown) revealed that poly-crystalline β-Ga2O3 is dominant in such areas, while the other mirror-like areas were confirmed to be single crystalline α-Ga2O3 through   Template for APEX (Jan. 2014) 4 pole figure measurement. The mirror-like area completely disappeared at 650°C. The temperature at which pure α-Ga2O3 is obtained is different depending on the report. Shinohara and Fujita reported that α-Ga2O3 was dominant under 630°C, and no β-Ga2O3 was detected below 470°C 17) in the Ga2O3 film grown by mist-CVD. In contrast, β-Ga2O3 was still dominant at 500°C in MBE-grown Ga2O3 films on c-plane sapphire. 21, 29) Such differences could be originated not only on the accuracy of the growth temperature, but also on the intrinsic nature of each growth method, therefore a detailed explanation is difficult at present. Because our HVPE and the mist-CVD both include H2O and HCl in the atmosphere, we speculate that such molecules could play a key role. The temperature dependence of the growth rate at the mirror-like area is presented in Fig.2. The growth rate exhibited a significant increase with the temperature rise. This could be explained by the change of the molar ratio of GaCl and GaCl3 generated in the Ga container. Figure 3 shows the calculated partial pressures of GaCl and GaCl3 in the equilibria Ga + HCl = GaCl + 1/2H2 and GaCl + 2HCl = GaCl3 + H2, respectively. The detailed procedure of the thermodynamic analysis has been described elsewhere. 28) The calculation was carried out assuming the injection of the HCl gas into the Ga container with a partial pressure of 5 kPa and using N2 as the carrier gas, in accordance with the actual experimental condition. Note that the Ga temperature should be close to the growth temperature, since the Ga container and the substrate holder were nearby in a single zone hot-wall furnace. It can be seen that the mole fraction of GaCl3 continuously decreases with increasing temperature, being negligible above 600°C. On the contrary, GaCl rises significantly with the temperature from 250 to 500°C, where it starts to saturate towards 600°C. Taking both into account, at the considered deposition temperatures the decrease of GaCl3 is largely compensated by the increase in GaCl. As a result, this enhancement of Ga transport can be responsible for the increase in deposition rate. Secondly, we describe the characterization of the α-Ga2O3 films (3.6 µm thick) grown at 550°C under the partial pressures of P(HCl) = 0.25 kPa and P(O2) = 1.0 kPa. The XRD ω-2θ scan profile of the film is shown in Fig.4(a) and (b). Apart from the peaks of the sapphire substrate, only the allowed reflections from the c-plane of α-Ga2O3 were observed. Therefore, the film is composed of only the (0001)-oriented α-Ga2O3 phase, with no detectable admixture of the β-Ga2O3 phase.   Template for APEX (Jan. 2014) 5  Figures 5(a) and (b) are the {101�2} pole figures of the film and the sapphire substrate, respectively. Three peaks appeared at the positions which are expected for single crystalline α-Ga2O3, indicating that the film is single crystalline. This result contrasts with the reported in Ref.17, where three additional reflections were observed with a rotation angle of 180° relative to the former ones. Therefore, our diffraction measurements prove that twin-free α-Ga2O3 was deposited heteroepitaxially on sapphire (0001) for the first time. The epitaxial relationships are elucidated as:  [101�0] α-Ga2O3 // [101�0] sapphire and (0001) α-Ga2O3 // (0001) sapphire  Figure 6 shows the XRC profiles of (0006) and (101�2) diffraction peaks measured in symmetric and skew-symmetric geometry, respectively. The FWHM of the (0006) diffraction, with 612 arcsec, reflects the tilting of the c-plane. The FWHM of (101�2) diffraction (whose plane is 57.6o inclined from the c-plane) in skew-symmetry, with 1296 arcsec, indicates the twisting around the c-axis. Smaller values of FWHM are notably favored by a slower growth rate during nucleation. Therefore, the structural quality of the HVPE epilayers can be remarkably improved by optimizing the growth conditions at the initial growth stage. The results of the SIMS measurement is given in Table I. [H], [C] were below the detection limit, in contrast to the high values [H] = 3 × 1019 cm-3 and [C] = 1 × 1019 cm-3 reported in Ref.19. This is probably due to the fact that HVPE does not use organic compounds as raw materials. [Al] was also below the detection limit. In addition, [Si] was also much lower than the reported value. 19) Although [Cl] cannot be compared because there is no past report, it might be worth to mention that a value as low as [Cl] = 1 × 1016 cm-3 was determined for a homoepitaxial β-Ga2O3 layer grown by HVPE at 1000°C. 26) A plausible explanation for the difference in [Cl] is that the desorption of chlorine atoms from the growth front is favored through the temperature rise. Figure 7 shows the transmittance T. Although the optical transition type of α-Ga2O3 is still under discussion, we estimated the bandgap energy to be 5.16 eV through the (hνα)2 - (hν) plot (inset of Fig.7). Finally, we tried further high-speed growth. The growth rate of α-Ga2O3 at 550°C as a   Template for APEX (Jan. 2014) 6 function of the partial pressures of P(O2) and P(HCl) is shown in Figs.8(a) and (b), respectively. The growth rate monotonically increased with increasing the partial pressures, showing a saturating tendency at high partial pressures. The growth rate reaches approximately 150 µm/h, which is over two orders of magnitude greater than those of other epitaxial techniques, such as mist-CVD or MBE. The surface was still mirror-like, and the pole figure was confirmed to be that of single crystal even at the largest growth rate. The saturation in Fig.8(a) is probably due to the shortage of gallium chloride. Actually, the further increase of the growth rate at P(O2) = 4.0 kPa by increasing the P(HCl) is confirmed in Fig.8(b). On the other hand, the saturation tendency in Fig.8(b) is likely to be caused by the limitation of the chemical reactions at the considered temperatures to generate gallium chloride and/or Ga2O3. Insufficient mixing of gallium chloride and O2 could also contribute to the saturation.  In summary, present work demonstrates for the first time the successful heteroepitaxial growth of twin-free α-Ga2O3 on c-plane sapphire substrates by HVPE. The epitaxial relationships were determined to be [101�0] α-Ga2O3 // [101�0] sapphire and (0001) α-Ga2O3 // (0001) sapphire, in good accordance with the corundum structure of both compounds. Impurity concentrations ([H], [C], and [Si]) were much lower than the reported values by factors in the range of 100-1000. The optical bandgap was estimated to be 5.16 eV. The growth by HVPE reached a maximum rate of approximately 150 µm/h, which is over two orders of magnitude greater than the reported so far. Present results confirm the high potential of the HVPE for the growth of thick films or even free-standing α-Ga2O3 wafers.  Acknowledgment This work was partly supported by a Grant-in-Aid for Scientific Research (C) No. 25420307 from Japan Society for the Promotion of Science (JSPS).        Template for APEX (Jan. 2014) 7 References 1) R. Roy, V. G. Hill, and E. F. Osborn, J. Am. Chem. Soc. 74, 719 (1952). 2) H. H. Tippins, Phys. Rev. 140, A316 (1965). 3) N. Suzuki, S. Ohira, M. Tanaka, T. Sugawara, K. Nakajima, and T. Shishido: Phys. Status. Solidi C 4, 2310 (2007). 4) E. G. Víllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, and K. Aoki: Appl. Phys. Lett 92, 202120 (2008). 5) E.G. Víllora, K. Shimamura, Y. Yoshikawa, K. Aoki, N. Ichinose, J. Cryst. Growth 270, 420 (2004).  6) H. Aida, K. Nishiguchi, H. Takeda, N. Aota, K. Sunakawa, Y. Yaguchi, Jpn. J. Appl. Phys. 47, 8506 (2008). 7) Z. Galazka, K. Irmscher, R. Uecker, R. Bertram, M. Pietsch, A. Kwasniewski, M. Naumann, T. Schulz, R. Schewski, D. Klimm, and M. Bickermann, J. Cryst. Growth 404, 184 (2014). 8) E.G. Víllora, K. Shimamura, K. Kitamura, and K. Aoki, Appl. Phys. Lett. 88, 031105 (2006). 9) T. Oshima, N. Arai, N. Suzuki, S. Ohira, and S. Fujita, Thin Solid Films 516, 5768 (2008). 10) K. Sasaki, A. Kuramata, T. Masui, E.G. Víllora, K. Shimamura, and S. Yamakoshi, Appl. Phys. Express 5, 035502 (2012). 11) K. Nomura, K.Goto, R.Togashi, H. Murakami, Y. Kumagai, A. Kuramata, S.  Yamakoshi, A. Koukitu, J. Cryst. Growth 405, 19 (2014). 12) K. Shimamura, E.G. Víllora, K. Domen, K. Yui, K. Aoki, N. Ichinose, Jpn. J. Appl. Phys. 44, L7 (2005).   Template for APEX (Jan. 2014) 8 13) E.G. Víllora, S. Arjoca, K. Shimamura, D. Inomata, K. Aoki, Proc. SPIE 8987, Oxide-based Materials and Devices V, 89871U, March 8 (2014). 14) T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, and S. Fujita, Appl. Phys. Express 1, 011202 (2008). 15) M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 100, 013504 (2012). 16) M. Higashiwaki, K. Sasaki, T. Kamimura, M.H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 103, 123511 (2013). 17) D. Shinohara and S. Fujita, Jpn. J. Appl. Phys. 47, 7311 (2008). 18) T. Kawaharamura, G. T. Dang, and M. Furuta, Jpn. J. Appl. Phys. 51, 040207 (2012). 19) K. Akaiwa and S. Fujita, Jpn. J. Appl. Phys. 51, 070203 (2012). 20) S. Fujita and K. Kaneko, J. Cryst. Growth 401, 588 (2014). 21) R. Kumaran, T. Tiedje, S.E. Webster, S. Penson, and W. Li, Opt. Lett. 35, 3793 (2010). 22) K. Kaneko, H. Kawanowa, H. Ito, and S. Fujita, Jpn. J. Appl. Phys. 51, 020201 (2012). 23) J. J. Tietjen and J. A. Amic, J. Electrochem. Soc. 113, 724 (1966). 24) Y. Oshima, T. Eri, M. Shibata, H. Sunakawa, K. Kobayashi, T. Ichihashi, and A. Usui, Jpn. J. Appl. Phys. 42, L1 (2003). 25) T. Matsumoto, M. Aoki, A. Kinoshita, T. Aono, Jpn. J. Appl. Phys. 13, 1578 (1974). 26) H. Murakami, K. Nomura, K. Goto, K. Sasaki, K. Kawara, Q. T. Thieu, R. Togashi, Y. Kumagai, M. Higashiwaki, A. Kuramata, S. Yamakoshi, B. Monemar and A. Koukitu, Appl. Phys. Express 8, 015503 (2015). 27) Y. Oshima. E. G. Víllora, and K. Shimamura, J. Cryst. Growth 410, 53 (2015). 28) V. S. Ban, M. Ettenberg, J. Phys. Chem. Solids 34, 1119 (1973). 29) T. Oshima, T. Okuno, and S. Fujita, Jpn. J. Appl. Phys. 46, 7217 (2007).   Template for APEX (Jan. 2014) 9 Figure Captions Fig. 1. SEM images of an α-Ga2O3 layer grown on a sapphire (0001) substrate. (a) surface image, (b) cross-sectional image. Fig. 2. Growth rate of α-Ga2O3 as a function of growth temperature. Fig. 3. Calculated partial pressures of GaCl and GaCl3 in equilibrium as a function of temperature (result of thermodynamic analysis). Fig. 4. XRD ω-2θ profile of an α-Ga2O3 layer grown on a sapphire (0001) substrate. (a) wide-scan profile, (b) narrow-scan profile around (0006). Fig. 5. X-ray {101�2} pole figures (log-scale) of (a) α-Ga2O3 layer and (b) sapphire substrate. Fig. 6. XRCs of an α-Ga2O3 film. Fig. 7. Transmittance spectra of α-Ga2O3. The inset shows the absorption coefficient in (hνα)2 vs hν. Fig. 8. Growth rate of α-Ga2O3 as a function of (a) O2 partial pressure, (b) HCl partial pressure.                   Template for APEX (Jan. 2014) 10                           Table I.  Impurity concentrations in α-Ga2O3 measured by SIMS.  Element Detection limit (D. L.) [cm-3] Concentration [cm-3] H 4 × 1017 < D. L. C 6 × 1017 < D. L. Al 4 × 1015 < D. L. Si 1 × 1016 2 × 1016 Cl 1 × 1016 7 × 1016    Template for APEX (Jan. 2014) 11 Figures                                Fig. 1  α-Ga2O3 Sapphire (b) (a) 3 µm 3 µm   Template for APEX (Jan. 2014) 12                                 Fig. 2  520 540 560 580 600 6202025303540Growth temperature [oC]Growth rate [µm/h]P(HCl) = 0.25 kPa P(O2) = 1.0 kPa Partly rough surface   Template for APEX (Jan. 2014) 13                                 Fig. 3  200 400 600 80001.02.03.04.05.0Temperaure [oC]Partial pressure [kPa]GaClGaCl3GaCl + GaCl3Carrier gas: N2  P0(HCl) = 5 kPa   Template for APEX (Jan. 2014) 14                                 Fig. 4  20 40 60 80 1002θ [deg.]Intensity [arb. units]38 39 40 41 42 432θ [deg.]Intensity [arb. units]α-Ga2O3 (0006) sapphire (0006) α-Ga2O3 (0006) sapphire (0006) α-Ga2O3 (00012) sapphire (00012) (a) (b)   Template for APEX (Jan. 2014) 15                                 Fig. 5          χ=20° χ=40° χ=60° (a) (b) 10-1 104 cps 10-1 103 cps 100 101 102 100 101 102 103   Template for APEX (Jan. 2014) 16                                 Fig. 6  -2 -1 0 1 2Intensity [arb. units]∆ω [deg.](0006) (101�2) 612 arcsec 1296 arcsec   Template for APEX (Jan. 2014) 17                                 Fig. 7  3 4 5020406080100Photon energy [eV]Transmittance [%]5.0 5.1 5.200.20.40.60.81.0Photon energy [eV](hνα)2 [1010eV2cm-2]  Template for APEX (Jan. 2014) 18                          Fig. 8 0 2 4 6050100150P(O2) [kPa]Growth rate [µm/h]P(HCl) = 0.25 kPa0 0.3 0.6 0.9P(HCl) [kPa]P(O2) = 4.0 kPa(a) (b)