- Email: [email protected]
Ga2O3 heterostructures on sapphire substrates
Journal of Crystal Growth 436 (2016) 150–154
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth characteristics of corundum-structured α-(AlxGa1 x)2O3/Ga2O3 heterostructures on sapphire substrates Kentaro Kaneko a,n, Kenta Suzuki b, Yoshito Ito b, Shizuo Fujita a a b
Photonics and Electronics Science and Engineering Center, Kyoto University, Katsura, Kyoto 615-8520, Japan Department of Electronic Science and Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 17 June 2015 Received in revised form 29 November 2015 Accepted 11 December 2015 Communicated by R. Fornari Available online 22 December 2015
We report improved growth conditions for corundum-structured α-(AlxGa1 x)2O3, followed by the growth characteristics of α-(AlxGa1 x)2O3/Ga2O3 heterostructures with the use of mist chemical vapor deposition (CVD) technology. Higher growth temperatures, 700–800 °C, were effective for better crystalline quality especially for higher Al composition x. Coherent growth of α-(AlxGa1 x)2O3 was achieved for x ¼0.03 and 0.11 with the film thickness of about 100 nm. The type-I band lineup was expected for the heterostructure. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A3. Chemical vapor deposition processes B1. Oxides B2. Semiconducting ternary compounds
1. Introduction Gallium oxide (Ga2O3) is an attractive semiconductor material from the viewpoint of its wide band gap (about 5 eV) [1]. Recent achievements showing solar-blind photodetectors [2], Shottkybarrier diodes (SBDs) [3,4], and field-effect transistors (FETs) [5,6] have markedly accelerated the interest and research on this material. Among five crystal structures of Ga2O3 (α, β, γ, δ, and ε) orthorhombic β-gallia-structured β-Ga2O3 was reported to be the most stable phase from the thermodynamics [7,8]. This has allowed the growth of β-Ga2O3 bulk crystals by solution methods such as the floating-zone (FZ) [9–12], the edge-defined film-fed growth(EFG) [13], and Czochralski (CZ) [14,15] methods, leading to evolution of β-Ga2O3-based devices by the surface treatment [2,16,17], ion implantation [4,18], and homoepitaxy [3] using the β-Ga2O3 substrates. On the other hand, we have paid attention to hexagonal corundum-structured α-Ga2O3 in response to the successful growth of α-Ga2O3, showing the x-ray (0006) diffraction rocking curve full-width at half-maximum (FWHM) of as small as o100 arcsec, on sapphire substrates [19]. Device-oriented research on α-Ga2O3 has recently shown rapid progress [20]. High growth rate of α-Ga2O3 by halide vapor phase epitaxy [21] is attractive for wide applications of α-Ga2O3. n
Corresponding author. E-mail address: [email protected] (K. Kaneko).
http://dx.doi.org/10.1016/j.jcrysgro.2015.12.013 0022-0248/& 2015 Elsevier B.V. All rights reserved.
One of the noteworthy advantages of α-Ga2O3 for device applications is that we can expect alloying of α-Ga2O3 with α-Al2O3, whose crystal structure is the same as that of α-Ga2O3, for the entire composition region of Al composition x in α-(AlxGa1 x)2O3. This may allow the tuning of band gap energy from that of α-Ga2O3 (5.3 eV [19]) to that of α-Al2O3 (8.8 eV [22]) and contribute to the formation of various heterostructure devices, in contrast to β-(AlxGa1 x)2O3, where the maximum composition of x taking the β-gallia-structure is limited [23]. In this paper, we show the growth of α-(AlxGa1 x)2O3 at improved conditions, followed by the growth characteristics of α-(AlxGa1 x)2O3/Ga2O3 heterostructures.
2. Growth technology For the growth, we employed the mist chemical vapor deposition (CVD) method. An example of the system used for the growth of semiconductor single crystals is shown in Fig. 1. In the mist CVD growth of a metal oxide, we prepare a water or alcohol solution of safe and inexpensice chemicals containing the metal, for example, acetate or acetylacetonate, and then small mist particles generated by applying ultrasonic power to the solution are transferred by carrier gas to the reaction area to form the target film. The details of the mist CVD are found in the earlier literatures [19,24–28]. We used aluminum(III) acetylacetonate and gallium(III) acetylacetonate as source chemicals. They were solved in water
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
151
Structural phase transition temperature [oC]
Fig. 1. An example of the system used for the growth of semiconductor single crystals.
1000
Fig. 3. X-ray diffraction 2θ/ω scanning spectra for different Al composition x.
900 800
700 600
0
0.2
0.4 0.6 0.8 Al composition x
1
Fig. 2. Structural phase transition temperature experimentally investigated in terms of the Al composition x in α-(AlxGa1 x)2O3.
together and small amount of hydrochloric acid was added to help solving the source chemicals completely. The amount of Al and Ga ions in the solvent was changed between 0.02 and 0.96 mol/L, with which the Al composition x in α-(AlxGa1 x)2O3 was controlled. Oxygen was used both as carrier and dilution gases. C-face (0001)-oriented sapphire (α-Al2O3) was employed as substrates.
3. Growth of
α-(AlxGa1 x)2O3 on sapphire
Since the α-phase is the metastable structure, the growth of α-Ga2O3 has been carried out at the temperature around 500 °C in our previous study. The higher growth temperature tended to result in inclusion of the β-phase [19]. However for α-(AlxGa1 x)2O3, the structure may be more stable compared to α-Ga2O3 due to the alloying with α-Al2O3 which is the stable phase [29]. Fig. 2 shows the structural phase transition temperature experimentally investigated from the x-ray diffraction data in terms of the Al composition x in α-(AlxGa1 x)2O3. It is apparent that the α-(AlxGa1 x)2O3 films of x¼ 0.2–0.8 can keep the α-phase at the temperatures higher than 800 °C. Therefore the growth temperature for α-(AlxGa1 x)2O3 was set at 700–800 °C for x40.2, because the higher growth temperature was desirable for decomposition of the Al precursor and for forming Al–O bonds, while at 500 °C for xo0.2. Fig. 3 shows the x-ray diffraction 2θ/ω scanning spectra for different Al composition x, which was systematically controlled by changing the amount of Al and Ga ions in the solvent and was determined by using the Vegard's law because we confirmed that the α-(AlxGa1 x)2O3 films grown for this experiment were free standing on sapphire substrates. Clear diffraction peaks without noticeable variation of the FWHM were seen for all samples. Here the growth time was kept constant at 30 min, and therefore the
Fig. 4. X-ray (0006) diffraction ω-scanning rocking curve FWHM in terms of the Al composition x in α-(AlxGa1 x)2O3.
films tend to be thin for higher x due to higher decomposition energy and thus fewer decomposition of the Al precursor (aluminum acetylacetonate) than those of the Ga precursor (gallium acetylacetonate). The weaker x-ray diffraction peaks for higher x can partly attributed to the thinner film thickness. Further, the x-ray scattering factor is lower for Al than for Ga, and this can also result in weaker x-ray diffraction for higher x. The authors wish to emphasize that the weaker x-ray diffraction peaks for higher x does not always mean poorer crystallinity. The x-ray (0006) diffraction ω-scanning rocking curve full width at half-maximum (FWHM) is summarized in Fig. 4 against the Al composition x. The FWHM takes the smallest value for x ¼0 (α-Ga2O3) and the increases with x. For x 40.5 it turns to decrease. This behavior is similar to that for Ga xIn1 xAs on GaAs [30], where the FWHM is maximum for x ¼0.4–0.5 and takes the small value for x ¼0 (InAs on GaAs). One may argue that the increase of x results in the reduction in lattice mismatch between an epilayer α-(AlxGa1 x)2O3 and a sapphire substrate, then the increase of the FWHM, that is, the degradation in crystalline quality is against the tendency of lattice mismatching. However, as we showed above, the epilayers are free standing, that is, the lattice relaxation occurs somewhere during the growth. For x¼ 0 (α-Ga2O3) the lattice mismatch is relaxed at the epilayer/substrate interface [31]; lattice relaxation of the epilayer occurs in a short range during the growth then the epilayer grows without severe inclusion of dislocation defects, showing good crystallinity. With the increase of x the lattice mismatch to the substrate is reduced but the lattice relaxation mechanism may not be the same as in the case for x ¼0.
152
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
If the lattice relaxation accompanies dislocation defects in the epilayer, the crystalline quality will be degraded. This might be a reason for the increase of FWHM with x for 0o xo0.5. In addition, more composition fluctuation is plausible for x 0.5 and this can also be responsible for large FWHM value especially at x 0.5. The reduction of FWHM with increase of x for x40.5 may be attributed to reduction in lattice mismatch to sapphire substrates and/or in composition fluctuation. In overall, however, the FWHM values for all samples were smaller than 200 arcsec. It should be noted that in our previous report on properties of α-(AlxGa1 x)2O3 on sapphire grown at 500 °C [32], the ω-scanning rocking curve FWHM values were larger as around 300 arcsec for x between 0.2 and 0.8, indicating that the present growth conditions, especially high temperature (800 °C) growth, was effective for the improvement of the crystal quality. Actually, the x-ray diffraction patterns shown in Fig. 3 were much definite, especially for x4 0,6, compared to those of the previous samples grown at 500 °C. This result can be understood by taking into account of that high temperature is indispensable for the growth of sapphire (Al2O3).
4. Growth of
The α-(AlxGa1 x)2O3 on α-Ga2O3 heterostructure is attractive for transistor applications, as AlGaAs on GaAs and AlGaN on GaN, because the interface defect density may markedly be low by the coherent growth. The stack of unintentionally doped highly resistive α-(AlxGa1 x)2O3 on semiconducting α-Ga2O3 is expected to work as metal-insulator–semiconductor structure with low interface state density compared to that of amorphous Al2O3 or SiO2 on semiconductors. In order for the fabrication of α-(AlxGa1 x)2O3 on α-Ga2O3 heterostructure, one of the problems to be overcome is the fact that the underlying α-Ga2O3 can partly be transferred to β-Ga2O3 at the temperature above 600 °C, as is evidenced in Fig. 2. Therefore, the growth temperature of α-(AlxGa1 x)2O3 should not be higher than 600 °C. We carefully optimized the growth conditions, for example, flow rate of carrier gases and/or concentration of precursors in source, so that the high quality α-(AlxGa1 x)2O3 can be grown at higher temperatures, but not exceeding 600 °C, without causing degradation of the underlying α-Ga2O3.
x = 0.03
0.70
x = 0.11
0.70
0.69 qy
qy
0.69
0.68
0.68
0.67
0.67
0.66 0.39
0.40
0.41
qx
0.42
0.66 0.39
0.43
x = 0.38
0.70
0.41 qx
0.42
0.43
0.42
0.43
x = 0.71
0.69 qy
0.68 0.67 0.66 0.39
2 30.40
0.70
0.69 qy
α-(AlxGa1 x)2O3 on α-Ga2O3
0.68 0.67
0.40
0.41 qx
0.42
0.43
0.66 0.39
0.40
0.41 qx
Fig. 5. X-ray (119) diffraction pole figures for the heterostructure of α-(AlxGa1 x)2O3 on α-Ga2O3, where the thickness of α-(AlxGa1 x)2O3 was about 100 nm.
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
153
5. Conclusions
Fig. 6. XPS spectra for α-(AlxGa1 x)2O3 of x¼ 0.03 and 0.11, in comparison to that for α-Ga2O3.
Fig. 5 illustrates the x-ray (119) diffraction pole figures for the
α-(AlxGa1 x)2O3 on α-Ga2O3 heterostructures, where the thickness of α-(AlxGa1 x)2O3 was about 100 nm. Here the Al composi-
tion x was determined by the x-ray photoemission spectroscopy (XPS), which is sensitive to the surface and suitable for analyzing the composition of the overlying layers. For the films whose Al composition x was too small to be measured accurately by the XPS, x was estimated by interpolating the date for other samples in terms of the concentration ratios of Al and Ga ions in the solvent. For x ¼0.03 and 0.11, the diffraction spots for α-(AlxGa1 x)2O3 and α-Ga2O3 were centered at almost the same qx value, indicating the coherent growth. For x ¼0.38 the diffraction spot for α-(AlxGa1 x)2O3 appeared at slightly larger qx value compared to that for α-Ga2O3, but not at the qx value estimated by the Vegard's law for the relaxed lattice, suggesting onset of the lattice relaxation. For x¼ 0.71, the diffraction spot for α-(AlxGa1 x)2O3 existed at the qx value far different from that of the α-Ga2O3 spot, showing the lattice relaxation or the free standing. Calculating the critical thickness of α-(AlxGa1 x)2O3 on α-Ga2O3 based on the People and Bean model [33], it turns out to be 2600, 210, 12, and 0.39 nm for x¼ 0.03, 0.11, 0.38, and 0.71, respectively. It is, therefore, reasonable that the 100-nm thick α(AlxGa1 x)2O3 layers grew coherently on α-Ga2O3 for x¼ 0.03 and 0.11, while exhibited slight lattice relaxation for x¼0.31 and free standing for x ¼0.71, as discussed in Fig. 5. The band alignment at the interface between α-(AlxGa1 x)2O3 and α-Ga2O3 was analyzed by the XPS measurement. The valence band offset was estimated from the energy difference of the emission edge for α-(AlxGa1 x)2O3 and α-Ga2O3, and the conduction band offset was calculated using their band gap energy. Fig. 6 shows the XPS spectra for α-(AlxGa1 x)2O3 of x ¼0.03 and 0.11, which were coherent on α-Ga2O3, in comparison to that for α-Ga2O3. It is difficult to estimate accurately the energy variation of the emission edge, but the left shift of the spectrum indicates the increase of the emission edge energy by about 0.2 eV for x ¼0.11. This results in the estimated valence band offset and conduction band offset of about 0.2 and 0.1 eV, respectively, with the type-I band lineup. Due to severe charge-up effect owing to the highly insulating wide band gap sapphire substrates, the qualitative values should include large uncertainty. Nevertheless the type-I band lineup is expected from the result. The type-I band lineup has been reported for amorphous Al2O3/β-Ga2O3 interface [34] and β-(AlxGa1 x)2O3/β-Ga2O3 interface [35], and is favorable not only for heterojunction transistor applications but also for multiple quantum wells of optical functions.
Growth conditions for α-(AlxGa1 x)2O3 were carefully optimized and the higher temperature growth was found to be desirable especially for higher Al composition x. Nevertheless, the growth temperature of α-(AlxGa1 x)2O3 on α-Ga2O3 for the fabrication of heterostructure was not able to be higher due to phase transition of α-Ga2O3 at higher temperatures (4600 °C). Structural characterizations were carried out for the heterostructures of α-(AlxGa1 x)2O3 on α-Ga2O3, and the nearly coherent growth was achieved for x ¼0.03, 011, and 0.38 with the film thickness of about 100 nm. The heterostructure was expected to take the type-I band lineup. Therefore, by appropriate design of the Al composition and the thickness of the α-(AlxGa1 x)2O3 film on α-Ga2O3, it will be possible to realize the heterostructure of large band discontinuity suitable for applications to transistors and optical devices. Further we have already found that the slight doping of Al into α-Ga2O3 (solid Al composition of less than 0.1 and without noticeable band gap enhancement) was effective to stabilize the corundum structure of α-Ga2O3 without noticeable modulation of the band gap [29]. The slightly Al-doped α-Ga2O3 can allow heat treatment at 700–750 °C [29] and may contribute to fabrication of higher quality α-(AlxGa1 x)2O3/Ga2O3 heterostructures.
Acknowledgements This work was supported in part by the Scientific Research Grant-in-Aid #25286050 from the Japan Society for the Promotion of Science (JSPS).
References [1] S. Fujita, Jpn. J. Appl. Phys. 54 (2015) 030101. [2] T. Oshima, T. Okuno, N. Arai1, N. Suzuki1, H. Hino, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 011605. [3] K. Sasaki, A. Kuramata, T. Masui, E.G. Víllora, K. Shimamura, S. Yamakoshi, Appl. Phys. Express 5 (2012) 035502. [4] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, IEEE Electron Device Lett. 34 (2013) 493. [5] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 100 (2012) 013504. [6] M. Higashiwaki, K. Sasaki, T. Kamimura, M.H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 103 (2013) 123511. [7] R. Roy, V.G. Hill, E.F. Osborn, J. Am. Chem. Soc. 74 (1952) 719. [8] H.H. Tippins, Phys. Rev. 140 (1965) A316. [9] N. Ueda, H. Hosono, R. Waseda, H. Kawazoe, Appl. Phys. Lett. 70 (1997) 3561. [10] E.G. Víllora, K. Shimamura, Y. Yoshikawa, K. Aoki, N. Ichinose, J. Cryst. Growth 270 (2004) 420. [11] N. Suzuki, S. Ohira, M. Tanaka, T. Sugawara, K. Nakajima, T. Shishido, Phys. Status Solidi C 4 (2007) 2310. [12] E.G. Víllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, K. Aoki, Appl. Phys. Lett. 92 (2008) 202120. [13] H. Aida, K. Nishiguchi, H. Takeda, N. Aota, K. Sunakawa, Y. Yaguchi, Jpn. J. Appl. Phys. 47 (2008) 8506. [14] Y. Tomm, P. Reiche, D. Klimm, T. Fukuda, J. Cryst. Growth 220 (2000) 510. [15] Z. Galazka, R. Uecker, K. Irmscher, M. Albrecht, D. Klimm, M. Pietsch, M. Brützam, R. Bertram, S. Ganschow, R. Fornari, Cryst. Res. Technol. 45 (2010) 1229. [16] T. Oshima, T. Okuno, N. Arai1, N. Suzuki, S. Ohira, S. Fujita, Appl. Phys. Express 1 (2008) 011202. [17] T. Oshima, K. Kaminaga, A. Mukai, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, S. Fujita, A. Ohtomo, Jpn. J. Appl. Phys. 52 (2013) 051101. [18] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Express 6 (2013) 086502. [19] D. Shinohara, S. Fujita, Jpn. J. Appl. Phys. 47 (2008) 7311. [20] M. Oda, A. Takatsuka, T. Hitora, J. Kikawa, K. Kaneko, S. Fujita, Presented at The 46th Int. Conf. Solid State Devices and Materials, Tsukuba, Japan, #N-1-5, 2014. [21] Y. Oshima, E.G. Villora, K. Shimamura, Appl. Phys. Express 8 (2015) 055501. [22] R.H. French, J. Am. Ceram. Soc. 13 (1990) 471. [23] T. Oshima, T. Okuno, N. Arai, Y. Kobayashi, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 070202. [24] T. Kawaharamura, Jpn. J. Appl. Phys. 53 (2014) 05FF08.
154
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
[25] H. Nishinaka, S. Fujita, J. Cryst. Growth 310 (2008) 5007. [26] K. Kaneko, T. Nomura, I. Kakeya, S. Fujita, Appl. Phys. Express 2 (2009) 075501. [27] H. Nishinaka, Y. Kamada, N. Kameyama, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 121103. [28] H. Nishinaka, Y. Kamada, N. Kameyama, S. Fujita, Phys. Status Solidi B 247 (2010) 1460. [29] S.-D. Lee, Y. Ito, K. Kaneko, S. Fujita, Jpn. J. Appl. Phys. 54 (2015) 030301. [30] S. Fujita, Y. Nakaoka., T. Uemura., M. Tabuchi, S. Noda, Y. Takeda, A. Sasaki, J. Cryst. Growth 95 (1989) 224. [31] K. Kaneko, H. Kawanowa, H. Ito, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 020201.
[32] H. Ito, K. Kaneko, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 100207. [33] R. People, J.C. Bean, Appl. Phys. Lett. 47 (1985) 322. [34] T. Kamimura, K. Sasaki, M.-H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, M. Higashiwaki, Appl. Phys. Lett. 104 (2014) 192104. [35] M. Hattori, R. Wakabayashi, T. Oshima, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, K. Horiba, H. Kumigashira, K. Yoshimatsu, A. Ohtomo, Presented at The 62nd JSAP (Japan Society of Applied Physics) Spring Meeting, Hiratsuka, Japan, #13p-D1-D8, 2015 (in Japanese).
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth characteristics of corundum-structured α-(AlxGa1 x)2O3/Ga2O3 heterostructures on sapphire substrates Kentaro Kaneko a,n, Kenta Suzuki b, Yoshito Ito b, Shizuo Fujita a a b
Photonics and Electronics Science and Engineering Center, Kyoto University, Katsura, Kyoto 615-8520, Japan Department of Electronic Science and Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 17 June 2015 Received in revised form 29 November 2015 Accepted 11 December 2015 Communicated by R. Fornari Available online 22 December 2015
We report improved growth conditions for corundum-structured α-(AlxGa1 x)2O3, followed by the growth characteristics of α-(AlxGa1 x)2O3/Ga2O3 heterostructures with the use of mist chemical vapor deposition (CVD) technology. Higher growth temperatures, 700–800 °C, were effective for better crystalline quality especially for higher Al composition x. Coherent growth of α-(AlxGa1 x)2O3 was achieved for x ¼0.03 and 0.11 with the film thickness of about 100 nm. The type-I band lineup was expected for the heterostructure. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A3. Chemical vapor deposition processes B1. Oxides B2. Semiconducting ternary compounds
1. Introduction Gallium oxide (Ga2O3) is an attractive semiconductor material from the viewpoint of its wide band gap (about 5 eV) [1]. Recent achievements showing solar-blind photodetectors [2], Shottkybarrier diodes (SBDs) [3,4], and field-effect transistors (FETs) [5,6] have markedly accelerated the interest and research on this material. Among five crystal structures of Ga2O3 (α, β, γ, δ, and ε) orthorhombic β-gallia-structured β-Ga2O3 was reported to be the most stable phase from the thermodynamics [7,8]. This has allowed the growth of β-Ga2O3 bulk crystals by solution methods such as the floating-zone (FZ) [9–12], the edge-defined film-fed growth(EFG) [13], and Czochralski (CZ) [14,15] methods, leading to evolution of β-Ga2O3-based devices by the surface treatment [2,16,17], ion implantation [4,18], and homoepitaxy [3] using the β-Ga2O3 substrates. On the other hand, we have paid attention to hexagonal corundum-structured α-Ga2O3 in response to the successful growth of α-Ga2O3, showing the x-ray (0006) diffraction rocking curve full-width at half-maximum (FWHM) of as small as o100 arcsec, on sapphire substrates [19]. Device-oriented research on α-Ga2O3 has recently shown rapid progress [20]. High growth rate of α-Ga2O3 by halide vapor phase epitaxy [21] is attractive for wide applications of α-Ga2O3. n
Corresponding author. E-mail address: [email protected] (K. Kaneko).
http://dx.doi.org/10.1016/j.jcrysgro.2015.12.013 0022-0248/& 2015 Elsevier B.V. All rights reserved.
One of the noteworthy advantages of α-Ga2O3 for device applications is that we can expect alloying of α-Ga2O3 with α-Al2O3, whose crystal structure is the same as that of α-Ga2O3, for the entire composition region of Al composition x in α-(AlxGa1 x)2O3. This may allow the tuning of band gap energy from that of α-Ga2O3 (5.3 eV [19]) to that of α-Al2O3 (8.8 eV [22]) and contribute to the formation of various heterostructure devices, in contrast to β-(AlxGa1 x)2O3, where the maximum composition of x taking the β-gallia-structure is limited [23]. In this paper, we show the growth of α-(AlxGa1 x)2O3 at improved conditions, followed by the growth characteristics of α-(AlxGa1 x)2O3/Ga2O3 heterostructures.
2. Growth technology For the growth, we employed the mist chemical vapor deposition (CVD) method. An example of the system used for the growth of semiconductor single crystals is shown in Fig. 1. In the mist CVD growth of a metal oxide, we prepare a water or alcohol solution of safe and inexpensice chemicals containing the metal, for example, acetate or acetylacetonate, and then small mist particles generated by applying ultrasonic power to the solution are transferred by carrier gas to the reaction area to form the target film. The details of the mist CVD are found in the earlier literatures [19,24–28]. We used aluminum(III) acetylacetonate and gallium(III) acetylacetonate as source chemicals. They were solved in water
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
151
Structural phase transition temperature [oC]
Fig. 1. An example of the system used for the growth of semiconductor single crystals.
1000
Fig. 3. X-ray diffraction 2θ/ω scanning spectra for different Al composition x.
900 800
700 600
0
0.2
0.4 0.6 0.8 Al composition x
1
Fig. 2. Structural phase transition temperature experimentally investigated in terms of the Al composition x in α-(AlxGa1 x)2O3.
together and small amount of hydrochloric acid was added to help solving the source chemicals completely. The amount of Al and Ga ions in the solvent was changed between 0.02 and 0.96 mol/L, with which the Al composition x in α-(AlxGa1 x)2O3 was controlled. Oxygen was used both as carrier and dilution gases. C-face (0001)-oriented sapphire (α-Al2O3) was employed as substrates.
3. Growth of
α-(AlxGa1 x)2O3 on sapphire
Since the α-phase is the metastable structure, the growth of α-Ga2O3 has been carried out at the temperature around 500 °C in our previous study. The higher growth temperature tended to result in inclusion of the β-phase [19]. However for α-(AlxGa1 x)2O3, the structure may be more stable compared to α-Ga2O3 due to the alloying with α-Al2O3 which is the stable phase [29]. Fig. 2 shows the structural phase transition temperature experimentally investigated from the x-ray diffraction data in terms of the Al composition x in α-(AlxGa1 x)2O3. It is apparent that the α-(AlxGa1 x)2O3 films of x¼ 0.2–0.8 can keep the α-phase at the temperatures higher than 800 °C. Therefore the growth temperature for α-(AlxGa1 x)2O3 was set at 700–800 °C for x40.2, because the higher growth temperature was desirable for decomposition of the Al precursor and for forming Al–O bonds, while at 500 °C for xo0.2. Fig. 3 shows the x-ray diffraction 2θ/ω scanning spectra for different Al composition x, which was systematically controlled by changing the amount of Al and Ga ions in the solvent and was determined by using the Vegard's law because we confirmed that the α-(AlxGa1 x)2O3 films grown for this experiment were free standing on sapphire substrates. Clear diffraction peaks without noticeable variation of the FWHM were seen for all samples. Here the growth time was kept constant at 30 min, and therefore the
Fig. 4. X-ray (0006) diffraction ω-scanning rocking curve FWHM in terms of the Al composition x in α-(AlxGa1 x)2O3.
films tend to be thin for higher x due to higher decomposition energy and thus fewer decomposition of the Al precursor (aluminum acetylacetonate) than those of the Ga precursor (gallium acetylacetonate). The weaker x-ray diffraction peaks for higher x can partly attributed to the thinner film thickness. Further, the x-ray scattering factor is lower for Al than for Ga, and this can also result in weaker x-ray diffraction for higher x. The authors wish to emphasize that the weaker x-ray diffraction peaks for higher x does not always mean poorer crystallinity. The x-ray (0006) diffraction ω-scanning rocking curve full width at half-maximum (FWHM) is summarized in Fig. 4 against the Al composition x. The FWHM takes the smallest value for x ¼0 (α-Ga2O3) and the increases with x. For x 40.5 it turns to decrease. This behavior is similar to that for Ga xIn1 xAs on GaAs [30], where the FWHM is maximum for x ¼0.4–0.5 and takes the small value for x ¼0 (InAs on GaAs). One may argue that the increase of x results in the reduction in lattice mismatch between an epilayer α-(AlxGa1 x)2O3 and a sapphire substrate, then the increase of the FWHM, that is, the degradation in crystalline quality is against the tendency of lattice mismatching. However, as we showed above, the epilayers are free standing, that is, the lattice relaxation occurs somewhere during the growth. For x¼ 0 (α-Ga2O3) the lattice mismatch is relaxed at the epilayer/substrate interface [31]; lattice relaxation of the epilayer occurs in a short range during the growth then the epilayer grows without severe inclusion of dislocation defects, showing good crystallinity. With the increase of x the lattice mismatch to the substrate is reduced but the lattice relaxation mechanism may not be the same as in the case for x ¼0.
152
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
If the lattice relaxation accompanies dislocation defects in the epilayer, the crystalline quality will be degraded. This might be a reason for the increase of FWHM with x for 0o xo0.5. In addition, more composition fluctuation is plausible for x 0.5 and this can also be responsible for large FWHM value especially at x 0.5. The reduction of FWHM with increase of x for x40.5 may be attributed to reduction in lattice mismatch to sapphire substrates and/or in composition fluctuation. In overall, however, the FWHM values for all samples were smaller than 200 arcsec. It should be noted that in our previous report on properties of α-(AlxGa1 x)2O3 on sapphire grown at 500 °C [32], the ω-scanning rocking curve FWHM values were larger as around 300 arcsec for x between 0.2 and 0.8, indicating that the present growth conditions, especially high temperature (800 °C) growth, was effective for the improvement of the crystal quality. Actually, the x-ray diffraction patterns shown in Fig. 3 were much definite, especially for x4 0,6, compared to those of the previous samples grown at 500 °C. This result can be understood by taking into account of that high temperature is indispensable for the growth of sapphire (Al2O3).
4. Growth of
The α-(AlxGa1 x)2O3 on α-Ga2O3 heterostructure is attractive for transistor applications, as AlGaAs on GaAs and AlGaN on GaN, because the interface defect density may markedly be low by the coherent growth. The stack of unintentionally doped highly resistive α-(AlxGa1 x)2O3 on semiconducting α-Ga2O3 is expected to work as metal-insulator–semiconductor structure with low interface state density compared to that of amorphous Al2O3 or SiO2 on semiconductors. In order for the fabrication of α-(AlxGa1 x)2O3 on α-Ga2O3 heterostructure, one of the problems to be overcome is the fact that the underlying α-Ga2O3 can partly be transferred to β-Ga2O3 at the temperature above 600 °C, as is evidenced in Fig. 2. Therefore, the growth temperature of α-(AlxGa1 x)2O3 should not be higher than 600 °C. We carefully optimized the growth conditions, for example, flow rate of carrier gases and/or concentration of precursors in source, so that the high quality α-(AlxGa1 x)2O3 can be grown at higher temperatures, but not exceeding 600 °C, without causing degradation of the underlying α-Ga2O3.
x = 0.03
0.70
x = 0.11
0.70
0.69 qy
qy
0.69
0.68
0.68
0.67
0.67
0.66 0.39
0.40
0.41
qx
0.42
0.66 0.39
0.43
x = 0.38
0.70
0.41 qx
0.42
0.43
0.42
0.43
x = 0.71
0.69 qy
0.68 0.67 0.66 0.39
2 30.40
0.70
0.69 qy
α-(AlxGa1 x)2O3 on α-Ga2O3
0.68 0.67
0.40
0.41 qx
0.42
0.43
0.66 0.39
0.40
0.41 qx
Fig. 5. X-ray (119) diffraction pole figures for the heterostructure of α-(AlxGa1 x)2O3 on α-Ga2O3, where the thickness of α-(AlxGa1 x)2O3 was about 100 nm.
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
153
5. Conclusions
Fig. 6. XPS spectra for α-(AlxGa1 x)2O3 of x¼ 0.03 and 0.11, in comparison to that for α-Ga2O3.
Fig. 5 illustrates the x-ray (119) diffraction pole figures for the
α-(AlxGa1 x)2O3 on α-Ga2O3 heterostructures, where the thickness of α-(AlxGa1 x)2O3 was about 100 nm. Here the Al composi-
tion x was determined by the x-ray photoemission spectroscopy (XPS), which is sensitive to the surface and suitable for analyzing the composition of the overlying layers. For the films whose Al composition x was too small to be measured accurately by the XPS, x was estimated by interpolating the date for other samples in terms of the concentration ratios of Al and Ga ions in the solvent. For x ¼0.03 and 0.11, the diffraction spots for α-(AlxGa1 x)2O3 and α-Ga2O3 were centered at almost the same qx value, indicating the coherent growth. For x ¼0.38 the diffraction spot for α-(AlxGa1 x)2O3 appeared at slightly larger qx value compared to that for α-Ga2O3, but not at the qx value estimated by the Vegard's law for the relaxed lattice, suggesting onset of the lattice relaxation. For x¼ 0.71, the diffraction spot for α-(AlxGa1 x)2O3 existed at the qx value far different from that of the α-Ga2O3 spot, showing the lattice relaxation or the free standing. Calculating the critical thickness of α-(AlxGa1 x)2O3 on α-Ga2O3 based on the People and Bean model [33], it turns out to be 2600, 210, 12, and 0.39 nm for x¼ 0.03, 0.11, 0.38, and 0.71, respectively. It is, therefore, reasonable that the 100-nm thick α(AlxGa1 x)2O3 layers grew coherently on α-Ga2O3 for x¼ 0.03 and 0.11, while exhibited slight lattice relaxation for x¼0.31 and free standing for x ¼0.71, as discussed in Fig. 5. The band alignment at the interface between α-(AlxGa1 x)2O3 and α-Ga2O3 was analyzed by the XPS measurement. The valence band offset was estimated from the energy difference of the emission edge for α-(AlxGa1 x)2O3 and α-Ga2O3, and the conduction band offset was calculated using their band gap energy. Fig. 6 shows the XPS spectra for α-(AlxGa1 x)2O3 of x ¼0.03 and 0.11, which were coherent on α-Ga2O3, in comparison to that for α-Ga2O3. It is difficult to estimate accurately the energy variation of the emission edge, but the left shift of the spectrum indicates the increase of the emission edge energy by about 0.2 eV for x ¼0.11. This results in the estimated valence band offset and conduction band offset of about 0.2 and 0.1 eV, respectively, with the type-I band lineup. Due to severe charge-up effect owing to the highly insulating wide band gap sapphire substrates, the qualitative values should include large uncertainty. Nevertheless the type-I band lineup is expected from the result. The type-I band lineup has been reported for amorphous Al2O3/β-Ga2O3 interface [34] and β-(AlxGa1 x)2O3/β-Ga2O3 interface [35], and is favorable not only for heterojunction transistor applications but also for multiple quantum wells of optical functions.
Growth conditions for α-(AlxGa1 x)2O3 were carefully optimized and the higher temperature growth was found to be desirable especially for higher Al composition x. Nevertheless, the growth temperature of α-(AlxGa1 x)2O3 on α-Ga2O3 for the fabrication of heterostructure was not able to be higher due to phase transition of α-Ga2O3 at higher temperatures (4600 °C). Structural characterizations were carried out for the heterostructures of α-(AlxGa1 x)2O3 on α-Ga2O3, and the nearly coherent growth was achieved for x ¼0.03, 011, and 0.38 with the film thickness of about 100 nm. The heterostructure was expected to take the type-I band lineup. Therefore, by appropriate design of the Al composition and the thickness of the α-(AlxGa1 x)2O3 film on α-Ga2O3, it will be possible to realize the heterostructure of large band discontinuity suitable for applications to transistors and optical devices. Further we have already found that the slight doping of Al into α-Ga2O3 (solid Al composition of less than 0.1 and without noticeable band gap enhancement) was effective to stabilize the corundum structure of α-Ga2O3 without noticeable modulation of the band gap [29]. The slightly Al-doped α-Ga2O3 can allow heat treatment at 700–750 °C [29] and may contribute to fabrication of higher quality α-(AlxGa1 x)2O3/Ga2O3 heterostructures.
Acknowledgements This work was supported in part by the Scientific Research Grant-in-Aid #25286050 from the Japan Society for the Promotion of Science (JSPS).
References [1] S. Fujita, Jpn. J. Appl. Phys. 54 (2015) 030101. [2] T. Oshima, T. Okuno, N. Arai1, N. Suzuki1, H. Hino, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 011605. [3] K. Sasaki, A. Kuramata, T. Masui, E.G. Víllora, K. Shimamura, S. Yamakoshi, Appl. Phys. Express 5 (2012) 035502. [4] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, IEEE Electron Device Lett. 34 (2013) 493. [5] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 100 (2012) 013504. [6] M. Higashiwaki, K. Sasaki, T. Kamimura, M.H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 103 (2013) 123511. [7] R. Roy, V.G. Hill, E.F. Osborn, J. Am. Chem. Soc. 74 (1952) 719. [8] H.H. Tippins, Phys. Rev. 140 (1965) A316. [9] N. Ueda, H. Hosono, R. Waseda, H. Kawazoe, Appl. Phys. Lett. 70 (1997) 3561. [10] E.G. Víllora, K. Shimamura, Y. Yoshikawa, K. Aoki, N. Ichinose, J. Cryst. Growth 270 (2004) 420. [11] N. Suzuki, S. Ohira, M. Tanaka, T. Sugawara, K. Nakajima, T. Shishido, Phys. Status Solidi C 4 (2007) 2310. [12] E.G. Víllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, K. Aoki, Appl. Phys. Lett. 92 (2008) 202120. [13] H. Aida, K. Nishiguchi, H. Takeda, N. Aota, K. Sunakawa, Y. Yaguchi, Jpn. J. Appl. Phys. 47 (2008) 8506. [14] Y. Tomm, P. Reiche, D. Klimm, T. Fukuda, J. Cryst. Growth 220 (2000) 510. [15] Z. Galazka, R. Uecker, K. Irmscher, M. Albrecht, D. Klimm, M. Pietsch, M. Brützam, R. Bertram, S. Ganschow, R. Fornari, Cryst. Res. Technol. 45 (2010) 1229. [16] T. Oshima, T. Okuno, N. Arai1, N. Suzuki, S. Ohira, S. Fujita, Appl. Phys. Express 1 (2008) 011202. [17] T. Oshima, K. Kaminaga, A. Mukai, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, S. Fujita, A. Ohtomo, Jpn. J. Appl. Phys. 52 (2013) 051101. [18] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Express 6 (2013) 086502. [19] D. Shinohara, S. Fujita, Jpn. J. Appl. Phys. 47 (2008) 7311. [20] M. Oda, A. Takatsuka, T. Hitora, J. Kikawa, K. Kaneko, S. Fujita, Presented at The 46th Int. Conf. Solid State Devices and Materials, Tsukuba, Japan, #N-1-5, 2014. [21] Y. Oshima, E.G. Villora, K. Shimamura, Appl. Phys. Express 8 (2015) 055501. [22] R.H. French, J. Am. Ceram. Soc. 13 (1990) 471. [23] T. Oshima, T. Okuno, N. Arai, Y. Kobayashi, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 070202. [24] T. Kawaharamura, Jpn. J. Appl. Phys. 53 (2014) 05FF08.
154
K. Kaneko et al. / Journal of Crystal Growth 436 (2016) 150–154
[25] H. Nishinaka, S. Fujita, J. Cryst. Growth 310 (2008) 5007. [26] K. Kaneko, T. Nomura, I. Kakeya, S. Fujita, Appl. Phys. Express 2 (2009) 075501. [27] H. Nishinaka, Y. Kamada, N. Kameyama, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 121103. [28] H. Nishinaka, Y. Kamada, N. Kameyama, S. Fujita, Phys. Status Solidi B 247 (2010) 1460. [29] S.-D. Lee, Y. Ito, K. Kaneko, S. Fujita, Jpn. J. Appl. Phys. 54 (2015) 030301. [30] S. Fujita, Y. Nakaoka., T. Uemura., M. Tabuchi, S. Noda, Y. Takeda, A. Sasaki, J. Cryst. Growth 95 (1989) 224. [31] K. Kaneko, H. Kawanowa, H. Ito, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 020201.
[32] H. Ito, K. Kaneko, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 100207. [33] R. People, J.C. Bean, Appl. Phys. Lett. 47 (1985) 322. [34] T. Kamimura, K. Sasaki, M.-H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, M. Higashiwaki, Appl. Phys. Lett. 104 (2014) 192104. [35] M. Hattori, R. Wakabayashi, T. Oshima, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, K. Horiba, H. Kumigashira, K. Yoshimatsu, A. Ohtomo, Presented at The 62nd JSAP (Japan Society of Applied Physics) Spring Meeting, Hiratsuka, Japan, #13p-D1-D8, 2015 (in Japanese).