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The orientational relationship between monoclinic β-Ga2O3 and cubic NiO
Journal of Crystal Growth 445 (2016) 73–77
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
The orientational relationship between monoclinic β-Ga2O3 and cubic NiO Shinji Nakagomi n, Shohei Kubo, Yoshihiro Kokubun Department of Information Technology and Electronics, Faculty of Science and Engineering, Ishinomaki Senshu University, Ishinomaki, Miyagi 986-8580, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2016 Received in revised form 11 April 2016 Accepted 12 April 2016 Available online 13 April 2016
The orientational relationship between β-Ga2O3 and NiO was studied by X-ray diffraction measurements and cross-sectional high resolution transmission electron microscopy. A β-Ga2O3 thin film was formed on a (100) NiO layer on a (100) MgO substrate by gallium evaporation in an oxygen plasma. It was found that the resulting β-Ga2O3 had a four-fold domain structure satisfying both (100) β-Ga2O3 ‖ (100) NiO and (010) β-Ga2O3 ‖ {011} NiO. A γ-Ga2O3 layer was observed at the interface between the β-Ga2O3 and the NiO. An NiO film was also formed on a (100) β-Ga2O3 single-crystal substrate by the sol–gel method. An epitaxial (100) NiO film was formed on a (100) β-Ga2O3 substrate, and satisfied (011) NiO ‖ (010) β-Ga2O3. The crystal orientations of β-Ga2O3 on (100) NiO and NiO on (100) β-Ga2O3 can be explained using atomic arrangement models of the (100) plane of NiO and the (100) plane of β-Ga2O3. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A1. X-ray diffraction B1. Gallium compounds B1. Oxides B2. Semiconducting gallium compounds
1. Introduction Nickel oxide, which has a wide band gap of 3.7 eV and is a p-type metal oxide semiconductor, is a candidate for application in heterojunction devices that contain n-type oxide semiconductors [1,2]. Based on the structure of the heterojunction between p-type NiO and n-type ZnO, UV light emission diodes, UV photodetectors, and visible light transparent solar cells have been demonstrated [3–5]. On the other hand, β-Ga2O3 has a band gap of 4.9 eV and is a promising candidate for UV photodetector material [6,7]. Moreover, β-Ga2O3-based devices are potentially superior to power devices based on SiC and GaN [8,9]. To date, there have been no reports of p-type β-Ga2O3, although n-type β-Ga2O3 can be obtained by Si or Sn doping [8,10]. One possible method for fabricating a β-Ga2O3 device with a p–n junction is to employ a heterojunction between β-Ga2O3 and a p-type semiconductor such as NiO. NiO has a rock salt cubic structure, whereas β-Ga2O3 has a monoclinic structure. There have been no studies of the heterostructures that form between β-Ga2O3 and NiO. We have demonstrated the epitaxial growth of NiO films on MgO substrates, which have the same cubic structure [11]. The lattice mismatch between NiO and MgO is 0.86%. A few studies of β-Ga2O3 formed n
Corresponding author. E-mail address: [email protected] (S. Nakagomi).
http://dx.doi.org/10.1016/j.jcrysgro.2016.04.023 0022-0248/& 2016 Elsevier B.V. All rights reserved.
on MgO substrates have been performed. For example, Kong et al. investigated β-Ga2O3 on MgO and revealed a four-domain structure with relationships (100) β-Ga2O3 ‖ (100) MgO and [001] βGa2O3 ‖ o0114 MgO [12]. However, because MgO is an insulator, heterostructures formed between β-Ga2O3 and MgO cannot be used in electronic devices. There has been no report describing the orientational relationship between β-Ga2O3 and NiO. In this study, we formed βGa2O3 on NiO and NiO on β-Ga2O3, and clarified the orientational relationship between β-Ga2O3 and NiO using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Whereas Kong et al. explained the crystal orientation mechanism based on the degree of lattice mismatch between βGa2O3 and MgO [12], we used the distance between oxygen atoms in atomic arrangement models of β-Ga2O3 and NiO to explain the mechanism.
2. Experimental We prepared two types of samples: β-Ga2O3 films formed on NiO (type A) and NiO films formed on β-Ga2O3 single-crystal substrates (type B). The type A samples were prepared by forming a β-Ga2O3 layer with a thickness of 170 nm on a thin (100) NiO layer on a (100) MgO substrate by gallium evaporation in an oxygen plasma. The substrate temperature was kept at 800 °C and the radio frequency
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power of the oxygen plasma was 100 W [13]. The NiO layer was formed on the (100) MgO substrate using the sol–gel method. A mixture of nickel acetate tetrahydrate was used as the starting material, and a 2-methoxyethanol solvent was used with monoethanolamine as a stabilizer. The films were spin-coated onto the substrates at 3000 rpm, followed by heat treatment at 90 °C for 10 min. Then, the film was heated at 400 °C for 20 min. After 12 repetitions of this procedure, the film was heated to 800 °C for 1 h to make a single crystal layer. All processes were carried out in air. The NiO layer was grown epitaxially on the (100) MgO substrate, and was approximately 230 nm thick. The sample with multi-layer structure of β-Ga2O3 and NiO had higher optical transmittance than 80% for longer wavelength light than 470 nm. The detail of NiO layer formed on (100) MgO substrate using the sol–gel method has been described in our previous work (Ref. [11]). The type B samples were prepared by forming NiO films on (100) β-Ga2O3 single-crystal substrates at 700 °C using the sol–gel method. The NiO layer was approximately 100 nm thick. The crystal orientations of the samples were evaluated by XRD 2θ ω scans and ϕ-scans using a Bruker D8 Discover X-ray diffraction system with CuKα radiation. To determine the atomic arrangement at the interface, cross-sectional HRTEM images were obtained using a JEOL JEM-ARM200F atomic resolution analytical electron microscope with an acceleration voltage of 200 kV. For the selected-area electron diffraction measurements, a camera length of 0.8 m and an approximately 2-nm-diameter circular observation area were employed.
3. Results and discussion 3.1.
β-Ga2O3 films formed on NiO layers on MgO substrates
3.1.1. X-ray diffraction Fig. 1 shows an XRD pattern (2θ ω scan) of a β-Ga2O3 film grown on a (100) NiO thin film on a (100) MgO substrate. The XRD intensity is represented on a logarithmic scale. Two strong diffraction peak pairs were observed, at 42.9° and 43.3°, and at 94° and 95.2°. These peaks correspond to 200 and 400 diffractions of the MgO substrate and NiO film, respectively. NiO films formed on (100) MgO substrates had the same crystal orientation (100), as reported in our previous paper [11]. The full width at half maximum (FWHM) value of the rocking curve of NiO 200 diffraction was 0.29°.
Fig. 1. X-ray diffraction pattern for β-Ga2O3 grown on (100) NiO on an MgO substrate.
Fig. 2. X-ray diffraction patterns (ϕ-scans) of (a) β-Ga2O3 002, (b) NiO 111, and (c) MgO 111.
Furthermore, the 400, 600, and 800 diffraction peaks of βGa2O3 were observed at 29.9°, 45.3°, and 61.8°, respectively. No other β-Ga2O3 peaks were observed. This indicates that (100) βGa2O3 ‖ (100) NiO ‖ (100) MgO. The FWHM values of the rocking curve of β-Ga2O3 600 and 400 diffractions were 0.38° and 1.20°, respectively. Therefore, the crystallinity of β-Ga2O3 thin film is lower than NiO layer. The in-plane orientations of the β-Ga2O3 film and the NiO layer were determined by XRD ϕ-scans for the following reflections: βGa2O3 002, NiO 111, and MgO 111 (Fig. 2). Only four diffraction peaks, separated by 90° from β-Ga2O3 002, were observed. This means that the β-Ga2O3 crystal had a four-domain structure rotated every 90°. In addition, the β-Ga2O3 002 reflection peaks were observed at the same rotation angle as the reflection peaks from NiO 111 and MgO 111. This means that four cases of (010) β-Ga2O3 ‖ (011) NiO, (010) β-Ga2O3 ‖ (011) NiO, (010) β-Ga2O3 ‖ (011) NiO, and (010) β-Ga2O3 ‖ (011) NiO. Consequently, β-Ga2O3 formed on an NiO layer on an MgO substrate satisfies both (100) β-Ga2O3 ‖ (100) NiO ‖ (100) MgO and (010) β-Ga2O3 ‖ {011} NiO. Villora et al. have reported that β-Ga2O3 formed on a (100) MgO substrate by plasma-assisted MBE has the orientational relationship (100) β-Ga2O3 ‖ (100) MgO [14]. Kong et al. have reported the same relationship in β-Ga2O3 formed on a (100) MgO substrate by metal-organic vapor phase epitaxy [12]. This relationship between (100) β-Ga2O3 and (100) MgO is the same as that observed for β-Ga2O3 films on (100) oriented NiO layers in the present study. Kong et al. have also reported that the four-domain structure of β-Ga2O3 satisfies [001] β-Ga2O3 ‖ o 011 4 MgO. This representation has the same meaning as our condition of (010) βGa2O3 ‖ {011} NiO described above. Because NiO has the same crystal structure as MgO, our results regarding β-Ga2O3 and NiO could be expected easily. However, the present study validated these expectations. Fig. 3 shows a cross-sectional TEM image of a (100)-oriented βGa2O3 thin film prepared on a (100) NiO layer on a (100) MgO substrate. The sample was observed in a region near the interface between the β-Ga2O3 film and the NiO layer, and this section was
S. Nakagomi et al. / Journal of Crystal Growth 445 (2016) 73–77
Fig. 3. Cross-sectional TEM image of (100)-oriented β-Ga2O3 on a (100)–oriented NiO layer formed on a (100) MgO substrate observed from the (011) plane of MgO.
parallel to the (011) plane of the MgO substrate. The clear crosshatching observed in the image of the NiO layer reveals that the layer formed epitaxially on the (100) MgO substrate [11]. The direction of observation in the present work differed by 45° from the direction from which Kong et al. observed the interface region between β-Ga2O3 and MgO in Ref. [12]. Two kinds of TEM image of the Ga2O3 layer were observed. The approximately 10-nm-thick interface layer generated a cross-hatched pattern, whereas the β-Ga2O3 above the interface layer produced a different pattern. Fig. 4(a) shows the electron diffraction pattern of a cross-section of the β-Ga2O3 at point A in Fig. 3. The diffraction spots were indexed as shown in Fig. 4(a). From the diffraction pattern, it was determined that the cross section of the β-Ga2O3 layer was parallel to the (010) plane. That is, the relationship (010) β-Ga2O3 ‖ {011} NiO was confirmed by crosssectional TEM analysis to be the same as that determined by X-ray diffraction. Fig. 4(b) shows the electron diffraction pattern of a Ga2O3 cross-section obtained at point B in Fig. 3. The electron diffraction
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spots at point C were the same as those at point B. We determined that the interface layer was not β-Ga2O3 but γ-Ga2O3 with a cubic structure [15,16], so the diffraction spots were indexed based on γGa2O3. We found that the Ga2O3 had a cubic structure on cubic NiO. Moreover, the (011) plane of the γ-Ga2O3 was parallel to the (011) plane of the NiO. An approximately 10-nm-thick γ-Ga2O3 layer was formed at the interface with the β-Ga2O3 layer. Fig. 5(a1) and (a2) show atomic arrangement models of βGa2O3. (a2) shows the view along the [010] direction, and the shape of the unit cell is also shown. In the {100} planes (shown by thick arrows), which correspond to the cleavage plane, the oxygen atoms are arranged in a square, as shown in (a1). The distance between neighboring oxygen atoms is 0.290 nm or 0.304 nm, and the distance between diagonal oxygen atoms is 0.420 nm. Fig. 5(b) shows an atomic arrangement model of the (100) plane of NiO. The length of 0.4177 nm corresponds to the lattice constant. The distance between closest oxygen atoms is 0.2953 nm. We believed that the distances between oxygen atoms resulted in the orientational arrangement of (010) β-Ga2O3 ‖ {011} NiO. That is, as shown in Fig. 5(a1), when the arrangement of βGa2O3 is rotated by 45°, the arrangement fits that of NiO, as shown in Fig. 5(b). Therefore, β-Ga2O3 with a 4-fold domain structure is formed on NiO. We also considered the atomic arrangement of the (100) plane of γ-Ga2O3 shown in Fig. 5(c). Because the lattice constant of γGa2O3 is 0.822 nm, the distance between oxygen atoms on the side of the unit cell is 0.422 nm. The distance between closest oxygen atoms is 0.290 nm. The arrangement shown in Fig. 5(c) matches the arrangement shown in Fig. 5(b). We believe that this is why γ-Ga2O3 was formed on NiO. We also believe that γ-Ga2O3 is easy to grow on NiO because the two have similar cubic structures unlike monoclinic β-Ga2O3. Because γ-Ga2O3 is unstable at temperatures above 550 °C, γ-Ga2O3 is likely to transform to the more stable β-Ga2O3. However, the cubic structure of γ-Ga2O3 remained in an approximately 10-nm-thick region close to the NiO, even at 800 °C. The Ga2O3 seems to be restricted to a cubic structure by the cubic structure of the NiO. This may lead to the formation of a γ-Ga2O3 interface layer. 3.2. NiO films grown on a β-Ga2O3 single-crystal substrate For comparison with the results obtained for β-Ga2O3 formed on NiO, we also studied NiO thin films formed on (100) β-Ga2O3 single-crystal substrates.
Fig. 4. Electron diffraction patterns of cross-sections taken at (a) point A and (b) point B of the TEM image shown in Fig. 3. The sample was observed from the (011) plane of the MgO substrate.
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Fig. 6. X-ray diffraction pattern of an NiO thin film grown on a (100) β-Ga2O3 substrate.
peaks at 30.1°, 45.8°, 62.5°, and 80.9° correspond to the 400, 600, 800, and 10 00 diffractions of the β-Ga2O3 substrate, respectively. Furthermore, the 200 and 400 diffraction peaks of the NiO film were observed at 43.3° and 95.3°. No other NiO peaks were observed. This reveals that the NiO film was preferentially oriented as (100) NiO ‖ (100) β-Ga2O3. Fig. 7(a), (b), and (c) show XRD ϕ-scans for the NiO 220, NiO 111, and β-Ga2O3 201 reflections, respectively. Four diffraction peaks, mutually separated by 90°, were observed for both NiO 111 and NiO 220. The rotation angles of NiO 111 were shifted by 45° from those of NiO 220. This suggests that the NiO film was well aligned in-plane on the β-Ga2O3 substrate. Moreover, the rotation angle for one of the four NiO 111 diffraction peaks coincided with
Fig. 5. Atomic arrangement model of (a1) (100) β-Ga2O3 and (a2) a side view from [010] β-Ga2O3. Top views of (b) (100) NiO and (c) (100) γ-Ga2O3.
Fig. 6 shows the XRD pattern (2θ ω scan) of an NiO thin film grown on a (100) β-Ga2O3 single-crystal substrate. The XRD intensity is represented on a logarithmic scale. The strong diffraction
Fig. 7. X-ray diffraction patterns (ϕ-scan) of (a) NiO 220, (b) NiO 111, and (c) βGa2O3 201.
S. Nakagomi et al. / Journal of Crystal Growth 445 (2016) 73–77
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4. Conclusions A (100) oriented β-Ga2O3 thin film was formed on a (100) NiO layer on a (100) MgO substrate using gallium evaporation in an oxygen plasma. The β-Ga2O3 thin film had a four-fold domain structure that satisfies the condition (010) β-Ga2O3 ‖ {011} NiO. We found a 10-nm-thick γ-Ga2O3 interface layer between the NiO and the β-Ga2O3. We explained how β-Ga2O3 is arranged on NiO using an atomic arrangement model. On the other hand, when an NiO film was formed on a (100) β-Ga2O3 single-crystal substrate, an epitaxial NiO layer satisfying (011) NiO‖(010) β-Ga2O3 was grown. The orientational relationship between β-Ga2O3 and NiO was clarified for two cases: β-Ga2O3 on NiO and NiO on β-Ga2O3.
Acknowledgment The authors acknowledge Nippon Light Metal Company, Ltd. for supplying the β-Ga2O3 single-crystal substrates.
References Fig. 8. Cross-sectional TEM image of a (100) oriented NiO film formed on a (100) βGa2O3 single crystal substrate observed from the (010) plane of β-Ga2O3. The inset shows the corresponding electron diffraction pattern of an NiO film.
that of the β-Ga2O3 201 diffraction peak. This implies that the NiO unit cell was rotated by 45° around the normal of the (100) plane of β-Ga2O3. This relationship can be described as (011) NiO ‖ (010) β-Ga2O3. Eventually, the orientational relationship of the NiO film formed on (100) β-Ga2O3 was the same as that of the β-Ga2O3 film formed on a (100) NiO layer, except for that the β-Ga2O3 formed on NiO had a four-fold domain structure. Fig. 8 shows a cross-sectional TEM image of a region near the interface between NiO and the (100) β-Ga2O3 single-crystal substrate, as observed from the (010) plane of β-Ga2O3. The atomic arrangement in the lattice image of the β-Ga2O3 single crystal was similar to that of the β-Ga2O3 layer shown in Fig. 3. Furthermore, the cross-hatched image of the (011) NiO was similar to that of the NiO layer shown in Fig. 3. The inset shows the corresponding electron diffraction pattern of the NiO film. It is clear that the NiO film was grown epitaxially on the β-Ga2O3 substrate. No interface layer such as that observed in β-Ga2O3 on NiO was observed, as can be seen in Fig. 8. When NiO layer is formed on the single crystal β-Ga2O3, the interface layer such as γ-Ga2O3 was not formed because β-Ga2O3 existed already as single crystal substrate. Therefore, the NiO layer grew on β-Ga2O3 substrate directly. The atomic arrangement of the NiO films on (100) β-Ga2O3 substrates can also be explained using the atomic arrangement model shown in Fig. 5. The nickel and oxygen atoms are formed with a 45° rotation on the oxygen atomic arrangement of the (100) β-Ga2O3 shown in Fig. 5 (a1).
[1] D. Adler, J. Feinleib, Electrical and optical properties of narrow-nband materials, Phys. Rev. B 2 (1970) 3112. [2] R.J. Powell, W.E. Spicer, Optical properties of NiO and CoO, Phys. Rev. B 2 (1970) 2182. [3] Y.Y. Xi, Y.F. Hsu, A.B. Djurišić, A.M.C. Ng, W.K. Chan, H.L. Tam, K.W. Cheah, NiO/ ZnO light emiting diodes by solution-based growth, Appl. Phys. Lett. 92 (2008) 113505. [4] H. Ohta, M. Kamiya, T. Kamiya, M. Hirano, H. Hosono, UV-detector based on pn-heterojunction diode composed of transparent oxide semiconductors, p-NiO/n-ZnO, Thin Solid Films 445 (2003) 317. [5] M. Warasawa, Y. Watanabe, J. Ishida, Y. Murata, S.F. Chichibu, M. Sugiyama, Fabrication of visible-light-transparent solar cells using p-type NiO films by low oxygen fraction reactive RF sputtering deposition, Jpn. J. Appl. Phys. 52 (2013) 021102. [6] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Sol–gel prepared β-Ga2O3 thin films for ultraviolet photodetectors, Appl. Phys. Lett. 90 (2007) 031912. [7] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Vertical solar-blind deep-ultraviolet Schottky photodetectors based on β-Ga2O3 substrates, Appl. Phys. Express 1 (2008) 011202. [8] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal β-Ga2O3 (010) substrates, Appl. Phys. Lett. 100 (2012) 013504. [9] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, MBE grown Ga2O3 and its power device applications, J. Cryst. Growth 378 (2013) 591. [10] K. Sasaki, A. Kuramata, S. Yamakoshi, T. Masui, E.G. Villora, K. Shimamura, Device-quality β-Ga2O3 epitaxial films fabricated by ozone molecular beam epitaxy, Appl. Phys. Express 5 (2012) 035502. [11] Y. Kokubun, Y. Amano, Y. Meguro, S. Nakagomi, NiO films grown epitaxially on MgO substrates by sol–gel method, Thin Solid Films 601 (2016) 76. [12] L. Kong, J. Ma, C. Luan, W. Mi, Y. Lv, Structural and optical properties of hetroepitaxial beta Ga2O3 films grown on MgO (100) substrates, Thin Solid Films 520 (2012) 4270. [13] S. Nakagomi, Y. Kokubun, Crystal orientation of β-Ga2O3 thin films formed on c-plane and a-plane sapphire substrate, J. Cryst. Growth 349 (2012) 12. [14] E.G. Villora, K. Shimamura, K. Kitamura, K. Aoki, Rf-plasma-assisted molecularbeam epitaxy of β-Ga2O3, Appl. Phys. Lett. 88 (2006) 031105. [15] T. Oshima, T. Nakazono, A. Mukai, A. Ohtomo, Epitaxial growth of γ–Ga2O3 films by mist chemical vapor deposition, J. Cryst. Growth 359 (2012) 60. [16] T. Oshima, K. Matsuyama, K. Yoshimatsu, A. Ohtomo, Conducting Si-doped γ– Ga2O3 epitaxial films grown by pulsed-laser deposition, J. Cryst. Growth 421 (2015) 23.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
The orientational relationship between monoclinic β-Ga2O3 and cubic NiO Shinji Nakagomi n, Shohei Kubo, Yoshihiro Kokubun Department of Information Technology and Electronics, Faculty of Science and Engineering, Ishinomaki Senshu University, Ishinomaki, Miyagi 986-8580, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2016 Received in revised form 11 April 2016 Accepted 12 April 2016 Available online 13 April 2016
The orientational relationship between β-Ga2O3 and NiO was studied by X-ray diffraction measurements and cross-sectional high resolution transmission electron microscopy. A β-Ga2O3 thin film was formed on a (100) NiO layer on a (100) MgO substrate by gallium evaporation in an oxygen plasma. It was found that the resulting β-Ga2O3 had a four-fold domain structure satisfying both (100) β-Ga2O3 ‖ (100) NiO and (010) β-Ga2O3 ‖ {011} NiO. A γ-Ga2O3 layer was observed at the interface between the β-Ga2O3 and the NiO. An NiO film was also formed on a (100) β-Ga2O3 single-crystal substrate by the sol–gel method. An epitaxial (100) NiO film was formed on a (100) β-Ga2O3 substrate, and satisfied (011) NiO ‖ (010) β-Ga2O3. The crystal orientations of β-Ga2O3 on (100) NiO and NiO on (100) β-Ga2O3 can be explained using atomic arrangement models of the (100) plane of NiO and the (100) plane of β-Ga2O3. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A1. X-ray diffraction B1. Gallium compounds B1. Oxides B2. Semiconducting gallium compounds
1. Introduction Nickel oxide, which has a wide band gap of 3.7 eV and is a p-type metal oxide semiconductor, is a candidate for application in heterojunction devices that contain n-type oxide semiconductors [1,2]. Based on the structure of the heterojunction between p-type NiO and n-type ZnO, UV light emission diodes, UV photodetectors, and visible light transparent solar cells have been demonstrated [3–5]. On the other hand, β-Ga2O3 has a band gap of 4.9 eV and is a promising candidate for UV photodetector material [6,7]. Moreover, β-Ga2O3-based devices are potentially superior to power devices based on SiC and GaN [8,9]. To date, there have been no reports of p-type β-Ga2O3, although n-type β-Ga2O3 can be obtained by Si or Sn doping [8,10]. One possible method for fabricating a β-Ga2O3 device with a p–n junction is to employ a heterojunction between β-Ga2O3 and a p-type semiconductor such as NiO. NiO has a rock salt cubic structure, whereas β-Ga2O3 has a monoclinic structure. There have been no studies of the heterostructures that form between β-Ga2O3 and NiO. We have demonstrated the epitaxial growth of NiO films on MgO substrates, which have the same cubic structure [11]. The lattice mismatch between NiO and MgO is 0.86%. A few studies of β-Ga2O3 formed n
Corresponding author. E-mail address: [email protected] (S. Nakagomi).
http://dx.doi.org/10.1016/j.jcrysgro.2016.04.023 0022-0248/& 2016 Elsevier B.V. All rights reserved.
on MgO substrates have been performed. For example, Kong et al. investigated β-Ga2O3 on MgO and revealed a four-domain structure with relationships (100) β-Ga2O3 ‖ (100) MgO and [001] βGa2O3 ‖ o0114 MgO [12]. However, because MgO is an insulator, heterostructures formed between β-Ga2O3 and MgO cannot be used in electronic devices. There has been no report describing the orientational relationship between β-Ga2O3 and NiO. In this study, we formed βGa2O3 on NiO and NiO on β-Ga2O3, and clarified the orientational relationship between β-Ga2O3 and NiO using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Whereas Kong et al. explained the crystal orientation mechanism based on the degree of lattice mismatch between βGa2O3 and MgO [12], we used the distance between oxygen atoms in atomic arrangement models of β-Ga2O3 and NiO to explain the mechanism.
2. Experimental We prepared two types of samples: β-Ga2O3 films formed on NiO (type A) and NiO films formed on β-Ga2O3 single-crystal substrates (type B). The type A samples were prepared by forming a β-Ga2O3 layer with a thickness of 170 nm on a thin (100) NiO layer on a (100) MgO substrate by gallium evaporation in an oxygen plasma. The substrate temperature was kept at 800 °C and the radio frequency
74
S. Nakagomi et al. / Journal of Crystal Growth 445 (2016) 73–77
power of the oxygen plasma was 100 W [13]. The NiO layer was formed on the (100) MgO substrate using the sol–gel method. A mixture of nickel acetate tetrahydrate was used as the starting material, and a 2-methoxyethanol solvent was used with monoethanolamine as a stabilizer. The films were spin-coated onto the substrates at 3000 rpm, followed by heat treatment at 90 °C for 10 min. Then, the film was heated at 400 °C for 20 min. After 12 repetitions of this procedure, the film was heated to 800 °C for 1 h to make a single crystal layer. All processes were carried out in air. The NiO layer was grown epitaxially on the (100) MgO substrate, and was approximately 230 nm thick. The sample with multi-layer structure of β-Ga2O3 and NiO had higher optical transmittance than 80% for longer wavelength light than 470 nm. The detail of NiO layer formed on (100) MgO substrate using the sol–gel method has been described in our previous work (Ref. [11]). The type B samples were prepared by forming NiO films on (100) β-Ga2O3 single-crystal substrates at 700 °C using the sol–gel method. The NiO layer was approximately 100 nm thick. The crystal orientations of the samples were evaluated by XRD 2θ ω scans and ϕ-scans using a Bruker D8 Discover X-ray diffraction system with CuKα radiation. To determine the atomic arrangement at the interface, cross-sectional HRTEM images were obtained using a JEOL JEM-ARM200F atomic resolution analytical electron microscope with an acceleration voltage of 200 kV. For the selected-area electron diffraction measurements, a camera length of 0.8 m and an approximately 2-nm-diameter circular observation area were employed.
3. Results and discussion 3.1.
β-Ga2O3 films formed on NiO layers on MgO substrates
3.1.1. X-ray diffraction Fig. 1 shows an XRD pattern (2θ ω scan) of a β-Ga2O3 film grown on a (100) NiO thin film on a (100) MgO substrate. The XRD intensity is represented on a logarithmic scale. Two strong diffraction peak pairs were observed, at 42.9° and 43.3°, and at 94° and 95.2°. These peaks correspond to 200 and 400 diffractions of the MgO substrate and NiO film, respectively. NiO films formed on (100) MgO substrates had the same crystal orientation (100), as reported in our previous paper [11]. The full width at half maximum (FWHM) value of the rocking curve of NiO 200 diffraction was 0.29°.
Fig. 1. X-ray diffraction pattern for β-Ga2O3 grown on (100) NiO on an MgO substrate.
Fig. 2. X-ray diffraction patterns (ϕ-scans) of (a) β-Ga2O3 002, (b) NiO 111, and (c) MgO 111.
Furthermore, the 400, 600, and 800 diffraction peaks of βGa2O3 were observed at 29.9°, 45.3°, and 61.8°, respectively. No other β-Ga2O3 peaks were observed. This indicates that (100) βGa2O3 ‖ (100) NiO ‖ (100) MgO. The FWHM values of the rocking curve of β-Ga2O3 600 and 400 diffractions were 0.38° and 1.20°, respectively. Therefore, the crystallinity of β-Ga2O3 thin film is lower than NiO layer. The in-plane orientations of the β-Ga2O3 film and the NiO layer were determined by XRD ϕ-scans for the following reflections: βGa2O3 002, NiO 111, and MgO 111 (Fig. 2). Only four diffraction peaks, separated by 90° from β-Ga2O3 002, were observed. This means that the β-Ga2O3 crystal had a four-domain structure rotated every 90°. In addition, the β-Ga2O3 002 reflection peaks were observed at the same rotation angle as the reflection peaks from NiO 111 and MgO 111. This means that four cases of (010) β-Ga2O3 ‖ (011) NiO, (010) β-Ga2O3 ‖ (011) NiO, (010) β-Ga2O3 ‖ (011) NiO, and (010) β-Ga2O3 ‖ (011) NiO. Consequently, β-Ga2O3 formed on an NiO layer on an MgO substrate satisfies both (100) β-Ga2O3 ‖ (100) NiO ‖ (100) MgO and (010) β-Ga2O3 ‖ {011} NiO. Villora et al. have reported that β-Ga2O3 formed on a (100) MgO substrate by plasma-assisted MBE has the orientational relationship (100) β-Ga2O3 ‖ (100) MgO [14]. Kong et al. have reported the same relationship in β-Ga2O3 formed on a (100) MgO substrate by metal-organic vapor phase epitaxy [12]. This relationship between (100) β-Ga2O3 and (100) MgO is the same as that observed for β-Ga2O3 films on (100) oriented NiO layers in the present study. Kong et al. have also reported that the four-domain structure of β-Ga2O3 satisfies [001] β-Ga2O3 ‖ o 011 4 MgO. This representation has the same meaning as our condition of (010) βGa2O3 ‖ {011} NiO described above. Because NiO has the same crystal structure as MgO, our results regarding β-Ga2O3 and NiO could be expected easily. However, the present study validated these expectations. Fig. 3 shows a cross-sectional TEM image of a (100)-oriented βGa2O3 thin film prepared on a (100) NiO layer on a (100) MgO substrate. The sample was observed in a region near the interface between the β-Ga2O3 film and the NiO layer, and this section was
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Fig. 3. Cross-sectional TEM image of (100)-oriented β-Ga2O3 on a (100)–oriented NiO layer formed on a (100) MgO substrate observed from the (011) plane of MgO.
parallel to the (011) plane of the MgO substrate. The clear crosshatching observed in the image of the NiO layer reveals that the layer formed epitaxially on the (100) MgO substrate [11]. The direction of observation in the present work differed by 45° from the direction from which Kong et al. observed the interface region between β-Ga2O3 and MgO in Ref. [12]. Two kinds of TEM image of the Ga2O3 layer were observed. The approximately 10-nm-thick interface layer generated a cross-hatched pattern, whereas the β-Ga2O3 above the interface layer produced a different pattern. Fig. 4(a) shows the electron diffraction pattern of a cross-section of the β-Ga2O3 at point A in Fig. 3. The diffraction spots were indexed as shown in Fig. 4(a). From the diffraction pattern, it was determined that the cross section of the β-Ga2O3 layer was parallel to the (010) plane. That is, the relationship (010) β-Ga2O3 ‖ {011} NiO was confirmed by crosssectional TEM analysis to be the same as that determined by X-ray diffraction. Fig. 4(b) shows the electron diffraction pattern of a Ga2O3 cross-section obtained at point B in Fig. 3. The electron diffraction
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spots at point C were the same as those at point B. We determined that the interface layer was not β-Ga2O3 but γ-Ga2O3 with a cubic structure [15,16], so the diffraction spots were indexed based on γGa2O3. We found that the Ga2O3 had a cubic structure on cubic NiO. Moreover, the (011) plane of the γ-Ga2O3 was parallel to the (011) plane of the NiO. An approximately 10-nm-thick γ-Ga2O3 layer was formed at the interface with the β-Ga2O3 layer. Fig. 5(a1) and (a2) show atomic arrangement models of βGa2O3. (a2) shows the view along the [010] direction, and the shape of the unit cell is also shown. In the {100} planes (shown by thick arrows), which correspond to the cleavage plane, the oxygen atoms are arranged in a square, as shown in (a1). The distance between neighboring oxygen atoms is 0.290 nm or 0.304 nm, and the distance between diagonal oxygen atoms is 0.420 nm. Fig. 5(b) shows an atomic arrangement model of the (100) plane of NiO. The length of 0.4177 nm corresponds to the lattice constant. The distance between closest oxygen atoms is 0.2953 nm. We believed that the distances between oxygen atoms resulted in the orientational arrangement of (010) β-Ga2O3 ‖ {011} NiO. That is, as shown in Fig. 5(a1), when the arrangement of βGa2O3 is rotated by 45°, the arrangement fits that of NiO, as shown in Fig. 5(b). Therefore, β-Ga2O3 with a 4-fold domain structure is formed on NiO. We also considered the atomic arrangement of the (100) plane of γ-Ga2O3 shown in Fig. 5(c). Because the lattice constant of γGa2O3 is 0.822 nm, the distance between oxygen atoms on the side of the unit cell is 0.422 nm. The distance between closest oxygen atoms is 0.290 nm. The arrangement shown in Fig. 5(c) matches the arrangement shown in Fig. 5(b). We believe that this is why γ-Ga2O3 was formed on NiO. We also believe that γ-Ga2O3 is easy to grow on NiO because the two have similar cubic structures unlike monoclinic β-Ga2O3. Because γ-Ga2O3 is unstable at temperatures above 550 °C, γ-Ga2O3 is likely to transform to the more stable β-Ga2O3. However, the cubic structure of γ-Ga2O3 remained in an approximately 10-nm-thick region close to the NiO, even at 800 °C. The Ga2O3 seems to be restricted to a cubic structure by the cubic structure of the NiO. This may lead to the formation of a γ-Ga2O3 interface layer. 3.2. NiO films grown on a β-Ga2O3 single-crystal substrate For comparison with the results obtained for β-Ga2O3 formed on NiO, we also studied NiO thin films formed on (100) β-Ga2O3 single-crystal substrates.
Fig. 4. Electron diffraction patterns of cross-sections taken at (a) point A and (b) point B of the TEM image shown in Fig. 3. The sample was observed from the (011) plane of the MgO substrate.
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Fig. 6. X-ray diffraction pattern of an NiO thin film grown on a (100) β-Ga2O3 substrate.
peaks at 30.1°, 45.8°, 62.5°, and 80.9° correspond to the 400, 600, 800, and 10 00 diffractions of the β-Ga2O3 substrate, respectively. Furthermore, the 200 and 400 diffraction peaks of the NiO film were observed at 43.3° and 95.3°. No other NiO peaks were observed. This reveals that the NiO film was preferentially oriented as (100) NiO ‖ (100) β-Ga2O3. Fig. 7(a), (b), and (c) show XRD ϕ-scans for the NiO 220, NiO 111, and β-Ga2O3 201 reflections, respectively. Four diffraction peaks, mutually separated by 90°, were observed for both NiO 111 and NiO 220. The rotation angles of NiO 111 were shifted by 45° from those of NiO 220. This suggests that the NiO film was well aligned in-plane on the β-Ga2O3 substrate. Moreover, the rotation angle for one of the four NiO 111 diffraction peaks coincided with
Fig. 5. Atomic arrangement model of (a1) (100) β-Ga2O3 and (a2) a side view from [010] β-Ga2O3. Top views of (b) (100) NiO and (c) (100) γ-Ga2O3.
Fig. 6 shows the XRD pattern (2θ ω scan) of an NiO thin film grown on a (100) β-Ga2O3 single-crystal substrate. The XRD intensity is represented on a logarithmic scale. The strong diffraction
Fig. 7. X-ray diffraction patterns (ϕ-scan) of (a) NiO 220, (b) NiO 111, and (c) βGa2O3 201.
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4. Conclusions A (100) oriented β-Ga2O3 thin film was formed on a (100) NiO layer on a (100) MgO substrate using gallium evaporation in an oxygen plasma. The β-Ga2O3 thin film had a four-fold domain structure that satisfies the condition (010) β-Ga2O3 ‖ {011} NiO. We found a 10-nm-thick γ-Ga2O3 interface layer between the NiO and the β-Ga2O3. We explained how β-Ga2O3 is arranged on NiO using an atomic arrangement model. On the other hand, when an NiO film was formed on a (100) β-Ga2O3 single-crystal substrate, an epitaxial NiO layer satisfying (011) NiO‖(010) β-Ga2O3 was grown. The orientational relationship between β-Ga2O3 and NiO was clarified for two cases: β-Ga2O3 on NiO and NiO on β-Ga2O3.
Acknowledgment The authors acknowledge Nippon Light Metal Company, Ltd. for supplying the β-Ga2O3 single-crystal substrates.
References Fig. 8. Cross-sectional TEM image of a (100) oriented NiO film formed on a (100) βGa2O3 single crystal substrate observed from the (010) plane of β-Ga2O3. The inset shows the corresponding electron diffraction pattern of an NiO film.
that of the β-Ga2O3 201 diffraction peak. This implies that the NiO unit cell was rotated by 45° around the normal of the (100) plane of β-Ga2O3. This relationship can be described as (011) NiO ‖ (010) β-Ga2O3. Eventually, the orientational relationship of the NiO film formed on (100) β-Ga2O3 was the same as that of the β-Ga2O3 film formed on a (100) NiO layer, except for that the β-Ga2O3 formed on NiO had a four-fold domain structure. Fig. 8 shows a cross-sectional TEM image of a region near the interface between NiO and the (100) β-Ga2O3 single-crystal substrate, as observed from the (010) plane of β-Ga2O3. The atomic arrangement in the lattice image of the β-Ga2O3 single crystal was similar to that of the β-Ga2O3 layer shown in Fig. 3. Furthermore, the cross-hatched image of the (011) NiO was similar to that of the NiO layer shown in Fig. 3. The inset shows the corresponding electron diffraction pattern of the NiO film. It is clear that the NiO film was grown epitaxially on the β-Ga2O3 substrate. No interface layer such as that observed in β-Ga2O3 on NiO was observed, as can be seen in Fig. 8. When NiO layer is formed on the single crystal β-Ga2O3, the interface layer such as γ-Ga2O3 was not formed because β-Ga2O3 existed already as single crystal substrate. Therefore, the NiO layer grew on β-Ga2O3 substrate directly. The atomic arrangement of the NiO films on (100) β-Ga2O3 substrates can also be explained using the atomic arrangement model shown in Fig. 5. The nickel and oxygen atoms are formed with a 45° rotation on the oxygen atomic arrangement of the (100) β-Ga2O3 shown in Fig. 5 (a1).
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