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Large misorientation of GaN films grown on r-plane sapphire substrates by metalorganic vapor-phase epitaxy
ARTICLE IN PRESS
Journal of Crystal Growth 298 (2007) 293–296 www.elsevier.com/locate/jcrysgro
Large misorientation of GaN films grown on r-plane sapphire substrates by metalorganic vapor-phase epitaxy Kazuhide Kusakabea,, Shizutoshi Andob, Kazuhiro Ohkawaa,c a
Department of Applied Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan b Department of Electrical Engineering, Tokyo University of Science, Tokyo, Japan c Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), Japan Available online 27 November 2006
Abstract The lattice orientation of epitaxial GaN films grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy was investigated. It is generally known that nonpolar a-plane GaN layers are grown on r-plane sapphire substrates. However, highresolution X-ray diffraction revealed the large misorientation of GaN grown on r-plane sapphire when the growth temperature was increased from1100 1C to 1150 1C. The c-axis was oriented to 251 from the surface normal toward the (1¯ 1 0 1)Sapphire orientation. In addition, the GaN grown at 1150 1C indicated crystal twinning. These results were attributed to the anisotropic strain that was enhanced by higher growth temperature. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 81.05.E Keywords: A1. Pole figure; A3. MOVPE; B1. GaN
1. Introduction III–V nitride semiconductors are very attractive materials for application to short-wavelength light emitters and high-power electronic devices [1]. Recent work generally reports that GaN epitaxial films have been grown on c-plane (0 0 0 1) sapphire substrates. In the earliest work, GaN was grown on r-plane (1 1¯ 0 2) sapphire since it was the most readily available substrate. The surface morphology was usually quite rough and the electrical property was poor [2]. Most of the research groups avoided using r-plane sapphire substrates to grow GaN devices, because the rough surface makes device fabrication more difficult. In recent years, however, the interest of growth on the r-plane sapphire has increased as revival [3–5]. This trend is due to the absence of polarization effects that reduce internal quantum efficiency in quantum well structures. We have reported morphological characteristics [6], improvement of electrical properties [7] and transmission electron microCorresponding author. Tel.: +81 3 3260 4280; fax: +81 3 3260 4280.
E-mail address: [email protected] (K. Kusakabe). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.095
scope analysis [8] of (In)GaN films grown on r-plane sapphire. In this paper, structural analysis of GaN films grown on r-plane sapphire is discussed. It has been reported that the epitaxial relationship of GaN and r-plane sapphire is assigned to (1 1 2¯ 0)GaN//(1 1¯ 0 2)Sapphire, (0 0 0 1)GaN//(1¯ 1 0 1)Sapphire, and (1¯ 1 0 0)GaN//(1 1 2¯ 0)Sapphire [3]. However, we found the large misorientation of the GaN films grown on r-plane sapphire. 2. Experiment The GaN films were grown on r-plane sapphire substrates by an atmospheric two-flow metalorganic vapor-phase epitaxy reactor [9–11]. Nominal r-plane sapphire substrates with 721 surface orientation tolerance were used. The sapphire substrate was thermally cleaned in a hydrogen stream before the deposition of a low temperature GaN (LT-GaN) layer. The growth temperature was ramped in the region of 1040–1150 1C. Trimethylgallium and ammonia were used as precursors. The detail of growth conditions was described in our previous report [6]. Macroscopic surface morphology was characterized
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through a Nomarski optical micrograph. A Panalytical X’Pert PRO MRD diffractometer with Cu-Ka1 radiation was used to determine the crystallographic orientation of the epitaxial GaN films 3. Results and discussion From our experiments, the GaN films grown on r-plane sapphire frequently indicated rough surface morphology. Moreover, the surface feature depended on the combination of growth temperature and LT-GaN layer thickness [6]. The plane-view images of the GaN films grown on r-plane sapphire substrates are shown in Fig. 1. The surface of the GaN film grown at 1040 1C appeared a faceted pattern that is a bidirectional feature along the orthogonal (1¯ 1 0 1)Sapphire and (1 1 2¯ 0)Sapphire orientations. The GaN films grown at 1100 1C and 1120 1C showed strip-like morphologies along the (1 1 2¯ 0)Sapphire orientation. On the contrary, the GaN film grown at 1150 1C was dominated by the undulating surface morphology. These surface morphologies categorized into three features were found to be associated with the result of crystallographic orientations. The crystallographic orientation of the GaN films on r-plane sapphire was determined by an X-ray pole figure scan. Fig. 2(a) shows the pole figure profile of a-axis of the GaN film grown at 1040 1C. Three peaks were detected at
Psi-angle of 01 and 601. These come from an identical unit cell of hexagonal structure as shown in Fig. 2(b). Since a-axis peaks were aligned to the (1 1 2¯ 0)Sapphire orientation, it is considered that the c-axis of GaN is parallel to the (1¯ 1 0 1)Sapphire orientation. Therefore, the GaN film grown on r-plane sapphire at 1040 1C is a-plane oriented that is coincident with the previous report [3]. On the other hand, the variety of crystallographic orientations appears in the GaN films grown at 1100–1150 1C. Fig. 3 shows the pole figure profiles of c-axis of the GaN films grown at 1100–1150 1C and corresponding geometrical models. Figs. 3(a) and (b) show almost the same result. The c-axis was detected at Psi-angle of 251. This clearly indicates the large inclination from the a-plane of GaN grown at 1100 1C and 1120 1C as shown in Fig. 3(d). This suggests that the c-axis is rotated 651 from the (1¯ 1 0 1)Sapphire orientation. Furthermore, Fig. 3(c) shows the crystal twinning characteristic of the GaN film grown at 1150 1C. Two peaks arising from the c-axis were positioned at Psi-angle of 7251. This result indicates the c-axis split into the two directions that are symmetry against the (1¯ 1 0 1)Sapphire orientation as shown in Fig. 3(e). Matsuoka and Hagiwara reported the similar result of twin-crystals in the GaN films grown on m-plane sapphire substrates [12]. It is considered that the large misorientation of GaN grown on r-plane sapphire is attributed to the anisotropic strain that is induced by the increase of growth temperature.
Fig. 1. Nomarski optical micrographs of GaN surfaces for growth temperatures of (a) 1040 1C, (b) 1100 1C, (c) 1120 1C and (d) 1150 1C.
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Fig. 2. (a) X-ray pole figure profile of a-axis of GaN grown at 1040 1C. (b) Schematic diagram of crystallographic orientation of GaN grown at 1040 1C.
Fig. 3. X-ray pole figure profiles of c-axes of GaN grown at (a) 1100 1C, (b) 1120 1C and (c) 1150 1C. Schematic diagrams of crystallographic orientation of GaN grown at (d) 1100 and 1120 1C and (e) 1150 1C.
4. Conclusion The crystallographic orientation of epitaxial GaN films grown on r-plane sapphire substrates by MOVPE was discussed. The X-ray pole figures showed the large misorientation of GaN grown on r-plane sapphire at the
growth temperature of 1100–1150 1C. The c-axis was oriented to 251 from the surface normal toward the (1 1¯ 0 1)Sapphire orientation. The lattice twinning of GaN was observed grown at 1150 1C. These were resulted from the anisotropic strain that was induced by higher growth temperature.
ARTICLE IN PRESS 296
K. Kusakabe et al. / Journal of Crystal Growth 298 (2007) 293–296
Acknowledgement The authors acknowledge S. Funazaki for his technical support. This work was partially supported by ‘‘Open Research Center’’ Project for Private Universities; matching fund subsidy from MEXT, 2002–2007. References [1] S. Nakamura, S. F. Chichibu, (Ed.), Introduction to Nitride Semiconductor Blue Laser and Light Emitting Diodes,Taylor & Francis, London and New York, 2000. [2] W.A. Melton, J.I. Pankove, J. Crystal. Growth 178 (1997) 168. [3] M.D. Craven, S.H. Lim, F. Wu, J.S. Speck, S.P. DenBaars, Appl. Phys. Lett. 81 (2002) 469.
[4] H.M. Ng, Appl. Phys. Lett. 80 (2002) 4369. [5] B.A. Haskell, F. Wu, S. Matsuda, M.D. Craven, P.T. Fini, S.P. DenBaars, J.S. Speck, S. Nakamura, Appl. Phys. Lett. 83 (2003) 1554. [6] K. Kusakabe, K. Ohkawa, Jpn. J. Appl. Phys. 44 (2005) 7931. [7] K. Kusakabe, T. Furuzuki, K. Ohkawa, Physica B 376–377 (2006) 520. [8] N. Nakanishi, K. Kusakabe, T. Yamazaki, K. Ohkawa, I. Hashimoto, Physica B 376–377 (2006) 527. [9] K. Ohkawa, A. Hirako, M. Yoshitani, Phys. Stat. Sol. (A) 188 (2001) 621. [10] A. Hirako, M. Yoshitani, M. Nishibayashi, Y. Nishikawa, K. Ohkawa, J. Crystal. Growth 237–239 (2002) 931. [11] A. Hirako, K. Ohkawa, Phys. Stat. Sol. (A) 194 (2002) 489. [12] T. Matsuoka, E. Hagiwara, Phys. Stat. Sol. (A) 188 (2001) 485.
Journal of Crystal Growth 298 (2007) 293–296 www.elsevier.com/locate/jcrysgro
Large misorientation of GaN films grown on r-plane sapphire substrates by metalorganic vapor-phase epitaxy Kazuhide Kusakabea,, Shizutoshi Andob, Kazuhiro Ohkawaa,c a
Department of Applied Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan b Department of Electrical Engineering, Tokyo University of Science, Tokyo, Japan c Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), Japan Available online 27 November 2006
Abstract The lattice orientation of epitaxial GaN films grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy was investigated. It is generally known that nonpolar a-plane GaN layers are grown on r-plane sapphire substrates. However, highresolution X-ray diffraction revealed the large misorientation of GaN grown on r-plane sapphire when the growth temperature was increased from1100 1C to 1150 1C. The c-axis was oriented to 251 from the surface normal toward the (1¯ 1 0 1)Sapphire orientation. In addition, the GaN grown at 1150 1C indicated crystal twinning. These results were attributed to the anisotropic strain that was enhanced by higher growth temperature. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 81.05.E Keywords: A1. Pole figure; A3. MOVPE; B1. GaN
1. Introduction III–V nitride semiconductors are very attractive materials for application to short-wavelength light emitters and high-power electronic devices [1]. Recent work generally reports that GaN epitaxial films have been grown on c-plane (0 0 0 1) sapphire substrates. In the earliest work, GaN was grown on r-plane (1 1¯ 0 2) sapphire since it was the most readily available substrate. The surface morphology was usually quite rough and the electrical property was poor [2]. Most of the research groups avoided using r-plane sapphire substrates to grow GaN devices, because the rough surface makes device fabrication more difficult. In recent years, however, the interest of growth on the r-plane sapphire has increased as revival [3–5]. This trend is due to the absence of polarization effects that reduce internal quantum efficiency in quantum well structures. We have reported morphological characteristics [6], improvement of electrical properties [7] and transmission electron microCorresponding author. Tel.: +81 3 3260 4280; fax: +81 3 3260 4280.
E-mail address: [email protected] (K. Kusakabe). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.095
scope analysis [8] of (In)GaN films grown on r-plane sapphire. In this paper, structural analysis of GaN films grown on r-plane sapphire is discussed. It has been reported that the epitaxial relationship of GaN and r-plane sapphire is assigned to (1 1 2¯ 0)GaN//(1 1¯ 0 2)Sapphire, (0 0 0 1)GaN//(1¯ 1 0 1)Sapphire, and (1¯ 1 0 0)GaN//(1 1 2¯ 0)Sapphire [3]. However, we found the large misorientation of the GaN films grown on r-plane sapphire. 2. Experiment The GaN films were grown on r-plane sapphire substrates by an atmospheric two-flow metalorganic vapor-phase epitaxy reactor [9–11]. Nominal r-plane sapphire substrates with 721 surface orientation tolerance were used. The sapphire substrate was thermally cleaned in a hydrogen stream before the deposition of a low temperature GaN (LT-GaN) layer. The growth temperature was ramped in the region of 1040–1150 1C. Trimethylgallium and ammonia were used as precursors. The detail of growth conditions was described in our previous report [6]. Macroscopic surface morphology was characterized
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through a Nomarski optical micrograph. A Panalytical X’Pert PRO MRD diffractometer with Cu-Ka1 radiation was used to determine the crystallographic orientation of the epitaxial GaN films 3. Results and discussion From our experiments, the GaN films grown on r-plane sapphire frequently indicated rough surface morphology. Moreover, the surface feature depended on the combination of growth temperature and LT-GaN layer thickness [6]. The plane-view images of the GaN films grown on r-plane sapphire substrates are shown in Fig. 1. The surface of the GaN film grown at 1040 1C appeared a faceted pattern that is a bidirectional feature along the orthogonal (1¯ 1 0 1)Sapphire and (1 1 2¯ 0)Sapphire orientations. The GaN films grown at 1100 1C and 1120 1C showed strip-like morphologies along the (1 1 2¯ 0)Sapphire orientation. On the contrary, the GaN film grown at 1150 1C was dominated by the undulating surface morphology. These surface morphologies categorized into three features were found to be associated with the result of crystallographic orientations. The crystallographic orientation of the GaN films on r-plane sapphire was determined by an X-ray pole figure scan. Fig. 2(a) shows the pole figure profile of a-axis of the GaN film grown at 1040 1C. Three peaks were detected at
Psi-angle of 01 and 601. These come from an identical unit cell of hexagonal structure as shown in Fig. 2(b). Since a-axis peaks were aligned to the (1 1 2¯ 0)Sapphire orientation, it is considered that the c-axis of GaN is parallel to the (1¯ 1 0 1)Sapphire orientation. Therefore, the GaN film grown on r-plane sapphire at 1040 1C is a-plane oriented that is coincident with the previous report [3]. On the other hand, the variety of crystallographic orientations appears in the GaN films grown at 1100–1150 1C. Fig. 3 shows the pole figure profiles of c-axis of the GaN films grown at 1100–1150 1C and corresponding geometrical models. Figs. 3(a) and (b) show almost the same result. The c-axis was detected at Psi-angle of 251. This clearly indicates the large inclination from the a-plane of GaN grown at 1100 1C and 1120 1C as shown in Fig. 3(d). This suggests that the c-axis is rotated 651 from the (1¯ 1 0 1)Sapphire orientation. Furthermore, Fig. 3(c) shows the crystal twinning characteristic of the GaN film grown at 1150 1C. Two peaks arising from the c-axis were positioned at Psi-angle of 7251. This result indicates the c-axis split into the two directions that are symmetry against the (1¯ 1 0 1)Sapphire orientation as shown in Fig. 3(e). Matsuoka and Hagiwara reported the similar result of twin-crystals in the GaN films grown on m-plane sapphire substrates [12]. It is considered that the large misorientation of GaN grown on r-plane sapphire is attributed to the anisotropic strain that is induced by the increase of growth temperature.
Fig. 1. Nomarski optical micrographs of GaN surfaces for growth temperatures of (a) 1040 1C, (b) 1100 1C, (c) 1120 1C and (d) 1150 1C.
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Fig. 2. (a) X-ray pole figure profile of a-axis of GaN grown at 1040 1C. (b) Schematic diagram of crystallographic orientation of GaN grown at 1040 1C.
Fig. 3. X-ray pole figure profiles of c-axes of GaN grown at (a) 1100 1C, (b) 1120 1C and (c) 1150 1C. Schematic diagrams of crystallographic orientation of GaN grown at (d) 1100 and 1120 1C and (e) 1150 1C.
4. Conclusion The crystallographic orientation of epitaxial GaN films grown on r-plane sapphire substrates by MOVPE was discussed. The X-ray pole figures showed the large misorientation of GaN grown on r-plane sapphire at the
growth temperature of 1100–1150 1C. The c-axis was oriented to 251 from the surface normal toward the (1 1¯ 0 1)Sapphire orientation. The lattice twinning of GaN was observed grown at 1150 1C. These were resulted from the anisotropic strain that was induced by higher growth temperature.
ARTICLE IN PRESS 296
K. Kusakabe et al. / Journal of Crystal Growth 298 (2007) 293–296
Acknowledgement The authors acknowledge S. Funazaki for his technical support. This work was partially supported by ‘‘Open Research Center’’ Project for Private Universities; matching fund subsidy from MEXT, 2002–2007. References [1] S. Nakamura, S. F. Chichibu, (Ed.), Introduction to Nitride Semiconductor Blue Laser and Light Emitting Diodes,Taylor & Francis, London and New York, 2000. [2] W.A. Melton, J.I. Pankove, J. Crystal. Growth 178 (1997) 168. [3] M.D. Craven, S.H. Lim, F. Wu, J.S. Speck, S.P. DenBaars, Appl. Phys. Lett. 81 (2002) 469.
[4] H.M. Ng, Appl. Phys. Lett. 80 (2002) 4369. [5] B.A. Haskell, F. Wu, S. Matsuda, M.D. Craven, P.T. Fini, S.P. DenBaars, J.S. Speck, S. Nakamura, Appl. Phys. Lett. 83 (2003) 1554. [6] K. Kusakabe, K. Ohkawa, Jpn. J. Appl. Phys. 44 (2005) 7931. [7] K. Kusakabe, T. Furuzuki, K. Ohkawa, Physica B 376–377 (2006) 520. [8] N. Nakanishi, K. Kusakabe, T. Yamazaki, K. Ohkawa, I. Hashimoto, Physica B 376–377 (2006) 527. [9] K. Ohkawa, A. Hirako, M. Yoshitani, Phys. Stat. Sol. (A) 188 (2001) 621. [10] A. Hirako, M. Yoshitani, M. Nishibayashi, Y. Nishikawa, K. Ohkawa, J. Crystal. Growth 237–239 (2002) 931. [11] A. Hirako, K. Ohkawa, Phys. Stat. Sol. (A) 194 (2002) 489. [12] T. Matsuoka, E. Hagiwara, Phys. Stat. Sol. (A) 188 (2001) 485.