- Email: [email protected]
Structural properties of GaN grown on Zn-face ZnO at room temperature
ARTICLE IN PRESS
Journal of Crystal Growth 305 (2007) 70–73 www.elsevier.com/locate/jcrysgro
Structural properties of GaN grown on Zn-face ZnO at room temperature Atsushi Kobayashia, Yuki Shirakurab, Kazuo Miyamurab, Jitsuo Ohtaa,c, Hiroshi Fujiokaa,c, a
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Department of Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan c Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan b
Received 9 January 2007; received in revised form 7 March 2007; accepted 16 April 2007 Communicated by R.M. Biefled Available online 19 April 2007
Abstract We have grown GaN films on atomically flat Zn-face ZnO at room temperature (RT) and 700 1C, and compared their structural properties. Although the film quality of GaN grown at 700 1C was quite poor due to the serious interface reaction between GaN and ZnO, GaN with an atomically flat stepped and terraced surface grows epitaxially at RT due to the suppression of the interface reaction. The growth of GaN at RT proceeds in the layer-by-layer mode, while at 700 1C it grows three-dimensionally. Atomic force microscope observations after alkali etching of GaN surfaces revealed that GaN grown at RT exhibits an N-polarity while that grown at 700 1C has a Ga-polarity. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Fg; 81.15.Aa; 81.05.Ea; 81.10.Aj Keywords: A1. Surface morphology; A3. Laser epitaxy; B1. Nitrides; B2. Semiconductiong II–IV materials
In order to obtain high-quality GaN films, the use of a substrate lattice matched to GaN has been proposed [1–9]. ZnO is one of the most promising materials among a large number of proposed substrates for heteroepitaxy of GaN because it has a small lattice mismatch of 1.9% and shares perfectly the same crystalline symmetries with GaN [5–9]. However, it is well known that ZnO is chemically vulnerable at high temperatures and easily reacts with GaN at the growth temperatures for metalorganic chemical vapor deposition and molecular beam epitaxy [8]. We have shown that pulsed laser deposition (PLD) is quite appropriate for epitaxial growth of GaN at low temperatures [10–14] and, in fact, succeeded in the layer-by-layer growth of GaN on atomically flat O-face ZnO(0 0 0 1), even at room temperature (RT). The low-temperature growth of Corresponding author. Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Tel.: +81 3 5452 6342; fax: +81 3 5452 6343. E-mail address: [email protected] (H. Fujioka).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.04.027
GaN using PLD can be attributed to the enhanced kinetic energies of Ga precursors that assist surface migration of the growth precursors. Most recently, we have demonstrated polarity control of GaN on O-face ZnO by changing the growth temperature. A first principles calculation explains well how Ga-polarity GaN grows on O-polarity ZnO at RT [13]. Although several groups have reported that GaN grown on Zn-face ZnO above 600 1C exhibits Ga-polarity; details of the structural properties of GaN grown on Zn-face ZnO at low temperatures have not been reported. In this letter, we report on the effect of the reduction in growth temperature down to RT on the structural properties of GaN on Zn-face ZnO. In order to obtain flat stepped ZnO surfaces, which are inherently important to take advantage of the nearly lattice-matched substrate, a hydrothermally grown Zn-face ZnO(0 0 0 1) wafer was annealed at 950 1C in a box made of ceramic ZnO. The details of this annealing method will be found elsewhere [15]. After annealing, the substrate was degreased in ethanol for 10 min and introduced into the
ARTICLE IN PRESS A. Kobayashi et al. / Journal of Crystal Growth 305 (2007) 70–73
PLD chamber at a base pressure of 2 1010 Torr. GaN films were grown on the ZnO substrates at RT and 700 1C. During the growth, nitrogen was supplied through an RF plasma source operated at an input power of 400 W and at a pressure of 4.5 106 Torr. A gallium liquid target (99.9999% purity) was ablated by a KrF excimer laser (l ¼ 248 nm, t ¼ 20 ns, 30 Hz) and the ablated species reacted with the activated nitrogen on the ZnO surface located above the target. The growth was monitored by in situ reflection high-energy electron diffraction (RHEED) in order to investigate the structural properties of the GaN films. After the growth, we performed grazing incidence Xray reflectivity (GIXR) measurements to investigate the interfacial reaction between GaN and ZnO. GaN surfaces were characterized using atomic force microscopy (AFM) before and after chemical etching in 1.8 M NaOH solution for 10 min at RT in order to investigate their crystal polarities. Fig. 1a shows an AFM image of a Zn-face ZnO surface annealed in a ceramic ZnO box at 950 1C for 1 h. A clear stepped and terraced structure can be seen, although the edges of the steps are not as straight as those for O-face ZnO(0 0 0 1) [15]. Fig. 1c shows a cross-sectional profile for the white line in Fig. 1a. We found that the step height of annealed Zn-face ZnO is approximately 1 nm, which corresponds to four monolayers of ZnO in the [0 0 0 1] direction. Fig. 1b shows a RHEED pattern of the annealed ZnO with the incident azimuth of the electron beam parallel to ½1 1 2¯ 0. A very clear pattern with Kikuchi lines was obtained, indicating that the surface is atomically flat and the crystallinity at the surface region is high. These results are quite consistent with the AFM observation. We compared the surface morphology of GaN films grown at 700 1C and RT. Figs. 2a and b show RHEED patterns of 50-nm-thick GaN grown at 700 1C and RT, respectively. Just after initiation of the growth at 700 1C, a
71
ring-like spotty RHEED pattern appeared, which was quite similar to pattern for the interfacial layer between GaN and O-face ZnO formed at 700 1C [13]. We speculate that the growth of GaN on Zn-face ZnO also causes the formation of interfacial layer. The blurry spotty RHEED pattern for 50-nm-thick GaN shown in Fig. 2a shows that recovery of the crystalline quality and surface morphology was not good enough even at this thickness. A rough surface with a root-mean-square (rms) value of 1.1 nm was confirmed by AFM observation, as shown in Fig. 2c. On
Fig. 2. RHEED patterns of GaN films grown on Zn-face ZnO at (a) 700 1C and (b) room temperature. The incident azimuth of the electron beam is parallel to the ½1 1 2¯ 0 direction. 1 1 mm2 AFM images of GaN films grown on Zn-face ZnO at (c) 700 1C and (d) room temperature. The root-mean-square values of the surface roughness are 1.1 and 0.2 nm for (c) and (d), respectively.
Z(nm)
6.0 4.0 1 nm
2.0 0.0 0
200
400
600
800
1000
X(nm)
Fig. 1. (a) AFM image and (b) RHEED pattern of Zn-face ZnO annealed in a box made of ceramic ZnO at 950 1C for 1 h. (c) A cross-sectional profile along the white line in (a).
ARTICLE IN PRESS A. Kobayashi et al. / Journal of Crystal Growth 305 (2007) 70–73
RHEED intensity (a.u.)
the other hand, the RHEED pattern for GaN grown at RT remained streaky during the growth, as shown in Fig. 2b, indicating that GaN grows epitaxially even at RT and that the growth mode is two dimensional. Fig. 2d shows an AFM image of GaN grown at RT. A clear stepped and terraced structure similar to the surface morphology of the ZnO substrate can be seen in this image. In order to investigate the growth mode of GaN grown on Zn-face ZnO at RT, the RHEED intensity was monitored during the growth. Fig. 3 shows the intensity profile for the specular spot of the RHEED pattern. We found that it oscillates from the beginning of the growth, which indicates that the growth proceeds in the layer-by-layer mode from the initial stage of the film growth. Film thickness measurements revealed that one period of the oscillation corresponds to the growth of one monolayer of GaN. These phenomena at RT suggest that the excimer laser imparts sufficient energy to the film precursors to give rise to migration across the surface and cause layer-by-layer growth. GIXR measurements were performed to investigate the heterointerface between GaN grown at RT and Zn-face ZnO. From the GIXR analysis, we can obtain parameters such as the film thickness, the film density, the surface roughness, and the interface roughness by curve-fitting of the experimental data with the Fresnel equation [16,17]. Fig. 4 shows the GIXR curve for GaN grown on Zn-face ZnO at RT and its theoretical fitting based on the Fresnel equation. The theoretical fit led us to conclude that there was no interfacial layer in the GaN/ZnO heterostructure grown at RT, which is consistent with the fact that the RHEED pattern of GaN remains streaky from the initial stage of RT growth. These results indicate that PLD is an appropriate technique for the fabrication of abrupt GaN/ZnO heterojunctions. To check the polarities of GaN grown on Zn-face ZnO at 700 1C and RT, we observed changes of the surface morphology after chemical etching by alkali solutions. Figs. 5a and b show AFM images of etched GaN surfaces grown at 700 1C and RT, respectively. Although the surface morphology of GaN grown at 700 1C remained almost unchanged (rms value: 1.1–1.6 nm), that grown at RT
0
50 100 150 Growth time (s)
200
Fig. 3. Intensity profile of the specular spot of the RHEED pattern during the room temperature growth.
experimental simulation
Log intensity (a.u.)
72
RT-GaN (50 nm) interface roughness<0.1 nm ZnO
0.5
1.0
1.5 2.0 2θ/ω (deg.)
2.5
3.0
Fig. 4. GIXR curve (gray dots) of GaN/Zn-face ZnO grown at room temperature and its theoretical fitting (solid line) based on the one-layer model.
Fig. 5. 1 1 mm2AFM images of GaN films grown on Zn-face ZnO at (a) 700 1C and (b) room temperature after alkali etching. The root-meansquare values of the surface roughness are 1.6 and 4.7 nm for (a) and (b), respectively.
became quite rough after chemical etching (rms value: 0.2–4.7 nm). Since it is well known that N-polarity GaN is easily etched in alkali solutions, and Ga-polarity GaN resists them, we concluded that RT-grown GaN has a N-polarity and that 700 1C-grown GaN has a Ga-polarity. This result indicates that inversion of the polarity from cation to anion occurs at the RT-GaN/ZnO(0 0 0 1) heterointerface in spite of the absence of the interfacial layer. It is interesting to note that RT-grown GaN on O-face ZnO substrates shows a Gapolarity [13] and its mechanism and interface structure have been well discussed using the results of the first principles calculations. To understand the mechanism of the polarity flip, further investigations, both theoretical and experimental, are necessary. In summary, we have succeeded in the epitaxial growth of GaN on atomically flat Zn-face ZnO at RT by the suppression of the interfacial reactions. GaN with an atomically flat stepped and terraced surface grows epitaxially at RT in the layer-by-layer mode. We also found that the polarity flips from cation face to anion face during the growth of GaN at RT in spite of the absence of any interfacial layers. One of the authors (A.K.) is supported by JSPS Research Fellowships for Young Scientists.
ARTICLE IN PRESS A. Kobayashi et al. / Journal of Crystal Growth 305 (2007) 70–73
References [1] S. Ito, J. Ohta, H. Fujioka, M. Oshima, Appl. Surf. Sci. 197/198 (2002) 384. [2] J. Ohta, H. Fujioka, H. Takahashi, M. Oshima, Phys. Status Solidi A 188 (2001) 497. [3] H. Takahashi, J. Ohta, H. Fujioka, M. Oshima, Thin Solid Films 407 (2002) 114. [4] J. Suda, H. Matsunami, J. Cryst. Growth 237–239 (2002) 1114. [5] T. Matsuoka, N. Yoshimoto, T. Sasaki, A. Katsui, J. Electron. Mater. 21 (1992) 157. [6] F. Hamdani, A. Botchkarev, W. Kim, H. Morkoc- , M. Yeadon, J.M. Gibson, et al., Appl. Phys. Lett. 70 (1997) 467. [7] X. Gu, M.A. Reshchikov, A. Teke, D. Johnstone, H. Morkoc- , B. Nemeth, et al., Appl. Phys. Lett. 84 (2004) 2268. [8] E.S. Hellman, D.N.E. Buchanan, D. Wiesmann, I. Brener, MRS Int. J. Nitride Semicond. Res. 1 (1996) 16.
73
[9] G. Namkoong, S. Burnham, K.-K. Lee, E. Trybus, W.A. Doolittle, M. Losurdo, et al., Appl. Phys. Lett. 87 (2005) 184104. [10] J. Ohta, H. Fujioka, S. Ito, M. Oshima, Appl. Phys. Lett. 81 (2002) 2373. [11] J. Ohta, H. Fujioka, M. Oshima, Appl. Phys. Lett. 83 (2003) 3060. [12] A. Kobayashi, H. Fujioka, J. Ohta, M. Oshima, Jpn. J. Appl. Phys. 43 (2004) L53. [13] A. Kobayashi, Y. Kawaguchi, J. Ohta, H. Fujioka, K. Fujiwara, A. Ishii, Appl. Phys. Lett. 88 (2006) 181907. [14] Y. Kawaguchi, J. Ohta, A. Kobayashi, H. Fujioka, Appl. Phys. Lett. 87 (2005) 221907. [15] A. Kobayashi, J. Ohta, H. Fujioka, Jpn. J. Appl. Phys. 45 (2006) 5724. [16] L.G. Parratt, Phys. Rev. 95 (1954) 359. [17] L. Nevot, P. Croce, Rev. Phys. Appl. 15 (1980) 761.
Journal of Crystal Growth 305 (2007) 70–73 www.elsevier.com/locate/jcrysgro
Structural properties of GaN grown on Zn-face ZnO at room temperature Atsushi Kobayashia, Yuki Shirakurab, Kazuo Miyamurab, Jitsuo Ohtaa,c, Hiroshi Fujiokaa,c, a
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Department of Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan c Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan b
Received 9 January 2007; received in revised form 7 March 2007; accepted 16 April 2007 Communicated by R.M. Biefled Available online 19 April 2007
Abstract We have grown GaN films on atomically flat Zn-face ZnO at room temperature (RT) and 700 1C, and compared their structural properties. Although the film quality of GaN grown at 700 1C was quite poor due to the serious interface reaction between GaN and ZnO, GaN with an atomically flat stepped and terraced surface grows epitaxially at RT due to the suppression of the interface reaction. The growth of GaN at RT proceeds in the layer-by-layer mode, while at 700 1C it grows three-dimensionally. Atomic force microscope observations after alkali etching of GaN surfaces revealed that GaN grown at RT exhibits an N-polarity while that grown at 700 1C has a Ga-polarity. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Fg; 81.15.Aa; 81.05.Ea; 81.10.Aj Keywords: A1. Surface morphology; A3. Laser epitaxy; B1. Nitrides; B2. Semiconductiong II–IV materials
In order to obtain high-quality GaN films, the use of a substrate lattice matched to GaN has been proposed [1–9]. ZnO is one of the most promising materials among a large number of proposed substrates for heteroepitaxy of GaN because it has a small lattice mismatch of 1.9% and shares perfectly the same crystalline symmetries with GaN [5–9]. However, it is well known that ZnO is chemically vulnerable at high temperatures and easily reacts with GaN at the growth temperatures for metalorganic chemical vapor deposition and molecular beam epitaxy [8]. We have shown that pulsed laser deposition (PLD) is quite appropriate for epitaxial growth of GaN at low temperatures [10–14] and, in fact, succeeded in the layer-by-layer growth of GaN on atomically flat O-face ZnO(0 0 0 1), even at room temperature (RT). The low-temperature growth of Corresponding author. Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Tel.: +81 3 5452 6342; fax: +81 3 5452 6343. E-mail address: [email protected] (H. Fujioka).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.04.027
GaN using PLD can be attributed to the enhanced kinetic energies of Ga precursors that assist surface migration of the growth precursors. Most recently, we have demonstrated polarity control of GaN on O-face ZnO by changing the growth temperature. A first principles calculation explains well how Ga-polarity GaN grows on O-polarity ZnO at RT [13]. Although several groups have reported that GaN grown on Zn-face ZnO above 600 1C exhibits Ga-polarity; details of the structural properties of GaN grown on Zn-face ZnO at low temperatures have not been reported. In this letter, we report on the effect of the reduction in growth temperature down to RT on the structural properties of GaN on Zn-face ZnO. In order to obtain flat stepped ZnO surfaces, which are inherently important to take advantage of the nearly lattice-matched substrate, a hydrothermally grown Zn-face ZnO(0 0 0 1) wafer was annealed at 950 1C in a box made of ceramic ZnO. The details of this annealing method will be found elsewhere [15]. After annealing, the substrate was degreased in ethanol for 10 min and introduced into the
ARTICLE IN PRESS A. Kobayashi et al. / Journal of Crystal Growth 305 (2007) 70–73
PLD chamber at a base pressure of 2 1010 Torr. GaN films were grown on the ZnO substrates at RT and 700 1C. During the growth, nitrogen was supplied through an RF plasma source operated at an input power of 400 W and at a pressure of 4.5 106 Torr. A gallium liquid target (99.9999% purity) was ablated by a KrF excimer laser (l ¼ 248 nm, t ¼ 20 ns, 30 Hz) and the ablated species reacted with the activated nitrogen on the ZnO surface located above the target. The growth was monitored by in situ reflection high-energy electron diffraction (RHEED) in order to investigate the structural properties of the GaN films. After the growth, we performed grazing incidence Xray reflectivity (GIXR) measurements to investigate the interfacial reaction between GaN and ZnO. GaN surfaces were characterized using atomic force microscopy (AFM) before and after chemical etching in 1.8 M NaOH solution for 10 min at RT in order to investigate their crystal polarities. Fig. 1a shows an AFM image of a Zn-face ZnO surface annealed in a ceramic ZnO box at 950 1C for 1 h. A clear stepped and terraced structure can be seen, although the edges of the steps are not as straight as those for O-face ZnO(0 0 0 1) [15]. Fig. 1c shows a cross-sectional profile for the white line in Fig. 1a. We found that the step height of annealed Zn-face ZnO is approximately 1 nm, which corresponds to four monolayers of ZnO in the [0 0 0 1] direction. Fig. 1b shows a RHEED pattern of the annealed ZnO with the incident azimuth of the electron beam parallel to ½1 1 2¯ 0. A very clear pattern with Kikuchi lines was obtained, indicating that the surface is atomically flat and the crystallinity at the surface region is high. These results are quite consistent with the AFM observation. We compared the surface morphology of GaN films grown at 700 1C and RT. Figs. 2a and b show RHEED patterns of 50-nm-thick GaN grown at 700 1C and RT, respectively. Just after initiation of the growth at 700 1C, a
71
ring-like spotty RHEED pattern appeared, which was quite similar to pattern for the interfacial layer between GaN and O-face ZnO formed at 700 1C [13]. We speculate that the growth of GaN on Zn-face ZnO also causes the formation of interfacial layer. The blurry spotty RHEED pattern for 50-nm-thick GaN shown in Fig. 2a shows that recovery of the crystalline quality and surface morphology was not good enough even at this thickness. A rough surface with a root-mean-square (rms) value of 1.1 nm was confirmed by AFM observation, as shown in Fig. 2c. On
Fig. 2. RHEED patterns of GaN films grown on Zn-face ZnO at (a) 700 1C and (b) room temperature. The incident azimuth of the electron beam is parallel to the ½1 1 2¯ 0 direction. 1 1 mm2 AFM images of GaN films grown on Zn-face ZnO at (c) 700 1C and (d) room temperature. The root-mean-square values of the surface roughness are 1.1 and 0.2 nm for (c) and (d), respectively.
Z(nm)
6.0 4.0 1 nm
2.0 0.0 0
200
400
600
800
1000
X(nm)
Fig. 1. (a) AFM image and (b) RHEED pattern of Zn-face ZnO annealed in a box made of ceramic ZnO at 950 1C for 1 h. (c) A cross-sectional profile along the white line in (a).
ARTICLE IN PRESS A. Kobayashi et al. / Journal of Crystal Growth 305 (2007) 70–73
RHEED intensity (a.u.)
the other hand, the RHEED pattern for GaN grown at RT remained streaky during the growth, as shown in Fig. 2b, indicating that GaN grows epitaxially even at RT and that the growth mode is two dimensional. Fig. 2d shows an AFM image of GaN grown at RT. A clear stepped and terraced structure similar to the surface morphology of the ZnO substrate can be seen in this image. In order to investigate the growth mode of GaN grown on Zn-face ZnO at RT, the RHEED intensity was monitored during the growth. Fig. 3 shows the intensity profile for the specular spot of the RHEED pattern. We found that it oscillates from the beginning of the growth, which indicates that the growth proceeds in the layer-by-layer mode from the initial stage of the film growth. Film thickness measurements revealed that one period of the oscillation corresponds to the growth of one monolayer of GaN. These phenomena at RT suggest that the excimer laser imparts sufficient energy to the film precursors to give rise to migration across the surface and cause layer-by-layer growth. GIXR measurements were performed to investigate the heterointerface between GaN grown at RT and Zn-face ZnO. From the GIXR analysis, we can obtain parameters such as the film thickness, the film density, the surface roughness, and the interface roughness by curve-fitting of the experimental data with the Fresnel equation [16,17]. Fig. 4 shows the GIXR curve for GaN grown on Zn-face ZnO at RT and its theoretical fitting based on the Fresnel equation. The theoretical fit led us to conclude that there was no interfacial layer in the GaN/ZnO heterostructure grown at RT, which is consistent with the fact that the RHEED pattern of GaN remains streaky from the initial stage of RT growth. These results indicate that PLD is an appropriate technique for the fabrication of abrupt GaN/ZnO heterojunctions. To check the polarities of GaN grown on Zn-face ZnO at 700 1C and RT, we observed changes of the surface morphology after chemical etching by alkali solutions. Figs. 5a and b show AFM images of etched GaN surfaces grown at 700 1C and RT, respectively. Although the surface morphology of GaN grown at 700 1C remained almost unchanged (rms value: 1.1–1.6 nm), that grown at RT
0
50 100 150 Growth time (s)
200
Fig. 3. Intensity profile of the specular spot of the RHEED pattern during the room temperature growth.
experimental simulation
Log intensity (a.u.)
72
RT-GaN (50 nm) interface roughness<0.1 nm ZnO
0.5
1.0
1.5 2.0 2θ/ω (deg.)
2.5
3.0
Fig. 4. GIXR curve (gray dots) of GaN/Zn-face ZnO grown at room temperature and its theoretical fitting (solid line) based on the one-layer model.
Fig. 5. 1 1 mm2AFM images of GaN films grown on Zn-face ZnO at (a) 700 1C and (b) room temperature after alkali etching. The root-meansquare values of the surface roughness are 1.6 and 4.7 nm for (a) and (b), respectively.
became quite rough after chemical etching (rms value: 0.2–4.7 nm). Since it is well known that N-polarity GaN is easily etched in alkali solutions, and Ga-polarity GaN resists them, we concluded that RT-grown GaN has a N-polarity and that 700 1C-grown GaN has a Ga-polarity. This result indicates that inversion of the polarity from cation to anion occurs at the RT-GaN/ZnO(0 0 0 1) heterointerface in spite of the absence of the interfacial layer. It is interesting to note that RT-grown GaN on O-face ZnO substrates shows a Gapolarity [13] and its mechanism and interface structure have been well discussed using the results of the first principles calculations. To understand the mechanism of the polarity flip, further investigations, both theoretical and experimental, are necessary. In summary, we have succeeded in the epitaxial growth of GaN on atomically flat Zn-face ZnO at RT by the suppression of the interfacial reactions. GaN with an atomically flat stepped and terraced surface grows epitaxially at RT in the layer-by-layer mode. We also found that the polarity flips from cation face to anion face during the growth of GaN at RT in spite of the absence of any interfacial layers. One of the authors (A.K.) is supported by JSPS Research Fellowships for Young Scientists.
ARTICLE IN PRESS A. Kobayashi et al. / Journal of Crystal Growth 305 (2007) 70–73
References [1] S. Ito, J. Ohta, H. Fujioka, M. Oshima, Appl. Surf. Sci. 197/198 (2002) 384. [2] J. Ohta, H. Fujioka, H. Takahashi, M. Oshima, Phys. Status Solidi A 188 (2001) 497. [3] H. Takahashi, J. Ohta, H. Fujioka, M. Oshima, Thin Solid Films 407 (2002) 114. [4] J. Suda, H. Matsunami, J. Cryst. Growth 237–239 (2002) 1114. [5] T. Matsuoka, N. Yoshimoto, T. Sasaki, A. Katsui, J. Electron. Mater. 21 (1992) 157. [6] F. Hamdani, A. Botchkarev, W. Kim, H. Morkoc- , M. Yeadon, J.M. Gibson, et al., Appl. Phys. Lett. 70 (1997) 467. [7] X. Gu, M.A. Reshchikov, A. Teke, D. Johnstone, H. Morkoc- , B. Nemeth, et al., Appl. Phys. Lett. 84 (2004) 2268. [8] E.S. Hellman, D.N.E. Buchanan, D. Wiesmann, I. Brener, MRS Int. J. Nitride Semicond. Res. 1 (1996) 16.
73
[9] G. Namkoong, S. Burnham, K.-K. Lee, E. Trybus, W.A. Doolittle, M. Losurdo, et al., Appl. Phys. Lett. 87 (2005) 184104. [10] J. Ohta, H. Fujioka, S. Ito, M. Oshima, Appl. Phys. Lett. 81 (2002) 2373. [11] J. Ohta, H. Fujioka, M. Oshima, Appl. Phys. Lett. 83 (2003) 3060. [12] A. Kobayashi, H. Fujioka, J. Ohta, M. Oshima, Jpn. J. Appl. Phys. 43 (2004) L53. [13] A. Kobayashi, Y. Kawaguchi, J. Ohta, H. Fujioka, K. Fujiwara, A. Ishii, Appl. Phys. Lett. 88 (2006) 181907. [14] Y. Kawaguchi, J. Ohta, A. Kobayashi, H. Fujioka, Appl. Phys. Lett. 87 (2005) 221907. [15] A. Kobayashi, J. Ohta, H. Fujioka, Jpn. J. Appl. Phys. 45 (2006) 5724. [16] L.G. Parratt, Phys. Rev. 95 (1954) 359. [17] L. Nevot, P. Croce, Rev. Phys. Appl. 15 (1980) 761.