- Email: [email protected]
Modulated beam growth method for MBE grown GaN layers
ARTICLE IN PRESS
Journal of Crystal Growth 263 (2004) 400–405
Modulated beam growth method for MBE grown GaN layers K.T. Liua,*, T. Tezukab, S. Sugitab, Y. Watarib, Y. Horikoshib, Y.K. Sua, S.J. Changa a
Institute of Microelectronics, Department of Electrical Engineering National Chen Kung University, 1 University Road, Tainan 70101, Taiwan b School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 13 October 2003; accepted 28 November 2003 Communicated by M. Schieber
Abstract A novel modulated beam growth method was proposed for the low temperature molecular beam epitaxy grown GaN layers. From reflection high-energy electron diffraction patterns, it was found that we could alternately achieve Nenriched surfaces and Ga-enriched surfaces during the growth. Scanning electron microscopic pictures also show that we could achieve better surface morphologies by using the modulated beam growth method. Improved X-ray diffraction characteristics were also demonstrated. These observations could all be attributed to the enhanced lateral growth of the modulated beam growth method. r 2003 Elsevier B.V. All rights reserved. Keywords: A1. Reflection high energy electron diffraction; A1. Scanning electron microscope; A1. X-ray diffraction; A3. Modulated beam growth; A3. RF plasma assisted molecular beam epitaxy; A3. Stoichiometric growth; B1. Ga-enrich; B1. N-enrich
1. Introduction GaN and the related group III-nitrides prepared by metalorganic chemical vapor deposition (MOCVD) have been proven to be useful for various device applications, such as light emitting diodes (LEDs) [1–5] and high power, high-speed transistors [6–8]. High-quality III-nitride crystals prepared by MOCVD, however, could only be grown at high temperatures (i.e. >1000 C) [9–10]. It has been reported that crystal qualities of the MOCVD grown GaN films prepared at low *Corresponding author. Tel.: +88662351864; +88662351864. E-mail address: [email protected] (K.T. Liu).
fax:
temperatures were poor [11]. High temperature growth often results in considerable impurity redistribution during the growth. For LED applications, high temperature growth might also result in severe intermixing in the quantum well active region. Previously, it has been shown that GaN films prepared at low temperatures (i.e. below 800 C) with reasonably good crystal quality could be achieved by RF plasma assisted molecular beam epitaxy (RF-MBE) [12]. However, it has also been shown that vertical striations were often found in the cleaved sections of the GaN films prepared by RF-MBE. Such a result indicates that the growth is predominately occurred in the vertical direction in these nitride films. The dense column structures observed from the low
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.100
ARTICLE IN PRESS K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
temperature grown GaN films are probably due to the low migration speed of Ga adatoms on the sample surface. As a result, GaN growth rates in the lateral directions are much smaller than that in the vertical (0 0 0 1) direction. It is very likely that these column structures could result in a significant degradation in the optical and the electrical properties of nitride-based devices. One possible way to solve this problem is to grow the low temperature GaN films under Ga-enriched growth conditions. Although the Ga-enriched growth conditions could enhance the lateral growth rate, a large Ga flux might result in the formation of Ga droplets on the sample surface. As a result, the sample surface will become rough. Fig. 1(a) and (b) show the schematic diagrams of the substrate surfaces under N- and Ga-enriched conditions, respectively. It can be seen clearly that N-enriched
401
condition leads to three-dimensional (3D) growth due to the small surface migration speed of Ga adatoms at low temperatures. Although we can uniformly cover the sample surfaces with the surplus Ga atoms under Ga-enriched condition, it might lead to the formation of Ga droplets. On the other hand, if we can modulate the Ga flow rate, we might be able to enhance the GaN lateral growth rate without the formation of Ga droplets. In this study, a novel modulated beam growth method is proposed to grow the low temperature GaN films by RF-MBE. Using such a modulated beam growth method, it was found that we could alternately achieve N- and Ga-enriched surfaces during the growth. Detailed growth procedures and the characteristics of the GaN samples prepared by such a method will be reported.
2. Experiments N-enriched condition Ga-flux : small
N-flux : large
Substrate
(a)
Ga-enriched condition Ga-flux : large
(b)
Ga atom N atom
Ga atom N atom
N-flux : small
Substrate
Fig. 1. Schematic diagrams of the substrate surfaces under (a) N-enriched and (b) Ga-enriched conditions.
Samples used in this study were all grown by RF-MBE on c-face (0 0 0 1) sapphire substrates. Prior to the growth, the initial nitridations of sapphire substrates were performed at 640 C. During nitridations, the N2 gas flow rate and the RF power were kept at 1.5 sccm and 300 W, respectively. AlN buffer layers with a thickness of approximately 17 mono-layers were subsequently deposited onto the sapphire substrates. At this stage, the N2 gas flow rate and the RF power were kept at 2 sccm and 300 W, respectively. We then raised the substrate temperature to 730 C to grow the unintentionally doped GaN epitaxial layers. In this study, two different conditions with two different Ga flows were used to grow the GaN epitaxial layers. During the growth, the N2 gas flow rate, RF power and the growth rate were fixed at 8 sccm, 470 W and 0.5 mm/h, respectively, under both conditions. Under stoichiometric condition, GaN epitaxial layers were grown under a constant Ga flow rate, which was the maximum flow rate without the formation of Ga droplets. Under Ga-flux modulation condition, we modulated the Ga-flux during the growth. The Ga-flux modulation was performed by using two Ga effusion cells with different temperatures so as to produce two different beam intensities.
ARTICLE IN PRESS 402
K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
Fig. 2 shows the time sequence of Ga-flux under Ga-flux modulation condition. The dash line shown in this figure depicts the Ga flux under stoichiometric condition (i.e., the maximum Ga flow rate without the formation of Ga droplets). We first opened the shutter of one Ga effusion cell only at t0 to achieve a Ga-flux F1 : In this period (i.e., in between t0 and t1 ), the sample surfaces were N-enriched. We then opened the shutter of the second Ga effusion cell at t1 to achieve a total Ga-flux F2 : In this period (i.e., in between t1 and t2 ), the sample surfaces were Ga-enriched. We subsequently closed the shutter of the second Ga effusion cell at t2 so as to achieve the N-enriched surfaces again. In other words, we kept the shutter of one Ga effusion cell constantly open while the shutter of the second Ga effusion cell was opened intermittently under Ga-flux modulation condition. By properly controlling the parameters of the Ga-flux modulation condition, we could alternately achieve N-enriched surfaces and Ga-enriched surfaces. In this study, the periodicity of the Ga-flux modulation we kept at 60 s while the average Ga-flux was the same as that under stoichiometric condition. The total thicknesses of the samples were all around 3 mm. Reflection highenergy electron diffraction (RHEED) was used to monitor the GaN surface morphology and reconstruction. Surface morphologies of the as-grown GaN samples were evaluated by scanning electron microscope (SEM). The as-grown GaN samples were also characterized by X-ray diffraction (XRD).
Fig. 2. Time sequence of Ga-flux under Ga-flux modulation condition. The dash line shown in this figure depicts the Ga flux under stoichiometric condition.
3. Results and discussion Fig. 3(a) and (b) show RHEED patterns of the modulated beam grown GaN epitaxial layers during growth. As shown in Fig. 3(a), we observed the (2 2) diffraction pattern during the period t0 2t1 : Such a pattern indicates that the sample surfaces were N-enriched [13]. The spotty pattern observed in this figure also suggests that sample surface was rough and was dominated by 3D growth during this period. On the other hand, the (1 1) streaky diffraction pattern was observed during the period t1 2t2 ; which suggests a smooth
Fig. 3. RHEED patterns of the modulated beam grown GaN epitaxial layers (a) in between t0 and t1 and (b) in between t1 and t2 :
ARTICLE IN PRESS K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
Ga-enriched surface. The smooth surface could be attributed to the enhanced migration of Ga adatoms due to the increased Ga-flux. These observations also indicate that we could indeed alternatively achieve N-enhanced and Ga-enhanced surfaces under Ga-flux modulation condition. In contrast, only spotty (2 2) diffraction pattern was observed under stoichiometric condition. Figs. 4(a) and (b) show the post-growth RHEED patterns observed with the electron beam directed along the ½1 1% 0 0 and ½2 1% 1% 0 directions for the GaN epitaxial layers grown under stoichiometric condition, respectively. It can be seen clearly that both figures show (1 1) spotty-like diffraction patterns. In contrast, the post-growth RHEED patterns observed with the electron beam directed along the ½1 1% 0 0 and ½2 1% 1% 0 directions for the GaN epitaxial layers grown under Ga-flux modulation condition were (4 4) streaky diffraction patterns, as shown in Fig. 4(c) and (d),
403
respectively. These results suggest that we could achieve a smoother sample surface by using the Ga-flux modulation method. Figs. 5(a) and (b) show SEM surface morphologies of the GaN epitaxial layers grown under stoichiometric and Ga-flux modulation conditions, respectively. As shown in Fig. 5(a), a large number of surface pits were found from the samples grown under stoichiometric condition. In contrast, the surface morphology was smoother for the samples grown under Ga-flux modulation condition as shown in Fig. 5(b). Such a result agrees well with that observed from post-growth RHEED patterns. This observation could again be attributed to the enhanced lateral growth of the modulated beam growth method. Fig. 6 compares the XRD rocking curves of the stoichiometry grown and modulated beam grown GaN epitaxial layers. The XRD rocking curve full-width-half-maximum (FWHM) of the modulated beam grown GaN is 340 s. Such a value is much smaller than the 462 s XRD
Fig. 4. Post-growth RHEED patterns observed with the electron beam directed along the (a) ½1 1% 0 0 direction under stoichiometric condition, (b) ½2 1% 1% 0 direction under stoichiometric condition, (c) ½1 1% 0 0 direction under Ga-flux modulation condition and (d) ½2 1% 1% 0 direction under Ga-flux modulation condition.
ARTICLE IN PRESS 404
K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
Intensity (arb. units)
Modulated beam grown GaN FWHM = 340 sec
Stoichiometry grown GaN FWHM = 462 sec
16.7
16.9
17.1
17.3
ω (degrees) Fig. 6. XRD rocking curves of the GaN epitaxial layers grown under stoichiometric and Ga-flux modulation conditions.
the growth. SEM pictures also show that we could achieve better surface morphologies by using the modulated beam growth method. Improved XRD characteristics were also demonstrated. These observations could all be attributed to the enhanced lateral growth of the modulated beam growth method.
Acknowledgements Fig. 5. SEM surface morphologies of the GaN epitaxial layers grown under (a) stoichiometric and (b) Ga-flux modulation conditions.
rocking curve observed from the stoichiometry grown GaN sample. This indicates that the Gaflux modulation drastically improves the crystal quality.
The work was partly supported by the Japan Society for the Promotion of Science ‘‘Research for the Future’’ Program (JSPS-RFTF96P00103), and also COE Program ‘‘Molecular Nano-Engineering’’ from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References 4. Conclusions A novel modulated beam growth method was proposed for the low temperature MBE grown GaN epitaxial layers. From RHEED patterns, it was found that we could alternately achieve Nenriched surfaces and Ga-enriched surfaces during
[1] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 74 (1995) L797. [2] S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, J.K. Sheu, T.C. Wen, W.C. Lai, J.F. Chen, J.M. Tsai, IEEE J. Sel. Top. Quan. Electron. 8 (2002) 744. [3] N. Maeda, T. Nishida, N. Kobayashi, M. Tomizawa, Appl. Phys. Lett. 73 (1998) 1856.
ARTICLE IN PRESS K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405 [4] C.H. Kuo, S.J. Chang, Y.K. Su, J.F. Chen, L.W. Wu, J.K. Sheu, C.H. Chen, G.C. Chi, IEEE Electron. Dev. Lett. 23 (2002) 240. [5] W.C. Lai, S.J. Chang, M. Yokoyama, J.K. Sheu, J.F. Chen, IEEE Photon. Technol. Lett. 13 (2001) 559. [6] Y.Z. Chiou, S.J. Chang, Y.K. Su, C.K. Wang, T.K. Lin, B.R. Huang, IEEE Trans. Electron. Dev. 50 (2003) 1748. [7] Y.K. Su, S.C. Wei, R.L. Wang, S.J. Chang, C.H. Ko, T.M. Kuan, IEEE Electron. Dev. Lett. 24 (2003) 622. [8] S.J. Chang, S.C. Wei, Y.K. Su, T.Y. Tsai, T.H. Hsu, S.C. Chen, C.H. Liu, Jpn. J. Appl. Phys. 42 (2003) 3316.
405
[9] S.J. Chang, W.C. Lai, Y.K. Su, J.F. Chen, C.H. Liu, U.H. Liaw, IEEE J. Sel. Top. Quan. Electron. 8 (2002) 278. [10] S.J. Chang, M.L. Lee, J.K. Sheu, W.C. Lai, Y.K. Su, C.S. Chang, C.J. Kao, G.C. Chi, J.M. Tsai, IEEE Electron. Dev. Lett. 24 (2003) 212. [11] O. Briot, J.P. Alexis, M. Tchounkeu, R.L. Aulombard, Mater. Sci. Eng. B43 (1997) 147. [12] L.T. Romano, B.S. Krusor, R. Singh, T.D. Moustakas, J. Electron. Mater. 26 (1997) 285. [13] P. Hacke, G. Feuillet, H. Okumura, S. Yoshida, Appl. Phys. Lett. 69 (1996) 2507.
Journal of Crystal Growth 263 (2004) 400–405
Modulated beam growth method for MBE grown GaN layers K.T. Liua,*, T. Tezukab, S. Sugitab, Y. Watarib, Y. Horikoshib, Y.K. Sua, S.J. Changa a
Institute of Microelectronics, Department of Electrical Engineering National Chen Kung University, 1 University Road, Tainan 70101, Taiwan b School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 13 October 2003; accepted 28 November 2003 Communicated by M. Schieber
Abstract A novel modulated beam growth method was proposed for the low temperature molecular beam epitaxy grown GaN layers. From reflection high-energy electron diffraction patterns, it was found that we could alternately achieve Nenriched surfaces and Ga-enriched surfaces during the growth. Scanning electron microscopic pictures also show that we could achieve better surface morphologies by using the modulated beam growth method. Improved X-ray diffraction characteristics were also demonstrated. These observations could all be attributed to the enhanced lateral growth of the modulated beam growth method. r 2003 Elsevier B.V. All rights reserved. Keywords: A1. Reflection high energy electron diffraction; A1. Scanning electron microscope; A1. X-ray diffraction; A3. Modulated beam growth; A3. RF plasma assisted molecular beam epitaxy; A3. Stoichiometric growth; B1. Ga-enrich; B1. N-enrich
1. Introduction GaN and the related group III-nitrides prepared by metalorganic chemical vapor deposition (MOCVD) have been proven to be useful for various device applications, such as light emitting diodes (LEDs) [1–5] and high power, high-speed transistors [6–8]. High-quality III-nitride crystals prepared by MOCVD, however, could only be grown at high temperatures (i.e. >1000 C) [9–10]. It has been reported that crystal qualities of the MOCVD grown GaN films prepared at low *Corresponding author. Tel.: +88662351864; +88662351864. E-mail address: [email protected] (K.T. Liu).
fax:
temperatures were poor [11]. High temperature growth often results in considerable impurity redistribution during the growth. For LED applications, high temperature growth might also result in severe intermixing in the quantum well active region. Previously, it has been shown that GaN films prepared at low temperatures (i.e. below 800 C) with reasonably good crystal quality could be achieved by RF plasma assisted molecular beam epitaxy (RF-MBE) [12]. However, it has also been shown that vertical striations were often found in the cleaved sections of the GaN films prepared by RF-MBE. Such a result indicates that the growth is predominately occurred in the vertical direction in these nitride films. The dense column structures observed from the low
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.100
ARTICLE IN PRESS K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
temperature grown GaN films are probably due to the low migration speed of Ga adatoms on the sample surface. As a result, GaN growth rates in the lateral directions are much smaller than that in the vertical (0 0 0 1) direction. It is very likely that these column structures could result in a significant degradation in the optical and the electrical properties of nitride-based devices. One possible way to solve this problem is to grow the low temperature GaN films under Ga-enriched growth conditions. Although the Ga-enriched growth conditions could enhance the lateral growth rate, a large Ga flux might result in the formation of Ga droplets on the sample surface. As a result, the sample surface will become rough. Fig. 1(a) and (b) show the schematic diagrams of the substrate surfaces under N- and Ga-enriched conditions, respectively. It can be seen clearly that N-enriched
401
condition leads to three-dimensional (3D) growth due to the small surface migration speed of Ga adatoms at low temperatures. Although we can uniformly cover the sample surfaces with the surplus Ga atoms under Ga-enriched condition, it might lead to the formation of Ga droplets. On the other hand, if we can modulate the Ga flow rate, we might be able to enhance the GaN lateral growth rate without the formation of Ga droplets. In this study, a novel modulated beam growth method is proposed to grow the low temperature GaN films by RF-MBE. Using such a modulated beam growth method, it was found that we could alternately achieve N- and Ga-enriched surfaces during the growth. Detailed growth procedures and the characteristics of the GaN samples prepared by such a method will be reported.
2. Experiments N-enriched condition Ga-flux : small
N-flux : large
Substrate
(a)
Ga-enriched condition Ga-flux : large
(b)
Ga atom N atom
Ga atom N atom
N-flux : small
Substrate
Fig. 1. Schematic diagrams of the substrate surfaces under (a) N-enriched and (b) Ga-enriched conditions.
Samples used in this study were all grown by RF-MBE on c-face (0 0 0 1) sapphire substrates. Prior to the growth, the initial nitridations of sapphire substrates were performed at 640 C. During nitridations, the N2 gas flow rate and the RF power were kept at 1.5 sccm and 300 W, respectively. AlN buffer layers with a thickness of approximately 17 mono-layers were subsequently deposited onto the sapphire substrates. At this stage, the N2 gas flow rate and the RF power were kept at 2 sccm and 300 W, respectively. We then raised the substrate temperature to 730 C to grow the unintentionally doped GaN epitaxial layers. In this study, two different conditions with two different Ga flows were used to grow the GaN epitaxial layers. During the growth, the N2 gas flow rate, RF power and the growth rate were fixed at 8 sccm, 470 W and 0.5 mm/h, respectively, under both conditions. Under stoichiometric condition, GaN epitaxial layers were grown under a constant Ga flow rate, which was the maximum flow rate without the formation of Ga droplets. Under Ga-flux modulation condition, we modulated the Ga-flux during the growth. The Ga-flux modulation was performed by using two Ga effusion cells with different temperatures so as to produce two different beam intensities.
ARTICLE IN PRESS 402
K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
Fig. 2 shows the time sequence of Ga-flux under Ga-flux modulation condition. The dash line shown in this figure depicts the Ga flux under stoichiometric condition (i.e., the maximum Ga flow rate without the formation of Ga droplets). We first opened the shutter of one Ga effusion cell only at t0 to achieve a Ga-flux F1 : In this period (i.e., in between t0 and t1 ), the sample surfaces were N-enriched. We then opened the shutter of the second Ga effusion cell at t1 to achieve a total Ga-flux F2 : In this period (i.e., in between t1 and t2 ), the sample surfaces were Ga-enriched. We subsequently closed the shutter of the second Ga effusion cell at t2 so as to achieve the N-enriched surfaces again. In other words, we kept the shutter of one Ga effusion cell constantly open while the shutter of the second Ga effusion cell was opened intermittently under Ga-flux modulation condition. By properly controlling the parameters of the Ga-flux modulation condition, we could alternately achieve N-enriched surfaces and Ga-enriched surfaces. In this study, the periodicity of the Ga-flux modulation we kept at 60 s while the average Ga-flux was the same as that under stoichiometric condition. The total thicknesses of the samples were all around 3 mm. Reflection highenergy electron diffraction (RHEED) was used to monitor the GaN surface morphology and reconstruction. Surface morphologies of the as-grown GaN samples were evaluated by scanning electron microscope (SEM). The as-grown GaN samples were also characterized by X-ray diffraction (XRD).
Fig. 2. Time sequence of Ga-flux under Ga-flux modulation condition. The dash line shown in this figure depicts the Ga flux under stoichiometric condition.
3. Results and discussion Fig. 3(a) and (b) show RHEED patterns of the modulated beam grown GaN epitaxial layers during growth. As shown in Fig. 3(a), we observed the (2 2) diffraction pattern during the period t0 2t1 : Such a pattern indicates that the sample surfaces were N-enriched [13]. The spotty pattern observed in this figure also suggests that sample surface was rough and was dominated by 3D growth during this period. On the other hand, the (1 1) streaky diffraction pattern was observed during the period t1 2t2 ; which suggests a smooth
Fig. 3. RHEED patterns of the modulated beam grown GaN epitaxial layers (a) in between t0 and t1 and (b) in between t1 and t2 :
ARTICLE IN PRESS K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
Ga-enriched surface. The smooth surface could be attributed to the enhanced migration of Ga adatoms due to the increased Ga-flux. These observations also indicate that we could indeed alternatively achieve N-enhanced and Ga-enhanced surfaces under Ga-flux modulation condition. In contrast, only spotty (2 2) diffraction pattern was observed under stoichiometric condition. Figs. 4(a) and (b) show the post-growth RHEED patterns observed with the electron beam directed along the ½1 1% 0 0 and ½2 1% 1% 0 directions for the GaN epitaxial layers grown under stoichiometric condition, respectively. It can be seen clearly that both figures show (1 1) spotty-like diffraction patterns. In contrast, the post-growth RHEED patterns observed with the electron beam directed along the ½1 1% 0 0 and ½2 1% 1% 0 directions for the GaN epitaxial layers grown under Ga-flux modulation condition were (4 4) streaky diffraction patterns, as shown in Fig. 4(c) and (d),
403
respectively. These results suggest that we could achieve a smoother sample surface by using the Ga-flux modulation method. Figs. 5(a) and (b) show SEM surface morphologies of the GaN epitaxial layers grown under stoichiometric and Ga-flux modulation conditions, respectively. As shown in Fig. 5(a), a large number of surface pits were found from the samples grown under stoichiometric condition. In contrast, the surface morphology was smoother for the samples grown under Ga-flux modulation condition as shown in Fig. 5(b). Such a result agrees well with that observed from post-growth RHEED patterns. This observation could again be attributed to the enhanced lateral growth of the modulated beam growth method. Fig. 6 compares the XRD rocking curves of the stoichiometry grown and modulated beam grown GaN epitaxial layers. The XRD rocking curve full-width-half-maximum (FWHM) of the modulated beam grown GaN is 340 s. Such a value is much smaller than the 462 s XRD
Fig. 4. Post-growth RHEED patterns observed with the electron beam directed along the (a) ½1 1% 0 0 direction under stoichiometric condition, (b) ½2 1% 1% 0 direction under stoichiometric condition, (c) ½1 1% 0 0 direction under Ga-flux modulation condition and (d) ½2 1% 1% 0 direction under Ga-flux modulation condition.
ARTICLE IN PRESS 404
K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405
Intensity (arb. units)
Modulated beam grown GaN FWHM = 340 sec
Stoichiometry grown GaN FWHM = 462 sec
16.7
16.9
17.1
17.3
ω (degrees) Fig. 6. XRD rocking curves of the GaN epitaxial layers grown under stoichiometric and Ga-flux modulation conditions.
the growth. SEM pictures also show that we could achieve better surface morphologies by using the modulated beam growth method. Improved XRD characteristics were also demonstrated. These observations could all be attributed to the enhanced lateral growth of the modulated beam growth method.
Acknowledgements Fig. 5. SEM surface morphologies of the GaN epitaxial layers grown under (a) stoichiometric and (b) Ga-flux modulation conditions.
rocking curve observed from the stoichiometry grown GaN sample. This indicates that the Gaflux modulation drastically improves the crystal quality.
The work was partly supported by the Japan Society for the Promotion of Science ‘‘Research for the Future’’ Program (JSPS-RFTF96P00103), and also COE Program ‘‘Molecular Nano-Engineering’’ from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References 4. Conclusions A novel modulated beam growth method was proposed for the low temperature MBE grown GaN epitaxial layers. From RHEED patterns, it was found that we could alternately achieve Nenriched surfaces and Ga-enriched surfaces during
[1] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 74 (1995) L797. [2] S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, J.K. Sheu, T.C. Wen, W.C. Lai, J.F. Chen, J.M. Tsai, IEEE J. Sel. Top. Quan. Electron. 8 (2002) 744. [3] N. Maeda, T. Nishida, N. Kobayashi, M. Tomizawa, Appl. Phys. Lett. 73 (1998) 1856.
ARTICLE IN PRESS K.T. Liu et al. / Journal of Crystal Growth 263 (2004) 400–405 [4] C.H. Kuo, S.J. Chang, Y.K. Su, J.F. Chen, L.W. Wu, J.K. Sheu, C.H. Chen, G.C. Chi, IEEE Electron. Dev. Lett. 23 (2002) 240. [5] W.C. Lai, S.J. Chang, M. Yokoyama, J.K. Sheu, J.F. Chen, IEEE Photon. Technol. Lett. 13 (2001) 559. [6] Y.Z. Chiou, S.J. Chang, Y.K. Su, C.K. Wang, T.K. Lin, B.R. Huang, IEEE Trans. Electron. Dev. 50 (2003) 1748. [7] Y.K. Su, S.C. Wei, R.L. Wang, S.J. Chang, C.H. Ko, T.M. Kuan, IEEE Electron. Dev. Lett. 24 (2003) 622. [8] S.J. Chang, S.C. Wei, Y.K. Su, T.Y. Tsai, T.H. Hsu, S.C. Chen, C.H. Liu, Jpn. J. Appl. Phys. 42 (2003) 3316.
405
[9] S.J. Chang, W.C. Lai, Y.K. Su, J.F. Chen, C.H. Liu, U.H. Liaw, IEEE J. Sel. Top. Quan. Electron. 8 (2002) 278. [10] S.J. Chang, M.L. Lee, J.K. Sheu, W.C. Lai, Y.K. Su, C.S. Chang, C.J. Kao, G.C. Chi, J.M. Tsai, IEEE Electron. Dev. Lett. 24 (2003) 212. [11] O. Briot, J.P. Alexis, M. Tchounkeu, R.L. Aulombard, Mater. Sci. Eng. B43 (1997) 147. [12] L.T. Romano, B.S. Krusor, R. Singh, T.D. Moustakas, J. Electron. Mater. 26 (1997) 285. [13] P. Hacke, G. Feuillet, H. Okumura, S. Yoshida, Appl. Phys. Lett. 69 (1996) 2507.