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GaN-filled carbon nanotubes: synthesis and photoluminescence
Chemical Physics Letters 381 (2003) 715–719 www.elsevier.com/locate/cplett
GaN-filled carbon nanotubes: synthesis and photoluminescence C.Y. Zhi, D.Y. Zhong, E.G. Wang
*
State Key Laboratory for Surface Physics and International Center for Quantum Structures, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-20, Beijing 100080, PR China Received 15 July 2003 Published online: 4 November 2003
Abstract Employing a GaAs substrate, GaN nanowires encapsulated in carbon nanotubes are synthesized by microwaveplasma-enhanced chemical vapor deposition. Almost 100% of the carbon nanotubes are filled with GaN. Both highresolution transmission electron microscopy and selected area electron diffraction pattern reveal high crystallization in the GaN nanowires. The formation mechanism of the GaN-core/C-shell structure is discussed, emphasizing chemistryÕs influence on the structure produced, particularly the role of Ga from the substrate. Photoluminescence measurements reveal an ultraviolet band located at 3.35 eV and a yellow band located at 2.20 eV. The redshift of the ultraviolet band is attributed to N vacancies that result from the Ga-rich conditions of growth. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Advances in novel GaN-based optoelectronic and electronic devices, such as light-emitting diodes [1], have made GaN a bright star in a variety of semiconducting materials. Great progress has been made in the fabrication of GaN-based films by adopting molecular beam epitaxy and metal– organic chemical vapor deposition [2,3]. Recently, GaN nanowires have attracted increased interest; these open up unique opportunities in fundamental research and in applications [4,5]. Many experi-
*
Corresponding author. Fax: +86-10-8264-9531. E-mail address: [email protected] (E.G. Wang).
mental techniques have been employed to grow GaN nanowires, such as a direct reaction of Ga with NH3 [6] and chemical vapor deposition [7]. On the other hand, filling materials into the hollow cavity of carbon nanotubes (CNTs) is particularly interesting because this structure is expected to exhibit novel physical properties [8]. Various kinds of metals have been filled into CNTs [9–11], but semiconducting-material-filled CNTs have been produced much less for study. Han et al. produced GaN-carbon composite nanotubes by arc-discharge and characterized the morphology and composition of the nanotubes [12]. However, the resulting samples were found not to be clean due to endemic to the arc-discharge method. Also, they provided no characterization of the samplesÕ properties.
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.141
716
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
In this Letter, we report the synthesis of GaN nanowires encapsulated in CNTs by microwaveplasma-enhanced chemical vapor deposition (MWCVD) using a GaAs substrate. The structure of the GaN-filled CNTs is determined by highresolution transmission electron microscopy (HRTEM) and the chemical composition is examined by electron energy loss spectroscopy (EELS). The formation mechanism of the coreshell structure is discussed with regard to the varied makeup of nanostructures under the influence of substrates of different chemicals. Photoluminescence (PL) measurements reveal an ultraviolet emission band and a yellow emission. A redshift is observed in the ultraviolet band and an explanation is presented.
2. Experimental The experimental procedure has been described in detail in [13]. In the present experiment, GaAs was employed as the substrate and coated with catalytic Fe particles by the sol–gel process. The precursors, N2 and CH4 , were introduced into the growth chamber with flow rates of 150 and 25 sccm, respectively. The temperature of the sample holder was kept at a constant 550 °C and the power of the microwave was 700 W. PL spectra of the GaN nanowires are measured during excitation by a He–Cd laser with a 325 nm wavelength at room temperature. The excitation power is 9.6 mW.
3. Results and discussion Fig. 1a shows a typical scanning electron microscopy image of an as-grown sample from the present experiment. It is very clean and the outside morphology shows nothing special in comparison with common nanotubes [14]. A tip growth mode is indicated by catalytic particles on the tips of the nanotubes [15]. Almost all of the CNTs are filled with GaN nanowires, as shown in Fig. 1b, the lowmagnification transmission electron microscopy image. The typical core-shell structure shown in Fig. 2 by HRTEM shows a CNT with diameter of
Fig. 1. (a) SEM image of CNTs filled with GaN. (b) Lowmagnification TEM image shows that almost 100% of the CNTs are filled with GaN.
about 46 nm. Inset A is the corresponding selected area electron diffraction (SAED) pattern, which indicates a core of hexagonal GaN. The incident direction of the electron beam is along h0 0 1i direction. The cross-lattice ripples plainly visible in the core indicate that the GaN is highly crystalline, as shown in inset B. The interface between the nanowire and nanotube is quite distinct, as shown by HRTEM. EELS shows that the Ga, C and N elements are present in the central part of the nanotube (such as region I in Fig. 2) and only C is found on the margin (such as region II in Fig. 2) in Fig. 3a and b, respectively. (The very inconspicuous peak at about 401 eV in Fig. 3b is attributed to the fact that our instrument cannot select a sample area small enough to include only the shell.) As reported earlier, carbon nitride nanobell structures have been obtained from N2 and CH4 precursors on a silicon substrate [16,17]. In contrast, core-
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
717
Fig. 2. HRTEM image of a nanotube filled with GaN. Inset A is the selected area diffraction pattern; the incidence direction of the electron beam is along the h0 0 1i direction. Inset B is the enlarged profile of one part of the nanotube.
shell structures form, and N is absent in the graphitic layers when GaAs is used as a substrate. Taking this substrate-chemistry effect into account along with the vapor–liquid–solid (VLS) growth mode [15], we consider the formation mechanism of the core-shell structures. Although the temperature of the sample holder is 550 °C, the actual temperature of the sample, heated by the plasma, can be at least as high as 580 °C during the growth process. GaN-cores do not appear if the temperature of the sample holder is kept at 500 °C or below, which highlights the crucial role of temperature for the growth of GaN. In fact, GaAs decomposes into atomic Ga and As at 580 °C easily, which provides the Ga for the growth of GaN. So C, N, Ga, and As containing species are included in the plasma and dissolve in the catalytic Fe droplets. Arsenic cannot bond with the three other kinds of atoms at so high temperature, so the
Fig. 3. (a) EELS taken from the central part of the nanotube; the inset is the peak corresponding to Ga. (b) EELS taken from the margin of the nanotube.
concentration of As ions in the Fe droplets saturates quickly. There are too few N ions free to bond with all of the C and Ga ions because the NBN bonds in the N2 molecules are too strong to be broken adequately by the plasma energy in these growth conditions. N tends to bond with Ga rather than with C under these conditions, primarily due to the influence of electronegativity. (For N, C, and Ga, the electronegativity numbers are 3.1, 2.5, and 1.8, respectively.) Therefore, Ga bonds with all the available N ions. That is, C cannot compete successfully with Ga to bond with
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C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
N, resulting in the formation of our filled CNTs rather than the carbon nitride nanobells that would have been expected in the absence of Ga. At the initial stage, the Fe melts on the substrate and forms droplets; the temperature of the bottom of the Fe droplets is lower due to contact with the substrate. GaN molecules diffuse through each droplet to the center of its bottom and condense there and the C ions condense around the GaN cluster. An Fe droplet is lifted up from the substrate by the ongoing condensation of GaN and C at its bottom, and the condensate gradually forms a stem beneath the rising droplet, with the GaNÕs concentration gradient between the top and bottom of the iron droplet driving the GaN molecules to the central part of the stem and the C ions similarly driven to the stemÕs margin. Thus, the GaN-core/C-shell structures form. An ultraviolet emission band at 3.35 eV and a yellow emission band at 2.20 eV are observed when samples were excited by a He–Ce laser, as shown in Fig. 4. Their full-widths at half maximum are 0.28 and 0.46 eV, respectively. The ultraviolet band is attributed to the band gap transition of GaN, while the yellow band is attributed to Ga or N vacancies or a related complex by some early studies [18,19], and to a C impurity by other studies [20–22]. Redshift of the ultraviolet band is 40 meV, in contrast with the well-known 3.39 eV band gap of GaN. It is believed that the redshift does not originate in a quantum confine-
ment effect. The calculated Bohr radius of GaN is 11 nm according to [23] – much smaller than the size of the sample GaN nanowires. On the other hand, the quantum confinement effect generally causes blueshift in the band gap transition process [24,25]. Another possible explanation is that the redshift is caused by a C impurity and the C impurity induces the yellow band simultaneously [20–22]. But the ionization energies of carbon have been predicted to be 0.2 eV regardless of whether the carbon atom is on a N site or a Ga site [26] – much larger than 40 meV. So a C impurity should not be the primary explanation for the redshift. As we discussed above, N ions are not numerous enough to bond with all the C and Ga ions. It is believed that N vacancies will appear under the Ga-rich conditions of growth. The energies of N vacancies lie roughly 40 meV below the conduction band of GaN [27], which matches the value of the redshift observed here very well. Most possibly, the redshift originates from the N vacancies caused by Ga-rich growth conditions.
4. Conclusions In summary, GaN nanowires filled in CNTs are fabricated by MWCVD using GaAs as a substrate. Almost 100% of the nanotubes are filled with GaN. The HRTEM and the SAED pattern reveal that the GaN nanowires are highly crystalline. Based on the analysis of chemical composition and VLS growth mode, the formation mechanism of GaN-core/C-shell structure is described. PL measurements are made. An ultraviolet band at 3.35 eV and a yellow band at 2.20 eV are observed. The redshift observed in the ultraviolet band is ascribed to the N vacancies resulting from the Ga-rich conditions of growth.
Acknowledgements
Fig. 4. PL spectrum of GaN nanowires encapsulated in CNTs.
The authors thank Wenchong Wang for PL measurements and acknowledge financial support from the NSF of China, the National Key Project for Basic Research (G2000067103) and the
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
National Key (2002AA311150).
Project
for
High-Tech.
References [1] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 76 (1994) 8189. [2] Y. Xie, Y. Qian, W. Wang, S. Zhang, Y. Zhang, Science 272 (1996) 1926. [3] T. Sasaki, S. Zembutsu, J. Appl. Phys. 61 (1987) 2533. [4] C.M. Lieber, Solid State Commun. 107 (1998) 607. [5] D. Snoke, Science 273 (1996) 1351. [6] M. He, I. Minus, P. Zhou, S.N. Mohammed, J.B. Halpern, R. Jacobs, W.L. Sarney, L. Salamanca-Riba, R.D. Vispute, Appl. Phys. Lett. 77 (2000) 3731. [7] H.Y. Peng, X.T. Zhou, N. Wang, Y.F. Zheng, L.S. Liao, W.S. Shi, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 327 (2000) 263. [8] T.W. Ebbsen, Phys. Today 26 (June) (1996). [9] J. Bao, C. Tie, Z. Suo, Q. Zhou, J. Hong, Adv. Mater. 14 (2002) 1483. [10] J. Bao, Q. Zhou, J. Hong, Z. Xu, Appl. Phys. Lett. 81 (2002) 4592. [11] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, M. Terrones, M. R€ uhle, R. Escudero, F. Morales, Appl. Phys. Lett. 75 (1999) 3363. [12] W. Han, P. Redlich, F. Ernst, M. R€ uhle, Appl. Phys. Lett. 76 (2000) 652.
719
[13] X. Ma, E.G. Wang, W. Zhou, D.A. Jefferson, J. Chen, S. Deng, N. Xu, J. Yuan, Appl. Phys. Lett. 75 (1999) 3105. [14] D.Y. Zhong, G.Y. Zhang, S. Liu, E.G. Wang, Q. Wang, H. Li, X.J. Huang, Appl. Phys. Lett. 79 (2001) 3500. [15] G.G. Tibbetts, J. Cryst. Growth 66 (1984) 632. [16] X. Ma, E.G. Wang, R.D. Tilley, D.A. Jefferson, W. Zhou, Appl. Lett. Phys. 77 (2000) 4136. [17] D.Y. Zhong, S. Liu, G. Zhang, E.G. Wang, J. Appl. Phys. 89 (2001) 5939. [18] T. Suski, P. Perlin, H. Teisseyre, M. Leszczy nski, I. Grzegory, J. Jun, M. Bockowski, S. Porowski, T.D. Moustakas, Appl. Phys. Lett. 67 (1995) 2188. [19] E.R. Glaser, T.A. Kennedy, K. Doverspike, L.B. Rowland, D.K. Gaskill, J.A. Freitas Jr., M.A. Khan, D.T. Olson, J.N. Kuznia, D.K. Wickenden, Phys. Rev. B 51 (1995) 13326. [20] J. Cao, D. Pavlidis, A. Eisenbach, A. Philippe, C. BruChevallier, G. Guillot, Appl. Phys. Lett. 71 (1997) 3880. [21] J. Kang, Y. Shen, Z. Wang, Mat. Sci. Eng. B 91–92 (2002) 303. [22] R. Zhang, T.F. Kuech, Appl. Lett. Phys. 72 (1998) 1611. [23] B.K. Ridley, Quantum Process in Semiconductors, Clarendon Press, Oxford, 1982, p. 62. [24] S. Shionoya, in: D.R. Vij (Ed.), Luminescence of Solids, Plenum Press, New York, 1998, p. 130. [25] S.M. Lee, Y.H. Lee, Y.G. Hwang, J. Elsner, D. Porezag, T. Frauenheim, Phys. Rev. B 60 (1999) 7788. [26] P. Bogusawski, E.L. Briggs, J. Bernholc, Appl. Phys. Lett. 69 (1996) 233. [27] H. Morkocß, Nitride Semiconductors and Devices, Springer, Berlin, 1998, p. 154.
GaN-filled carbon nanotubes: synthesis and photoluminescence C.Y. Zhi, D.Y. Zhong, E.G. Wang
*
State Key Laboratory for Surface Physics and International Center for Quantum Structures, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-20, Beijing 100080, PR China Received 15 July 2003 Published online: 4 November 2003
Abstract Employing a GaAs substrate, GaN nanowires encapsulated in carbon nanotubes are synthesized by microwaveplasma-enhanced chemical vapor deposition. Almost 100% of the carbon nanotubes are filled with GaN. Both highresolution transmission electron microscopy and selected area electron diffraction pattern reveal high crystallization in the GaN nanowires. The formation mechanism of the GaN-core/C-shell structure is discussed, emphasizing chemistryÕs influence on the structure produced, particularly the role of Ga from the substrate. Photoluminescence measurements reveal an ultraviolet band located at 3.35 eV and a yellow band located at 2.20 eV. The redshift of the ultraviolet band is attributed to N vacancies that result from the Ga-rich conditions of growth. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Advances in novel GaN-based optoelectronic and electronic devices, such as light-emitting diodes [1], have made GaN a bright star in a variety of semiconducting materials. Great progress has been made in the fabrication of GaN-based films by adopting molecular beam epitaxy and metal– organic chemical vapor deposition [2,3]. Recently, GaN nanowires have attracted increased interest; these open up unique opportunities in fundamental research and in applications [4,5]. Many experi-
*
Corresponding author. Fax: +86-10-8264-9531. E-mail address: [email protected] (E.G. Wang).
mental techniques have been employed to grow GaN nanowires, such as a direct reaction of Ga with NH3 [6] and chemical vapor deposition [7]. On the other hand, filling materials into the hollow cavity of carbon nanotubes (CNTs) is particularly interesting because this structure is expected to exhibit novel physical properties [8]. Various kinds of metals have been filled into CNTs [9–11], but semiconducting-material-filled CNTs have been produced much less for study. Han et al. produced GaN-carbon composite nanotubes by arc-discharge and characterized the morphology and composition of the nanotubes [12]. However, the resulting samples were found not to be clean due to endemic to the arc-discharge method. Also, they provided no characterization of the samplesÕ properties.
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.141
716
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
In this Letter, we report the synthesis of GaN nanowires encapsulated in CNTs by microwaveplasma-enhanced chemical vapor deposition (MWCVD) using a GaAs substrate. The structure of the GaN-filled CNTs is determined by highresolution transmission electron microscopy (HRTEM) and the chemical composition is examined by electron energy loss spectroscopy (EELS). The formation mechanism of the coreshell structure is discussed with regard to the varied makeup of nanostructures under the influence of substrates of different chemicals. Photoluminescence (PL) measurements reveal an ultraviolet emission band and a yellow emission. A redshift is observed in the ultraviolet band and an explanation is presented.
2. Experimental The experimental procedure has been described in detail in [13]. In the present experiment, GaAs was employed as the substrate and coated with catalytic Fe particles by the sol–gel process. The precursors, N2 and CH4 , were introduced into the growth chamber with flow rates of 150 and 25 sccm, respectively. The temperature of the sample holder was kept at a constant 550 °C and the power of the microwave was 700 W. PL spectra of the GaN nanowires are measured during excitation by a He–Cd laser with a 325 nm wavelength at room temperature. The excitation power is 9.6 mW.
3. Results and discussion Fig. 1a shows a typical scanning electron microscopy image of an as-grown sample from the present experiment. It is very clean and the outside morphology shows nothing special in comparison with common nanotubes [14]. A tip growth mode is indicated by catalytic particles on the tips of the nanotubes [15]. Almost all of the CNTs are filled with GaN nanowires, as shown in Fig. 1b, the lowmagnification transmission electron microscopy image. The typical core-shell structure shown in Fig. 2 by HRTEM shows a CNT with diameter of
Fig. 1. (a) SEM image of CNTs filled with GaN. (b) Lowmagnification TEM image shows that almost 100% of the CNTs are filled with GaN.
about 46 nm. Inset A is the corresponding selected area electron diffraction (SAED) pattern, which indicates a core of hexagonal GaN. The incident direction of the electron beam is along h0 0 1i direction. The cross-lattice ripples plainly visible in the core indicate that the GaN is highly crystalline, as shown in inset B. The interface between the nanowire and nanotube is quite distinct, as shown by HRTEM. EELS shows that the Ga, C and N elements are present in the central part of the nanotube (such as region I in Fig. 2) and only C is found on the margin (such as region II in Fig. 2) in Fig. 3a and b, respectively. (The very inconspicuous peak at about 401 eV in Fig. 3b is attributed to the fact that our instrument cannot select a sample area small enough to include only the shell.) As reported earlier, carbon nitride nanobell structures have been obtained from N2 and CH4 precursors on a silicon substrate [16,17]. In contrast, core-
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
717
Fig. 2. HRTEM image of a nanotube filled with GaN. Inset A is the selected area diffraction pattern; the incidence direction of the electron beam is along the h0 0 1i direction. Inset B is the enlarged profile of one part of the nanotube.
shell structures form, and N is absent in the graphitic layers when GaAs is used as a substrate. Taking this substrate-chemistry effect into account along with the vapor–liquid–solid (VLS) growth mode [15], we consider the formation mechanism of the core-shell structures. Although the temperature of the sample holder is 550 °C, the actual temperature of the sample, heated by the plasma, can be at least as high as 580 °C during the growth process. GaN-cores do not appear if the temperature of the sample holder is kept at 500 °C or below, which highlights the crucial role of temperature for the growth of GaN. In fact, GaAs decomposes into atomic Ga and As at 580 °C easily, which provides the Ga for the growth of GaN. So C, N, Ga, and As containing species are included in the plasma and dissolve in the catalytic Fe droplets. Arsenic cannot bond with the three other kinds of atoms at so high temperature, so the
Fig. 3. (a) EELS taken from the central part of the nanotube; the inset is the peak corresponding to Ga. (b) EELS taken from the margin of the nanotube.
concentration of As ions in the Fe droplets saturates quickly. There are too few N ions free to bond with all of the C and Ga ions because the NBN bonds in the N2 molecules are too strong to be broken adequately by the plasma energy in these growth conditions. N tends to bond with Ga rather than with C under these conditions, primarily due to the influence of electronegativity. (For N, C, and Ga, the electronegativity numbers are 3.1, 2.5, and 1.8, respectively.) Therefore, Ga bonds with all the available N ions. That is, C cannot compete successfully with Ga to bond with
718
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
N, resulting in the formation of our filled CNTs rather than the carbon nitride nanobells that would have been expected in the absence of Ga. At the initial stage, the Fe melts on the substrate and forms droplets; the temperature of the bottom of the Fe droplets is lower due to contact with the substrate. GaN molecules diffuse through each droplet to the center of its bottom and condense there and the C ions condense around the GaN cluster. An Fe droplet is lifted up from the substrate by the ongoing condensation of GaN and C at its bottom, and the condensate gradually forms a stem beneath the rising droplet, with the GaNÕs concentration gradient between the top and bottom of the iron droplet driving the GaN molecules to the central part of the stem and the C ions similarly driven to the stemÕs margin. Thus, the GaN-core/C-shell structures form. An ultraviolet emission band at 3.35 eV and a yellow emission band at 2.20 eV are observed when samples were excited by a He–Ce laser, as shown in Fig. 4. Their full-widths at half maximum are 0.28 and 0.46 eV, respectively. The ultraviolet band is attributed to the band gap transition of GaN, while the yellow band is attributed to Ga or N vacancies or a related complex by some early studies [18,19], and to a C impurity by other studies [20–22]. Redshift of the ultraviolet band is 40 meV, in contrast with the well-known 3.39 eV band gap of GaN. It is believed that the redshift does not originate in a quantum confine-
ment effect. The calculated Bohr radius of GaN is 11 nm according to [23] – much smaller than the size of the sample GaN nanowires. On the other hand, the quantum confinement effect generally causes blueshift in the band gap transition process [24,25]. Another possible explanation is that the redshift is caused by a C impurity and the C impurity induces the yellow band simultaneously [20–22]. But the ionization energies of carbon have been predicted to be 0.2 eV regardless of whether the carbon atom is on a N site or a Ga site [26] – much larger than 40 meV. So a C impurity should not be the primary explanation for the redshift. As we discussed above, N ions are not numerous enough to bond with all the C and Ga ions. It is believed that N vacancies will appear under the Ga-rich conditions of growth. The energies of N vacancies lie roughly 40 meV below the conduction band of GaN [27], which matches the value of the redshift observed here very well. Most possibly, the redshift originates from the N vacancies caused by Ga-rich growth conditions.
4. Conclusions In summary, GaN nanowires filled in CNTs are fabricated by MWCVD using GaAs as a substrate. Almost 100% of the nanotubes are filled with GaN. The HRTEM and the SAED pattern reveal that the GaN nanowires are highly crystalline. Based on the analysis of chemical composition and VLS growth mode, the formation mechanism of GaN-core/C-shell structure is described. PL measurements are made. An ultraviolet band at 3.35 eV and a yellow band at 2.20 eV are observed. The redshift observed in the ultraviolet band is ascribed to the N vacancies resulting from the Ga-rich conditions of growth.
Acknowledgements
Fig. 4. PL spectrum of GaN nanowires encapsulated in CNTs.
The authors thank Wenchong Wang for PL measurements and acknowledge financial support from the NSF of China, the National Key Project for Basic Research (G2000067103) and the
C.Y. Zhi et al. / Chemical Physics Letters 381 (2003) 715–719
National Key (2002AA311150).
Project
for
High-Tech.
References [1] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 76 (1994) 8189. [2] Y. Xie, Y. Qian, W. Wang, S. Zhang, Y. Zhang, Science 272 (1996) 1926. [3] T. Sasaki, S. Zembutsu, J. Appl. Phys. 61 (1987) 2533. [4] C.M. Lieber, Solid State Commun. 107 (1998) 607. [5] D. Snoke, Science 273 (1996) 1351. [6] M. He, I. Minus, P. Zhou, S.N. Mohammed, J.B. Halpern, R. Jacobs, W.L. Sarney, L. Salamanca-Riba, R.D. Vispute, Appl. Phys. Lett. 77 (2000) 3731. [7] H.Y. Peng, X.T. Zhou, N. Wang, Y.F. Zheng, L.S. Liao, W.S. Shi, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 327 (2000) 263. [8] T.W. Ebbsen, Phys. Today 26 (June) (1996). [9] J. Bao, C. Tie, Z. Suo, Q. Zhou, J. Hong, Adv. Mater. 14 (2002) 1483. [10] J. Bao, Q. Zhou, J. Hong, Z. Xu, Appl. Phys. Lett. 81 (2002) 4592. [11] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, M. Terrones, M. R€ uhle, R. Escudero, F. Morales, Appl. Phys. Lett. 75 (1999) 3363. [12] W. Han, P. Redlich, F. Ernst, M. R€ uhle, Appl. Phys. Lett. 76 (2000) 652.
719
[13] X. Ma, E.G. Wang, W. Zhou, D.A. Jefferson, J. Chen, S. Deng, N. Xu, J. Yuan, Appl. Phys. Lett. 75 (1999) 3105. [14] D.Y. Zhong, G.Y. Zhang, S. Liu, E.G. Wang, Q. Wang, H. Li, X.J. Huang, Appl. Phys. Lett. 79 (2001) 3500. [15] G.G. Tibbetts, J. Cryst. Growth 66 (1984) 632. [16] X. Ma, E.G. Wang, R.D. Tilley, D.A. Jefferson, W. Zhou, Appl. Lett. Phys. 77 (2000) 4136. [17] D.Y. Zhong, S. Liu, G. Zhang, E.G. Wang, J. Appl. Phys. 89 (2001) 5939. [18] T. Suski, P. Perlin, H. Teisseyre, M. Leszczy nski, I. Grzegory, J. Jun, M. Bockowski, S. Porowski, T.D. Moustakas, Appl. Phys. Lett. 67 (1995) 2188. [19] E.R. Glaser, T.A. Kennedy, K. Doverspike, L.B. Rowland, D.K. Gaskill, J.A. Freitas Jr., M.A. Khan, D.T. Olson, J.N. Kuznia, D.K. Wickenden, Phys. Rev. B 51 (1995) 13326. [20] J. Cao, D. Pavlidis, A. Eisenbach, A. Philippe, C. BruChevallier, G. Guillot, Appl. Phys. Lett. 71 (1997) 3880. [21] J. Kang, Y. Shen, Z. Wang, Mat. Sci. Eng. B 91–92 (2002) 303. [22] R. Zhang, T.F. Kuech, Appl. Lett. Phys. 72 (1998) 1611. [23] B.K. Ridley, Quantum Process in Semiconductors, Clarendon Press, Oxford, 1982, p. 62. [24] S. Shionoya, in: D.R. Vij (Ed.), Luminescence of Solids, Plenum Press, New York, 1998, p. 130. [25] S.M. Lee, Y.H. Lee, Y.G. Hwang, J. Elsner, D. Porezag, T. Frauenheim, Phys. Rev. B 60 (1999) 7788. [26] P. Bogusawski, E.L. Briggs, J. Bernholc, Appl. Phys. Lett. 69 (1996) 233. [27] H. Morkocß, Nitride Semiconductors and Devices, Springer, Berlin, 1998, p. 154.