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Self-catalyzed vapor–liquid–solid growth of large-scale single crystal GaN whiskers
Materials Letters 57 (2003) 3880 – 3883 www.elsevier.com/locate/matlet
Self-catalyzed vapor–liquid–solid growth of large-scale single crystal GaN whiskers Shao-Min Zhou * Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China Lingling University, Yongzhou 425000, PR China Received 7 December 2002; accepted 11 March 2003
Abstract Based on self-catalyzed vapor – liquid – solid mechanism, large-scale hexagonal GaN whiskers were synthesized via a twostep reaction of GaS powders with H2 and NH3 at 1000 jC. These whiskers were characterized using XRD, SEM, TEM, SAED, HRTEM, and Raman spectroscopy, in which the results obtained show that these GaN whiskers have the single crystal wurtzite structure with width of 20 – 200 nm and length of several micrometers. D 2003 Elsevier Science B.V. All rights reserved. Keywords: GaN nanomaterials; Self-catalyzed vapor – liquid – solid; III – V semiconductors; Single crystal
1. Introduction As a wide and direct band gap (3.4 eV) semicondictor [1], nanometer-scale GaN is the focus of current research due to the promise for use as ultraviolet or blue emitters, detectors, high-speed fieldeffect transistors, and high-temperature electronic devices [1 –3]. Various methods have been made to manufacture quasi-one-dimensional GaN nanostructured materials, such as GaN nanowires, nanorods, and nanobelts. Han et al. [4] and Cheng et al. [5] have synthesized GaN nanorods and GaN nanowires using carbon-nanotube-confined and anodic alumina template, respectively. Recently, Bae et al. [6] reported
* Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China. Fax: +86-551-5591434. E-mail address: [email protected] (S.-M. Zhou).
the synthesis of GaN nanobelts via a catalyst-assisted CVD and Chen et al. [7] fabricated GaN nanowires by the catalytic growth. However, these methods mentioned above inevitably lead to impure products because GaN samples can hardly be separated from the templates used or the catalysts used. In fact, it is very necessary to produce large-scale high pure GaN whiskers (nanoscale materials) without involvement of any template or patterned catalyst, particular for fundamental research and the further electronics applications of GaN. In this paper, we report a new and simple process to prepare large quantities of high pure GaN whiskers based on the first reduction of GaS powders in H2 and the following direct reaction of metal Ga with NH3 under controlled conditions, where there is no template or catalyst. It is envisioned that this kind of high pure GaN whiskers may have potential applications.
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00233-7
S.-M. Zhou / Materials Letters 57 (2003) 3880–3883
3881
2. Experimental procedure Firstly, a quartz crucible was cleaned using a standard treatment in piranha solution (30% H2O2 + 20% H2SO4), and rinsed with deionized water. Then GaS powders were placed at the bottom of the quartz crucible, which was inserted into half of a quartz-tube reactor in the horizontal tube furnace. Prior to heating, the system was flushed with high-purity Ar gas to eliminate O2. Under a constant flow of Ar mixed with 5% H2, temperature of the reactor was rapidly increased to 1000 jC, held at this temperature for 2 h. Subsequently, the Ar/H2 was shifted into NH3 and the system was maintained 3 h under the constant flow of NH3. During the whole heating period, pressure inside the tube was kept at 300 Torr. After the furnace was cooled to the room temperature, the pale-yellow wool-like product deposited on the inner wall of the tube was cleaned and then collected for characterization, where they were characterized by X-ray diffraction (XRD) (Philips PW 1710 with Cu Kia radiation), scanning electron microscopy (SEM) (JEOL JSM-630), transmission electron microscopy (TEM)(JEM-200CX), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy (Spex-1403, a 514-nm line of an Ar+ laser and 200 mW out power). In experiment, all chemicals
Fig. 1. X-ray diffraction patterns of GaN whiskers [Cu Ka radiation and a scanning speed of 0.02 s 1 in the 2h range 30 – 80j (20 – 200 nm in width and several micrometers in typical length)].
Fig. 2. A typical SEM image of large-scale GaN whiskers (width of about 20 – 200 nm and length of several micrometers).
used were of analytical grade and without further purification.
3. Results and discussion Fig. 1 shows XRD spectra of the as-synthesized GaN, in which Miller indices are marked on each diffraction peak. From these indices, the overall crystal structure and phase of GaN sample can be indicated. Also, it can be seen that the whole spectrum can be indexed in peak position to a crystalline GaN phase, which is consistent with hexagonal (wurtzite) structure with a lattice constant a = 0.318 nm, c = 0.516 nm (JCPDS 74-0243). Moreover, no diffraction peaks from GaS or other impurities are found in Fig. 1. SEM observation (Fig. 2) shows that the sample is composed of large quantities of whisker-like nanostructures with width of 20 – 200 nm and typical length up to several micrometers and these whiskers appear straight. A typical TEM image of GaN sample is presented in Fig. 3, where a smaller whisker with a width bout 20 nm can be seen and a small white ring region is used for SAED and HRTEM detection. From Fig. 3, one can also see that the whisker has smooth surface like the result from the-above-mentioned SEM image. SAED patterns in the inset of Fig. 3, taken from the white section of the single whisker, are composed of the regular clear diffraction dots, together with HRTEM
3882
S.-M. Zhou / Materials Letters 57 (2003) 3880–3883
onal GaN [9]. Not any peaks of impure elements are found in Fig. 4. In fact, these results are consistent with that of previous lateral epitaxial growth GaN [10] and also in good agreement with results of XRD. The formation process of GaN composition could be simple two-step reaction from a chemical reaction point of view, that is,
Fig. 3. TEM image of single GaN whisker (width of about 20 nm); the inset of top in the figure shows a typical selected area electron diffraction from the stem of the whisker (this section is marked with a white district).
(for the spaces limited, the streaky patterns are not shown here but it is presented in reply to the reviewer where clear streaky patterns can be viewed.), which reveal the single crystalline nature of the whisker and high pure GaN composition. And moreover, in other sections in the whisker SAED patterns and HRTEM images are the same the two figures, respectively. Raman spectrum of the as-fabricated GaN whiskers is shown in Fig. 4, in which three features of GaN are clearly seen. The most intense peak at 567.6 cm 1 is the well-known mode of E2 symmetry for hexagonal GaN [8]. The peaks at 532 and 556 cm 1 agree with phonon vibration frequencies of A1 (TO) and E1 (TO) modes of the hexag-
GaS ðgÞ þ H2 ðgÞ ! Ga ðlÞ þ H2 S ðgÞ
ð1Þ
Ga ðlÞ þ NH3 ðgÞ ! GaN ðsÞ þ H2 ðgÞ
ð2Þ
GaN ðnucleiÞ ! GaN ðwhiskerÞ
ð2VÞ
However, the formation of counterpart whiskers would involve a very complicated process although only step (2V) forms GaN whisker. Because of the fact that the whisker can only be formed above a certain process temperature (900 jC) and only in a place away from the center of the tube furnace (lower temperature range), we can reasonably speculate the whisker formation process as follows: GaS (gas) is carried by the processing gases and travels to the lower temperature region in the furnace tube and meanwhile reduced into metal Ga (liquid) by H2, where Ga deposits in the form of a liquid droplet on the inner wall of the tube. Then
Fig. 4. Room temperature Raman spectra of GaN whiskers with a 514-nm line of an Ar+ laser.
S.-M. Zhou / Materials Letters 57 (2003) 3880–3883
the liquidized droplet reacts with NH3 and forms GaN (nuclei), which further serves as a seed for GaN whisker growth, and the above growth is most likely to be controlled by the vapor – liquid – solid (VLS) mechanism [11]. In the present VLS growth of these GaN whiskers, no additional transition metals are added as catalysts; it is, therefore, believed that the formation of these GaN whiskers undergoes a selfcatalyzed VLS growth process. The metals not only act as the reactant but also provide an energetically favored site for the absorption of NH3. The newly formed GaN functions as a whisker seed, which further grows to a GaN whisker in the presence of Ga (liquid) and NH3 (gas), and the size and the shape of the whiskers are predecided by the size of the new reduced liquid metal droplet. In conclusion, self-catalyzed vapor – liquid – solid has been employed to synthesize large-scale GaN whiskers via a process of the first reduction of GaS in H2 and the following direct reaction of Ga and NH3 (template-catalyst-free growth). XRD, EDAS, Raman spectroscopy, SEM, TEM and HRTEM show that the GaN whiskers are hexagonal single crystalline with the width of about 20– 200 nm and the typical length up to several micrometers. The technique is new and simple, and it is reasonable to expect that the method can be extended to obtain other semiconductor whiskers.
3883
Acknowledgements The author thanks Prof. L.D. Zhang and W.P. Cai for useful advice and this work was supported by the Nature Science Foundation of China (Grant No. 10074064) and the National 973 Project (Nanomaterials and Nanostructures).
References [1] J. Zhang, X. Peng, X. Wang, Y. Wang, L. Zhang, Chem. Phys. Lett. 345 (2001) 372. [2] H. Morkoc, S. Mohammad, Science 267 (1995) 51. [3] G. Gasol, Nature 272 (1996) 1751. [4] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 1287. [5] G. Chen, L. Zhang, Y. Zhu, G. Fei, Y. Li, C. Mo, Appl. Phys. Lett. 75 (1999) 2455. [6] S. Bae, H. Seo, J. Park, Appl. Phys. Lett. 81 (2002) 126. [7] C. Chen, C. Yeh, C. Yeh, C. Chen, M. Yu, H. Liu, J. Wu, K. Chen, L. Chen, J. Peng, Y. Chen, J. Am. Chem. Soc. 123 (2001) 2791. [8] A. Cros, H. Angerer, O. Ambacher, M. Stuzmann, R. Hopler, T. Metager, Solid State Commun. 104 (1997) 35. [9] L. Bergman, M. Bremser, W. Perry, R. Davis, M. Dutta, R. Nemanich, Appl. Phys. Lett. 71 (1997) 2157. [10] E. Oh, S. Lee, S. Park, K. Lee, I. Song, J. Han, Appl. Phys. Lett. 78 (2001) 273. [11] Y. Wang, G. Meng, L. Zhang, C. Liang, J. Zhang, Chem. Mater. 14 (2002) 1743.
Self-catalyzed vapor–liquid–solid growth of large-scale single crystal GaN whiskers Shao-Min Zhou * Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China Lingling University, Yongzhou 425000, PR China Received 7 December 2002; accepted 11 March 2003
Abstract Based on self-catalyzed vapor – liquid – solid mechanism, large-scale hexagonal GaN whiskers were synthesized via a twostep reaction of GaS powders with H2 and NH3 at 1000 jC. These whiskers were characterized using XRD, SEM, TEM, SAED, HRTEM, and Raman spectroscopy, in which the results obtained show that these GaN whiskers have the single crystal wurtzite structure with width of 20 – 200 nm and length of several micrometers. D 2003 Elsevier Science B.V. All rights reserved. Keywords: GaN nanomaterials; Self-catalyzed vapor – liquid – solid; III – V semiconductors; Single crystal
1. Introduction As a wide and direct band gap (3.4 eV) semicondictor [1], nanometer-scale GaN is the focus of current research due to the promise for use as ultraviolet or blue emitters, detectors, high-speed fieldeffect transistors, and high-temperature electronic devices [1 –3]. Various methods have been made to manufacture quasi-one-dimensional GaN nanostructured materials, such as GaN nanowires, nanorods, and nanobelts. Han et al. [4] and Cheng et al. [5] have synthesized GaN nanorods and GaN nanowires using carbon-nanotube-confined and anodic alumina template, respectively. Recently, Bae et al. [6] reported
* Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China. Fax: +86-551-5591434. E-mail address: [email protected] (S.-M. Zhou).
the synthesis of GaN nanobelts via a catalyst-assisted CVD and Chen et al. [7] fabricated GaN nanowires by the catalytic growth. However, these methods mentioned above inevitably lead to impure products because GaN samples can hardly be separated from the templates used or the catalysts used. In fact, it is very necessary to produce large-scale high pure GaN whiskers (nanoscale materials) without involvement of any template or patterned catalyst, particular for fundamental research and the further electronics applications of GaN. In this paper, we report a new and simple process to prepare large quantities of high pure GaN whiskers based on the first reduction of GaS powders in H2 and the following direct reaction of metal Ga with NH3 under controlled conditions, where there is no template or catalyst. It is envisioned that this kind of high pure GaN whiskers may have potential applications.
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00233-7
S.-M. Zhou / Materials Letters 57 (2003) 3880–3883
3881
2. Experimental procedure Firstly, a quartz crucible was cleaned using a standard treatment in piranha solution (30% H2O2 + 20% H2SO4), and rinsed with deionized water. Then GaS powders were placed at the bottom of the quartz crucible, which was inserted into half of a quartz-tube reactor in the horizontal tube furnace. Prior to heating, the system was flushed with high-purity Ar gas to eliminate O2. Under a constant flow of Ar mixed with 5% H2, temperature of the reactor was rapidly increased to 1000 jC, held at this temperature for 2 h. Subsequently, the Ar/H2 was shifted into NH3 and the system was maintained 3 h under the constant flow of NH3. During the whole heating period, pressure inside the tube was kept at 300 Torr. After the furnace was cooled to the room temperature, the pale-yellow wool-like product deposited on the inner wall of the tube was cleaned and then collected for characterization, where they were characterized by X-ray diffraction (XRD) (Philips PW 1710 with Cu Kia radiation), scanning electron microscopy (SEM) (JEOL JSM-630), transmission electron microscopy (TEM)(JEM-200CX), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy (Spex-1403, a 514-nm line of an Ar+ laser and 200 mW out power). In experiment, all chemicals
Fig. 1. X-ray diffraction patterns of GaN whiskers [Cu Ka radiation and a scanning speed of 0.02 s 1 in the 2h range 30 – 80j (20 – 200 nm in width and several micrometers in typical length)].
Fig. 2. A typical SEM image of large-scale GaN whiskers (width of about 20 – 200 nm and length of several micrometers).
used were of analytical grade and without further purification.
3. Results and discussion Fig. 1 shows XRD spectra of the as-synthesized GaN, in which Miller indices are marked on each diffraction peak. From these indices, the overall crystal structure and phase of GaN sample can be indicated. Also, it can be seen that the whole spectrum can be indexed in peak position to a crystalline GaN phase, which is consistent with hexagonal (wurtzite) structure with a lattice constant a = 0.318 nm, c = 0.516 nm (JCPDS 74-0243). Moreover, no diffraction peaks from GaS or other impurities are found in Fig. 1. SEM observation (Fig. 2) shows that the sample is composed of large quantities of whisker-like nanostructures with width of 20 – 200 nm and typical length up to several micrometers and these whiskers appear straight. A typical TEM image of GaN sample is presented in Fig. 3, where a smaller whisker with a width bout 20 nm can be seen and a small white ring region is used for SAED and HRTEM detection. From Fig. 3, one can also see that the whisker has smooth surface like the result from the-above-mentioned SEM image. SAED patterns in the inset of Fig. 3, taken from the white section of the single whisker, are composed of the regular clear diffraction dots, together with HRTEM
3882
S.-M. Zhou / Materials Letters 57 (2003) 3880–3883
onal GaN [9]. Not any peaks of impure elements are found in Fig. 4. In fact, these results are consistent with that of previous lateral epitaxial growth GaN [10] and also in good agreement with results of XRD. The formation process of GaN composition could be simple two-step reaction from a chemical reaction point of view, that is,
Fig. 3. TEM image of single GaN whisker (width of about 20 nm); the inset of top in the figure shows a typical selected area electron diffraction from the stem of the whisker (this section is marked with a white district).
(for the spaces limited, the streaky patterns are not shown here but it is presented in reply to the reviewer where clear streaky patterns can be viewed.), which reveal the single crystalline nature of the whisker and high pure GaN composition. And moreover, in other sections in the whisker SAED patterns and HRTEM images are the same the two figures, respectively. Raman spectrum of the as-fabricated GaN whiskers is shown in Fig. 4, in which three features of GaN are clearly seen. The most intense peak at 567.6 cm 1 is the well-known mode of E2 symmetry for hexagonal GaN [8]. The peaks at 532 and 556 cm 1 agree with phonon vibration frequencies of A1 (TO) and E1 (TO) modes of the hexag-
GaS ðgÞ þ H2 ðgÞ ! Ga ðlÞ þ H2 S ðgÞ
ð1Þ
Ga ðlÞ þ NH3 ðgÞ ! GaN ðsÞ þ H2 ðgÞ
ð2Þ
GaN ðnucleiÞ ! GaN ðwhiskerÞ
ð2VÞ
However, the formation of counterpart whiskers would involve a very complicated process although only step (2V) forms GaN whisker. Because of the fact that the whisker can only be formed above a certain process temperature (900 jC) and only in a place away from the center of the tube furnace (lower temperature range), we can reasonably speculate the whisker formation process as follows: GaS (gas) is carried by the processing gases and travels to the lower temperature region in the furnace tube and meanwhile reduced into metal Ga (liquid) by H2, where Ga deposits in the form of a liquid droplet on the inner wall of the tube. Then
Fig. 4. Room temperature Raman spectra of GaN whiskers with a 514-nm line of an Ar+ laser.
S.-M. Zhou / Materials Letters 57 (2003) 3880–3883
the liquidized droplet reacts with NH3 and forms GaN (nuclei), which further serves as a seed for GaN whisker growth, and the above growth is most likely to be controlled by the vapor – liquid – solid (VLS) mechanism [11]. In the present VLS growth of these GaN whiskers, no additional transition metals are added as catalysts; it is, therefore, believed that the formation of these GaN whiskers undergoes a selfcatalyzed VLS growth process. The metals not only act as the reactant but also provide an energetically favored site for the absorption of NH3. The newly formed GaN functions as a whisker seed, which further grows to a GaN whisker in the presence of Ga (liquid) and NH3 (gas), and the size and the shape of the whiskers are predecided by the size of the new reduced liquid metal droplet. In conclusion, self-catalyzed vapor – liquid – solid has been employed to synthesize large-scale GaN whiskers via a process of the first reduction of GaS in H2 and the following direct reaction of Ga and NH3 (template-catalyst-free growth). XRD, EDAS, Raman spectroscopy, SEM, TEM and HRTEM show that the GaN whiskers are hexagonal single crystalline with the width of about 20– 200 nm and the typical length up to several micrometers. The technique is new and simple, and it is reasonable to expect that the method can be extended to obtain other semiconductor whiskers.
3883
Acknowledgements The author thanks Prof. L.D. Zhang and W.P. Cai for useful advice and this work was supported by the Nature Science Foundation of China (Grant No. 10074064) and the National 973 Project (Nanomaterials and Nanostructures).
References [1] J. Zhang, X. Peng, X. Wang, Y. Wang, L. Zhang, Chem. Phys. Lett. 345 (2001) 372. [2] H. Morkoc, S. Mohammad, Science 267 (1995) 51. [3] G. Gasol, Nature 272 (1996) 1751. [4] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 1287. [5] G. Chen, L. Zhang, Y. Zhu, G. Fei, Y. Li, C. Mo, Appl. Phys. Lett. 75 (1999) 2455. [6] S. Bae, H. Seo, J. Park, Appl. Phys. Lett. 81 (2002) 126. [7] C. Chen, C. Yeh, C. Yeh, C. Chen, M. Yu, H. Liu, J. Wu, K. Chen, L. Chen, J. Peng, Y. Chen, J. Am. Chem. Soc. 123 (2001) 2791. [8] A. Cros, H. Angerer, O. Ambacher, M. Stuzmann, R. Hopler, T. Metager, Solid State Commun. 104 (1997) 35. [9] L. Bergman, M. Bremser, W. Perry, R. Davis, M. Dutta, R. Nemanich, Appl. Phys. Lett. 71 (1997) 2157. [10] E. Oh, S. Lee, S. Park, K. Lee, I. Song, J. Han, Appl. Phys. Lett. 78 (2001) 273. [11] Y. Wang, G. Meng, L. Zhang, C. Liang, J. Zhang, Chem. Mater. 14 (2002) 1743.