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1 This article was downloaded by:[university of Michigan] [University of Michigan] On: 30 January 2007 Access Details: [subscription number ] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Philosophical Magazine Letters Publication details, including instructions for authors and subscription information: A new modulated structure in GaN nanoparticles H. Li; H. P. Sun; F. Shen; Z. Zhang To link to this article: DOI: / URL: Full terms and conditions of use: This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Taylor and Francis 2007

2 PHILOSOPHICAL MAGAZINE LETTERS, 2002 VOL. 82, NO. 2, 91±98 A new modulated structure in GaN nanoparticles H. Li, H. P. Sun, F. Shen and Z. Zhangy Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, PO Box 2724, Beijing, PR China [Received in nal form 15 October 2001 and accepted 17 October 2001] Abstract A new modulated structure with a superlattice having parameters a ˆ 2:209 nm, b ˆ 3:826 nm, c ˆ 1:037 nm and ˆ ˆ ˆ 90 has been found in GaN nanoparticles synthesized by a dc arc plasma method. The nanoparticles transformed further into particles with holes at their centres under electron-beam irradiation during high-resolution electron microscopy observations. At the same time, Ga atoms were extruded on to the surface of the nanoparticles and formed an amorphous layer. A series of simulations of high-resolution images and electron diœraction patterns revealed that the modulation could be attributed to aggregations of N vacancies founded during the electron bombardment. Molecular mechanics calculations show that the aggregation of N vacancies is far more energetically favourable than that of Ga vacancies. The stability of the GaN particles is discussed. } 1. Introduction Dislocations and domain boundaries in GaN have been extensively studied (Northrup et al. 1996, Cheng et al a, b, 1999, Nakamura and Faso 1997). Although there are some theoretical studies on point defects in GaN (Gorczyca et al. 1999, Chow and Watkins 2000, Lee et al. 2000), few experimental observations have been reported. Owing to the variation in experimental conditions close to the entrance nozzle of metal±organic sources during the synthesis process, the as-synthesized GaN lms could be de cient in nitrogen because of the presence of nitrogen vacancies. The vacancy distribution is normally varied by post-annealing or beam irradiation and aggregated into voids during device processing. These voids can fully deplete the free carriers, rendering it semi-insulating and causing photoluminescence failure (Wang et al. 2001). Furthermore, when the size of GaN devices is decreased to the nanometre scale as required by the development of nanotechnology, the structural evolution due to imperfections impacts on the stability of GaN. Accordingly, GaN with a nanostructure is worthwhile studying and indeed has attracted increasing research interest (Xie et al. 1996, Chen et al. 2000). Because electron beams are widely used in semiconductor device processing and their structural characterization, electron irradiation was employed in the present study to simulate the structural evolution of GaN nanoparticles during transmission electron microscopy (TEM) observations. Electron irradiation can induce rearrangements and/or breaking of chemical bonds, knock-on displacement, and sputtering (Buckett et al. 1989, Medlin et al. 1989, Medlin and Howitt 1991). A related study is yauthor for correspondence. zhang@blem.ac.cn Philosophical Magazine Letters ISSN 0950±0839 print/issn 1362±3036 online # 2002 Taylor & Francis Ltd DOI: /

3 92 H. Li et al. the structural evolution from graphite to diamond and the coalescence of C nanotubes reported by Egerton and Rauf (1999) and Banhart (1997). Here we report, for the rst time, a new N-vacancy-modulate d structure and a sequence of structural evolutions of GaN nanocrystals induced by focused electron-beam irradiation. } 2. Experimental details GaN nanoparticles were synthesized using a dc arc plasma method. The specimens for TEM investigation were prepared by collecting the nanoparticles, after supersonic dispersion in ethanol, on to a Formvar- lm-covered grid. TEM and high-resolution electron microscopy (HREM) studies were carried out in a Philips CM200 electron microscope with a eld emission gun operated at 200 kv. A GIF100 image lter with a slow-scan charge-couple d detector system attached to the microscope was used for the image data recording and parallel electron-energy-loss spectroscopy (PEELS) analysis. A molecular mechanics calculation (MMC) was used for simulation of the variation in the total energy with diœerent modulation models using the HyperChem (HyperCube 1996) program with a traditional potential model (MM ) (Allinger 1977, Leach 1996). } 3. Results and discussion Figure 1 (a) shows a typical electron diœraction contrast image of the as-synthesized GaN nanoparticles with hexagonal facets and with a diameter of about 150 nm. Electron diœraction analysis shows that the nanoparticles normally have the wurtzite structure. When nanoparticles were subjected to an intense electron beam for a few minutes during the HREM study, the original wurtzite structure gradually changed. On increasing the electron irradiation dose, a hole (indicated by a white arrow) was eventually introduced in the centre of the irradiated region and an abundance of particles were extruded on to the surface of the GaN nanoparticle as indicated in gure 1 (b) by a black arrow. PEELS analysis showed that these particles are metallic Ga. The extruded gallium particles can be seen more clearly in gures 1 (c) and (d), and a typical HREM image shown in gure 1 (e) reveals that an extruded Ga particle (indicated by an arrow) has an amorphous structure and is formed on the surface of the crystalline GaN nanoparticles. By careful examination of the sample region close to the edge of the hole, we observed a modulated structure, as shown in gure 2. The corresponding selectedarea electron diœraction (SAED) pattern showed six obvious satellite spots around each of the basic diœraction spots as shown in gure 3 (a), which revealed again the occurrence of a modulated structure that had formed under the electron beam irradiation. The satellite spots in this SAED pattern can be indexed as 1 12 h and 1 k, which implies that the modulation periodicity is 12 times larger than the original 12 lattice in the close-packed atom plane of the wurtzite structure, that is 12a 12b, where a and b are the lattice parameters of the wurtzite structure. Moire eœects, originating from double diœraction by superimposed GaN nanoparticles, have been excluded by theoretical calculations. To simulate the above experiment results, we introduced a modulation model with size 12a 12b 2c, based on the hexagonal coordination of the wurtzite structure, and converted it into a rectangular superlattice of dimensions 4a* 12b 12c for the convenience of calculation. Here, a* is 30 anticlockwise rotated with respect to a and a* ˆ 3 1=2 a. Accordingly, the superlattice of the modulated structure had the parameters a 0 ˆ 4a* ˆ 2:209 nm, b 0 ˆ 12b ˆ 3:826 nm, c 0 ˆ 2c ˆ 1:037 nm and

4 New modulated structure in GaN nanoparticles 93 Figure 1. Electron diœraction contrast images of as-fabricated GaN nanoparticles (a) before and (b) after electron beam irradiation with a high dose. Note that a hole at the centre of the particle was introduced (white arrow) and some Ga particles (black arrow) were extruded on to the surface of the particle after electron beam irradiation as shown in (b). (c), (d) The extruded Ga particles can be seen more clearly. (e) The amorphous characteristics of the Ga particles can be seen from a typical HREM image where the amorphous Ga formed on the surface of the GaN nanoparticle. ˆ ˆ ˆ 90. Considering two constitutant elements of Ga and N in the nanoparticles, there are two possible modulations resulting from Ga or N vacancies. The former can be easily excluded from the disagreement between the simulated and experimental HREM, as well as the electron diœraction results. It can also be con- rmed by a MMC ( gure 4) that N-vacancy modulation is more energetically favourable than Ga-vacancy modulation; the total energy of the whole system for the former is only 20% higher with respect to the wurtzite structure, while the latter is about three times higher. Accordingly, we use a N-vacancy modulation model. In order to have a reasonable structural model to describe the N-vacancy modulation, two restrictions have to be taken into account. Firstly, the sixfold or threefold rotational symmetry of the modulated structure, as revealed from the above experimental results ( gure 3 (a)), should remain. Secondly, the overlapping of N

5 94 H. Li et al. Figure 2. HREM image of a GaN nanoparticle with a modulated structure taken from the specimen area close to the hole shown in gure 1 (b) and along the [0001] axis of the wurtzite structure. Figure 3. (a) SAED pattern from the specimen area with modulation, corresponding to gure 2. Note the six satellite spots around each of the basic re ections from the wurtzite structure. (b) Simulated diœraction pattern from the model with clustered N vacancies in a superlattice of 4a* 12b 2c.

6 New modulated structure in GaN nanoparticles 95 Figure 4. A comparison of total energy versus two diœerent types of vacancy, namely Ga (*) and N (&), in a two-dimensional supercell of the modulated structure. This shows clearly that the N vacancies are more energetically favourable. Note that there are only 96 atom sites in the two-dimensional supercell of 4a* 12b. vacancies in successive N, N 0 layers should be avoided; otherwise it introduces an increase in the total energy by the formation of unbonded Ga atomic chains along the c axis. Accordingly, we have a N-vacancy-containin g superstructur e model with a lattice of size 4a* 12b 2c, which includes four pairs of N±Ga atom planes and two N-atom planes containing clustered N vacancies. These clustered vacancies are located at the centre and vertices respectively of the two-dimensional face-centred rectangular supercell. To investigate the vacancy behaviour and corresponding total energy variation of the system, we carried out a systematic MMC. This method has advantages for many particle systems (Leach 1996) and was therefore employed in our calculations for GaN nanoparticles with 768 atoms and vacancies in the supercell. We have calculated the system energy variations versus diœerent vacancy concentrations and aggregation stages. Figure 5 (a) shows the variation in the total energy for models of modulated structure with the number of clustered vacancies varying from 0 to 28 in one of the two N-vacancy-containin g planes in the supercell. The various symbols represent the energy change using models with diœerent number of vacancies. From these curves, two energy-favoure d trends can be seen. Firstly, the system energy varies with respect to the number of clustered vacancies. For example, for the model with 25 N vacancies (curve denoted by open circles), the total energy increased by 6.7% with respect to the wurtzite structure (3300 kcal m 1 ), when only two N vacancies clustered together and the remaining 23 were randomly distributed in the supercell. For the case where 21 N vacancies were aggregated together and only four randomly distributed, the total energy decreased more rapidly than in the previous case. Secondly, some energetically favourable stages with minimum energy

7 96 H. Li et al. Figure 5. (a) Series of MMC calculations of the system energy E versus the number of N clustered vacancies. E 0 represents the system energy with a wurtzite structure. Note that energetically favourable stages are reached when triangular con gurations of the aggregated N vacancies were formed in the close-packed atom planes of the N layers. (b) A structural model with a triangular con guration of 21 N vacancies, 19 clustered (*) and two randomly (with 22% occupation) distributed at the nine sites (*) of the corners of the triangle. (c) A similar con guration, in another N plane a distance c apart, but rotated 60 around the c axis. (d) A projection of the supercell superimposed on (b) and (c).

8 New modulated structure in GaN nanoparticles 97 occur when three, six, ten, 15 and 21 vacancies are clustered and the rest distributed randomly for any of the vacancy models, where the vacancy aggregation stages decrease sharply in the total energy. These clustered vacancy aggregations imply a set of triangular con gurations of N vacancies in the N-atom plane (0001) as shown, for example, in gure 5 (b), where 28 N vacancies (represented by all open and full circles) aggregated into a triangle con guration. It should be pointed out that the triangle con guration of 28 N clustered vacancies is a maximum in the 4a* 12b supercell, because the bottom corner of the triangle in one supercell ( gure 5 (b)) is connected to the edge of a neighbouring supercell in the same N-atom plane, hence resulting in an unstable state with higher system energy. Accordingly, a triangle con guration consisting of 21 vacancies will be a reasonable model with a more favourable total energy. It should also be noted that, to maintain the threefold or sixfold symmetry around the c axis, the triangle con guration of clustered vacancies in another N-vacancy-containin g plane should be rotated by 60 ( gure 5 (c)) around the c axis with respect to each other, thereby introducing a 2c* modulation along the c axis. Figure 5 (d) shows a projection of a pair of successive triangle con gurations of N vacancies. Note that 19 of each of the clustered vacancies superimpose on to the vacancy sites of a neighbouring plane, resulting in a projected hexagonal con guration of N vacancies along the c axis. We shall see below that these hexagonal con- gurations of the N-vacancy modulation model agree well with the experimental HREM images ( gure 6). Based on the above total-energy considerations of the N-vacancy modulated structure, we carried out a systematic simulation of HREM images using the multislide methods. The simulated imaging conditions are as follows: an acceleration voltage of 200 kv, spherical aberration C s ˆ 0:50 mm, a defocus variation from 40 to 60 nm and a specimen thickness in the range 5.2±15.6 nm. From a series of simulations, we found that the best t between the experimental ( gure 6 (a)) and the simulated HREM images ( gure 6 (b)) is of a modulated structure with two Figure 6. (a) Experimental HREM image of a GaN nanoparticle showing periodic hexagonal modulation; (b) corresponding simulated image with a structural model having N vacancies in the supercell. Imaging conditions for (b) were f ˆ 50 nm, a specimen thickness of 15.6 nm, C s ˆ 0:5 mm and a voltage of 200 kv.

9 98 New modulated structure in GaN nanoparticles clustered 21 N vacancies, located at the centre and the corners respectively of the two-dimensional face-centred rectangular supercell. For the clustered 21 N vacancies, 19 of these are located at the sites indicated as full circles in gures 5 (b) and (c) and the remaining two (indicated as open circles) are distributed randomly among the nine vacancy positions at the three corners of the triangle, with a site occupation density of 22.2%. The best t was reached with HREM imaging conditions of 50 nm defocusing and a specimen thickness of 15.6 nm. This agreement between the experimental and simulated SAED patterns can also be seen in gures 3 (a) and (b). It should be noted that there are ˆ 84 N vacancies in the two N- vacancy-containin g atom planes within the 4a* 12b 2c supercell. The N-vacancy density in this model is therefore about 22%. With the described above HREM, SAED and MMC studies on GaN nanoparticles, we can conclude that N vacancies play a dominant role in the modulated structure. The randomly distributed N vacancies in the GaN nanoparticles can be attributed either to a N de ciency during the synthesis process or to the high-dose electron bombardment during TEM observation. The N vacancies are either rearranged as a modulated structure or aggregate d further into voids or holes under the in-situ electron-beam irradiation. Ga atoms originally sited in the vicinity of N vacancies were extruded on to the surface of the irradiated GaN nanoparticles, owing to the aggregation of N vacancies, and formed some amorphous Ga clusters under electron irradiation. This process cannot be energetically favourabl e at room temperature and proper external energy excitation is required to make the individual vacancies aggregate into voids. This may be responsible for the failure of devices (Wang et al. 2001). Our results provide useful information for GaN lm and/or nanoparticle synthesis and also for device fabrication. REFERENCES Allinger, N. L., 1977, J. Am. chem. Soc., 99, Banhart, F., 1997, J. appl. Phys., 81, Buckett, M. I., Strane, J., Luzzi, D. E., Zhang, J. P., Wessels, B. W., and Marks, L. D., 1989, Ultramicroscopy, 29, 217. Chen, X. L., Cao, Y. G., Lan, Y. C., Xu, X. P., Li, J. Q., Lu, K. Q., Jiang, P. Z., Xu, T., Bai, Z. G., Yu, Y. D., and Liang. J. K., 2000, J. Cryst. Growth, 209, 208. Cheng, L. S., Zhang, G. Y., Yu, D. P., and Zhang, Z., 1997a, Appl. Phys. Lett., 70, 1408; 1997b, ibid., 71, Cheng, L. S., Zhou, K., and Zhang, Z. 1999, Appl. Phys. Lett., 74, 661. Chow, K. H., and Watkins, G. D., 2000, Phys. Rev. Lett., 85, Egerton, R. F., and Rauf, I., 1999, Ultramicroscopy, 80, 247. Gorczyca, I., Svane, A., and Christensen, N. E., 1999, Phys. Rev. B, 60, HyperCube, 1996, HyperChem 1 Computational Chemistry Manual (HyperCube, Inc.). Leach,A. R., 1996, Molecular Modeling (London: Addison-Wesley Longman). Lee, S. M., Belkhir, M. A., Zhu, X. Y., Lee, Y. H., Hwang, Y. G., and Frauenheim, T., 2000, Phys. Rev. B, 61, Medlin, D. L., and Howitt, D. G., 1991, Phil. Mag. Lett., 64, 133. Medlin, D. L., Thomas, L. E., and Howitt, D. G., 1989, Ultramicroscopy, 29, 228. Nakamura, S., and Fasol, G., 1997, The Blue Laser Diode (New York: Springer). Northrup, J. E., Neugebauer, J., and Romano, L. T., 1996, Phys. Rev. Lett., 77, 103. Wang, Y. G., Li, W., Han, P. D., and Zhang, Z., 2001, preprint. Xie, Y., Qian, Y. T., Wang, W. Z., Zhang, S. Y., and Zhang, Y. H., 1996, Science, 272, 1926.