Exploration of Ba3N2 flux for GaN single-crystal growth

Exploration of Ba3N2 flux for GaN single-crystal growth

ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 2955–2959 www.elsevier.com/locate/jcrysgro Exploration of Ba3N2 flux for GaN single-crystal gro...

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ARTICLE IN PRESS

Journal of Crystal Growth 310 (2008) 2955–2959 www.elsevier.com/locate/jcrysgro

Exploration of Ba3N2 flux for GaN single-crystal growth H.Q. Bao, H. Li, G. Wang, B. Song, W.J. Wang, X.L. Chen Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, PR China Received 23 September 2007; received in revised form 18 February 2008; accepted 20 February 2008 Communicated by R. Fornari Available online 4 March 2008

Abstract Here, we report the exploration of Ba3N2 as a new flux to grow GaN single crystals from a Ga melt at 900 1C and under a nitrogen pressure of about 2 atm. Scanning electron microscope (SEM) observations indicated that regular pyramidal crystals with a size of 10–20 mm were obtained. Raman scattering measurement confirmed the axial direction of crystals as the c-direction. Morphological features of crystals were compared with that of GaN crystals grown by using Li3N, Na and Ca3N2 flux. These results prove that Ba3N2 is an effective new flux for growing GaN crystals. r 2008 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.10.Dn Keywords: A1. Crystal morphology; A1. Characterization; A1. X-ray diffraction; A2. Single crystal growth; B1. Nitrides; B2. Semiconducting III–V materials

1. Introduction GaN is an ideal candidate for nitride-based optoelectronic devices due to its wide direct bandgap, high breakdown voltage, high mechanical hardness and high thermal conductivity [1,2]. To date, GaN-based lightemitting diodes (LEDs) and laser diodes (LDs) have been fabricated by hetero-epitaxial growth on the SiC, GaAs and sapphire substrates [3–5]. However, high-density dislocations, induced by large lattice constant and thermal expansion mismatches between the substrate and GaN film, inevitably deteriorate performance and lifetime of these devices. Ideal improvement is to select the GaN single crystal as a substrate for the homo-epitaxial growth of high-quality films. However, it is still intractable for growing high-quality and large-sized GaN crystals. To date, the growth of high-quality GaN crystals has been extensively investigated by several research groups using different methods. Among these routes, hydride vapor phase epitaxy (HVPE) [6,7] has been successfully Corresponding author. Tel.: +86 10 8264 9039; fax: +86 10 8264 9646.

E-mail address: [email protected] (X.L. Chen). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.02.023

used for the commercial production of GaN substrates. However, high dislocation density in crystals remains a challenge because of the nature of the hetero-epitaxial growth process. High-quality GaN single crystals over 10 mm with a dislocation density of less than 102 cm2 have been achieved by the high-pressure solution method [8,9]. However, growing large crystals in a production scale by this method is difficult due to the rigorous growth conditions (temperatures over 1500 1C and N2 pressures over 15 kbar). Recently, large GaN crystals were successfully obtained by an ammonothermal growth method [10,11]. GaN single crystals of 1 inch with high quality were grown under conditions of about 500 1C and less than 130 MPa system pressure, indicating that ammonothermal growth is a promising technique to prepare large GaN crystals. The Sodium flux method [12–14], developed by Yamane et al., was applied to grow GaN single crystals of mm scales at 600–800 1C under a nitrogen pressure of 5–100 atm. The Li3N flux method under more moderate conditions was explored to synthesize GaN crystals up to 4 mm at about 800 1C under a nitrogen pressure of 1–2 atm [15–17]. These presented investigations suggest that the flux method is perhaps promising for low-cost and large-scale

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growth of GaN crystals. More recently, Jian et al. [18] synthesized prismatic GaN crystals with lengths up to 1.5 mm by the Ca3N2 flux method, indicating that alkalineearth metal nitrides are potential fluxes for GaN singlecrystal growth. Hence, it is speculated that Ba3N2 may be another new flux in the crystal growth of GaN. In this paper, we report that GaN crystals are successfully obtained by the Ba3N2 flux method. Pyramidal crystals grown under moderate conditions reveal that Ba3N2 is an effective flux for GaN single-crystal growth. SEM and Raman investigations confirm the axial direction of crystals as the c-direction. In addition, the morphological features and growth mode of crystals are also discussed. 2. Experimental procedure The flux Ba3N2 was synthesized by the direct reaction of metal Ba grains (499% purity) and high-purity N2 gas (99.999%) at about 600 1C (X-ray diffraction (XRD) analysis indicates that there are impurity phases of BaO or Ba(OH)2 in the as-grown Ba3N2). The starting materials used for GaN crystal growth were Ga (99.999%) and Ba3N2 in a proper proportion. The growth apparatus used for synthesizing Ba3N2 and growing GaN crystals was the same with one used for Li3N flux growth [15]. After evacuating to a vacuum of 3.0  103 Pa, the system was charged with 2 atm of N2 gas at room temperature. The tungsten crucible filled with starting materials was heated to 900 1C, kept at this temperature for 12 h, and then was slowly cooled at a rate of 2 1C day1. The whole growth process lasted about 130 h. After that, the system was cooled to room temperature by switching off the power. Some of the as-obtained products were taken out and analyzed by XRD. Then, the products were washed by HCl solution and distilled water, and gray powders were obtained. XRD data were collected on a MAC-M18XHF diffractometer with Cu Ka radiation. Morphology of the crystals was characterized by SEM (FEI, XL-30). The compositions of samples were addressed by an energydispersive X-ray (EDX) spectroscope equipped on the SEM. Raman scattering measurement was performed at room temperature by a Raman system (JY-HR800) using 532 nm line of a solid-state laser as the excitation source. The resolution of the Raman spectrometer is lower than 1 cm1 and the diameter of the incident light spot is about 1 mm. 3. Results and discussion Fig. 1 presents a typical XRD pattern of the gray products prepared at a molar ratio of Ba3N2:Ga ¼ 1:10. The main reflections can be indexed based on a hexagonal cell with lattice constants of a ¼ 3.189 A˚ and c ¼ 5.185 A˚ (ICCD-PDF no: 50-0792), suggesting that GaN was successfully obtained. In this pattern, the (1 0 1) reflection is obviously preferential by comparing with the standard data card, probably implying the special morphological

Fig. 1. XRD pattern of the products, Cu Ka radiation.

features of the obtained GaN crystals. Several unindexed reflections in this pattern are due to unknown impurities. The SEM image (Fig. 2a) shows that pyramidal GaN crystals with the size of 10–20 mm and small floccules entwist together. Fig. 2b–d further presents the morphological features of these crystals. It can be seen that the pyramidal shape with a flat top and a hillock-like bottom is the typical crystal morphology. Moreover, almost all the crystals have growth stripes on their side surfaces. According to the hexagonal structure of GaN and the (1 0 1) reflection-preferred XRD pattern, the top surface of crystals should be the c-plane, and the side surfaces should be {1 0 1¯ 1}. The difference in the morphology between the top and the bottom perhaps originated from the polar wurtzite-type structure, as reported by Ponce et al. [19]. The flat top of pyramidal crystals corresponds to the Ga-terminated (0 0 0 1) plane, and the bottom is the N-terminated (0 0 0 1¯ ) plane. The typical SEM images of the stripe-like side surfaces are shown in Fig. 3a and b, respectively. Fig. 3b is a magnified image of the selected region in Fig. 3a, clearly showing the growth stripes of GaN crystals. Considering that these facets are not the whole and smooth planes, they are identified to be pseudo-facets of {1 0 1¯ 1}. It is also considered that the pseudo-facets are actually built by the step-like figure of alternating planes {1 0 1¯ 1} and (0 0 0 1), as schematically shown in Fig. 3c. Raman scattering measurement is employed to further characterize the orientation of crystals. The configuration of Raman examination is designed by the incident light perpendicular to the hexagonal top surface of the crystal and the polarization of light perpendicular to the edge of the hexagonal top surface. The obtained Raman spectrum is shown in Fig. 4. Four phonon modes are observed at 146, 530, 570 and 734 cm1, which are in good agreement with the reported E2(low), A1(TO), E2(high) and A1(LO) [20]. According to the Raman selection rules of GaN [20], Raman scattering examination reveals the top surface of crystals as the c-plane. A1(TO), the forbidden mode in this

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Fig. 2. SEM images of GaN crystals: (a) overall observation of products, (b–d) the top, side surface and bottom morphology of GaN crystals, respectively.

scattering geometry, is also observed, maybe because the incident light is not exactly perpendicular to the top surface of the crystal. Fig. 5 schematically illustrates the size and typical morphology of GaN crystals grown by using Li3N, Ca3N2 and Ba3N2 flux [15,18], respectively. The crystals are in the form of a platelet by using Li3N flux. However, the crystals grown by using Ca3N2 flux are hexagonal prisms with {1 0 1¯ 0} as the side surfaces and capped with the {1 0 1¯ 1} and (0 0 0 1) surfaces. The striking morphological feature in crystals formed by using Ba3N2 flux is pyramidal, and no side surfaces {1 0 1¯ 0} are observed. The difference in morphology reflects the varying anisotropy in the growth rates along the axial direction (Vc) over along the direction normal to the axis (along the basal plane), for example Va. The crystals grown by using Ca3N2 flux exhibit the largest Vc/Va, while those obtained by Li3N show the least, and those grown by Ba3N2 have between the least and the largest Vc/Va. Other factors such as surface tensions induced by different fluxes may also contribute to the variety in morphology to some extent. As for the crystals grown by the sodium flux method [21], the morphological features of the obtained GaN crystals strongly depend on the molar ratio of starting

materials. Though the prismatic morphology indicates the crystal growth near the equilibrium condition, the prismatic crystals are formed under Ga-rich condition and the platelet crystals are obtained in a low Ga concentration. Here, binary alloys of BaGa4 and BaGa2 and melt Ga were found in the as-products, implying the GaN crystals grown under the Ga-rich condition. Besides, the high binding energy of Ba with nitrogen evidenced by the related experiments [22–24] may have much effect on the crystal growth. It can be speculated that the nitrogen in the Ga–Ba system is tightly bound, and the transportation of the nitrogen in the system is very slow. As a result, crystal growth carries out under low supersaturation, even near the equilibrium condition. The two sides perhaps explain the pyramidal morphology of obtained GaN crystals. Moreover, the pyramidal crystals is smaller than those grown using Li3N and Ca3N2 flux, as shown in Fig. 6. One reason may be from the high viscosity of Ga–Ba melt in terms of our former work [15,17,18]. High viscosity that will retard the nitrogen diffusion in the growth system and lead to the local fluctuation of nitrogen concentration. Thus, the stable growth environment for large GaN crystals is difficult to achieve due to the fast consumption of local nitrogen. It seems that when the N concentration in

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Fig. 4. Raman spectrum of the GaN single crystal.

Fig. 5. Size and morphology of GaN single crystals obtained using different fluxes.

Fig. 3. (a) GaN single crystal, (b) the magnified image of the selected region in (a), (c) the schematic view of stripe-like side surfaces.

the Ga–Ba system reached a critical value, spontaneous nucleations occurred rapidly, and then the degree of supersaturation dropped not to drive further growth of the grains. In terms of the growth strips on side surfaces and plenty of small hillocks on the crystal bottom (see Fig. 6), we consider the unbalance growth of the non-polar face responsible for the crystal growth process. Obviously, the development of planes {1 0 1¯ 1¯ } enclosing the small hillock is slower than that of crystal side planes {1 0 1¯ 1}. It is well known that the crystal edge is a nucleation site and the

Fig. 6. SEM image of the bottom edge of the crystal.

corners are even better ones. Thus, the small hillocks can form on the bottom of the pyramid, but they cannot efficiently grow due to the faster development of planes

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{1 0 1¯ 1}. This kind of unbalanced development of crystalline planes may induce unstable morphological feature such as growth stripe [25]. The yield of GaN crystals was closely related to the proportion of the starting materials. When the molar ratio of Ga to Ba3N2 decreased from 10:1 to 8:1, morphology of the obtained GaN crystals had no change, but the yield was much lower. Further research focusing on optimizing growth conditions to enlarge the crystal size is necessary. 4. Conclusions Ba3N2 as a new flux was exploited to grow hexagonal GaN single crystals under moderate conditions (900 1C, 2 atm of N2 pressure). Pyramidal crystals with a flat top and hillock-like bottom were obtained. SEM and Raman investigations confirmed the axial direction of the GaN crystals as the c-direction. The mechanism responding for crystal growth is discussed based on the unbalance growth of the non-polar face. The results reported here provide an alternative choice for the flux growth of GaN single crystals. Acknowledgments The authors are grateful to Prof. J. K. Jian for many helpful discussion. This work was financially supported by the National Natural Science Foundation of China (Grant no. 50472075) and by the 863 Program (Grant no. 2006AA03A107). References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74.

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