CRYSTAL GROWTH & DESIGN
Growth of GaN Single Crystals Using Ca-Li3N Composite Flux Gang Wang,† Jikang Jian,†,‡ Wenxia Yuan,# and Xiaolong Chen*,† Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, P. R. China, Center of Condensed Matter Physics, School of Sciences, Beihang UniVersity, Beijing 100083, P. R. China, and Department of Chemistry, School of Applied Science, UniVersity of Science and Technology Beijing, Beijing 100083, P. R. China
2006 VOL. 6, NO. 5 1157-1160
ReceiVed December 16, 2005; ReVised Manuscript ReceiVed February 19, 2006
ABSTRACT: A Ca-Li3N composite flux was introduced to grow GaN single crystals from Ga melts. The obtained GaN crystals were colorless and up to 4 mm at 800 °C under N2 pressure of about 2 atm. The effects of introducing the composite flux on the size distribution, morphologies, and yield of the GaN crystals were studied. It was found that the addition of Ca retarded the homogeneous nucleations to some extent and resulted in a more uniform size distribution of the GaN crystals in comparison with that grown using Li3N as flux only. The layered morphological feature on the GaN surface suggested that the layer growth mechanism was probably enhanced by the composite flux. The yield of the crystals was strongly dependent on the flux compositions. These results provided a possible new route for improving the growth of GaN crystals from melts in the future by optimizing the composite flux. Introduction GaN is recognized as one of the most promising materials for the fabrication of optoelectronic devices due to its wide direct band gap, high thermal stability, and high breakdown voltage. Currently, the GaN-based thin film devices have been fabricated mainly on foreign substrates by epitaxial growth.1,2 However, the high density of dislocations that result from heteroepitaxial growth due to the lattice and thermal expansion coefficient mismatch negatively affects the performance of GaN-based devices. Therefore, high-quality substrates of GaN are strongly desired to improve the performance of the nitride-based optoelectronic devices such as light-emitting diodes (LEDs) and laser diode (LDs).3,4 Up to now, several techniques have been exploited to grow bulk GaN single crystals. Among these techniques, the hydride vapor-phase epitaxy (HVPE) has been used for the commercial production of GaN substrates (up to 2 in diameter wafers).5,6 But, the dislocation density of the best HVPE samples is approximately 106 cm-2.6 Better quality GaN single crystals over 10 mm with dislocation densities less than 102 cm-2 have been achieved by the high-pressure solution method.7 However, growing large crystals in a production scale by this method is still a great challenge since the growth conditions (temperatures over 1500 °C and N2 pressures over 15 kbar) are stringent. A flux method under less stringent conditions reported by Yamane et al. was applied to grow GaN single crystals with a size of several millimeters at 600-800 °C and under 50-100 atm of N2 from the Na-Ga melts.8-10 A more moderate method was employed to synthesize platelet GaN single crystals up to 4 mm at about 800 °C and 1 atm N2.11-16 Alkali metal Li is thought to play an important role in the growth of bulk GaN single crystals under such moderate conditions. The method, however, suffers from homogeneous nucleations occurring in random orientations during growth processes, hampering the further increase of the crystal sizes. A possible solution is to increase the critical supersaturation at which the melts begin to nucleate * To whom correspondence should be addressed. Tel.: +86-10-82649039. Fax: +86-10-8264-9646. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Beihang University. # University of Science and Technology Beijing.
under the same N concentration, growth temperature, and pressure. It is known that metal Ca has a stronger power to dissolve N compared to metal Li from the related binary phase diagrams.17-19 Our thermodynamic calculations confirm this result, too.20,21 In addition, it has been demonstrated that the addition of Ca to Na flux can grow large and transparent GaN single crystals by the liquid-phase epitaxy.22 Hence, we expect that the addition of metallic Ca to the Li3N flux system should be helpful to control homogeneous nucleations. In the present work, we reported the results of the growth of GaN single crystals by using a composite Ca-Li3N flux at 800 °C under N2 pressure up to 2 atm. It was found that the introduction of the composite flux could retard the homogeneous nucleations to some extent, resulting in GaN crystals with a more even distribution in sizes. Most of the crystals were 0.2-1.5 mm in size and 4 mm maximum. The effects of the composite flux on the size distribution, morphologies, and the yield of crystals are discussed. Experimental Section The starting materials used for the growth of the GaN single crystals were Ga (99.999%), Ca (99.5%), and Li3N. Li3N was synthesized by the reaction of metallic Li (99.9%) and N2 (99.999%) in a quartz tube at about 400 °C. These starting materials in a proper proportion (Ga: 63.4 g, Ga and Li3N with molar ratio 4:1, Ca and Li with molar ratio 0.25:1 or 0.35:1) were put in a tungsten crucible (50 mm inner diameter, 60 mm depth) and heated to 800 °C in a growth furnace, which was charged with N2 gas up to 2 atm at room temperature. Then, the furnace was slowly cooled at a rate of about 3 °C/day and such process lasted for about 6 days. After that, the system was cooled to room temperature by shutting off the power. GaN single crystals were obtained and separated from the residual substances by soaking the products in HCl solution. Powder X-ray diffraction (PXRD) was performed on a MAC-M18XHF diffractometer with Cu KR radiation. X-ray rocking curve was carried out on a high-resolution X-ray diffractometer (X’Pert-MRD) equipped with a four-crystal Ge (220) monochromator. Raman spectrum of the products was collected at room temperature by a multichannel modular triple Raman system (JY-64000) using a 532 nm line of a solid-state laser as the excitation source. The morphologies of the crystals were observed by an optical microscope and a scanning electron microscope (SEM, FEI, XL-30).
Results Plenty of shining platelets were obtained after soaking the products. Figure 1 shows the optical photograph of three
10.1021/cg050666a CCC: $33.50 © 2006 American Chemical Society Published on Web 03/24/2006
1158 Crystal Growth & Design, Vol. 6, No. 5, 2006
Wang et al.
Figure 1. Optical photograph of selected GaN single crystals obtained by the Ca-Li3N flux method. Figure 3. The X-ray rocking curve of GaN single crystals.
Figure 2. Powder X-ray diffraction pattern of GaN, Cu KR radiation.
platelets selected from the sample having sizes of 2-3 mm with irregular shapes. The PXRD pattern of the milled platelets is shown in Figure 2. All diffraction lines could be indexed based on a hexagonal cell with lattice constants of a ) 3.189 Å and c ) 5.185 Å (ICDD-PDF No: 50-0792), indicating that those platelets were GaN crystals. There was no impurity phase detected under the resolution of the diffractometer. The (002) reflection was the strongest in the pattern, instead of the (101) reflection in the standard data card, revealing a preferred texture with fiber along [002]. The EDS spectrum (the inset in Figure 6a) showed that the crystal consisted of N and Ga, and no impurity element was detected. But the atomic ratio of N to Ga was about 37:63, indicating the deficiency of N in the crystals. The quality of the crystals was examined by the X-ray rocking curve. Figure 3 shows the rocking curve from the (100) reflection of a platelet. The full-width at half-maximum (FWHM) value was 3.96′. The diffraction peak split was perhaps due to the twin crystals. Figure 4 displays the size distribution of GaN crystals grown from the Ca-Li3N composite flux. As shown in the figure, more than 73% of the crystals had sizes of 0.2-1.5 mm, while only about 4.4% of the crystals had sizes smaller than 0.1 mm. The size distribution of GaN crystals was much more uniform than that obtained in the Li3N flux. Raman scattering was employed to further characterize the GaN crystals. Room temperature Raman spectrum of the GaN crystals is shown in Figure 5. Four phonon modes could be observed at 144, 557, 568, and 734 cm-1 in the spectrum, which were in good agreement with the reported E2 (low), E1 (TO),
Figure 4. The size distribution of GaN crystals.
Figure 5. Raman spectrum of GaN crystals.
E2 (high), and A1 (LO) phonons of GaN.23 E2 (high) mode was the strongest band in the spectrum. Scanning electron microscopy (SEM) observations revealed that the crystal morphologies were quite distinct. Figure 6 presents the typical morphological features of the obtained GaN crystals. The crystals had a thickness on the micron scale, much thinner than the crystals grown in the Li3N flux (20-300 µm).11
Growth of GaN Single Crystals
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Figure 6. SEM images of GaN crystals. (a) The typical layered structure of the GaN crystals. (b) An SEM image of the smooth cross-section of a layer. (c) An SEM image of numerous pits on a thinner layer. The inset is an enlarged image of the hole. (d) Smooth surface of a layer.
Moreover, the crystals were found to have a layered structure, as seen in Figure 6a. Figure 6b shows the smooth cross-section of a layer with a thickness of about 0.5 µm. Much thinner layers with thickness of about tens of nanometers were observed in other crystals, as shown in Figure 6c. The layers were very smooth in large scale, as displayed in Figure 6d. Such morphologies, different from that of the crystals grown from the Li3N and Na flux,12,24 suggested that a new mechanism was involved in the present growth process by introducing the CaLi3N composite flux. This mechanism should be layer-based growth, like that frequently observed in gas phase growth process. Another morphological feature was the existence of pits on the crystal surfaces, as shown in Figure 6c. Some of them were even across the entire thickness of crystals and formed holes. These holes had diameters on the micron scale. The inset in Figure 6c is an enlarged image of the hole, clearly indicating that the holes can block the growth of layers. Discussion The results presented above suggested that the addition of Ca to the Li3N flux affected the size distribution, morphologies, and yield of GaN crystals. The size distribution of crystals is considered to be closely related to the nucleation process in the crystal growth. The uniform size distribution of the GaN crystal means that the nucleation occurs at a narrower temperature range compared to that in the case with the Li3N flux.16 In the latter case, the nucleation may already occur prior to cooling, and the process continues in the subsequent cooling at a wide temperature range, leading to a wide size distribution of crystals. In the case with the composite flux, the critical supersaturation is promoted to a higher level, resulting in the nucleation occurring only at temperatures in the cooling process where the supersaturations are critical. The increase in the critical supersaturation is attributed to the enhanced solubility of N in the melts by adding Ca and retards the homogeneous nucleations to some extent. Our calculations on the phase diagrams,20,21 and related experiments,25 showed that the N solubility in melts containing Ca and Li3N should be higher than melts containing Li3N only under the same conditions.
Kawamura and co-workers observed that a less N2 atmosphere (10 atm) was required to grow GaN crystals in Na-Ca-Ga melts compared to in Na-Ga melts. Meanwhile, the crystals became transparent from black due to the decrease of vacancies of N. What is shared here in both our system and the Na-CaGa system is enhancing N solubility by adding Ca. But in our case, only 2 atm or less of N2 is required to grow GaN crystals due to higher affinity of N with the Li-Ca system. The introduction of composite flux resulted in thinner layers and pits on the surfaces of GaN crystals, enhancing the layerbased growth. Two possible reasons can contribute to the changes in the morphology. One is that metallic Ca plays a role of surfactant in changing the growth habit by slowing the growth rate along the c-axis direction, while it enhances the rates along the lateral directions. Another reason is due to the impurities in the melts that are absorbed on the crystal growth front and form inclusions in crystals, as seen from Figure 6c. These impurities perhaps are compounds related to Ca. The yield of GaN crystals by the composite flux method was closely related to the flux compositions. The influence of the addition of Ca on yield was substantial. The yield was about 0.4 mol % at a molar ratio 0.25:1 of Ca and Li. When the molar ratio of Ca to Li increased from 0.25 to 0.35, the GaN crystals in the products were from plenty to zero. Conclusions Colorless GaN platelet crystals of up to 4 mm have been prepared by the Ca-Li3N flux method. The effects of introducing the Ca-Li3N composite flux can be summarized as follows: (1) The size distribution of the GaN crystals grown from the composite flux is more uniform. (2) The morphologies of the GaN crystals obtained using the composite flux is quite different from that of the Li3N flux. (3) Excessive Ca in the composite flux prevents the formation of GaN. These results imply a helpful route for the growth of better quality GaN crystals from melts by optimizing the composite flux.
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Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 59972040 and 59925206) and by the Ministry of Science and Technology of the People’s Republic of China (Grant No. 2002 AA311210). References (1) Nakamura, S.; Mukai, T.; Senoh, M.; Iwasa, N. Jpn. J. Appl. Phys. 1992, 31, L139-L142. (2) Amano, H.; Kito, M.; Hiramatsu, K.; Akasaki, I. Jpn. J. Appl. Phys. 1989, 28, L2112-L2114. (3) Nakamura, S. Science 1998, 281, 956-961. (4) Nakamura, S. Phys. World 1998, 11, 31-35. (5) Xu, X. P.; Vaudo, R. P.; Loria, C.; Salant, A.; Brandes, G. R.; Chaudhuri, J. J. Cryst. Growth 2002, 246, 223-229. (6) Motoki, K.; Okahisa, T.; Nakahata, S.; Matsumoto, N.; Kimura, H.; Kasai, H.; Takemoto, K.; Uematsu, K.; Ueno, M.; Kumagai, Y.; Koukitu, A.; Seki H. J. Cryst. Growth 2002, 237-239, 912-921. (7) Porowski, S. J. Cryst. Growth 1998, 189-190, 153-158. (8) Aoki, M.; Yamane, H.; Shimada, M.; Sarayama, S.; Disalvo, F. J. J. Cryst. Growth 2002, 242, 70-76. (9) Aoki, M.; Yamane, H.; Shimada, M.; Sarayama, S.; Disalvo, F. J. Cryst. Growth Des. 2001, 1, 119-122. (10) Aoki, M.; Yamane, H.; Shimada, M.; Sarayama, S.; Disalvo, F. J. Cryst. Growth Des. 2002, 2, 55-58. (11) Song, Y. T.; Wang, W. J.; Yuan, W. X.; Wu, X.; Chen, X. L. J. Cryst. Growth 2003, 247, 275-278.
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