CRYSTAL GROWTH & DESIGN
GaN Single Crystals: Growth Mechanism and Temperature-Modulated Growth Using Li3N Flux
2009 VOL. 9, NO. 1 611–615
Huiqiang Bao, Bo Song, Hui Li, Gang Wang, Wenjun Wang, Wanyan Wang, and Xiaolong Chen* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, P. R. China ReceiVed August 22, 2008; ReVised Manuscript ReceiVed October 6, 2008
ABSTRACT: GaN single crystals were successfully obtained by using Li3GaN2 flux at 800 °C under a nitrogen pressure of 2 atm, providing direct experimental evidence for GaN crystals grown from melts in Li-Ga-N ternary system. Then, a temperaturemodulated process was introduced to grow GaN single-crystal by Li3N flux at the temperature range from 800 °C to T1 (T1 ) 820, 810, and 794 °C). It is found that this process suppresses the negative effect of homogeneous nucleations distinctly and results in a more uniform size-distribution of GaN crystals in comparison with that grown under no modulated conditions; 800-810 °C is a suitable modulating region and about 68 wt % of crystals has a size larger than 1 mm. The layered growth mechanism is also assured by the micro-observation on crystal morphology. These results indicate a potential route for growing large GaN crystals by optimizing the modulating conditions. Introduction GaN has attracted great interest in the field of opoelectronic devices due to its wide band gap, high thermal stability, and high breakdown voltage.1,2 For the conventional SiC, GaAs and sapphire substrates used in GaN-based light-emitting diodes (LEDs) and laser diodes (LDs), large lattice constant and thermal expansion mismatches between the substrate and epitaxial film, will inevitably induce high-density defects, which deteriorates the performance of these devices.3-5 An ideal improvement is to select the GaN substrate for the homoepitaxial growth of highquality films. Thus, developing high-quality and large GaN crystals is concernful. The growth of GaN crystals has been extensively investigated by several research groups using different methods such as hydride vapor phase epitaxy (HVPE),6 high-pressure solution method7 and ammonothermal growth route.8 However, highdefect-density, rigorous growth conditions, and corrosion of reactant to equipment in the growth process restrict the commercial production of GaN substrates in a large scale by these methods. Sodium flux method, developed by Yamane et al., was applied to grow GaN single crystals of mm scales at 600-800 °C under a nitrogen pressure of 5-100 atm.9 Simultaneously, Chen et al. explored Li3N flux method and grew GaN crystals up to 4 mm under more moderate conditions (nitrogen pressure of 1-2 atm, 800 °C),10 suggesting that the flux method is promising pathway to grow GaN crystals. Subsequently, Wang et al. proposed a two-step-reaction responsible for GaN growth mechanism in Li3N flux, and reported that GaN crystals was grown from the Li-Ga-N melt.11 Up to now, however, direct experiment evidence is still in lack, not benefiting to further design experiment route for enlarging the crystal size. In addition, Li3N flux method suffers from the homogeneous nucleations occurring in random orientations during growth process, hampering the further increase of crystal sizes. To improve this situation, we try a simple and low-cost way, that is, controlling the GaN nucleation in melts by modulating growth temperature. As well-known, nucleation occurs in melts at critical supersaturation and gradually grows, * Corresponding author. Tel.: +86-10-8264-9039; fax: +86-10-8264-9646 E-mail address:
[email protected].
Table 1. Experiment Parameters of Modulating Growth case
1
2
3
V1 (°C /h) T1 (°C) Keeping time at T1 (h) V2 (°C /h) T0 (°C) Keeping time at T0 (h) Cycling times
20 820 0.5 1 800 5 5
10 810 0.5 0.5 800 5 5
0.125 794 0.5 3 800 5 3
but GaN nucleations or grown grains will completely or partially redissolve in Li-Ga-N solution zone by elevating system temperature again. Hence, it is expected to obtain more sizeuniform and larger crystals by controlling homogeneous nucleations. In this work, we focused on growth mechanism of GaN single crystals using Li3N flux, GaN crystals up to 2 mm grown by Li3GaN2 flux provided experiment evidence for the twostep-reaction process and crystals grown from melt in Li-Ga-N ternary system. Then, we try to grow GaN single-crystals by modulating growth temperature in the region of 800 °C to T1 (T1 ) 820, 810, and 794 °C). As expected, modulating process can effectively control the homogeneous nucleations, resulting in GaN crystal distributing more uniform in size. It is found that 800-810 °C is suitable modulating range. The micromorphology of obtained crystals indicates the lay-like growth mechanism involved in this system. Experimental Sections GaN Single-Crystal Growth by Li3GaN2. Li3GaN2 was synthesized by the direct reaction of metal Li (>99%) and Ga in a flowing atmosphere of N2 (99.999%) at 450 °C. The molar ratio of Li to Ga is 3:1. Then, Li3GaN2 and Ga with a molar ratio of 1:2 were put into tungsten crucible for subsequent crystal growth. The apparatus used here for growing GaN crystals was same as described before.10 Detailed experiment process is same to that used for Li3N flux.10 The whole experiment process holds about 120 h. Temperature-Modulated Growth of GaN Crystals. The Li3N was synthesized by the reaction of metal Li (>99%) and high purity N2 gas (99.999%) at 450 °C. The starting materials used for GaN crystal growth were Ga (99.999%) and Li3N with a molar ratio of 3:1. After evacuated to a vacuum of 3.0 × 10-3 Pa, the system was charged with 2 atm of N2 gas at room temperature. The tungsten crucible filled with starting materials was heated to 800 °C (T0), kept at this temperature
10.1021/cg800925t CCC: $40.75 2009 American Chemical Society Published on Web 11/20/2008
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Figure 1. (a) Typical optical image, (b) EDX spectrum, and (c) XRD pattern of the obtained crystals using Li3GaN2 flux.
Figure 2. Optical image of the products obtained at different modulated conditions: (a) and (b) gray products and plate-like crystals grown in case 1, respectively; (c) and (d) crystals obtained in case 2 and 3, respectively. for 12 h, and then carried out the modulating process: heating/cooling system to a premeditated temperature (T1) at certain velocity (V1) and keeping 0.5 h, then cooling/heating to T0 at V2 again and holding 5 h. After cycling this process several times, the system was cooled to room temperature by switching off the power. The detailed parameters for modulating experiments were listed in Table 1. After experiments, the products were washed by HCl solution and distilled water, the products left were collected for further characterizations. Characterization. X-ray diffraction (XRD) data of collected products were collected on a MAC-M18XHF diffractometer with Cu KR radiation. Morphology of the crystals was characterized by optical and scan electron microscope (SEM, FEI. XL-30). The element composition of the crystals was characterized by energy dispersive X-ray spectroscope (EDX) attached to 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 excitation source.
Results GaN Crystal Growth Using Li3GaN2. Figure 1a shows the optical image of collected products, and amount of plate-like
crystals are obtained. The crystals up to 2 mm are smaller than ones grown in Li3N flux. EDX spectrum (Figure 1b) shows that the crystal consists of N and Ga, and the atomic ratio of N to Ga is about 39: 61. Light-black in color may be induced by nitrogen vacancies.9 Figure 1c presents the typical XRD pattern of the milled crystals and a standard wurzite GaN pattern (ICCDPDF> No: 50-0792) is also inserted at the bottom for reference. This pattern confirms the obtained crystals being wurtzite GaN by comparing with the reference one. Especially, the (0002) reflection is obviously preferential, implying the basal plane of crystal platelet as c-plane. GaN Crystal Growth by Temperature-Modulated Design. Figure 2 displays the optical images of collected products at different modulating temperatures. From these images, it is found that the sizes and morphological features of these crystals are intimately related to the modulating conditions. In case 1, gray products (Figure 2a) and a few colorless and transparent crystals with size less than 1 mm (Figure 2b) were obtained.
GaN Single Crystals
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synthesized under the no modulating conditions, only 33.7 wt % crystals has the size larger than 1 mm. Obviously, 800-810 °C is suitable temperature-modulated range for the growth of more uniform and larger crystals in present work. Discussion
Figure 3. Typical XRD patterns of products: (a) pray products in case 1, and (b) milled crystals and (c) platelet GaN ones.
Figure 3a and b presents the typical XRD patterns of the gray products and milled crystals, respectively. The reflections from both of the patterns confirm that GaN was successfully obtained (ICDD-PDF No: 50-0792). Similarly, preferred (0002) reflection is also observed in Figure 3b. In case 2 and 3, plenty of colorless and transparent crystals with the size larger that 1 mm can be collected (Figure 2c and d), only differ in size with that from case 1. The typical XRD pattern of plate-like crystals obtained in all of cases is shown in the Figure 3c, indicating the basal plane of crystal platelets being c-plane. Typical N/Ga atomic ratio of platelet crystals, detected by EDX, is about 45: 55, which is much more than that of crystals grown by Li3GaN2. Gray products is found to be consisted of large numbers of small spheres with internal platelets in interior (Figure 4a). Figure 4b shows the typical magnified morphology of these platelets, they are about 300 nm in thickness with one hillocklike side and one smooth side. It is also found that partial platelets link tightly together by big hillocks within some spheres, as shown in Figure 4c. For the obtained crystals, layered structure can be also observed distinctly (Figure 4d). The stripes in the side face of crystals have a height of 300-400 nm, similar to that of the platelets in sphere-like GaN. Thus, in this study, it is reasonable to describe the growth process by layer-based growth mechanism, like that frequently observed in gas phase growth process.12 Raman scattering was employed to detect the vibration states of GaN, which is sensitive to microscopic disorders. A typical Raman spectrum of the platelet GaN crystals is shown in Figure 5. Four phonon modes are observed at 146, 530, 570, and 734 cm-1, which are in good agreement with the reported E2(low), A1(TO), E2(high), and A1(LO).13 A1(TO), the forbidden mode in this scattering geometry, is also observed, probably since the incident light is not exactly perpendicular to the top surface of crystal. In general, the full-width at half-maximum (fwhm) of the E2(high) peak can reflect the crystalline quality. In the spectrum, the fwhm of E2(high) peak is 6 cm-1, indicating that obtained GaN crystals have high crystalline quality. Figure 6 illustrates the weight distribution of crystals in same size range obtained at different modulating temperatures. Crystals obtained in no modulating process of about 120 h is also shown as reference. As seen from the figure, all the crystals grown at the modulating temperature of 820 °C have size smaller than 1 mm. The ratio of crystals with size larger than 1 mm is 67.7 wt %, 43.4 wt % in case 2 and 3, respectively. For ones
Wang et al. proposed that a two-step-reaction was involved in the Ga-Li3N system: Li3N + Ga f Li3GaN2 + Li (1) and Li3GaN2 + Ga f GaN + Li (2). 11 Using Li3GaN2 flux, GaN single crystals up to 2 mm were obtained only by reactions (2) in this work, providing the convincing experimental evidence for this mechanism. In this process, Li produced in reaction (2) and Ga form Li-Ga alloy at early stage of experiment, and then Li3GaN2 dissolves in Li-Ga melt to form Li-Ga-N ternary solution. Upon cooling, GaN nucleates from Li-Ga-N melt or grows on the existing GaN fine particles by reaction (2), and then gradually grow up. But the size of crystals is smaller than that obtained using Li3N flux, low Li concentration due to the lack of reaction (1) may be responsible for this. Relatively low Li concentration may result in low N content and further low GaN surpersturation in forming Li-Ga-N ternary solution zone, which is not beneficial for the growth of large GaN crystals. The investigation of growth mechanism builds foundation for further designing effective experiment route to obtain crystals in large size. In Li3N flux method, the size distribution of crystals mainly depends on GaN nucleation process in melts. Crystal nucleation occurs in a wide temperature range, lots of GaN spontaneously nucleates and grows up in the growth process, resulting the obtained crystals have a wide size distribution and small average size. But in the temperature-modulated process, the nucleation and growth of GaN crystals can be selected and controlled to some extent. Taking the case 2 as example, critical supersaturation perhaps does not occur when system is heated to 810 °C. During decreasing the temperature to 800 °C, a number of nucleations gradually form in melts, and grow up until the first modulating process finished. With increasing temperature to 810 °C again, nucleuses will redissolve in melts and grown grains partially dissolve. By cycling the process for several runs, as natural seeds, these small grains have more chances to grow continuously based on more enough N in melt supplied by system and the solution of many nucleuses. As a result, more uniform and larger crystals are obtained. For the case of 794 °C modulating growth, similar process will are also encountered. But relatively low modulating temperature may result in more nucleuses formed in melts, since relatively high supersaturation is available in growth system. More nucleations than that in case 2 have opportunity to grow, resulting in relatively small crystals formed. In case 1, 820 °C is perhaps too high to be used as a suitable modulating temperature in terms of the decomposition temperature (813 °C) of Li3N.14 When the system is heated to temperature higher than 813 °C, partial Li3N which does not transform to Li3GaN2 begins to decompose.15 This leads to the lack of N and Li in melts, which retards the diffusion of N and Li in growth system and induces the local fluctuation of N and Li concentration. As a result, the stable environment for growing large crystals is hardly achieved, and different Li and N concentrations in melts will also confine the crystal size and induce difference in morphology. It is considered that crystals of hundreds of µm grew in the zone with high concentration of Li and N, and sphere-like GaN formed in melt with relatively low one. In modulating growth, the fringe of crystals is easy to dissolve due to the
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Figure 4. (a) and (b) Low and (c) high magnitude images of gray products. (d) Typical lay-like morphology of platelet crystals.
Figure 5. Raman spectrum of the platelet GaN crystals.
existence of amount of handing bonds. This is also speculated as the reason why round edge and sphere-like contour are common in crystal morphology. For sphere-like GaN, the structure of multiplatelets should be caused by unstable growth environment during temperature modulation. Lots of hillocks may indicate an edge-nucleation lay-like growth mechanism. Platelet-like structure in spheres substantially reveals the lay-like growth mechanism of GaN crystals in Li3N method, consistent well with that observed in bulk crystals. Conclusions Growth mechanism of GaN crystals in Li3N flux was investigated by Li3GaN flux method, confirming that GaN crystals were grown from Li-Ga-N melts. Based on this result, a temperature-modulated process was designed. The obtained GaN crystals, with more uniform and larger size, suggest that the homogeneous nucleations occurring in Li-Ga-N system
Figure 6. Weight dependence of crystal size at different modulating temperatures.
can be greatly restrained by temperature-modulated process. It is found that 800-810 °C is a suitable modulating region by now. The layer-like growth mechanism is also assured to be responsible for crystal growth. The results reported here provide a helpful way for getting more size-uniform and larger GaN single crystals. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50472075) and by the Ministry of Science and Technology of China (Grant No. 2002AA311210).
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