Effects of Al additives on growth of GaN polycrystals by the Na flux method

Effects of Al additives on growth of GaN polycrystals by the Na flux method

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Optical Materials xxx (2016) 1e4

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Effects of Al additives on growth of GaN polycrystals by the Na flux method Hiroki Imabayashi*, Kosuke Murakami, Daisuke Matsuo, Masatomo Honjo, Masayuki Imanishi, Mihoko Maruyama, Mamoru Imade, Masashi Yoshimura, Yusuke Mori Department of Electrical Electronic and Information Engineering, Osaka University, Suita, Osaka, 565-0871, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 April 2016 Received in revised form 9 September 2016 Accepted 9 September 2016 Available online xxx

In this study, we investigated the growth of GaN polycrystals using the Al-added Na flux method. We studied the effects of Al on accelerating the nucleation and purity of GaN polycrystals. The yields of GaN crystals grown in Al-added Na flux were dramatically increased from those in Al-free Na flux, and the polycrystals grown by the Al-added Na flux method were highly transparent. As observed in secondary ion mass spectroscopy measurements, the Al content of the polycrystals was below the detection limit of 3  1016 atoms/cm3. From these results, the Al-added Na flux method is found to be appropriate for fabricating a large amount of GaN polycrystals without deteriorating the crystal quality. © 2016 Elsevier B.V. All rights reserved.

Keywords: Gallium nitride (GaN) Polycrystals Crystal growth Na flux method

1. Introduction High-quality and low-cost growth methods have been extensively investigated for gallium nitride (GaN) growth because of its potential applications in optoelectronic devices as well as highpower, high-frequency devices [1e3]. Some approaches such as hydride vapor phase epitaxy (HVPE) [4e6], high-pressure solution growth [7e9], ammonothermal growth [10e12], and Na-flux growth [13e15] have been developed to obtain GaN crystals. Among these methods, Na flux growth is a promising candidate for growing high-quality GaN crystals with low impurity concentrations [16]. In our previous study, we have investigated GaN crystal growth technologies using the Na flux method, such as the liquid phase epitaxy (LPE) method for restricting the nucleation site on the seed [17e19], the point seed growth technique [20e22], and the coalescence growth technique [23,24]. Among these technologies, controlling spontaneous nucleation of GaN polycrystals on any area other than the seed is quite important in terms of raw material efficiency and controlling the size of grown crystals. For growing bulk GaN crystals by the Na flux method, it is required to suppress the generation of polycrystals because generation of polycrystals causes reduces the raw material efficiency

* Corresponding author. E-mail address: [email protected] (H. Imabayashi).

and growth rate. In our previous study, we have achieved suppression of nucleation of GaN crystals in the Na flux method by addition of carbon [25,26] and solution stirring [27], resulting in the growth of high-quality bulk GaN crystals for fabricating GaN substrate [19,20,28e30]. In recent years, GaN polycrystals obtained by spontaneous nucleation have also attracted attention because of their low impurity concentration. In case of some methods for GaN bulk and thin film growth, such as the sputtering method and the ammonothermal method, GaN is used as one of raw materials. Therefore, high-purity and highdensity GaN materials are required. GaN powder or sintered body, which are made of polycrystals grown by the HVPE method or prepared from Ga2O3 and ammonia gas, are used. However, these GaN materials increase the cost of fabrication and have high concentration impurities such as oxygen. Additionally, sintered GaN is fragile because of low sinter density. In contrast, Na flux growth is a promising candidate for growing high-quality GaN crystals with low impurity concentrations [16] that may be suitable for sputtering target materials. For fabricating GaN polycrystals for use as raw materials in the Na flux method, new methodologies must be developed for preparing a large amount of polycrystals because few studies have been reported on improving nucleation in the Na flux method. In this study, we investigated the effects of additives into the GaNa flux because small amounts of impurities in the solutions are

http://dx.doi.org/10.1016/j.optmat.2016.09.030 0925-3467/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: H. Imabayashi, et al., Effects of Al additives on growth of GaN polycrystals by the Na flux method, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.09.030

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Fig. 1. Total GaN yield in the growth with and without an aluminum additive. The addition of aluminum promotes crystal generation.

known to have dramatic effects on crystal growth [25,31]. In particular, we studied the growth of GaN polycrystals by using the Al-added Na flux method, and investigated the effects of Al on accelerating the nucleation and size of GaN polycrystals. 2. Experimental In an Ar-filled glove box, metal Ga (purity: 6 N), metal Na (purity: 4 N), carbon (purity: 6 N) and metal aluminum (purity: 5 N)

were placed in an yttrium oxide (yttria) crucible (17-mm inner diameter and 25-mm height), and this crucible was enclosed in a stainless tube. The Ga:Na molar ratio was 27:73, for 2.0-g Ga and 1.8-g Na. C content relative to the total Ga/Na amount was fixed at 0.5 mol% (0.006 g). Al contents relative to the total Ga/Na amount were fixed at 0.05 mol% (0.0015 g), 0.1 mol% (0.003 g) and 0.5 mol% (0.015 g). After removing the tube from the glove box, the crystal growth was conducted using the following procedure. First, the tube was connected to an N2 gas line and heated to 870  C using an electric furnace. Then, the tube was maintained at 870  C and 3.2 MPa N2 gas for 96 h. After the tube cooled naturally, the crucible was removed from the tube. Residual Na and Ga-Na alloy were removed from the crucible with ethanol and water. The crystal yields were derived from the weight of metal Ga placed in the crucible and the grown polycrystals. The crystal size was evaluated using scanning electron microscopy (SEM) images. The concentration of Al impurity in the grown GaN crystals was evaluated by secondary ion mass spectrometry (SIMS), and the true density was measured by an AccuPyc II 1340 gas displacement pycnometer. 3. Results and discussion Fig. 1 shows the total GaN yield as a function of the amount of added Al. In this experiment, the yield of GaN crystals grown in Alfree Na flux was almost 0%. However, the yields of GaN crystals grown with 0.05 and 0.1mol% Al-additives were 84.79% and 89.94%,

Fig. 2. SEM image (a) and optical microscope image (b) of the polycrystals grown with 0.05 mol% Al addition, and photograph of typical polycrystals grown using the Na flux method without an Al additive (c). Schematic illustration of the growth of polycrystals on the crucible wall with and without 0.05 mol% Al additive (d).

Please cite this article in press as: H. Imabayashi, et al., Effects of Al additives on growth of GaN polycrystals by the Na flux method, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.09.030

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respectively. SEM image and optical microscopy image of the polycrystals grown with 0.05 mol% Al are shown in Fig. 2. From the schematic illustration of the growth of GaN polycrystals (shown in Fig. 2(d), these polycrystals were grown on the crucible wall. Fig. 2(a) shows that a large amount of polycrystals with sizes smaller than or comparable to 1 mm was grown with the addition of Al. In the case of the typical Na flux method without addition of Al, 1e2 mm polycrystals were obtained. (The detailed growth equipment is described in ref. [31]). These results indicate that the addition of Al increases the nucleation of GaN polycrystals in the Na flux method. The polycrystals grown using the Al-added Na flux method had high transparency (shown in Fig. 2(b)). In the previous paper, typical GaN polycrystals grown using the Na flux method without Al additive showed blackening (shown in Fig. 2(c)), and the black color was attributed to nitrogen deficiency [32]. In addition, hightransparency GaN crystals were grown by the addition of Ca in the Na flux [33], and the transparency was caused by increasing nitrogen concentration into the flux as a result of Ca addition. This suggests that the addition of Al into the Na flux results in increasing nitrogen concentration into the flux and decreasing nitrogen deficiency in the GaN polycrystals. Therefore, Al addition facilitates obtaining a larger amount of polycrystals. A photograph and SEM image of the GaN polycrystals grown

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using the 0.5 mol% Al-added Na flux method are shown in Fig. 3. In the previous studies, almost all polycrystals were grown on the crucible wall. This indicates that nucleation near the crucible wall is more difficult than that on the crucible wall. However, the polycrystals shown in Fig. 3 were grown not only on the crucible wall but also in the flux near the gas-liquid interface (far from the crucible wall). Thus, an increasing amount of additive Al enhanced the nucleation of GaN. From the surface SEM image in Fig. 3(b), there are no remarkable voids in the aggregation of polycrystals grown with 0.5 mol% Al addition (The size of the crystal shown in Fig. 3(b) could not be measured because the surface of the aggregate was covered with a GaN layer). We also measured the real density of GaN polycrystal aggregation. The density was 6.01 g/cm3, and the relative density to the density of bulk GaN was 97.3%. This indicates that the aggregation of the polycrystals obtained in this study had very few closed pores. As observed in SIMS measurements, the Al content of the polycrystals shown in Fig. 3(a) was below the detection limit of 3  1016 atoms/cm3, regardless of the Al content in the solution. This result indicates that the Al is not readily incorporated into the GaN crystal. Hence, the Al-added Na flux method is appropriate for fabricating a large amount of GaN polycrystals without deterioration of the crystal quality. 4. Conclusions In this study, we investigated the growth of GaN polycrystals using the Al-added Na flux method. The total GaN crystal yield changed from almost 0% to nearly 90% by the addition of Al, and the Al content of the polycrystals was below the detection limit. These results indicate that the Al-added Na flux method is appropriate for fabricating a large amount of GaN polycrystals without deterioration of the crystal quality. Thus, this method could be highly advantageous for fabricating high-quality GaN sputtering targets. Funding We gratefully acknowledge funding from the New Energy and Industrial Technology Development Organization (NEDO) (Project No. 15102169-0). Acknowledgements We thank T. Iwai and A. Michishita (Shimadzu, Corp.) for their help in the bulk true density measurements. We thank K. Neishi, and N. Takahashi (Tokyo Electron Ltd.) for providing the SIMS data. References

Fig. 3. Optical image (a) and SEM image (b) of the polycrystals grown with 0.5 mol% Al addition.

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Please cite this article in press as: H. Imabayashi, et al., Effects of Al additives on growth of GaN polycrystals by the Na flux method, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.09.030