Ammonothermal Crystal Growth of GaN Using an NH4F Mineralizer

Low-resistivity m -plane freestanding GaN substrate with very low point-defect concentrations grown by hydride vapor phase epitaxy on a GaN seed cryst...
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Ammonothermal Crystal Growth of GaN Using an NH4F Mineralizer Quanxi Bao,†,§ Makoto Saito,†,‡ Kouji Hazu,† Kentaro Furusawa,† Yuji Kagamitani,‡ Rinzo Kayano,§ Daisuke Tomida,† Kun Qiao,*,† Tohru Ishiguro,† Chiaki Yokoyama,† and Shigefusa F. Chichibu† †

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Mitsubishi Chemical Corp., 1000 Higashi-Mamiana, Ushiku, Ibaraki 300-1295, Japan § Japan Steel Works, Ltd., 11-1, Osaki 1-chome, Shinagawa-ku, Tokyo 141-0032, Japan ‡

ABSTRACT: NH4F is demonstrated to be a promising mineralizer for the acidic ammonothermal crystal growth of GaN. In comparison with other acidic mineralizers such as NH4Cl, NH4Br, and NH4I, NH4F behaves distinctively different. First, NH4F affords a negative temperature gradient for crystal growth of GaN in supercritical NH3 at a temperature range from 550 to 650 °C. Second, it enables GaN crystal growth in polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH4F remarkably increases both the growth rate and quality of the GaN crystal. With the aid of NH4F, self-nucleation of GaN and bulk growth of hexagonal GaN crystals from the self-nucleated seed have been realized. temperatures from 400 to 600 °C in the presence of NH4Cl, NH4Br, and NH4I, a positive temperature gradient is currently employed in the acidic ammonothermal method, where crystal growth of GaN occurs in the cold zone.9,10 In contrast, the basic ammonothermal version can provide a negative temperature coefficient of GaN solubility within the same temperature range, which has the advantage of GaN growth in the hot zone and leads to better crystal quality.4,11 In this study, we report the use of NH4F as a mineralizer for the acidic ammonothermal crystal growth of GaN. Although NH4F is a member of the ammonium halide family, it behaves distinctively different from its Cl, Br, and I counterparts in the acidic ammonothermal crystal growth of GaN, providing advantages that can address most of the problems currently encountered with the acidic method. First, NH4F affords a negative temperature gradient for crystal growth of GaN in supercritical NH3 at a temperature range of 550 to 650 °C. This is the first example of the achievement of a reverse temperature gradient under ammonoacidic conditions. Second, it can enable GaN crystal growth in both polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH4F can remarkably increase the growth rate and quality of GaN crystals. Growth speeds achieved in the c plane and m plane can be up to 250 and 300 μm/day in the presence of NH4F. These are the highest rates reported so far for the ammonothermal method. With the aid of NH4F, we have also realized the selfnucleation of GaN seeds and bulk growth of hexagonal GaN crystals from the self-nucleated seeds. This work thus

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allium nitride (GaN) is a wide band gap semiconductor with unique material and electronic properties. It has wide application as the key material for manufacturing next generation high-power and high-frequency devices. To meet the great demand for high-quality GaN crystals, a variety of crystal growth methods, such as hydride vapor phase epitaxy (HVPE), Na-flux, N2 high pressure, and ammonothermal methods, have been proposed.1−4 The ammonothermal method is an analogue of hydrothermal crystallization, which has applications in the industrial crystal growth of quartz and ZnO. Both basic and acidic mineralizers have found application in the ammonothermal crystal growth of GaN.4,5 Owing to advantages such as scalability, crystal quality, and cost effectiveness, the method is generally believed to be the most promising to realize the mass production of high quality GaN crystals, especially the acidic process because it can be operated under milder pressure (100−200 MPa). Over the past decade, our efforts have been focused on acidic ammonothermal crystal growth using mineralizers including NH4Cl, NH4Br, and NH4I.6,7 Unfortunately, however, these mineralizers only allow the homoepitaxial growth of GaN layers in the polar direction (c plane). GaN substrates with different orientations, including the polar c plane, semipolar m plane, and nonpolar a plane, are all in market demand. The latter two are the most suitable for green optoelectronics due to their largely or completely eliminated electric fields.8 This means that the acidic ammonothermal method must provide true bulk crystal growth of GaN in semipolar and nonpolar directions, as well as the polar direction so that any arbitrary crystal plane may be cut. In addition, the acidic ammonothermal method needs to improve the quality of the resultant GaN crystal. Because the temperature coefficient of GaN solubility is positive at © 2013 American Chemical Society

Received: May 22, 2013 Revised: August 16, 2013 Published: August 23, 2013 4158

dx.doi.org/10.1021/cg4007907 | Cryst. Growth Des. 2013, 13, 4158−4161

Crystal Growth & Design

Communication

represents the first step toward a process for true bulk crystal growth of GaN, using the acidic ammonothermal method. The experimental configuration used for the ammonothermal crystal growth of GaN with the NH4F mineralizer is illustrated in Figure 1. A negative temperature gradient was employed,

Figure 3. Top view photographs of typical GaN crystal samples grown using different mineralizers: (a) NH4Cl, (b) NH4Br, (c) NH4I, and (d) NH4F. Figure 1. Experimental configuration of ammonothermal GaN growth using NH4F as a mineralizer. A negative temperature gradient was employed, where the GaN seed was placed at the bottom of the autoclave (hot zone, 650 °C) and polycrystalline GaN nutrient was placed at the top of the autoclave (cold zone, 550 °C).

growth of GaN in polar (c plane), semipolar, and nonpolar directions (a plane and m plane) was achieved. NH4F also remarkably increases the growth rate of GaN. The pressure dependence of the growth rate of the GaN crystals obtained using different mineralizers is summarized in Figure 4.

where the GaN seed was placed at the bottom of the autoclave (hot zone, 650 °C) and the polycrystalline GaN nutrient was placed at the top (cold zone, 550 °C). Commercially available NH4F (Aldrich, 99.99%) was used as received, and the experimental procedure was the same as that described in our previous studies.7 A cross-sectional photograph of an as-grown GaN crystal is shown in Figure 2. XRD analysis of a pulverized GaN crystal

Figure 4. Pressure dependence of typical GaN crystal samples grown using different mineralizers.

As can be seen, the GaN crystal growth rate increases with NH3 pressure, and NH4F gives a much better performance than NH4Cl, NH4Br, and NH4I in terms of c plane crystal growth. The m plane growth rate achieved using NH4F was up to 300 μm/day. These are the highest growth rates reported so far for an acidic ammonothermal method. The crystal quality of as-grown GaN is also improved with NH4F mineralizer by reducing the concentrations of impurities. For example, Si and O are the major impurities in acidic ammonothermal GaN. The average concentrations of Si and O in the GaN layers grown using NH4Cl, NH4Br, and NH4I were approximately 1019 and 1−1.5 × 1020 cm−3, respectively. Secondary-ion mass spectrometry (SIMS) analysis of a typical GaN crystal grown using NH 4 F revealed that NH 4 F significantly decreased the concentrations of Si and O, especially that of Si, to about 9 × 1014 and 8 × 1018 cm−3, respectively. We think this improvement can be attributed to the following two reasons. One is that the temperature of the

Figure 2. XRD pattern of pulverized GaN crystal grown using NH4F as a mineralizer and cross-sectional photograph of as-grown GaN crystal under UV light.

confirmed that it had a pure hexagonal phase. These results clearly demonstrate the achievement of the ammonothermal growth of GaN crystal using NH4F as a mineralizer. This is the first example of ammonothermal crystal growth of GaN using a negative temperature gradient under ammonoacidic conditions. NH4F is the only acidic mineralizer that enables the crystal growth of GaN in polar (c plane), semipolar, and nonpolar directions (a and m planes). Top view photographs of typical GaN samples grown using ammonium halide mineralizers on a HVPE seed GaN crystal are presented in Figure 3. The shape of the original HVPE seed is marked by the red-dashed line. As can be seen, GaN growth only occurred in a polar direction (c plane) using NH4Cl, NH4Br, and NH4I. With NH4F, full crystal 4159

dx.doi.org/10.1021/cg4007907 | Cryst. Growth Des. 2013, 13, 4158−4161

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Communication

crystallization zone in the case of NH4F is higher than that of NH4Cl, NH4Br, and NH4I thanks to the retrograde solubility provided by NH4F. The other is possibly associated with the nature of fluoride because it has the smallest radius/charge ratio of the halide anions. Development of the ammonothermal method for GaN crystal growth is regarded as comparable to that of the hydrothermal method used for quartz and ZnO growth. This requires that the ammonothermal method should also allow the growth of GaN seed crystals, as already achieved for quartz and ZnO. The preparation of self-nucleated GaN seeds, and the crystal growth of GaN on the seeds were carried out in the presence of NH4F.12 High-quality self-nulceated GaN seeds can be obtained by recrystallization of polycrystalline GaN in the presence of NH4F. X-ray rocking curve measured on the (100) and (110) planes in the c axis direction of a typical self-nulceated GaN seed is shown in Figure 5. The full width at half-maximum

Figure 7. Room temperature photoluminescence spectra of typical GaN crystal samples. (a) epitaxial GaN film grown using NH4I as a mineralizer, (b) epitaxial GaN film grown using NH4F as a mineralizer, (c) a seed-grown hexagonal GaN crystal, and (d) a standard HVPE GaN seed crystal.

suggesting that use of NH4F results in better GaN crystal quality. In the case of the seed-grown hexagonal GaN crystal, a 67 meV FWHM was observed, which is very close to that of a standard HVPE GaN seed crystal. This is the first time that an acidic ammonothermal method has been used to grow GaN crystal with quality on par with that of HVPE GaN crystal. In conclusion, NH4F is demonstrated to be a very promising acidic mineralizer for the ammonothermal crystal growth of GaN. With its ability to provide retrograde temperature gradient, full crystal growth of GaN in all directions, high growth rate, and high crystal quality, NH4F is able to address some of the most demanding requirements for the mass production of GaN using the acidic ammonothermal method. With the aid of NH4F, self-nucleation of GaN seeds and bulk growth of hexagonal GaN crystal from the self-nucleated seeds have been realized. The present study represents the first step toward a process that can truly realize bulk crystal growth of GaN, and mass production of high quality GaN crystal at a reasonable price can be highly expected in the near future.

Figure 5. X-ray rocking curve measured on the (100) and (110) planes in the c axis direction of a typical self-nucleated GaN seed.

(FWHM) values are 28.6 arsec and 28.9 arsec, respectively, suggesting that the self-nucleated seed has excellent crystallinity. Images of a typical 5 mm self-nucleated GaN seed along the c axis and a 7 mm hexagonal GaN crystal grown from the seed are shown in Figure 6. This is the first example of the seed growth of a single GaN crystal under the ammonoacidic conditions, and NH4F plays a crucial role in realizing this process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 6. Images of a typical 5 mm self-nucleated GaN seed along the c axis and a 7 mm hexagonal GaN crystal grown from the seed.

REFERENCES

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Room temperature photoluminescence (PL) spectra of various GaN crystal samples, including epitaxial GaN films grown using NH4I and NH4F as mineralizers, a seed-grown hexagonal GaN crystal, and a standard HVPE GaN seed crystal are shown in Figure 7. PL is a powerful tool to evaluate the quality of wide-bandgap semiconductors. As can be seen, the FWHM value of the GaN epitaxial film grown using NH4F is 142 meV, which is lower than that of NH4I (168 meV), 4160

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(6) Kagamitani, Y.; Ehrentraut, D.; Yoshikawa, A.; Hoshino, N.; Fukuda, T.; Kawabata, S.; Inaba, K. Jpn. J. Appl. Phys. 2006, 45 (5A), 4018. (7) Kagamitani, Y.; Kuribayashi, T.; Hazu, K.; Onuma, T.; Tomida, D.; Simura, R.; Chichibu, S. F.; Sugiyama, K.; Yokoyama, C.; Ishiguro, T.; Fukuda, T. J. Cryst. Growth 2010, 312, 3384. (8) Kucharski, R.; Zajac, M.; Doradzinski, R.; Rudzinski, M.; Kudrawiec, R.; Dwilinski, R. Semicond. Sci. Technol. 2012, 27, 024007. (9) Tomida, D.; Kuribayashi, T.; Suzuki, K.; Kagamitani, Y.; Ishiguro, T.; Fukuda, T.; Yokoyama, C. J. Cryst. Growth 2011, 325, 52. (10) Tomida, D.; Kuroda, K.; Hoshino, N.; Suzuki, K.; Kagamitani, Y.; Ishiguro, T.; Fukuda, T.; Yokoyama, C. J. Cryst. Growth 2010, 312, 3161. (11) Dwilinski, R.; Doradzinski, R.; Garczynski, J.; Sierzputowski, L.; Kucharski, R.; Zajac, M.; Rudzinski, M.; Kudrawiec, R.; Strupinski, W.; Misiewicz, J. Phys. Status Solidi A 2011, 208, 1489. (12) Preparation of self-nucleated GaN seed was carried out using polycrystalline GaN as raw material in the presence of NH4F (0.5−5 mol %) at the temperature range from 550 to 650 °C. Seed-growth of GaN crystal was carried out using the condition shown in Figure 1. Details of preparation of self-nucleated GaN seed and growth on the seed will be reported elsewhere.

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dx.doi.org/10.1021/cg4007907 | Cryst. Growth Des. 2013, 13, 4158−4161