Bulk GaN single crystals: growth conditions by flux method

Bulk GaN single crystals: growth conditions by flux method

Journal of Crystal Growth 247 (2003) 275–278 Bulk GaN single crystals: growth conditions by flux method Youting Songa, Wenjun Wanga, Wenxia Yuanb, Xin...

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Journal of Crystal Growth 247 (2003) 275–278

Bulk GaN single crystals: growth conditions by flux method Youting Songa, Wenjun Wanga, Wenxia Yuanb, Xing Wua, Xiaolong Chena,* a

Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China b Department of Applied Sciences, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Received 3 October 2002; accepted 8 October 2002 Communicated by M. Schieber

Abstract Hexagonal GaN platelet crystals with a size of 1–4 mm have been grown by a Li-based flux method. The influence of growth conditions such as the molar ratio of starting materials, temperature, pressure, the position of Li3N in the crucible on the growth of GaN single crystals was studied. The quality of GaN single crystal was checked by optical microscope and X-ray rocking curve. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.10.Dn Keywords: A2. Growth from solutions; A2. Single crystal growth; B1. Gallium compounds; B1. Nitrides; B2. Semiconducting III–V materials

1. Introduction Wurtzite structure GaN is a wide direct band gap semiconductor and finds applications in blue light emitting diode (LED) and laser diode (LD) [1]. Performances of LED and LD depend on, to some extent, the quality of GaN epitaxy film, which is closely related to its substrate materials. Up to now, substrate materials used for GaN film include sapphire, silicon carbide, LiGaO2, LiAlO2, etc. But all substrate materials except GaN may introduce stresses and dislocations into GaN epitaxy film due to lattice mismatch and thermal *Corresponding author. Tel.: +86-010-82649039; fax: +86010-82649531. E-mail address: [email protected] (X. Chen).

mismatch, and thus lower quality of GaN film. GaN bulk single crystals are undoubtedly to be an ideal substrate for homoepitaxial growth of high quality GaN films. However, it is very difficult to obtain large size of GaN single crystals, for GaN decomposes at the temperature much lower than its melting temperature. Bulk GaN single crystals with centimeter size have been grown at high temperatures (B15001C) and high pressures (B14000 atm) [2], since this growth condition is stringent, is difficult to popularize this method. In recently years, people are actively search for methods to grow GaN bulk single crystals at low temperatures and low pressures. Yamane et al reported on the growth of GaN single crystals with millimeter size at a temperature of B8001C and N2 pressure of B100 atm by a Na flux [3]. Recently,

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 2 0 1 4 - 6

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We found that the yields of GaN are closely related to the molar ratio of Li3N and Ga. If Li3N is excessive, only Li3GaN2 can be obtained; if the molar ratio of Li3N and Ga is 1:1, GaN and Li3GaN2 coexist in final forming substances and Li3GaN2 always appears at upper parts of the crucible, GaN at the bottom of the crucible. It may be due to Li3N at the upper part of the crucible not fully contacting with liquid Ga. If Ga is excessive, the main products are GaN. The suitable proportion of raw materials is Ga: Li3N=1:1–4:1. Usually, only chemical reactions occur, GaN granules, no crystals of millimeter size can be

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ð2Þ

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Li3 GaN2 þ Ga ¼ 2GaN þ 3Li:

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ð1Þ

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2Li3 N þ Ga ¼ Li3 GaN2 þ 3Li;

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The growth of GaN single crystals undergoes two-step chemical reactions:

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3. Results and discussion

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The starting materials for the growth of GaN single crystals are Li3N and Ga (99.999%). Li3N was synthesized by metal Li and N2 gas in a quartz tube at about 3001C. These starting materials in a proper proportion were put in a tungsten crucible and were heated to 8001C in growth furnace which was charged with N2 gas of 2 atm at room temperature, and then was slowly cooled at a rate of 2–31C/d. The growth process lasts 120–180 h. GaN single crystals were obtained and, separated from residual substances by soaking in HCl solution. The as-grown crystals were observed using an optical microscope and the quality of the crystals was checked by X-ray rocking curve.

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2. Experimental procedure

obtained, therefore, the nucleation and growth of GaN by this method were certainly completed in some liquid phase. It has been reported that there is a large solubility of Li3N in liquid Li [5], so it is possible that the displaced Li in above reaction further dissolves Li3N to form Li–N–Ga liquid phase with liquid Ga, in which GaN nucleates and grows. It is found that the pressure of N2 affects the growth of GaN single crystals greatly and the pressure of N2 is at least 1 atm. If the pressure of N2 is lower than 0.5 atm, no GaN can be formed, no matter what proportion of raw materials was used, which can be caused by the rapid decomposition of Li3N under N2 pressure lower than 0.5 atm at 8001C. The proper growth temperature should be 740– 8001C. When the temperature exceeds 8501C, Li3N can corrode W crucible and transform in to Li3WN6, GaN cannot be obtained. Below 7001C, also, no GaN can be produced. It may be because the forming free energy of Li3N is smaller than that of GaN at the temperatures lower than 7001C. At room temperature, the standard forming free energy of Li3N is smaller than that of GaN. In addition, the positions of Li3N in the crucible have a great influence on the growth rate and quality of GaN crystals. If Li3N floats on the surface of liquid Ga, the growth rate of the crystals is low, the crystals crystallize with regular hexagonal outer shape. If Li3N is pressed by W block at bottom of W crucible, the rapid infiltration of Ga into bulk Li3N quickens the

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bulk GaN single crystals with a diameter of 2.5 inch have been grown at about 9001C in an ambient pressure not exceeding 2 atm [4]. Here we reported an alternative method to grow bulk GaN single crystals at low temperature and low pressure.

Intensity

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Fig. 1. The X-ray powder diffraction pattern of GaN.

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Fig. 2. GaN single crystals grown at temperatures 8001C–7501C and N2 pressure of 2 atm. (a, b): GaN single crystals with regular hexagonal outer shape; (c, d): GaN single crystals with irregular shape; (e, f): GaN single crystals with the thickness of B300 mm.

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278 14000

700

12000

600

Intensity [cps]

8000 6000 (004)

Indensity [cps]

10000

4000

500 400 11 arcsec

300 200

2000

100 0

0 20

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60

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Fig. 3. The X-ray diffraction pattern of GaN single crystals.

growth rate, as a result, the quality of the crystals deteriorates. The GaN single crystals prepared by this method were identified by X-ray powder diffraction as hexagonal wurtzite type, no cubic phase was found. All diffraction lines can be indexed (Fig. 1). The lattice constants obtained from ( and c ¼ diffraction data are a ¼ 3:1903ð3Þ A, ( 5:1864ð6Þ A. These GaN crystals are colorless and transparent. However, it has been reported that bulk GaN single crystals prepared by some other methods are slightly yellowish [2], green, and black amber [3], probably due to nitrogen deficiency. Majority of crystals obtained by this method are platelets (Fig. 2), their sizes are up to 1–4 mm and crystal thickness ranges from 20 to 300 mm. The normal axis of the crystal platelets was c-axis oriented as determined by XRD study of the crystal, as shown in Fig. 3. It implies the growth of GaN single crystals performs at low supersaturation. When N concentration in Ga–N–Li liquid phase are high, the growth in the c-direction should be favored. The quality of the crystal was determined from the X-ray rocking curve (Fig. 4). The typical full-width at half-maximum (FWHM) of the X-ray rocking curve for the (1 0 1) reflection is 10–30 arcsec. It indicates that the crystals obtained by this method have good crystallinity.

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21

θ (deg)

2θ (deg.)

Fig. 4. The X-ray rocking curve of GaN single crystals.

4. Conclusions Hexagonal GaN platelet crystals with a size of 1–4 mm have been prepared by a Li-based flux method. The growth conditions to obtain good quality GaN single crystals were elaborated. Determined from X-ray rocking curve and optical microscopy observation of GaN crystals, these crystals have good crystallinity. Li-based flux is a good promise candidate for flux growth of GaN single crystals.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 59972040, 59925206).

References [1] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) 139. [2] S. Porowski, J. Crystal Growth 166 (1996) 583. [3] Hisanon Yamane, Masahiko Shimada, Simon. J. Clarke, F. J. Disalvo, Chem. Mater. 9 (1997) 413. [4] V. Soukhoveev, et al., Phys. Stat. Sol. A 188 (1) (2001) 411. [5] R.M. Yonco, et al., J. Nucl. Mater. 57 (1975) 317.