GaN crystals prepared through solid-state metathesis reaction from NaGaO2 and BN under high pressure and high temperature

Article abstract of DOI:10.1016/j.jallcom.2010.12.087

GaN crystals are successfully obtained through solid-state metathesis (SSM) reaction between sodium gallium oxide (NaGaO₂) and boron nitride (BN) under high pressure and high temperature. X-ray diffraction (XRD) pattern indicates that the attained GaN crystals possess a hexagonal wurtzite-type structure. Scanning electron microscopy (SEM) is used to estimate the size and morphology of GaN crystals, and results show that GaN grains with the size over 100 μm can be prepared at 5 GPa and 1600 °C. Moreover, pressure-temperature (P-T) formation region of GaN has been discussed. Our results suggest a promising novel route for synthesizing GaN crystals from SSM reactions under high pressure.

Full text of DOI:10.1016/j.jallcom.2010.12.087

Journal of Alloys and Compounds 509 (2011) L124–L127
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
GaN crystals prepared through solid-state metathesis reaction from NaGaO2 and
BN under high pressure and high temperature
Huan Maa, Duanwei Hea,∗, Li Leib, Shanmin Wanga, Ying Chena, Haikuo Wanga
a Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China
b Geodynamics Research Center, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2010
Received in revised form 6 December 2010
Accepted 14 December 2010
Available online 22 December 2010
GaN crystals are successfully obtained through solid-state metathesis (SSM) reaction between sodium
gallium oxide (NaGaO2) and boron nitride (BN) under high pressure and high temperature. X-ray diffrac-
tion (XRD) pattern indicates that the attained GaN crystals possess a hexagonal wurtzite-type structure.
Scanning electron microscopy (SEM) is used to estimate the size and morphology of GaN crystals, and
results show that GaN grains with the size over 100 ␮m can be prepared at 5 GPa and 1600 ◦C. Moreover,
pressure–temperature (P–T) formation region of GaN has been discussed. Our results suggest a promising
novel route for synthesizing GaN crystals from SSM reactions under high pressure.
Keywords:
Gallium nitride
Solid-state metathesis reaction
High pressure and high temperature
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
as impurities in the products cannot be easily removed. In the
present work, NaGaO2 and BN as the initial precursors for SSM
GaN has attracted considerable attention as a semiconductor
material for fabrication of optoelectronic devices, such as blue or
green light emitting diodes (LEDs) and violet laser diodes (LDs)
due to its wide band gap, high thermal stability, and high break-
down voltage [1–4]. Up to now, the growth of GaN crystals has been
extensively investigated by several research groups using different
preparation approaches mainly including flux growth [5,6], hydride
vapor-phase epitaxy [7,8], pressure-controlled solution growth
[9], high-pressure ammonothermal growth [10], mechanochem-
ical reaction method [11] and so on. Because of the importance
of application of GaN on optoelectronic industry, the pursuit of
the optimal synthetic route to attain well-crystallized GaN crys-
tals has not stopped so far. We are attempting to synthesize high
quality crystals of GaN by using effective methods involving high
pressure. Recently, a high-pressure solid-state metathesis (SSM)
method [12,13] has been developed to synthesize GaN crystals
from LiGaO2 and BN [14]. In spite of its remarkable advantages,
such as high growth rate, simple assembly and substrate free,
there are still some inevitable disadvantages in this synthetic route.
The precursor LiGaO2 is usually unstable and may decompose
into lithium-poor phase LiGa5O8 under high temperature [14–16].
The unreacted LiGaO2 and the decomposition product LiGa5O8
reaction are used to synthesize GaN crystals under high pressure
and high temperature (HPHT). Compared with LiGaO2, NaGaO2
will be a better choice to maintain the previous merits and over-
come the aforementioned difficulties. It is because that NaGaO2
does not decompose at our HPHT conditions and easily dissolves
in water, implying that the purification process of GaN crystals
will become simple. By optimizing the P–T conditions, the well-
crystallized GaN crystals with the size over 100 ␮m are prepared
at 5 GPa and 1600 ◦C. Moreover, forming regions for GaN crystals
are also discussed. Our results not only provide a new method for
synthesizing GaN crystals from the SSM reaction of NaGaO2 with
BN under HPHT but also give us a better understanding of SSM
reactions under high pressure. The method with respect to high-
pressure SSM reaction can also be extended to synthesize other
materials.
2. Experimental procedure
The starting materials for synthesizing GaN crystals are NaGaO2 (synthesized
from Ga2O3 and Na2CO3 at 1100 ◦C for 12 h under ambient pressure, ∼99.99% purity)
and BN (hexagonal BN or cubic BN, ∼99% purity) powders. Before high-pressure
experiments, NaGaO2 and BN powders were compacted into a disk shape of 8.0 mm
in diameter and 6.0 mm in height, respectively. And the disk was placed in a capsule
made of hexagonal BN (hBN) or Mo with a sandwich-like configuration, and then
contained in a pyrophyllite high-pressure cell [14]. This cell assembly was put into
a high pressure furnace chamber [17]. HPHT experiments were performed on a
DS6×8 MN cubic press [17,18]. The experimental details were described elsewhere
[14,19,20]. The samples were first pressurized to a certain pressure, and then heated
to a desired temperature for 30 min. Before decompression, the samples were
∗
Corresponding author. Fax: +86 28 85466676.
E-mail address: duanweihe@scu.edu.cn (D. He).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.12.087

H. Ma et al. / Journal of Alloys and Compounds 509 (2011) L124–L127
L125
˚
˚
on a hexagonal cell with a = 3.189 A and c = 5.186 A (ICCD-PDF NO.:
50-0792), implying the attained yellowish crystals are GaN crystals.
Note that these GaN crystals exhibit a highly (0 0 2) preferred orien-
tation, indicating the basal plane of crystals as c-plane. Besides the
phase GaN, the other phase hBN is also found under the resolution
of the X-ray diffractometer. XRD pattern contains only two phases
(GaN and hBN) because the unreacted NaGaO2 powder and the
byproduct sodium metaborate (NaBO2) were removed in washing
process. Obviously, gallium and nitrogen of GaN come from NaGaO2
and hBN, respectively. This also suggests that GaN is the product of
the exchange reaction between NaGaO2 and hBN. Therefore, the
reaction should be a high-pressure SSM reaction and explained as
the equation NaGaO2 + BN = GaN + NaBO2, which is similar to the
reaction of LiGaO2 with BN under HPHT [14].
SEM was used to investigate the size and morphology of GaN
crystals. Prominent features observed are presence of well defined
grains shown in Fig. 2(a)–(d). To get initial appearance of GaN crys-
tals, fracture surface of the samples was observed directly by SEM
without taking any further treatment. The fracture surface images
of the sample formed from SSM reaction of NaGaO2 and hBN are
shown in Fig. 2(a) and (b). There mainly exist two strikingly dif-
ferent regions of the images which reflect morphologies of GaN
and NaGaO2. And the corresponding regions are marked as GaN or
NaGaO2 (confirmed by EDXA). The well-crystallized GaN crystals
have a wurtzite-type structure, which is in good agreement with
XRD measurement. Combined with XRD in Fig. 1, it indicates that
the c axis is the preferential growth direction of GaN crystals under
high pressure. As shown in Fig. 2(a), the biggest crystal dimension
along the c axis direction can be over 100 ␮m, while the dimension
along other crystallographic orientation is only ∼50 ␮m. Fig. 2(b)
shows a closed image of the GaN crystals prepared at 5.0 GPa and
1600 ◦C for 30 min. Moreover, EDXA reveals that the main element
components of the crystals are Ga and N, which further indicate that
the obtained crystals are GaN. GaN crystals were also attained when
cubic boron nitride (cBN) was used as the nitrogen precursor for
Fig. 1. XRD pattern of the sample formed from SSM reaction between NaGaO2 and
hBN at 5.0 GPa and 1600 ◦C for 30 min.
quenched to room temperature with the cooling rate about 2 ◦C/s. The recovered
samples were cleaned in water and alcohol with an ultrasonic washer before
examination by X-ray diffraction (XRD), scanning electron microscopy (SEM), and
energy-dispersive X-ray analysis (EDXA).
3. Results and discussion
At first, we tried to use NaGaO2 and hBN as the starting materi-
als to synthesize GaN crystals under HPHT. In this reaction, NaGaO2
and hBN were as the precursors to provide gallium and nitro-
gen resources, respectively. When the reaction between the two
compounds was occurred under HPHT, the massive yellowish crys-
tals were found to grow on the interface of two precursors after
sandwich-like precursor layers were separated and washed by the
pure water and alcohol. Fig. 1 is the XRD pattern of the sample
growth surface. The major diffraction peaks are well indexed based
Fig. 2. SEM images of the samples prepared at 5.0 GPa and 1600 ◦C for 30 min. (a) and (b), the fracture surface of the sample prepared from SSM reaction of NaGaO2 and hBN.
(c) and (d), GaN crystals formed by the SSM reaction between NaGaO2 and cBN in the cross-section.

L126
H. Ma et al. / Journal of Alloys and Compounds 509 (2011) L124–L127
High pressure served as an effective way can suppress GaN
decomposition up to much higher temperature [21], which means
that GaN crystal can grow up under high temperature and does
not decompose when high pressure is applied. Previous studies
show that the mobility of all species in the reaction is increased
and subsequent reaction can rapidly occur if the temperature at this
point is high enough to melt the byproduct salt [13], which has also
proved in our study. Note that the melting point of the byproduct
NaBO2 is only 966 ◦C at ambient pressure [25], when the tempera-
ture is above the melting point, the mobility of NaGaO2 with BN is
enhanced and the SSM reaction of the two precursors under HPHT
can unceasingly progress. Therefore, we suggest that GaN crystals
are originally in the GaN–NaBO2 solution at HPHT and precipitate
from the melt during quenching [26]. NaBO2 may also be a new
solution of GaN for large single crystal growth. However, further
investigation is still needed to optimize the synthetic conditions of
GaN crystals from SSM reaction at HPHT.
Fig. 3. Forming region for GaN crystals by the SSM reaction of NaGaO2 with BN under
high pressures and temperatures. Inverse triangle indicates the experimental con-
ditions at which GaN can be detected by XRD. Rhombus represents the pressure and
temperature (P–T) conditions at which GaN crystal fails to form. The dashed line is
the boundary line of synthesizing GaN crystal, and the solid line is the decomposition
line for GaN from Ref. [21].
4. Conclusion
In summary, GaN crystals are synthesized through SSM reaction
of NaGaO2 and BN at HPHT. hBN and cBN can all be as nitro-
gen source to synthesize GaN crystals and the bigger crystals over
100 ␮m in size are attained at 5 GPa and 1600 ◦C by using hBN. The
formation region of GaN is also described as the conditions of high
pressure (1.0–5.0 GPa) and high temperature (1000–1600 ◦C). We
suggest that GaN crystals are originally in the GaN–NaBO2 solution
and precipitate from the melt during quenching. NaBO2 may also
be a new solution of GaN for large single crystal growth. The results
indicate a potential new route not only for preparing GaN crystals
through SSM reaction under HPHT but also for other materials.
metathesis reaction under the same synthetic conditions. In order
to get rid of unreacted NaGaO2, the sample was washed by pure dis-
tilled water and alcohol. As shown in Fig. 2(c), these GaN crystals are
different from those prepared from hBN precursor in morphology,
and also possess a wurtzite-type structure and exhibit well-defined
grains with the size range from several micron to 30 ␮m. Fig. 2(d)
presents a closer SEM picture of the GaN crystals synthesized from
NaGaO2 and cBN at 5.0 GPa and 1600 ◦C. It can be readily seen that
these crystals have clearly visible grain boundaries. Compared with
hBN and cBN situations described above, it suggests that GaN crys-
tal growing rate may be related to the polytypism of BN under
high pressure. It can be seen that hBN as nitrogen resource is more
favorable for GaN crystal grain growth.
A series of experiments are conducted to investigate the pos-
sibility for the synthesis of GaN crystals through SSM reaction of
NaGaO2 with BN under different P–T conditions. Fig. 3 summarizes
the P–T regions for GaN formation. Inverse triangle indicates the
P–T conditions at which GaN crystals can be detected by XRD, and
the rhombus represents that GaN crystals fail to form. GaN crystals
can be synthesized at the P–T conditions above the dashed line (as
shown in Fig. 3). The solid line is the decomposition line of GaN.
At high temperature and low pressure, above the solid line, GaN
will decompose into Ga and N2 [21]. According to our experimen-
tal observation, under the pressure below 1.0 GPa and temperature
above 1100 ◦C, the reaction cannot propagate and a mass of gallium
metal will appear in the sample chamber, which may be caused
by the decomposition of GaN [14]. As can be seen from Fig. 3, the
synthesis temperature of GaN decreases as the pressure increases
from 1.0 GPa to 2.0 GPa. We speculate that the pressure increases
the contact between the two precursors, which speeds up the local
reaction. So the synthesis temperature decreases under relatively
low pressures (1.0–2.0 GPa) in our experiments. This has also been
observed for some other materials, such as WB synthesis under
HPHT [22]. The synthesis temperature of GaN increases when the
pressure is elevated from 2.0 GPa to 5.0 GPa. In this case, a higher
pressure may suppress the nucleation and growth of GaN crys-
tals. However, high temperature usually prompts grain growth.
Therefore, the dashed line at low pressure (<2 GPa) has a negative
slope and the dashed line at high pressure (>2 GPa) has a positive
slope. And the synthetic dashed line near the solid decomposition
line may have some discrepancy with the normal synthesis model.
Similar phenomena have also been observed by other researches
[14,22–24].
Acknowledgements
This work is supported by National Natural Science Foundation
of China (Grant Nos. 10772126 & 11027405), National Natural Sci-
ence Foundation of China – NSAF (Grant No. 10976018), and the
China 973 Program (Grant No. 2011CB808200).
References
[1] S. Nakamura, Science 281 (1998) 956–961.
[2] F.A. Ponce, D.P. Bour, Nature 386 (1997) 351–359.
[3] T. Miyajima, T. Tojyo, T. Asano, K. Yanashima, S. Kijima, T. Hino, M. Takeya,
S. Uchida, S. Tomiya, K. Funato, T. Asatsuma, T. Kobayashi, M. Ikeda, J. Phys.
Condens. Matter. 13 (2001) 7099–7114.
[4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H.
Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74–L76.
[5] X.L. Chen, Sci. Technol. Adv. Mater. 6 (2005) 766–771.
[6] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, F.J. DiSalvo, J. Cryst. Growth 242
(2002) 70–76.
[7] S. Hontsu, N. Mukai, J. Ishii, T. Kawai, S. Kawai, Appl. Phys. Lett. 61 (1992)
2709–2711.
[8] H. Morkoc, Mater. Sci. Eng. R 33 (2001) 135–207.
[9] T. Inoue, Y. Seki, O. Oda, S. Kurai, Y. Yamada, T. Taguchi, J. Cryst. Growth 229
(2001) 35–40.
[10] M.P. D’Evelyn, H.C. Hong, D.-S. Park, H. Lu, E. Kaminsky, R.R. Melkote, P. Perlin,
M. Lesczynski, S. Porowski, R.J. Molnar, J. Cryst. Growth 300 (2007) 11–16.
[11] J. Kano, E. Kobayashi, W. Tongamp, F. Saito, J. Alloys Compd. 464 (2008)
337–339.
[12] A.M. Nartowski, I.P. Parkin, M. Mackenzie, A.J. Craven, J. Mater. Chem. 11 (2001)
3116–3119.
[13] E.G. Gillan, R.B. Kaner, Chem. Mater. 8 (1996) 333–343.
[14] L. Lei, D. He, Cryst. Growth Des. 9 (2009) 1264–1266.
[15] M. Marezio, J.P. Remeika, J. Phys. Chem. Solid 26 (1965) 1277–1280.
[16] C.J. Rawn, J. Chaudhuri, J. Cryst. Growth 225 (2001) 214–220.
[17] S.M. Wang, D.W. He, W.D. Wang, L. Lei, High Pressure Res. 29 (2009) 806–814.
[18] C.M. Sung, High Temp.–High Press. 29 (1997) 253–293.
[19] D.W. He, M. Akaishi, T. Tanaka, Diamond Relat. Mater. 10 (2001) 1465–1469.
[20] J. Qin, D. He, L. Lei, P. An, L. Fang, Y. Li, F. Wang, Z. Kou, J. Alloys Compd. 476
(2009) L8–L10.
[21] W. Utsumi, H. Saitoh, H. Kaneko, T. Watanuki, K. Aoki, O. Shimomura, Nat. Mater.
2 (2003) 735–738.

H. Ma et al. / Journal of Alloys and Compounds 509 (2011) L124–L127
L127
[22] Y. Chen, D. He, J. Qin, Z. Kou, S. Wang, J. Wang, J. Mater. Res. 25 (2010)
[24] H. Xiao, R. Liu, H. Ma, Z. Lin, J. Ma, F. Zong, L. Mei, J. Alloys Compd. 465 (2008)
340–343.
637.
[23] H. Xiao, H. Ma, Z. Lin, J. Ma, F. Zong, X. Zhang, Mater. Chem. Phys. 106 (2007)
5–7.
[25] H.H. Nersisyan, J.H. Lee, C.W. Won, Mater. Chem. Phys. 89 (2005) 283–288.
[26] D. He, M. Akaishi, B.L. Scott, Y. Zhao, J. Mater. Res. 17 (2002) 284–290.