CRYSTAL GROWTH & DESIGN
Morphology and Polarity of GaN Single Crystals Synthesized by the Na Flux Method Aoki,†
Yamane,*,†
Masato Hisanori Masahiko Seiji Sarayama,§ and Francis J. DiSalvo#
Shimada,†
Takashi
Kajiwara,‡
2002 VOL. 2, NO. 1 55-58
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8577, Japan, Department 5, R & D Center, Research and Development Group, Ricoh Company Ltd., Natori, 981-1241, Japan, and Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA Received August 23, 2001
ABSTRACT: The correlation between morphology and polarity was investigated for GaN single crystals synthesized by the Na flux method. The polarity was identified by using anomalous X-ray dispersion. Colorless transparent prismatic single crystals were classified into three types. The first had a smooth basal plane face and no hexagonal pyramidal faces, the second had smooth pyramidal faces, and the third had a rough basal plane face and rough pyramidal faces. The prismatic crystals with smooth surfaces had N-polarity and grew in the -c direction, whereas the crystals having rough surfaces had Ga-polarity and grew in the +c direction. In colorless transparent platelet single crystals, one side of the basal planes had a mirror smooth surface, while the other side had many step edges and hexagonal pits. The smooth basal plane and the rough basal plane were the (0001 h ) N-face and (0001) Ga-face, respectively. Black pyramidal crystals had a smooth (0001) basal plane face corresponding to the Ga-face. Introduction Recently, GaN has attracted a lot of interest as a material for short wavelength optical devices. GaN crystallizes into the polar hexagonal wurtzite-type structure (space group P63mc). In polar crystals, the two faces perpendicular to the polar axis consist of different atoms, and the surface morphology of each face is different. Since the polar axis of GaN is the c-axis, GaN single crystals have the {0001} basal planes formed by either Ga atoms or N atoms. It is known that the surface morphology of GaN single crystals is different for the two basal plane faces, and one face is flat but the other one is rough.1-3 The polarity determination of GaN has been carried out mainly on thin film samples by convergent beam electron diffraction (CBED),4-6 coaxial impact collision ion scattering spectroscopy (CAICISS),7,8 and Auger electron spectroscopy (AES).9,10 However, few investigations on the polarity of GaN bulk single crystals using X-ray anomalous dispersion have been performed. We have synthesized GaN bulk single crystals by the Na flux method. Although the crystal growth of GaN by the direct reaction of molten Ga and N2 gas requires high temperatures (about 1500 °C) and high N2 pressures (around 1 GPa),11,12 GaN single crystals can grow at relatively low temperatures (650-800 °C) and * Corresponding author. Hisanori Yamane, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, Tel./Fax: 81-22-2175160, E-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]. † Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. ‡ Department of Chemistry, Graduate School of Science, Tohoku University. § Ricoh Company Ltd. # Cornell University.
low N2 pressures (1-5 MPa) using Na as a flux. In our previous studies, GaN crystals were prepared in sealed stainless steel tubes by using NaN3 as nitrogen source.3,13-16 The relation between the crystal morphology and polarity of the GaN single crystals obtained by the sealed tube method has been reported.14 Because the N2 pressure decreased with formation of GaN crystals in the sealed tube, it was difficult to obtain GaN single crystals with large size and good quality. Recently, GaN single crystals were synthesized under a constant N2 pressure by introducing N2 gas from outside the container. The GaN single crystals have larger size (longest dimension of 5 mm) and better quality than those prepared by the sealed tube method.17,18 In this study, we determined the polarity of GaN single crystals grown at a constant N2 pressure by using X-ray anomalous dispersion and investigated the relation between the {0001} surface morphology and the direction of the polar axis. We also present a growth model for the different GaN single crystals on the basis of these results. Experimental Section GaN single crystals were synthesized by heating Na-Ga melts in a BN crucible under a nitrogen atmosphere. Details of the synthesis procedure and the apparatus used were described in the previous paper.17 The morphology of GaN single crystals depends on the growth temperature, N2 pressure, and starting composition of the Na-Ga melt. In the present study, crystal growth experiments were carried out for 150-450 h at the temperatures of 750-800 °C, N2 pressures of 3-5 MPa and Na/(Ga + Na) mole ratios (rNa) of 0.360.60. The crystal habit and surface morphology were observed using a scanning electron microscope (SEM; Hitachi S2150). Single-crystal X-ray diffraction intensities were collected by a diffractometer equipped with a two-dimensional chargecoupled-device (CCD) area detector (Bruker, SMART 1000).
10.1021/cg015548b CCC: $22.00 © 2002 American Chemical Society Published on Web 12/08/2001
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Figure 3. SEM image of a black GaN pyramidal single crystal. Figure 1. SEM images of colorless transparent GaN prismatic single crystals. (a) prismatic crystal with a smooth basal plane face (type A), (b) prismatic crystal with smooth pyramidal faces (type B), and (c) prismatic crystal with a rough basal plane face and rough pyramidal faces (type C).
Figure 2. SEM images of basal planes of a GaN platelet single crystal. (a,c) Smooth surface with lower and higher magnification, respectively; (b,d) rough surface with lower and higher magnification, respectively. Graphite-monochromated MoKR radiation (λ ) 0.71073 Å) and the ω scan mode were used for the intensity measurements. An analytical absorption correction was performed using the program XPREP.19 The dimensions of the crystals for the absorption correction were measured using the SEM. Crystal structure parameters were refined by the program SHELXL97.20
Results and Discussion Colorless transparent prismatic single crystals were synthesized at 750 °C, 5 MPa of N2, and rNa ) 0.54 for 450 h. All the Ga source reacted with nitrogen to form GaN crystals under this experimental condition. The colorless prismatic crystals are classified into three types based on their crystal forms. Scanning electron micrographs of the typical crystals are shown in Figure 1. The colorless prismatic crystals of type A are columnar with a smooth basal plane face and no hexagonal pyramidal faces. Type B crystals have smooth pyramidal faces. Skeletal-crystal like facets are observed on the basal plane and pyramidal facets of type C crystals. Colorless transparent platelet GaN single crystals were obtained at 750 °C, 5 MPa of N2, and rNa ) 0.60 for 360 h. No unreacted Ga remained after this experiment. Figure 2 shows SEM images of a platelet single crystal. One side of the basal plane faces is smooth
(Figure 2a,c), but the other side has many step edges and hexagonal pits (Figure 2b,d). Black pyramidal single crystals grew into the melt from the crucible wall at 800 °C, 3 MPa of N2, and rNa ) 0.36 for 150 h. A large amount of unreacted Ga precipitated as a Na-Ga intermetallic compound in the crucible on cooling. As shown in Figure 3, the black crystals have a smooth basal plane face and smooth pyramidal faces. As discussed in the previous papers,13,15 the black color may be related to nitrogen deficiency in the crystals. Table 1 lists the measurement conditions for the single-crystal X-ray diffraction experiments and the results of structure analysis. The lattice parameters of the crystals agree with those reported for bulk GaN single crystals prepared by the high-pressure solution method (a ) 3.189 Å, c ) 5.186 Å)21 within the standard deviations. After the analytical X-ray absorption correction, values of R1 indices were within 3.1%. The values of the extinction coefficient are larger than those refined for general inorganic single crystals. This may be attributed to a high degree of crystal lattice perfection. The smaller extinction coefficient values for the black pyramidal crystal and colorless type C prismatic crystal probably indicate lower lattice perfection. Since the absolute structure parameters are close to zero within the range of three estimated standard deviations, the absolute structure was clearly determined for all the samples.20,22 The correlations between the crystal form and polarity are illustrated schematically in Figure 4. Following the previous studies on the other wurtzite-type compounds,23-25 the two polar surfaces formed by metallic atoms (Ga) and nonmetallic atoms (N) are represented by (0001) and (0001 h ), respectively. The colorless transparent prismatic crystals of type A and type B with smooth surfaces grew from the crucible wall in the direction of -c and have N-polarity. On the contrary, type C crystals having rough surfaces grew in the +c direction and have Ga-polarity. These three types of colorless prismatic crystals with the growth directions of -c and +c were found in the same crucible. This means that there is no regulation of polarity in the nucleation and growth of prismatic GaN crystals in the crucible. In the colorless transparent platelet crystals, the smooth basal plane face is the (0001h ) N-face and the other side with many step edges and hexagonal pits is the (0001) Ga-face. The smooth basal face of the black pyramidal crystals corresponds to the (0001) Ga-face. These crystals have the growth direction of +c and Ga-polarity.
Morphology and Polarity of GaN Single Crystals
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Table 1. Crystal Data and Structure Refinements for GaN Single Crystals colorless prismatic type A 0.06 × 0.06 × 0.15 7.39 to 29.77 h -4 to 4 k -4 to 4 l -7 to 7 reflection numbers 483 unit cell dimensions a [Å] 3.190(3) c [Å] 5.187(4) volume [Å3] 45.7(1) absorption coefficient [mm-1] 28.963 data/restraints/parameters 67/1/8 goodness-of-fit on F2 1.349 R indices (all data)a R1 0.0180 wR2 0.0387 absolute structure parameter -0.25(12) extinction coefficient 5.3(5) crystal size [mm3] θ range for data collection index ranges
type B
type C
colorless platelet
black pyramidal
0.15 × 0.15 × 0.50 7.38 to 28.96 -4 to 4 -4 to 4 -7 to 5 336 3.192(3) 5.187(4) 45.8(1) 28.810 54/1/8 1.294 0.0310 0.0728 0.0(3) 3.1(7)
0.20 × 0.20 × 0.20 7.40 to 29.80 -4 to 4 -4 to 4 -7 to 7 482 3.188(3) 5.183(4) 45.6(1) 29.028 67/1/7 1.285 0.0209 0.0480 -0.04(14) 1.4(2)
0.08 × 0.05 × 0.02 7.39 to 29.78 -4 to 4 -4 to 4 -7 to 7 479 3.189(3) 5.185(4) 45.7(1) 28.990 69/1/8 1.220 0.0225 0.0457 0.06(14) 5.2(6)
0.15 × 0.15 × 0.10 7.39 to 29.75 -4 to 4 -4 to 4 -7 to 7 488 3.190(3) 5.190(4) 45.7(1) 28.950 68/1/8 1.002 0.0188 0.0370 0.00(13) 0.59(7)
a R1 ) ∑||Fo| - |Fc||/∑ |Fo|, wR2 ) [Σ w(Fo2 - Fc2)2/∑ (wFo2)2]1/2, w ) 1/[σ2(Fo2) + (0.0216P)2 + 0.0000P] for type A colorless prism, w ) 1/[σ2 (Fo2) + (0.0473P)2 + 0.0771P] for type B colorless prism, w ) 1/[σ2(Fo2) + (0.0306P)2 + 0.0506P] for type C colorless prism, w ) 1/[σ2(Fo2) + (0.0241P)2 + 0.0000P] for plate and w ) 1/[σ2(Fo2) + (0.0000P)2 + 0.3859P] for black pyramid, where P ) [max (Fo2, 0) + 2Fc2]/3.
Figure 5. Wurtzite-type structure of GaN single crystals with the growth directions toward c (a) and -c (b). Figure 4. Schematic descriptions of GaN crystal morphology and polarity.
The rough basal plane faces of both the colorless prismatic and colorless platelet single crystals correspond to the Ga-face. In our crystal growth experiments, the Ga concentration in the Na-Ga melt decreases with the formation of GaN crystals, but nitrogen is supplied from gas phase into the melt under the constant pressure. Therefore, the growth environment becomes N-rich and Ga-poor at the latter stage of the experiment. We presume that the growth environment effects on the surface morphology. As shown in Figure 5, each Ga and N atom in the wurtzite-type GaN crystal structure is tetrahedrally coordinated by each other. In view of the growth direction, the difference of polarity gives two types of tetrahedral Ga sites: a T+ site of Ga-polarity and a Tsite of N-polarity (Figure 5). The crystal growth of GaN along the c-axis proceeds by stacking Ga and N atoms alternately. From the view of growth direction, each T+
site is a hollow position between three of the N atoms, and each T- site is directly atop the N atom. In the case of placing a Ga layer over a N layer, it is obvious that T+ sites are favored in contrast to T- sites. Although the actual atomic arrangement at the crystal surface or growth front is not so simple, the atomic configuration is related to the difference of growth mechanism between the crystals of Ga-polarity and N-polarity. In the N-rich environment at the late stage of the synthesis, N atoms may cover all the crystal faces quickly. The growth surface of N-polarity crystals was terminated with the smooth N-face. Figure 6 illustrates a growth model of a GaN single crystal of Ga-polarity at a low Ga concentration in a Na-Ga melt. The Ga atoms sit at the T+ sites, but before all the basal plane surface was covered with Ga atoms, N atoms could bond to the Ga atoms and form new T+ sites in the next upper layer. Thus, many step edges and pits are introduced at the (0001) crystal plane, and sometimes defects and voids with Na-melt inclusion are included in the crystals. The low crystal
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pyramidal crystals having a smooth basal plane face of the Ga-face grew under the Ga-rich environment. Acknowledgment. This work was supported in part by the NEDO International Joint Research Program and by a grant from the Ministry of Education, Culture, Sports, Science and Technology. References
Figure 6. Growth model of a GaN single crystal with Gapolarity at low Ga concentration in a Na-Ga melt.
quality of the type C colorless prismatic crystals with the Ga-polarity was suggested by the small value of extinction coefficient (Table 1). The black pyramidal crystals have the smooth basal plane face corresponding to the Ga-face. Since a large amount of Ga remained in the crucible and the black color of the crystals suggested nitrogen deficiency, the black pyramidal crystals grew in the Ga-rich environment. This could be explained with the above model by replacing the positions of Ga and N each other. Summary The polarity of GaN single crystals synthesized by the Na flux method was determined using X-ray anomalous dispersion. The polarity of the smooth basal plane face depends on the growth environment. The colorless prismatic and platelet crystals prepared at the N-rich conditions have smooth surfaces of the N-face and rough surface of the Ga-face for the basal plane. The black
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CG015548B
