ARTICLE pubs.acs.org/crystal
Control of Growth Facets and Dislocation Propagation Behavior in the Na-Flux Growth of GaN Mamoru Imade,* Yasuhiro Hirabayashi, Naoya Miyoshi, Masashi Yoshimura, Yasuo Kitaoka, Takatomo Sasaki, and Yusuke Mori Division of Electric, Electronic and Information Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT: We investigated changes in the growth mode and dislocation propagation behavior of Na-flux-grown GaN on GaN (0001) templates with changing flux composition (Ga/Na). Results suggested that deliberate control of the flux composition enables fast growth of thick GaN with a low dislocation density. The growth mode was evaluated by scanning electron microscopy (SEM). The distribution and density of dislocations were investigated using chemical etching. SEM observation revealed that the growth mode changed from a two-dimensional (2D) mode developing (0001) facets to a three-dimensional (3D) mode developing (0001) and {1011} facets as the Ga composition in the flux was increased. A reduction of dislocation density was promoted in the 3D growth mode favored at high Ga compositions, and the minimum dislocation density observed was on the order of 103 cm2 (280 μm 200 μm) after growth for 96 h at a Ga composition of 40 mol %. On the other hand, the growth rate was higher in the 2D growth mode favored at low Ga compositions. Based on the observed changes in the growth mode, a growth sequence that is effective for the growth of thick GaN substrates with a low dislocation density is proposed.
’ INTRODUCTION GaN-based electronic devices would benefit from the availability of a native substrate. However, GaN substrates tend to have a high density of dislocations, particularly edge and screw dislocations, preventing the realization of high-efficiency, reliable electronic devices.1 Therefore, true GaN substrates with a low defect density are desired. The growth of GaN bulk single crystals has been attempted by various methods, such as hydride vapor phase epitaxy (HVPE),24 high-pressure solution growth (HPSG),5 ammonothermal growth,68 and Na-flux growth.915 HVPE has the advantage of a high growth rate of over 100 μm/h, and GaN substrates produced by this method are now commercially available. In HVPE, epitaxial lateral overgrowth (ELO), based on selective growth through an SiO2 mask/window striped pattern formed on a pregrown GaN layer, is a promising technique, capable of producing GaN substrates with a very low threading dislocation density.2,3 However, the increased number of process steps in this method increases the fabrication cost of the resulting GaN substrates. The Na-flux method is a promising candidate for the mass production of high-quality GaN substrates. A decrease in dislocation density from 108 cm2 in a seed to 104 cm2 in the liquidphase epitaxy (LPE) layers and a growth rate over 20 μm/h were attained with 2-in. GaN LPE.11 Two dislocation reduction mechanisms occurred during the Na-flux LPE growth, but no artificial process, such as the ELO technique, was applied. Our previous paper revealed that the natural change in growth mode during LPE growth altered the propagation behavior of dislocations, resulting in two different mechanisms of dislocation reduction.12 However, the reason for the change in growth mode r 2011 American Chemical Society
during LPE growth was not clear, and the propagation behavior of dislocations could not be controlled. Intentional control of the growth mode would enable a further reduction in dislocation density during Na-flux LPE growth and would enable the production of GaN wafers with a very low dislocation density. In this paper, we discuss the change in growth mode in relation to the flux composition [Ga]/([Ga] þ [Na]) for growth on a c-GaN template, and we present a favorable growth mode for reducing the dislocation density and increasing the growth rate of c-GaN LPE layers. The growth mode changed from a twodimensional (2D) mode to a three-dimensional (3D) mode with increasing Ga composition in the flux. The dislocation density of an LPE layer grown via the 3D growth mode was lower than that of layers grown via the 2D growth mode, while the growth rate was found to be higher for lower Ga compositions. Based on the change in the growth mode, a growth sequence that is effective for the growth of thick GaN substrates with a low dislocation density is proposed.
’ EXPERIMENTAL SECTION GaN (0001) films (10 μm thick) grown on sapphire (0001) by HVPE (dislocation density: ∼109 cm2) with dimensions of 13 mm 18 mm were used as seed substrates. A seed substrate was placed in an alumina (Al2O3) crucible 16 mm in diameter and 50 mm in height, and the starting materials of metallic Ga (purity, 6 N; 0.040.07 mol), metallic Na (purity, 4 N), and graphite grains (purity, 6 N) were added to the Received: January 12, 2011 Revised: April 25, 2011 Published: May 02, 2011 2346
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Figure 2. Growth rate (solid circle) and fwhm's of X-ray rocking curves of GaN (0002) (open circles) and (1010) (open triangles) as a function of flux composition. the crystallinity of the LPE layers was evaluated from the full width at half-maximum (fwhm) of the X-ray rocking curves of GaN (0002) (Rigaku RINT2500). Dislocations were decorated with chemical etching, because it was difficult to find dislocations at a density lower than the order of 106 cm2 by cross-sectional transmission electron microscopy (TEM) imaging. The dislocation density was estimated from the etch pit density (EPD), determined after etching in a mixture of H2SO4 and H3PO4 solutions (H2SO4/H3PO4 = 1:3) at 250 C for 90 min. The EPD was measured by laser scanning microscopy (LSM) (KEYENCE VK-9700; wavelength of incident laser, 408 nm; maximum output power of the laser, 0.9 mW).
Figure 1. Bird’s-eye SEM images of LPE layers grown for 50 h with a Ga composition in the flux of (a) 18 mol %, (b) 27 mol %, and (c) 40 mol %. crucible in an Ar-filled globe box. The ratio of carbon content to the total Ga/Na amount was fixed at 0.5 mol %. Graphite grains were added in order to prevent the growth of polycrystals on the crucible wall. Details on the effect of C addition were presented in our previous paper.11 The starting compositions of Ga/Na/C were 18:82:0.5, 27:73:0.5, and 40:60:0.5. The detailed growth equipment was described in ref 11. After the crucible was transferred into a stainless-steel tube 25.4 mm in diameter and 126 mm in height, the tube was evacuated and connected to an N2 gas line. N2 gas was introduced into the tube, and the temperature of the tube was increased to 860 C over a 1-h period using a resistive heater. The temperature and N2 pressure in the tube were maintained at 860 C and 4.0 MPa, respectively, for either 50 or 96 h. After the tube cooled naturally, the crucible was removed from the stainless-steel tube and immersed in cold ethanol and water to dissolve the residual flux. The growth rate was calculated from the weight change of the substrate. The growth morphology of the LPE layers was observed by scanning electron microscopy (SEM) (JEOL2100 FE-SEM). After the as-grown LPE surface was polished using a diamond-based abrasive,
’ RESULTS AND DISCUSSION The growth mode changed when the flux composition was altered. Parts a, b, and c of Figure 1 show bird’s-eye SEM images of LPE layers grown for 50 h at Ga compositions of 18, 27, and 40 mol %, respectively. When the Ga composition was 18 mol %, the LPE layer grew via a 2D growth mode, developing (0001) facets. On the other hand, pyramidal structures with inclined facets and (0001) facets formed at higher Ga compositions in the flux. At a Ga composition of 40 mol %, the inclined facets developed predominantly, indicating a 3D growth mode. The angle between the inclined facets and the (0001) facets was approximately 62, which is nearly equal to the angle between {1011} and (0001) of GaN. Therefore, the growth mode changed from a 2D mode to a 3D mode with increasing Ga composition in the flux. The growth rate in the c-axis direction and the fwhm of the GaN (0002) and (1010) rocking curves as a function of flux composition are shown in Figure 2. The average LPE layer thickness, which was calculated from the weight change of the substrate and the LPE area, increased with decreasing Ga composition in the flux, reaching 650 μm (13 μm/h for 50-h growth) at a Ga composition of 18 mol %. The fwhm values of the GaN(0002) and GaN(1010) reflections were 260 arcsec and 151 arcsec, respectively, for a Ga composition of 18 mol %, and they decreased with increasing Ga composition in the flux, reaching 95 arcsec (GaN(0002) reflection) and 83 arcsec (GaN(1010) reflection) for a Ga composition of 40 mol %. LSM images of the LPE surface after chemical etching are shown in Figure 3. Two types of hexagonal etch pits of different sizes were observed when the surface of LPE GaN was etched 2347
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Figure 3. LSM images of LPE layers grown for 50 h at Ga compositions of 18 mol % (a and d), 27 mol % (b, e, and g), and 40 mol % (c, f, and h) after chemical etching in a mixture of H2SO4 and H3PO4 solutions (H2SO4/H3PO4 = 1:3) at 250 C for 90 min. The left column shows low-magnification images (a, b, and c), and the middle and right columns show high-magnification images of the high EPD region (d, e, and f) and low EPD region (g and h), respectively. Black arrows in panels b and c indicate etch pit arrays.
with a mixture of H2SO4 and H2PO4 solutions (H2SO4/H3PO4 = 1:3) at 250 C for 90 min. According to Zhang et al., large etch pits are formed on screw and mixed dislocations, while small etch pits are formed on edge dislocations.16 In this study, both types of dislocations were observed in all samples. The etch pits of an LPE layer grown with 18 mol % Ga (Figure 3a and d) were homogeneously distributed over the LPE layer, and the EPD was 6.2 105 to 3.0 106 cm2. On the other hand, LPE layers grown with 27 mol % Ga (Figure 3b) and 40 mol % Ga (Figure 3c) Ga had regions with a high EPD (as shown in Figure 3e,f) and a low EPD (as shown in Figure 3g,h), which were separated by etch pit arrays (as indicated by black arrows in Figure 3b,c). The LPE layer grown with 40 mol % Ga had a low EPD region with a larger area than that of the layer grown with 27 mol % Ga. Figure 4 shows the EPD of LPE layers grown for 50 and 96 h, respectively, as a function of the flux composition. The open circles and solid squares show the EPD of a randomly selected region (more than five points) and an average EPD value determined by counting the etch pits in a 1.0 mm 1.0 mm area, including both high and low EPD regions, respectively. The EPD of LPE layers grown at high Ga compositions (27 and 40 mol %) could be clearly separated into high and low values, which corresponded to the EPD values in the high and low EPD regions shown in Figure 3. The ratio of the low EPD region area to the 1.0 mm 1.0 mm area was zero at 18 mol % Ga, and it increased with increasing Ga composition, reaching 0.33 at 27 mol % Ga and as high as 0.79 at 40 mol % Ga. Figure 4 shows that the EPD decreased with increasing Ga composition and growth period, reaching minimum
values on the order of 103 cm2 in low EPD regions (280 μm 200 μm) after 96-h growth at a Ga composition of 40 mol %. The relationship between Ga composition and growth mode will be discussed here. The growth facet dependence on flux composition agreed well with results published by Yamane9 and with our previous report.13 In these reports, platelike GaN with (0001) facets and prismatic GaN with (0001), {1010}, and {1011} facets were formed at low and high Ga compositions, respectively, on a crucible and/or a pyramidal seed. These tendencies are caused by differences in the ratio of the growth rate along the Æ1011æ direction compared to those along the [0001] and Æ1010æ directions. In the case of growth on a GaN (0001) template, the platelike and prismatic habits correspond to the 2D growth mode developing (0001) facets and the 3D growth mode developing (0001) and {1011} facets, respectively. The change in the growth mode and growth rate as a function of Ga composition is likely to be caused by a change in nitrogen solubility in the flux and the degree of supersaturation, as reported by Kawahara et al.14 Figure 2 suggests that the degree of supersaturation decreased with increasing Ga composition because the growth rate strongly depends upon the degree of supersaturation. Aoki et al. reported that pyramidal GaN single crystals developing (1011) facets were grown at a low N2 pressure and high Ga composition.15 These results indicate that a low supersaturation favors the growth of pyramidal crystals, developing (1011) facets in the Na flux method. Although the details of this mechanism remain unclear, a low Ga composition does offer the advantage of a high growth rate. 2348
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Figure 4. EPDs of LPE layers grown for (a) 50 h and (b) 96 h as a function of flux composition. Open circles and solid squares indicate EPD values in a randomly selected region (more than five points) and an average EPD value determined by counting the etch pits in a 1.0 mm 1.0 mm area including both high and low EPD regions, respectively.
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The distribution of EPD was determined by the propagation behavior of dislocations. It is apparent from Figures 1 and 3 that the dislocation propagation behavior changed along with the growth mode of the LPE layers. The distribution of dislocations in the LPE grown with 27 mol % Ga (Figure 3b), which showed two regions with different EPDs separated by etch pit arrays, was quite similar to that reported previously by the authors, implying that the dislocation propagation behavior in this sample was also similar.12 We reported that dislocation reduction occurs via a two-step mechanism during LPE growth as follows: At the initial growth stage of LPE, many {1011} facets are formed on the GaN (0001) template during regrowth after dissolution of the GaN seed film surface. Dislocations propagating from the GaN seed film bend so as to align along the normal direction to the {1011} facet and become concentrated. This results in a reduction of the dislocation density from 108109 to the order of 106 cm2 (denoted “the first-step reduction”). After the first-step reduction, dislocations located at concave regions on the growth surface between preferentially grown grains and their neighbors propagate along the grain boundary and coalesce, resulting in a dislocation density of the order of 104105 cm2 and an increased LPE thickness (denoted “the second-step reduction”). In this stage, the average density of dislocations over the LPE layer is reduced, while areas of low dislocation density spread toward areas of high dislocation density as the growth proceeds. The detailed mechanisms of dislocation reduction were presented in refs 10 and 12. In this study, the LPE layer grown with 40 mol % Ga had a larger area of low EPD regions compared to the layer grown with 27 mol % Ga. In addition, it can be seen from Figure 4 that the EPD decreased significantly at high Ga compositions as the growth proceeded. These results suggest that the second-step reduction mechanism was favored at higher Ga compositions in the flux. Schematic growth models and the dislocation behaviors for LPE layers grown at high and low Ga compositions are shown in parts a and b, respectively, of Figure 5. As mentioned in the previous paragraph, the surface after the first-step reduction had a
Figure 5. Schematic growth models and dislocation behavior in LPE layers. (a) At higher Ga compositions, the growth transitions to a 3D mode after the first-step reduction. (b) At lower Ga compositions, the growth transitions to a 2D mode after the first-step reduction. 2349
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Crystal Growth & Design rough morphology with many (1011) facets. At higher Ga compositions (Figure 5a), the growth transitions to a 3D growth mode, which is favorable for the second-step reduction, after the first-step reduction. In this case, since the fronts of the dislocations terminate on the inclined {1011} facet, the dislocations propagate normal to the {1011} facet as the grains grow, which promotes the coalescence of dislocations positioning at concave regions between preferentially grown grains and their neighbors. This preferential growth results in a low dislocation density region. Etch pit arrays, as shown in Figure 3b,c, originate from dislocations concentrated at a concave region. On the other hand, the LPE layer grown with 18 mol % Ga grew in a 2D growth mode to produce a flat (0001) surface, as seen in Figure 1a. The dislocations in Figure 5b will not bend after the first-step reduction, since the fronts of the threading dislocations were terminated on the horizontal (0001) facet. Therefore, the dislocations penetrated the LPE layer normal to the (0001) surface and were distributed homogeneously, as seen in Figure 3a. At lower Ga compositions, after the first-step reduction, the growth transitioned to the 2D growth mode, which is unfavorable for the second-step reduction but has the advantage of a high growth rate.
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(9) Yamane, H.; Shimada, M.; Sekiguchi, T.; DiSalvo, F. J. J. Cryst. Growth 1998, 186, 8–12. (10) Kawamura, F.; Umeda, H.; Kawahara, M.; Yoshimura, M.; Mori, Y.; Sasaki, T.; Okado, H.; Arakawa, K.; Mori, H. Jpn. J. Appl. Phys., Part 1: Regul. Pap. Short Note Rev. Pap. 2006, 45, 2528–2530. (11) Kawamura, F.; Morishita, M.; Tanpo, M.; Imade, M.; Yoshimura, M.; Kitaoka, Y.; Mori, Y.; Sasaki, T. J. Cryst. Growth 2008, 310, 3946–3949. (12) Kawamura, F.; Tanpo, M.; Miyoshi, N.; Imade, M.; Yoshimura, M.; Mori, Y.; Kitaoka, Y.; Sasaki, T. J. Cryst. Growth 2009, 311, 3019–3024. (13) Imade, M.; Hirabayashi, Y.; Konishi, Y.; Ukegawa, H.; Miyoshi, N.; Yoshimura, M.; Sasaki, T.; Kitaoka, Y.; Mori, Y. Appl. Phys. Express 2010, 3, 075501-1–075501-3. (14) Kawahara, M.; Kawamura, F.; Yoshimura, M.; Mori, Y.; Sasaki, T.; Yanagisawa, S.; Morikawa, Y. J. Cryst. Growth 2007, 303, 34–36. (15) Aoki, M.; Yamane, H.; Shimada, M.; Kajiwara, T.; Sarayama, S.; DiSalvo, F. J. J. Cryst. Growth Des. 2002, 2, 55–58. (16) Zhang, L.; Shao, Y.; Wu, Y.; Hao, X.; Chen, X.; Qu, S.; Xu, X. J. Alloys Compd. 2010, 504, 186–191.
’ CONCLUSION In conclusion, during growth by the Na-flux method, the growth mode can be controlled by changing the flux composition, which is similar to the ELO technique in the HVPE method. Based on the changes in the growth mode of Na-flux GaN under different flux compositions, it is possible to control the dislocation propagation behavior. The following growth sequence is recommended to obtain a thick GaN substrate with a low dislocation density: At the initial growth stage, a high Ga composition in the flux decreases the dislocation density significantly, after which changing the Ga composition to a low value enables the rapid growth of thick GaN by LPE. In the near future, we will establish the capability to deliberately control the flux composition during LPE growth, in order to optimize the growth sequence. ’ AUTHOR INFORMATION Corresponding Author
*Phone/Fax: þ81-6-6879-7705/þ81-6-6879-7708. E-mail: imade@ cryst.eei.eng.osaka-u.ac.jp.
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