Centimeter-Sized Bulk GaN Single Crystals Grown by the Na-Flux

Article pubs.acs.org/crystal

Centimeter-Sized Bulk GaN Single Crystals Grown by the Na-Flux Method with a Necking Technique Mamoru Imade,* Kosuke Murakami, Daisuke Matsuo, Hiroki Imabayashi, Hideo Takazawa, Yuma Todoroki, Akira Kitamoto, Mihoko Maruyama, Masashi Yoshimura, and Yusuke Mori Division of Electric, Electronic and Information Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT: Centimeter-sized bulk GaN single crystals with large dislocation-free areas were fabricated by the Na-flux method with a necking technique. This necking, which is the key technique in Czochralski growth of dislocation-free Si ingots, was realized using a newly developed GaN point seed. Structural properties of grown crystals were investigated using panchromatic cathodoluminescence (CL) measurements and X-ray diffraction. Prism-shape and well-faceted bulk GaN crystals with dimensions as large as 0.85 cm (width) and 1 cm (length) were grown by this technique. The GaN single crystal grown for 400 h have full-width at half-maximum values for c// and c⊥ as narrow as 42.8 and 32.5 arcsec, respectively, indicating an extremely high quality. Panchromatic CL images of (0001) GaN wafers sliced from grown crystals revealed that large areas of the wafers were dislocation free. We concluded that the necking technique in Na flux GaN growth may be a major breakthrough for fabricating large dislocation-free GaN ingots.

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INTRODUCTION Gallium nitride (GaN) has attracted great interest due to its potential to be used in ultraviolet light emitting diodes,1 ultraviolet laser diodes2,3 and high-power, high-frequency electronic devices.4 However, GaN wafers and overgrown epilayers have high dislocation densities, reducing device performance. In recent years, much effort has been expended in growing bulk GaN single crystals that are dislocation free. However, the techniques that have been developed for realizing this suffer from various problems.5−18 Fujito et al. grew a colorless, freestanding c-plane bulk GaN crystal (diameter: 52 mm; thickness: 5.8 mm; dislocation density: ∼106 cm−2) by hydride vapor phase epitaxy.5 Dwilinski et al. reported the ammonothermal growth of large GaN crystals with high crystallinities, a curvature radius of greater than 1000 m, and a low dislocation density (5 × 103 cm−2 at 1 in.).8 The Na-flux method is promising for mass producing low dislocation density GaN crystals because a reduction in the dislocation density from 108 cm−2 in a seed to 104 cm−2 in liquid-phase epitaxy (LPE) layers and a growth rate of over 20 μm/h have been attained for 2-in. GaN LPE.12,13 However, further progress is required to realize dislocation-free GaN single crystals by this technique. To realize this, we considered the necking technique used in Czochralski growth of Si ingots in which dislocations generated at the interface between the seed and the crystal are eliminated in a thin neck, resulting in dislocation-free Si crystals.19 We report here for the first time Na-flux GaN growth by the necking technique using a newly developed GaN point seed. We describe typical structural characteristics of the grown crystals, which are centimeter-sized bulk GaN crystals with © 2012 American Chemical Society

large dislocation-free areas. Detailed investigation of the interface between the seed and the crystal (i.e., the necking region) by cross-sectional cathodoluminescence (CL) mapping revealed that almost all dislocations propagated from the GaN seed were bent and terminated at the initial growth stage, resulting in a dislocation-free GaN crystal. The necking technique in Na flux GaN growth may be a major breakthrough for fabricating large dislocation-free GaN ingots. Furthermore, GaN wafers fabricated by this process will significantly advance the development of GaN-based devices.

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EXPERIMENTAL SECTION

As shown in Figure 1a,b, a GaN point seed was established by mounting a sapphire plate (thickness: 430 μm) with a small hole (0.5− 1.5 mm in diameter) freely on a GaN template, which is a 10-μm-thick (0001) GaN film grown on a (0001) sapphire substrate by hydride vapor phase epitaxy. The GaN point seed was placed in a ceramic crucible (diameter: 17 mm; height: 50 mm) and the starting materials of metallic Ga (purity: 6N), metallic Na (purity: 4N), graphite grains (purity: 6N), and Sr (purity: 4N) were added to the crucible in an Arfilled glovebox (Figure 1c). The ratio of carbon content to the total Ga/Na amount was fixed at 0.5 mol %. Graphite grains were added to prevent polycrystals growing on the crucible wall.13 Addition of Sr to the solution allows the habit of the growing crystal to be easily changed to give a prism-shaped crystal.14 The starting composition of Ga:Na:C was fixed at 40:60:0.5. The ratio of the Sr content to the Na content was 0.03 mol %. Growth on a GaN point seed was performed by the following procedure. After the crucible was transferred to a pressure-resistant vessel (diameter: 25.4 mm; height: 126 mm) in the glovebox, the tube Received: May 11, 2012 Published: June 6, 2012 3799

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Figure 1. Schematic diagram of experimental setup. (a) Configuration of GaN point seed. The GaN point seed was established by mounting a sapphire plate (thickness: 430 μm) with a small hole (diameter: 0.5−1.5 mm) (1) freely on a GaN template (10-μm-thick (0001) GaN film grown on (0001) sapphire substrate by hydride vapor phase epitaxy) (2). (b) Cross-section of GaN point seed. (c) Arrangement of Ga−Na based solution (4) and GaN point seed in the crucible. The GaN point seed was placed at the bottom of the crucible. (d) Arrangement of the pressure-resistant vessel including the crucible (3) in the electric furnace. The pressure-resistant vessel (5) was placed in an electric furnace equipped with a resistive heater (6). Nitrogen source gas was pressurized through the stainless-steel tube (diameter: 6.4 mm).

in detail by Iwahashi et al.14 However, in the crystals grown in the present study, the (0001) facet had a skeletal shape. This morphological instability may be due to nonuniform supersaturation over the seed surface, which is known as the Berg effect. This phenomenon produces a higher supersaturation at the edges of a finite crystal. The crystal surface then becomes morphologically unstable due to preferential growth of the edges and the corners. This leads to the formation of “negative” facets, which can be seen in the (0001) sector. Skeletal growth was promoted after the formation of these negative facets. To eliminate the nonuniform supersaturation, we are currently investigating effects of stirring the solution on the morphology using a specially made chamber equipped with a stirring machine (see ref 12 for details); the results will be presented in a future paper. The structural properties of grown crystals were evaluated from the fwhm of XRCs, SIMS, and panchromatic CL images. Figure 4a,b shows XRCs of GaN (1010̅ ) when the incident Xray beam is perpendicular (c⊥, Figure 4a) and parallel (c//, Figure 4b) to the ⟨0001⟩ direction. As Figure 4a,b shows, the XRCs of all crystals exhibit a single peak, indicating that they consist of a single domain. The GaN single crystal grown for 400 h have fwhms for c// and c⊥ as narrow as 42.8 and 32.5 arcsec, respectively, indicating an extremely high quality. In SIMS measurements, the Sr content of the (101̅0) facet of the crystal was below the detection limit of 2 × 1013 atoms/cm3. After slicing the crystals parallel to the (0001) face (Figure 5a) and performing chemical mechanical polishing, CL measurements were performed to estimate the dislocation densities of the crystals. This was done because GaN crystals grown by the Na-flux method exhibit dislocation densities below the transmission electron microscopy resolution of the order of 106 cm−2. The dark spots and lines in the panchromatic CL images correspond to nonradiative carrier

was evacuated and connected to an N2 gas line (Figure 1d). N2 gas was introduced into the tube, and the tube temperature was increased to 890 °C over a 1-h period using a resistive heater (Figure 1d). The temperature and N2 pressure in the tube were respectively maintained at 890 °C and 4.0 MPa during the growth periods of 200−600 h. After the tube had cooled naturally, the crucible was removed from the tube and immersed in cold ethanol and water to dissolve the residual flux. The aspect ratio of crystals was defined as the ratio of the length of the as-grown crystal along the ⟨0001⟩ direction to that along the direction. The crystallinity of as-grown GaN crystals was evaluated from the full width at half-maximum (fwhm) of the X-ray rocking curves (XRCs) of GaN (1010̅ ) with incident X-ray directions of c⊥ and c// (Rigaku, SmartLab-ES; Cu−Kα; 40 kV; 30 mA). The Sr content of a (1010̅ ) facet of a grown crystal was measured by secondary ion mass spectrometry (SIMS). Wafers were sliced from grown GaN crystals so that each wafer surface was parallel to the (0001) face. They were then mechanically polished and chemical mechanical polished (CMP). The dislocation density of wafers was investigated by panchromatic cathodoluminescence (CL) measurements (Horiba, Imaging CL DF-100).

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RESULTS AND DISCUSSION Prism-shaped and well-faceted bulk GaN crystals could be grown on the GaN point seed through a hole. Figure 2a−c shows photographs of crystals grown for 200, 400, and 600 h, respectively. After 600 h growth, the crystal had dimensions as large as 0.85 cm (width) and 1 cm (length). This is the largest bulk GaN crystal grown by this method.17,18 Figure 3 shows the relationship between the crystal mass, the aspect ratio, and the growth period. It shows that the crystal mass increases dramatically with increasing growth period. Additionally, the aspect ratio (c/a) increases with increasing growth period; this is probably due to the Sr concentration in the solution increasing during the later growth stage as a result of Ga consumption by crystallization of GaN. The increase in the aspect ratio with increasing Sr concentration has been reported 3800

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Figure 3. Mass and aspect ratio of grown crystal as a function of growth period. Aspect ratio is defined as the ratio of the crystal length in the ⟨0001⟩ direction to that in the direction.

Figure 4. X-ray rocking curves with full width at half-maximum values for X-ray incident beams (a) perpendicular (c⊥) and (b) parallel (c//) to the c-direction.

density of the dark spots and lines will be close to the actual dislocation density. Figure 5b shows a photograph of a sliced (0001) GaN wafer. Some holes near the center of the wafer are due to the above-described skeletal growth. Panchromatic CL measurements were performed at more than 10 regions on the wafer. Figure 5c−e shows scanning electron microscopy (SEM) images of three randomly selected regions, and Figure 5f−h are panchromatic CL images of the same regions as Figure 5c−e, respectively. By correlating the features in the SEM images with those in the CL images, contrast from scratches and contamination could be distinguished from that from dislocations. In this case, by comparing the SEM images with

Figure 2. Photographs of GaN crystals grown on the point seed for (a) 200, (b) 400, and (c) 600 h. Prism-shaped and well-faceted GaN single crystals could be grown on the point seed.

recombination at dislocations, which is strongly localized due to the short-hole carrier diffusion length of GaN.20 Therefore, the 3801

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Figure 5. SEM and panchromatic CL images of (0001) GaN wafer sliced from bulk GaN crystal. (a) Schematic diagram of method used to slice (0001) GaN wafers from a bulk GaN crystal. (b) Photograph of a (0001) GaN wafer. (c−e), SEM images of randomly selected regions on the (0001) GaN wafer. (f−h) Panchromatic CL images from regions shown in (c−e), respectively. By comparing the SEM and CL images, contrast from surface artifacts can be distinguished from those produced by dislocations. The panchromatic CL images contain no dark spots due to dislocations in the 120 μm × 120 μm area.

other CL images in Figure 6 are high magnification images of various regions in Figure 6d, specifically, regions near the sapphire sidewall (Figure 6e,g), a region near the seed layer (Figure 6f), and a region near the center of the hole (Figure 6h). Two regions exhibit different CL contrasts: the initial growth layer (Figure 6f) and the right and left edges that were in contact with the sapphire sidewall (Figure 6e,g) show a high contrast (i.e., they are defective regions consisting of many grains), whereas the contrast in the central part of the hole (Figure 6h) was homogeneous and the striations are parallel to (0001) and {101̅1}, indicating a single grain structure. The interface between these two regions was clearly observed (Figure 6d). Many dark spots are observed in the initial growth layer (Figure 6f); its density is about 4 × 105 cm−2. These dark spots seem to be dislocations propagating toward the sapphire

the CL images of the wafer, scratches and contamination could be readily correlated; these surface artifacts were probably produced during handling. No dark spots due to dislocations were observed in panchromatic CL images (120 μm × 120 μm) over a large area of the wafer, indicating that this large area was free from dislocations. The alternating black and white lines in the CL images appear to be impurity striations due to temporal variations in the growth rate as the result of temporal variations in the nitrogen concentration near the growing surface. Cross-sectional CL mapping was performed to clarify the dislocation geometry of a typical GaN crystal grown on a GaN point seed. The specimen was sliced from a crystal parallel to the (101̅0) face so that the necking region was included (see Figure 6a). Figure 6b,c shows a CL image and a photograph of the specimen, respectively. Figure 6d shows a CL image of a region in the hole, corresponding to the necking region. The 3802

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Figure 6. Panchromatic CL mapping images of a cross-section of a typical GaN crystal grown on a point seed. (a) Schematic diagram of method used to slice specimen. The specimen was sliced from a grown crystal parallel to (101̅0) face so that the necking region was included. (b) Panchromatic CL image and (c) optical microscopy image of the specimen. (d) Panchromatic CL image near the seed, which corresponds to the region grown in the hole. High-magnification CL images of regions in (d): (e, g) regions near sapphire sidewall, (f) regions near the seed layer, and (h) near the center of the hole.

results can also be explained by the high contrast at the boundaries of the respective regions in the CL image (Figure 6d). As shown in Figure 7d, only the single grain continued to grow preferentially after the intermediate growth stage. On the basis of the above observation results, the growth in the hole is likely to progress as follows. In the initial growth stage, numerous grains with (1011̅ ) facets form on the (0001) GaN seed layer and they grow by coalescing with each other. During this process, a single grain starts to grow preferentially from one grain. Once this single grain starts to grow, the supplied nitrogen is consumed by the growth of the single grain. Thus, the grain becomes large and covers the initial growth layer. Only the single grain preferentially grows and single crystals shown in Figure 2a−c eventually grow. Figure 8a,b shows schematic diagrams of the elimination mechanism of dislocations, which is predicted from the above

sidewall. No dark spots or lines were observed in the region near the center of the hole (Figure 6h). To gain a detailed understanding of the growth mode and the elimination mechanism of dislocations, SEM was used to observe the change in the growth morphology in the hole during the initial and intermediate growth stages. Figure 7a shows an SEM image of a crystal in the hole in the initial growth stage (the sapphire plate has been removed). In the hole shown in Figure 7a, an initial growth layer with a rough surface over the entire area in the hole and a single pyramidal grain at its center are observed. From a high-magnification SEM image of the surface of the initial growth layer (Figure 7b), the initial growth layer was found to consist of numerous grains with (101̅1) facets. The boundary between the single grain and the initial growth layer was spatially separated and these regions were not crystallographically connected (Figure 7c). These 3803

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Figure 7. (a) Bird’s eye and plane-view (inset) SEM images of crystal grown in the hole in an initial stage (the sapphire plate has been removed). The initial growth started with the formation of many grains with (101̅1) facets. High-magnification SEM images of (b) the surface of initial growth layer and (c) the boundary between the preferentially grown single grain and the initial growth layer. A selected GaN grain was preferentially and laterally grown over the initial growth layer. (d) SEM image of crystal immediately after its top came out of the hole.

propagate perpendicularly to the (101̅1) facets, and they are bent (Figure 8a). The growth of (101̅1) facets in this initial growth layer and the dislocation behavior are unique phenomena to the Na flux method; they have already been demonstrated by Kawamura et al.15 The conventional LPE by the Na flux method suffers from the problem in that a portion of bent dislocations remains in the final crystal surface.16 However, in the present study, these bent dislocations terminate at the sapphire sidewall so that further propagation does not occur. In addition, the dislocations that propagate in the ⟨0001⟩ direction also terminate at the boundary between the initial growth layer and the preferentially grown single grain so that propagation of dislocations to a single grain cannot occur (Figure 8b). This is because these regions are crystallographically not connected, as mentioned above. Consequently, almost all the dislocations originating from the GaN seed layer are eliminated in the hole (i.e., the necking region), resulting in a GaN crystal free from threading dislocations. This process is analogous to Si ingot growth by the Czochralski technique in which dislocations generated at the interface between the seed and the crystal are eliminated in a thin neck, resulting in dislocation-free Si crystals.19 The necking technique in the Na flux GaN growth may be a major breakthrough for fabricating dislocation-free GaN ingots. GaN wafers fabricated by this process will significantly advance the development of GaN-based devices.

growth mode. A dislocation generally propagates perpendicular to the growing surface. Therefore, in the initial growth layer that consists of numerous (101̅1) facets, the dislocations

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CONCLUSION In this work, centimeter-sized bulk GaN single crystals could be grown on the GaN point seed by the Na-flux method with the necking technique. For 600 h growth, GaN crystal reached dimensions as large as 0.85 cm (width) and 1 cm (length). fwhms of GaN (101̅0) XRC for the X-ray incident directions of c// and c⊥ were 42.8 arcsec and 32.5 arcsec, respectively, showing an extremely high crystallinity. Furthermore, panchromatic CL images showed that no dark spots were observed in large areas of the sliced (0001) GaN wafer. Detail CL measurements near the interface between the seed layer and the grown crystal revealed that almost all dislocations terminated in the hole. This elimination process of dislocations is quite similar to the necking technique in the Czochralski growth of a dislocation-free Si ingot. We concluded that the necking technique in the Na flux method allows the elimination of

Figure 8. Illustrations of the growth model in (a) the initial growth stage and (b) at a later growth stage. Almost all dislocations originate from the seed GaN layer eliminated in the hole (i.e., the necking region), resulting in a GaN crystal free from threading dislocations. 3804

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(18) Konishi, Y.; Masumoto, K.; Murakami, K.; Imabayashi, H.; Takazawa, H.; Todoroki, Y.; Matsuo, D.; Maruyama, M.; Imade, M.; Yoshimura, M.; Sasaki, T.; Mori, Y. Appl. Phys. Express 2012, 5, 025503. (19) Dash, W. C. J. Appl. Phys. 1959, 30, 459−474. (20) Rosner, S. J.; Girolami, G.; Marchand, H.; Fini, P. T.; Ibbetson, J. P.; Zhao, L.; Keller, S.; Mishra, U. K.; DenBaars, S. P.; Speck, J. S. Appl. Phys. Lett. 1999, 74, 2035−2037.

dislocations propagating from a seed layer and may be a major breakthrough for fabricating large dislocation-free GaN ingots.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-6-6879-7705. Fax: +81-6-6879-7708. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding from the New Energy and Industrial Technology Development Organization (Project No. P09024). We would like to thank Dr. Aoyama and Dr. Nishikata (Horiba, Ltd.) for supporting the CL measurements. We thank E. Sawai (Frontier Alliance, LLC) for slicing and polishing specimens.

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REFERENCES

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