Influence of Solution Flow on Step Bunching in ... - ACS Publications

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Influence of Solution Flow on Step Bunching in Solution Growth of SiC Crystals Can Zhu,*,†,‡ Shunta Harada,‡ Kazuaki Seki,§ Huayu Zhang,† Hiromasa Niinomi,‡ Miho Tagawa,‡ and Toru Ujihara‡ †

Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, HIT Campus, University Town, Xili, Shenzhen 518055, China ‡ Department of Materials Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan § Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: The control of step bunching by solution flow in 4H-SiC solution growth is proposed. We achieved the solution flow control with the specially designed top-seeded solution growth method as follows: by deviating a seed crystal from the center of a crucible and rotating the crucible in one direction, the solution flow direction was controlled to be parallel or antiparallel to the step-flow direction. After the growth, the widely spaced, accumulated macrosteps were observed and the surface of the grown crystal became rough under the parallel flow. On the other hand, the development of the macrosteps was suppressed under the antiparallel flow. As the growth proceeds, the surface roughness of the growth surface increases under the parallel flow, while the surface roughness decreases under the antiparallel flow. This fact suggests the solution flow control can be an effective method to suppress the step bunching during the solution growth of SiC single crystals.

1. INTRODUCTION Silicon carbide (SiC) is considered to be a promising semiconductor material for power devices due to the excellent physical properties such as wide band gap and high breakdown voltage.1 The high-quality SiC crystal is a key issue for the applications in power devices. The commercial SiC crystal is grown by a sublimation method. Over the last decades, great efforts have been made to improve the quality of the commercial SiC crystals,2−8 and the density of micropipes (MPs), which cause the degradation of breakdown voltage, has been dramatically decreased below 0.1 cm−1.9 However, thousands of dislocations still remain in the crystal. To obtain higher-quality SiC crystals with low dislocation density, solution growth has been intensively investigated.10−19 The solution growth method yields high-quality SiC crystals because the crystal growth proceeds under the condition close to thermal equilibrium.13 So far, the reduction of MPs11 and basal plane dislocations,15,17 in solution growth method, have been reported. Recently, we reported that most of the threading screw dislocations (TSDs), having the Burgers vector of [0001], converted to the extended defects composed of 4 Frank partial dislocations and stacking faults (SFs) between them on the (0001) basal planes. This TSD conversion could be attributed to the extrusion effect by a propagating macrostep on the basal plane toward the outside of the crystal.19,20 A macrostep is formed by the step bunching during the solution growth, consists of a large number of superimposed microsteps © 2013 American Chemical Society

of several Si−C bilayers, and usually has the height of 100− 1000 nm.12 A Frank-type SF propagates laterally on the basal plane and finally gets out of the crystal completely. Therefore, once a TSD converts to a Frank-type SF, it never propagates toward the direction of crystal growth.21 This result indicates that macrosteps play important roles in reducing the TSDs. On the other hand, accumulated macrosteps lead to the micrometer-sized defects (e.g., trenches, solvent inclusions, etc.) after long time growth due to their abnormal evolution, which makes the growth surface rough and degrades the quality of crystal significantly.14 In order to stabilize the growth surface, it is necessary to control the formation and the evolution of macrosteps during the solution growth. The step bunching phenomenon is understood from the several aspects. Schwoebel et al. proposed an model which indicated that an additional energy barrier existed when adatoms diffuse down a step than when they hop on a flat terrace.22,23 Frank et al. explained that impurity atoms absorbed on a terrace impede step advance and initiate step bunching.24,25 Kimoto et al. explained the step bunching is a kind of surface equilibrium process to minimize the free energy during crystal growth.3,25 However, the mechanism of macrostep formation and evolution during the solution growth has Received: May 7, 2013 Revised: June 3, 2013 Published: June 13, 2013 3691

dx.doi.org/10.1021/cg400706u | Cryst. Growth Des. 2013, 13, 3691−3696

Crystal Growth & Design

Article

not been systematically understood. Chernov et al. pointed out that the formation of macrosteps due to the step−step interaction is strongly influenced by the solute transport in solution.26−29 They demonstrated that macrosteps were formed when steps were moving in the same direction as the solution flows (parallel flow), while macrosteps disappeared when the steps and solution flow were opposite (antiparallel flow) in the solution growth of ADP (NH4KH2PO4) and KDP (KH2PO4) crystals.28,30 In this study, in order to control the formation and the evolution of macrosteps, the effect of solution flow on step bunching during the solution growth of 4H-SiC crystal on the (0001) Si face was investigated. The Si face was chosen because highly efficient TSD conversions were reported during the solution growth on an off-axis (0001) Si face. We demonstrated that the evolution of step bunching can be suppressed by the controlled solution flow.

2. EXPERIMENTAL DETAILS SiC crystals were grown in an RF-heated furnace (Nisshin-Giken Company, Ltd.) by the specially designed top-seeded solution growth (TSSG) method. In order to apply the parallel and antiparallel flows above the crystal surface, the growth configuration was built as shown in Figure 1a. Unlike the conventional TSSG configuration, the seeds

Figure 2. The morphology analysis of the crystal surface just after melt-back. (a) An AFM image of the crystal surface. (b) The section profile corresponding to the white line AB in the AFM image. (c) The close-up section profile of the macroterrace corresponds to the white line CD. directions were the opposite as shown in Figure 1b. Off-axis 4H-SiC (0001) Si-face (10 mm ×10 mm) crystals were used as seeds. The off angle was 1° and the off direction was [1120]. The step flow proceeds along the off direction. The rotation speed of the crucible was 27 rpm, corresponding to the solution flow velocity of 8.6 cm/s above the crystal surface. The solution was placed in a graphite crucible and kept in a vertical temperature gradient of 20 K/cm under a high-purity (>99.9999 vol%) helium gas flow. The graphite crucible was 100 mm in inner diameter and 105 mm in height. High-purity silicon (11N) was used as a solvent. Carbon was supplied from the graphite crucible. Prior to growth, the 4H-SiC seed crystals and the polycrystalline silicon blocks were cleaned by sonication in methanol, acetone, and purified water (18 MΩ cm). The growth time varied from 0 to 20 min. Figure 1c illustrates the growth procedure: (1) the crucible was maintained at 1873 K for 1 h to melt and homogenize the raw material, (2) the seed crystals were immersed in the high-temperature zone (near the bottom of crucible, as shown in the Figure 1a) of the solution to melt back for 20 min by raising the temperature to 1973 K to clean the surface of the seed crystals, (3) the seed crystals were moved to the low temperature zone (near the surface of solution) and held there during the growth period, and (4) the grown crystals were then removed from the solution. After growth, residual solvent was removed in HF and HNO3 solution (HF:HNO3 = 1:2). Surface morphology was observed by a differential interference contrast (DIC) microscope (Leica, DM4000 M) using Nomarski-type prism and an atomic force microscope (AFM) (Molecular Imaging PicoScan2500). The surface roughness was measured by the

Figure 1. Growth procedure of the solution flow control in the newly developed top-seeded solution growth method. Schematic illustrations of the experimental configuration from (a) side view and (b) top view. (c) The temperature sequence of the experiment. were deviated by 30 mm from the center of the crucible. By keeping the graphite rod fixed and only rotating the crucible, parallel flow was obtained when step flow and solution flow directions were the same, and antiparallel flow was obtained when step and solution flow 3692



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Figure 4. The morphology analysis of the grown crystal under the antiparallel flow after the growth for 10 min. (a) A Nomarski image of the grown crystal surface. (b) An AFM image of the grown crystal surface. (c) The section profile corresponding to the white line AB.

observed on the macroterrace as shown in Figure 2c. In this study, the macroterrace means the micrometer-sized quasiterrace between the macrosteps. The surface morphology of the crystal after 10 min of growth under the parallel flow is shown in Figure 3. In the Nomarski image (Figure 3a), the zigzag-shaped macrosteps were observed on the grown crystal. Wide macroterraces were frequently observed on the crystal surface. Most of them have the width above 10 μm. The RMS value of the grown crystal under the parallel flow was evaluated to be 110 nm. In the AFM image (Figure 3b), the wide and flat macroterraces were observed similarly to the Nomarski image in Figure 3a. From the section profile, the accumulated macrosteps were confirmed and microsteps were hardly observed on the macroterraces (Figure 3c). On some macroterraces, the islands were observed and most of them were hexagonally shaped (Figure 3d). It should be noted that trenches were often found in front of a macrostep (marked by a white arrow in Figure 3c). This kind of trench would be formed by the following process: A two-dimensional (2D) nucleation island is

Figure 3. The morphology analysis of the grown crystal under the parallel flow after the growth for 10 min. (a) A Nomarski image of the grown crystal surface. (b) An AFM image of the grown crystal surface. (c) The section profile corresponding to the white line AB. (d) An AFM image of an island on the macroterrace. stylus surface profiler (Dektak 150) as the RMS value. The thickness of the grown layer was measured by an optical microscope (Keyence VHX-500F).

3. RESULTS Figure 2 shows an AFM image of the crystal surface just after melt-back. On the surface of the crystal, macrosteps were already formed under both parallel and antiparallel flow. The height of macrosteps ranged from 100 to 200 nm (Figure 2b). The microsteps with the height of several nanometers were also 3693



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formed on the macroterrace and then it expands to the advancing macrostep. The macrostep and the 2D island meet and finally a trench is formed in front of the macrostep. On the other hand, under the antiparallel flow, the grown surface was different. Figure 4 shows the surface morphology of the crystal after 10 min of growth under the antiparallel flow. Step trains with narrow macroterraces were observed (Figure 4a). Most of the macroterraces have the width below 10 μm. In the AFM image, the wide macroterraces, as shown in Figure 3b, were rarely observed (Figure 4b). And no trench was found in front of the macrosteps. From a section profile in Figure 4c, we can see that the growth surface was smoother than that under the parallel flow. The RMS value of the grown crystal under the antiparallel flow was evaluated to be 70 nm, which was much smaller than that under the parallel flow. These results indicate that a smooth growth surface is achieved under the antiparallel flow. Figure 5 shows the temporal evolutions of growth thickness and the values of RMS under the parallel flow and the antiparallel flow. The thicknesses of grown layers increase with the growth time both under the parallel flow and the antiparallel flow. The growth rates under the parallel flow and the antiparallel flow are 42 μm/h and 45 μm/h, respectively. On the other hand, the values of RMS show the different trend by the solution flow direction as shown in Figure 5b. Although no obvious difference about the surface roughness after melt-back under both the parallel and the antiparallel

Figure 5. Temporal evolutions of (a) growth thickness and (b) the values of RMS under the parallel flow and antiparallel flow.

Figure 6. Schematic illustrations of contour plot of solute concentration distribution above the crystal surface and the evolution of the macrosteps (a) in the stagnant case, (b and c) under the parallel flow, and (d−e) under the antiparallel flow. The solid lines represent the surfaces with equal solute concentrations. The solid line farther from the surface indicates higher concentration. 3694



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plays a role as a sink of solute. Thus, the solute concentration is low near a macrostep. On the other hand, the solute concentration is high in the region of a macroterrace. In the stagnant case, the solute concentration distribution is symmetric. By applying the flow, the solution moves tangentially to the crystal surface,33 and the solute concentration distribution is changed to be asymmetric. Under the parallel flow, the solution distribution changes as schematically illustrated in Figure 6b. The solute concentration gradient near the edge of the macroterrace is larger than that in front of the macrostep (L1 < L2). In this case, the microsteps on the macroterraces are accelerated by the larger solute concentration gradient near the edge of macroterraces and run into the front macrosteps, as shown in Figure 6c. On the other hand, it is difficult for the microsteps to run out from the macrosteps because of the limitation of solute supply due to the low solute concentration gradient in front of the macrosteps. Thus, the flat macroterraces are formed between the macrosteps, which provide the opportunity for the formation of 2D nucleation islands. As the growth proceeds, the 2D nucleation islands are expanding and meet the macrosteps. Because of the lower solute supply in front of the macrosteps, the advance of macrosteps is prevented. Finally, trenches are formed near the macrosteps. The step flow growth mode is expected to be hindered by the trench. As the growth proceeds, the surface roughness would increase. On the other hand, under the antiparallel flow, the solute concentration distribution was dragged by the solution flow, as shown in Figure 6d. Because of the larger solute concentration gradient in front of the macrosteps (L1> L2), the microsteps which bunched in the macrosteps are accelerated and then run out from the macrosteps (Figure 6e). On the other hand, the microsteps on the macroterraces are controlled by the smaller solute concentration gradient, which move toward the macrosteps with lower velocity. Thus, the macrosteps dissipate gradually and the height of macrosteps decreases as the growth proceeds. Actually, the 2D nucleation under the parallel flow and the dissipation of the macrosteps under the antiparallel flow were observed, as shown in Figure 7. Figure 7a shows the AFM image of the crystal after 10 min of growth under the parallel flow. An island (marked by a white arrow) was observed near the edge of the macroterrace. This indicates that the 2D nucleation island is formed on the macroterrace under the parallel flow. On the other hand, under the antiparallel flow, the process of the macrostep dissipation was observed, as shown in Figure 7b. In this position, the microsteps leaving from the bunching macrosteps were observed, which would decrease the height of macrosteps. These observations were consistent with the proposed model during the solution growth under the parallel and antiparallel flow.

Figure 7. (a) An AFM image of the grown crystal surface which includes a 2D nucleation island formed on the macroterrace under the parallel flow. (b) The dissipating process of the macrostep under the antiparallel flow: an AFM deviation image of the grown crystal surface (upper) and the section profile corresponding to the red line AB.

flow was observed, as the growth proceeds, the values of RMS increase under the parallel flow, while the values decrease under the antiparallel flow. Even though the thickness of grown layer increases, the surface roughness decreases under the antiparallel flow. This indicates that the solution flow direction plays an important role in controlling the evolution of macrosteps.

5. CONCLUSION The influence of solution flow on step bunching in the solution growth of SiC was investigated. Under the parallel flow, accumulated macrosteps and wide macroterraces were observed on the surface of the grown crystal by Nomarski and AFM imaging. On the other hand, step trains with narrow macroterraces were observed under the antiparallel flow. As the growth proceeds, the surface roughness of the grown crystal increased under a parallel flow, while it decreases under the antiparallel flow. This fact suggests that the direction of solution flow affects the formation and the evolution of macrosteps and

4. DISCUSSION The behaviors of the macrosteps observed here can be explained by modifying Chernov’s model.31 The step bunching is closely related to the surface diffusion and volume diffusion. In the solution growth, volume diffusion is predominant because the growth rate is limited by the diffusion resistance of the solution.32 Figure 6a shows the contour plot of solute concentration distribution above the crystal surface. A macrostep 3695



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an antiparallel flow is an effective method to suppress the step bunching during the solution growth of SiC crystals.

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(30) Booth, N.; Chernov, A.; Vekilov, P. J. Cryst. Growth 2002, 237, 1818−1824. (31) Chernov, A. A.; Coriell, S.; Murray, B. J. Cryst. Growth 1993, 132, 405−413. (32) Chernov, A. A. Phys.−Usp. 1961, 4, 116. (33) Markov, I. V. In Crystal Growth for Beginners: Fundamentals of Nucleation, Growth and Epitaxy; Markov, I. V., Eds.; World Scientific: Singapore, 2003; Chapter 3, pp 208.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-52-789-3249. Fax: +81-52-789-3248. Notes

The authors declare no competing financial interest.

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REFERENCES

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Influence of Solution Flow on Step Bunching in ... - ACS Publications

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