Polytype Transformation by Replication of Stacking Faults Formed by

Article pubs.acs.org/crystal

Polytype Transformation by Replication of Stacking Faults Formed by Two-Dimensional Nucleation on Spiral Steps during SiC Solution Growth Shunta Harada,*,† Alexander,‡ Kazuaki Seki,‡ Yuji Yamamoto,† Can Zhu,† Yuta Yamamoto,§ Shigeo Arai,§ Jun Yamasaki,§ Nobuo Tanaka,§ and Toru Ujihara† †

Department of Materials Science and Engineering, ‡Department of Crystalline Materials Science, and §EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: Polytype transformations on the 4H-SiC(0001) Si face during top-seeded solution growth have been investigated by transmission electron microscopy and microRaman spectroscopy. 4H-, 15R-, and 6H-SiC were grown on the 4H-SiC(0001) Si face via spiral growth. Once a polytype transformation from 4H-SiC to 15R- or 6H-SiC occurs, the polytype rarely returns to 4H-SiC. Just before the polytype transformation, a disturbance in the stacking sequence involving the introduction of stacking faults was observed. A polytype transformation during spiral growth can be understood in terms of the replication of stacking faults due to twodimensional nucleation of a single bilayer on the spiral steps. This polytype transformation model shows good agreement with observed polytype transformation pathways as well as the disturbance in the stacking sequence at the interface between 4H-SiC and 15R-SiC.

1. INTRODUCTION Silicon carbide (SiC) has attracted a great deal of attention from researchers because of its excellent physical properties for power devices such as a wide band gap, high electric breakdown field, large saturated drift velocity, and high thermal conductivity.1,2 Solution growth is a proposed method to obtain high quality SiC crystals.3−7 However, solution growth of SiC has problems concerning polytypism. SiC exhibits a large number of different polytype structures with different stacking sequences of Si−C bilayers.8 Among the various SiC polytype structures, 4H-SiC is the most attractive for application to power devices. However, it has often been difficult to achieve stable growth of 4H-SiC because it can easily transform to another polytype. During physical vapor transport (PVT) growth, the 4H-SiC was reported to be stabilized by controlling growth parameters such as growth temperature,9,10 supersaturation,10−12 Si/C ratio,10−12 and the polarity of the seed crystal.13,14 The growth of 4H-SiC by PVT has been reported to be more favored at relatively high temperatures (around 2673 K), low supersaturation, and C-rich conditions.10−12 During the PVT growth of 4H-SiC, the polytype is believed to be stabilized by spiral growth. During chemical vapor deposition (CVD) growth of SiC, the grown polytype has also been reported to be stabilized by spiral growth.15 On the other hand, we recently reported a polytype transformation from 4H-SiC to 6H-SiC via spiral growth during solution growth,16 However, the specific polytype transformation behavior on 4H-SiC during solution growth is still not clear. © 2012 American Chemical Society

In the present study, we investigated polytype transformation during solution growth on the 4H-SiC Si face, specifically focusing on how the polytype transformation occurs during spiral growth. We propose a polytype transformation model involving replication of stacking faults (SFs) formed by twodimensional (2D) nucleation on spiral steps.

2. POLYTYPE STRUCTURES OF SIC The structure of SiC can be described as an assembly of cornersharing tetrahedrons composed of a carbon atom at the center of the tetrahedron and four silicon atoms at the vertices.17,18 The carbon and silicon atoms are bonded to each other by predominantly covalent bonds. Different polytype structures are formed by different tetrahedron stacking sequences, as shown in Figure 1. The tetrahedrons can be stacked in three different positions (A, B, and C) in two different ways related to each other by twinning. Henceforth, twinned stacking will be denoted using a prime sign (′) as A′, B′, and C′. The polytype structures are described using Ramsdell’s notation,17,19 in which the polytype is represented by the number of tetrahedrons (or Si−C bilayers) in the unit cell and the crystal system (“C” for cubic, “H” for hexagonal, and “R” for rhombohedral). Schematic illustrations of the structure of 4H-SiC, 15R-SiC, Received: March 18, 2012 Revised: May 8, 2012 Published: May 18, 2012 3209

dx.doi.org/10.1021/cg300360h | Cryst. Growth Des. 2012, 12, 3209−3214

Crystal Growth & Design

Article

with JEOL JEM-ARM200 and HITACHI H-800 transmission electron microscopes, both operated at 200 kV.

4. RESULTS Figure 2 shows the results of micro-Raman mapping of the grown crystals for 1 h (a−c) and 2 h (d−f). 3C-SiC was not

Figure 1. Schematic illustrations of polytype structure for 4H-SiC (a), 15R-SiC (b), and 6H-SiC (c) projected along [112̅0]. Corner-sharing tetrahedron composed of a carbon atom and silicon atoms are represented by triangles.

and 6H-SiC are shown in Figure 1 and the stacking sequence of these polytypes is tabulated in Table 1. Table 1. Stacking Sequence of 4H-SiC, 15R-SiC, and 6H-SiC polytype

stacking sequence

3C 4H 15R 6H

-ABC-ABC′B′-ABCA′C′ BCAB′A′ CABC′B′-ABCA′C′B′-

Figure 2. Polytype distribution of grown crystal characterized by micro-Raman mapping after 1 h growth (a−c) and 2 h growth (d−f). Mapping measurements were carried out using the Raman peak at 776 cm−1 (4H-SiC TO mode) (a, d), 789 cm−1 (6H-SiC TO mode) (b, e), and 785 cm−1 (15R-SiC TO mode) (c, f).

observed by Raman investigation, which was in fairly good agreement with our previous study about polytype stability of 3C-SiC.23,24 After solution growth for 1 h, a polytype transformation from 4H-SiC to 6H-SiC and 15R-SiC occurred in about 20% and 50% of the grown crystal, respectively, while 30% of the grown crystal remained unchanged. After crystal growth for an additional 1 h, further polytype transformation occurred. The 4H-SiC was transformed to 15R- and 6H-SiC, and the 15R-SiC was transformed to 6H-SiC. These results indicate that the polytype transformation pathways are limited as shown in Figure 3; that is, once the polytype transformation from 4H-SiC to 15R- or 6H-SiC occurs, the polytype of the grown crystal rarely changes back to 4H-SiC during solution growth. Figure 4 shows Nomarski images of the crystal grown for 1 h. Many hillocks are observed over the entire surface regardless of grown polytypes as seen in Figure 4a. This indicates that 4H-, 6H-, and 15R-SiC grow via spiral growth on the 4H-SiC Si face during solution growth. The shape of the hillocks varies with the polytype. In the areas identified as 4H-SiC, two different

3. EXPERIMENTAL PROCEDURES SiC was grown in a radio frequency-heated graphite hot-zone furnace (Nisshin-Giken Co., Ltd.) by top-seeded solution growth (TSSG). The solution was placed in a graphite crucible and kept in a vertical temperature gradient of 32 K/cm under a high-purity (>99.9999 vol %) Ar gas flow. The graphite crucible had an inner diameter of 45 mm and was 50 mm high, and the graphite rod was 10 mm in diameter. A 4H-SiC(0001) Si face on-axis crystal (5 mm ×10 mm) was used as the seed. The Si for the solvents had a purity of 11N (Tokuyama Co., Ltd.). Carbon was supplied from the graphite crucible. Prior to growth, the 4H-SiC seed crystal and the Si were cleaned by sonication in methanol, acetone, and purified water (18 MΩ·cm). The growth procedure was as follows: (1) the crucible was heated to 1853 K for 1.5 h; (2) the seed crystal, mounted on a graphite rod using carbon adhesive (Nisshinbo, ST-201), was immersed in the solution and held there during the growth period; (3) the grown crystal was then removed from the solution. The crucible was rotated by applying the accelerated crucible rotation technique (ACRT).20,21 The crucible and the seed crystal were counter rotated with alternating rotation directions. The maximum crucible and seed rotation speeds were 20 rpm in each case. Residual solvent on the crystal was removed by etching in an HNO3 + HF solution (HNO3/HF = 2:1). The grown crystal was characterized by micro-Raman spectroscopy using a Renishaw inVia Raman microscope, and the SiC polytypes were identified from the Raman spectra.22 The wavelength of the incident laser was 532 nm and the spot size was about 10 μm. MicroRaman mapping was carried out with a mapping interval of 100 μm. The surface morphology of the grown crystals was examined using a differential interference contrast (DIC) microscope (Leica DM4000 M) with a Nomarski-type prism, as well as an atomic force microscope (AFM) (Molecular Imaging PicoScan2500). Cross-sectional transmission electron microscopy (TEM) observations were carried out

Figure 3. Possible polytype transformation pathways on the 4HSiC(0001) Si face. 3210



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4H-SiC changes to (ABCAB′A′). Similarly, at the SF labeled “(2)”, one bilayer (C′) is inserted into 4H-SiC (-B′A′CA-) and the stacking sequence changes to (B′A′C′BC). Such a disturbance in the stacking sequence, involving the introduction of SFs just before the polytype transformation, was often observed, which indicates that the SFs are a trigger for the polytype transformation. The SFs are possibly introduced during spiral growth by 2D nucleation on the spiral steps. Figure 6 shows an AFM image taken from the area where the polytype transformation from 4H-SiC to 6H-SiC occurred during growth. 2D nucleation of a single bilayer is observed on a three bilayer double spiral structure. If these 2D nuclei disturb the stacking sequence, SFs are expected to be introduced.

5. DISCUSSION Vand first suggested that 2D nucleation on spiral steps leads to the formation of SFs, and the SFs on the original growth layer affect the polytype structure.25,26 Subsequently, Pandey and Krishna showed a model for the growth of an anomalous polytype structure by the introduction of SFs near the surface at the time of origin of a screw dislocation ledge during PVT growth of SiC.27 Sanchez et al. also reported that SF formation could be attributed to 2D nucleation during the initial stages of PVT growth.28 Here, we consider the 2D nucleation of a single bilayer on four bilayers with bunched steps as schematically illustrated in Figure 7. The possible stacking sequence of the single layer on the step is A or A′. If a single layer of A is nucleated on the step (Figure 7a), nothing happens. However, if a single layer of A′ is nucleated on the step (Figure 7b), two kinds of SFs may possibly be formed. If the spiral step overgrows the single layer, an extrinsic Frank type SF is formed and the stacking sequence changes to -ABC′B′-A′-CAB′A′-. On the other hand, if the spiral step does not overgrow the single layer, a Shockley type SF is formed and the stacking sequence changes to -ABC′B′A′CA′C′-. However, a SF containing the 2H structure (described as -A′CA′-, for example) is known to be thermally unstable.27,29,30 Thus, Shockley type SFs are expected to be rarely formed. Therefore, 2D nucleation of a single bilayer on a 4H-SiC spiral step generally results in an extrinsic Frank type SF. An important point to note is that a stacking sequence of this type is identical to that of 15R-SiC. If the stacking sequences of the SF are replicated by spiral growth, a polytype transformation from 4H-SiC to 15R-SiC occurs. However, a polytype transformation does not always occur when SFs are formed during spiral growth. Whether a polytype transformation occurs or not is determined by the position where 2D nucleation of the single bilayer occurs. Figure 8 schematically illustrates 2D nucleation on a 4H-SiC spiral step during growth. If nucleation of single bilayers occurs at the middle of the spiral steps, as shown in Figure 8a, an SF is formed. However, the stacking sequence is not replicated because the SF becomes covered with 4H-SiC by the grown spiral step above the SF. On the other hand, if nucleation of single bilayers occurs at the front of the spiral, as shown in Figure 8b, a polytype transformation from 4H-SiC to 15R-SiC occurs. Similarly, a polytype transformation from 15R-SiC to 6H-SiC can be explained by the 2D nucleation of a single bilayer. Four types of SF can be considered to form by 2D nucleation of a single bilayer on a 15R-SiC spiral step, as schematically illustrated in Figure 9. Among them, the Shockley type SF shown in Figure 9b is expected to be rarely formed because the

Figure 4. Nomarski image obtained from the crystal grown for 1 h (a). Two different types of hillocks were observed in areas identified as 4HSiC by micro-Raman mapping (b, c). Only regular hexagonal hillocks were observed in areas identified as 6H-SiC (d). A different hexagonal shape of the hillocks was observed in the areas of 15R-SiC (e).

types of hillocks were observed as shown in Figure 4b,c. One has a regular hexagonal shape (Figure 4b), while the other has a regular triangular shape (Figure 4c). In areas identified as 6HSiC, only regular hexagonal hillocks were observed (Figure 4d). In the 15R-SiC areas, different hexagonal hillocks were observed as shown in Figure 4e. A bright-field (BF) TEM image of the 4H/15R interface taken along the [112̅0] direction is shown in Figure 5a. Selected area electron diffraction (SAED) patterns taken from the upper and lower parts (the insets in the upper left and lower left, respectively) clearly indicate that a polytype transformation from 4H-SiC to 15R-SiC occurs. While SFs were scarcely observed in the grown 4H-SiC, a number of stripes indicating the existence of SFs were observed in the 15R-SiC. Figure 5b shows a HREM image of the 4H/15R interface. The stacking sequence of 4H-SiC (-ABC′B′-) changes to that of 15R-SiC (-ABCA′C′-BCAB′A′- CABC′B′-). Just below the interface, a disturbance in the stacking sequence is observed, as shown in Figure 5b. Two Frank type SFs, indicated by arrows labeled “(1)” and “(2)”, are introduced into the 4H-SiC. At the SF labeled “(1)”, two bilayers (CA) are inserted into 4H-SiC (-ABC′B′-) and the stacking sequence of 3211



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Figure 5. Bright-field TEM image of the 4H/15R interface taken along the [112̅0] direction and corresponding selected area electron diffraction patterns taken from the upper and lower parts (a). High resolution electron microscopy image of the 4H/15R interface taken along the [112̅0] direction. The positions of SFs are indicated by arrows (b).

Figure 6. AFM image obtained from an area identified as 6H-SiC in the crystal grown for 1 h. Single bilayer 2D nucleation sites are indicated by arrows. Figure 7. Schematic illustration of possible SFs in 4H-SiC formed by 2D nucleation on steps during spiral growth. If a single layer of A is nucleated on the step, it is incorporated in the grown step (a). If a single layer of A′ is nucleated on the step, extrinsic Frank type SFs or Shockley type SFs are formed (b).

SF contains the 2H structure. The other Shockley type SF results in the formation of a different 15R-SiC stacking sequence (Figure 9a). The stacking sequence of the two different extrinsic Frank type SFs is identical to that in 6H-SiC (Figure 9a) and 18R-SiC (Figure 9b). Thus, if 6H-SiC type SFs are replicated by spiral growth, a polytype transformation from 15R-SiC to 6H-SiC occurs. Furthermore, a polytype transformation from 4H-SiC to 6H-SiC can occur when successive SFs are introduced by multiple 2D nucleation steps. In this polytype transformation model, a polytype transformation from 4H-SiC to 15R- or 6H-SiC is possible; however, polytype transformations involving a decrease in the number of stacking layers, such as from 15R-SiC and 6H-SiC to 4H-SiC, do not occur. This is in good agreement with the observed polytype transformation pathways during solution growth, as shown in Figure 3.

Although in many cases 2D nucleation on spiral steps results in the formation of SFs, if it occasionally takes place near the front of the spiral, a polytype transformation occurs. In that case, several 2D nuclei are often formed near the front of the spiral. Thus, a disturbance in the stacking sequence with the introduction of multiple SFs was often observed immediately before polytype transformation, as shown in Figure 5.

6. CONCLUSION Polytype transformations on the 4H-SiC(0001) Si face during top-seeded solution growth have been investigated by transmission electron microscopy and micro-Raman spectroscopy. 3212



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growth can be understood in terms of 2D nucleation of a single bilayer on the spiral steps. Although in many cases, 2D nucleation on the spiral steps results in the formation of SFs, if it occasionally takes place near the front of the spiral, a polytype transformation occurs by the replication of SFs. Thus, the disturbance in the stacking sequence with the introduction of SFs was often observed immediately before polytype transformation. Furthermore, this polytype transformation model provides good agreement with observed polytype transformation pathways.

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Figure 8. Schematic illustration of 2D nucleation on the spiral terrace during spiral growth. If the nucleation of a single bilayer occurs at the middle of the spiral step, a SF is formed; however the stacking sequence is not replicated (a). If the nucleation of one bilayer takes place at the front of the spiral, the stacking sequence of the SF is replicated and a polytype transformation from 4H-SiC to 15R-SiC occurs (b).

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by the Novel Semiconductor Power Electronics Project Realizing Low Carbon-Emission Society of the Ministry of Economy, Trade and Industry through an R&D Partnership for Future Power Electronics Technology (FUPET) and the Ministry of Education, Culture, Sports, Science and Technology Program for Fostering Innovation (Global Type) “Tokai Region Nanotechnology Manufacturing Cluster”. The authors acknowledge fruitful discussions with Dr. Kazuhisa Kurashige (FUPET, Hitachi Chemical Co., Ltd.), Dr. Tomohisa Kato (FUPET, Advanced Power Electronics Research Center (APERC), National Institute of Advanced Industrial Science and Technology (AIST)), Prof. Yuji Matsumoto (Tokyo Institute of Technology) and their co-workers. The authors acknowledge the assistance of Mr. Yoshida with AFM measurements.

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REFERENCES

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Figure 9. Schematic illustration of possible SFs in 15R-SiC formed by 2D nucleation on steps during spiral growth. Considering 2D nucleation on 15R-SiC spiral steps, two configurations are possible as shown in (a) and (b). Two extrinsic Frank type SFs and two Shockley type SFs may then be formed.

4H-, 15R-, and 6H-SiC were grown on the 4H-SiC(0001) Si face via spiral growth. Once a polytype transformation from 4H-SiC to 15R- or 6H-SiC occurs, the polytype rarely returns to 4H-SiC. Just before the polytype transformation, a disturbance in the stacking sequence involving the introduction of SFs was observed. A polytype transformation during spiral 3213



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Polytype Transformation by Replication of Stacking Faults Formed by

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