High-Temperature Chemical Vapor Deposition for SiC Single Crystal

Article pubs.acs.org/crystal

High-Temperature Chemical Vapor Deposition for SiC Single Crystal Bulk Growth Using Tetramethylsilane as a Precursor Deok-Hui Nam,†,‡ Byeong Geun Kim,† Ji-Young Yoon,†,§ Myung-Hyun Lee,† Won-Seon Seo,† Seong-Min Jeong,*,† Cheol-Woong Yang,‡ and Won-Jae Lee∥ †

Energy and Environmental Division, Korea Institute of Ceramic Engineering and Technology, 233-5 Gasan-dong, Guemcheon-gu, Seoul 153-801, Korea ‡ School of Advanced Materials Science and Engineering, Sungkyunkwan University, Gyeonggi-do 440-746, Korea § Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea ∥ Department of Materials and Component Engineering, Dong-Eui University, Busan 614-714, Korea ABSTRACT: SiC single bulk crystals were grown using a high-temperature chemical vapor deposition (HTCVD) method, with SiH4 and hydrocarbons as the source materials. SiH4 is a pyrophoric gas, which frequently causes fatal accidents in experiments. In this study, therefore, we propose the use of a HTCVD method using tetramethylsilane (TMS), a cheap and safe precursor, for growing SiC bulk crystals. Although TMS contains C four times more than Si in its chemical formula, a stoichiometric SiC layer was successfully synthesized from TMS in the presence of a high concentration of H2 based on the thermodynamic process design. 6H-SiC single crystals were successfully grown on a 6H-SiC seed crystal using the same process conditions. The resulting single crystalline layer was evaluated by rocking curve analysis by X-ray diffraction, which showed that the crystal properties of the grown SiC layer were improved compared to the seed crystals. This suggests that the TMS-based HTCVD method is feasible for practical use in SiC bulk growth.

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INTRODUCTION

the facilities used in the HTCVD method cannot be as simple as those of the PVT method. Therefore, a new safer and cheap precursor would be useful for the HTCVD method as the next generation technique for producing bulk SiC crystals. To simplify the growing process, a single source containing both Si and C in its chemical formula could be considered first. There were several candidate precursors that meet this requirement, such as tetramethylsilane (TMS, Si(CH3)4), diethylsilane (DES, (C2H5)2SiH2), tripropylsilane (TPS, (CH3)3SiH), triethylsilane (TES, (CH3)3SiH), methyltrichlorosilane (MTS, (CH3)SiCl3), and more.5−11 Although these precursors have essentially not been used as a source for producing SiC bulk crystals, they have been used as the sources for producing SiC powder or thin films at relatively low temperatures.5−11 Especially, TMS is a cheap and safe precursor even though it contains much more C than Si in its chemical formula. At temperatures in excess of ∼1300 °C, high C/Si ratio of TMS makes it difficult to form pure SiC deposits but bring about the formation of unwanted C in addition to SiC.6 This is why TMS has been used at relatively low temperatures, to synthesize β-SiC (3C-SiC) films.5−7 TMS had not been used as the precursor for preparing α-SiC (4H/6H-

As an alternative method for growing SiC bulk crystals, hightemperature chemical vapor deposition (HTCVD) methods were introduced and have been in use for the last two decades.1−4 While the commercialized bulk SiC growing technique, namely, the physical vapor transport (PVT) method, uses SiC powder as a source in a closed system, the HTCVD method uses gas phase precursors, which are continuously fed into the reactor. The HTCVD method, therefore, involves the continuous feeding of the source material and is free from the fundamental issues associated with the use of close systems, such as a shortage of the source powder during growing experiments. Furthermore, it is possible to produce high quality SiC crystals using the HTCVD method as demonstrated in the literature,2 which makes this technique interesting as an alternative method for manufacturing bulk SiC crystals. Nearly all of the HTCVD methods reported to date have used dual sources, SiH4 as the Si source and a hydrocarbon, such as CH4 or C3H8, as the C source.1−4 The HTCVD method, which uses dual sources, is advantageous, in that the stoichiometry of the deposits can be controlled more readily. However, this process is risky because the Si source, SiH4, is flammable and explosive. Therefore, an explosion-proof local exhaust system with a sufficient flow velocity is necessary to prevent experimental accidents. That is one of the reasons why © 2014 American Chemical Society

Received: June 5, 2014 Revised: July 31, 2014 Published: September 19, 2014 5569

dx.doi.org/10.1021/cg5008186 | Cryst. Growth Des. 2014, 14, 5569−5574

Crystal Growth & Design

Article

Figure 1. Thermodynamic stability of vapors and solids in the TMS-based HTCVD process (a) at Si/H = 7.14 × 10−2, 0.001 atm, and (b) at Si/H ratio = 4.97 × 10−4, 1 atm (dashed lines, vapor species; solid lines, solid species).

SiC), which could be used as the substrate in power electronic or light emitting devices (LED). In our previous study, however, TMS was proposed as a source for α-SiC, via thermodynamic calculations.12 The synthesis of stoichiometric α-SiC particles was then experimentally demonstrated via the HTCVD method using TMS as a precursor.13 In this work, in an extension of a previous work, we examined the feasibility of TMS as a source for SiC bulk crystal through the HTCVD method. First, we provided process modeling for the TMSbased HTCVD method based on thermodynamics as functions of Si/H ratio, temperature, and working pressure. The theoretically determined growth conditions were then validated in experiments in which single 6H-SiC crystals were grown on SiC seed crystals.

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THERMODYNAMIC MODELING Depending on temperature, the growth of SiC is determined by kinetics, mass transfer, and thermodynamics.14 At a typical growth temperature for HTCVD (usually above 1700 °C1−4), both the mass transfer and surface reaction kinetics are sufficiently fast, where the growth of SiC is mainly determined by thermodynamics.14 Therefore, thermodynamic modeling is especially effective in terms of analyzing the deposition behavior under specified process conditions at such high temperatures. To determine the process conditions for SiC synthesis, thermodynamic calculations were conducted using the FactSage software program in conjunction with the FactPS database,15 where the chemical species of H, H2, C, C2, C3, C4, C5, CH, CH2, CH3, CH4, C2H, C2H2, C2H3, C2H4, C2H6, Si, Si2, Si3, SiH, SiH4, Si2H6, SiC, Si2C, SiC2, Si(CH3)4 were considered for calculating thermodynamically stable phases under various conditions. Figure 1a,b represents the calculated thermodynamic equilibria among the chemical species in the temperature range from 1000 to 3000 °C in the two extreme conditions: a high Si/H ratio, a low working pressure of Figure 1a, and a low Si/H ratio, a high working pressure of Figure 1b. Most chemical species, including solid C and many vapors, show quite different behaviors in the two extreme conditions, which revealed their strong dependencies on the Si/H ratio and the

Figure 2. Evolution of maximum temperature of solid SiC and solid C as a function of Si/H ratio and working pressure calculated by thermodynamic modeling: (a) three-dimensional grid surface plot, (b) the effect of Si/H ratio, and (c) the effect of the working pressure.

working pressure. Exceptionally, solid SiC showed almost identical thermodynamic behaviors in Figure 1a,b, indicating that SiC is relatively less sensitive to process conditions. As represented in Figure 1a,b, it is possible to classify temperature ranges based on existing solid phases. As shown in Figure 1a, there are three distinguished regions: a mixed solid phase region at low temperature, a C region at mid temperature, and a no deposit region at high temperature. When a high Si/H ratio and low working pressure are used, pure SiC cannot be formed at any temperature. In the case of a low Si/H ratio and a high working pressure, as shown in Figure 1b, however, a pure SiC region exists in the temperatures between ∼1800 °C to ∼2100 °C. To quantitatively evaluate the dependencies of the thermodynamic stabilities of solid phases on the Si/H ratio 5570



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Figure 3. (a) Optical images of SiC crystal synthesized at 2000 °C (working pressure = 0.724 atm; Si/H ratio = 6.24 × 10−4); (b) XRD spectra showing characteristic peaks of 4H-, 6H-, and 15R-SiC; and (c) Raman spectra obtained at various points showing characteristic peaks of 4H-, 6H-, and 15R-SiC. water-cooled quartz tube. The reactions occurred inside the graphite crucible that was thermally insulated with graphite felt. The total pressure in the reactor was set to 0.724 atm as high as possible considering the limitations of the equipment. To produce pure SiC in the deposits, TMS was extensively diluted with H2 to achieve a low Si/ H ratio. The TMS source in the bubbler was transported to the reactor with H2 carrier gas, and additional H2 gas was allowed to flow into the reactor to satisfy the required Si/H ratio. The growth of SiC was carried out with or without SiC seeds on the graphite lid as a susceptor. The seed crystal was a N2 doped on-axis SiC wafer grown by the PVT method. The target temperature was measured by an optical pyrometer through a hole at the bottom of the crucible. The deposits after each experiment were analyzed by various methods including optical microscopy, scanning electron microscopy (SEM, JSM-5600, JEOL, Japan), high-resolution X-ray diffraction (HRXRD, X’pert PRO MED, PANalytical, The Netherlands), and micro-Raman spectroscopy (NRS-3100, JASCO, Japan)

and working pressure, the maximum stable temperatures of solid SiC and solid C were calculated as functions of the Si/H ratio (0.0001−0.1) and working pressure (0.001−1 atm). The 3-dimensional grid surfaces shown in Figure 2a represent the maximum stable temperatures for solid SiC and solid C, respectively. Because pure SiC could be synthesized only when the maximum temperature of solid SiC is above that of solid C, as shown in Figure 1b, the process parameters should be in the region where the grid surface for solid SiC is above the grid surface of solid C. To provide better readability of the effects of the Si/H ratio and the working pressure, the 2-dimensional projections of Figure 2a on the XZ plane and YZ planes were plotted as Figure 2b,c, which show the effects of Si/H ratio and the effect of working pressure, respectively. From Figure 2b,c, it can clearly be seen that a pure SiC region is formed only at high working pressures and low Si/H ratios.

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RESULTS AND DISCUSSION As shown in Figure 3a, green colored deposits were grown on the graphite lid at 2000 °C within 1 h without a SiC seed. The Si/H ratio and the working pressure were 6.24 × 10−4 and 0.724 atm, respectively. Some deposits were found to have been delaminated and were deposited on the bottom of the reactor.

EXPERIMENTAL SECTION

On the basis of the thermodynamic modeling described above, the growth of SiC was experimentally carried out in a vertical type hot-wall reactor using TMS as the precursor and H2 as the carrier gas. The HTCVD reactor was equipped with an induction heating system and a 5571



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Figure 4. (a) Schematic of the experimental setup used to fix a small rectangular SiC seed on a graphite lid with a graphite cap. (b) Optical images of deposits on the graphite cap and seed at 2000 °C for 1 h (working pressure = 0.724 atm; Si/H ratio = 6.24 × 10−4).

Figure 7. Raman spectra of selected positions before and after growth.

Figure 5. Growth rate as a function of Si/H ratio.

Figure 6. Optical images of a broken SiC wafer with many polytypes before and after growth at 2000 °C for 1 h (working pressure = 0.724 atm; Si/H ratio = 6.24 × 10−4). Figure 8. (a) Schematic of experimental setup to fixed 2 in. full-scale 6H-SiC seed on a graphite lid with a graphite cap. (b) Obtained crystal after growing experiment at 2000 °C (working pressure = 0.724 atm; Si/H ratio = 6.24 × 10−4). (c) HRXRD data showing 6H-SiC single crystal. (d) Rocking curve analysis obtained with HRXRD.

The inserted image of Figure 3a is an optical microscopic image showing the particles of the green deposits. In our previous study, while large amounts of 3C-SiC were formed at temperatures lower than 1800 °C, α-SiC became abundant with increasing temperatures.13 In the processing conditions of the current study, the deposits were characterized as a mixture of 4H-, 6H-, and 15R-SiC by XRD, as shown in Figure 3b, which was consistent with results reported in a previous study.13 Compared to XRD, micro-Raman spectroscopy provides a much better spatial resolution of ∼1 μm, which allows individual particles to be characterized. For some selected areas, three representative Raman spectra of 4H-, 6H-, and 15R-SiC were obtained, as shown in Figure 3c, where the characteristic spectra of the SiC polytypes were clearly distinguished from each other. Support for this conclusion can be found in Raman spectra reported in the literature.16,17 The mixed polytypes in the deposits were assumed due to the small

differences in the standard enthalpies of formation of 4H-, 6H-, and 15R-SiC, which were reported as 15.9, 15.6, and 15.7 kcal/ mol, respectively.18 Figure 4a represents a schematic setup used for growing a rectangular, on-axis 6H-SiC seed with dimensions of 10 × 10 mm. The SiC seed was mechanically fixed on the graphite lid screwed with a graphite cap. The graphite cap had a 10 mm diameter hole in the center. The deposited graphite cap and the seed after a 1 h growth period are shown in Figure 4b. The mechanical seeding technique used here, however, cannot be considered to be a good production technique because the step 5572



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improve crystal quality by reducing the crystal mosaicity, as occurs in the classical HTCVD method,2 which is advantageous to improve SiC crystal quality prepared by PVT method. TMSbased HTCVD process should be further improved to demonstrate a wafer scale SiC single crystal by adjusting the reactor design and the process conditions, which are the issues of our future studies.

between the graphite cap and the seed inevitably cause a discontinuity in the gas flow and temperature distribution, which influence the crystal quality of the grown layer. However, because the objective of the current study was only to demonstrate the feasibility of TMS-based HTCVD for bulk SiC growth, the mechanical seeding technique was considered to be sufficient and even effective for reducing the process cost and time for seeding process. The growth rates of the proposed TMS-based HTCVD shown in Figure 5 were measured in cross-sectional scanning probe microscopy (SEM) images. The flow rates almost proportionally increase with increasing flow rate of the TMS precursor. The maximum growth rate among the experimental conditions was obtained as 64.84 μm/h for an Si/H ratio of 1.25 × 10−3, but the resulting layer was polycrystalline. The maximum growth rate with single crystalline growth was found to be 33.12 μm/h at a Si/H ratio of 6.24 × 10−4. Polycrystalline crystal growth is determined by a variety of reasons, which are not clearly explained with our experimental results at this time. The objective of the current study was to demonstrate the feasibility of TMS-based HTCVD for the growth of bulk SiC crystals. Therefore, the issue of polycrystalline formation at high growth rates remains to be addressed in a future study. The Si/H ratio was fixed at 6.24 × 10−4 in subsequent experiments, in which stable single crystal growth was observed. The mechanical seeding technique was also applied to the growth of SiC on a seed crystal containing many polytypes, as shown in Figure 6. From the optical image of the SiC wafer, the polytypes of 4H-, 6H-, and 15R-SiC could be clearly identified in yellow, dark green, and light green colors, as confirmed by micro-Raman spectroscopy. From the Raman spectra obtained at several selected points before and after the growing experiments are shown in Figure 7. Homoepitaxial growth was observed in the 4H- and 6H-SiC regions, where the polytype of the SiC grown layer matched the polytype of the seed. Polytype transition was observed only in the 15R-SiC region, which transformed to 6H-SiC. Because the formation enthalpy of 15R-SiC shows small deviation of 0.1 kcal/mol with 6H-SiC,18 15R-SiC was assumed to be a metastable phase in the given thermodynamic conditions and was then transformed to 6H-SiC, resulting in a more stable state. However, no polytype transition was observed in 15R-4H and 6H-4H with formation enthalpy differences larger than that of 15R-6H. Therefore, we propose that TMS-based HTCVD provides a stable process for the growth of 4H- and 6H-SiC crystals without polytype transition. Finally, to demonstrate the feasibility of the proposed HTCVD method for use in SiC bulk growth, growth experiments were carried out using 2 in. full-scale on-axis 6HSiC wafers as seeds. In this experiment, the hole size in the graphite cap was 30 mm in diameter, as shown in Figure 8a, which resulted in the formation of a SiC layer with a diameter of 30 mm. The SiC grown layer was characterized by highresolution X-ray diffraction (HRXRD), which confirmed the grown layer to be a single, 6H-SiC crystal that was identical to the seed crystal. A rocking curve analysis with HRXRD provided information about the crystal qualities based on full width at half-maximum (fwhm) values as shown in Figure 8d. Although the TMS-based HTCVD method described in this study is not completely optimized at this time, the layer produced using this technique showed a smaller fwhm value of 89 arcsec compared to that for the seed crystal (122 arcsec). This suggests that a TMS-based HTCVD process could

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CONCLUSIONS Thermodynamic modeling was effective for designing a novel SiC technique for crystal growth, namely, the TMS-based HTCVD method. It is interesting to note that single crystals composed of 6H-SiC could be produced using a safe and cheap liquid source, TMS, in spite of the large amount of C in its chemical formula. From a growth test on a 6H-SiC seed, a good quality, single crystal was produced, suggesting that the TMSbased HTCVD would be feasible for use in commercial production.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a grant from the World Premiere Materials (WPM) funded by Ministry of Trade, Industry and Energy (MOTIE), Korea. The authors wish to thank all the colleagues in Dong-Eui University who provided the seeds used in this study. The authors are grateful to Dr. M.C. Chun (POSCO), Dr. T.-H. Eun (Pohang Research Institute of Science and Technology), and Prof. I.-H. Jung (McGill University, Canada) for extensive discussions regarding this study.

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REFERENCES

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High-Temperature Chemical Vapor Deposition for SiC Single Crystal

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