Thermodynamic approach to the synthesis of silicon carbide using tetramethylsilane as the precursor at high temperature

Journal of Crystal Growth 357 (2012) 48–52

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Thermodynamic approach to the synthesis of silicon carbide using tetramethylsilane as the precursor at high temperature Seong-Min Jeong a, Kyung-Hun Kim a, Young Joon Yoon b, Myung-Hyun Lee c, Won-Seon Seo c,n a

Business Support Division, Korea Institute of Ceramic Engineering and Technology (KICET), Seoul 153-801, Republic of Korea Future Convergence Ceramics Division, Korea Institute of Ceramic Engineering and Technology (KICET), Seoul 153-801, Republic of Korea c Green Ceramics Division, Korea Institute of Ceramic Engineering and Technology (KICET), 233-5, Gasan-dong, Geumcheon-gu, Seoul 153-801, Republic of Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 May 2012 Received in revised form 18 June 2012 Accepted 19 July 2012 Communicated by R. Kern Available online 3 August 2012

Tetramethylsilane (TMS) is commonly used as a precursor in the production of SiC(b) films at relatively low temperatures. However, because TMS contains much more C than Si, it is difficult to produce solid phase SiC at high temperatures. In an attempt to develop a more efficient TMS-based SiC(a) process, computational thermodynamic simulations were performed under various temperatures, working pressures and TMS/H2 ratios. The findings indicate that each solid phase has a different dependency on the H2 concentration. Consequently, a high H2 concentration results in the formation of a single, solid phase SiC region at high temperatures. Finally, TMS appears to be useful as a precursor for the high temperature production of SiC(a). & 2012 Elsevier B.V. All rights reserved.

Keywords: A1. Computational thermodynamics A2. High temperature chemical vapor deposition B1. SiC B1. Tetramethylsilane

1. Introduction Because silicon carbide (SiC) is thermodynamically stable and has good mechanical properties even at high temperatures, it is widely used in various industrial applications [1–3]. The advantageous material properties of SiC also include a wide band gap, a high dielectric breakdown field and a high thermal conductivity. Consequently, SiC is considered to be a promising material for electronic and optoelectronic applications, especially for LED and power devices [4]. SiC does not melt at atmospheric pressure but can be sublimed at very high temperatures of  2700 1C. A previous study reported that a stoichiometric melt of SiC was only observed at pressures exceeding 10 atm and temperatures of 3200 1C [5]. Therefore, SiC is usually synthesized from the gas phase, even though there also exist several liquid phase epitaxy techniques available that use incongruent melts as starting materials. However, using a melt of SiC is currently not feasible due to its high cost. At present, the most successful commercialized method for producing single SiC crystal growth is the physical vapor transport (PVT) technique which is based on sublimation starting from powdered SiC. It is also possible to prepare SiC by chemical vapor deposition (CVD) using various vaporized or liquid precursors [6–14].

n

Corresponding author. Tel.: þ82 2 3282 2496; fax: þ 82 2 3282 2470. E-mail address: [email protected] (W.-S. Seo).

0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.034

Epitaxial films or powdered SiC can be produced by metal organic chemical vapor deposition (MOCVD) from various liquid precursor such as tetramethylsilane (TMS), diethylsilane (DES) and tripropylsilane (TPS) [6–8]. The most common processes used to produce SiC thin films are a high temperature chemical vapor deposition (HTCVD) technique starting from a hydrocarbon and silane, diluted in hydrogen, performed at high temperature (1600–2000 1C) and reduced pressure (100–500 mbar) [9–11]. HTCVD performed at even higher temperatures in the range of 2100–2300 1C was proposed as an alternative technique for producing bulk SiC crystal growth [12–14]. Meanwhile, TMS is assumed as a safe precursor that is not explosive like SiH4 nor does it produce hazardous gases like SiCl4. Hence, TMS has been widely used as a precursor in the deposition of SiC films at relatively low temperatures, in which a SiC(b) phase is formed [7,8]. Moreover, TMS has been used to produce bulk SiC(b) single crystals by means of a continuous feed physical vapor transport (CF-PVT) technique, in which a solid SiC phase is dynamically generated from TMS in a CVD reactor and then sublimed in a PVT chamber [15,16]. However, a hexagonal polytype of SiC, SiC(a), is preferred to tetragonal polytypes of SiC, SiC(b), in commercialized applications including single crystalline substrates for LED/power electronic devices because it exhibits a high breakdown field and a high thermal conductivity. However, TMS has not been used to produce SiC(a), such as 4 H/ 6 H-SiC. The reason why TMS is not used for SiC(a) is probably because higher temperatures of over  2100 1C are required to

S.-M. Jeong et al. / Journal of Crystal Growth 357 (2012) 48–52

produce SiC(a), where the excess C concentration in TMS hinders the formation of a single SiC phase without secondary solid phases. Meanwhile, due to recent progress in the field of computational thermodynamics, it is now possible to easily determine the chemical conditions, including temperature, working pressure and the relative amount of chemical species needed in a chemical process. A thermodynamic analysis of the equilibrium gas composition not only provides information related to the chemical reactions but also the thermodynamic yields and the formation of intermediate species [17]. TMS has been used with H2, as the carrier gas, for the deposition of SiC at relatively low temperatures, so that thermodynamic calculations in this area have been limited to the relatively narrow ranges [18–20]. The focus of the previous studies was mainly on the region where SiC(b) is formed. Since an increase in temperature increases the probability of SiC(a) undergoing nucleation [21], it is necessary to expand the thermodynamic study to the high temperature range when TMS is being considered as a precursor for SiC(a). Therefore, in the present study, thermodynamic calculations were carried out to find process conditions for forming SiC at high temperatures where the SiC(a) phase is stable. We explored various process conditions for forming a solid SiC single phase by varying the temperature, working pressure and TMS/H2 ratio to find an alternative procedure for producing SiC from TMS.

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equilibrium components at temperatures over 1500 K. Si(CH3)4 is completely discomposed at 1000 K while CH4 is a major species at temperatures less than  1300 K. Si2C is the dominant species contributing to the formation of Si over wide temperature range, while SiC2 becomes meaningful only at high temperatures. Significant changes in the species that contain C atoms were found with changes in temperature: C2H2, Si2C and SiC2 were dominant at high temperatures, while CH4, Si2C and C2H4 were dominant at low temperature. Next, the phase equilibria shown in Fig. 1 were recalculated including solid phases from 500 K to 2800 K, and the solid phase composition is shown in Fig. 2(a). Since only very small Gibbs free energy difference of less than 0.5 Kcal/mol between SiC(b) and SiC(a) was found [23], the phase transformation between SiC(b) and SiC(a) is influenced by a combination of many effects, including seed polarity, impurities, grain boundary, temperature, pressure, Si/C ratio, the presence of dislocations and so on. The present study does not cover those effects, but concentrates on thermodynamic calculations concerning the process conditions of temperature, partial pressures of gases and working pressure. Therefore, both the SiC(b) and SiC(a) phases are considered to be the same in the calculation, since SiC(b) is generally assumed to be most stable in the nucleation stage, as evidenced by experimental thermodynamic measurements [23]. The Fact53 database in FactSageTM software contains thermodynamic data for C, Si, Si2H6 and SiC as solid phases [22],

2. Thermodynamic calculations and discussion To carry out the thermodynamic calculations in the present study, we adopted the software FactSageTM with Fact53 database [22], 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 the thermodynamic stable phases under various conditions. Early thermodynamic analyses were based on calculations using a database that contained only limited numbers of chemical species. Hence, to verify the thermodynamic calculations, we first performed calculations that were identical to those reported in a previous study [19] which is for TMS:H2 ¼1:1 at a working pressure of 10 Torr without considering the formation of any solid phases. Fig. 1 represents the thermodynamic equilibria among the vapor phases from 500 K to 2000 K, and the results were in good agreement with data reported in previous studies [19]. It was found that H2, C2H2, Si2C and SiC2 are major

Fig. 1. Equilibrium compositions of vapor phases as a function of temperature in the range of 500–2000 K for TMS:H2 ¼ 1:1 at a working pressure of 10 Torr.

Fig. 2. Equilibrium compositions of the solid phases as a function of temperature at a working pressure of 10 Torr (a) when the ratio TMS to H2 is 1 and (b) 0.001, respectively.

Mole fraction of SiC in the solid phases

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1.0

TMS: H2 = 1:1000 TMS: H2 = 1:10 TMS: H2 = 1:0 (No H2)

TMS: H2 = 1:100 TMS: H2 = 1:1

0.8

0.6

0.4

0.2

0.0 500

1000

1500

2000

2500

Temperature (K) Fig. 3. Solid phase compositions as a function of temperature at a working pressure of 10 Torr for the ratio TMS to H2 of 1 (TMS:H2 ¼ 1:1), 0.1 (TMS:H2 ¼ 1:10), 0.01 (TMS:H2 ¼1:100) and 0.001 (TMS:H2 ¼ 1:1000).

Fig. 4. Equilibrium compositions of the solid phases as a function of temperature at a working pressure of 500 Torr (a) when the ratio TMS to H2 is 1 and (b) 0.001, respectively.

single SiC regions at high temperature range

Mole fraction of SiC in the solid phases

however, only C and SiC are the thermodynamically stable solid phases at the conditions investigated in the temperature range of 500–2800 K, a working pressure of 10 Torr and a TMS/H2 ratio of 1 (TMS:H2 ¼1:1). The results show that a C-rich deposit will occur over wide range. This result is reasonable, since TMS is chemically composed of one Si atom with four CH3 groups bonded to it, which provides the following C-containing species: CH4, C2H4, C2H2, Si2C and SiC2. Therefore, various working pressures and TMS/H2 ratios were evaluated to eliminate the C phase. Fig. 2(b) shows the effect of the TMS/H2 ratio for the same conditions shown in Fig. 2(a), except the TMS/H2 ratio was changed from 1 (TMS:H2 ¼1:1) to 0.001 (TMS:H2 ¼1:1000). In both cases of Fig. 2(a) and (b), the estimated composition of solid phases is still dominated by large amounts of C and small amounts of SiC over a wide temperature range. If the dilution of H2 is increased, the upper limits for the deposition of C and SiC are shifted to lower temperatures. Fig. 3 shows the calculation results for the mole fraction of SiC in the solid phase as a function of temperature. This result is in good agreement with data reported in a previous study [19], in which thermodynamic calculations were performed in a limited temperature range up to 2000 K. The simulation results show that a pure SiC solid phase is produced, not at high temperatures, but at low temperatures of less than 1000 K. The mole fraction of SiC decreases to 0.25 which means a C/SiC ratio of 3. At this condition, gaseous TMS is completely decomposed to solid C and solid SiC. Since solid SiC is decomposed at a lower temperature than solid C in all cases, except for high concentrations of H2 (TMS:H2 ¼1:1000), the mole fraction of SiC decreases to 0 in the high temperature region from 0.25 in the intermediate temperature range around 1500 K. However, under high concentrations of H2 (TMS:H2 ¼1:1000), the mole fraction of SiC increases from 0.25 at high temperatures and approaches a local maximum value. This indicates that the temperature needed for C etching by hydrogen could be reduced to be lower than that of SiC under a high dilution of H2. If we control the temperature of C etching to be sufficiently low, the deposition of a single SiC phase could be obtained at high temperature ranges. An increase in temperature increases the probability of forming SiC(a), as theoretically explained in a previous study [21]. Since the aim of the present study was to evaluate methodology for producing TMS-based SiC(a), it is necessary to find a process condition that permits the formation of a single SiC solid phase at high temperature. Fig. 4(a) and (b) show the calculation results

1.0

0.8

0.6

0.4

0.2

TMS: H2 = 1:1000 TMS: H2 = 1:10 TMS: H2 = 1:0 (No H2)

TMS: H2 = 1:100 TMS: H2 = 1:1

0.0 500

1000

1500

2000

2500

Temperature (K) Fig. 5. Solid phase compositions as a function of temperature at a working pressure of 500 Torr for the ratio TMS to H2 of 1 (TMS:H2 ¼1:1), 0.1 (TMS:H2 ¼ 1:10), 0.01 (TMS:H2 ¼ 1:100) and 0.001 (TMS:H2 ¼ 1:1000).

for TMS/H2 ratios for 1 and 0.001, respectively, which are the same conditions as those used in Fig. 2(a) and (b) except that the working pressure was changed from 10 Torr to 500 Torr. While no

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H2 / TMS ratio range for single SiC phase Mole fraction of SiC in the solid phases Temperature (K)

0

200

400

600

800

1000

2600 2500 2400 2300 2200 1.0 0.8 0.6 2200K 2300K 2400K 2500K 2600K

0.4 0.2 0.0 0

200

400 600 H2 / TMS ratio

800

1000

Fig. 6. Mole fraction of SiC in the solid phases and optimized hydrogen dilution ratio (H2/TMS) range for single SiC phase varying with temperature at working pressure of 500 Torr.

significant change was found in Fig. 4(a) compared to Fig. 2(a), Fig. 4(b) shows that the single solid phase region of SiC forms at high temperatures of over 2100 K. The mole fraction of SiC in the solid phase shown in Fig. 5 represents the effect of working pressure on the deposition of a solid. In all calculations, single SiC regions were formed in the low temperature range under 1400 K. This supports the conclusion that TMS is thermodynamically a good precursor for producing single phase SiC at temperatures below 1400 K. Meanwhile, a single SiC solid phase region at a high temperature over  2000 K is estimated to occur only in cases of TMS/H2 ratios of 0.01 and 0.001. Compared to Fig. 3, Fig. 5 clearly shows that a high pressure makes it possible to form a single SiC solid region at high temperatures. However, the single SiC region is formed in a different temperature range, since it is influenced by the TMS/H2 ratio. Under higher concentrations of H2, a single SiC solid is estimated to form at higher temperatures but in a narrower range. This means that the concentration of H2 has a stronger influence on the deposition of C than on the deposition of SiC. Consequently, the single SiC solid region is determined by the working pressure and the TMS/H2 ratio. Fig. 6 shows the process conditions required for producing a single SiC solid phase, which are dependent on the hydrogen dilution ratio, H2/TMS, in the temperature range from 2200 K to 2600 K. In this case, the working pressure was fixed at 500 Torr, the same as that shown in Fig. 5. A higher temperature led to a narrower range for the hydrogen dilution ratio, which implies that the process becomes more difficult to control. Beside the temperature range for the single SiC region, the hydrogen dilution ratio is also an important determinant of the process condition. Fig. 6 shows the mole fraction of SiC in the solid phases as a function of the hydrogen dilution ratio with an optimized hydrogen dilution ratio for a single SiC phase at various temperatures under a working pressure of 500 Torr. This indicates that a single SiC phase can be formed at temperatures up to  2600 K under certain conditions (hydrogen dilution ratio ¼  100). Since

previous experimental studies showed that a single SiC(a) phase is formed in the temperature range of 2300 K to 2600 K, TMS could be used to deposit a SiC(a) phase under a working pressure of 500 Torr, even though the process range is narrow and would need to be precisely controlled. If the temperature range for the single SiC region is too narrow, increasing the working pressure could be a thermodynamic solution to expanding the temperature range. Considering other hazardous precursors like SiH4 or SiCl4, TMS is an attractive alternative precursor for the SiC(a) HTCVD process if the process conditions are precisely designed and optimized. To apply TMS to a real system, however, it will be necessary to perform additional calculations in which the limitations of the CVD reactor are taken into consideration as well as the localized distribution of chemical species. This is a subject of a future study.

3. Conclusion A thermodynamic approach was performed, in an attempt to identify a CVD process that can be used to produce single SiC solid using TMS at a high temperature range over 2000 K. TMS generally produces C-rich solid deposits over a wide range of processing conditions, including temperature variations, working pressures and TMS/H2 ratios. The thermodynamic calculations proved that the deposition of C is strongly dependent on the H2 concentration used. Consequently, at high temperatures, single SiC solid regions are formed under high pressures and a high dilution ratio for H2, suggesting that TMS is acceptable as a precursor for producing SiC(a).

Acknowledgments This work was financially supported by a grant from the World Premier Materials (WPM) Program funded by Ministry of Knowledge

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