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Thermodynamical calculations on the chemical vapour transport of silicon carbide
Materials Science and Engineering B61 – 62 (1999) 98 – 101
Thermodynamical calculations on the chemical vapour transport of silicon carbide D. Chaussende a,*, Y. Monteil a, P. Aboughe-nze a, C. Brylinski b, J. Bouix a a
Laboratoire des Multimate´riaux et Interfaces, UMR No. 5615, Uni6ersite´ Claude Bernard Lyon I, 69622 Villeurbanne Cedex, France b Laboratoire Central de Recherches, Thomson CSF, Domaine de Corbe6ille, 91404 Orsay Cedex, France
Abstract We have performed thermodynamical calculations in order to oversee the potential of a new method for growing silicon carbide: the chemical vapour transport process. In this way we have been able to select possible transporting agents and to show the nature of the deposits with varying parameters. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Thermodynamical calculations; Chemical vapour transport; Silicon carbide
1. Introduction
2. Calculation details
Silicon carbide is a wide gap semiconductor which exhibits outstanding electronic properties associated with chemical and thermal stability. The SiC-based devices ability to function at high temperature, high power and high radiation conditions will enable large performance enhancements to a wide variety of systems and applications. Obtaining good quality SiC is of great interest and many laboratories are studying SiC growth in different ways. Chemical vapour deposition, molecular beam epitaxy and sublimation process have been reported for epitaxial growth of thin films and bulk SiC respectively. New methods for SiC like liquid phase epitaxy and high temperature CVD have also been reported at the ECSCRM’98 congress. However, to our knowledge, the chemical vapour transport process (CVT) has never be performed for the growth of SiC. Moreover, a comparison between ZnSe growth by seeded CVT (SCVT) with iodine as transporting agent and seeded physical vapour transport (SPVT) has revealed a better crystalline quality for the first method at a lower temperature [1,2]. Thermodynamical calculations have been carried out for different chemical systems in order to select the best transporting agent for silicon carbide and to foresee the nature of the deposits with varying parameters.
The thermodynamic calculations based on the total Gibbs free energy minimisation were obtained from the GEMINI1 computer program from the scientific group Thermodata (S.G.T.E. Saint-Martin d’Heres, FRANCE) and a coherent set of thermodynamical data from this same society. All these calculations were accomplished in isochore conditions. The parameters involved in this process are temperatures of the source and the substrate and initial amount of the transporting agent. First, we have investigated SiC etching at the source zone by calculating the equilibrium partial pressures of the gases. It shows silicon and carbon behaviour with the transporting agent. Secondly, we have examined the solid phases deposited at the other end of the tube, called the substrate zone. For these two types of simulations we have studied the Si–C–X systems where X is an halogen (F, Cl, Br, I). Then, we have studied the Si–C–H–X systems in order to investigate the influence of hydrogen on the CVT process.
* Corresponding author. E-mail address: [email protected] (D. Chaussende)
We have particularly studied the Si–C–Cl system because its thermodynamical data are quite accurate.
3. Results
3.1. Chemical etching of SiC
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D. Chaussende et al. / Materials Science and Engineering B61–62 (1999) 98–101
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Fig. 1. Equilibrium partial pressures of gas species obtained for a initial amount of Cl2 of 10 − 3 mol l − 1.
We can assume that general trends are similar in the halogen column of the periodic table of elements (F, Cl, Br, I). As Fig. 1 shows, silicon only gives gaseous species. All the silicon chlorides are present in different concentrations in the gas phase, from SiCl to SiCl4. Carbon stays in the solid state like graphite. Other authors have already reported this behaviour [3–5]. Extraction of metals from carbides by halogens or their compounds can lead to the formation of free carbon. For SiC, it has been reported that the Si component is preferentially attacked and a carbon rich layer builds up in the reaction with chlorine containing gas. Carbon will give gaseous species only if silicon is entirely con-
sumed, i.e. if halogen is in excess. This situation is not of practical possibility because an excess of halogen involves too high a total pressure in the sealed tube. The same calculations in the system Si– C–Cl–H show interesting results (Fig. 2). Chlorine has the same behaviour with silicon as that in the previous chemical system. However, in this case carbon can lead to gaseous species with hydrogen such as methane. These trends are similar with all the hydrogen halide series (HF, HCl, HBr and HI). So, if both silicon and carbon can pass trough the gas phase, what about the nature of the deposit obtained in the other end of the sealed tube?
Fig. 2. Equilibrium partial pressures of gas species obtained for a initial amount of HCl of 10 − 3 mol l − 1.
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D. Chaussende et al. / Materials Science and Engineering B61–62 (1999) 98–101
Fig. 3. Nature of the deposits obtained in a temperature gradient for the systems Si −C– H–F with a initial amount of HF of 10 − 2 mol l − 1.
3.2. Nature of the deposits obtained With the assumption that the diffusion rate of each gas species is the same in a temperature gradient, we have examined the solid phase deposited in the substrate zone. For this we have used the partial pressure obtained for the first time in a second calculus at a different temperature. We have represented this simulation in a graph where the temperature of the source is the X-axis and the temperature of the substrate is the Y-axis. So, these figures show domains where solid phases appear. So, the top left part of the graph shows a positive gradient (Td \ Ts) and the lower part reports a negative gradient (Td B Ts). The equation line y= x is the inversion line of the sign of the thermal gradient. On this line, Ts= Td and there is no chemical transport. Figs. 3–6 show simulations for the systems containing HF, HI, HCl and HBr, respectively. First, we can see that HF has a specific behaviour in the hydrogen halides series. Indeed, transport is achieved in both directions. Silicon carbide is obtained from the hot zone to the cold zone. With the opposite thermal gradient, only carbon should be transported. This transporting agent is too reactive and very difficult to use. For the three other chemical systems, the transport appears with a positive thermal gradient (Td \Ts). If this is negative, all the species remain in the gaseous state and there is no deposit at the other end of the tube. The case of HI is a little particular as this graph is doubtful. Indeed, the thermodynamical data on the system Si– C – I –H are not enough to describe the phenomena over all the temperature range. However, the general trends are reliable. So, SiC transport needs too high a temper-
Fig. 4. Nature of the deposits obtained in a temperature gradient for the systems Si– C – H – I with a initial amount of HI of 10 − 2 mol l − 1.
ature gradient, unfavourable for obtaining good quality single crystals. It is well known that a large thermal gradient leads to a higher nucleation density but not to a large single crystal of good quality [6]. However, with HCl and HBr, although several small domains of mixed deposits (C, Si and SiC) appear, there is large zone where SiC alone can be obtained over a large range of temperature gradients. The transport is achieved in one direction from low to the high temperature. This point must be taken into account if we consider that higher temperature growth leads to a better crystalline quality.
Fig. 5. Nature of the deposits obtained in a temperature gradient for the systems Si– C – H – Cl with a initial amount of HCl of 10 − 2 mol l − 1.
D. Chaussende et al. / Materials Science and Engineering B61–62 (1999) 98–101
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dynamical calculations. The main condition is to make silicon and carbon pass through the gas phase. From the results we have shown that applying the classical halide process is not possible for the chemical vapour transport of SiC as carbon does not lead to gaseous species with halogen under CVT conditions. However, the adjunction of hydrogen in the chemical system Si–C–halogen shows that transport of carbon is possible through hydrocarbons. Silicon would be transported with the halogens. So, transporting agents like HCl and HBr seem interesting as SiC alone should be deposited through a large range of temperature gradients and at a lower temperature than by the other known methods.
References Fig. 6. Nature of the deposits obtained in a temperature gradient for the systems Si – C– H –Br with a initial amount of HBr of 10 − 2 mol l − 1.
9 4. Conclusion The aim of this study was to estimate the feasibility of a new method for growing silicon carbide by thermo-
.
[1] K. Bo¨ttcher, H. Hartmann, J. Cryst. Growth 146 (1995) 53–58. [2] K. Bo¨ttcher, H. Hartmann, R. Ro¨stel, J. Cryst. Growth 159 (1996) 161 – 166. [3] M.J. McNallan, S.Y. Ip, S.Y. Lee, C. Park, Ceram. Trans. 10 (1990) 309. [4] W.S. Pan, A.J. Stekl, J. Electrochem Soc. 137 (1990) 212. [5] M. Balooch, D.R. Olander, Surf. Sci. 126 (1992) 321. [6] H. Scha¨fer, Chemical Transport Reactions, Academic Press, New York, 1964, p. 1964).
Thermodynamical calculations on the chemical vapour transport of silicon carbide D. Chaussende a,*, Y. Monteil a, P. Aboughe-nze a, C. Brylinski b, J. Bouix a a
Laboratoire des Multimate´riaux et Interfaces, UMR No. 5615, Uni6ersite´ Claude Bernard Lyon I, 69622 Villeurbanne Cedex, France b Laboratoire Central de Recherches, Thomson CSF, Domaine de Corbe6ille, 91404 Orsay Cedex, France
Abstract We have performed thermodynamical calculations in order to oversee the potential of a new method for growing silicon carbide: the chemical vapour transport process. In this way we have been able to select possible transporting agents and to show the nature of the deposits with varying parameters. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Thermodynamical calculations; Chemical vapour transport; Silicon carbide
1. Introduction
2. Calculation details
Silicon carbide is a wide gap semiconductor which exhibits outstanding electronic properties associated with chemical and thermal stability. The SiC-based devices ability to function at high temperature, high power and high radiation conditions will enable large performance enhancements to a wide variety of systems and applications. Obtaining good quality SiC is of great interest and many laboratories are studying SiC growth in different ways. Chemical vapour deposition, molecular beam epitaxy and sublimation process have been reported for epitaxial growth of thin films and bulk SiC respectively. New methods for SiC like liquid phase epitaxy and high temperature CVD have also been reported at the ECSCRM’98 congress. However, to our knowledge, the chemical vapour transport process (CVT) has never be performed for the growth of SiC. Moreover, a comparison between ZnSe growth by seeded CVT (SCVT) with iodine as transporting agent and seeded physical vapour transport (SPVT) has revealed a better crystalline quality for the first method at a lower temperature [1,2]. Thermodynamical calculations have been carried out for different chemical systems in order to select the best transporting agent for silicon carbide and to foresee the nature of the deposits with varying parameters.
The thermodynamic calculations based on the total Gibbs free energy minimisation were obtained from the GEMINI1 computer program from the scientific group Thermodata (S.G.T.E. Saint-Martin d’Heres, FRANCE) and a coherent set of thermodynamical data from this same society. All these calculations were accomplished in isochore conditions. The parameters involved in this process are temperatures of the source and the substrate and initial amount of the transporting agent. First, we have investigated SiC etching at the source zone by calculating the equilibrium partial pressures of the gases. It shows silicon and carbon behaviour with the transporting agent. Secondly, we have examined the solid phases deposited at the other end of the tube, called the substrate zone. For these two types of simulations we have studied the Si–C–X systems where X is an halogen (F, Cl, Br, I). Then, we have studied the Si–C–H–X systems in order to investigate the influence of hydrogen on the CVT process.
* Corresponding author. E-mail address: [email protected] (D. Chaussende)
We have particularly studied the Si–C–Cl system because its thermodynamical data are quite accurate.
3. Results
3.1. Chemical etching of SiC
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 4 5 4 - 1
D. Chaussende et al. / Materials Science and Engineering B61–62 (1999) 98–101
99
Fig. 1. Equilibrium partial pressures of gas species obtained for a initial amount of Cl2 of 10 − 3 mol l − 1.
We can assume that general trends are similar in the halogen column of the periodic table of elements (F, Cl, Br, I). As Fig. 1 shows, silicon only gives gaseous species. All the silicon chlorides are present in different concentrations in the gas phase, from SiCl to SiCl4. Carbon stays in the solid state like graphite. Other authors have already reported this behaviour [3–5]. Extraction of metals from carbides by halogens or their compounds can lead to the formation of free carbon. For SiC, it has been reported that the Si component is preferentially attacked and a carbon rich layer builds up in the reaction with chlorine containing gas. Carbon will give gaseous species only if silicon is entirely con-
sumed, i.e. if halogen is in excess. This situation is not of practical possibility because an excess of halogen involves too high a total pressure in the sealed tube. The same calculations in the system Si– C–Cl–H show interesting results (Fig. 2). Chlorine has the same behaviour with silicon as that in the previous chemical system. However, in this case carbon can lead to gaseous species with hydrogen such as methane. These trends are similar with all the hydrogen halide series (HF, HCl, HBr and HI). So, if both silicon and carbon can pass trough the gas phase, what about the nature of the deposit obtained in the other end of the sealed tube?
Fig. 2. Equilibrium partial pressures of gas species obtained for a initial amount of HCl of 10 − 3 mol l − 1.
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D. Chaussende et al. / Materials Science and Engineering B61–62 (1999) 98–101
Fig. 3. Nature of the deposits obtained in a temperature gradient for the systems Si −C– H–F with a initial amount of HF of 10 − 2 mol l − 1.
3.2. Nature of the deposits obtained With the assumption that the diffusion rate of each gas species is the same in a temperature gradient, we have examined the solid phase deposited in the substrate zone. For this we have used the partial pressure obtained for the first time in a second calculus at a different temperature. We have represented this simulation in a graph where the temperature of the source is the X-axis and the temperature of the substrate is the Y-axis. So, these figures show domains where solid phases appear. So, the top left part of the graph shows a positive gradient (Td \ Ts) and the lower part reports a negative gradient (Td B Ts). The equation line y= x is the inversion line of the sign of the thermal gradient. On this line, Ts= Td and there is no chemical transport. Figs. 3–6 show simulations for the systems containing HF, HI, HCl and HBr, respectively. First, we can see that HF has a specific behaviour in the hydrogen halides series. Indeed, transport is achieved in both directions. Silicon carbide is obtained from the hot zone to the cold zone. With the opposite thermal gradient, only carbon should be transported. This transporting agent is too reactive and very difficult to use. For the three other chemical systems, the transport appears with a positive thermal gradient (Td \Ts). If this is negative, all the species remain in the gaseous state and there is no deposit at the other end of the tube. The case of HI is a little particular as this graph is doubtful. Indeed, the thermodynamical data on the system Si– C – I –H are not enough to describe the phenomena over all the temperature range. However, the general trends are reliable. So, SiC transport needs too high a temper-
Fig. 4. Nature of the deposits obtained in a temperature gradient for the systems Si– C – H – I with a initial amount of HI of 10 − 2 mol l − 1.
ature gradient, unfavourable for obtaining good quality single crystals. It is well known that a large thermal gradient leads to a higher nucleation density but not to a large single crystal of good quality [6]. However, with HCl and HBr, although several small domains of mixed deposits (C, Si and SiC) appear, there is large zone where SiC alone can be obtained over a large range of temperature gradients. The transport is achieved in one direction from low to the high temperature. This point must be taken into account if we consider that higher temperature growth leads to a better crystalline quality.
Fig. 5. Nature of the deposits obtained in a temperature gradient for the systems Si– C – H – Cl with a initial amount of HCl of 10 − 2 mol l − 1.
D. Chaussende et al. / Materials Science and Engineering B61–62 (1999) 98–101
101
dynamical calculations. The main condition is to make silicon and carbon pass through the gas phase. From the results we have shown that applying the classical halide process is not possible for the chemical vapour transport of SiC as carbon does not lead to gaseous species with halogen under CVT conditions. However, the adjunction of hydrogen in the chemical system Si–C–halogen shows that transport of carbon is possible through hydrocarbons. Silicon would be transported with the halogens. So, transporting agents like HCl and HBr seem interesting as SiC alone should be deposited through a large range of temperature gradients and at a lower temperature than by the other known methods.
References Fig. 6. Nature of the deposits obtained in a temperature gradient for the systems Si – C– H –Br with a initial amount of HBr of 10 − 2 mol l − 1.
9 4. Conclusion The aim of this study was to estimate the feasibility of a new method for growing silicon carbide by thermo-
.
[1] K. Bo¨ttcher, H. Hartmann, J. Cryst. Growth 146 (1995) 53–58. [2] K. Bo¨ttcher, H. Hartmann, R. Ro¨stel, J. Cryst. Growth 159 (1996) 161 – 166. [3] M.J. McNallan, S.Y. Ip, S.Y. Lee, C. Park, Ceram. Trans. 10 (1990) 309. [4] W.S. Pan, A.J. Stekl, J. Electrochem Soc. 137 (1990) 212. [5] M. Balooch, D.R. Olander, Surf. Sci. 126 (1992) 321. [6] H. Scha¨fer, Chemical Transport Reactions, Academic Press, New York, 1964, p. 1964).