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Epitaxially grown β-SiC on Si(100) and Si(111) substrates by low pressure chemical vapour deposition
Materials Science and Engineering, B11 (1992) 317- 319
317
Epitaxially grown fl-SiC on Si(100) and Si( 111 ) substrates by low pressure chemical vapour deposition Wolfgang Just, Lucia Miihlhoff, Christoph Scholz and Thomas Weber c s GmbH Semiconductor and Solar Technology, Gollierstrafle 70, W-8000 Munich 2 (F.R.G.)
Abstract Owing to its unique physical and electronic properties, fl-SiC is a promising semiconductor material for specialized high temperature, high power and high speed device applications. The development of SiC technology is hindered by the unavailability of good quality, large area fl-SiC material. Heteroepitaxial growth of fl-SiC on silicon substrates by chemical vapour deposition (CVD) promises to be a cost-effective way of growing large area SiC single crystals. Silicon wafers are cheap and have excellent qualities, and the technology for good wafer surface preconditioning is available. It is well established that ultraclean process conditions are important for CVD processes. A low pressure CVD apparatus has been developed utilizing state-of-the-art ultraclean system technology. The reactor is an ultrahigh vacuum system that can be loaded with up to eight substrates. The process temperature is limited to the melting point of silicon. The gas supply installation provides excellent gas quality at the point of use. Commercial high purity process gases and point-of-use purification are utilized. Comparative growth studies have been carried out by simultaneously depositing SiC layers on Si(100) and Si( 111 ) substrates. All the relevant process parameters such as temperature, pressure, reaction gas flows, etc. have been varied and investigated. Crystallographic characterization of the CVD-grown SiC films has been carried out. X-ray diffraction measurements reveal 0.08 ° FWHM (full width at half-maximum) for the SIC(200) rocking curve peak and 0.10 ° FWHM for the SiC( 111 ) rocking curve peak.
I. Introduction This paper is a status report on our work of heteroepitaxially depositing fl-SiC layers on silicon substrates. T h e low pressure chemical vapour deposition (LPCVD) process and the crystallographic characterization of fl-SiC single-crystal films are presented. T h e L P C V D growth process still has to be improved. Optimization of the growth process and electrical SiC film characterization will be carried out in the future.
2. SiC layer deposition Epitaxial growth of SiC single-crystal films on Si(100) and S i ( l l l ) substrates has been carried out in an L P C V D apparatus developed by CS G m b H . Eight samples are arranged concentric with the cylindrically shaped graphite heater. T h e gas flow is normal to the sample surface. T h e size of the silicon substrates is 22.5 cm 2. Special attention has been paid to ultraclean residual gas con0921-5107/92/$5.00
ditions in the reactor and ultraclean delivery of gases. T h e all-metal sealed ultrahigh vacuum reactor, which is evacuated by a turbomolecular pump, achieves a base pressure of 5 x 10 -8 mbar at r o o m temperature. Point-of-use gas purity specifications of the carrier gas and process gases are 7.0 and 5.5 respectively. H 2 has been utilized as carrier gas (maximum flow rate 5000 standard cm 3 m i n - 1 (sccm)), with Sill 4 (maximum flow rate 100 sccm) and C H 4 (maximum flow rate 500 sccm) as process gases. T h e pressure can be adjusted to values between 0.1 and 100 mbar. T h e temperature has been varied between 1250 and 1400 °C. T h e highest temperature utilized is limited to the melting point of the silicon substrate. Before insertion into the reactor, the silicon substrates are cleaned by a standard wet chemical cleaning procedure. HESO4 and H202 treatment for removing inorganic contaminants is followed by H F treatment for removing metallic contami© Elsevier Sequoia/Printed in The Netherlands
318
nants. Additionally, a thermal etch is carried out in vacuum at 1250 °C. After cool-down an interface layer is deposited on the silicon substrate by introducing a CH 4 :H 2 gas mixture in the ratio of 1 : 3 into the reactor and quickly heating the system to the CVD temperature (1250-1400°C). For CVD deposition a Sill 4 :CH 4 ratio of about 1 : 1 and a (CH 4 + Sill 4): H 2 ratio of about 40 : 1 at a total gas flow of 1500 sccm and about 10 mbar pressure are utilized. For crystallographic characterization the SiC film can be removed from the silicon substrate by a 1:1 etch of concentrated HNO 3 and HE 3. Crystallographic characterization of SiC film
Good quality SiC films on silicon substrates are transparent and show mirror-like surfaces. The appearance of the SiC films is yellowish, indicating intrinsic SiC semiconductor material. The pale yellowish colour is distinctly seen when the silicon substrate is etched away. Deposition times of 3 h yielded a self-supporting SiC layer 25 /~m thick. Growth rates are about 8/~m h-1 for both Si(100)- and Si(lll)-oriented substrates. The growth rates reported here have been deter-
mined by weight increase measurements or from scanning electron micrographs and are two to three times higher than those reported for atmospheric pressure CVD [l, 2]. Figure 1 shows a reflection Lane pattern of an SiC layer which had been deposited on an Si(100) substrate and subsequently removed from the substrate. The fourfold symmetry of the diffraction spots identifies the cubic SiC polytype. No other SiC polytype is detected. X-ray diffraction studies confirm fl-SiC to be the only SiC polytype deposited on the silicon substrates. Figures 2 and 3 show the 2:1 X-ray diffraction scans for CVD-grown SiC on Si(100) and Si(111) substrates respectively. The tilt of the SiC crystal relative to the silicon substrate lattice orientation is less than 0.1 °. The 2 : 1 X-ray diffraction scan of the SiC film on Si(100) substrate (Fig. 2) shows the SIC(200) and SIC(400) diffraction peaks. The Si(400) peak is attributed to the silicon substrate (back curve)
Counts [10'] 2.00 -
2:1 SCAN ~-SiC/Si (100) & SiC-Film
SiC (200) 1.50 SiC (400) 1.00 -
Si(400)
0.50
0.00
SiC/Si SiC
/
h
I[
' 1 '
40.0
6d.O '
'8d.o
' 1;o 20 [°]
Fig. 2. X-ray diffraction pattern (Cu Ka radiation) of fl-SiC film epitaxially grown on Si(100) substrate.
2:1 SCAN ~-SiC/Si (111)
Counts
[1 o,]
2.00 -
1.50 SiC (111) 1.00 -
0,50 " Si (111)
2d.o' Fig. 1. Reflection Laue pattern of fl-SiC film 25/~m thick,
SiC (222)
k__
0.00
' 'd.o'
.
' 'do'
'~o'.o'
20 [°]
Fig. 3. X-ray diffraction pattern (Cu Ka radiation) of fl-SiC film epitaxially grown on Si( 11 l ) substrate.
319
and is not seen when the silicon substrate is etched away (front curve). Correspondingly, the 2:1 X-ray diffraction scan of the SiC film on Si(111) substrate (Fig. 3) shows the SiC( 111 ) and SIC(222) peaks in addition to the Si(111) peak attributed to the silicon substrate. Thorough measurements of the diffraction angles have been carried out for the calculation of the lattice constants of the SiC layers. The lattice constant of the (100)-oriented SiC layer is 4.360 A and the lattice constant of the ( 111 )-oriented SiC layer is 4.362 A. Allowing for the accuracy of the X-ray diffraction method, these values are in accordance with the 4.359 A value of the ASTM table 1-1119. X-ray diffraction rocking curves show 0.08 ° FWHM (full width at half-maximum) for the SIC(200) peak and 0.10 ° FWHM for the SIC(111) peak [3]. Raman and Rutherford backscattering spectroscopy (RBS) studies have been carried out as well. The results are listed below.
(7) The Raman spectrum of a fl-SiC film on Si(100) shows the fl-SiC longitudinal optic (LO) phonon peak at 971.3 cm -1 and the fl-SiC transverse optic (TO) phonon peak at 794.3 c m - i [4, 5]. (8) RBS shows an intensity ratio of more than 30:1 for spectra recorded in random and (100) channel directions. (9) The lattice constants determined by X-ray diffraction are 4.362 A for fl-SiC/Si(111) and 4.360 A for fl-SiC/Si(100).
4. Results
Acknowledgments
We have epitaxially grown fl-SiC single crystals on silicon substrates. The following film characteristics have been obtained to date. (1) Transparent SiC films have been deposited on silicon substrates. (2) On etching the silicon substrate away, the SiC films are self-supporting and yellowish. (3) In 3 h we obtained SiC layers 25/~m thick, corresponding to growth rates of about 8/~m h - 1. (4) The SiC films are fl-SiC single crystals. No other crystallographic modification is detected by the Laue pattern, X-ray diffraction or Raman spectroscopy. (5) The SiC film is epitaxially deposited and continues the silicon crystal orientation. (6) X-ray diffraction studies reveal 0.08 ° FWHM SIC(200) and 0.10 ° FWHM SIC(111) rocking curves.
5. Outlook
Further electrical and optical properties of the grown films are under investigation. First measurements resulted in a sheet resistivity of 888 ~ cm and p doping, which indicates a very high purity of the film material. These results have yet to be confirmed independently.
The authors would like to acknowledge the SiC film characterization of the FraunhoferInstitut f/it Festk6rpertechnologie (Munich, ER.G.) and the Gesellschaft fiir Werkstoffprfifung mbH (Zorneding, ER.G.). This work has been partially supported by the Bundesministerium fiir Forschung und Technolgie project TOU 1115. References 1 P.E.R. Nordquist, G. Kelner, M. L. Gipe and P. H. Klein, Mater. Lett., 8 (1989) 209. 2 S. Yoshida, K. Saski, E. Sakuma, S. Misawa and S. Gonda, Appl. Phys. Lett., 46 (1985) 766. 3 H. S. Kong, B. L. Jiang, J. T. Glass, G. A. Rozgonyi and K. L. More, J. Appl. Phys., 63 (1988) 2645. 4 Z. C. Feng, W. J. Choyke and J. A. Powell, J. Appl. Phys., 64 (1988) 6827. 5 H. Okumara, E. Sakuma, J. H. Lee, H. Mukaida, S. Misawa, K. Endo and S. Yoshida, J. Appl. Phys., 61 (1987) 1134.
317
Epitaxially grown fl-SiC on Si(100) and Si( 111 ) substrates by low pressure chemical vapour deposition Wolfgang Just, Lucia Miihlhoff, Christoph Scholz and Thomas Weber c s GmbH Semiconductor and Solar Technology, Gollierstrafle 70, W-8000 Munich 2 (F.R.G.)
Abstract Owing to its unique physical and electronic properties, fl-SiC is a promising semiconductor material for specialized high temperature, high power and high speed device applications. The development of SiC technology is hindered by the unavailability of good quality, large area fl-SiC material. Heteroepitaxial growth of fl-SiC on silicon substrates by chemical vapour deposition (CVD) promises to be a cost-effective way of growing large area SiC single crystals. Silicon wafers are cheap and have excellent qualities, and the technology for good wafer surface preconditioning is available. It is well established that ultraclean process conditions are important for CVD processes. A low pressure CVD apparatus has been developed utilizing state-of-the-art ultraclean system technology. The reactor is an ultrahigh vacuum system that can be loaded with up to eight substrates. The process temperature is limited to the melting point of silicon. The gas supply installation provides excellent gas quality at the point of use. Commercial high purity process gases and point-of-use purification are utilized. Comparative growth studies have been carried out by simultaneously depositing SiC layers on Si(100) and Si( 111 ) substrates. All the relevant process parameters such as temperature, pressure, reaction gas flows, etc. have been varied and investigated. Crystallographic characterization of the CVD-grown SiC films has been carried out. X-ray diffraction measurements reveal 0.08 ° FWHM (full width at half-maximum) for the SIC(200) rocking curve peak and 0.10 ° FWHM for the SiC( 111 ) rocking curve peak.
I. Introduction This paper is a status report on our work of heteroepitaxially depositing fl-SiC layers on silicon substrates. T h e low pressure chemical vapour deposition (LPCVD) process and the crystallographic characterization of fl-SiC single-crystal films are presented. T h e L P C V D growth process still has to be improved. Optimization of the growth process and electrical SiC film characterization will be carried out in the future.
2. SiC layer deposition Epitaxial growth of SiC single-crystal films on Si(100) and S i ( l l l ) substrates has been carried out in an L P C V D apparatus developed by CS G m b H . Eight samples are arranged concentric with the cylindrically shaped graphite heater. T h e gas flow is normal to the sample surface. T h e size of the silicon substrates is 22.5 cm 2. Special attention has been paid to ultraclean residual gas con0921-5107/92/$5.00
ditions in the reactor and ultraclean delivery of gases. T h e all-metal sealed ultrahigh vacuum reactor, which is evacuated by a turbomolecular pump, achieves a base pressure of 5 x 10 -8 mbar at r o o m temperature. Point-of-use gas purity specifications of the carrier gas and process gases are 7.0 and 5.5 respectively. H 2 has been utilized as carrier gas (maximum flow rate 5000 standard cm 3 m i n - 1 (sccm)), with Sill 4 (maximum flow rate 100 sccm) and C H 4 (maximum flow rate 500 sccm) as process gases. T h e pressure can be adjusted to values between 0.1 and 100 mbar. T h e temperature has been varied between 1250 and 1400 °C. T h e highest temperature utilized is limited to the melting point of the silicon substrate. Before insertion into the reactor, the silicon substrates are cleaned by a standard wet chemical cleaning procedure. HESO4 and H202 treatment for removing inorganic contaminants is followed by H F treatment for removing metallic contami© Elsevier Sequoia/Printed in The Netherlands
318
nants. Additionally, a thermal etch is carried out in vacuum at 1250 °C. After cool-down an interface layer is deposited on the silicon substrate by introducing a CH 4 :H 2 gas mixture in the ratio of 1 : 3 into the reactor and quickly heating the system to the CVD temperature (1250-1400°C). For CVD deposition a Sill 4 :CH 4 ratio of about 1 : 1 and a (CH 4 + Sill 4): H 2 ratio of about 40 : 1 at a total gas flow of 1500 sccm and about 10 mbar pressure are utilized. For crystallographic characterization the SiC film can be removed from the silicon substrate by a 1:1 etch of concentrated HNO 3 and HE 3. Crystallographic characterization of SiC film
Good quality SiC films on silicon substrates are transparent and show mirror-like surfaces. The appearance of the SiC films is yellowish, indicating intrinsic SiC semiconductor material. The pale yellowish colour is distinctly seen when the silicon substrate is etched away. Deposition times of 3 h yielded a self-supporting SiC layer 25 /~m thick. Growth rates are about 8/~m h-1 for both Si(100)- and Si(lll)-oriented substrates. The growth rates reported here have been deter-
mined by weight increase measurements or from scanning electron micrographs and are two to three times higher than those reported for atmospheric pressure CVD [l, 2]. Figure 1 shows a reflection Lane pattern of an SiC layer which had been deposited on an Si(100) substrate and subsequently removed from the substrate. The fourfold symmetry of the diffraction spots identifies the cubic SiC polytype. No other SiC polytype is detected. X-ray diffraction studies confirm fl-SiC to be the only SiC polytype deposited on the silicon substrates. Figures 2 and 3 show the 2:1 X-ray diffraction scans for CVD-grown SiC on Si(100) and Si(111) substrates respectively. The tilt of the SiC crystal relative to the silicon substrate lattice orientation is less than 0.1 °. The 2 : 1 X-ray diffraction scan of the SiC film on Si(100) substrate (Fig. 2) shows the SIC(200) and SIC(400) diffraction peaks. The Si(400) peak is attributed to the silicon substrate (back curve)
Counts [10'] 2.00 -
2:1 SCAN ~-SiC/Si (100) & SiC-Film
SiC (200) 1.50 SiC (400) 1.00 -
Si(400)
0.50
0.00
SiC/Si SiC
/
h
I[
' 1 '
40.0
6d.O '
'8d.o
' 1;o 20 [°]
Fig. 2. X-ray diffraction pattern (Cu Ka radiation) of fl-SiC film epitaxially grown on Si(100) substrate.
2:1 SCAN ~-SiC/Si (111)
Counts
[1 o,]
2.00 -
1.50 SiC (111) 1.00 -
0,50 " Si (111)
2d.o' Fig. 1. Reflection Laue pattern of fl-SiC film 25/~m thick,
SiC (222)
k__
0.00
' 'd.o'
.
' 'do'
'~o'.o'
20 [°]
Fig. 3. X-ray diffraction pattern (Cu Ka radiation) of fl-SiC film epitaxially grown on Si( 11 l ) substrate.
319
and is not seen when the silicon substrate is etched away (front curve). Correspondingly, the 2:1 X-ray diffraction scan of the SiC film on Si(111) substrate (Fig. 3) shows the SiC( 111 ) and SIC(222) peaks in addition to the Si(111) peak attributed to the silicon substrate. Thorough measurements of the diffraction angles have been carried out for the calculation of the lattice constants of the SiC layers. The lattice constant of the (100)-oriented SiC layer is 4.360 A and the lattice constant of the ( 111 )-oriented SiC layer is 4.362 A. Allowing for the accuracy of the X-ray diffraction method, these values are in accordance with the 4.359 A value of the ASTM table 1-1119. X-ray diffraction rocking curves show 0.08 ° FWHM (full width at half-maximum) for the SIC(200) peak and 0.10 ° FWHM for the SIC(111) peak [3]. Raman and Rutherford backscattering spectroscopy (RBS) studies have been carried out as well. The results are listed below.
(7) The Raman spectrum of a fl-SiC film on Si(100) shows the fl-SiC longitudinal optic (LO) phonon peak at 971.3 cm -1 and the fl-SiC transverse optic (TO) phonon peak at 794.3 c m - i [4, 5]. (8) RBS shows an intensity ratio of more than 30:1 for spectra recorded in random and (100) channel directions. (9) The lattice constants determined by X-ray diffraction are 4.362 A for fl-SiC/Si(111) and 4.360 A for fl-SiC/Si(100).
4. Results
Acknowledgments
We have epitaxially grown fl-SiC single crystals on silicon substrates. The following film characteristics have been obtained to date. (1) Transparent SiC films have been deposited on silicon substrates. (2) On etching the silicon substrate away, the SiC films are self-supporting and yellowish. (3) In 3 h we obtained SiC layers 25/~m thick, corresponding to growth rates of about 8/~m h - 1. (4) The SiC films are fl-SiC single crystals. No other crystallographic modification is detected by the Laue pattern, X-ray diffraction or Raman spectroscopy. (5) The SiC film is epitaxially deposited and continues the silicon crystal orientation. (6) X-ray diffraction studies reveal 0.08 ° FWHM SIC(200) and 0.10 ° FWHM SIC(111) rocking curves.
5. Outlook
Further electrical and optical properties of the grown films are under investigation. First measurements resulted in a sheet resistivity of 888 ~ cm and p doping, which indicates a very high purity of the film material. These results have yet to be confirmed independently.
The authors would like to acknowledge the SiC film characterization of the FraunhoferInstitut f/it Festk6rpertechnologie (Munich, ER.G.) and the Gesellschaft fiir Werkstoffprfifung mbH (Zorneding, ER.G.). This work has been partially supported by the Bundesministerium fiir Forschung und Technolgie project TOU 1115. References 1 P.E.R. Nordquist, G. Kelner, M. L. Gipe and P. H. Klein, Mater. Lett., 8 (1989) 209. 2 S. Yoshida, K. Saski, E. Sakuma, S. Misawa and S. Gonda, Appl. Phys. Lett., 46 (1985) 766. 3 H. S. Kong, B. L. Jiang, J. T. Glass, G. A. Rozgonyi and K. L. More, J. Appl. Phys., 63 (1988) 2645. 4 Z. C. Feng, W. J. Choyke and J. A. Powell, J. Appl. Phys., 64 (1988) 6827. 5 H. Okumara, E. Sakuma, J. H. Lee, H. Mukaida, S. Misawa, K. Endo and S. Yoshida, J. Appl. Phys., 61 (1987) 1134.