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Low-temperature chemical-vapor deposition of 3C–SiC films on Si(1 0 0) using SiH4–C2H4–HCl–H2
Journal of Crystal Growth 191 (1998) 439—445
Low-temperature chemical-vapor deposition of 3C—SiC films on Si(1 0 0) using SiH —C H —HCl—H 4 2 4 2 Y. Gao!, J.H. Edgar!,*, J. Chaudhuri", S.N. Cheema", M.V. Sidorov#, D.N. Braski$ ! Department of Chemical Engineering, Kansas State University, Durland Hall, Manhattan, KS 66506-5102, USA " Mechanical Engineering Department, Wichita State University, Wichita, KS 67260-0133, USA # Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA $ High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, USA Received 8 October 1997; accepted 27 January 1998
Abstract The benefits of adding HCl on the low-temperature (1000°C) epitaxial growth of 3C—SiC on Si(1 0 0) were examined. At either silicon rich (C/Si(1) or carbon rich (C/Si'1) inlet gas ratios, the SiC film composition approached stoichiometry by adding HCl, but only at an inlet C/Si ratio of 1 was the composition of the SiC film approximate 1.0 with a Cl/Si input ratio of 50. The structure of the films improved with the addition of HCl, confirmed by both X-ray diffraction and TEM. For epitaxial films, the FWHM of X-ray diffraction rocking curves decreased to 0.37° or 1348 arcs with increasing Cl/Si to 50 at a C/Si ratio of 1. The film dislocation density was reduced from 1.1]1010 cm~2 for a 2.0 lm thick film for a Cl/Si ratio of 0 to 4.27]109 cm~2 for a 0.75 lm thick film at a Cl/Si ratio of 50. The benefits of adding HCl are attributed to the suppression of pure silicon nucleation and the reduction in growth rate. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: 3C—SiC; CVD; Crystal growth; Epitaxy
1. Introduction Silicon carbide’s excellent physical and electrical properties, such as high thermal stability, large band gap, resistance to chemical attack, and high breakdown electric field [1], provide SiC based electronic devices with superior operating characteristics compared to either Si or GaAs based devices [2—4]. Relatively high deposition temperatures,
* Corresponding author. E-mail: [email protected].
1350°C or greater, are usually necessary to ensure high-quality epitaxial SiC films from the standard reactants of SiH and either C H or C H in 4 3 8 2 4 H as the carrier gas [5,6]. For high-quality 2 3C—SiC/Si heterostructures, reducing the temperature necessary for epitaxy is important to reduce the strain produced by the thermal expansion mismatch between the substrate and the epitaxial film, and to minimize the generation of crystal defects. In addition, lower epitaxial growth temperatures are attractive for minimizing damage to SiO masks in 2 the selective epitaxial growth of SiC.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 1 2 - 7
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Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
Prior studies suggest that the temperature necessary for 3C—SiC epitaxy can be reduced by including chlorine in the source gases. Reduced temperature epitaxial growth of SiC using some of the many chlorine containing carbon and silicon sources has been demonstrated by Baranov et al. [7] (1100°C with C HCl ), Furumura et al. [8] 2 3 (1000°C with SiHCl ), and Kunstamann and Vep3 rek [9] (1150°C with CH SiCl ). Evidently, Cl 3 3 plays a key role in reducing the temperature required for epitaxy, no matter how it is introduced into the system, either in the carbon or silicon source. In all previous studies, the Cl/Si ratio in the input was fixed by the silicon and carbon sources employed, and no systematic study of the effects of Cl on the quality of SiC has been previously reported. In this paper, we report on the effects of HCl on low temperature SiC CVD. A wide range of HCl flow rates were added to the standard SiC deposition system, SiH and C H , which allowed us to 4 2 4 change the Cl/Si systematically. Three different C/Si input ratios were employed to gain an insight into how chlorine affected the film composition and consequently its crystal quality. The composition, crystal quality, and growth rate of SiC thin films deposited on Si(1 0 0) by CVD were investigated by Auger electron spectrometry (AES), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) techniques.
2. Experimental procedure The 3C—SiC films were deposited on Si(1 0 0) substrates in a cold-wall horizontal reactor, with a resistantly heated boron nitride coated graphite susceptor. The susceptor temperature was 1000°C and the pressure was 1.0 atm for all depositions. The silicon and carbon sources were 1% SiH 4 diluted in H and 1% C H diluted in He, respec2 2 4 tively. The net flow rate of SiH was kept constant 4 at 1 sccm throughout all experiments, while the flow rate of C H was 0.125, 0.5, or 1 sccm for 2 4 a C/Si ratio of 0.25, 1, or 2, respectively. Pure HCl served as the chlorine source and the Cl/Si ratios examined ranged from 0 to 100. The carrier gas was
high-purity H at a constant flow rate of 8000 sccm. 2 Depositions were run for 40 min, producing films 0.5—2.5 lm thick. The experimental conditions are summarized in Table 1. The compositions of the films were determined using a PHI 660 Scanning Auger Microprobe after bombarding with Ar` for 2 min to remove surface oxides and contamination. The atomic concentration in the SiC film was obtained by correcting the AES peak-to-peak line intensities of silicon and carbon (the Si (LVV) and C (KLL) transition peaks were at around 90 and 275 eV, respectively) by their sensitivity factors (Si : 0.150; C : 0.080). The crystal texture of the SiC films was evaluated by symmetric h—2h XRD scans. For epitaxial films exhibiting only a single 3C—SiC (2 0 0) diffraction peak in the h—2h scans, the crystal quality was determined by measuring the (2 0 0) peak full-width at half-maximum (FWHM) by XRD rocking curves. Symmetric (0 0 4) and asymmetric (1 1 5) double crystal X-ray rocking curves were taken, and from these measurements the dislocation densities in the films were calculated. The film structure and interface between the film and substrate were determined by a Philips CM 200 FEG TEM operating at 200 kV. Plan-view and cross-sectional specimens for TEM were prepared by standard techniques consisting of mechanical thinning and subsequent Ar-ion beam milling. The surface morphology and the film thickness were evaluated by SEM. To measure the film thickness, SiC/Si samples were cleaved and mounted vertically on a sample holder. The SEM image of the SiC/Si cross section distinguished between Si and SiC heterostructures and hence the SiC image width could be converted to the SiC grown layer thickness.
3. Results The compositional analysis from AES of the SiC films are summarized in Table 2. All but two samples contained small amounts of oxygen (2.4—3.8 at%), possibly from atmospheric contamination while the samples were stored after deposition. The ratio of the relative concentrations of carbon and silicon (C/Si) reflects the SiC film
Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
441
Table 1 Summary of the experimental conditions P"1 atm Process
Reaction gas
Flow rate (sccm)
Time (min)
Temperature (K)
Etching Carbonization
H 2 H 2 C H 2 4 H 2 C H 2 4 SiH 4 HCl
3000 3000 1 8000 0.125, 0.5, 1 1 0, 5, 10, 20, 40, 50
10 3
1223 1323
40
1273
Growth
Table 2 Summary of AES results of SiC grown at 1273 K, 1 atm, H flow 2 rate is 8 slm, SiH flow rate is 1 sccm 4 C/Si of reactant Cl/Si Si gases (at%)
C (at%)
O (at%)
C/Si in the SiC film
0.25 0.25 1 1 2 2
26.1 48.0 55.2 51.6 57.1 53.4
2.4 0 2.6 0 3.8 3.5
0.4 0.9 1.3 1.1 1.5 1.2
0 50 0 50 0 50
71.5 52.0 42.2 48.4 39.1 43.1
stoichiometry; however, for those samples containing oxygen, this ratio may be slightly higher than the actual C/Si ratio in the bulk of the film, as the carbon from contamination may still be incorporated in the film. Two important observations can be made based on the film compositions given in Table 2: without HCl, the film’s C/Si ratio was not stoichiometric; by adding HCl up to a Cl/Si ratio of 50, the C/Si ratios in the films improved (approaching stoichiometry), but were still either carbon or silicon rich if the reactant mixture was carbon or silicon rich. Stoichiometric growth was achieved at a C/Si ratio of 1; therefore, HCl worked best with the C/Si ratio of 1 for stoichiometric film composition. The results of adding HCl on the stoichiometry are consistent with those of Ohshita [10], who concluded that the ratio of silicon to carbon approached one with increasing HCl flow rate. Since in his studies he always used excess silicon, the films
were always silicon rich. For the silicon rich case, excess silicon is etched away by HCl gas so stoichiometric may be achieved. The X-ray diffraction results of the SiC samples listed in Table 2 are summarized in Table 3. Films prepared without HCl at C/Si ratio of 0.25 were polycrystalline, while films prepared at other conditions were epitaxial. Furthermore, adding HCl improved the SiC crystal quality dramatically, as reflected by the reduction in the X-ray rocking curve FWHM, with the best quality obtained at a C/Si ratio of 1. The double crystal X-ray rocking curve FWHM for a SiC film deposited with a Cl/Si ratio of 50 input ratio was 0.37°. From the standpoints of both stoichiometry and crystal quality, the optimum gas inlet C/Si ratio was 1; neither silicon rich or carbon rich was beneficial for SiC deposition. There is an overall reduction in defect density as compared to the previous published results as shown in Table 3. The dislocation density was reduced from 1.1]1010 cm~2 for a SiC film grown without HCl to 4.27]109 cm~2 for a SiC film grown with a Cl/Si ratio of 50. The thicknesses of these two samples were 2.0 and 0.75 lm, respectively. The reported defect density (mainly stacking faults, and the number of dislocations is roughly equal to the number of stacking faults) of SiC films was 4]1010 cm~2 at 700 nm and 5]109 cm~2 at the surface of a 2.5 lm film [11]. A most recently reported defect density of a SiC film (with a thickness of 1.5 lm) on Si(1 0 0) substrate grown at 900°C was 4.06]1011 cm~2; another sample from Cree Research, which was much thicker (6 lm), had
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Table 3 Summary of XRD results of SiC grown at 1273 K, 1 atm, H flow rate is 8 slm, SiH flow rate is 1 sccm. 2 4 C/Si
Cl/Si
FWHM
Dislocation density (cm~2)
Thickness (lm)
0.25
0 50 0 50 0 50
Polycrystalline 0.86 0.56 0.37 0.59 0.46
—! 2.63]1010 1.05]1010 4.27]109 1.19]1010 8.37]109
2.0 0.2 2.0 0.75 1.5 1.0
1 2
!Dislocation density could not be measured since it was a polycrystalline sample.
a dislocation density of 9.83]108 cm~2 [12]. Since the dislocation density decreases as the thickness increases, taking into account of the thickness effect, the SiC film grown by adding HCl in this study had a crystal quality comparable to the state-ofthe-art cubic SiC films. TEM observations of samples with Cl/Si ratio of 50 and C/Si of 1 and 0.25 showed that the films were highly epitaxial and contained only a small fraction of slightly ((1°) misoriented grains in the upper parts of the films. The grains are slightly rotated with respect to one another around their [0 0 1] directions (which are parallel to the [0 0 1] of the (0 0 1) substrate). A cross-sectional TEM image of the sample grown with a Cl/Si ratio of 50 and C/Si"1 is shown in Fig. 1 and shows the epitaxial nature of the film as well as the SiC stacking faults typical for SiC grown on Si (1 0 0). TEM revealed that the films grown with C/Si of 0.25 had a larger fraction of slightly misoriented grains, and also occasional (1 1 1) and randomly oriented grains (with the major part of the film being highly epitaxial). The morphologies of the surfaces of the SiC films for various Cl/Si ratios are shown in Fig. 2. At higher Cl/Si ratios (40 and 50), the films have smaller grain sizes and more oriented surfaces than those at lower Cl/Si (5) or without HCl. The similarity between the SiC film grown without HCl and that grown with Cl/Si ratio of 5 indicates that a small amount of HCl was not enough to improve SiC film morphology. On the other hand, it appears that the SiC films grown with Cl/Si ratio of 40 and
Fig. 1. A cross-section TEM image of the Si/SiC interface for the sample with Cl/Si ratio of 50.
50 had similar grain sizes and roughness, which means the morphology cannot be improved significantly by increasing HCl concentration further after it reaches a certain value. These surface morphologies are similar to those films grown at a much higher temperature of 1350°C with SiH —C H —H system by Kong et al. [13]. 4 2 4 2 The grown SiC film thicknesses were measured by the cross-sectional SEM. At a Cl/Si ratio of 50, the deposition rate was approximately 30% of the deposition rate without HCl. The almost linear decrease in SiC deposition rate with the Cl/Si ratio is shown in Fig. 3. This is in agreement with the results of Ohshita [10], who attributed the decrease in growth rate with increasing amount of HCl to the hindering effect of adsorbed HCl.
Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
443
Fig. 2. SEM micrographs of the surfaces of the SiC films for various Cl/Si ratios. (a) Cl/Si"0, (b) Cl/Si"5, (c) Cl/Si"40, (d) Cl/Si"50. Magnification of 10 000.
4. Discussion The key question this research raises is: why does HCl improve the SiC crystal quality? Two explanations are proposed: that HCl eliminates simultaneous Si nucleation which interferes with epitaxy, and that HCl allows the adsorbed species more time to find the crystallographically appropriate positions. These two explanations are in agreement
with our observations of a change in composition toward more stoichiometric C/Si ratios in the deposited films, and a decrease in the growth rate with the addition of HCl. At 1000°C, ethylene decomposes more slowly than silane, and in the absence of HCl the excess Si formed from silane decomposition may nucleate simultaneously with the SiC. This Si nucleation can be quite small, resulting in only a slight change in
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Fig. 3. The effect of HCl flow rate on the SiC growth rate.
the film’s composition; nevertheless, it may interfere with the epitaxial growth of the SiC films, causing the deposit to become polycrystalline. Adding HCl suppresses the nucleation of silicon, but has only a small effect on silicon carbide nucleation; consequently, the film quality is improved. To test this hypothesis, we compared the etch rates of Si and SiC in a separate set of experiments. Our measured Si etch rate was at least 100 times greater than that of SiC at 1000°C under the same conditions. Thus, Si nuclei are unstable in the presence of excess HCl, making SiC the only stable solid phase kinetically possible. We speculate that the decomposition of ethylene and the incorporation of carbon into the film is rate limited by the availability of adsorbed silicon with which it can react. With the high H/C reactant ratios employed, carbon deposition in the absence of silicon is thermodynamically unfavorable. Thus, by reducing the surface absorbed silicon by including HCl, the adsorption of carbon and carbonbearing species is suppressed as well. However, we cannot surmise the efficiency of HCl in removing pre-existing carbon from the silicon surface, such as residual carbon contamination from improper substrate cleaning for example. The second proposed explanation is that the film quality is improved by simply reducing the deposition rate. Whether the deposited film is polycrystalline or epitaxial depends on the arrival rate of the reactant species (the growth rate) and the ability of
adsorbed species to diffuse on the surface to the appropriate crystallographic positions. Polycrystalline films form, when the arrival rate is high and the surface diffusion rate is low; the absorbed atoms can diffuse only a short distance before they are buried beneath newly arriving species from the gas phase. Epitaxial films can form even if the diffusion rate is slow provided that deposition rate is low, so the adsorbed atoms can still move to the appropriate location. To test this hypothesis, we deposited a SiC film at half the normal SiH and C H 4 2 4 concentrations without HCl, to see if the quality of this film was better than that grown at higher concentrations and thus faster growth rates. The resulting SiC thin film did indeed have more narrow X-ray diffraction double crystal rocking curve FWHM, suggesting that the reduction in growth rate caused by HCl is a contributing factor to the improved SiC crystal quality. It would be worthwhile to compare the composition of films grown at several different growth rates with and without HCl to test whether HCl is required to achieve stoichiometry, or if stoichiometry is simply a matter of the growth rate. We hope to address this issue and the effect of HCl on the incorporation of impurities into the epitaxial SiC film in future work.
5. Conclusion The temperature required for epitaxial growth of SiC can be reduced to 1000°C with the addition of HCl at the Cl/Si gas input ratios above 40 to the standard deposition sources (SiH and C H ) used 4 2 4 for the chemical vapor deposition. A C/Si ratio of 1 was superior to either silicon rich or carbon rich gas input in taking advantage of the benefits of adding HCl, including to enhance the film stoichiometry, improve the crystal quality, but at the cost of a reduced growth rate (by 70%) at a Cl/Si ratio of 50 compared to no HCl case.
Acknowledgements Support for this research from the National Science Foundation (Grants No. CTS-9319770 and
Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
No. DMR-967333), Kansas State University, and K*STAR EPSCoR is greatly appreciated. References [1] M. Tairov, Y.A. Vodakov, in: J.I. Pankove (Ed.), Electroluminescence, Springer, New York, 1977, p. 31. [2] R.F. Davis, J.T. Glass, Adv. Solid-State Chem. 2 (1991) 1. [3] R.F. Davis, G. Kelner, M. Shur, J.W. Palmour, J.A. Edmond, IEEE Proc. 79 (1991) 677. [4] J.H. Edgar, J. Mater. Res. 7 (1992) 235. [5] S. Nishino, J.A. Powell, H.A. Will, Appl. Phys. Lett. 42 (1983) 460. [6] J.T. Glass, Y.C. Wang, H.S. Kong, R.F. Davis, Mater. Res. Soc. Symp. Proc. 116 (1988) 337.
445
[7] I.M. Baranov, V.A. Dmitriev, I.P. Nikitina, T.S. Kondrateva, in: C.Y. Yang, M.M. Rahman, G.L. Harris (Eds.), Amorphous and Crystalline Silicon Carbide IV; Proc. Physics, vol. 71, Springer, Berlin, 1992, p. 116. [8] Y. Furumura, M. Doki, F. Mieno, T. Eshita, T. Suzuki, M. Maeda, J. Electrochem. Soc. 135 (1988) 1255. [9] Th. Kunstamann, S. Veprek, Appl. Phys. Lett. 67 (1995) 3126. [10] K. Ohshita, Mater. Res. Soc. Symp. Proc. 162 (1990) 433. [11] J. Stoemenos, C. Dezauzier, G. Arnaud, S. Contreras, J. Camassel, J. Pascual, J.L. Robert, Mater. Sci. Eng. B 29 (1995) 160. [12] J. Chaudhuri, X. Cheng, C. Yuan, A.J. Steckl, Thin Solid Films 292 (1997) 1. [13] H.S. Kong, Y.C. Wang, J.T. Glass, R.F. Davis, J. Mater. Res. 3 (3) (1988) 521.
Low-temperature chemical-vapor deposition of 3C—SiC films on Si(1 0 0) using SiH —C H —HCl—H 4 2 4 2 Y. Gao!, J.H. Edgar!,*, J. Chaudhuri", S.N. Cheema", M.V. Sidorov#, D.N. Braski$ ! Department of Chemical Engineering, Kansas State University, Durland Hall, Manhattan, KS 66506-5102, USA " Mechanical Engineering Department, Wichita State University, Wichita, KS 67260-0133, USA # Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA $ High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, USA Received 8 October 1997; accepted 27 January 1998
Abstract The benefits of adding HCl on the low-temperature (1000°C) epitaxial growth of 3C—SiC on Si(1 0 0) were examined. At either silicon rich (C/Si(1) or carbon rich (C/Si'1) inlet gas ratios, the SiC film composition approached stoichiometry by adding HCl, but only at an inlet C/Si ratio of 1 was the composition of the SiC film approximate 1.0 with a Cl/Si input ratio of 50. The structure of the films improved with the addition of HCl, confirmed by both X-ray diffraction and TEM. For epitaxial films, the FWHM of X-ray diffraction rocking curves decreased to 0.37° or 1348 arcs with increasing Cl/Si to 50 at a C/Si ratio of 1. The film dislocation density was reduced from 1.1]1010 cm~2 for a 2.0 lm thick film for a Cl/Si ratio of 0 to 4.27]109 cm~2 for a 0.75 lm thick film at a Cl/Si ratio of 50. The benefits of adding HCl are attributed to the suppression of pure silicon nucleation and the reduction in growth rate. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: 3C—SiC; CVD; Crystal growth; Epitaxy
1. Introduction Silicon carbide’s excellent physical and electrical properties, such as high thermal stability, large band gap, resistance to chemical attack, and high breakdown electric field [1], provide SiC based electronic devices with superior operating characteristics compared to either Si or GaAs based devices [2—4]. Relatively high deposition temperatures,
* Corresponding author. E-mail: [email protected].
1350°C or greater, are usually necessary to ensure high-quality epitaxial SiC films from the standard reactants of SiH and either C H or C H in 4 3 8 2 4 H as the carrier gas [5,6]. For high-quality 2 3C—SiC/Si heterostructures, reducing the temperature necessary for epitaxy is important to reduce the strain produced by the thermal expansion mismatch between the substrate and the epitaxial film, and to minimize the generation of crystal defects. In addition, lower epitaxial growth temperatures are attractive for minimizing damage to SiO masks in 2 the selective epitaxial growth of SiC.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 1 2 - 7
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Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
Prior studies suggest that the temperature necessary for 3C—SiC epitaxy can be reduced by including chlorine in the source gases. Reduced temperature epitaxial growth of SiC using some of the many chlorine containing carbon and silicon sources has been demonstrated by Baranov et al. [7] (1100°C with C HCl ), Furumura et al. [8] 2 3 (1000°C with SiHCl ), and Kunstamann and Vep3 rek [9] (1150°C with CH SiCl ). Evidently, Cl 3 3 plays a key role in reducing the temperature required for epitaxy, no matter how it is introduced into the system, either in the carbon or silicon source. In all previous studies, the Cl/Si ratio in the input was fixed by the silicon and carbon sources employed, and no systematic study of the effects of Cl on the quality of SiC has been previously reported. In this paper, we report on the effects of HCl on low temperature SiC CVD. A wide range of HCl flow rates were added to the standard SiC deposition system, SiH and C H , which allowed us to 4 2 4 change the Cl/Si systematically. Three different C/Si input ratios were employed to gain an insight into how chlorine affected the film composition and consequently its crystal quality. The composition, crystal quality, and growth rate of SiC thin films deposited on Si(1 0 0) by CVD were investigated by Auger electron spectrometry (AES), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) techniques.
2. Experimental procedure The 3C—SiC films were deposited on Si(1 0 0) substrates in a cold-wall horizontal reactor, with a resistantly heated boron nitride coated graphite susceptor. The susceptor temperature was 1000°C and the pressure was 1.0 atm for all depositions. The silicon and carbon sources were 1% SiH 4 diluted in H and 1% C H diluted in He, respec2 2 4 tively. The net flow rate of SiH was kept constant 4 at 1 sccm throughout all experiments, while the flow rate of C H was 0.125, 0.5, or 1 sccm for 2 4 a C/Si ratio of 0.25, 1, or 2, respectively. Pure HCl served as the chlorine source and the Cl/Si ratios examined ranged from 0 to 100. The carrier gas was
high-purity H at a constant flow rate of 8000 sccm. 2 Depositions were run for 40 min, producing films 0.5—2.5 lm thick. The experimental conditions are summarized in Table 1. The compositions of the films were determined using a PHI 660 Scanning Auger Microprobe after bombarding with Ar` for 2 min to remove surface oxides and contamination. The atomic concentration in the SiC film was obtained by correcting the AES peak-to-peak line intensities of silicon and carbon (the Si (LVV) and C (KLL) transition peaks were at around 90 and 275 eV, respectively) by their sensitivity factors (Si : 0.150; C : 0.080). The crystal texture of the SiC films was evaluated by symmetric h—2h XRD scans. For epitaxial films exhibiting only a single 3C—SiC (2 0 0) diffraction peak in the h—2h scans, the crystal quality was determined by measuring the (2 0 0) peak full-width at half-maximum (FWHM) by XRD rocking curves. Symmetric (0 0 4) and asymmetric (1 1 5) double crystal X-ray rocking curves were taken, and from these measurements the dislocation densities in the films were calculated. The film structure and interface between the film and substrate were determined by a Philips CM 200 FEG TEM operating at 200 kV. Plan-view and cross-sectional specimens for TEM were prepared by standard techniques consisting of mechanical thinning and subsequent Ar-ion beam milling. The surface morphology and the film thickness were evaluated by SEM. To measure the film thickness, SiC/Si samples were cleaved and mounted vertically on a sample holder. The SEM image of the SiC/Si cross section distinguished between Si and SiC heterostructures and hence the SiC image width could be converted to the SiC grown layer thickness.
3. Results The compositional analysis from AES of the SiC films are summarized in Table 2. All but two samples contained small amounts of oxygen (2.4—3.8 at%), possibly from atmospheric contamination while the samples were stored after deposition. The ratio of the relative concentrations of carbon and silicon (C/Si) reflects the SiC film
Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
441
Table 1 Summary of the experimental conditions P"1 atm Process
Reaction gas
Flow rate (sccm)
Time (min)
Temperature (K)
Etching Carbonization
H 2 H 2 C H 2 4 H 2 C H 2 4 SiH 4 HCl
3000 3000 1 8000 0.125, 0.5, 1 1 0, 5, 10, 20, 40, 50
10 3
1223 1323
40
1273
Growth
Table 2 Summary of AES results of SiC grown at 1273 K, 1 atm, H flow 2 rate is 8 slm, SiH flow rate is 1 sccm 4 C/Si of reactant Cl/Si Si gases (at%)
C (at%)
O (at%)
C/Si in the SiC film
0.25 0.25 1 1 2 2
26.1 48.0 55.2 51.6 57.1 53.4
2.4 0 2.6 0 3.8 3.5
0.4 0.9 1.3 1.1 1.5 1.2
0 50 0 50 0 50
71.5 52.0 42.2 48.4 39.1 43.1
stoichiometry; however, for those samples containing oxygen, this ratio may be slightly higher than the actual C/Si ratio in the bulk of the film, as the carbon from contamination may still be incorporated in the film. Two important observations can be made based on the film compositions given in Table 2: without HCl, the film’s C/Si ratio was not stoichiometric; by adding HCl up to a Cl/Si ratio of 50, the C/Si ratios in the films improved (approaching stoichiometry), but were still either carbon or silicon rich if the reactant mixture was carbon or silicon rich. Stoichiometric growth was achieved at a C/Si ratio of 1; therefore, HCl worked best with the C/Si ratio of 1 for stoichiometric film composition. The results of adding HCl on the stoichiometry are consistent with those of Ohshita [10], who concluded that the ratio of silicon to carbon approached one with increasing HCl flow rate. Since in his studies he always used excess silicon, the films
were always silicon rich. For the silicon rich case, excess silicon is etched away by HCl gas so stoichiometric may be achieved. The X-ray diffraction results of the SiC samples listed in Table 2 are summarized in Table 3. Films prepared without HCl at C/Si ratio of 0.25 were polycrystalline, while films prepared at other conditions were epitaxial. Furthermore, adding HCl improved the SiC crystal quality dramatically, as reflected by the reduction in the X-ray rocking curve FWHM, with the best quality obtained at a C/Si ratio of 1. The double crystal X-ray rocking curve FWHM for a SiC film deposited with a Cl/Si ratio of 50 input ratio was 0.37°. From the standpoints of both stoichiometry and crystal quality, the optimum gas inlet C/Si ratio was 1; neither silicon rich or carbon rich was beneficial for SiC deposition. There is an overall reduction in defect density as compared to the previous published results as shown in Table 3. The dislocation density was reduced from 1.1]1010 cm~2 for a SiC film grown without HCl to 4.27]109 cm~2 for a SiC film grown with a Cl/Si ratio of 50. The thicknesses of these two samples were 2.0 and 0.75 lm, respectively. The reported defect density (mainly stacking faults, and the number of dislocations is roughly equal to the number of stacking faults) of SiC films was 4]1010 cm~2 at 700 nm and 5]109 cm~2 at the surface of a 2.5 lm film [11]. A most recently reported defect density of a SiC film (with a thickness of 1.5 lm) on Si(1 0 0) substrate grown at 900°C was 4.06]1011 cm~2; another sample from Cree Research, which was much thicker (6 lm), had
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Y. Gao et al. / Journal of Crystal Growth 191 (1998) 439–445
Table 3 Summary of XRD results of SiC grown at 1273 K, 1 atm, H flow rate is 8 slm, SiH flow rate is 1 sccm. 2 4 C/Si
Cl/Si
FWHM
Dislocation density (cm~2)
Thickness (lm)
0.25
0 50 0 50 0 50
Polycrystalline 0.86 0.56 0.37 0.59 0.46
—! 2.63]1010 1.05]1010 4.27]109 1.19]1010 8.37]109
2.0 0.2 2.0 0.75 1.5 1.0
1 2
!Dislocation density could not be measured since it was a polycrystalline sample.
a dislocation density of 9.83]108 cm~2 [12]. Since the dislocation density decreases as the thickness increases, taking into account of the thickness effect, the SiC film grown by adding HCl in this study had a crystal quality comparable to the state-ofthe-art cubic SiC films. TEM observations of samples with Cl/Si ratio of 50 and C/Si of 1 and 0.25 showed that the films were highly epitaxial and contained only a small fraction of slightly ((1°) misoriented grains in the upper parts of the films. The grains are slightly rotated with respect to one another around their [0 0 1] directions (which are parallel to the [0 0 1] of the (0 0 1) substrate). A cross-sectional TEM image of the sample grown with a Cl/Si ratio of 50 and C/Si"1 is shown in Fig. 1 and shows the epitaxial nature of the film as well as the SiC stacking faults typical for SiC grown on Si (1 0 0). TEM revealed that the films grown with C/Si of 0.25 had a larger fraction of slightly misoriented grains, and also occasional (1 1 1) and randomly oriented grains (with the major part of the film being highly epitaxial). The morphologies of the surfaces of the SiC films for various Cl/Si ratios are shown in Fig. 2. At higher Cl/Si ratios (40 and 50), the films have smaller grain sizes and more oriented surfaces than those at lower Cl/Si (5) or without HCl. The similarity between the SiC film grown without HCl and that grown with Cl/Si ratio of 5 indicates that a small amount of HCl was not enough to improve SiC film morphology. On the other hand, it appears that the SiC films grown with Cl/Si ratio of 40 and
Fig. 1. A cross-section TEM image of the Si/SiC interface for the sample with Cl/Si ratio of 50.
50 had similar grain sizes and roughness, which means the morphology cannot be improved significantly by increasing HCl concentration further after it reaches a certain value. These surface morphologies are similar to those films grown at a much higher temperature of 1350°C with SiH —C H —H system by Kong et al. [13]. 4 2 4 2 The grown SiC film thicknesses were measured by the cross-sectional SEM. At a Cl/Si ratio of 50, the deposition rate was approximately 30% of the deposition rate without HCl. The almost linear decrease in SiC deposition rate with the Cl/Si ratio is shown in Fig. 3. This is in agreement with the results of Ohshita [10], who attributed the decrease in growth rate with increasing amount of HCl to the hindering effect of adsorbed HCl.
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Fig. 2. SEM micrographs of the surfaces of the SiC films for various Cl/Si ratios. (a) Cl/Si"0, (b) Cl/Si"5, (c) Cl/Si"40, (d) Cl/Si"50. Magnification of 10 000.
4. Discussion The key question this research raises is: why does HCl improve the SiC crystal quality? Two explanations are proposed: that HCl eliminates simultaneous Si nucleation which interferes with epitaxy, and that HCl allows the adsorbed species more time to find the crystallographically appropriate positions. These two explanations are in agreement
with our observations of a change in composition toward more stoichiometric C/Si ratios in the deposited films, and a decrease in the growth rate with the addition of HCl. At 1000°C, ethylene decomposes more slowly than silane, and in the absence of HCl the excess Si formed from silane decomposition may nucleate simultaneously with the SiC. This Si nucleation can be quite small, resulting in only a slight change in
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Fig. 3. The effect of HCl flow rate on the SiC growth rate.
the film’s composition; nevertheless, it may interfere with the epitaxial growth of the SiC films, causing the deposit to become polycrystalline. Adding HCl suppresses the nucleation of silicon, but has only a small effect on silicon carbide nucleation; consequently, the film quality is improved. To test this hypothesis, we compared the etch rates of Si and SiC in a separate set of experiments. Our measured Si etch rate was at least 100 times greater than that of SiC at 1000°C under the same conditions. Thus, Si nuclei are unstable in the presence of excess HCl, making SiC the only stable solid phase kinetically possible. We speculate that the decomposition of ethylene and the incorporation of carbon into the film is rate limited by the availability of adsorbed silicon with which it can react. With the high H/C reactant ratios employed, carbon deposition in the absence of silicon is thermodynamically unfavorable. Thus, by reducing the surface absorbed silicon by including HCl, the adsorption of carbon and carbonbearing species is suppressed as well. However, we cannot surmise the efficiency of HCl in removing pre-existing carbon from the silicon surface, such as residual carbon contamination from improper substrate cleaning for example. The second proposed explanation is that the film quality is improved by simply reducing the deposition rate. Whether the deposited film is polycrystalline or epitaxial depends on the arrival rate of the reactant species (the growth rate) and the ability of
adsorbed species to diffuse on the surface to the appropriate crystallographic positions. Polycrystalline films form, when the arrival rate is high and the surface diffusion rate is low; the absorbed atoms can diffuse only a short distance before they are buried beneath newly arriving species from the gas phase. Epitaxial films can form even if the diffusion rate is slow provided that deposition rate is low, so the adsorbed atoms can still move to the appropriate location. To test this hypothesis, we deposited a SiC film at half the normal SiH and C H 4 2 4 concentrations without HCl, to see if the quality of this film was better than that grown at higher concentrations and thus faster growth rates. The resulting SiC thin film did indeed have more narrow X-ray diffraction double crystal rocking curve FWHM, suggesting that the reduction in growth rate caused by HCl is a contributing factor to the improved SiC crystal quality. It would be worthwhile to compare the composition of films grown at several different growth rates with and without HCl to test whether HCl is required to achieve stoichiometry, or if stoichiometry is simply a matter of the growth rate. We hope to address this issue and the effect of HCl on the incorporation of impurities into the epitaxial SiC film in future work.
5. Conclusion The temperature required for epitaxial growth of SiC can be reduced to 1000°C with the addition of HCl at the Cl/Si gas input ratios above 40 to the standard deposition sources (SiH and C H ) used 4 2 4 for the chemical vapor deposition. A C/Si ratio of 1 was superior to either silicon rich or carbon rich gas input in taking advantage of the benefits of adding HCl, including to enhance the film stoichiometry, improve the crystal quality, but at the cost of a reduced growth rate (by 70%) at a Cl/Si ratio of 50 compared to no HCl case.
Acknowledgements Support for this research from the National Science Foundation (Grants No. CTS-9319770 and
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