Characterization of 4H-SiC Homoepitaxial Films on Porous 4H-SiC

Journal of The Electrochemical Society, 150 共2兲 G90-G95 共2003兲

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0013-4651/2003/150共2兲/G90/6/$7.00 © The Electrochemical Society, Inc.

Characterization of 4H-SiC Homoepitaxial Films on Porous 4H-SiC from Bis„trimethylsilyl…methane Precursor Jae Kyeong Jeong and Hyeong Joon Kim*,z School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea 4H-SiC homoepitaxial films were grown on 8° off-axis porous 4H-SiC 共0001兲 faces in the temperature range of 1280-1510°C by chemical vapor deposition from bis共trimethylsilyl兲methane 共BTMSM兲 precursor. The activation energy for growth was 5.6 kcal/ mol, indicating that the film growth is dominated by the diffusion-limited mechanism. Triangular stacking faults were incorporated in the SiC thin film grown at low temperature of 1280°C due to the formation of 3C-SiC polytype. Moreover, super-screw dislocations appeared seriously in the SiC film grown below 1320°C. Clean and featureless morphology was observed in the SiC film grown below 25 standard cubic centimeters per minute 共sccm兲 H2 carrier gas flow rate of BTMSM at 1380°C while 3C-SiC polytype with double positioning boundaries grew at 30 sccm flow rate of BTMSM. The dislocation density of the epi layer was strongly influenced by the growth temperature and flow rate of BTMSM. Double axis crystal X-ray diffraction and optical microscopy analysis revealed that the dislocation density decreased at the higher growth temperature and lower flow rate of BTMSM. The full width at half maximum of the rocking curve of the film grown at optimized condition was 7.6 arcsec and the sharp free exciton and Al bound exciton lines appear in the epi layer, which indicates that the 4H-SiC film was of very high quality. © 2003 The Electrochemical Society. 关DOI: 10.1149/1.1532329兴 All rights reserved. Manuscript submitted March 4, 2002; revised manuscript received July 15, 2002. Available electronically January 2, 2003.

Silicon carbide 共SiC兲 has been considered as a promising wide bandgap material for high power, high frequency, and high temperature devices due to its high breakdown field (⬃3 ⫻ 106 V/cm), high thermal conductivity, high saturated electron drift velocity (⬃2 ⫻ 107 cm/s), and chemical stability.1-2 Among the many polytypes of SiC, the most recent work has focused on 4H-SiC and 6H-SiC because high quality epi layers can be grown on the commercially available 4H- or 6H-SiC wafers with 2 in. diam. However, dislocation and micropipes in the SiC substrate still limit the performance of electronic devices such as metal semiconductor field effect transistor, metal oxide semiconductor field effect transistor, and surveillance bug detector. Therefore, it is essential to reduce the dislocation density in order to exploit the full potential of SiC material. A porous SiC substrate may be one of the ways to reduce the dislocation during epitaxial growth. In the case of heteroepitaxy such as GaAs/Si,3-4 CoSi2 /Si, 5 and GaN/Si,6 the stress due to the differences in the lattice constants and thermal expansion coefficients 共TECs兲 between the thin film and substrate material generates a high defect density. Such stress may be relaxed by using a porous substrate; high quality epitaxial layers have been obtained on porous Si.3-6 In the case of SiC, there are a few reports concerning the epitaxial growth on porous silicon carbide 共PSC兲.7-9 Saddow et al.7 reported the defect density reduction of the epi layer on PSC using photoluminescence. Mynbaeva et al.8 reported that the dislocation density of the epi layer (5 ⫻ 104 cm⫺2 ) on PSC was lower than that of the substrate (⬃106 cm⫺2 ) and the deep level defects in the epi layer on PSC were reduced compared to those on conventional SiC substrates. However, the growth mechanism and the defect density reduction mechanism have not been studied. Moreover, there have been no studies relating the effect of growth temperature and the flow rate of source material on the defect generation during crystal growth. In this paper, 4H-SiC thin films were grown on an 8° off-oriented PSC substrate using bis共trimethylsilyl兲methane 兵关 (CH3 ) 3 Si兴 2 CH2 , BTMSM其 precursor. The growth mechanism of 4H-SiC homoepitaxy on PSC substrate is discussed. The effect of the growth conditions such as growth temperature and source flow rate on the surface morphology, the polytype formation, and the dislocation density of the epi layer is also reported.

* Electrochemical Society Active Member. z

E-mail: [email protected]

Experimental The 8° off-axis porous 4H-SiC共0001兲 Si faces were used as a substrate for 4H-SiC epitaxial growth. Porous SiC wafers with a 2 in. diam, were prepared by TDI10 via surface anodization of the wafers in an aqueous solution of hydrogen fluoride, which was carried out in the electrochemical cell under UV illumination from a mercury lamp. The thickness and pore size of porous silicon carbide layers were about 1.3 ␮m and in the range of 30 to 120 nm, respectively. And the fraction of the surface area containing voids was estimated to be 0.05-0.1 by scanning electron microscopy 共SEM兲. Epitaxial layer was deposited in a cold-wall, horizontal-type 共chemical vapor deposition兲 reactor, where the substrates were heated using a graphite susceptor by radio-frequency induction heating. The deposition temperature was measured by an optical pyrometer and calibrated using the melting temperature of Si 共1418°C兲. Prior to being loaded into the growth chamber, the substrates were dipped into a 10% HF solution in order to remove the native oxide layer. The substrates were then dried with an Ar gas gun. After being loaded, the SiC substrates were subjected to a 10 min H2 etch at 1510°C. The flow rate of the H2 carrier gas through the liquid BTMSM source in the bubbler was varied from 5 to 30 sccm and the growth temperature was varied from 1280 to 1510°C. The bubbler temperature was kept at room temperature. The actual transport rate of BTMSM cannot be determined accurately because its vapor pressure has not been reported. The input flow rate of the diluent gas, H2 , was kept at 3000 standard cubic centimeters per minute 共sccm兲 and the pressure in the growth chamber was fixed to 360 Torr. The film thickness and surface morphology were observed by scanning electron microscopy 共SEM, Cambridge Instruments S.240兲. X-ray diffraction 共XRD兲 analysis was conducted on a Bede Scientific Instruments HRXRD with 2 kW Cu radiation source. Triple axis crystal XRD 共TCD兲 as well as double axis crystal XRD 共DCD兲 were utilized to map the reciprocal space. TCD employs the monochrometer with double dual channel, which produces incident X-rays with the K␣ 2 and K␤ radiation removed and allows K␣ 1 radiation to the sample. A third analyzer crystal is placed between the sample and the detector to increase the resolution in 2␪ further, and to improve the signal-to-noise ratio. A computer-controlled goniometer with a precision of 0.055 arcsec was used with high intensity beam geometry for a reciprocal space mapping 共RSM兲 of the off-axis 共0004兲 plane. Photoluminescence spectra were measured at 10 K using a He-Cd laser of 325 nm wavelength with 30 mW power. The crystallographic relationships between domains were examined by electron back-scattering diffraction 共EBSD, Oxford instruments Ltd.兲. Because EBSD is an add-on package to SEM, it has the ca-


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lower temperature. The dependence of the growth rate on the flow rate of BTMSM at a deposition temperature of 1380°C is shown in Fig. 1b. As the flow rate of BTMSM carrier gas (H2 ) increases, the growth rate is linearly increased initially and then is saturated. Because the vapor pressure of liquid BTMSM source in the bubbler system of the room temperature is fixed, net feed rate of BTMSM vapor into the reactor chamber seems to be saturated at high enough rate of carrier gas.

Figure 1. Growth rate of 4H-SiC films on PSC as a function of 共a兲 growth temperature and 共b兲 H2 carrier gas flow rate of BTMSM.

pability of diffraction and imaging in real time with a spatial resolution of 0.5 ␮m. Results and Discussion Growth mechanism of 4H-SiC on PSC substrate.—An Arrhenius plot for the growth rate of the SiC film on PSC substrate as a function of growth temperature is shown in Fig. 1a. The carrier gas flow rate of BTMSM is 10 sccm. The growth rate for 4H-SiC films increased with temperature with the activation energy of 5.6 kcal/mol. For 6H-SiC homoepitaxy on well-oriented 兵0001其 faces, Jennings et al.11 reported the large activation energy of 20 kcal/mol, which was attributed to Si adsorption based on thermodynamic consideration. In that case, surface reaction limited growth due to barrier of two-dimensional 共2-D兲 nucleation is expected. However, on off-axis substrate, there are many steps and kinks to incorporate adatoms. Therefore, lateral growth by step flow without the formation of 2-D nucleation will proceed, which results in the small activation energy. Kimoto et al.12 reported similar activation energy of 3 kcal/mol for 6H-SiC thin film growth on 6° off-axis 6H-SiC兵0001其 faces. The above results indicate that the growth on PSC is also diffusion limited. The slightly large activation energy of 5.6 kcal/mol might be ascribed to the diminished decomposition efficiency of BTMSM at

Effect of the growth temperature and flow rate of BTMSM on the morphology and polytype formation of SiC films.—The dependence of the surface morphology on the growth temperature was investigated at 10 sccm H2 gas carrier flow rate of BTMSM. The effect of deposition temperature on the surface morphology of the films is clearly shown in Fig. 2. Growth at 1280°C produced triangular stacking faults 共TSFs兲 inclusions in the epi layer 共Fig. 2a兲, which were always formed at super-screw dislocation sites toward the ¯ 0 兴 direction, as shown in the magnified micrograph of the film 关 112 grown at 1280°C 共Fig. 2b兲. If TSFs are formed at the interface between the substrate and film, the thickness of the epi layer can be estimated to 1.39 ␮m because the thickness of thin films is given by the height of TSF 共⬃10 ␮m兲 times sin 8°. Since the thickness of the film grown at 1280°C observed by cross-sectional SEM was 1.4 ␮m, TSFs seem to be generated at the super-screw dislocation defects at the initial stage of CVD growth. RSM of 共0004兲 Bragg spot of the epi layers revealed the polytype of TSFs in the epi layer to be 3C-SiC. The film grown at 1320°C 共Fig. 2c兲 has no TSFs, but superscrew dislocation defects appeared seriously in certain areas. It is no wonder that low temperature growth results in the sporadic occurrence of super-screw dislocations and formation of TSFs due to low adatom mobility. In contrast, all the films grown above 1380°C are featureless and specular, as shown in Fig. 2d. The effect of the BTMSM flow rate on the film structure was investigated. In the second series of growth experiments, the growth temperature was kept at 1380°C. All other conditions, except the flow rate of BTMSM, were also fixed. All the film grown below 25 sccm H2 carrier gas flow rate of BTMSM are similar to Fig. 2d, that is, clean and featureless. However, the surface microstructure of the film grown at 30 sccm H2 flow rate of BTMSM 共Fig. 3a兲 showed a marked change in that it consisted of two types of triangles. The intensity of 3C-SiC共111兲 peak of the film grown at 30 sccm, 1380°C was much higher than that of the film grown at 1280°C, indicating that the most region of the grown film is the 3C-SiC polytype. The crystallographic relationship between two type triangles was analyzed by EBSD coupled with SEM. The 兵001其 pole figures for each domains with a pixel size of 1 um were mapped by the indexing of Kikuchi lines. The 关111兴 zone axis of the 兵100其 poles is tilted from the center due to the off-axis of the substrate. The 兵001其 pole figures of A region and B region in Fig. 3a are shown in Fig. 3b and c, respectively. The two triangles have 60° rotational symmetry with each other, suggesting these regions are twins and the boundaries between them are double positioning boundaries 共DPBs兲.13 This result can be explained in terms of supersaturation. At low flow rate, the impinging atoms can reach the step so that the polytype of the substrate is duplicated in the epi layer. A higher supersaturation, however, can result in the simultaneous nucleation in different sites rotated from each other by 60° on the terraces, which produces the 3C-SiC polytype which is thermodynamically stable at the growth temperature of 1380°C. Effect of the growth temperature and flow rate of BTMSM on the dislocation density of the SiC films.—Figure 4 shows the deposition temperature dependence of the 4H-SiC共0004兲 full width at halfmaximum 共FWHM兲 of rocking curve in the DCD mode for SiC epi layers. For comparison, the rocking curves were taken before and after the epitaxial growth. FWHM of all the substrates lies in the range of 15-17 arcsec. The wafer pieces having the strong mosaicity or FWHM of more than 20 arcsec were not used as the substrates for CVD growth. FWHM of the rocking curve for the epi layers mo-

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Figure 3. 共a兲 SEM morphologies of the film grown at 30 sccm H2 carrier gas flow rate of BTMSM and 1380°C. The 兵001其 pole figures of 共b兲 A region and 共c兲 B region marked in Fig. 4a, which were obtained from EBSD patterns coupled with SEM.

peak width broadening caused by instruments ␤ instr , and the peak width broadening caused by dislocation ␤ dis 2 2 2 ␤m 共 hkl 兲 ⫽ ␤ 20 共 hkl 兲 ⫹ ␤ instr 共 hkl 兲 ⫹ ␤ dis 共 hkl 兲

␤ 0 (hkl), which is the intrinsic rocking curve width for the measured crystal, can be calculated as15 ␤ 0 共 hkl 兲 ⫽ 关 r e␭ 2 共 1 ⫹ 兩 cos 2␪ 兩兲兩 F hkl 兩兴 ⫻ 关 sin共 ␪ ⫺ ␾ 兲 /sin共 ␪ ⫹ ␾ 兲兴 1/2/ 关 ␲V sin共 2␪ 兲兴

Figure 2. SEM micrographs showing the surface morphologies of the SiC films grown at 共a兲 1280°C, 共c兲 1320°C, 共d兲 1380°C, and 共b兲 magnified micrograph of 共a兲.

notonously decreases at increasing growth temperature from 1280 to 1510°C, indicating higher quality crystallinity at higher temperature. Moreover, the FWHM of the epi layers grown above 1380°C is slightly lower than that of the substrates. The measured FWHM ␤ m can be expressed as the sum of the following contributions:14 the intrinsic Bragg peak width ␤ 0 , the

Figure 4. The variation of 4H-SiC共0004兲 Bragg spot width of SiC thin films as a function of the growth temperature at 10 sccm H2 carrier gas flow rate of BTMSM. For comparison, the substrates were analyzed using the RSM technique before and after epitaxial growth.


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Figure 5. The variation of EPD of the SiC thin films as a function of the growth temperature at 10 sccm H2 carrier gas flow rate of BTMSM.

where r e is the classical electron radius, 2.818 ⫻ 10⫺5 Å, ␭ is the X-ray wavelength, ␪ is the Bragg angle, 兩 F hkl 兩 is the magnitude of the structure factor for the 共hkl兲 reflection, V is the unit cell volume, and ␾ is the angle between the crystal surface and the diffracting planes. Intrinsic rocking curve width is approximately 3.73 arcsec for 共0004兲 Bragg spot of the 8° off-axis 4H-SiC共0001兲 crystal. ␤ instr(hkl) is the intrinsic rocking curve width for the first crystal in the case of DCD, which is also constant for the same X-ray geometry 关共0004兲 Bragg spot兴. Therefore, the measured FWHM variations of the rocking curve are mainly dominated by the dislocation density. The FWHM of the 4H-SiC films grown at the low temperature of 1280°C is 40 arcsec, while that of the substrate is 15.1 arcsec. This fact suggests that a lot of new dislocations are generated at the defect sites due to low adatoms mobility during crystal growth as well as the threading dislocations extended from the substrate. However, FWHM of the 4H-SiC epi layers grown above 1380°C is slightly lower than that of the substrate. Therefore, it is tentatively concluded that the dislocations existing in the epi layers grown at relatively high temperature are mainly the threading dislocations replicated from the substrate. Moreover, the slight reduction of FWHM of the epi layers compared to that of substrates might be attributed to the partial annihilation of threading dislocations with opposite Burgers vector during CVD growth. To confirm the correlation between the FWHM and dislocation density, etch pit densities 共EPD兲 of the epi layers were investigated. EPDs of the epi layers were examined by wet etching, which was performed in molten KOH at 550°C for 10 min to reveal the defects, followed by optical microscopy. Figure 5 shows the variation of EPD of the epi layers grown on PSC as a function of growth temperature. As expected, EPDs of the epi layers decreased as the growth temperature increases. A typical EPD of the substrate before the epi growth is from 1 ⫻ 104 to 4 ⫻ 104 cm⫺2 . EPDs of the epi layers grown between 1380 and 1440°C are comparable to those of substrate, and EPDs of the epi layers at 1510°C are lower than the typical values of the substrate. Figure 6a and b shows the variations of 4H-SiC共0004兲 Bragg spot width and EPDs as a function of the H2 carrier gas flow rate of BTMSM, respectively. The growth temperature and pressure were fixed to 1380°C and 360 Torr, respectively. FWHM of the epi layers grown between 10 and 25 sccm is slightly lower than that of substrates, while FWHM of the epi layer at 30 sccm rapidly increased to 37.7 arcsec. Similar tendency is observed in EPD variations. There is a critical flow rate of BTMSM for crystalline quality to

Figure 6. The variation of 共a兲 4H-SiC共0004兲 Bragg spot width and 共b兲 EPD of SiC thin films as a function of H2 carrier gas flow rate of BTMSM at 1380°C.

deteriorate. Because the growth rate is proportional to the flow rate of BTMSM, the critical flow rate of BTMSM corresponds to the critical growth rate. Therefore, the growth rate which can be increased without the degradation of crystalline quality of 4H-SiC epitaxial films at 1380°C was 1.5 ␮m/h. Of course, the critical growth rate should be increased with increasing deposition temperature. Under the critical growth rate, dislocation density is insensitive to the flow rate of BTMSM, as shown in Fig. 6b. However, dislocation density of the films grown above the critical growth rate abruptly increased and 3C-SiC polytype with twins grew as a results of high supersaturation of source material. High quality 4H-SiC epitaxial growth on PSC substrate.—Figure 7 shows low temperature photoluminescence 共LTPL兲 spectra of the substrate, and epi layers grown at differing flow rates of BTMSM. Although only broad donor-to-acceptor pair transition peaks appear from the substrate 共Fig. 7a兲,16 the intrinsic free exciton (I 76.4) and Al bound exciton peaks17 were shown in the spectra of the epi layers 共Fig. 7b, c兲. Free exciton related peaks in the LTPL spectrum of SiC film have been interpreted as an indicator of high quality of the epi layer, because they appear only in the high quality film with lower defects density.17-19 On the other hand, aluminum seems to be incorporated unintentionally from the SiC coated graphite susceptor during epitaxial growth process, which was confirmed by secondary

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Figure 7. LTPL spectra of 共a兲 PSC substrate, and epi layers grown at 共b兲 10 and 共c兲 15 sccm H2 flow rate of BTMSM, 1380°C.

Figure 9. A series of rocking curves of substrate and epi layer for 共0004兲 Bragg spot along a line at different positions of off-axis 共0001兲 surface.

ion mass spectrometry depth profiling 共not shown兲. The existence of the sharp free exciton and Al bound exciton lines indicate that the 4H-SiC is of very high quality. The rocking curve of the 共0004兲 Bragg spot of 4H-SiC film grown at 1510°C with 10 sccm H2 carrier flow rate of BTMSM is shown in Fig. 8. FHWM of the rocking curve of the film in TCD mode was 7.6 arcsec, which is an excellent value when compared with that of 13 arcsec of the substrate. It is interesting to note the crystalline quality of the films grown on a normal 8° off-axis 4HSiC共0001兲 substrate. The 4H-SiC epitaxial growth on off-axis 4H 共0001兲 faces by CVD using BTMSM precursor was previously reported by the authors.20 It was found that the structural perfection of 4H-SiC film is improved at the higher deposition temperature and lower source flow rate, which is the same tendency as in this study. However, at 1510°C, the film quality on normal substrate was not better than that of substrate.20 This results might be attributed to the difference of the stress relaxation of the epi layer surface during epitaxial growth. Because the X-ray penetration depth is approximately 10 ␮m, the diffracted X-ray beam consisted of both that from the epi layer and

substrate. In order to evaluate the structural perfection of only the epi layer, a thick 4H-SiC film with 16 ␮m thickness was grown at 1510°C with the 20 sccm flow rate of BTMSM for 8 h. A series of rocking curves for 共0004兲 Bragg spot before and after epitaxial growth were taken along a line at different positions of off-axis 共0001兲 surface, as shown in Fig. 9. Although the SiC thin film grown is thick enough to avoid any XRD signal from the substrate, the FWHM variation of the epi layer along a line at different positions resembles that of substrate. This fact confirmed that the dislocations existing in the epi layers grown at optimized growth condition are mainly the threading dislocations replicated from the substrate. Threading dislocation density of the epi layer is expected to be higher at the region of more defective substrate areas having a lot of dislocation. Therefore, the distribution of dislocation density of the epi layer along the different positions is similar to that of the substrate. The average value of these FWHM is 20.9 and 19.6 arcsec for the substrate and epi layer, respectively. Although the dislocation density of the epi layer was strongly influenced by the substrate, the structural perfection of epi layer on PSC substrate was slightly better than that of substrate itself. Conclusions 4H-SiC thin films have been epitaxially grown on 8° off-oriented PSC substrate using BTMSM source. The activation energy was determined to be 5.6 kcal/mol. The growth on PSC is dominated by a diffusion-limited mechanism. TSFs of 3C-SiC were incorporated in the film grown even at low temperature below 1280°C and high carrier gas flow rate of source material above 30 sccm at the growth temperature of 1380°C resulted in the appearance of 3C-SiC polytype with DPBs. As the growth temperature increased and the flow rate of the source material decreased, the structural perfection of 4H-SiC epi layer was improved, which could be explained by the reduction of dislocation density in the grown films. FWHM of the rocking curve of the film grown at optimized condition is 7.6 arcsec and the sharp free exciton and Al bound exciton lines appear in the epi layer, which indicate that 4H-SiC film is of very high quality. Acknowledgments This work was supported by SiC Device Development Program 共SiCDDP兲 under Ministry of Commerce, Industry, and Energy, Korea, and partially by the Share-ISRC Program through Interuniversity Semiconductor Research Center in the year of 2002.

Figure 8. Rocking curve of 4H-SiC共0004兲 Bragg spot of the epi layer grown on PSC substrate at 1510°C.

Seoul National University assisted in meeting the publication costs of this article.

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