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Fabrication of Si–C–N compounds in silicon carbide by ion implantation
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1294–1298
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Fabrication of Si–C–N compounds in silicon carbide by ion implantation Alexandra A. Suvorova a,*, Tim Nunney b, Alexander V. Suvorov c a b c
Centre for Microscopy, Analysis and Characterisation, The University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia Thermo Fisher Scientific, East Grinstead, West Sussex, United Kingdom CREE Inc., Durham, NC 27703, USA
a r t i c l e
i n f o
Article history: Available online 29 January 2009 PACS: 73.40.T 61.72.T,V,W 68.55.L Keywords: Ion implantation Silicon carbide Silicon nitride Electron microscopy Electron energy loss-spectroscopy X-ray photoelectron spectroscopy
a b s t r a c t The chemical variation and depth profile of silicon carbide implanted with nitrogen and overgrown with epitaxial layer has been studied using X-ray photoelectron spectroscopy (XPS). The results of this study have been supplemented by transmission electron microscopy (TEM) imaging and electron energy lossspectroscopy (EELS) in an attempt to correlate the chemical and structural information. Our results indicate that the nitrogen implantation into silicon carbide results in the formation of the Si–C–N layer. XPS revealed significant change in the bonding structure and chemical states in the implanted region. XPS results can be interpreted in terms of the silicon nitride and silicon carbonitride nanocrystals formation in the implanted region which is supported by the electron microscopy and spectroscopy results. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Si–C–N materials are of considerable interest as high temperature engineering materials combining the properties of silicon carbide and silicon nitride [1]. A number of experimental and theoretical investigations of the electronic and structural properties of the silicon carbonitride films have been carried out using different growth techniques for the Si–C–N film synthesis. Ion implantation provides a practical method of synthesis of Si–C–N compounds and has been successfully employed to produce Si– C–N composite layers in silicon [2,3]. The combination of carbon and nitrogen implantation in silicon has been investigated to produce a silicon carbonitride layers with tailored stoichiometries [2]. It has been shown that Si–C–N films with carbon concentration above the solubility limit in Si3N4 remained amorphous after annealing at 1250 °C. IR and XPS analysis of the Si–C–N films has suggested the formation of an amorphous network of mixed Si(C, N)4 tetrahedrons. Ion implantation has only rarely been applied to the fabrication of Si–C–N layers in silicon carbide material. Early examples studied nitrogen implantation in silicon carbide at room temperature [4]. It has been reported that nitrogen implantation into SiC at room tem-
* Corresponding author. Tel.: +61 8 6488 8095. E-mail address: [email protected] (A.A. Suvorova). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.036
perature associated with the formation of SixCyNz composite. McHargue et al. [5] have shown the possibility of more significant replacement of carbon by nitrogen atoms during nitrogen implantation in silicon carbide at high temperatures. Miyagawa et al. [6] have observed b-Si3N4 crystallites formation in a polycrystalline b-SiC after nitrogen implantation at 1100 °C. The ion implantation with nitrogen ions at room temperature into 70% SiC–C films has also been reported to form SiCyNz compound [7]. In this paper we study the structural, chemical and bonding variations in silicon carbide by implanting nitrogen ions at high dose and high temperature.
2. Experimental 14 + N ions at 200 keV were implanted into the 4H silicon carbide wafers, using Varian300XP ion implanter. The dose of implantation was 1.4 1018 at. cm 2. During implantation the wafers were maintained at temperature of 650 °C. After the implantation, a silicon carbide epitaxial layer (0.65 lm thick) has been deposited on as-implanted layer by CVD method at 1600 °C to produce structure for applications in new integrated devices. The resulted structure is shown in Fig. 1(a). X-ray photoelectron spectroscopy (XPS) was used to assess the chemistry and bonding variations of the implanted layers. The XPS studies were carried out using K-Alpha (Thermo Scientific, UK). The
A.A. Suvorova et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1294–1298
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Fig. 1. (a) The schematic representation of the nitrogen implanted SiC structure with epitaxial layer; (b) TEM image; (c) HREM image of a nanocrystalline inclusion in the implanted layer.
system base pressure was less than 5 10 9 mbar, however the pressure in the analysis chamber during etching and analysis was 2 10 7 mbar due to use of the charge neutralization system which uses a combination of low energy electrons and low energy argon ion to compensate for the loss of photoelectrons from an insulating sample. The sample was etched using an 1000 eV Ar+ ion beam (Argon (99.999%), Scientific and Technical Gases Ltd., UK). Etch rates were calibrated using the position of the implanted N-layer from the TEM images of the sample, and assume that the etch rate remains constant throughout the profile. The local compositional and structural variations of implanted region have been studied using a combination of high resolution electron microscopy (HREM) and electron energy loss-spectroscopy (EELS). The work has been carried out using a JEOL 3000F field emission gun TEM equipped with a Gatan Image Filter. The crosssectional TEM samples were prepared using standard methods (mechanical thinning and Ar-ion milling). 3. Results and discussion 3.1. TEM imaging The TEM image shown in Fig. 1(b) demonstrates the structure of the 4H-SiC film implanted at 200 keV with a nitrogen dose of 1.4 1018 at. cm 2 (14N+) and substrate temperature of 650 °C followed by the 4H-SiC epitaxial film deposition. The implanted region is clearly visible as a bright region with two dark defective regions. The bright region corresponds to the projected range depth of nitrogen. The dark defective regions exhibit significant diffraction contrast due to the strains related to the defects. High resolution imaging has been applied to study the structure of the implanted layer and revealed the formation of nanocrystalline
inclusions in the implanted region (Fig. 1(c)). The interplanar distances of the nanoscrystals measured from high resolution images are closely matching the lattice parameters of hexagonal SiCN (Si2CN4) and Si3N4 structures. The difficulty with the HREM analysis is that the distinction of the silicon carbonitride phase from silicon nitride phase is highly problematic due to the small difference in lattice parameters between the two phases. TEM also revealed that the overgrown layer is a single crystal silicon carbide layer. The diffraction pattern obtained from the 0] zone axis is shown as inset in Fig. 1(b). epi-layer at the [1 1 2 The diffraction pattern exhibits a regular distribution of reflections resulting from 4H crystal arrangement. 3.2. XPS analysis 3.2.1. Depth profiles The depth profiles obtained by X-ray photoelectron spectroscopy (XPS) are shown in Fig. 2. The etching rate was calculated using the information available from TEM measurements. Non-linear least squares fitting and peak fitting at every level was used to generate the profile. The average atomic concentration of each element in the implanted layer has been estimated from the depth profile and was approximately 40% for silicon, 30% for carbon and 30% for nitrogen. Fig. 3 is the XPS depth profiles displayed as a two-dimensional (2-D) image, with intensity represented by a colour scale (from black through a colour to white in this case). Displaying the data in this way helps to visualize how the peaks shift through the profile and how the chemistry changes throughout the profile. The N implantation affects the chemical state of both C and Si, moving them both to higher binding energy compared to the layers either side of the implantation, suggesting some interaction between SiC
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Atomic concentration, %
50
40 XPS depth profile Si2p-SiC Si2p-Si3N4 C1s-SiC C1s C-C C1s C-O/C-N N1s
30
20
10
0
0
200
400
600
800 1000 1200 1400 1600 1800 2000 2200 2400
Depth, nm Fig. 2. X-ray photoelectron spectroscopy depth profiles.
and nitrogen. The observed variation in C 1s depth profile (Fig. 3(a)) may indicate a disordering effect after ion implantation and a reduction of carbon in the implanted layer as the redistribution is taken place.The implanted nitrogen can clearly be seen in the nitrogen depth profile (Fig. 3(b)). Only one major chemical state appears for nitrogen. In addition, the SiC in the upper, near surface layer and the near implantation layer appeared to be chemically different to the bulk of the sample below the N-implanted layer. Here it can be seen that the SiC in the surface layer and around 400 nm below the N-layer produces peaks with lower binding energy than the bulk SiC. This shift towards lower energies relative to the bulk SiC may indicate a change in the SiC structure in these regions, e.g. the disorder-induced bond length deviations. 3.2.2. High resolution XPS Fig. 4 shows the high resolution Si 2p, C 1s and N 1s XPS spectra from the middle of the implanted layer. All XPS peak positions observed in the nitrogen implanted region are listed in Table 1. Deconvolution of the C 1s peaks reveals two main components at 283.4 eV and 284.4 eV well known for carbidic and graphitic binding states, respectively [8,9] and one minor component at 286.0 eV attributed to C(sp2)–N bonds. This is in agreement with the energy shift of the main Si 2p peak (101.8 eV) from pure Si3N4 (102 eV) to pure SiC (100.6 eV). The chemical shift of the Si 2p peak observed in the implanted region can be interpreted in terms of the Si3N4/ SiCN formation in the implanted layer (which is supported by the previous electron microscopy results). The deconvoluted N 1s spectrum (Fig. 4(c)) revealed that the low binding energy (BE) peak is more Si–N in character, but has some C component as the binding energy is too high for pure Si3N4 (397.5 eV). The higher BE peak (the smaller peak) could be some kind of CN bonding (a mixture of cyanide 399.37 eV and C(sp2)–N 400 eV bonds), but is most likely to be C–N (as seen in the C1s spectrum). In addition to main peaks, all deconvoluted XPS spectra reveal small high-energetic components attributed to the surface oxidation during XPS measurements. This could result from the mounting tape on the sample holder outgasing CO/CO2 which is adsorbing on the sample. O1s composition is measured at 1% which corresponds well with the quantification of the high BE component in the C1s spectrum – also consistent with C@O as would be expected for adsorbed CO/CO2. 3.3. EELS analysis The electronic states of the atoms can be obtained from the fine structure of the elemental peaks observed in the EELS spectra. This permits phase identification and bonding variations to be studied at
Fig. 3. Two-dimensional (2-D) C 1s (a), N 1s (b) and Si 2p (c) profiles with the chemistry changes through the profiles.
A.A. Suvorova et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1294–1298
a
1297
Si2p Scan - N-layer
2.00E+04
Counts / s
1.50E+04
1.00E+04
5.00E+03
0.00E+00 110
108
106
104
102
100
98
96
Binding Energy (eV)
b
C1s Scan - N-layer
1.80E+04 1.60E+04
Counts / s
1.40E+04 1.20E+04 1.00E+04 8.00E+03 6.00E+03 4.00E+03 2.00E+03 298 296 294 292 290 288 286 284 282 280
Binding Energy (eV)
c
Si2p Scan - N-layer
2.00E+04
Fig. 5. Electron energy loss spectra (EELS) taken from various regions across the implanted structure: (a) SiC crystalline region; (b) SiCN implanted region.
Counts / s
1.50E+04
1.00E+04
5.00E+03
0.00E+00 110 108
106 104 102 100
98
96
Binding Energy (eV) Fig. 4. High resolution C 1s, N 1s and Si 2p XPS spectra of the N-doped implanted regions.
Table 1 Summary of Si 2p, C 1s and N 1s peak positions determined by X-ray photoelectron spectroscopy. Binding energies
Si 2p (eV)
C 1s (eV)
N 1s (eV)
Si–C Si–C–N or Si–N C–N C–C
100.8 101.8
283.4 284.4 286 284.4
398.4 399.7
the atomic level. The results can be compared with reference data obtained from standard materials or related material systems. Fig. 5 shows the Si L2,3 and C K-edge EELS spectra following the background subtraction. EELS spectrum obtained from Si–C–N region (Fig. 5(b)) demonstrates relatively weak peak at 285 eV and can be assigned to the p* states of sp2-bonded carbon. This pre-edge p* peak indicates that substantial percentage of carbon atoms in the Si–C–N layer have graphite-like sp2 bonding. This is in agreement with high concentration of C–C bonds in the implanted layer observed by XPS. It suggests that carbon replaced by implanted nitrogen in SiC could form the amorphous component in the SiCN layer. The spectrum of Si–C–N region also exhibits a shift (1.6 eV for Si L2,3 ionization edge) towards higher energy relative to the bulk SiC spectrum. This is consistent with the previous reported elemental and chemical effects in the near-edge onset regions of the Si L2,3 edge in SiC and Si3N4 compounds [10]. It may indicate the formation of Si3N4/Si2CN4 nanocrystalline inclusions in the implanted region which is correlated with the structural data obtained by high resolution electron microscopy. 4. Conclusion Our results indicate that the nitrogen implantation into silicon carbide results in the formation of the silicon carbonitride layer.
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The epitaxial overgrowth of the implanted SiCN layer results in high quality single crystal 4H-SiC layer. High resolution imaging of structural defects revealed the formation of Si3N4/Si2CN4 nanocrystalline inclusions and amorphous graphitic component in the implanted layer. XPS revealed significant change in the bonding structure and chemical states in the various layers across the structure. XPS N 1s peak has shown one major component at 398.0 eV and has been attributed to SiCN elemental composition in the implanted layer. The SiC chemistry was found to change in the implantation layer, with both Si 2p and C 1s peaks moving to higher binding energy compared to the layers either side of the implantation layer. The XPS studies of the chemistry in SiCN layer were correlated with HREM and EELS measurements in the implanted layer. Acknowledgements The authors acknowledge the facilities, scientific and technical assistance of the Australian Microscopy and Microanalysis Re-
search Facility at the Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, a facility funded by The University, State and Commonwealth Governments. References [1] R. Riedel, H.J. Kleebe, H. Schonfelder, F. Aldinger, Nature 374 (1995) 526. [2] M. Rudolphi, M. Bruns, H. Baumann, U. Geckle, Diam. Relat. Mater. 16 (2007) 1273. [3] A.A. Suvorova, A.V. Suvorov, Nucl. Instr. and Meth. B 257 (2007) 217. [4] A. Nakao, M. Iwaki, H. Sakairi, K. Terasima, Nucl. Instr. and Meth. B 65 (1992) 352. [5] C.J. McHargue, J.M. Williams, Nucl. Instr. and Meth. B 80–81 (1993) 889. [6] S. Miyagawa, K. Baba, M. Ikeyama, K. Saitoh, S. Nakao, Y. Miyagawa, Nucl. Instr. and Meth. B 127–128 (1997) 651. [7] J.-R. Lei, D.Z. Wang, N.K. Huang, J. Kor. Phys. Soc. 46 (2005) S11. [8] N.I. Fainer et al., Nucl. Instr. and Meth. A 470 (2001) 193. [9] T.P. Smirnova et al., Thin Solid Films 429 (2003) 144. [10] W.M. Skiff, R.W. Carpenter, S.H. Lin, J. Appl. Phys. 62 (1987) 2439.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Fabrication of Si–C–N compounds in silicon carbide by ion implantation Alexandra A. Suvorova a,*, Tim Nunney b, Alexander V. Suvorov c a b c
Centre for Microscopy, Analysis and Characterisation, The University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia Thermo Fisher Scientific, East Grinstead, West Sussex, United Kingdom CREE Inc., Durham, NC 27703, USA
a r t i c l e
i n f o
Article history: Available online 29 January 2009 PACS: 73.40.T 61.72.T,V,W 68.55.L Keywords: Ion implantation Silicon carbide Silicon nitride Electron microscopy Electron energy loss-spectroscopy X-ray photoelectron spectroscopy
a b s t r a c t The chemical variation and depth profile of silicon carbide implanted with nitrogen and overgrown with epitaxial layer has been studied using X-ray photoelectron spectroscopy (XPS). The results of this study have been supplemented by transmission electron microscopy (TEM) imaging and electron energy lossspectroscopy (EELS) in an attempt to correlate the chemical and structural information. Our results indicate that the nitrogen implantation into silicon carbide results in the formation of the Si–C–N layer. XPS revealed significant change in the bonding structure and chemical states in the implanted region. XPS results can be interpreted in terms of the silicon nitride and silicon carbonitride nanocrystals formation in the implanted region which is supported by the electron microscopy and spectroscopy results. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Si–C–N materials are of considerable interest as high temperature engineering materials combining the properties of silicon carbide and silicon nitride [1]. A number of experimental and theoretical investigations of the electronic and structural properties of the silicon carbonitride films have been carried out using different growth techniques for the Si–C–N film synthesis. Ion implantation provides a practical method of synthesis of Si–C–N compounds and has been successfully employed to produce Si– C–N composite layers in silicon [2,3]. The combination of carbon and nitrogen implantation in silicon has been investigated to produce a silicon carbonitride layers with tailored stoichiometries [2]. It has been shown that Si–C–N films with carbon concentration above the solubility limit in Si3N4 remained amorphous after annealing at 1250 °C. IR and XPS analysis of the Si–C–N films has suggested the formation of an amorphous network of mixed Si(C, N)4 tetrahedrons. Ion implantation has only rarely been applied to the fabrication of Si–C–N layers in silicon carbide material. Early examples studied nitrogen implantation in silicon carbide at room temperature [4]. It has been reported that nitrogen implantation into SiC at room tem-
* Corresponding author. Tel.: +61 8 6488 8095. E-mail address: [email protected] (A.A. Suvorova). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.036
perature associated with the formation of SixCyNz composite. McHargue et al. [5] have shown the possibility of more significant replacement of carbon by nitrogen atoms during nitrogen implantation in silicon carbide at high temperatures. Miyagawa et al. [6] have observed b-Si3N4 crystallites formation in a polycrystalline b-SiC after nitrogen implantation at 1100 °C. The ion implantation with nitrogen ions at room temperature into 70% SiC–C films has also been reported to form SiCyNz compound [7]. In this paper we study the structural, chemical and bonding variations in silicon carbide by implanting nitrogen ions at high dose and high temperature.
2. Experimental 14 + N ions at 200 keV were implanted into the 4H silicon carbide wafers, using Varian300XP ion implanter. The dose of implantation was 1.4 1018 at. cm 2. During implantation the wafers were maintained at temperature of 650 °C. After the implantation, a silicon carbide epitaxial layer (0.65 lm thick) has been deposited on as-implanted layer by CVD method at 1600 °C to produce structure for applications in new integrated devices. The resulted structure is shown in Fig. 1(a). X-ray photoelectron spectroscopy (XPS) was used to assess the chemistry and bonding variations of the implanted layers. The XPS studies were carried out using K-Alpha (Thermo Scientific, UK). The
A.A. Suvorova et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1294–1298
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Fig. 1. (a) The schematic representation of the nitrogen implanted SiC structure with epitaxial layer; (b) TEM image; (c) HREM image of a nanocrystalline inclusion in the implanted layer.
system base pressure was less than 5 10 9 mbar, however the pressure in the analysis chamber during etching and analysis was 2 10 7 mbar due to use of the charge neutralization system which uses a combination of low energy electrons and low energy argon ion to compensate for the loss of photoelectrons from an insulating sample. The sample was etched using an 1000 eV Ar+ ion beam (Argon (99.999%), Scientific and Technical Gases Ltd., UK). Etch rates were calibrated using the position of the implanted N-layer from the TEM images of the sample, and assume that the etch rate remains constant throughout the profile. The local compositional and structural variations of implanted region have been studied using a combination of high resolution electron microscopy (HREM) and electron energy loss-spectroscopy (EELS). The work has been carried out using a JEOL 3000F field emission gun TEM equipped with a Gatan Image Filter. The crosssectional TEM samples were prepared using standard methods (mechanical thinning and Ar-ion milling). 3. Results and discussion 3.1. TEM imaging The TEM image shown in Fig. 1(b) demonstrates the structure of the 4H-SiC film implanted at 200 keV with a nitrogen dose of 1.4 1018 at. cm 2 (14N+) and substrate temperature of 650 °C followed by the 4H-SiC epitaxial film deposition. The implanted region is clearly visible as a bright region with two dark defective regions. The bright region corresponds to the projected range depth of nitrogen. The dark defective regions exhibit significant diffraction contrast due to the strains related to the defects. High resolution imaging has been applied to study the structure of the implanted layer and revealed the formation of nanocrystalline
inclusions in the implanted region (Fig. 1(c)). The interplanar distances of the nanoscrystals measured from high resolution images are closely matching the lattice parameters of hexagonal SiCN (Si2CN4) and Si3N4 structures. The difficulty with the HREM analysis is that the distinction of the silicon carbonitride phase from silicon nitride phase is highly problematic due to the small difference in lattice parameters between the two phases. TEM also revealed that the overgrown layer is a single crystal silicon carbide layer. The diffraction pattern obtained from the 0] zone axis is shown as inset in Fig. 1(b). epi-layer at the [1 1 2 The diffraction pattern exhibits a regular distribution of reflections resulting from 4H crystal arrangement. 3.2. XPS analysis 3.2.1. Depth profiles The depth profiles obtained by X-ray photoelectron spectroscopy (XPS) are shown in Fig. 2. The etching rate was calculated using the information available from TEM measurements. Non-linear least squares fitting and peak fitting at every level was used to generate the profile. The average atomic concentration of each element in the implanted layer has been estimated from the depth profile and was approximately 40% for silicon, 30% for carbon and 30% for nitrogen. Fig. 3 is the XPS depth profiles displayed as a two-dimensional (2-D) image, with intensity represented by a colour scale (from black through a colour to white in this case). Displaying the data in this way helps to visualize how the peaks shift through the profile and how the chemistry changes throughout the profile. The N implantation affects the chemical state of both C and Si, moving them both to higher binding energy compared to the layers either side of the implantation, suggesting some interaction between SiC
1296
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Atomic concentration, %
50
40 XPS depth profile Si2p-SiC Si2p-Si3N4 C1s-SiC C1s C-C C1s C-O/C-N N1s
30
20
10
0
0
200
400
600
800 1000 1200 1400 1600 1800 2000 2200 2400
Depth, nm Fig. 2. X-ray photoelectron spectroscopy depth profiles.
and nitrogen. The observed variation in C 1s depth profile (Fig. 3(a)) may indicate a disordering effect after ion implantation and a reduction of carbon in the implanted layer as the redistribution is taken place.The implanted nitrogen can clearly be seen in the nitrogen depth profile (Fig. 3(b)). Only one major chemical state appears for nitrogen. In addition, the SiC in the upper, near surface layer and the near implantation layer appeared to be chemically different to the bulk of the sample below the N-implanted layer. Here it can be seen that the SiC in the surface layer and around 400 nm below the N-layer produces peaks with lower binding energy than the bulk SiC. This shift towards lower energies relative to the bulk SiC may indicate a change in the SiC structure in these regions, e.g. the disorder-induced bond length deviations. 3.2.2. High resolution XPS Fig. 4 shows the high resolution Si 2p, C 1s and N 1s XPS spectra from the middle of the implanted layer. All XPS peak positions observed in the nitrogen implanted region are listed in Table 1. Deconvolution of the C 1s peaks reveals two main components at 283.4 eV and 284.4 eV well known for carbidic and graphitic binding states, respectively [8,9] and one minor component at 286.0 eV attributed to C(sp2)–N bonds. This is in agreement with the energy shift of the main Si 2p peak (101.8 eV) from pure Si3N4 (102 eV) to pure SiC (100.6 eV). The chemical shift of the Si 2p peak observed in the implanted region can be interpreted in terms of the Si3N4/ SiCN formation in the implanted layer (which is supported by the previous electron microscopy results). The deconvoluted N 1s spectrum (Fig. 4(c)) revealed that the low binding energy (BE) peak is more Si–N in character, but has some C component as the binding energy is too high for pure Si3N4 (397.5 eV). The higher BE peak (the smaller peak) could be some kind of CN bonding (a mixture of cyanide 399.37 eV and C(sp2)–N 400 eV bonds), but is most likely to be C–N (as seen in the C1s spectrum). In addition to main peaks, all deconvoluted XPS spectra reveal small high-energetic components attributed to the surface oxidation during XPS measurements. This could result from the mounting tape on the sample holder outgasing CO/CO2 which is adsorbing on the sample. O1s composition is measured at 1% which corresponds well with the quantification of the high BE component in the C1s spectrum – also consistent with C@O as would be expected for adsorbed CO/CO2. 3.3. EELS analysis The electronic states of the atoms can be obtained from the fine structure of the elemental peaks observed in the EELS spectra. This permits phase identification and bonding variations to be studied at
Fig. 3. Two-dimensional (2-D) C 1s (a), N 1s (b) and Si 2p (c) profiles with the chemistry changes through the profiles.
A.A. Suvorova et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1294–1298
a
1297
Si2p Scan - N-layer
2.00E+04
Counts / s
1.50E+04
1.00E+04
5.00E+03
0.00E+00 110
108
106
104
102
100
98
96
Binding Energy (eV)
b
C1s Scan - N-layer
1.80E+04 1.60E+04
Counts / s
1.40E+04 1.20E+04 1.00E+04 8.00E+03 6.00E+03 4.00E+03 2.00E+03 298 296 294 292 290 288 286 284 282 280
Binding Energy (eV)
c
Si2p Scan - N-layer
2.00E+04
Fig. 5. Electron energy loss spectra (EELS) taken from various regions across the implanted structure: (a) SiC crystalline region; (b) SiCN implanted region.
Counts / s
1.50E+04
1.00E+04
5.00E+03
0.00E+00 110 108
106 104 102 100
98
96
Binding Energy (eV) Fig. 4. High resolution C 1s, N 1s and Si 2p XPS spectra of the N-doped implanted regions.
Table 1 Summary of Si 2p, C 1s and N 1s peak positions determined by X-ray photoelectron spectroscopy. Binding energies
Si 2p (eV)
C 1s (eV)
N 1s (eV)
Si–C Si–C–N or Si–N C–N C–C
100.8 101.8
283.4 284.4 286 284.4
398.4 399.7
the atomic level. The results can be compared with reference data obtained from standard materials or related material systems. Fig. 5 shows the Si L2,3 and C K-edge EELS spectra following the background subtraction. EELS spectrum obtained from Si–C–N region (Fig. 5(b)) demonstrates relatively weak peak at 285 eV and can be assigned to the p* states of sp2-bonded carbon. This pre-edge p* peak indicates that substantial percentage of carbon atoms in the Si–C–N layer have graphite-like sp2 bonding. This is in agreement with high concentration of C–C bonds in the implanted layer observed by XPS. It suggests that carbon replaced by implanted nitrogen in SiC could form the amorphous component in the SiCN layer. The spectrum of Si–C–N region also exhibits a shift (1.6 eV for Si L2,3 ionization edge) towards higher energy relative to the bulk SiC spectrum. This is consistent with the previous reported elemental and chemical effects in the near-edge onset regions of the Si L2,3 edge in SiC and Si3N4 compounds [10]. It may indicate the formation of Si3N4/Si2CN4 nanocrystalline inclusions in the implanted region which is correlated with the structural data obtained by high resolution electron microscopy. 4. Conclusion Our results indicate that the nitrogen implantation into silicon carbide results in the formation of the silicon carbonitride layer.
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The epitaxial overgrowth of the implanted SiCN layer results in high quality single crystal 4H-SiC layer. High resolution imaging of structural defects revealed the formation of Si3N4/Si2CN4 nanocrystalline inclusions and amorphous graphitic component in the implanted layer. XPS revealed significant change in the bonding structure and chemical states in the various layers across the structure. XPS N 1s peak has shown one major component at 398.0 eV and has been attributed to SiCN elemental composition in the implanted layer. The SiC chemistry was found to change in the implantation layer, with both Si 2p and C 1s peaks moving to higher binding energy compared to the layers either side of the implantation layer. The XPS studies of the chemistry in SiCN layer were correlated with HREM and EELS measurements in the implanted layer. Acknowledgements The authors acknowledge the facilities, scientific and technical assistance of the Australian Microscopy and Microanalysis Re-
search Facility at the Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, a facility funded by The University, State and Commonwealth Governments. References [1] R. Riedel, H.J. Kleebe, H. Schonfelder, F. Aldinger, Nature 374 (1995) 526. [2] M. Rudolphi, M. Bruns, H. Baumann, U. Geckle, Diam. Relat. Mater. 16 (2007) 1273. [3] A.A. Suvorova, A.V. Suvorov, Nucl. Instr. and Meth. B 257 (2007) 217. [4] A. Nakao, M. Iwaki, H. Sakairi, K. Terasima, Nucl. Instr. and Meth. B 65 (1992) 352. [5] C.J. McHargue, J.M. Williams, Nucl. Instr. and Meth. B 80–81 (1993) 889. [6] S. Miyagawa, K. Baba, M. Ikeyama, K. Saitoh, S. Nakao, Y. Miyagawa, Nucl. Instr. and Meth. B 127–128 (1997) 651. [7] J.-R. Lei, D.Z. Wang, N.K. Huang, J. Kor. Phys. Soc. 46 (2005) S11. [8] N.I. Fainer et al., Nucl. Instr. and Meth. A 470 (2001) 193. [9] T.P. Smirnova et al., Thin Solid Films 429 (2003) 144. [10] W.M. Skiff, R.W. Carpenter, S.H. Lin, J. Appl. Phys. 62 (1987) 2439.