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Carrier concentration profiles in 6H-SiC by scanning capacitance microscopy
Materials Science in Semiconductor Processing 4 (2001) 195–199
Carrier concentration profiles in 6H-SiC by scanning capacitance microscopy F. Giannazzoa, P. Musumecia, L. Calcagnoa,*, A. Makhtarib, V. Rainerib a
INFM and Dipartimento di Fisica ed Astronomia } Corso Italia 57 } 95129 Catania, Italy b CNR-IMETEM } Stradale Primosole 50 } 95121 Catania, Italy
Abstract We have used scanning capacitance microscopy to determine two-dimensional carrier distributions in 6H-SiC on both epitaxial layers and implanted samples. Measurements were carried out on cross-sections using metal-covered Si tips. The sample preparation, surface passivation and tip selection have been investigated to obtain an optimised procedure. Implants were performed on p-type 6H-SiC at a substrate temperature of 5008C with N ions at different fluences. The defect profiles were determined by Rutherford backscattering spectrometry. The influence of the implanted damage on the measurements has been investigated by implanted Si-self ions. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Doping; Carrier concentration; Ion implantation; Defects
1. Introduction The research on silicon carbide material growth and device technology is presently in progress due to its excellent properties and promising device characteristics. In fact, SiC is a wide band gap (3 eV) semiconductor, which exhibits a high electron mobility, high thermal conductivity, high temperature stability and good chemical resistance. All these properties make this material attractive for electronic devices operating at high power, high frequency and high temperature [1,2]. The electrical properties of devices can be controlled by dopant incorporation, and doping of SiC is generally achieved during epitaxial growth. However, this method does not offer planar-selective doping which is essential for making SiC devices. Furthermore, doping by thermal diffusion, which is very popular in Si technology, is not feasible for SiC due to very low diffusion coefficients of all useful acceptor and donor dopants in SiC [3]. Doping by ion implantation is an alternative method to overcome these problems, but its main *Corresponding author. Tel.: +39-095-7195420; fax: +39095-383023. E-mail address: [email protected] (L. Calcagno).
disadvantage is the formation of damage due to the collision of introduced ions with the target atoms. To produce electrically active regions, the damage must be removed by post annealing processes, but in SiC the requested temperatures are very high (>15008C) [4,5]. One possible way to reduce the damage is the implantation of SiC at elevated temperatures. High-temperature ion implantation produces dynamic annealing with a decrease of damage and simultaneously with the incorporation of a considerable fraction of dopants onto substitutional sites. It has been shown by several authors [6–8] that implantation of N, Al and B in the temperature range 400–7008C reduces considerably the amount of defects and prevents amorphization of the material. Because the electrical properties of doped region have a huge impact on the operation of devices, there is a pressing need to profile the number of carriers in doped SiC. The determination of carrier profiling in SiC present many difficulties due to the material characteristics (hardness, band gap, and Schottky barrier). Many of the standard techniques used for silicon devices cannot be adopted for silicon carbide due to technical or physical limits (SiC hardness, sample preparation, etc.). The capacitance measurements can be efficiently used,
1369-8001/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 0 ) 0 0 1 2 9 - 3
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F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199
however, this technique, based mainly on mercury probes, present many limits on carrier delineation as a poor contact capability and a limited dynamic range. Even a two-dimensional characterisation is required to define the edge termination in power device. However, not so many techniques have been developed to date in particular for two-dimensional carrier profiling. Recently, scanning capacitance microscopy (SCM) has been developed for two-dimensional carrier profiling in silicon [9,10]. The method produces carrier-sensitive images of semiconductor device even if many effects distorting them have been reported. Imaging carrier distribution involves a correct sample preparation, an accurate measurement and the elimination of potential distortions. In this work we demonstrate that SCM can be successfully applied to image carrier concentration in SiC. However, artefacts can be introduced when measurements are performed in ion-implanted layers. The damage due to ion implantation produces contrast in the SCM image: this effect is carefully investigated and described.
2. Experimental details In this work we have used two different kinds of 6 HSiC wafers (50 mm diameter) Si faced supplied by CREE: (a) n-type wafers with a bulk doping level of 3 1018 cm3 and a first n-type epilayer of 3 1015 cm3 and a second epilayer 1 1019 cm3, (b) p-type wafers with a bulk doping level of 3 1016 cm3. The p-type wafers were implanted with 200 keV N+ at two different fluences, 2 1014 and 2 1015 cm2, and with 250 keV Si+ at a fluence of 1015 cm2. The substrate temperature during implantation was maintained at 5008C. The profiles of defects produced by ion implantation were analysed by Rutherford backscattering spectrometry in channeling configuration using 1.6 MeV He beam incident normally to the sample surface. The aligned spectra were taken along the h0 0 0 1i sample axis. SCM data were acquired using Digital Instruments Dimension 3100 SCM. The AC amplitude applied to the sample has been optimised considering the C–V curve. DC bias is applied to the sample and its value was determined by sample optimising the dC/dV signal. Sample preparation is a fundamental step of measurements because it guarantees their success and reproducibility. We adopted a method described in the following to ensure their reproducibility. Cross-section sample preparation was performed in some steps. First, samples were cut on the region of interest; then, on the rear a thin ( 10 nm) gold layer was deposited. Afterwards, samples were glued together to obtain a sandwich. By a diamond saw, the sandwich was cut in a parallelepiped
of roughly 0.5 0.5 0.2 mm. On one of the two parallel sides with the specimen sections (the longer dimensions) a gold layer ( 10 nm) was deposited. On the opposite side, the sandwich was polished to obtain the flat and clean surface required by scanning C–V measurements. The sandwich was mounted on a fixed support standing on a lapping machine and incrementally polished. We used first 10 mm diamond lapping film to remove saw marks and flatten the sandwich side. We achieved a finer and finer grind by using decrement size lapping films (3, 1, 0.5, 0.1 mm). Before measurements, the sample was cleaned by a dip in HF (5%) and then reoxidised by immersion in H2O2 for 10 min. This process produces a uniform oxide thickness on the polished surface of 2.0 nm. Capacitance measurements are influenced by surface states, directly related to the surface treatments, and by the oxide thickness. Due to the SiC hardness polishing with 0.1 mm diamond lapping film for enough time to remove all surface scratches due to the previous grind polishing is enough to obtain a flat surface. By atomic force measurements, we measured a surface roughness better than 1 nm. The uniformity in the oxide layer was ensured by the wet oxidation and it has been checked by measuring on the sample surface of uniformly doped samples. In this case, change in the oxide thickness should appear as variation in the dC/dV signal.
3. Results and discussion SCM technique presents many difficulties and, in order to test the reliability of our procedure, we have performed measurements on epitaxial layers. The SCM data obtained in SiC samples containing two epitaxial layers are reported in Fig. 1. The SCM image (Fig. 1a) shows the presence of a first layer about 5.5 mm thick on the SiC substrate followed by a second layer about 2 mm thick. According to the manufacturer’s specifications, the substrate is an n-type high concentration (3.3 1018 cm3) substrate. The first epitaxial layer is also ntype doped with an uniform concentration of 2.6 1015 cm3, while the second layer is p-doped with an uniform concentration of 1.3 1019 cm3. The image was obtained on a sandwich of two equal samples. The profile reported in Fig. 1b is obtained from the SCM image and represents the dC/dV versus depth signal collected during the scan. The dC/dV scale is given in arbitrary units. It should be noted that the shape of the dC/dV profile resembles qualitatively the concentration steps of the epitaxial sample, but the dC/dV dependence on the concentration value is quite complex, also taking into account the different flat band voltage values in the p- and n-type doped zones. It is also interesting to note how the transition between two different doping regions is very steep. This is a confirmation of the high spatial
F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199
197
Fig. 2. dC/dV signal versus depth for SiC samples implanted with 200 keV N+ at two different fluences. The substrate temperature during implantation was maintained at 5008C.
Fig. 1. SCM image of the dC/dV signal reported in false colour for a SiC sample with two epitaxial layers grown on a 6H-SiC substrate (a), dC/dV signal versus depth in a plot for an image line (b).
resolution of the SCM measurement, essentially determined by the tip dimension. The SCM measurements were carried out on p-type SiC implanted with N ions, which is a donor dopant. The obtained two-dimensional SCM profiles are reported in Fig. 2: these profiles refer to implantation performed at a substrate temperature of 5008C with two different fluences 2 1014 and 2 1015 cm2. The SCM profile of low-fluence implanted sample shows a constant signal up to a depth of 0.2 mm, then decreases and reaches the substrate level at 0.43 mm. The region up to 0.43 mm corresponds to the implanted layer. The profile of high-fluence implanted sample shows the same characteristics, but contrary to expectations, the signal is lower with respect to low fluence one. This result, surprisingly, evidences that the total amount of carrier concentration is lower in high-fluence
implantations. Moreover in both samples the depth of implanted layer (0.43 mm) is larger than the range of implanted ion (0.32 mm) and the profile shape is not a Gaussian as expected for a single-energy implantation. On the other hand, by measurements on van der Pauw patterns, we determined an active fraction of 1–5%. The results obtained on ion-implanted SiC samples indicate that SCM measurements are not only sensitive to the electrically active part of the dopant, but are affected by some other factors. Generally, the presence of defects in a semiconductor crystal introduces some trap centres in the band gap. C–V measurements are sensitive to these levels; then a dC/dV signal can be related to defect rich although not doped sample regions. This could justify an SCM signal extending in depth much further than the ion-projected range. Moreover, a high-defect density strongly modifies the dC/dV signal associated with free carriers in the doped zone. In order to detect the defect profile, Rutherford backscattering analysis in channeling configuration were performed in both samples and the obtained spectra are shown in Fig. 3. The spectra are normalised to the same random level. The channeling backscattering yield increases in the near-surface region right after implantation with a fluence of 1 1014 cm2 and the damage peak is located at a depth of about 300 nm below the surface. The channeling backscattering yield increases after the higher fluence implantation; however, the shape is almost the same. The thickness of damaged region is about 400 nm, and the depth scale has been calculated using the density of crystalline SiC (3.2 g/cm3), as our analysis are restricted to low-level damage samples and change in the material density is observed when an amorphous layer is formed [11]. Moreover, at the used substrate
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F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199
Fig. 3. RBS-channeling spectra for SiC samples implanted at 5008C with 200 keV N+ at two different fluences. The depth scale is also shown.
be related to the implanted layer. Rutherford backscattering spectra (not reported) shows the presence of defects up to a depth of 1.0 mm. These results evidence that SCM measurements are strongly affected by the presence of defects. This statement is also supported by the thickness of implanted layer measured with SCM, which in implanted samples is larger than the range of implanted Si ions (0.35 mm). In fact, as reported in literature [12] high-temperature implantation produces damaged regions larger than the ion range because of the defect diffusion, while dopant profile does not change with the implantation temperature. The obtained results show that scanning capacitance microscopy is able to measure carrier concentration profiles in silicon carbide, however, in ion-implanted samples, the contribution of defects strongly affects the data. More experiments are required in order to distinguish the dopant and the defect contribution; these include post-implantation annealing processes in order to reduce the amount of defects. Furthermore, calibration must be performed in order to quantify the carrier concentration profiles.
4. Conclusions
Fig. 4. SCM image of SiC samples implanted with 250 keV Si ions at a fluence of 1 1015 cm2 and at a substrate temperature of 5008C.
temperature SiC amorphization is never reached because of the dynamic annealing of defects during implantation [6,7]. In order to check the influence of the defects on the SCM data, measurements were carried out also in SiC samples implanted with Si ions at a fluence of 1015 cm2 and at a substrate temperature of 5008C. This implantation produces defects without any doping effect. The SCM image of this sample is shown in Fig. 4. The measurement was carried out on the cross-section of a sample obtained by gluing a Si-implanted SiC sample and a crystalline SiC sample. A contrast region about 1.05 mm thick is observed at each side of the glue. It can
Scanning capacitance microscopy has been developed in SiC on both epitaxial and ion-implanted samples. The sample preparation has been improved in order to obtain an optimised procedure. The measurements on epitaxial layers show that this technique provides the great ability to measure carrier concentration profile in SiC, although up to now only qualitative measurements are obtained. Measurements performed in ion-implanted samples indicate that the presence of defects strongly affects the SCM data. Although several crucial points for the quantification of carrier concentration profile need to be addressed in the future, we believe that this work represent an important step on the way to determine two-dimensional quantification of carrier profile in silicon carbide by SCM technique. Further experiments are necessary in order to obtain quantitative profiles and to optimise the activation of the ion-implanted dopant.
References [1] Ruff M, Mitlehner H, Helbig R. IEEE Trans Electron Devices 1994;41:1040. [2] Shnai K, Scott RS, Baliga BJ. IEEE Trans Electron Devices 1989;36:1811. [3] Davis RF, Kelner G, Shur M, Palmour JM, Edmond JA. Proc IEEE 1991;79:677. [4] Rao MV, Tucker JB, Ridgway MC, Holland OW, Papanicolau N, Mittered J. J Appl Phys 1999;86:752.
F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199 [5] Rao MV, Griffiths P, Holland OW, Kelner G, Freitas JA, Simons DS, Chi PH, Ghezzo M. J Appl Phys 1995; 77:2479. [6] Gardner JA, Edwards A, Rao M, Papanicolaou N, Kelner G, Holland OW, Capano MA, Ghezzo M, Kretchmer J. J Appl Phys 1998;83:5119. [7] Hirano Y, Inada T. J Electrochem Soc 1994;141:3489. [8] Handy EM, Rao MV, Jones KA, Derenge MA, Chi PH, Vispute RD, Venkatesan T, Papanicolaou NA, Mittereder J. J Appl Phys 1999;86:746.
199
[9] Zavayalov VV, McMurray JS, Williams CC. J Appl Phys 1999;85:7774. [10] Edwards H, Ukraintsev VA, Martin RS, Johnson FS, Menz P, Walsh SW, Ashburn S, Will KS, Harvey K. J Appl Phys 2000;87:1485. [11] Nipoti R, Albertazzi E, Bianconi M, Lotti R, Lulli G, Cervera M, Carnera A. Appl Phys Lett 1997;70:3425. [12] Wesh W, Heft A, Heindl J, Strunk HP, Bachmann T, Glaser E, Wendler E. Nucl Instr and Meth B 1995;106:339.
Carrier concentration profiles in 6H-SiC by scanning capacitance microscopy F. Giannazzoa, P. Musumecia, L. Calcagnoa,*, A. Makhtarib, V. Rainerib a
INFM and Dipartimento di Fisica ed Astronomia } Corso Italia 57 } 95129 Catania, Italy b CNR-IMETEM } Stradale Primosole 50 } 95121 Catania, Italy
Abstract We have used scanning capacitance microscopy to determine two-dimensional carrier distributions in 6H-SiC on both epitaxial layers and implanted samples. Measurements were carried out on cross-sections using metal-covered Si tips. The sample preparation, surface passivation and tip selection have been investigated to obtain an optimised procedure. Implants were performed on p-type 6H-SiC at a substrate temperature of 5008C with N ions at different fluences. The defect profiles were determined by Rutherford backscattering spectrometry. The influence of the implanted damage on the measurements has been investigated by implanted Si-self ions. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Doping; Carrier concentration; Ion implantation; Defects
1. Introduction The research on silicon carbide material growth and device technology is presently in progress due to its excellent properties and promising device characteristics. In fact, SiC is a wide band gap (3 eV) semiconductor, which exhibits a high electron mobility, high thermal conductivity, high temperature stability and good chemical resistance. All these properties make this material attractive for electronic devices operating at high power, high frequency and high temperature [1,2]. The electrical properties of devices can be controlled by dopant incorporation, and doping of SiC is generally achieved during epitaxial growth. However, this method does not offer planar-selective doping which is essential for making SiC devices. Furthermore, doping by thermal diffusion, which is very popular in Si technology, is not feasible for SiC due to very low diffusion coefficients of all useful acceptor and donor dopants in SiC [3]. Doping by ion implantation is an alternative method to overcome these problems, but its main *Corresponding author. Tel.: +39-095-7195420; fax: +39095-383023. E-mail address: [email protected] (L. Calcagno).
disadvantage is the formation of damage due to the collision of introduced ions with the target atoms. To produce electrically active regions, the damage must be removed by post annealing processes, but in SiC the requested temperatures are very high (>15008C) [4,5]. One possible way to reduce the damage is the implantation of SiC at elevated temperatures. High-temperature ion implantation produces dynamic annealing with a decrease of damage and simultaneously with the incorporation of a considerable fraction of dopants onto substitutional sites. It has been shown by several authors [6–8] that implantation of N, Al and B in the temperature range 400–7008C reduces considerably the amount of defects and prevents amorphization of the material. Because the electrical properties of doped region have a huge impact on the operation of devices, there is a pressing need to profile the number of carriers in doped SiC. The determination of carrier profiling in SiC present many difficulties due to the material characteristics (hardness, band gap, and Schottky barrier). Many of the standard techniques used for silicon devices cannot be adopted for silicon carbide due to technical or physical limits (SiC hardness, sample preparation, etc.). The capacitance measurements can be efficiently used,
1369-8001/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 0 ) 0 0 1 2 9 - 3
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F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199
however, this technique, based mainly on mercury probes, present many limits on carrier delineation as a poor contact capability and a limited dynamic range. Even a two-dimensional characterisation is required to define the edge termination in power device. However, not so many techniques have been developed to date in particular for two-dimensional carrier profiling. Recently, scanning capacitance microscopy (SCM) has been developed for two-dimensional carrier profiling in silicon [9,10]. The method produces carrier-sensitive images of semiconductor device even if many effects distorting them have been reported. Imaging carrier distribution involves a correct sample preparation, an accurate measurement and the elimination of potential distortions. In this work we demonstrate that SCM can be successfully applied to image carrier concentration in SiC. However, artefacts can be introduced when measurements are performed in ion-implanted layers. The damage due to ion implantation produces contrast in the SCM image: this effect is carefully investigated and described.
2. Experimental details In this work we have used two different kinds of 6 HSiC wafers (50 mm diameter) Si faced supplied by CREE: (a) n-type wafers with a bulk doping level of 3 1018 cm3 and a first n-type epilayer of 3 1015 cm3 and a second epilayer 1 1019 cm3, (b) p-type wafers with a bulk doping level of 3 1016 cm3. The p-type wafers were implanted with 200 keV N+ at two different fluences, 2 1014 and 2 1015 cm2, and with 250 keV Si+ at a fluence of 1015 cm2. The substrate temperature during implantation was maintained at 5008C. The profiles of defects produced by ion implantation were analysed by Rutherford backscattering spectrometry in channeling configuration using 1.6 MeV He beam incident normally to the sample surface. The aligned spectra were taken along the h0 0 0 1i sample axis. SCM data were acquired using Digital Instruments Dimension 3100 SCM. The AC amplitude applied to the sample has been optimised considering the C–V curve. DC bias is applied to the sample and its value was determined by sample optimising the dC/dV signal. Sample preparation is a fundamental step of measurements because it guarantees their success and reproducibility. We adopted a method described in the following to ensure their reproducibility. Cross-section sample preparation was performed in some steps. First, samples were cut on the region of interest; then, on the rear a thin ( 10 nm) gold layer was deposited. Afterwards, samples were glued together to obtain a sandwich. By a diamond saw, the sandwich was cut in a parallelepiped
of roughly 0.5 0.5 0.2 mm. On one of the two parallel sides with the specimen sections (the longer dimensions) a gold layer ( 10 nm) was deposited. On the opposite side, the sandwich was polished to obtain the flat and clean surface required by scanning C–V measurements. The sandwich was mounted on a fixed support standing on a lapping machine and incrementally polished. We used first 10 mm diamond lapping film to remove saw marks and flatten the sandwich side. We achieved a finer and finer grind by using decrement size lapping films (3, 1, 0.5, 0.1 mm). Before measurements, the sample was cleaned by a dip in HF (5%) and then reoxidised by immersion in H2O2 for 10 min. This process produces a uniform oxide thickness on the polished surface of 2.0 nm. Capacitance measurements are influenced by surface states, directly related to the surface treatments, and by the oxide thickness. Due to the SiC hardness polishing with 0.1 mm diamond lapping film for enough time to remove all surface scratches due to the previous grind polishing is enough to obtain a flat surface. By atomic force measurements, we measured a surface roughness better than 1 nm. The uniformity in the oxide layer was ensured by the wet oxidation and it has been checked by measuring on the sample surface of uniformly doped samples. In this case, change in the oxide thickness should appear as variation in the dC/dV signal.
3. Results and discussion SCM technique presents many difficulties and, in order to test the reliability of our procedure, we have performed measurements on epitaxial layers. The SCM data obtained in SiC samples containing two epitaxial layers are reported in Fig. 1. The SCM image (Fig. 1a) shows the presence of a first layer about 5.5 mm thick on the SiC substrate followed by a second layer about 2 mm thick. According to the manufacturer’s specifications, the substrate is an n-type high concentration (3.3 1018 cm3) substrate. The first epitaxial layer is also ntype doped with an uniform concentration of 2.6 1015 cm3, while the second layer is p-doped with an uniform concentration of 1.3 1019 cm3. The image was obtained on a sandwich of two equal samples. The profile reported in Fig. 1b is obtained from the SCM image and represents the dC/dV versus depth signal collected during the scan. The dC/dV scale is given in arbitrary units. It should be noted that the shape of the dC/dV profile resembles qualitatively the concentration steps of the epitaxial sample, but the dC/dV dependence on the concentration value is quite complex, also taking into account the different flat band voltage values in the p- and n-type doped zones. It is also interesting to note how the transition between two different doping regions is very steep. This is a confirmation of the high spatial
F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199
197
Fig. 2. dC/dV signal versus depth for SiC samples implanted with 200 keV N+ at two different fluences. The substrate temperature during implantation was maintained at 5008C.
Fig. 1. SCM image of the dC/dV signal reported in false colour for a SiC sample with two epitaxial layers grown on a 6H-SiC substrate (a), dC/dV signal versus depth in a plot for an image line (b).
resolution of the SCM measurement, essentially determined by the tip dimension. The SCM measurements were carried out on p-type SiC implanted with N ions, which is a donor dopant. The obtained two-dimensional SCM profiles are reported in Fig. 2: these profiles refer to implantation performed at a substrate temperature of 5008C with two different fluences 2 1014 and 2 1015 cm2. The SCM profile of low-fluence implanted sample shows a constant signal up to a depth of 0.2 mm, then decreases and reaches the substrate level at 0.43 mm. The region up to 0.43 mm corresponds to the implanted layer. The profile of high-fluence implanted sample shows the same characteristics, but contrary to expectations, the signal is lower with respect to low fluence one. This result, surprisingly, evidences that the total amount of carrier concentration is lower in high-fluence
implantations. Moreover in both samples the depth of implanted layer (0.43 mm) is larger than the range of implanted ion (0.32 mm) and the profile shape is not a Gaussian as expected for a single-energy implantation. On the other hand, by measurements on van der Pauw patterns, we determined an active fraction of 1–5%. The results obtained on ion-implanted SiC samples indicate that SCM measurements are not only sensitive to the electrically active part of the dopant, but are affected by some other factors. Generally, the presence of defects in a semiconductor crystal introduces some trap centres in the band gap. C–V measurements are sensitive to these levels; then a dC/dV signal can be related to defect rich although not doped sample regions. This could justify an SCM signal extending in depth much further than the ion-projected range. Moreover, a high-defect density strongly modifies the dC/dV signal associated with free carriers in the doped zone. In order to detect the defect profile, Rutherford backscattering analysis in channeling configuration were performed in both samples and the obtained spectra are shown in Fig. 3. The spectra are normalised to the same random level. The channeling backscattering yield increases in the near-surface region right after implantation with a fluence of 1 1014 cm2 and the damage peak is located at a depth of about 300 nm below the surface. The channeling backscattering yield increases after the higher fluence implantation; however, the shape is almost the same. The thickness of damaged region is about 400 nm, and the depth scale has been calculated using the density of crystalline SiC (3.2 g/cm3), as our analysis are restricted to low-level damage samples and change in the material density is observed when an amorphous layer is formed [11]. Moreover, at the used substrate
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F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199
Fig. 3. RBS-channeling spectra for SiC samples implanted at 5008C with 200 keV N+ at two different fluences. The depth scale is also shown.
be related to the implanted layer. Rutherford backscattering spectra (not reported) shows the presence of defects up to a depth of 1.0 mm. These results evidence that SCM measurements are strongly affected by the presence of defects. This statement is also supported by the thickness of implanted layer measured with SCM, which in implanted samples is larger than the range of implanted Si ions (0.35 mm). In fact, as reported in literature [12] high-temperature implantation produces damaged regions larger than the ion range because of the defect diffusion, while dopant profile does not change with the implantation temperature. The obtained results show that scanning capacitance microscopy is able to measure carrier concentration profiles in silicon carbide, however, in ion-implanted samples, the contribution of defects strongly affects the data. More experiments are required in order to distinguish the dopant and the defect contribution; these include post-implantation annealing processes in order to reduce the amount of defects. Furthermore, calibration must be performed in order to quantify the carrier concentration profiles.
4. Conclusions
Fig. 4. SCM image of SiC samples implanted with 250 keV Si ions at a fluence of 1 1015 cm2 and at a substrate temperature of 5008C.
temperature SiC amorphization is never reached because of the dynamic annealing of defects during implantation [6,7]. In order to check the influence of the defects on the SCM data, measurements were carried out also in SiC samples implanted with Si ions at a fluence of 1015 cm2 and at a substrate temperature of 5008C. This implantation produces defects without any doping effect. The SCM image of this sample is shown in Fig. 4. The measurement was carried out on the cross-section of a sample obtained by gluing a Si-implanted SiC sample and a crystalline SiC sample. A contrast region about 1.05 mm thick is observed at each side of the glue. It can
Scanning capacitance microscopy has been developed in SiC on both epitaxial and ion-implanted samples. The sample preparation has been improved in order to obtain an optimised procedure. The measurements on epitaxial layers show that this technique provides the great ability to measure carrier concentration profile in SiC, although up to now only qualitative measurements are obtained. Measurements performed in ion-implanted samples indicate that the presence of defects strongly affects the SCM data. Although several crucial points for the quantification of carrier concentration profile need to be addressed in the future, we believe that this work represent an important step on the way to determine two-dimensional quantification of carrier profile in silicon carbide by SCM technique. Further experiments are necessary in order to obtain quantitative profiles and to optimise the activation of the ion-implanted dopant.
References [1] Ruff M, Mitlehner H, Helbig R. IEEE Trans Electron Devices 1994;41:1040. [2] Shnai K, Scott RS, Baliga BJ. IEEE Trans Electron Devices 1989;36:1811. [3] Davis RF, Kelner G, Shur M, Palmour JM, Edmond JA. Proc IEEE 1991;79:677. [4] Rao MV, Tucker JB, Ridgway MC, Holland OW, Papanicolau N, Mittered J. J Appl Phys 1999;86:752.
F. Giannazzo et al. / Materials Science in Semiconductor Processing 4 (2001) 195–199 [5] Rao MV, Griffiths P, Holland OW, Kelner G, Freitas JA, Simons DS, Chi PH, Ghezzo M. J Appl Phys 1995; 77:2479. [6] Gardner JA, Edwards A, Rao M, Papanicolaou N, Kelner G, Holland OW, Capano MA, Ghezzo M, Kretchmer J. J Appl Phys 1998;83:5119. [7] Hirano Y, Inada T. J Electrochem Soc 1994;141:3489. [8] Handy EM, Rao MV, Jones KA, Derenge MA, Chi PH, Vispute RD, Venkatesan T, Papanicolaou NA, Mittereder J. J Appl Phys 1999;86:746.
199
[9] Zavayalov VV, McMurray JS, Williams CC. J Appl Phys 1999;85:7774. [10] Edwards H, Ukraintsev VA, Martin RS, Johnson FS, Menz P, Walsh SW, Ashburn S, Will KS, Harvey K. J Appl Phys 2000;87:1485. [11] Nipoti R, Albertazzi E, Bianconi M, Lotti R, Lulli G, Cervera M, Carnera A. Appl Phys Lett 1997;70:3425. [12] Wesh W, Heft A, Heindl J, Strunk HP, Bachmann T, Glaser E, Wendler E. Nucl Instr and Meth B 1995;106:339.