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Quantitative analysis of complexes in electron irradiated CZ silicon
ARTICLE IN PRESS
Physica B 401–402 (2007) 477–482 www.elsevier.com/locate/physb
Quantitative analysis of complexes in electron irradiated CZ silicon N. Inouea,, H. Ohyamab, Y. Gotoc, T. Sugiyamad a
RIAST, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan KNCT, Kumamoto National College of Technology, Koshi, Kumamoto 861-1102, Japan c Vehicle Engineering G., Toyota Motor Corporation, Kirigahora, Nishihirose, Toyota, Aichi 470-0309, Japan d Power Device Division, Toyota Central R&D Laboratories Inc., Nagakute, Aichi 480-1192, Japan b
Abstract Complexes in helium or electron irradiated silicon are quantitatively analyzed by highly sensitive and accurate infrared (IR) absorption spectroscopy. Carbon concentration (1 1015–1 1017 cm3) and helium dose (5 1012–5 1013 cm2) or electron dose (1 1015–1 1017 cm2) are changed by two orders of magnitude in relatively low regime compared to the previous works. It is demonstrated that the carbon-related complex in low carbon concentration silicon of commercial grade with low electron dose can be detected clearly. Concentration of these complexes is estimated. It is clarified that the complex configuration and thermal behavior in low carbon and low dose samples is simple and almost confined within the individual complex family compared to those in high concentration and high dose samples. Well-established complex behavior in electron-irradiated sample is compared to that in Heirradiated samples, obtained by deep level transient spectroscopy (DLTS) or cathodoluminescence (CL), which had close relation to the Si power device performance. r 2007 Published by Elsevier B.V. Keywords: Experiment; Group IV and compounds; CZ silicon; Electron irradiation; Infrared absorption
1. Introduction Particle irradiation is now widely used to improve the Si power device performance. Deep level transient spectroscopy (DLTS) and photoluminescence (PL) are used for the characterization of complexes controlling the performance. Irradiation-induced complexes have also been examined by electron paramagnetic resonance (EPR) and infrared (IR) absorption spectroscopy. The former two techniques are highly sensitive and mainly examine the device region near the substrate surface. The latter two, on the other hand, are less sensitive and examine the whole thickness of the sample. PL and DLTS signals, however, are affected by many factors other than the complex concentration. IR absorption is the most quantitative technique among the three methods. Complex concentration is obtained from the absorption coefficient using the
Corresponding author. Tel.: +81 722 54 9831; fax: +81 722 54 9935.
E-mail address: [email protected] (N. Inoue). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.09.003
conversion coefficient which includes the oscillator strength of the complex. We have recently examined the complexes introduced by He ion irradiation by DLTS and cathodoluminescence (CL) [1]. Candidates for the important complexes were revealed and it was shown that their thermal behavior was also crucial for the device performance. We had established the quantitative measurement of oxygen and carbon concentration by IR [2]. Recently, we have improved the IR sensitivity down to 1 104 of peak absorbance to detect trace amount of carbon in silicon [3] and detected He- and electron-irradiation-induced complexes in low carbon concentration (1 1016 cm3) and low electron dose (1 1016 cm2) samples [4]. In this paper, IR absorption spectroscopy is done on various samples and the result is compared to those by DLTS and CL. Electron irradiation introduces complexes uniformly throughout the samples, in contrast to He irradiation. Therefore, more reliable results can be obtained by the quantitative analysis combined with quantitative IR. Previous IR studies had employed high carbon concentration samples and high
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dose which are not used for Si advanced power device fabrication now. Therefore we tried to reduce the carbon concentration and dose as low as possible. Highly sensitive and accurate IR spectrometry made it possible to quantitatively analyze the formation and annealing behavior of complexes in low carbon concentration and low dose samples. 2. Experimental Helium or electron irradiation was performed at RT with acceleration voltage of 23 or 2–6 MeV. Annealing was done after irradiation between 100 and 400 1C for 10 min. DLTS and CL measurements were done on irradiated and annealed samples after fabricating the diodes. The experimental details were reported elsewhere [1]. The carbon concentration ranges between o1 1015 cm3 (device grade) and 8 1016 cm3. IR absorption spectroscopy on local vibration mode (LVM) of complexes was performed at RT using Varian FTS-575 on the samples, 2 mm thick with both surfaces mirror polished. Either non-carbon containing or non-irradiated sample from the same origin was used as a reference. Absorbance of small peaks down to 5 105 was accurately determined. Several samples from different ingots but with nearly equal carbon concentration were examined to obtain the reliable result at the lowest signal level. Complex concentration was estimated from the absorbance of the peaks using the reasonable conversion coefficient. Conversion coefficient for the dominant light impurities in silicon from the absorption coefficient to the concentration is around 1.5 1017 cm2 within factors from 1/2 to 2; 3.1, 0.82 and 1.82 (for N2), for O, C and N, respectively. Therefore, the concentration of complex containing light element can be estimated roughly by assuming the equal conversion coefficient of 1.5 1017 cm2 for all the complexes. This was justified by, for example, that the absorbance from CiOi or VO was roughly equal (within a factor of 2) to the reduction of absorbance from Cs or Oi after irradiation and annealing, as will be shown later. As far as we choose the dominant peaks from the individual complex, quantitative analysis of annealing behavior can be made. Previous studies have not discussed the complex concentration quantitatively and not related the concentration to the irradiation condition. We compared the present results to the previous ones also. 3. Results and discussion 3.1. Comparison among DLTS, CL and IR results from Heirradiated samples DLTS on the samples of MCZ substrate with [O] about 5 1017 cm3, [C]o1 1015 cm3 and He dose 1 1012 cm2 and annealing at 380 1C for 2 h revealed several electron traps and hole traps such as Ec0.10, 0.16(VO), 0.23, 0.40 (vacancy pair (VV)), Ev+0.20(VV) and
°
Fig. 1. Annealing temperature dependence of dominant luminescence from helium irradiated and annealed samples. [C]o1 1015 cm3 and He dose 1 1013 cm2 at 23 MeV.
Ev+0.35 eV (CiOi or CiOiVV) (assignment follows those in Ref. [5]). Fig. 1 shows the annealing temperature dependence of CL lines in the diodes from CZ substrate with [O] ¼ 1.0 1018 cm3, [C]o1 1015 cm3 and He dose 1 1013 cm2 [1]. C(CiOi), G(CiCs), W(I3) and X(I4) [6] lines were the dominant luminescence. G-line reduced below 300 1C, while W- and X-lines developed above 300 1C. It is to be noted that C-line was stable up to about 400 1C. Fig. 2 shows the IR spectra from the He-irradiated samples. The sample was MCZ substrate with [O] ¼ 5 1017 cm3, [C] ¼ 9.5 1015 cm3 and He dose 5 1013 cm2 and 5 1012 cm2. CiOi (corresponding to Cline), VO and CiOiI were the only complexes detected clearly (wavenumbers shown below and in the figures are the numbers indicated by the FTIR machine and are not the exact peak location at RT). From the absorbance, [CiOi] was estimated to be about 4 1014 cm3, which was about 5% of the included carbon. There are a few weak peaks, as marked by the open circles in the figure, such as CiCs (corresponding to G-line), CiCsI, CiOiI and CiI. Lowest absorbance of the peaks was about 5 105. [CiOiI] and [CiI] was about one order of magnitude lower than [CiOi]. In other words, the yield of these secondary defects from the two elementary defects was about 10%. Some peaks are uncertain here. CiOi and VO were clearly observed in the sample of [He] ¼ 5 1012 cm2. VV expected from the DLTS result and located at 2766 cm1 was not detected, which had been observed in very high dose samples. IR absorption related to W- or X-line has not been reported, though LVM were reported by PL. Also, no IR absorption or PL from CiOiVV or COV was established. 3.2. IR absorption from the electron-irradiated samples To detect and quantitatively analyze the IR absorption from the complexes observed by the DLTS and CL, the
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Fig. 2. Absorption spectra from helium irradiated samples. Carbon concentration 9.5 1015 cm3, He dose 5 1013 cm2 (top), 5 1012 cm2 (bottom).
samples by electron irradiation were examined. The dose or carbon concentration was lower than those employed in the previous studies. Fig. 3 shows the spectra from very low [C] (around 1 1015 cm3) and relatively high electron dose samples. It is to be noted that the various complexes were detected in the samples of device grade [C]. VO peak was much stronger than that of CiOi, compared to the results of low dose He irradiation shown in Fig. 2 and low dose electron irradiation shown later. The peaks at 537.24, 575.82 (CiCs) and 988.63 and 994.42 cm1 (CiCsI) [7] were detected clearly at the level of A ¼ 1 104 in absorbance. In the previous report, CiCs was observed at A ¼ 0.005 for e-irradiated to 1 1018 cm2 for [C] ¼ 4.5 1017 cm3 [8] and A ¼ 0.01 for n-irradiated to 1 1017 cm2 at 5 MeV for [C] ¼ 1.5 1017 cm3 [7]. Owing to the high sensitivity attained here, the threshold condition for detection was reduced to 1 1016 cm2 for e and 5 1013 cm2 for He at [C] ¼ 1 1016 cm3, similar to the device fabricating condition. The signal intensity, A ¼ 0.0002 in this case is compared to the previous ones: 1/25 of that from Lavrov where [C] was 450 and [e] was 10 and measured at 10 K, and 1/50 of that of Londos where [C] was 100 and [n] is 1 and measured at RT. In our previous study, on the other hand, [CiOi] and [VO] was proportional to the electron dose up to 1 1016 cm2 and [CiOi] was proportional to [C] [4]. This comparison suggests the saturation of formation in high dose regime. As shown in Fig. 2, [CiOi] and [VO] is not proportional to the dose in case of He irradiation. CiCsI was reported only by Londos and the absorbance was about 0.01 compared to about 0.0002 here, almost the same ratio as that of CiCs. Some other features are observed but uncertain yet.
As shown in Fig. 1, W-line and X-line are the dominant luminescence peaks. There have been no reports on the IR absorption from the complexes related to these lines. Recently, the origin of these lines has been assigned to I3 and I4, respectively [6], and their LVM sidebands have also been reported. LVM are located between 500 and 650 cm1, 564 cm1 for W-line [9] and 534, 548 and 557 cm1 (Raman active) for X-line [10] (all wavenumbers at low temperature). There are some unidentified absorption bands observed in Fig. 3. Unfortunately, none of them apparently match with the reported LVM here. In case of C-line, high wavenumber LVM sidebands were reported and most of them between 1116 and 525 cm1 were observed also by IR, as shown in the figures here. LVM sideband study in PL in high wavenumber region will help the detection of IR absorption from I3 and I4. Fig. 4 shows the absorption spectra of electronirradiated and annealed samples with relatively low [C] and low dose. It is to be noted that the spectra were very simple with a few dominant bands and a few weak bands only for the individual temperature, in contrast to the previous studies. It is due to the low [C] and low dose. In addition to the peaks observed already, peaks were observed at 947.2 and 966.5 cm1 in the sample annealed at 300 1C. These were identical to the reported ones at 945 and 965 cm1 whose wavenumber was underestimated by 2 cm1. They were attributed to CiCsI2 [7] which may be formed from either CiCs or CiCsI. The peak intensity here was about 0.0001, while they were about 0.01. The product of [C] [e] was (9 1015 cm3) (1 1016 cm2) against [C] [n] which was (1 1017 cm3) (1 1017 cm2). In the present case, signal intensity ratio among CiCs family is about CiCs:CiCsI:CiCsI2 ¼ 2:1:3. This equality suggests
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Fig. 3. Absorption spectra from electron-irradiated samples. [C] around 1 1015 cm3, electron dose 1 1017 cm2 at 2 MeV. Top: between 500 and 650 cm1, bottom: between 500 and 1150 cm1.
that the oscillator strength of these CiCs family is nearly equal, if 100% conversion among the three complexes takes place by the annealing. In case of high dose n-irradiation it was 0.008:0.01:0.014, so a little different tendency was observed. 3.3. Annealing behavior revealed by IR It is practically important to reveal the behavior in the low [C] and low dose regime, though the behavior had been examined in the high [C] and high dose regime. Moreover,
low concentration and low dose experiment makes process simple, and gives a clear image as shown below. Annealing temperature dependence of the complexes is summarized in Fig. 5. Overall features of VO, CiOi and CsO, and those of CiCs, CiI and others, are nearly the same as those in the previous reports [7,11], respectively. However, many other kinds of complexes such as VmOn and CsCs had been included in the previous studies, and CsOi concentration, for example, was much smaller in our case. VO and CiOi were the dominant complexes in asirradiated samples, changed little up to 300 1C where steep
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Fig. 4. Absorption spectra from electron-irradiated and annealed samples. [C] 9.5 1015 cm3, e dose 1 1016 cm2 at 6 MeV. From bottom to top, asirradiation, 100, 200, 300 and 400 1C.
° °
Fig. 5. Temperature dependence of absorbance from the complexes in the samples shown in Fig. 4. Left: strong peaks, right: weak peaks.
decrease was observed simultaneously. VO2 and CsOi appeared around 400 1C instead. It is considered that the process is very simple and isolated within only a few families such as CiOiX, CiCsY and CsOZ, and the interaction between the families was small as follows: V emitted from VO either interacted with CiOi to form CsOi (and CsO2i) or with another VO to form VO2 [11]. CiOiI reduced at the lowest temperature among the various complexes. They might be unstable and some probably emitted I to turn to CiOi, others emitted Oi to turn to CiI. CiI increase around 100 1C and reduction around 200 1C was also reported [7]. In case of CiCs family, as already described also [7], CiCs reduced much above 250 1C,
whereas CiCsI increased around 200 1C and then reduced gradually above 250 1C, and CiCsI2 showed maxima at about 250 1C but reduced gradually above. These change took place by the sequential catch of I. CiOi may turn into CsOi around 400 1C. Return to Cs is not observed up to 400 1C as shown by the almost constant loss of Cs over the whole temperature range. Other interaction between the different families was ignored here. 3.4. Comparison with the CL result Now, some of the complexes responsible for the DLTS or CL signals are detected by IR in the low [C] and low e
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dose samples and their thermal behavior is revealed. Next, the thermal behavior is compared to the CL temperature dependence. G-line started to reduce at about 250 1C, which was consistent with the previous studies [12] and with the IR result here. V shape increase of W-line around 400 1C was observed when the proton dose was increased above 1 1012 cm2 [13]. Comparison between the C-line and CiOi is practically most important because the behavior of C-line and Ev+0.35 eV trap (CiOi or CiOiVV [5]) had close relationship to the device performance [1]. C-line started to reduce at about 400 1C. In IR, CiOi disappeared almost always below 400 1C [7,11]. Annealing behavior of C-line in implanted samples had been studied [14]. In C implanted sample, C-line reduced below 300 1C, while it disappeared at 500 1C in C–O co-implanted sample. One possible reason of the discrepancy here is the device fabrication process before CL measurement. Also, as discussed above, CL and DLTS examined the shallow part of the substrate and IR examines the average over the whole thickness. Nonuniform and localized point defect depth distribution may induce the difference. COVV has been a candidate for the Ev+0.4 eV hole trap [5]. Unfortunately, there has been no established information about the PL and IR signal from this structure. As described above, direct evidence from I3 and I4 was not obtained, though the increase of CiCsI, for example, suggested the presence of I related complexes. There were some unreported peaks observed in this study, being the candidates for these complexes. Anyway, the quantitatively analyzed reaction in e-irradiated homogeneous sample will be a base to understand the more complicated and practically important reaction in Heirradiated sample. 4. Summary In summary, highly sensitive and quantitative IR absorption spectroscopy is performed on Si samples with low carbon concentration and relatively low electron dose. Irradiation-induced complexes were detected in such device-grade substrates. CiOi, CiCs and some of their complexes with I or V were detected. Complex concentration was estimated accurately also. Thermal behavior of complexes which are related to device performance was revealed in low carbon concentration and dose samples. Due to low carbon concentration and low dose, simple phenomena were observed with almost isolated reaction within the individual complex family such as CiOiX, CiCsY and CsOiZ. Then they were compared to the results by the three techniques on He-irradiated samples. CiOi, for
example, showed a little different thermal behavior to that observed by CL. This may be due to the different depth of the sample, mainly characterized by the respective method. Quantitative reaction analysis in homogeneous samples then will be applied to more complicated phenomena in He-irradiated samples which are directly related to device fabrication process. Though LVMs were reported by PL from W- and X-lines, no corresponding absorption was yet detected. Highly sensitive and accurate IR, combined with DLTS and CL (PL) will clarify the complex formation process and the role of complexes better than that by the individual technique.
Acknowledgments The authors are grateful to C.A. Londos for discussion. N. Inoue is grateful to T. Takeguchi for adjusting the IR apparatus and E. Hazama for helping in IR data processing.
References [1] T. Sugiyama, S. Yamazaki, S. Nakagaki, M. Ishiko, in: Proceedigs of the 17th International Symposium, Power Semiconductor Devices & ICs Santa Barbara, 2004, p. 243. [2] N. Inoue, T. Arai, T. Nozaki, K. Endo, K. Mizuma, Am. Soc. Test. Mater. STP960 (1987) 365. [3] N. Inoue, M. Nakatsu, V.D. Akhmetov, Silicon Materials Science and Technology X (Electrochem. Soc., Pennington 2006), p. 461. [4] N. Inoue, Y. Goto, T. Sugiyama, S. Yamazaki, T. Kushida, High Purity Silicon IX, ECS Trans. 3 (4) (Electrochem. Soc., Pennington 2006), p. 313. [5] R. Siemieniec, F.-J. Niedernostheide, H.-J. Schlzer, W. Sudkamp, U. Kellner-Werdehausen. J. Lutz, High Purity Silicon VIII, (Electrochem. Soc., Pennington 2004), p. 369. [6] A. Carvalho, R. Jones, J. Coutinho, P.R. Briddon, Phys. Rev. B 72 (2005) 155208. [7] C.A. Londos, G.D. Antonaras, M.S. Potsidi, D.N. Aliprantis, A. Misiuk, J. Mater. Sci.: Mater. Electron. 18 (2007) 721. [8] E.V. Lavrov, L. Hoffmann, B.B. Nielsen, Phys. Rev. B 60 (1999) 8081. [9] G. Davies, E.C. Lightowlers, M.C. do Carmo, J.G. Wilkes, G.R. Wolstenholme, Solid State Commun. 50 (1984) 1057. [10] S. Hayama, G. Davies, M. Itoh, J. Appl. Phys. 96 (2004) 1754. [11] L.I. Murin, V.P. Markevich, J.L. Lindstroem, M. Kleverman, J. Hermansson, T. Hallberg, B.G. Svensson, Solid State Phenom. 82–84 (2002) 57. [12] G. Davies, E.C. Lightowlers, Z.E. Cienchanowska, J. Phys. C 20 (1987) 191. [13] M. Nakamura, Jpn. J. Aool. Phys. 40 (2001) L1000. [14] O.O. Awadelkarim, A. Henry, B. Monemar, J.L. Lindstroem, Y. Zhang, J.W. Corbett, Phys. Rev. B 42 (1990) 5635.
Physica B 401–402 (2007) 477–482 www.elsevier.com/locate/physb
Quantitative analysis of complexes in electron irradiated CZ silicon N. Inouea,, H. Ohyamab, Y. Gotoc, T. Sugiyamad a
RIAST, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan KNCT, Kumamoto National College of Technology, Koshi, Kumamoto 861-1102, Japan c Vehicle Engineering G., Toyota Motor Corporation, Kirigahora, Nishihirose, Toyota, Aichi 470-0309, Japan d Power Device Division, Toyota Central R&D Laboratories Inc., Nagakute, Aichi 480-1192, Japan b
Abstract Complexes in helium or electron irradiated silicon are quantitatively analyzed by highly sensitive and accurate infrared (IR) absorption spectroscopy. Carbon concentration (1 1015–1 1017 cm3) and helium dose (5 1012–5 1013 cm2) or electron dose (1 1015–1 1017 cm2) are changed by two orders of magnitude in relatively low regime compared to the previous works. It is demonstrated that the carbon-related complex in low carbon concentration silicon of commercial grade with low electron dose can be detected clearly. Concentration of these complexes is estimated. It is clarified that the complex configuration and thermal behavior in low carbon and low dose samples is simple and almost confined within the individual complex family compared to those in high concentration and high dose samples. Well-established complex behavior in electron-irradiated sample is compared to that in Heirradiated samples, obtained by deep level transient spectroscopy (DLTS) or cathodoluminescence (CL), which had close relation to the Si power device performance. r 2007 Published by Elsevier B.V. Keywords: Experiment; Group IV and compounds; CZ silicon; Electron irradiation; Infrared absorption
1. Introduction Particle irradiation is now widely used to improve the Si power device performance. Deep level transient spectroscopy (DLTS) and photoluminescence (PL) are used for the characterization of complexes controlling the performance. Irradiation-induced complexes have also been examined by electron paramagnetic resonance (EPR) and infrared (IR) absorption spectroscopy. The former two techniques are highly sensitive and mainly examine the device region near the substrate surface. The latter two, on the other hand, are less sensitive and examine the whole thickness of the sample. PL and DLTS signals, however, are affected by many factors other than the complex concentration. IR absorption is the most quantitative technique among the three methods. Complex concentration is obtained from the absorption coefficient using the
Corresponding author. Tel.: +81 722 54 9831; fax: +81 722 54 9935.
E-mail address: [email protected] (N. Inoue). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.09.003
conversion coefficient which includes the oscillator strength of the complex. We have recently examined the complexes introduced by He ion irradiation by DLTS and cathodoluminescence (CL) [1]. Candidates for the important complexes were revealed and it was shown that their thermal behavior was also crucial for the device performance. We had established the quantitative measurement of oxygen and carbon concentration by IR [2]. Recently, we have improved the IR sensitivity down to 1 104 of peak absorbance to detect trace amount of carbon in silicon [3] and detected He- and electron-irradiation-induced complexes in low carbon concentration (1 1016 cm3) and low electron dose (1 1016 cm2) samples [4]. In this paper, IR absorption spectroscopy is done on various samples and the result is compared to those by DLTS and CL. Electron irradiation introduces complexes uniformly throughout the samples, in contrast to He irradiation. Therefore, more reliable results can be obtained by the quantitative analysis combined with quantitative IR. Previous IR studies had employed high carbon concentration samples and high
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dose which are not used for Si advanced power device fabrication now. Therefore we tried to reduce the carbon concentration and dose as low as possible. Highly sensitive and accurate IR spectrometry made it possible to quantitatively analyze the formation and annealing behavior of complexes in low carbon concentration and low dose samples. 2. Experimental Helium or electron irradiation was performed at RT with acceleration voltage of 23 or 2–6 MeV. Annealing was done after irradiation between 100 and 400 1C for 10 min. DLTS and CL measurements were done on irradiated and annealed samples after fabricating the diodes. The experimental details were reported elsewhere [1]. The carbon concentration ranges between o1 1015 cm3 (device grade) and 8 1016 cm3. IR absorption spectroscopy on local vibration mode (LVM) of complexes was performed at RT using Varian FTS-575 on the samples, 2 mm thick with both surfaces mirror polished. Either non-carbon containing or non-irradiated sample from the same origin was used as a reference. Absorbance of small peaks down to 5 105 was accurately determined. Several samples from different ingots but with nearly equal carbon concentration were examined to obtain the reliable result at the lowest signal level. Complex concentration was estimated from the absorbance of the peaks using the reasonable conversion coefficient. Conversion coefficient for the dominant light impurities in silicon from the absorption coefficient to the concentration is around 1.5 1017 cm2 within factors from 1/2 to 2; 3.1, 0.82 and 1.82 (for N2), for O, C and N, respectively. Therefore, the concentration of complex containing light element can be estimated roughly by assuming the equal conversion coefficient of 1.5 1017 cm2 for all the complexes. This was justified by, for example, that the absorbance from CiOi or VO was roughly equal (within a factor of 2) to the reduction of absorbance from Cs or Oi after irradiation and annealing, as will be shown later. As far as we choose the dominant peaks from the individual complex, quantitative analysis of annealing behavior can be made. Previous studies have not discussed the complex concentration quantitatively and not related the concentration to the irradiation condition. We compared the present results to the previous ones also. 3. Results and discussion 3.1. Comparison among DLTS, CL and IR results from Heirradiated samples DLTS on the samples of MCZ substrate with [O] about 5 1017 cm3, [C]o1 1015 cm3 and He dose 1 1012 cm2 and annealing at 380 1C for 2 h revealed several electron traps and hole traps such as Ec0.10, 0.16(VO), 0.23, 0.40 (vacancy pair (VV)), Ev+0.20(VV) and
°
Fig. 1. Annealing temperature dependence of dominant luminescence from helium irradiated and annealed samples. [C]o1 1015 cm3 and He dose 1 1013 cm2 at 23 MeV.
Ev+0.35 eV (CiOi or CiOiVV) (assignment follows those in Ref. [5]). Fig. 1 shows the annealing temperature dependence of CL lines in the diodes from CZ substrate with [O] ¼ 1.0 1018 cm3, [C]o1 1015 cm3 and He dose 1 1013 cm2 [1]. C(CiOi), G(CiCs), W(I3) and X(I4) [6] lines were the dominant luminescence. G-line reduced below 300 1C, while W- and X-lines developed above 300 1C. It is to be noted that C-line was stable up to about 400 1C. Fig. 2 shows the IR spectra from the He-irradiated samples. The sample was MCZ substrate with [O] ¼ 5 1017 cm3, [C] ¼ 9.5 1015 cm3 and He dose 5 1013 cm2 and 5 1012 cm2. CiOi (corresponding to Cline), VO and CiOiI were the only complexes detected clearly (wavenumbers shown below and in the figures are the numbers indicated by the FTIR machine and are not the exact peak location at RT). From the absorbance, [CiOi] was estimated to be about 4 1014 cm3, which was about 5% of the included carbon. There are a few weak peaks, as marked by the open circles in the figure, such as CiCs (corresponding to G-line), CiCsI, CiOiI and CiI. Lowest absorbance of the peaks was about 5 105. [CiOiI] and [CiI] was about one order of magnitude lower than [CiOi]. In other words, the yield of these secondary defects from the two elementary defects was about 10%. Some peaks are uncertain here. CiOi and VO were clearly observed in the sample of [He] ¼ 5 1012 cm2. VV expected from the DLTS result and located at 2766 cm1 was not detected, which had been observed in very high dose samples. IR absorption related to W- or X-line has not been reported, though LVM were reported by PL. Also, no IR absorption or PL from CiOiVV or COV was established. 3.2. IR absorption from the electron-irradiated samples To detect and quantitatively analyze the IR absorption from the complexes observed by the DLTS and CL, the
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Fig. 2. Absorption spectra from helium irradiated samples. Carbon concentration 9.5 1015 cm3, He dose 5 1013 cm2 (top), 5 1012 cm2 (bottom).
samples by electron irradiation were examined. The dose or carbon concentration was lower than those employed in the previous studies. Fig. 3 shows the spectra from very low [C] (around 1 1015 cm3) and relatively high electron dose samples. It is to be noted that the various complexes were detected in the samples of device grade [C]. VO peak was much stronger than that of CiOi, compared to the results of low dose He irradiation shown in Fig. 2 and low dose electron irradiation shown later. The peaks at 537.24, 575.82 (CiCs) and 988.63 and 994.42 cm1 (CiCsI) [7] were detected clearly at the level of A ¼ 1 104 in absorbance. In the previous report, CiCs was observed at A ¼ 0.005 for e-irradiated to 1 1018 cm2 for [C] ¼ 4.5 1017 cm3 [8] and A ¼ 0.01 for n-irradiated to 1 1017 cm2 at 5 MeV for [C] ¼ 1.5 1017 cm3 [7]. Owing to the high sensitivity attained here, the threshold condition for detection was reduced to 1 1016 cm2 for e and 5 1013 cm2 for He at [C] ¼ 1 1016 cm3, similar to the device fabricating condition. The signal intensity, A ¼ 0.0002 in this case is compared to the previous ones: 1/25 of that from Lavrov where [C] was 450 and [e] was 10 and measured at 10 K, and 1/50 of that of Londos where [C] was 100 and [n] is 1 and measured at RT. In our previous study, on the other hand, [CiOi] and [VO] was proportional to the electron dose up to 1 1016 cm2 and [CiOi] was proportional to [C] [4]. This comparison suggests the saturation of formation in high dose regime. As shown in Fig. 2, [CiOi] and [VO] is not proportional to the dose in case of He irradiation. CiCsI was reported only by Londos and the absorbance was about 0.01 compared to about 0.0002 here, almost the same ratio as that of CiCs. Some other features are observed but uncertain yet.
As shown in Fig. 1, W-line and X-line are the dominant luminescence peaks. There have been no reports on the IR absorption from the complexes related to these lines. Recently, the origin of these lines has been assigned to I3 and I4, respectively [6], and their LVM sidebands have also been reported. LVM are located between 500 and 650 cm1, 564 cm1 for W-line [9] and 534, 548 and 557 cm1 (Raman active) for X-line [10] (all wavenumbers at low temperature). There are some unidentified absorption bands observed in Fig. 3. Unfortunately, none of them apparently match with the reported LVM here. In case of C-line, high wavenumber LVM sidebands were reported and most of them between 1116 and 525 cm1 were observed also by IR, as shown in the figures here. LVM sideband study in PL in high wavenumber region will help the detection of IR absorption from I3 and I4. Fig. 4 shows the absorption spectra of electronirradiated and annealed samples with relatively low [C] and low dose. It is to be noted that the spectra were very simple with a few dominant bands and a few weak bands only for the individual temperature, in contrast to the previous studies. It is due to the low [C] and low dose. In addition to the peaks observed already, peaks were observed at 947.2 and 966.5 cm1 in the sample annealed at 300 1C. These were identical to the reported ones at 945 and 965 cm1 whose wavenumber was underestimated by 2 cm1. They were attributed to CiCsI2 [7] which may be formed from either CiCs or CiCsI. The peak intensity here was about 0.0001, while they were about 0.01. The product of [C] [e] was (9 1015 cm3) (1 1016 cm2) against [C] [n] which was (1 1017 cm3) (1 1017 cm2). In the present case, signal intensity ratio among CiCs family is about CiCs:CiCsI:CiCsI2 ¼ 2:1:3. This equality suggests
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Fig. 3. Absorption spectra from electron-irradiated samples. [C] around 1 1015 cm3, electron dose 1 1017 cm2 at 2 MeV. Top: between 500 and 650 cm1, bottom: between 500 and 1150 cm1.
that the oscillator strength of these CiCs family is nearly equal, if 100% conversion among the three complexes takes place by the annealing. In case of high dose n-irradiation it was 0.008:0.01:0.014, so a little different tendency was observed. 3.3. Annealing behavior revealed by IR It is practically important to reveal the behavior in the low [C] and low dose regime, though the behavior had been examined in the high [C] and high dose regime. Moreover,
low concentration and low dose experiment makes process simple, and gives a clear image as shown below. Annealing temperature dependence of the complexes is summarized in Fig. 5. Overall features of VO, CiOi and CsO, and those of CiCs, CiI and others, are nearly the same as those in the previous reports [7,11], respectively. However, many other kinds of complexes such as VmOn and CsCs had been included in the previous studies, and CsOi concentration, for example, was much smaller in our case. VO and CiOi were the dominant complexes in asirradiated samples, changed little up to 300 1C where steep
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Fig. 4. Absorption spectra from electron-irradiated and annealed samples. [C] 9.5 1015 cm3, e dose 1 1016 cm2 at 6 MeV. From bottom to top, asirradiation, 100, 200, 300 and 400 1C.
° °
Fig. 5. Temperature dependence of absorbance from the complexes in the samples shown in Fig. 4. Left: strong peaks, right: weak peaks.
decrease was observed simultaneously. VO2 and CsOi appeared around 400 1C instead. It is considered that the process is very simple and isolated within only a few families such as CiOiX, CiCsY and CsOZ, and the interaction between the families was small as follows: V emitted from VO either interacted with CiOi to form CsOi (and CsO2i) or with another VO to form VO2 [11]. CiOiI reduced at the lowest temperature among the various complexes. They might be unstable and some probably emitted I to turn to CiOi, others emitted Oi to turn to CiI. CiI increase around 100 1C and reduction around 200 1C was also reported [7]. In case of CiCs family, as already described also [7], CiCs reduced much above 250 1C,
whereas CiCsI increased around 200 1C and then reduced gradually above 250 1C, and CiCsI2 showed maxima at about 250 1C but reduced gradually above. These change took place by the sequential catch of I. CiOi may turn into CsOi around 400 1C. Return to Cs is not observed up to 400 1C as shown by the almost constant loss of Cs over the whole temperature range. Other interaction between the different families was ignored here. 3.4. Comparison with the CL result Now, some of the complexes responsible for the DLTS or CL signals are detected by IR in the low [C] and low e
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dose samples and their thermal behavior is revealed. Next, the thermal behavior is compared to the CL temperature dependence. G-line started to reduce at about 250 1C, which was consistent with the previous studies [12] and with the IR result here. V shape increase of W-line around 400 1C was observed when the proton dose was increased above 1 1012 cm2 [13]. Comparison between the C-line and CiOi is practically most important because the behavior of C-line and Ev+0.35 eV trap (CiOi or CiOiVV [5]) had close relationship to the device performance [1]. C-line started to reduce at about 400 1C. In IR, CiOi disappeared almost always below 400 1C [7,11]. Annealing behavior of C-line in implanted samples had been studied [14]. In C implanted sample, C-line reduced below 300 1C, while it disappeared at 500 1C in C–O co-implanted sample. One possible reason of the discrepancy here is the device fabrication process before CL measurement. Also, as discussed above, CL and DLTS examined the shallow part of the substrate and IR examines the average over the whole thickness. Nonuniform and localized point defect depth distribution may induce the difference. COVV has been a candidate for the Ev+0.4 eV hole trap [5]. Unfortunately, there has been no established information about the PL and IR signal from this structure. As described above, direct evidence from I3 and I4 was not obtained, though the increase of CiCsI, for example, suggested the presence of I related complexes. There were some unreported peaks observed in this study, being the candidates for these complexes. Anyway, the quantitatively analyzed reaction in e-irradiated homogeneous sample will be a base to understand the more complicated and practically important reaction in Heirradiated sample. 4. Summary In summary, highly sensitive and quantitative IR absorption spectroscopy is performed on Si samples with low carbon concentration and relatively low electron dose. Irradiation-induced complexes were detected in such device-grade substrates. CiOi, CiCs and some of their complexes with I or V were detected. Complex concentration was estimated accurately also. Thermal behavior of complexes which are related to device performance was revealed in low carbon concentration and dose samples. Due to low carbon concentration and low dose, simple phenomena were observed with almost isolated reaction within the individual complex family such as CiOiX, CiCsY and CsOiZ. Then they were compared to the results by the three techniques on He-irradiated samples. CiOi, for
example, showed a little different thermal behavior to that observed by CL. This may be due to the different depth of the sample, mainly characterized by the respective method. Quantitative reaction analysis in homogeneous samples then will be applied to more complicated phenomena in He-irradiated samples which are directly related to device fabrication process. Though LVMs were reported by PL from W- and X-lines, no corresponding absorption was yet detected. Highly sensitive and accurate IR, combined with DLTS and CL (PL) will clarify the complex formation process and the role of complexes better than that by the individual technique.
Acknowledgments The authors are grateful to C.A. Londos for discussion. N. Inoue is grateful to T. Takeguchi for adjusting the IR apparatus and E. Hazama for helping in IR data processing.
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