- Email: [email protected]
Effect of high hydrostatic pressure on small oxygen-related clusters in silicon: LVM studies
ARTICLE IN PRESS
Physica B 340–342 (2003) 565–569
Effect of high hydrostatic pressure on small oxygen-related clusters in silicon: LVM studies . b, A. Misiukc L.I. Murina,*, J.L. Lindstrom a
Institute of Solid State and Semiconductor Physics, P. Brovki Street 17, Minsk 220072, Belarus b Lund University, Division of Solid State Physics, Lund, Sweden c Institute of Electron Technology, Warsaw, Poland
Abstract Local vibrational mode (LVM) spectroscopy is used to explore the effect of high hydrostatic pressure (HP) on the formation of small oxygen-related clusters (dimers, trimers, thermal donors, and C–O complexes) at 450 C and 650 C in Cz–Si crystals with different impurity content and prehistory. It is found, in agreement with previous studies, that HP enhances the oxygen clustering in Cz–Si at elevated temperatures. The effect of HP is related mainly to enhancement in the diffusivity of single oxygen atoms and small oxygen aggregates. HP does not noticeably increase the binding energies of the most simple oxygen related complexes like O2i, CsOni : The biggest HP effect on the thermal double donor (TDDs) generation is revealed in hydrogenated samples. Heat-treatment of such samples at 450 C under HP results in extremely high TDD introduction rates as well as in a strong increase in the concentration of the first TDD species. r 2003 Elsevier B.V. All rights reserved. Keywords: Silicon; Oxygen clustering; Pressure; LVMs
1. Introduction The most common material in semiconductor industry, Czochralski-grown silicon (Cz–Si), typically contains B1018 cm3 of interstitial oxygen atoms (Oi) and is in a very large temperature range highly supersaturated. As a consequence clustering of oxygen atoms occurs, resulting in formation of different oxygen complexes from small clusters like dimers (O2i) and trimers (O3i) to thermal double donors (TDDs) and quartz precipitates [1,2]. Due to its technological importance the oxygen clustering and TDD formation in Si have been *Corresponding author. Tel.: +375-17-184-1290; fax: +37517-284-0888. E-mail address: [email protected] (L.I. Murin).
widely studied [1–3]. However, in spite of intense work, the understanding of the TDD microscopic structure and formation mechanism is still far from complete. A normally rather low concentration of the small clusters (O2i, O3i and the earliest TDD species) prevents their identification by spectroscopic studies. To overcome the problem a number of new approaches in oxygen defect engineering were recently developed. Among them, the hydrogenation of Si crystals [3–5] and electron irradiation at elevated temperatures [6,7] were found to be efficient ways to increase the concentration of small oxygen clusters. Another new and very promising method of defect engineering is related to a strong enhancement effect of high hydrostatic pressure (HP) on the oxygen agglomeration [8–11]. Several explanations of the
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.134
ARTICLE IN PRESS L.I. Murin et al. / Physica B 340–342 (2003) 565–569
2. Experimental The samples used in this investigation were prepared from the n- and p-type commercial Cz–Si crystals (r ¼ 5280 O cm) with different content of oxygen and carbon impurities. The concentrations of Oi and Cs were monitored by measuring the absorption bands at 1107 and 605 cm1 [3]. The samples were polished to an optical surface on two sides and the dimensions were 10 5 3 mm3. Some of the samples were 2 MeV electron irradiated at 350 C in order to get a dimer-rich material. A few samples were hydrogenated by annealing at 1250 C in H2 gas ambient followed by fast cooling. Heat-treatments (HTs) at atmospheric pressure were performed in a nitrogen ambient or in air. HP treatments (see details in Ref. [9]) were performed under purified argon pressure of 10.7 kbar at 450 C and 650 C for 1 h. IR absorption analysis was carried out using a Bruker IFS 113v spectrometer. A spectral resolution of 0.5–1.0 cm1 was used and the samples were measured at 15 and 300 K in the range 400– 2000 cm1. Electrical properties of the samples were controlled by four-probe resistivity measurements at RT.
3. Results and discussion
3.1. Treatments at 450 C
eliminate such aggregates and standardise the conditions for further studies of oxygen clustering at lower temperatures [1,16]. Spectrum 1 in Fig. 1 is a typical one for samples annealed at 650 C for 1 h. Only the band at 1013 cm1 related to the oxygen dimer is seen and its intensity corresponds to an equilibrium level of O2i at 650 C [14]. The following HT at 450 C results in appearance of the band at 1105 cm1 related to oxygen trimer [5,17] and in a growth of the 1013 cm1 band (spectrum 2). The concentration of TDDs was about 5 1013 cm3 and the TDD vibrational bands [12,13] were practically undetectable. Such relatively low initial introduction rate of TDDs is typical for Cz–Si preannealed at 600–800 C [16]. Annealing at 450 C under HP results in a more significant increase of the O2i and O3i bands and appearance of the TDD1 and TDD2 LVMs (spectrum 3). Since the formation of oxygen dimer is expected to occur via a reaction Oi+Oi ) O2i, the enhanced generation of O2i implies the HP enhancement in the diffusivity of single oxygen atoms. A rapid growth of the O3i band indicates an enhancement in the dimer mobility as well. According to the recent models of the TDD formation [15,17,18], not only the oxygen dimers, but all other small oxygen clusters (chains) are mobile at elevated temperatures and the sequential formation of the TDD family occurs mainly via migration of the TDD species and their interaction 0.05 -1
HP effect were suggested, including HP activation of the nucleation centres [8,9] and HP enhancement in the oxygen diffusivity [10,11], but no direct evidences were presented for them. Local vibrational mode (LVM) spectroscopy has recently appeared to be a very powerful tool in the studies of small oxygen-related clusters in Si [5–7,12–15]. In this work, LVM spectroscopy is used to explore the effect of HP on the binding energy and diffusivity of such clusters.
Absorption coefficient, cm
566
T = 300 K
O2i
0.04
O3i
0.03 TDD 1
0.02
TDD 2
3 2
0.01 1 0.00
950
975
1000
-1
1025
Wavenumber, cm
As-grown Cz–Si crystals normally contain some amounts of small oxygen clusters like TDDs and their precursors. Short-time preheat-treatment at about 650 C followed by fast cooling allows us to
Fig. 1. Fragments of absorption spectra at 300 K for Cz–Si ([Oi]=1.1 1018, [Cs ]=2 1015 cm3) samples heat-treated at: 1–650 C for 1 h; 2–650 C for 1 h followed by 450 C for 1 h ; 3– 650 C for 1 h followed by 450 C for 1 h under HP.
ARTICLE IN PRESS
Absorption coefficient, cm
-1
T = 300 K
VO2
TDD
VO3
0.8
0.6
VO VO3 4
3 0.4
2
O3i
0.2
O2i O 3i 0.0
720
O2i
1 800
880
960 -1
1040
Wavenumber, cm
Fig. 2. Fragments of absorption spectra at 300 K for Cz–Si ([Oi]=1 1018, [Cs ]=5 1015 cm3) samples after: 1–2 MeV electron irradiation at 350 C with a fluence of 4 1018 cm2; 2—the irradiation followed by annealing at 450 C for 1 h; 3— the irradiation followed by annealing at 450 C for 1 h under HP.
Absorption coefficient, cm
with single O atoms. Hence, the enhanced generation of TDD1 and TDD2 under HP should be due to an enhancement in the diffusivity of the oxygen trimer, TDD0 and TDD1. Further support to such interpretation of the HP effect was found in the study of TDD generation in the dimer-rich samples (Fig. 2). Preirradiation at 350 C increased the dimer concentration significantly (spectrum 1). HT of such a material at 450 C for 1 h resulted in a decrease of the O2i bands (spectrum 2) and in generation of donor centers in concentration of about 5 1015 cm3 (r ¼ 0:55 O cm). The TDD bands in the region 975–1020 cm1 are overlapping with the more strong bands arising from the radiation-induced defects (RDs) [19], and their intensities could not be determined accurately. Another group of the TDD bands located at about 730 cm1 [12,15], however, manifests the presence of TDDs clearly. A similar sample was annealed under HP (spectrum 3). Both effects, elimination of O2i and growth of TDDs, are much more pronounced in this case. It should be noted that the 730 cm1 band is hardly seen in the spectrum of the sample with normal O2i content treated under HP (Fig. 3, spectrum 1). Evidently, all the excess TDDs formed in the dimer-rich material are associated
-1
L.I. Murin et al. / Physica B 340–342 (2003) 565–569
567
1.4 O2i
T = 300 K
1.2 TDD
1.0 0.8 TDD 3
0.6
TDD
0.4
3
0.2
2 1
0.0
550
750
800
O2i
TDD 2
850
900
950 1000 1050 -1
Wavenumber, cm
Fig. 3. Fragments of absorption spectra at 300 K for Cz–Si ([Oi]=1.1 1018, [Cs ]=2 1015 cm3) samples heat-treated at: 1–650 C for 1 h followed by 450 C for 1 h under HP (10.66 kbar); 2–1250 C for 40 min in H2 gas ambient followed by 450 C for 1 h; 3–1250 C for 40 min in H2 gas ambient followed by 450 C for 1 h under HP.
with the transformation of O2i and O3i (oxygen trimer was also developed upon ‘hot’ irradiation) into larger clusters. The HP enhancement of this process is most likely related to an increase in the migration ability of the oxygen clusters. Because of high concentration of RDs, an interaction between them and the oxygen dimer is expected to occur as well. The band at 985 cm1 was suggested to arise from a VO4 defect [19,20]. Its formation is normally observed upon annealing of the VO2 and VO3 bands at temperatures of about 500 C. In dimer-rich material, the generation of VO4 occurs rapidly already at 450 C, probably due to a reaction O2i+VO2 ) VO4. Elimination of the VO2 band and growth of the VO4 band are again more pronounced under HP which is consistent with the HP enhanced diffusivity of the oxygen dimer. The presence of hydrogen in Si also results in a strong enhancement of the oxygen clustering [3–5]. So, it was interesting to compare the both effects, hydrogenation and HP, on the formation of oxygen clusters. LVM studies of the TDD generation at 450 C (Fig. 3, spectra 1 and 2) have shown that TDDs are generated less efficiently in HP treated samples as compared to hydrogenated ones. However, when the hydrogenated samples were subjected to a similar HP annealing, the HP
ARTICLE IN PRESS L.I. Murin et al. / Physica B 340–342 (2003) 565–569
568
enhancement effect was enormously high (spectrum 3). The total concentration of TDDs was even higher than in dimer-rich material treated under HP and all the groups of TDD LVMs were clearly seen. The broad band centred at about 900–950 cm1 is related to electronic transitions in TDDs being in a positive charge state at room temperature (r ¼ 0:45 O cm). The dominant species was TDD3, but the TDD2 concentration was also extremely high with the main part of the defect being in a neutral X configuration. Only one mode at 1020 cm1 was found previously for this configuration [13]. The present study allowed us to reveal another mode located at 942 cm1. 3.2. Treatments at 650 C
Absorption coefficient, cm
-1
At temperatures X600 C, the quasi-equilibrium concentration of O2i [14,17] and CsOni (nX1) complexes [21] is gained rapidly and governed only by the defect binding energy and impurity content. To explore possible effects of HP on binding energy of the defects we have performed LVM studies of Cz–Si crystals with different carbon content annealed at 650 C for 1 h under atmospheric and high pressure. The typical spectra for carbon-rich material are shown in Fig. 4. It can be inferred from the difference spectrum 3 that the HP treatment results in slightly enhanced intensity (up to 10–15%) of the bands related to O2i, CsOi
0.5
CsOi
T = 15 K
and CsO2i defects. More pronounced increase in the intensity of the oxygen dimer band and appearance of the trimer band were observed in the carbon-lean samples with a high oxygen concentration (1.3 1018 cm3) after the HP treatment at 650 C. However, this may be due to a relatively low cooling rate (B100 C/min) after HP treatment. The lifetime of oxygen dimers is only about a few seconds at 650 C and their equilibrium concentration increases with a decrease in the temperature down to 500–450 C [14]. Apparently, only a small increase in the binding energy of O2i, O3i and CsOni occurs under HP. It cannot been excluded, however, that the HP effect on binding energy might be more significant for larger clusters like TDDs and precipitate nuclei [8,9].
4. Conclusion LVM studies have shown that, in agreement with previous findings, high HP enhances oxygen clustering in Cz–Si at elevated temperatures. Strong indications are found that the effect of HP is related mainly to an enhancement in the diffusivity of single oxygen atoms and small oxygen aggregates. HP does not increase noticeably the binding energies of the simple oxygenrelated complexes like O2i, CsOi, etc. The most striking HP effect in enhancement of the TDD generation occurs in hydrogenated Cz–Si. Heattreatment of such material at 450 C under HP results in extremely high TDD introduction rate.
0.4
Acknowledgements
0.3 18
O2i 0.1 0.0 1000
Oi
CsO3i
0.2
1020
We thank KVA, SI and the Nanometer Consortium at Lund University in Sweden for financial support. We also acknowledge support from the grant INTAS-01-486.
CsO2i 2 1 3 1040
1060
1080
1100
1120
-1
Wavenumber, cm
Fig. 4. Fragments of absorption spectra at 15 K for Cz–Si ([Oi]=7.5 1017, [Cs ]=4.5 1017 cm3) samples heat-treated at: 1–650 C for 1 h; 2–650 C for 1 h under HP; 3-difference between the spectra 2 and 1.
References [1] F. Shimura (Ed.), Oxygen in Silicon, Semiconductors and Semimetals, Vol. 42, Academic Press, London, 1994.
ARTICLE IN PRESS L.I. Murin et al. / Physica B 340–342 (2003) 565–569 [2] R. Jones (Ed.), Early Stages of Oxygen Precipitation in Silicon, High Technology, Vol. 17, NATO ASI Series, 3, Kluwer, Dordrecht, 1996. [3] R.C. Newman, J. Phys.: Condens. Matter 12 (2000) R335. [4] V.P. Markevich, I.F. Medvedeva, L.I. Murin, in: R. Jones (Ed.), Early Stages of Oxygen Precipitation in Silicon, High Technology, Vol. 17, NATO ASI Series, 3, Kluwer, Dordrecht, 1996, pp. 103–122. [5] L.I. Murin, et al., Physica B 302–303 (2001) 180. [6] S. Oberg, et al., Phys. Rev. Lett. 81 (1998) 2930. . [7] J.L. Lindstrom, et al., Nuclear Instrum. Methods Phys. Res. B 186 (2002) 121. [8] A. Misiuk, et al., Solid State Phenomena 57–58 (1997) 393. [9] A. Misiuk, Mater. Phys. Mech. 1 (2000) 119. [10] V.V. Emtsev, et al., Appl. Phys. Lett. 71 (1997) 254. [11] V.V. Emtsev Jr., et al., Phys. Stat. Sol. B 235 (2003) 75. . [12] J.L. Lindstrom, T. Hallberg, in: R. Jones (Ed.), Early Stages of Oxygen Precipitation in Silicon, High Techno-
[13] [14] [15] [16]
[17]
[18] [19] [20] [21]
569
logy, Vol. 17, NATO ASI Series, 3, Kluwer, Dordrecht, 1996, pp. 41–60. . T. Hallberg, J.L. Lindstrom, Appl. Phys. Lett. 68 (1996) 3458. L.I. Murin, et al., Phys. Rev. Lett. 80 (1998) 93. J. Coutinho, et al., Phys. Rev. Lett. 87 (2001) 235501. L.I. Murin, V.P. Markevich, in: K. Sumino (Ed.), Defect Control in Semiconductors, Elsevier, North-Holland, 1990, pp. 199–210. L.I. Murin, V.P. Markevich, in: R. Jones (Ed.), Early Stages of Oxygen Precipitation in Silicon, High Technology, Vol. 17, NATO ASI Series, 3, Kluwer, Dordrecht, 1996, pp. 329–336. Y.J. Lee, et al., Phys. Rev. B 66 (2002) 165221. . J.L. Lindstrom, B.G. Svensson, Mater. Res. Soc. Symp. Proc. 59 (1986) 45. C.A. Londos, L.G. Fytros, J. Appl. Phys. 89 (2001) 928. Y. Shirakawa, H. Yamada-Kaneta, H. Mori, J. Appl. Phys. 77 (1994) 92.
Physica B 340–342 (2003) 565–569
Effect of high hydrostatic pressure on small oxygen-related clusters in silicon: LVM studies . b, A. Misiukc L.I. Murina,*, J.L. Lindstrom a
Institute of Solid State and Semiconductor Physics, P. Brovki Street 17, Minsk 220072, Belarus b Lund University, Division of Solid State Physics, Lund, Sweden c Institute of Electron Technology, Warsaw, Poland
Abstract Local vibrational mode (LVM) spectroscopy is used to explore the effect of high hydrostatic pressure (HP) on the formation of small oxygen-related clusters (dimers, trimers, thermal donors, and C–O complexes) at 450 C and 650 C in Cz–Si crystals with different impurity content and prehistory. It is found, in agreement with previous studies, that HP enhances the oxygen clustering in Cz–Si at elevated temperatures. The effect of HP is related mainly to enhancement in the diffusivity of single oxygen atoms and small oxygen aggregates. HP does not noticeably increase the binding energies of the most simple oxygen related complexes like O2i, CsOni : The biggest HP effect on the thermal double donor (TDDs) generation is revealed in hydrogenated samples. Heat-treatment of such samples at 450 C under HP results in extremely high TDD introduction rates as well as in a strong increase in the concentration of the first TDD species. r 2003 Elsevier B.V. All rights reserved. Keywords: Silicon; Oxygen clustering; Pressure; LVMs
1. Introduction The most common material in semiconductor industry, Czochralski-grown silicon (Cz–Si), typically contains B1018 cm3 of interstitial oxygen atoms (Oi) and is in a very large temperature range highly supersaturated. As a consequence clustering of oxygen atoms occurs, resulting in formation of different oxygen complexes from small clusters like dimers (O2i) and trimers (O3i) to thermal double donors (TDDs) and quartz precipitates [1,2]. Due to its technological importance the oxygen clustering and TDD formation in Si have been *Corresponding author. Tel.: +375-17-184-1290; fax: +37517-284-0888. E-mail address: [email protected] (L.I. Murin).
widely studied [1–3]. However, in spite of intense work, the understanding of the TDD microscopic structure and formation mechanism is still far from complete. A normally rather low concentration of the small clusters (O2i, O3i and the earliest TDD species) prevents their identification by spectroscopic studies. To overcome the problem a number of new approaches in oxygen defect engineering were recently developed. Among them, the hydrogenation of Si crystals [3–5] and electron irradiation at elevated temperatures [6,7] were found to be efficient ways to increase the concentration of small oxygen clusters. Another new and very promising method of defect engineering is related to a strong enhancement effect of high hydrostatic pressure (HP) on the oxygen agglomeration [8–11]. Several explanations of the
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.134
ARTICLE IN PRESS L.I. Murin et al. / Physica B 340–342 (2003) 565–569
2. Experimental The samples used in this investigation were prepared from the n- and p-type commercial Cz–Si crystals (r ¼ 5280 O cm) with different content of oxygen and carbon impurities. The concentrations of Oi and Cs were monitored by measuring the absorption bands at 1107 and 605 cm1 [3]. The samples were polished to an optical surface on two sides and the dimensions were 10 5 3 mm3. Some of the samples were 2 MeV electron irradiated at 350 C in order to get a dimer-rich material. A few samples were hydrogenated by annealing at 1250 C in H2 gas ambient followed by fast cooling. Heat-treatments (HTs) at atmospheric pressure were performed in a nitrogen ambient or in air. HP treatments (see details in Ref. [9]) were performed under purified argon pressure of 10.7 kbar at 450 C and 650 C for 1 h. IR absorption analysis was carried out using a Bruker IFS 113v spectrometer. A spectral resolution of 0.5–1.0 cm1 was used and the samples were measured at 15 and 300 K in the range 400– 2000 cm1. Electrical properties of the samples were controlled by four-probe resistivity measurements at RT.
3. Results and discussion
3.1. Treatments at 450 C
eliminate such aggregates and standardise the conditions for further studies of oxygen clustering at lower temperatures [1,16]. Spectrum 1 in Fig. 1 is a typical one for samples annealed at 650 C for 1 h. Only the band at 1013 cm1 related to the oxygen dimer is seen and its intensity corresponds to an equilibrium level of O2i at 650 C [14]. The following HT at 450 C results in appearance of the band at 1105 cm1 related to oxygen trimer [5,17] and in a growth of the 1013 cm1 band (spectrum 2). The concentration of TDDs was about 5 1013 cm3 and the TDD vibrational bands [12,13] were practically undetectable. Such relatively low initial introduction rate of TDDs is typical for Cz–Si preannealed at 600–800 C [16]. Annealing at 450 C under HP results in a more significant increase of the O2i and O3i bands and appearance of the TDD1 and TDD2 LVMs (spectrum 3). Since the formation of oxygen dimer is expected to occur via a reaction Oi+Oi ) O2i, the enhanced generation of O2i implies the HP enhancement in the diffusivity of single oxygen atoms. A rapid growth of the O3i band indicates an enhancement in the dimer mobility as well. According to the recent models of the TDD formation [15,17,18], not only the oxygen dimers, but all other small oxygen clusters (chains) are mobile at elevated temperatures and the sequential formation of the TDD family occurs mainly via migration of the TDD species and their interaction 0.05 -1
HP effect were suggested, including HP activation of the nucleation centres [8,9] and HP enhancement in the oxygen diffusivity [10,11], but no direct evidences were presented for them. Local vibrational mode (LVM) spectroscopy has recently appeared to be a very powerful tool in the studies of small oxygen-related clusters in Si [5–7,12–15]. In this work, LVM spectroscopy is used to explore the effect of HP on the binding energy and diffusivity of such clusters.
Absorption coefficient, cm
566
T = 300 K
O2i
0.04
O3i
0.03 TDD 1
0.02
TDD 2
3 2
0.01 1 0.00
950
975
1000
-1
1025
Wavenumber, cm
As-grown Cz–Si crystals normally contain some amounts of small oxygen clusters like TDDs and their precursors. Short-time preheat-treatment at about 650 C followed by fast cooling allows us to
Fig. 1. Fragments of absorption spectra at 300 K for Cz–Si ([Oi]=1.1 1018, [Cs ]=2 1015 cm3) samples heat-treated at: 1–650 C for 1 h; 2–650 C for 1 h followed by 450 C for 1 h ; 3– 650 C for 1 h followed by 450 C for 1 h under HP.
ARTICLE IN PRESS
Absorption coefficient, cm
-1
T = 300 K
VO2
TDD
VO3
0.8
0.6
VO VO3 4
3 0.4
2
O3i
0.2
O2i O 3i 0.0
720
O2i
1 800
880
960 -1
1040
Wavenumber, cm
Fig. 2. Fragments of absorption spectra at 300 K for Cz–Si ([Oi]=1 1018, [Cs ]=5 1015 cm3) samples after: 1–2 MeV electron irradiation at 350 C with a fluence of 4 1018 cm2; 2—the irradiation followed by annealing at 450 C for 1 h; 3— the irradiation followed by annealing at 450 C for 1 h under HP.
Absorption coefficient, cm
with single O atoms. Hence, the enhanced generation of TDD1 and TDD2 under HP should be due to an enhancement in the diffusivity of the oxygen trimer, TDD0 and TDD1. Further support to such interpretation of the HP effect was found in the study of TDD generation in the dimer-rich samples (Fig. 2). Preirradiation at 350 C increased the dimer concentration significantly (spectrum 1). HT of such a material at 450 C for 1 h resulted in a decrease of the O2i bands (spectrum 2) and in generation of donor centers in concentration of about 5 1015 cm3 (r ¼ 0:55 O cm). The TDD bands in the region 975–1020 cm1 are overlapping with the more strong bands arising from the radiation-induced defects (RDs) [19], and their intensities could not be determined accurately. Another group of the TDD bands located at about 730 cm1 [12,15], however, manifests the presence of TDDs clearly. A similar sample was annealed under HP (spectrum 3). Both effects, elimination of O2i and growth of TDDs, are much more pronounced in this case. It should be noted that the 730 cm1 band is hardly seen in the spectrum of the sample with normal O2i content treated under HP (Fig. 3, spectrum 1). Evidently, all the excess TDDs formed in the dimer-rich material are associated
-1
L.I. Murin et al. / Physica B 340–342 (2003) 565–569
567
1.4 O2i
T = 300 K
1.2 TDD
1.0 0.8 TDD 3
0.6
TDD
0.4
3
0.2
2 1
0.0
550
750
800
O2i
TDD 2
850
900
950 1000 1050 -1
Wavenumber, cm
Fig. 3. Fragments of absorption spectra at 300 K for Cz–Si ([Oi]=1.1 1018, [Cs ]=2 1015 cm3) samples heat-treated at: 1–650 C for 1 h followed by 450 C for 1 h under HP (10.66 kbar); 2–1250 C for 40 min in H2 gas ambient followed by 450 C for 1 h; 3–1250 C for 40 min in H2 gas ambient followed by 450 C for 1 h under HP.
with the transformation of O2i and O3i (oxygen trimer was also developed upon ‘hot’ irradiation) into larger clusters. The HP enhancement of this process is most likely related to an increase in the migration ability of the oxygen clusters. Because of high concentration of RDs, an interaction between them and the oxygen dimer is expected to occur as well. The band at 985 cm1 was suggested to arise from a VO4 defect [19,20]. Its formation is normally observed upon annealing of the VO2 and VO3 bands at temperatures of about 500 C. In dimer-rich material, the generation of VO4 occurs rapidly already at 450 C, probably due to a reaction O2i+VO2 ) VO4. Elimination of the VO2 band and growth of the VO4 band are again more pronounced under HP which is consistent with the HP enhanced diffusivity of the oxygen dimer. The presence of hydrogen in Si also results in a strong enhancement of the oxygen clustering [3–5]. So, it was interesting to compare the both effects, hydrogenation and HP, on the formation of oxygen clusters. LVM studies of the TDD generation at 450 C (Fig. 3, spectra 1 and 2) have shown that TDDs are generated less efficiently in HP treated samples as compared to hydrogenated ones. However, when the hydrogenated samples were subjected to a similar HP annealing, the HP
ARTICLE IN PRESS L.I. Murin et al. / Physica B 340–342 (2003) 565–569
568
enhancement effect was enormously high (spectrum 3). The total concentration of TDDs was even higher than in dimer-rich material treated under HP and all the groups of TDD LVMs were clearly seen. The broad band centred at about 900–950 cm1 is related to electronic transitions in TDDs being in a positive charge state at room temperature (r ¼ 0:45 O cm). The dominant species was TDD3, but the TDD2 concentration was also extremely high with the main part of the defect being in a neutral X configuration. Only one mode at 1020 cm1 was found previously for this configuration [13]. The present study allowed us to reveal another mode located at 942 cm1. 3.2. Treatments at 650 C
Absorption coefficient, cm
-1
At temperatures X600 C, the quasi-equilibrium concentration of O2i [14,17] and CsOni (nX1) complexes [21] is gained rapidly and governed only by the defect binding energy and impurity content. To explore possible effects of HP on binding energy of the defects we have performed LVM studies of Cz–Si crystals with different carbon content annealed at 650 C for 1 h under atmospheric and high pressure. The typical spectra for carbon-rich material are shown in Fig. 4. It can be inferred from the difference spectrum 3 that the HP treatment results in slightly enhanced intensity (up to 10–15%) of the bands related to O2i, CsOi
0.5
CsOi
T = 15 K
and CsO2i defects. More pronounced increase in the intensity of the oxygen dimer band and appearance of the trimer band were observed in the carbon-lean samples with a high oxygen concentration (1.3 1018 cm3) after the HP treatment at 650 C. However, this may be due to a relatively low cooling rate (B100 C/min) after HP treatment. The lifetime of oxygen dimers is only about a few seconds at 650 C and their equilibrium concentration increases with a decrease in the temperature down to 500–450 C [14]. Apparently, only a small increase in the binding energy of O2i, O3i and CsOni occurs under HP. It cannot been excluded, however, that the HP effect on binding energy might be more significant for larger clusters like TDDs and precipitate nuclei [8,9].
4. Conclusion LVM studies have shown that, in agreement with previous findings, high HP enhances oxygen clustering in Cz–Si at elevated temperatures. Strong indications are found that the effect of HP is related mainly to an enhancement in the diffusivity of single oxygen atoms and small oxygen aggregates. HP does not increase noticeably the binding energies of the simple oxygenrelated complexes like O2i, CsOi, etc. The most striking HP effect in enhancement of the TDD generation occurs in hydrogenated Cz–Si. Heattreatment of such material at 450 C under HP results in extremely high TDD introduction rate.
0.4
Acknowledgements
0.3 18
O2i 0.1 0.0 1000
Oi
CsO3i
0.2
1020
We thank KVA, SI and the Nanometer Consortium at Lund University in Sweden for financial support. We also acknowledge support from the grant INTAS-01-486.
CsO2i 2 1 3 1040
1060
1080
1100
1120
-1
Wavenumber, cm
Fig. 4. Fragments of absorption spectra at 15 K for Cz–Si ([Oi]=7.5 1017, [Cs ]=4.5 1017 cm3) samples heat-treated at: 1–650 C for 1 h; 2–650 C for 1 h under HP; 3-difference between the spectra 2 and 1.
References [1] F. Shimura (Ed.), Oxygen in Silicon, Semiconductors and Semimetals, Vol. 42, Academic Press, London, 1994.
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