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Interaction between self-interstitials and the oxygen dimer in silicon
Physica B 308–310 (2001) 284–289
Interaction between self-interstitials and the oxygen dimer in silicon . a,*, T. Hallbergb, J. Hermanssona, L.I. Murinc, B.A. Komarovc, J.L. Lindstrom V.P. Markevichc,d, M. Klevermana, B.G. Svenssone a
Department of PhysicsFSolid State Physics, University of Lund, PO Box 118, SE-221 00 Lund, Sweden b Defence Research Establishment, PO Box 1165, SE-581 11, Linkoping, Sweden c Institute of Solid State and Semiconductor Physics, 220072 Minsk, Belarus d Centre for Electronic Materials, UMIST, Manchester M60 1QP, UK e Solid State Electronics, Royal Institute of Technology, SE-164 40 Kista, Sweden
Abstract Interactions between the oxygen dimer (O2i) and silicon self-interstitials (I) and vacancies (V) have been studied in Czochralski-grown silicon (Cz–Si) crystals using infrared absorption and deep level transient spectroscopies. The focus in this report is on reactions of O2i with I. The first step in this interaction is found to be the formation of a selfinterstitial-dioxygen centre (IO2i) with oxygen-related local vibrational mode (LVM) bands at 922 and 1037 cm1. During the second formation step, another centre, I2O2i, with LVM bands at 918 and 1034 cm1 is suggested to appear. A Si-related band at about 545 cm1 is also assigned to both the IO2i and I2O2i centres. The IO2i centre is found to be electrically active with an acceptor level at Ec 0:11 eV. The both defects, IO2i and I2O2i, are stable at room temperature and anneal out at about 400 and 550 K, respectively. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Self-interstitials; Oxygen; Electron irradiation
1. Introduction Silicon wafers used for integrated circuits require the presence of oxygen, due to its beneficial effects such as wafer hardening and intrinsic gettering [1,2]. The most common material, Czohralski-grown silicon, 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, trimers (O3i) to oxygen-related thermal double donors (TDDs) and quartz precipitates. *Corresponding author. Tel.: +46-46-2220929; fax: +46-462223637. E-mail address: [email protected] . (J.L. Lindstrom).
It was found that oxygen atoms as well as oxygenrelated complexes could interact with intrinsic defects of Si lattice, vacancies (V) and self-interstitials (I) [1,2]. V and I can be easily created upon irradiation of Si crystals with high energy particles (electrons, protons, ions, etc.). Electron-irradiated Cz–Si was a subject of many investigations [3–10]. Several oxygen-vacancy (Vn On ) complexes have been identified, but very few complexes incorporating oxygen atoms and self-interstitials are known. Experimental studies of samples irradiated at around 80 K have showed that Oi can trap I and form IOi complex [11–14], which is stable below 300 K. This defect has been found to trap an additional I and form I2Oi [15,16]. Silicon self-interstitials were suggested to play an important role in the formation of oxygen-related TDDs [17,18]. The structure of these complexes, which were discovered almost 50 years ago is still a matter of debate.
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 4 - 9
J.L. Lindstrom . et al. / Physica B 308–310 (2001) 284–289
According to one of the most popular models of TDDs, their core consists of a complex of two oxygen atoms and a silicon self-interstitial (O2iI) [17]. The O2iI was suggested to be a double donor. Recently, local vibrational mode bands due to oxygen dimers were identified in Si [19–21], and a method of enhanced formation of O2i complexes was proposed [22,23]. In this work we present results of infrared (IR) absorption and deep level transient spectroscopy (DLTS) studies of interaction of oxygen dimers with V and I in electron- and gamma-rays-irradiated Cz–Si crystals.
2. Experimental The samples used in this investigation were as-grown Cz–Si, doped with phosphorus to resistivities of about 60 O cm. The concentrations of interstitial oxygen (Oi) and substitutional carbon (Cs ) were monitored by measuring the well-known absorption bands at 1107 [24] and 605 cm1 [9], respectively. We also included samples doped with the isotope 18O. The samples were polished to an optical surface on two sides and the dimensions were 10 5 3 mm3. The electron irradiation was performed in a temperature regulated holder in air at temperatures in the range 320–650 K to different doses from 5 1016 to 4 1018 cm2, using 2.5 MeV electrons. The beam current was in the range 1–12 mA/cm2. Some samples were irradiated with gamma-rays from a 60Co source. Heat-treatments (HTs) were performed in a nitrogen ambient or in air. IR absorption studies of LVMs were carried out using Fourier Transform IR (FT-IR) spectrometers. A spectral resolution of 0.5–1.0 cm1 was used and the samples were measured at 10 K and at RT. DLTS measurements were carried out using a Semitrap DLS83D system. Capture cross sections of electrons by deep traps (sn ) were determined from the analysis of trap filling process upon applying pulses of different lengths. Gold Schottky diodes were prepared for the measurements.
3. Results and discussion 3.1. Formation of the oxygen dimer Since the concentration of oxygen dimers in as-grown Cz–Si crystals is relatively low (around 1 1015 cm3) and IR studies of LVM bands typically require defect concentrations an order of magnitude higher for detailed investigations, a special technique has been developed [22,23], where the dimers are produced by electron irradiation (MeV) at about 650 K in carbon-
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lean material through the reaction I þ VO2 ) O2i :
ð1Þ
Using this technique we could prepare silicon samples with the dimer concentration up to 5 1016 cm3. It has been found that there is a considerable difference between carbon-lean and carbon-rich CZ-silicon in terms of possible reactions during irradiation [22,23]. The effective interaction of self-interstitials with substitutional carbon atoms according to the Watkins replacement mechanism, Cs þI ) Ci ;
ð2Þ
is the main reason for that. One consequence of the selfinterstitial-carbon interaction is that a high concentration of oxygen dimers according to (1) cannot be achieved in carbon-rich silicon. Another efficient trap for I is the vacancy-oxygen centre (VO) through the reaction I þ VO ) Oi :
ð3Þ
3.2. Irradiation of dimer-rich samples at 300 K In carbon-lean silicon samples irradiated at 650 K the VO concentration after irradiation is much lower than the concentration of O2i and the main traps for I at a repeated irradiation at 300 K will therefore initially be according to I þ Oi ) IOi ;
ð4Þ
I þ IOi ) I2 Oi ;
ð5Þ
I þ O2i ) IO2i ;
ð6Þ
I þ IO2i ) I2 O2i :
ð7Þ
The IOi complex is not stable above 300 K, so reactions (4) and (5) are not effective during the irradiation at 300 K and the main reactions should be (6), (7) and (1). The further irradiation at 300 K will, however, lead to a rapidly increasing concentration of VO centres due to the dominating concentration of Oi and reaction (3) will very soon compete with reactions (6), (7) and (1). One can compare the concentration of O2i (1012 cm1 band) in as-grown samples and after irradiation at 3501C (F ¼ 8 1017 e/cm2) by an examination of spectra 1 and 2 in Fig. 1. The strong increase in [O2i] after the irradiation makes its concentration comparable to that of the VO2 complex (889 cm1 band). The well-known VO-related band at 830 cm1 is not seen in the spectrum 2. Several new bands are appearing in absorption spectra after subsequent irradiation at room temperature (spectra 3–10 in Fig. 1). After irradiation with a fluence of 5 1016 e/cm2, absorption bands at 911 and 1034 cm1 are seen (spectrum 4 in Fig. 1), further irradiation leads to the development of bands at 916
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Fig. 1. Room temperature absorption spectra for C-lean n-Cz– Si (r ¼ 50 O cm): (1) as-grown; (2) after electron irradiation at 3501C, F=8 1017 cm2; (3–10) after RT irradiation. F(cm2): (3) 1 1016, (4) 5 1016, (5) 1017, (6) 2 1017, (7) 4 1017, (8) 7 1017, (9) 1.1 1018, (10) 6 1018.
Fig. 2. Generation kinetics of some radiation-induced oxygenrelated complexes upon electron irradiation at room temperature of a dimer-rich Cz–Si sample.
Fig. 3. Absorption spectra measured at 10 and 300 K for a Clean Cz–Si sample after two-step electron irradiation: at 3501C with a fluence of 8 1017+at RT with a fluence of 7 1017 cm2.
into account differences in capture cross sections of selfinterstitials by VO, VO2, O2i, and IO2i complexes. An analysis of absorption spectra of irradiated oxygen-dimer-rich samples doped with 18O reveals that the pairs of bands at 911 and 1034 cm1, and at 916 and 1031 cm1 are oxygen-related LVMs, while the weak band at 540 cm1 does not show any shift upon replacement of oxygen isotopes and therefore is suggested to be a Si-related LVM. In Fig. 3 one can see the temperature shifts of the bands upon the change of measurement temperature from 300 to 10 K. The band at 911 cm–1 shifts to 922 cm1, while the band at 916 cm1 shifts to 918 cm1 with the decrease of the measurement temperature. Typical shifts of oxygenrelated LVMs in this wavenumber range is about 5– 6 cm1. Exceptions are the LVMs at 1012 and 1060 cm1 due to the oxygen dimer, the temperature shifts of which are extremely small (Fig. 3) [25]. 3.3. Annealing studies
and 1031 cm1 (spectra 6–10 in Fig. 1). It is suggested that the first pair of bands appears when IO2i starts to form according to reaction (6) and after that reaction (7) can be activated resulting in the formation of I2O2i. There is also a weak band at 540 cm1, the development of which seems to correlate with that of the above mentioned pairs of bands. As the irradiation fluence increases the concentration of the vacancy-oxygen complex grows rapidly and the VO-related line at 830 cm1 dominates in the spectra soon (spectra 6–10 in Fig. 1). This is also illustrated in Fig. 2, where the dose dependencies of the integrated intensities of the considered bands are shown. The observed dependencies can be described by reactions (1), (3), (6) and (7) taking
In order to investigate the thermal stability of the new defects (IO2i and I2O2i ), an isochronal annealing study has been done. Absorption spectra of an electronirradiated sample after isochronal anneals are shown in Fig. 4. An examination of the spectra shows that the first appearing defect, IO2i, has completely annealed out at 1601C and the second one, I2O2i, at 2501C. Disappearance of the IO2i and I2O2i complexes is accompanied by an increase in intensities of the bands at 1012 and 1060 cm1 due to the oxygen dimer, suggesting that reactions (6) and (7) have corresponding back-reactions. Self-interstitials released according to these back-reactions can take part in reactions like (1) and (3). Several other relatively weak bands have been observed upon
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Fig. 4. Absorption spectra at 10 K for Cz–Si: (1) after two-step electron irradiation (3501C, F=8 1017+RT, F= 7 1017 cm2) and at different stages of 20 min isochronal anneal, (2) 1201C, (3) 1601C, (4) 2101C, (5) 2501C, (6) 3001C, (7) 3501C.
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Fig. 5. Absorption spectra at 10 K for electron irradiated (RT, F ¼ 5 1016 cm2) as-grown Cz–Si crystals with low (o1015 cm3, spectra 2 and 3) and intermediate carbon content (5 1016 cm3, spectrum 1). Spectrum 3 is a difference between the spectra of irradiated and as-grown samples.
annealing of the irradiated dimer-rich samples and an investigation of these bands is in progress. After anneals at higher temperatures the band at 895 cm1 due to the VO2 complex appears in the spectra (spectra 6 and 7 in Fig. 4). 3.4. Irradiation of as-grown materials Simultaneously, with the irradiation of dimer-rich samples at 300 K, an as-grown sample with a high concentration of Oi (1.3 1018 cm3) and a very low carbon content (o1015 cm3) has been irradiated. Infrared absorption spectra for this sample are shown in Fig. 5. Weak bands have appeared after irradiation at the same positions as in dimer-rich samples. This result shows that in as-grown carbon-lean Cz–Si samples oxygen dimers are effective traps for self-interstitials at initial stages of electron irradiation.
Fig. 6. DLTS spectra of Cz–Si samples with different carbon concentration after irradiation with gamma-rays from a 60Co source. NC : (1) 5 1016 cm3, (2) o1 1015 cm3. Fluence of irradiation was 5 1016 cm2.
3.5. DLTS results It was found that DLTS spectra of electron- or gamma-rays-irradiated carbon-lean Cz–Si samples differ significantly from those of Cz–Si samples containing carbon. This difference is clearly seen in Fig. 6, where DLTS spectra in the temperature range 60–120 K for Cz–Si samples irradiated with gamma-rays from a 60Co source are presented. The irradiation of a carbon-lean sample resulted in introduction of two dominant deep level centres with peak maxima at about 73 K (E73 ) and 93 K (E93 ) (spectrum 2 in Fig. 6), while for a sample with higher concentration of carbon only one dominant centre (E93 ) is observed (spectrum 1 in Fig. 6). Fig. 7 shows Arrhenius plots of T 2 -corrected emission rates for both dominant centres induced by the irradiation.
Activation energies for electron emission (En ) were found to be 0.16370.001 and 0.12870.001 eV for E93 and E73 traps, respectively. Based on the comparison of the obtained trap parameters (En and sn ) with those known from the literature on radiation-induced centres in Cz–Si [26,27], E93 peak was assigned to the oxygenvacancy pair [3,4]. It was found that the electron capture cross section for the E73 trap is temperature dependent. The energy barrier for the electron capture was estimated to be about 0.02 eV. Taking this into account, the position of an energy level of the E73 trap was determined as Ec 0:11 eV. The obtained parameters of the E73 trap do not coincide with those of any known radiation-induced traps stable at room temperature
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behaviour. The polarisation of the components strongly indicates that the symmetry of centres is monoclinic I or lower. All the obtained experimental results are well in line with preliminary theoretical calculations for the IO2 centre which points to a defect made up of a staggered oxygen dimer and a /1 1 0S-split selfinterstitial [28]. 3.7. Other self-interstitial-oxygen complexes
Fig. 7. Arrhenius plot of T 2 -corrected electron emission rates from (1) E73 and (2) E93 traps.
During irradiation at 650 K there is a band appearing at 1006 cm1 which has been suggested to be related to the oxygen trimer, O3i [29]. This band can be observed in Fig. 4 as well as two bands at 956 and 1054 cm1, appearing together with the bands due to the IO2i and I2O2i. An analysis of the spectra in Fig. 4 shows that the intensity of the 1006 cm1 band increases upon disappearance of the bands at 956 and 1054 cm1. It is suggested that the O3i can also trap self-interstials and the pair of lines at 956 and 1054 cm1 is tentatively assigned to an IO3i complex.
4. Summary
Fig. 8. Isochronal annealing behaviour (1) of the concentration of a centre with an acceptor level at Ec 0:11 eV and (2) of the integrated intensity of the absorption band at 922 cm1.
[26,27]. The effect of electric field on the electron emission rate from the E73 trap is found to be negligible, indicating that the centre is of acceptor nature. An isochronal annealing study shows that the E73 trap anneals out in the temperature range of 100–1501C and the elimination process is very similar to that of the defect responsible for the 922 and 1037 cm1 lines in the absorption spectra (Fig. 8). Based on the similarities in formation and annealing conditions it is suggested that the E73 trap is the same as a centre giving rise to the 922 and 1037 cm1 lines in IR absorption spectra of irradiated carbon-lean Cz–Si samples. 3.6. Uniaxial stress and an atomic model of IO2i complex It was found from uniaxial stress measurements that the 918 and 922 cm1 lines show the same splitting
During electron irradiation at 300 K of oxygen-rich carbon-lean samples a new group of defects has been revealed by IR spectroscopy. These defects are suggested to be formed by reactions between oxygen dimers and silicon self-interstitials. The first step in these reactions is the formation of the IO2i centre with oxygen-related LVMs at 922 and 1037 cm1. The second step is the formation of the I2O2i centre with the corresponding LVMs at 918 and 1034 cm1. A Si-related band at 545 cm1 is common for both the complexes. Annealing studies of these centres show that the IO2i anneals out below 450 K and the I2O2i below 550 K. An acceptor level at Ec 0:11 eV has been identified as related to the IO2i complex on the basis of similarities in the formation and annealing behaviour of the level and the absorption bands at 922 and 1037 cm1. According to preliminary ab initio calculations [28], the obtained experimental characteristics of the IO2i centre are consistent with those expected for a complex incorporating a staggered oxygen dimer and /1 1 0Ssplit silicon self-interstitial.
Acknowledgements We thank TFR, KVA, SI, and FOA in Sweden for financial support. We also acknowledge financial support from the grant INTAS-Belarus 97-824. The authors would like to thank Prof. R. Jones and J. Coutinho for helpful discussions.
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References [1] F. Shimura (Ed.), Oxygen in Silicon, Semiconductors and Semimetals, Vol. 42, Academic Press, London, 1994. [2] R. Jones (Ed.), Early Stages of Oxygen Precipitation in Silicon, High Technology, Vol. 17, NATO ASI Series, 3, Kluwer, Dordrecht, 1996. [3] G.D. Watkins, J.W. Corbett, Phys. Rev. 121 (1961) 1001. [4] J.W. Corbett, et al., Phys. Rev. 121 (1961) 1015. [5] Y.H. Lee, J.W. Corbett, Phys. Rev. B 13 (1976) 2653. [6] J.W. Corbett, G.D. Watkins, R.S. McDonald, Phys. Rev. 135 (1964) A1381. [7] J.L. Lindstrom, B.G. Svenson, Mater. Res. Soc. Symp. Proc. 59 (1986) 45. [8] B.G. Svenson, J.L. Lindstrom, Phys. Rev. B 34 (1986) 59. [9] G. Davies, R.C. Newman, in: T.S. Moss (Ed.), Handbook on Semiconductors, Vol. 3b, Elsevier Science, Amsterdam, 1994, p. 1557pp. . [10] J.L. Lindstrom, et al., Physica B 273–274 (1999) 291. [11] R.E. Whan, F.L. Vook, Phys. Rev. 153 (1967) 814. [12] Brelot, J. Charmagne, in: J.W. Corbett, G.D. Watkins (Eds.), Radiation Effects in Semiconductors, Gordon and Breach, London, 1971, 125pp. [13] H.J. Stein, Appl. Phys. Lett. 55 (1989) 870. [14] Kh.A. Abdulin, B.N. Mukashev, Yu.V. Gorelkinskii, Semicond. Sci. Technol. 11 (1996) 1696.
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[15] L.I. Murin, et al., in: C.L. Claeys (Ed.), Proceedings of the Second ENDEASD Workshop, Kista-Stockholm, 2000, 248pp. [16] J. Hermansson, et al., Physica B 302–303 (2001) 188. [17] P. Deak, L.C. Snyder, J.W. Corbett, Phys. Rev. B 45 (1992) 11612. [18] R.C. Newman, J. Phys.: Condens. Matter 12 (2000) R335. [19] L.I. Murin, et al., Phys. Rev. Lett. 80 (1998) 93. [20] S. Oberg, et al., Phys. Rev. Lett. 81 (1998) 2930. [21] M. Pesola, et al., Phys. Rev. Lett. 82 (1999) 4022. . [22] J.L. Lindstrom, et al., Mater. Sci. Forum 258–263 (1997) 367. . [23] J.L. Lindstrom, et al., Solid State Phenom. 69–70 (1999) 297. [24] Baghdadi, et al., J. Electrochem. Soc. 136 (1989) 2015. [25] T. Hallber, et al., Mater. Sci. Forum 258–263 (1997) 361. [26] L.C. Kimerling, in: Radiation Effects in Semiconductors, Vol. 31 of IOP Series, London-Bristol, 1977, 221pp. [27] S.D. Brotherton, P. Bradley, J. Appl. Phys. 53 (1982) 5720. [28] J. Coutinho, R. Jones, private communication. [29] L.I. Murin, V.P. Markevich, in: R. Jones (Ed.), Early Stages of Oxygen Precipitation in Silicon, High Technology, Vol. 17, Kluwer, Dordrecht, NATO ASI Series, 3, 1996, 329pp.
Interaction between self-interstitials and the oxygen dimer in silicon . a,*, T. Hallbergb, J. Hermanssona, L.I. Murinc, B.A. Komarovc, J.L. Lindstrom V.P. Markevichc,d, M. Klevermana, B.G. Svenssone a
Department of PhysicsFSolid State Physics, University of Lund, PO Box 118, SE-221 00 Lund, Sweden b Defence Research Establishment, PO Box 1165, SE-581 11, Linkoping, Sweden c Institute of Solid State and Semiconductor Physics, 220072 Minsk, Belarus d Centre for Electronic Materials, UMIST, Manchester M60 1QP, UK e Solid State Electronics, Royal Institute of Technology, SE-164 40 Kista, Sweden
Abstract Interactions between the oxygen dimer (O2i) and silicon self-interstitials (I) and vacancies (V) have been studied in Czochralski-grown silicon (Cz–Si) crystals using infrared absorption and deep level transient spectroscopies. The focus in this report is on reactions of O2i with I. The first step in this interaction is found to be the formation of a selfinterstitial-dioxygen centre (IO2i) with oxygen-related local vibrational mode (LVM) bands at 922 and 1037 cm1. During the second formation step, another centre, I2O2i, with LVM bands at 918 and 1034 cm1 is suggested to appear. A Si-related band at about 545 cm1 is also assigned to both the IO2i and I2O2i centres. The IO2i centre is found to be electrically active with an acceptor level at Ec 0:11 eV. The both defects, IO2i and I2O2i, are stable at room temperature and anneal out at about 400 and 550 K, respectively. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Self-interstitials; Oxygen; Electron irradiation
1. Introduction Silicon wafers used for integrated circuits require the presence of oxygen, due to its beneficial effects such as wafer hardening and intrinsic gettering [1,2]. The most common material, Czohralski-grown silicon, 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, trimers (O3i) to oxygen-related thermal double donors (TDDs) and quartz precipitates. *Corresponding author. Tel.: +46-46-2220929; fax: +46-462223637. E-mail address: [email protected] . (J.L. Lindstrom).
It was found that oxygen atoms as well as oxygenrelated complexes could interact with intrinsic defects of Si lattice, vacancies (V) and self-interstitials (I) [1,2]. V and I can be easily created upon irradiation of Si crystals with high energy particles (electrons, protons, ions, etc.). Electron-irradiated Cz–Si was a subject of many investigations [3–10]. Several oxygen-vacancy (Vn On ) complexes have been identified, but very few complexes incorporating oxygen atoms and self-interstitials are known. Experimental studies of samples irradiated at around 80 K have showed that Oi can trap I and form IOi complex [11–14], which is stable below 300 K. This defect has been found to trap an additional I and form I2Oi [15,16]. Silicon self-interstitials were suggested to play an important role in the formation of oxygen-related TDDs [17,18]. The structure of these complexes, which were discovered almost 50 years ago is still a matter of debate.
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 4 - 9
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According to one of the most popular models of TDDs, their core consists of a complex of two oxygen atoms and a silicon self-interstitial (O2iI) [17]. The O2iI was suggested to be a double donor. Recently, local vibrational mode bands due to oxygen dimers were identified in Si [19–21], and a method of enhanced formation of O2i complexes was proposed [22,23]. In this work we present results of infrared (IR) absorption and deep level transient spectroscopy (DLTS) studies of interaction of oxygen dimers with V and I in electron- and gamma-rays-irradiated Cz–Si crystals.
2. Experimental The samples used in this investigation were as-grown Cz–Si, doped with phosphorus to resistivities of about 60 O cm. The concentrations of interstitial oxygen (Oi) and substitutional carbon (Cs ) were monitored by measuring the well-known absorption bands at 1107 [24] and 605 cm1 [9], respectively. We also included samples doped with the isotope 18O. The samples were polished to an optical surface on two sides and the dimensions were 10 5 3 mm3. The electron irradiation was performed in a temperature regulated holder in air at temperatures in the range 320–650 K to different doses from 5 1016 to 4 1018 cm2, using 2.5 MeV electrons. The beam current was in the range 1–12 mA/cm2. Some samples were irradiated with gamma-rays from a 60Co source. Heat-treatments (HTs) were performed in a nitrogen ambient or in air. IR absorption studies of LVMs were carried out using Fourier Transform IR (FT-IR) spectrometers. A spectral resolution of 0.5–1.0 cm1 was used and the samples were measured at 10 K and at RT. DLTS measurements were carried out using a Semitrap DLS83D system. Capture cross sections of electrons by deep traps (sn ) were determined from the analysis of trap filling process upon applying pulses of different lengths. Gold Schottky diodes were prepared for the measurements.
3. Results and discussion 3.1. Formation of the oxygen dimer Since the concentration of oxygen dimers in as-grown Cz–Si crystals is relatively low (around 1 1015 cm3) and IR studies of LVM bands typically require defect concentrations an order of magnitude higher for detailed investigations, a special technique has been developed [22,23], where the dimers are produced by electron irradiation (MeV) at about 650 K in carbon-
285
lean material through the reaction I þ VO2 ) O2i :
ð1Þ
Using this technique we could prepare silicon samples with the dimer concentration up to 5 1016 cm3. It has been found that there is a considerable difference between carbon-lean and carbon-rich CZ-silicon in terms of possible reactions during irradiation [22,23]. The effective interaction of self-interstitials with substitutional carbon atoms according to the Watkins replacement mechanism, Cs þI ) Ci ;
ð2Þ
is the main reason for that. One consequence of the selfinterstitial-carbon interaction is that a high concentration of oxygen dimers according to (1) cannot be achieved in carbon-rich silicon. Another efficient trap for I is the vacancy-oxygen centre (VO) through the reaction I þ VO ) Oi :
ð3Þ
3.2. Irradiation of dimer-rich samples at 300 K In carbon-lean silicon samples irradiated at 650 K the VO concentration after irradiation is much lower than the concentration of O2i and the main traps for I at a repeated irradiation at 300 K will therefore initially be according to I þ Oi ) IOi ;
ð4Þ
I þ IOi ) I2 Oi ;
ð5Þ
I þ O2i ) IO2i ;
ð6Þ
I þ IO2i ) I2 O2i :
ð7Þ
The IOi complex is not stable above 300 K, so reactions (4) and (5) are not effective during the irradiation at 300 K and the main reactions should be (6), (7) and (1). The further irradiation at 300 K will, however, lead to a rapidly increasing concentration of VO centres due to the dominating concentration of Oi and reaction (3) will very soon compete with reactions (6), (7) and (1). One can compare the concentration of O2i (1012 cm1 band) in as-grown samples and after irradiation at 3501C (F ¼ 8 1017 e/cm2) by an examination of spectra 1 and 2 in Fig. 1. The strong increase in [O2i] after the irradiation makes its concentration comparable to that of the VO2 complex (889 cm1 band). The well-known VO-related band at 830 cm1 is not seen in the spectrum 2. Several new bands are appearing in absorption spectra after subsequent irradiation at room temperature (spectra 3–10 in Fig. 1). After irradiation with a fluence of 5 1016 e/cm2, absorption bands at 911 and 1034 cm1 are seen (spectrum 4 in Fig. 1), further irradiation leads to the development of bands at 916
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Fig. 1. Room temperature absorption spectra for C-lean n-Cz– Si (r ¼ 50 O cm): (1) as-grown; (2) after electron irradiation at 3501C, F=8 1017 cm2; (3–10) after RT irradiation. F(cm2): (3) 1 1016, (4) 5 1016, (5) 1017, (6) 2 1017, (7) 4 1017, (8) 7 1017, (9) 1.1 1018, (10) 6 1018.
Fig. 2. Generation kinetics of some radiation-induced oxygenrelated complexes upon electron irradiation at room temperature of a dimer-rich Cz–Si sample.
Fig. 3. Absorption spectra measured at 10 and 300 K for a Clean Cz–Si sample after two-step electron irradiation: at 3501C with a fluence of 8 1017+at RT with a fluence of 7 1017 cm2.
into account differences in capture cross sections of selfinterstitials by VO, VO2, O2i, and IO2i complexes. An analysis of absorption spectra of irradiated oxygen-dimer-rich samples doped with 18O reveals that the pairs of bands at 911 and 1034 cm1, and at 916 and 1031 cm1 are oxygen-related LVMs, while the weak band at 540 cm1 does not show any shift upon replacement of oxygen isotopes and therefore is suggested to be a Si-related LVM. In Fig. 3 one can see the temperature shifts of the bands upon the change of measurement temperature from 300 to 10 K. The band at 911 cm–1 shifts to 922 cm1, while the band at 916 cm1 shifts to 918 cm1 with the decrease of the measurement temperature. Typical shifts of oxygenrelated LVMs in this wavenumber range is about 5– 6 cm1. Exceptions are the LVMs at 1012 and 1060 cm1 due to the oxygen dimer, the temperature shifts of which are extremely small (Fig. 3) [25]. 3.3. Annealing studies
and 1031 cm1 (spectra 6–10 in Fig. 1). It is suggested that the first pair of bands appears when IO2i starts to form according to reaction (6) and after that reaction (7) can be activated resulting in the formation of I2O2i. There is also a weak band at 540 cm1, the development of which seems to correlate with that of the above mentioned pairs of bands. As the irradiation fluence increases the concentration of the vacancy-oxygen complex grows rapidly and the VO-related line at 830 cm1 dominates in the spectra soon (spectra 6–10 in Fig. 1). This is also illustrated in Fig. 2, where the dose dependencies of the integrated intensities of the considered bands are shown. The observed dependencies can be described by reactions (1), (3), (6) and (7) taking
In order to investigate the thermal stability of the new defects (IO2i and I2O2i ), an isochronal annealing study has been done. Absorption spectra of an electronirradiated sample after isochronal anneals are shown in Fig. 4. An examination of the spectra shows that the first appearing defect, IO2i, has completely annealed out at 1601C and the second one, I2O2i, at 2501C. Disappearance of the IO2i and I2O2i complexes is accompanied by an increase in intensities of the bands at 1012 and 1060 cm1 due to the oxygen dimer, suggesting that reactions (6) and (7) have corresponding back-reactions. Self-interstitials released according to these back-reactions can take part in reactions like (1) and (3). Several other relatively weak bands have been observed upon
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Fig. 4. Absorption spectra at 10 K for Cz–Si: (1) after two-step electron irradiation (3501C, F=8 1017+RT, F= 7 1017 cm2) and at different stages of 20 min isochronal anneal, (2) 1201C, (3) 1601C, (4) 2101C, (5) 2501C, (6) 3001C, (7) 3501C.
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Fig. 5. Absorption spectra at 10 K for electron irradiated (RT, F ¼ 5 1016 cm2) as-grown Cz–Si crystals with low (o1015 cm3, spectra 2 and 3) and intermediate carbon content (5 1016 cm3, spectrum 1). Spectrum 3 is a difference between the spectra of irradiated and as-grown samples.
annealing of the irradiated dimer-rich samples and an investigation of these bands is in progress. After anneals at higher temperatures the band at 895 cm1 due to the VO2 complex appears in the spectra (spectra 6 and 7 in Fig. 4). 3.4. Irradiation of as-grown materials Simultaneously, with the irradiation of dimer-rich samples at 300 K, an as-grown sample with a high concentration of Oi (1.3 1018 cm3) and a very low carbon content (o1015 cm3) has been irradiated. Infrared absorption spectra for this sample are shown in Fig. 5. Weak bands have appeared after irradiation at the same positions as in dimer-rich samples. This result shows that in as-grown carbon-lean Cz–Si samples oxygen dimers are effective traps for self-interstitials at initial stages of electron irradiation.
Fig. 6. DLTS spectra of Cz–Si samples with different carbon concentration after irradiation with gamma-rays from a 60Co source. NC : (1) 5 1016 cm3, (2) o1 1015 cm3. Fluence of irradiation was 5 1016 cm2.
3.5. DLTS results It was found that DLTS spectra of electron- or gamma-rays-irradiated carbon-lean Cz–Si samples differ significantly from those of Cz–Si samples containing carbon. This difference is clearly seen in Fig. 6, where DLTS spectra in the temperature range 60–120 K for Cz–Si samples irradiated with gamma-rays from a 60Co source are presented. The irradiation of a carbon-lean sample resulted in introduction of two dominant deep level centres with peak maxima at about 73 K (E73 ) and 93 K (E93 ) (spectrum 2 in Fig. 6), while for a sample with higher concentration of carbon only one dominant centre (E93 ) is observed (spectrum 1 in Fig. 6). Fig. 7 shows Arrhenius plots of T 2 -corrected emission rates for both dominant centres induced by the irradiation.
Activation energies for electron emission (En ) were found to be 0.16370.001 and 0.12870.001 eV for E93 and E73 traps, respectively. Based on the comparison of the obtained trap parameters (En and sn ) with those known from the literature on radiation-induced centres in Cz–Si [26,27], E93 peak was assigned to the oxygenvacancy pair [3,4]. It was found that the electron capture cross section for the E73 trap is temperature dependent. The energy barrier for the electron capture was estimated to be about 0.02 eV. Taking this into account, the position of an energy level of the E73 trap was determined as Ec 0:11 eV. The obtained parameters of the E73 trap do not coincide with those of any known radiation-induced traps stable at room temperature
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behaviour. The polarisation of the components strongly indicates that the symmetry of centres is monoclinic I or lower. All the obtained experimental results are well in line with preliminary theoretical calculations for the IO2 centre which points to a defect made up of a staggered oxygen dimer and a /1 1 0S-split selfinterstitial [28]. 3.7. Other self-interstitial-oxygen complexes
Fig. 7. Arrhenius plot of T 2 -corrected electron emission rates from (1) E73 and (2) E93 traps.
During irradiation at 650 K there is a band appearing at 1006 cm1 which has been suggested to be related to the oxygen trimer, O3i [29]. This band can be observed in Fig. 4 as well as two bands at 956 and 1054 cm1, appearing together with the bands due to the IO2i and I2O2i. An analysis of the spectra in Fig. 4 shows that the intensity of the 1006 cm1 band increases upon disappearance of the bands at 956 and 1054 cm1. It is suggested that the O3i can also trap self-interstials and the pair of lines at 956 and 1054 cm1 is tentatively assigned to an IO3i complex.
4. Summary
Fig. 8. Isochronal annealing behaviour (1) of the concentration of a centre with an acceptor level at Ec 0:11 eV and (2) of the integrated intensity of the absorption band at 922 cm1.
[26,27]. The effect of electric field on the electron emission rate from the E73 trap is found to be negligible, indicating that the centre is of acceptor nature. An isochronal annealing study shows that the E73 trap anneals out in the temperature range of 100–1501C and the elimination process is very similar to that of the defect responsible for the 922 and 1037 cm1 lines in the absorption spectra (Fig. 8). Based on the similarities in formation and annealing conditions it is suggested that the E73 trap is the same as a centre giving rise to the 922 and 1037 cm1 lines in IR absorption spectra of irradiated carbon-lean Cz–Si samples. 3.6. Uniaxial stress and an atomic model of IO2i complex It was found from uniaxial stress measurements that the 918 and 922 cm1 lines show the same splitting
During electron irradiation at 300 K of oxygen-rich carbon-lean samples a new group of defects has been revealed by IR spectroscopy. These defects are suggested to be formed by reactions between oxygen dimers and silicon self-interstitials. The first step in these reactions is the formation of the IO2i centre with oxygen-related LVMs at 922 and 1037 cm1. The second step is the formation of the I2O2i centre with the corresponding LVMs at 918 and 1034 cm1. A Si-related band at 545 cm1 is common for both the complexes. Annealing studies of these centres show that the IO2i anneals out below 450 K and the I2O2i below 550 K. An acceptor level at Ec 0:11 eV has been identified as related to the IO2i complex on the basis of similarities in the formation and annealing behaviour of the level and the absorption bands at 922 and 1037 cm1. According to preliminary ab initio calculations [28], the obtained experimental characteristics of the IO2i centre are consistent with those expected for a complex incorporating a staggered oxygen dimer and /1 1 0Ssplit silicon self-interstitial.
Acknowledgements We thank TFR, KVA, SI, and FOA in Sweden for financial support. We also acknowledge financial support from the grant INTAS-Belarus 97-824. The authors would like to thank Prof. R. Jones and J. Coutinho for helpful discussions.
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