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Infrared absorption study of a new dicarbon center in silicon
Physica B 273}274 (1999) 256 } 259
Infrared absorption study of a new dicarbon center in silicon E.V. Lavrov!,", B. Bech Nielsen",*, J. Byberg#, J.L. LindstroK m$ !Institute of Radioengineering and Electronics of RAS, Mokhovaya 11, 103907 Moscow, Russia "Institute of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus, Denmark #Institute of Chemistry, University of Aarhus, DK-8000 Aarhus, Denmark $Linko( ping University, S-581 83, Linko( ping, Sweden
Abstract Infrared absorption measurements on n-type silicon doped with carbon and irradiated with electrons at room temperature have revealed new absorption lines at 527.4 and 748.7 cm~1. The 748.7 cm~1 line is observed only when the sample is cooled down in the dark and the spectra are measured through a low-pass "lter with cut-o! frequency below 6000 cm~1. Light of frequency above 6000 cm~1 removes this line and generates the 527.4-cm~1 line. Spectra recorded on silicon doped with 13C show that the two lines represent local vibrational modes of a carbon defect. The annealing behavior of the 748.7-cm~1 line and of the EPR signal of two neighboring substitutional carbon atoms, (C }C )~, are 4 4 identical. The 527.4- and 748.7-cm~1 modes are identi"ed as the modes of C }C in neutral and negative charge states, 4 4 respectively. The formation of C }C is investigated, and it is shown that the center may arise when a vacancy is trapped 4 4 by the metastable substitutional carbon-interstitial carbon center, C }C . ( 1999 Elsevier Science B.V. All rights 4 * reserved. Keywords: Silicon; Carbon; Infrared absorption
1. Introduction In crystalline silicon, carbon atoms are common and important impurities, which are present mainly as substitutional carbon, C [1]. Interstitial carbon, C , is pro4 * duced by electron irradiation when mobile silicon interstitials become trapped by C [1}3]. At room tem4 perature, C migrates through the lattice and becomes * trapped at C , whereby a dicarbon center, C }C , is 4 4 * formed [4]. A recent EPR study of carbon-doped silicon, Si : C, showed that C }C is not the only dicarbon center in 4 * silicon. After electron irradiation of n-type Si : C at room temperature a new paramagnetic center was observed and identi"ed as a dicarbon center comprising two neighboring subsitutional carbon atoms, C }C [5]. 4 4 While an EPR experiment probes the electronic wave function, information about the identity of light impurity * Corresponding author. Tel.: #45-8942-3716; fax: #458612-0740 . E-mail address: bbn@d".aau.dk (B. Bech Nielsen)
atoms can be obtained from the frequencies of their local vibrational modes (LVMs). Moreover, the LVMs serve to characterize the bonding of these atoms. In the present work the LVMs of C }C are investigated by means of 4 4 infrared absorption spectroscopy. Unlike EPR, this technique allows a study of both the negative and the neutral charge states of C }C . Moreover, the mechanism of 4 4 formation of C }C is addressed: From the electron dose 4 4 dependence of the LVM intensities of C , C , C }C , and 4 * 4 * C }C it is shown that C }C may arise when vacancies 4 4 4 4 become trapped by C }C . 4 * 2. Experimental Samples with dimension &10]7]2 mm3 were cut from three di!erent #oat-zone (FZ) silicon crystals doped either with 12C(Si : 12C) or predominantly with 13C (Si : 13C). The samples were mechanically polished on the two opposite 10]7 mm2 faces to ensure maximum transmission of infrared light. One Si : 12C crystal was n-type and contained 4]1017 cm~3 12C, 1]1016 cm~3
0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 4 6 9 - X
E.V. Lavrov et al. / Physica B 273}274 (1999) 256}259
oxygen, 5]1016 cm~3 phosphorus, and &1]1018 cm~3 tin. A second Si : 12C crystal was nearly intrinsic (high-resistivity n-type) and contained 4.5]1017 cm~3 12C and 1]1016 cm~3 oxygen. The Si : 13C crystal was n-type and contained 8]1017 cm~3 13C, 1]1017 cm~3 12C, 1]1017 cm~3 oxygen, and 4]1014 cm~3 phosphorus. The samples were irradiated with 2 MeV electrons supplied by a 5 MeV Escher Holland van de Graa! accelerator. The total irradiation dose varied from 5]1017 to 6]1018 cm~2. The irradiation was carried out either at room temperature or below !203C depending on the purpose of the experiment. In order to study the annealing characteristics of the absorption lines, infrared absorbance spectra were recorded at 10 K after each step in a series of isochronal heat treatments at temperatures in the range from 3003C to 5503C. Each heat treatment had a duration of 40 min. and was carried out in a nitrogen atmosphere. The infrared absorbance spectra were recorded with a Nicolet, System 800, Fourier-transform spectrometer equipped with a Ge-on-KBr beamsplitter, a glowbar as light source, and a mercury}cadmium}telluride detector. The spectra were recorded at 10 K with an apodized resolution of 2 cm~1. Low-pass "lters could be inserted between the glowbar and the sample. Spectra recorded with "lters were obtained after cooling the sample in the dark from *200 K to the temperature of measurement. A reference spectrum, recorded on pure silicon with a low carbon content, was subtracted from all spectra, unless otherwise stated. Moreover, the spectra shown in Figs. 1 and 3 were baseline corrected for presentation purposes, which could be done without introducing or removing any sharp absorption features. In order to establish a correlation between the EPR signal of (C }C )~ and the observed LVMs, EPR and 4 4 infrared absorption measurements were performed on the same sample. The EPR spectra were recorded at
Fig. 1. Sections of absorbance spectra of electron-irradiated (&1]1018 cm~2) Si : 12C recorded at 10 K: (a) through a lowpass "lter with cut-o! frequency at 3000 cm~1 and (b) without "lter. The absorption line denoted by ] is not related to the centers discussed in this work.
257
room temperature with a Bruker ESP300E spectrometer operated in the absorption mode at &9.5 GHz (X band).
3. Results and discussion 3.1. Basic properties of absorption lines Absorbance spectra of an n-type Si : 12C crystal irradiated with 2 MeV electrons at room temperature are shown in Fig. 1. The spectrum (a) was recorded with a low-pass "lter with cut-o! frequency at 3000 cm~1, after cooling the sample to 10 K in the dark. Then the "lter was removed, and the spectrum (b) was obtained. The intense absorption line at 607 cm~1 associated with the LVM of 12C dominates in the spectra. Much weaker 4 absorption lines at 540.4, 543.3, 579.8, 640.6, and 730.4 cm~1 are also seen. Recently, these were shown to represent the LVMs of C }C (see Ref. [6]). In addition, 4 * several other absorption lines appear in the spectra, among which those observed at 527.4 and 748.7 cm~1 are discussed in this work. They lie in the region characteristic of LVMs of carbon defects in silicon. The 748.7cm~1 line is observed only when the sample is cooled in the dark and the spectrum is recorded with a low-pass "lter with cut-o! frequency below 6000 cm~1. Illumination with light of frequency above 6000 cm~1 swiftly removes the 748.7-cm~1 line and generates the line at 527.4 cm~1, as can be seen from comparison of spectra (a) and (b) in the "gure. This process is reversible: After heating to 200 K and subsequent cooling in the dark, the line at 748.7 cm~1 is restored, whereas the 527.4-cm~1 line disappears from the spectra. The 527.4- and 748.7cm~1 lines display perfect anticorrelation, which implies that the same process controls the light-induced removal of the 748.7-cm~1 line and appearance of the 527.4-cm~1 line. At the same time, the annealing behaviors of these two lines are identical. These facts strongly indicate that the lines at 527.4 cm~1 and 748.7 cm~1 originate from two di!erent charge states of the same defect. Moreover, since the 527.4-cm~1 line can be observed in both n-type and intrinsic material, whereas the 748.7-cm~1 line is observed only in n-type silicon we may expect the 527.4cm~1 line to originate from a more positive charge state than the 748.7-cm~1 line. Measurements on the Si : 13C sample showed that the 748.7-cm~1 line shifts down to 726.3 cm~1. The frequency ratio between the two absorption lines is 1.031, which is very close to the value expected for a carbon atom bound to a silicon atom by a harmonic spring. Therefore, we ascribe the 748.7-cm~1 line to a LVM of a carbon defect presumably containing Si}C bonds. No line corresponding to the 527.4-cm~1 line was observed in the Si : 13C sample. However, with the same frequency ratio as found above (1.031), this line should shift down to &512 cm~1 and thus fall below the Raman
258
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frequency 524 cm~1 in silicon [7]. Such modes normally couple strongly to the lattice phonons and cannot be detected. Hence, we may also ascribe the 527.4-cm~1 line to a LVM of carbon.
and intrinsic material, and no EPR signal correlates with it. Therefore, we identify this line as a LVM of (C }C )0. 4 4
3.2. Assignment of absorption lines
Electron irradiation of virgin Si : C at low temperatures does not create any signals related to C }C . On the 4 4 other hand, room-temperature irradiation is the most e!ective way to produce the center. Among the identi"ed carbon-related centers, only C is mobile at room temper* ature, but as shown previously, C forms C }C rather * 4 * than C }C (see Ref. [4]). We propose that C }C is 4 4 4 * a precursor for C }C and that the transformation of 4 4 C }C into C }C may occur when C }C traps a vacancy 4 * 4 4 4 * produced by further electron irradiation. Accordingly, the proposed reaction scheme leading to formation of C }C is 4 4 C #Si PC : Low temperatures, 4 * * C #C PC }C : Room temperature, 4 * 4 * C }C #VPC }C : Above &K, 4 * 4 4 where Si denotes a silicon self-interstitial. * In Fig. 3, the experimental evidence for this reaction scheme is presented. As shown in part (a) of the "gure, the dominant center produced by electron irradiation of virgin Si : C below room temperature is C , which has two * infrared active LVMs at 922 and 932 cm~1. When the sample is heated to room temperature the intensities of these LVMs decrease as a function of time, and after 210 min they can no longer be detected, see part (b) of the "gure. The concurrent formation of C }C is demon4 * strated by the appearance of the LVMs of this complex at
So far, we have established that the center responsible for the 527.4 and 748.7-cm~1 lines is carbon related. Neither the number of carbon atoms involved nor their sites have been determined. The EPR signal arising from the negative charge state of C }C qualitatively displays 4 4 the same sensitivity to light as discussed here for the 748.7-cm~1 line [5], which suggests that it may originate from (C }C )~. To investigate this further, an n-type 4 4 Si : 12C sample was "rst irradiated below !203C with 2 MeV electrons to a dose of 5]1017 cm~2, in order to compensate the free carriers without creating much C }C (see Section 3.3). This compensation was required 4 4 to ensure reliable intensities of the (C }C )~ EPR signal 4 4 at room temperature. Subsequently, the sample was electron irradiated at room temperature to a dose of 1.5]1017 cm~2 and the EPR as well as the infrared absorption spectra were recorded. Then the room temperature irradiation was repeated and the spectra remeasured until the total dose reached 9]1017 cm~2. The results of this series of measurements are presented in Fig. 2, in which the intensity of the EPR signal from (C }C )~ is plotted against the intensity of the 748.74 4 cm~1 line. As can be seen from the "gure, the two data sets display a linear correlation, showing that the 748.7cm~1 line originates from (C }C )~. Thus, from the re4 4 sults described in the previous subsection we may conclude that the 748.7-cm~1 line represents a LVM of (C }C )~. The 527.4-cm~1 line is observed both in n-type 4 4
Fig. 2. The intensity (integrated absorbance) of the 748.7-cm~1 line shown against the intensity of the EPR signal associated with (C }C )~. The solid line represents the least-square "t to 4 4 the data.
3.3. Formation of C }C s s
Fig. 3. Sections of absorbance spectra of Si : 12C recorded at 10 K: (a) just after electron irradiation below room temperature to a dose of 8]1017 cm~2, (b) after subsequent room-temperature heat treatment for 210 min. The spectrum (c) is obtained by subtraction of spectrum (b) from a spectrum recorded after a second electron irradiation (6]1017 cm~2) below room temperature. The absorption line denoted by ] is not related to C }C . 4 4
E.V. Lavrov et al. / Physica B 273}274 (1999) 256}259
540.4, 543.3, 579.8, 640.6, 730.4, and 842.4 cm~1 (see Ref. [6]). No C }C is formed at this stage. However, if the 4 4 sample containing C }C is reirradiated with electrons 4 * below room temperature, but above &200 K, the 527.4 cm~1 LVM of (C }C )0 grows in and the LVMs of C }C 4 4 4 * decrease in intensity, as seen clearly from Fig. 3(c), in which the di!erence between the spectra recorded just after and before the second irradiation is shown. The irradiation procedure described above was repeated several times on the same sample. For accumulated doses below &2]1018 cm~2, we "nd that the increase of the intensity of the (C }C )0 line at 527.4-cm~1 is 4 4 a linear function of the decrease of the intensities of the C }C lines, thus supporting the mechanism proposed 4 * above. When the accumulated dose exceeds 2]1018 cm~2, the increase of the C }C lines falls below the 4 4 linear relationship, and the intensity of the 527.4-cm~1 line saturates at &5]1018 cm~2. We take this to indicate that the accumulated radiation damage gives rise to additional processes that interfere with the simple reaction scheme proposed above.
4. Conclusions Electron-irradiated n-type silicon doped with carbon has been studied by IR absorption spectroscopy. Absorption lines at 527.4 and 748.7 cm~1 have been identi"ed as LVMs of (C }C )0 and (C }C )~, respectively. 4 4 4 4
259
The formation mechanism of the center has been investigated and it is proposed that C }C is formed by the 4 4 process C }C #VPC }C . 4 * 4 4 Acknowledgements We thank Pia Bomholt for preparing the samples for optical measurements. This work was supported by the Danish National Research Foundation through the Aarhus Center for Atomic Physics. E.V. Lavrov also acknowledges a grant from the Russian Foundation for Basic Research (grant No. 99-02-16652).
References [1] G. Davies, R.C. Newman, in: T.S. Moss (Ed.), Handbook on Semiconductors, Vol. 3b, Elsevier Science, Amsterdam, 1994, p. 1557 and references therein. [2] A.R. Bean, R.C. Newman, Solid State Commun. 8 (1970) 175. [3] G.D. Watkins, K.L. Brower, Phys. Rev. Lett. 36 (1976) 1329. [4] G.D. Watkins, in: M. Hulin (Ed.), Radiation E!ects in Semiconductors, Dunod, Paris, 1965. [5] J.R. Byberg, B. Bech Nielsen, M. Fancuilli, S.K. Estreicher, P.A. Fedders, Phys. Rev. B, submitted for publication. [6] E.V. Lavrov, L. Ho!mann, B. Bech Nielsen, Phys. Rev. B, in press. [7] J. Menendez, M. Cardona, Phys. Rev. B 29 (1984) 2051.
Infrared absorption study of a new dicarbon center in silicon E.V. Lavrov!,", B. Bech Nielsen",*, J. Byberg#, J.L. LindstroK m$ !Institute of Radioengineering and Electronics of RAS, Mokhovaya 11, 103907 Moscow, Russia "Institute of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus, Denmark #Institute of Chemistry, University of Aarhus, DK-8000 Aarhus, Denmark $Linko( ping University, S-581 83, Linko( ping, Sweden
Abstract Infrared absorption measurements on n-type silicon doped with carbon and irradiated with electrons at room temperature have revealed new absorption lines at 527.4 and 748.7 cm~1. The 748.7 cm~1 line is observed only when the sample is cooled down in the dark and the spectra are measured through a low-pass "lter with cut-o! frequency below 6000 cm~1. Light of frequency above 6000 cm~1 removes this line and generates the 527.4-cm~1 line. Spectra recorded on silicon doped with 13C show that the two lines represent local vibrational modes of a carbon defect. The annealing behavior of the 748.7-cm~1 line and of the EPR signal of two neighboring substitutional carbon atoms, (C }C )~, are 4 4 identical. The 527.4- and 748.7-cm~1 modes are identi"ed as the modes of C }C in neutral and negative charge states, 4 4 respectively. The formation of C }C is investigated, and it is shown that the center may arise when a vacancy is trapped 4 4 by the metastable substitutional carbon-interstitial carbon center, C }C . ( 1999 Elsevier Science B.V. All rights 4 * reserved. Keywords: Silicon; Carbon; Infrared absorption
1. Introduction In crystalline silicon, carbon atoms are common and important impurities, which are present mainly as substitutional carbon, C [1]. Interstitial carbon, C , is pro4 * duced by electron irradiation when mobile silicon interstitials become trapped by C [1}3]. At room tem4 perature, C migrates through the lattice and becomes * trapped at C , whereby a dicarbon center, C }C , is 4 4 * formed [4]. A recent EPR study of carbon-doped silicon, Si : C, showed that C }C is not the only dicarbon center in 4 * silicon. After electron irradiation of n-type Si : C at room temperature a new paramagnetic center was observed and identi"ed as a dicarbon center comprising two neighboring subsitutional carbon atoms, C }C [5]. 4 4 While an EPR experiment probes the electronic wave function, information about the identity of light impurity * Corresponding author. Tel.: #45-8942-3716; fax: #458612-0740 . E-mail address: bbn@d".aau.dk (B. Bech Nielsen)
atoms can be obtained from the frequencies of their local vibrational modes (LVMs). Moreover, the LVMs serve to characterize the bonding of these atoms. In the present work the LVMs of C }C are investigated by means of 4 4 infrared absorption spectroscopy. Unlike EPR, this technique allows a study of both the negative and the neutral charge states of C }C . Moreover, the mechanism of 4 4 formation of C }C is addressed: From the electron dose 4 4 dependence of the LVM intensities of C , C , C }C , and 4 * 4 * C }C it is shown that C }C may arise when vacancies 4 4 4 4 become trapped by C }C . 4 * 2. Experimental Samples with dimension &10]7]2 mm3 were cut from three di!erent #oat-zone (FZ) silicon crystals doped either with 12C(Si : 12C) or predominantly with 13C (Si : 13C). The samples were mechanically polished on the two opposite 10]7 mm2 faces to ensure maximum transmission of infrared light. One Si : 12C crystal was n-type and contained 4]1017 cm~3 12C, 1]1016 cm~3
0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 4 6 9 - X
E.V. Lavrov et al. / Physica B 273}274 (1999) 256}259
oxygen, 5]1016 cm~3 phosphorus, and &1]1018 cm~3 tin. A second Si : 12C crystal was nearly intrinsic (high-resistivity n-type) and contained 4.5]1017 cm~3 12C and 1]1016 cm~3 oxygen. The Si : 13C crystal was n-type and contained 8]1017 cm~3 13C, 1]1017 cm~3 12C, 1]1017 cm~3 oxygen, and 4]1014 cm~3 phosphorus. The samples were irradiated with 2 MeV electrons supplied by a 5 MeV Escher Holland van de Graa! accelerator. The total irradiation dose varied from 5]1017 to 6]1018 cm~2. The irradiation was carried out either at room temperature or below !203C depending on the purpose of the experiment. In order to study the annealing characteristics of the absorption lines, infrared absorbance spectra were recorded at 10 K after each step in a series of isochronal heat treatments at temperatures in the range from 3003C to 5503C. Each heat treatment had a duration of 40 min. and was carried out in a nitrogen atmosphere. The infrared absorbance spectra were recorded with a Nicolet, System 800, Fourier-transform spectrometer equipped with a Ge-on-KBr beamsplitter, a glowbar as light source, and a mercury}cadmium}telluride detector. The spectra were recorded at 10 K with an apodized resolution of 2 cm~1. Low-pass "lters could be inserted between the glowbar and the sample. Spectra recorded with "lters were obtained after cooling the sample in the dark from *200 K to the temperature of measurement. A reference spectrum, recorded on pure silicon with a low carbon content, was subtracted from all spectra, unless otherwise stated. Moreover, the spectra shown in Figs. 1 and 3 were baseline corrected for presentation purposes, which could be done without introducing or removing any sharp absorption features. In order to establish a correlation between the EPR signal of (C }C )~ and the observed LVMs, EPR and 4 4 infrared absorption measurements were performed on the same sample. The EPR spectra were recorded at
Fig. 1. Sections of absorbance spectra of electron-irradiated (&1]1018 cm~2) Si : 12C recorded at 10 K: (a) through a lowpass "lter with cut-o! frequency at 3000 cm~1 and (b) without "lter. The absorption line denoted by ] is not related to the centers discussed in this work.
257
room temperature with a Bruker ESP300E spectrometer operated in the absorption mode at &9.5 GHz (X band).
3. Results and discussion 3.1. Basic properties of absorption lines Absorbance spectra of an n-type Si : 12C crystal irradiated with 2 MeV electrons at room temperature are shown in Fig. 1. The spectrum (a) was recorded with a low-pass "lter with cut-o! frequency at 3000 cm~1, after cooling the sample to 10 K in the dark. Then the "lter was removed, and the spectrum (b) was obtained. The intense absorption line at 607 cm~1 associated with the LVM of 12C dominates in the spectra. Much weaker 4 absorption lines at 540.4, 543.3, 579.8, 640.6, and 730.4 cm~1 are also seen. Recently, these were shown to represent the LVMs of C }C (see Ref. [6]). In addition, 4 * several other absorption lines appear in the spectra, among which those observed at 527.4 and 748.7 cm~1 are discussed in this work. They lie in the region characteristic of LVMs of carbon defects in silicon. The 748.7cm~1 line is observed only when the sample is cooled in the dark and the spectrum is recorded with a low-pass "lter with cut-o! frequency below 6000 cm~1. Illumination with light of frequency above 6000 cm~1 swiftly removes the 748.7-cm~1 line and generates the line at 527.4 cm~1, as can be seen from comparison of spectra (a) and (b) in the "gure. This process is reversible: After heating to 200 K and subsequent cooling in the dark, the line at 748.7 cm~1 is restored, whereas the 527.4-cm~1 line disappears from the spectra. The 527.4- and 748.7cm~1 lines display perfect anticorrelation, which implies that the same process controls the light-induced removal of the 748.7-cm~1 line and appearance of the 527.4-cm~1 line. At the same time, the annealing behaviors of these two lines are identical. These facts strongly indicate that the lines at 527.4 cm~1 and 748.7 cm~1 originate from two di!erent charge states of the same defect. Moreover, since the 527.4-cm~1 line can be observed in both n-type and intrinsic material, whereas the 748.7-cm~1 line is observed only in n-type silicon we may expect the 527.4cm~1 line to originate from a more positive charge state than the 748.7-cm~1 line. Measurements on the Si : 13C sample showed that the 748.7-cm~1 line shifts down to 726.3 cm~1. The frequency ratio between the two absorption lines is 1.031, which is very close to the value expected for a carbon atom bound to a silicon atom by a harmonic spring. Therefore, we ascribe the 748.7-cm~1 line to a LVM of a carbon defect presumably containing Si}C bonds. No line corresponding to the 527.4-cm~1 line was observed in the Si : 13C sample. However, with the same frequency ratio as found above (1.031), this line should shift down to &512 cm~1 and thus fall below the Raman
258
E.V. Lavrov et al. / Physica B 273}274 (1999) 256}259
frequency 524 cm~1 in silicon [7]. Such modes normally couple strongly to the lattice phonons and cannot be detected. Hence, we may also ascribe the 527.4-cm~1 line to a LVM of carbon.
and intrinsic material, and no EPR signal correlates with it. Therefore, we identify this line as a LVM of (C }C )0. 4 4
3.2. Assignment of absorption lines
Electron irradiation of virgin Si : C at low temperatures does not create any signals related to C }C . On the 4 4 other hand, room-temperature irradiation is the most e!ective way to produce the center. Among the identi"ed carbon-related centers, only C is mobile at room temper* ature, but as shown previously, C forms C }C rather * 4 * than C }C (see Ref. [4]). We propose that C }C is 4 4 4 * a precursor for C }C and that the transformation of 4 4 C }C into C }C may occur when C }C traps a vacancy 4 * 4 4 4 * produced by further electron irradiation. Accordingly, the proposed reaction scheme leading to formation of C }C is 4 4 C #Si PC : Low temperatures, 4 * * C #C PC }C : Room temperature, 4 * 4 * C }C #VPC }C : Above &K, 4 * 4 4 where Si denotes a silicon self-interstitial. * In Fig. 3, the experimental evidence for this reaction scheme is presented. As shown in part (a) of the "gure, the dominant center produced by electron irradiation of virgin Si : C below room temperature is C , which has two * infrared active LVMs at 922 and 932 cm~1. When the sample is heated to room temperature the intensities of these LVMs decrease as a function of time, and after 210 min they can no longer be detected, see part (b) of the "gure. The concurrent formation of C }C is demon4 * strated by the appearance of the LVMs of this complex at
So far, we have established that the center responsible for the 527.4 and 748.7-cm~1 lines is carbon related. Neither the number of carbon atoms involved nor their sites have been determined. The EPR signal arising from the negative charge state of C }C qualitatively displays 4 4 the same sensitivity to light as discussed here for the 748.7-cm~1 line [5], which suggests that it may originate from (C }C )~. To investigate this further, an n-type 4 4 Si : 12C sample was "rst irradiated below !203C with 2 MeV electrons to a dose of 5]1017 cm~2, in order to compensate the free carriers without creating much C }C (see Section 3.3). This compensation was required 4 4 to ensure reliable intensities of the (C }C )~ EPR signal 4 4 at room temperature. Subsequently, the sample was electron irradiated at room temperature to a dose of 1.5]1017 cm~2 and the EPR as well as the infrared absorption spectra were recorded. Then the room temperature irradiation was repeated and the spectra remeasured until the total dose reached 9]1017 cm~2. The results of this series of measurements are presented in Fig. 2, in which the intensity of the EPR signal from (C }C )~ is plotted against the intensity of the 748.74 4 cm~1 line. As can be seen from the "gure, the two data sets display a linear correlation, showing that the 748.7cm~1 line originates from (C }C )~. Thus, from the re4 4 sults described in the previous subsection we may conclude that the 748.7-cm~1 line represents a LVM of (C }C )~. The 527.4-cm~1 line is observed both in n-type 4 4
Fig. 2. The intensity (integrated absorbance) of the 748.7-cm~1 line shown against the intensity of the EPR signal associated with (C }C )~. The solid line represents the least-square "t to 4 4 the data.
3.3. Formation of C }C s s
Fig. 3. Sections of absorbance spectra of Si : 12C recorded at 10 K: (a) just after electron irradiation below room temperature to a dose of 8]1017 cm~2, (b) after subsequent room-temperature heat treatment for 210 min. The spectrum (c) is obtained by subtraction of spectrum (b) from a spectrum recorded after a second electron irradiation (6]1017 cm~2) below room temperature. The absorption line denoted by ] is not related to C }C . 4 4
E.V. Lavrov et al. / Physica B 273}274 (1999) 256}259
540.4, 543.3, 579.8, 640.6, 730.4, and 842.4 cm~1 (see Ref. [6]). No C }C is formed at this stage. However, if the 4 4 sample containing C }C is reirradiated with electrons 4 * below room temperature, but above &200 K, the 527.4 cm~1 LVM of (C }C )0 grows in and the LVMs of C }C 4 4 4 * decrease in intensity, as seen clearly from Fig. 3(c), in which the di!erence between the spectra recorded just after and before the second irradiation is shown. The irradiation procedure described above was repeated several times on the same sample. For accumulated doses below &2]1018 cm~2, we "nd that the increase of the intensity of the (C }C )0 line at 527.4-cm~1 is 4 4 a linear function of the decrease of the intensities of the C }C lines, thus supporting the mechanism proposed 4 * above. When the accumulated dose exceeds 2]1018 cm~2, the increase of the C }C lines falls below the 4 4 linear relationship, and the intensity of the 527.4-cm~1 line saturates at &5]1018 cm~2. We take this to indicate that the accumulated radiation damage gives rise to additional processes that interfere with the simple reaction scheme proposed above.
4. Conclusions Electron-irradiated n-type silicon doped with carbon has been studied by IR absorption spectroscopy. Absorption lines at 527.4 and 748.7 cm~1 have been identi"ed as LVMs of (C }C )0 and (C }C )~, respectively. 4 4 4 4
259
The formation mechanism of the center has been investigated and it is proposed that C }C is formed by the 4 4 process C }C #VPC }C . 4 * 4 4 Acknowledgements We thank Pia Bomholt for preparing the samples for optical measurements. This work was supported by the Danish National Research Foundation through the Aarhus Center for Atomic Physics. E.V. Lavrov also acknowledges a grant from the Russian Foundation for Basic Research (grant No. 99-02-16652).
References [1] G. Davies, R.C. Newman, in: T.S. Moss (Ed.), Handbook on Semiconductors, Vol. 3b, Elsevier Science, Amsterdam, 1994, p. 1557 and references therein. [2] A.R. Bean, R.C. Newman, Solid State Commun. 8 (1970) 175. [3] G.D. Watkins, K.L. Brower, Phys. Rev. Lett. 36 (1976) 1329. [4] G.D. Watkins, in: M. Hulin (Ed.), Radiation E!ects in Semiconductors, Dunod, Paris, 1965. [5] J.R. Byberg, B. Bech Nielsen, M. Fancuilli, S.K. Estreicher, P.A. Fedders, Phys. Rev. B, submitted for publication. [6] E.V. Lavrov, L. Ho!mann, B. Bech Nielsen, Phys. Rev. B, in press. [7] J. Menendez, M. Cardona, Phys. Rev. B 29 (1984) 2051.