- Email: [email protected]
Characterization of carrier recombination and trapping processes in proton irradiated silicon by microwave absorption transients
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112 www.elsevier.com/locate/nima
Characterization of carrier recombination and trapping processes in proton irradiated silicon by microwave absorption transients E. Gaubasa,, J. Vaitkusa, G. Niaurab, J. Ha¨rko¨nenc, E. Tuovinenc, P. Luukkac, E. Fretwurstd a
Institute of Materials Science and Applied Research, Vilnius University, Lithuania b Institute of Chemistry, Lithuania c Helsinki Institute of Physics, Helsinki University, Finland d Institute for Experimental Physics, Hamburg University, Germany Available online 18 April 2005
Abstract Carrier lifetime variations dependent on proton irradiation with fluences in the range from 5 1012 to 1015 cm2 were investigated in high resistivity oxygenated silicon wafers and pad detectors. The fast recombination and slow trapping constituents within recombination transients have been distinguished by combining analyses of the excess carrier decay dependence on the excitation intensity, bias illumination and temperature, measured using the technique of microwave absorption by free carriers. Differences in the rate of formation and type of defects in the ranges of moderate and highest proton irradiation fluences have been revealed from the inverse lifetime dependence on irradiation fluence. The activation factors of the capture centres have been evaluated from carrier lifetime variations in the range of low and elevated temperatures. r 2005 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 61.72.Ji; 61.82.Fk Keywords: Silicon; Microwave absorption; Carrier lifetime; Recombination; Trapping; Radiation defects
1. Introduction
Corresponding
author. Tel.: +370 2 769503; fax: +370 2 769313. E-mail address: [email protected] (E. Gaubas).
One of the prospects to improve silicon material radiation hardness is linked to the application of oxygenated high-resistivity (HR) silicon for the fabrication of particle detectors [1,2]. Formation of oxygen–radiation defect complexes and of
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.03.106
ARTICLE IN PRESS E. Gaubas et al. / Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112
oxygen associated thermal donors allows the deactivation of dopants and manipulation of the density of active recombination centres. In recent years, strong interest has appeared in the improvement of heat treatment technologies and in control and profiling of oxygen concentrations in differently doped Si, attaining temperature de-activation of the harmful defects [3–5]. In this work, carrier lifetime variations in starting and irradiated material wafers and diode structures, fabricated on oxygenated silicon processed by a high-temperature diffusion step and by low-temperature formation of thermal donors, were investigated. The aim is to understand the transformations of the electrically active radiation defects in silicon detectors. These high-resistivity (HR) oxygenated silicon wafers and diode structures were irradiated with protons at various radiation fluences in the range from 5 1012 to 1015 p cm2. The fast recombination and slow trapping constituents within recombination transients were distinguished by combining analyses of the excess carrier decay dependence on excitation intensity, bias illumination and temperature, measured using the technique of the microwave absorption by free carriers (MWA). The inverse lifetime dependence on proton irradiation fluence exhibited differences in defect formation rate and defect types in the range of moderate to highest proton irradiation fluences.
2. Samples and experimental techniques Two batches of samples were investigated. The first was composed of homogeneous n- and p-type magnetic Czochralski (M-Cz) Si wafers with dimensions of 20 20 mm2 and 300 mm thickness. Twin samples of thermally treated and non-heated wafers were fabricated to control the role of thermal donors. These twin sample pairs were simultaneously irradiated with a proton beam of 25 mm spot diameter. 10(50) MeV proton irradiations with fluences of 5 1012 and 1013 p cm2 (9 1012 and 2 1013 p cm2) were utilized. Thus, part of the wafer area was irradiated, allowing a comparison of the characteristics of irradiated and non-irradiated material as well as monitoring the
109
homogeneity of the irradiation. The wafer surfaces of the starting material were passivated with thick thermal oxide films. Thermal diffusion (TD) processing of one of the twin samples was carried out by heating at 450 1C for 30 min in an inert N2 atmosphere. The second batch of samples consisted of pad-detectors fabricated on oxygenated n-type DOFZ Si, with oxygen introduced by a 24 h high-temperature diffusion step, and non-processed FZ Si diodes for comparison. These detectors were irradiated by 24 GeV/c protons to fluences of 4 1014 and 1015 p cm2. A 2 mm diameter optical window in the centre of the detector and the 1 mm wide boundary of the detector area were left non-metallized, and utilized for microwave absorption (MWA) investigations. Using a Perkin-Elmer GX FTIR spectrometer, room temperature FTIR spectroscopy was employed to measure the concentration of interstitial oxygen (Oi), and to track transformations of oxygen related defects. Excess carrier recombination transients were examined by combining analyses of the dependence of the excess carrier decays on the excitation intensity, bias illumination and temperature, measured using the microwave absorption technique [6]. Excess carriers were generated in the bulk of the samples by light of 1064 nm wavelength from a 10 ns pulse length YAG: Nd3+ laser and probed by MWA (at 10 GHz). Continuous wave broad band bias illumination (BI) was employed to suppress trapping by emptying the capture levels. The samples were placed on a cold/hot finger to measure the temperature dependence of the lifetime characteristics. From these the trap activation factors were deduced.
3. Characteristics of carrier recombination and trapping processes MWA transients in the TD processed, unheated p- and n-type M-Cz Si wafers, measured at relatively low excitation without CW bias illumination exhibited a two-component decay characterized by very long lifetimes (up to tens ms), manifesting competition of carrier recombination and trapping processes. The recombination and
ARTICLE IN PRESS E. Gaubas et al. / Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112
102
mm
20 mm
25
trapping lifetimes decrease with excitation intensity, and the trapping constituent almost disappears at the highest excitation intensities. Bias illumination (BI) partially or fully suppresses the trapping long-tail component, which is more pronounced in the TD treated material. Trapping is also more conspicuous in the p-type material. These observations are consistent with FTIR spectra, where the differential spectrum, comparing the untreated and TD processed material, shows more intense peaks in the p-type starting material than those in n-Si. The interstitial oxygen concentration estimated from FTIR spectra at 1106 cm1, employing a baseline method [3,4] and using the IOC88 standard calibration factor F RT ¼ 3:14 1017 cm2 [8], is less than 9 1017 cm3 and nearly the same in oxygenated M-Cz n-Si and p-Si starting material. TD processing creates more changes of Oi concentration in the p-Si. Together with Oi, a peak at 1220 cm1 observable in FTIR spectra is attributed to precipitates of SiOx. Pronounced changes of peaks attributed to Oi and SiOx were observed in the differential FTIR spectra for both n- and p-type, comparing proton irradiated wafers with unirradiated starting material. At room temperature, the carrier lifetime decreases dramatically in the irradiated sample part even at the lowest fluences applied, and the decay shows only one component. This lifetime change is illustrated in Fig. 1. The steep increase of lifetime correlates well with the geometrical dimensions of the irradiation spot. Within the area of the proton beam spot, sketched in the inset of Fig. 1, nearly the same lifetime values were measured. In the non-irradiated sample area, values of carrier lifetime are close to those obtained in the starting material at the same excitation density. Significant carrier lifetime variations with irradiation fluence were obtained at room temperature (RT). Fig. 2a shows the reciprocal lifetime ðt1 Þ as a function of proton irradiation fluence. Such a plot is useful to extract defect introduction rates and to monitor radiation defect concentrations, since t1 is roughly proportional to the radiationdefect density. Different slopes can be determined from a linear approximation of segments of this
τ (µs)
110
101
20 mm n-Si TD processed 10 MeV 5x1012 p/cm2
100
10-1
0
5
10 x (mm)
15
20
Fig. 1. Lateral lifetime variation due to irradiation with a proton beam spot of 25 mm. Inset: circle—proton beam spot, square—the sample.
characteristic in various fluence ranges. Here, the t1 dependence on fluence is generalized for both n- and p-type wafers and detector samples. The slope variation in different fluence ranges implies that not only the defect concentrations, R, but also their type can vary with fluence. In the range of low fluences the slope appeared to depend on the distance between recombination centres, when 1=3 t1 . Defect conglomerates or clusters act R / R as extended centres characterized by a linear slope at higher fluences. This is partially corroborated by the different activation factors found for the trapping levels (0.23 eV and 0.3 eV) estimated for n-type wafers and pad detectors in the moderate and high fluence ranges, respectively. Nevertheless, the carrier lifetime in the samples irradiated with a fixed fluence appeared to depend on temperature, varying in the range from 90 to 450 K. These dependencies are illustrated in Figs. 2b and c. Two peak values of carrier lifetime were obtained in the ranges of low and elevated (above RT) temperature (Figs. 2b, c). The first, lowtemperature lifetime peak, is formed by the longtail trapping decay component, which appears together with an initial short component when cooling the sample, and is nearly independent of BI (in the range of cw BI intensities applied). This lifetime peak is inherent for all the irradiated samples, while the absolute lifetime values vary in samples irradiated by different fluences, as shown in Fig. 2b. In the starting material, these lifetime
ARTICLE IN PRESS E. Gaubas et al. / Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112
τeff-1 (µs-1)
10
diodes fabricated on oxygenated Si, 10 MeV
2
M-Czn-Si wafers TD treated, protons 10 and 50 MeV M-Czp-Si wafers TD treated, protons 10 and 50 MeV pad detectors DOFZ Si, protons 24 GeV/c
101 100
10-1 9
10
10
10
(a)
11
12
13
14
10 10 10 10 Irradiation fluence (p/cm2)
15
10
irradiated M-Cz n-Si
10
2
3
τ (µs)
2
1
10 MeV protons 1013 cm-2
2
10 MeV protons 5x1012 cm-2
3
50 MeV protons 9x1012 cm-2
0.23 eV
101
0.56 eV 0.1 eV
100
1
10-1 100 150 200 250 300 350 400 T (K)
(b)
M-Cz p-Si TD processed
101
1
50 MeV protons 2x1013 cm-2
2
10 MeV protons 5x1012 cm-2
τ (µs)
0.4 eV 1
0.19 eV
100 0.1 eV
2
10-1
(c)
100 150 200 250 300 350 400 T (K)
Fig. 2. (a) Reciprocal lifetime vs. proton irradiation fluence, at RT; (b) carrier lifetime temperature variations in n-type and (c) p-type irradiated wafers due to trapping and recombination.
111
variations with temperature were not resolvable. Variations of the short decay component, which prevails in the irradiated samples at elevated temperatures, form the high-temperature peak, shown for one sample of both n- and p-type M-Cz Si, in Figs. 2b and c, respectively. The trap activation factors were deduced by using plots of the lifetime as a function of 1/kT. In n-type wafers, the dominant slopes exhibited effective trap activation energies of 0.56, 0.23 and 0.1 eV. In the n-type DOFZ pad detectors irradiated with fluences of 4 1014 and 1015 p cm2, the dominant trapping level is characterized by an activation energy of 0.3 eV. In p-type M-Cz Si material, effective trap activation energy values of 0.4, 0.19 and 0.1 eV were extracted. The oxygenation and heat treatments (TD process) probably introduce shallow levels: characteristic activation energies of 0.07 and 0.15 eV have been measured in TD samples [4]. Trapping centres with activation energy close to 0.1 eV, determined from the lifetime temperature dependences (Figs. 2b, c), can therefore be attributed to TD. The two-component decay in the starting material wafers demonstrates that the relaxation process of excess carrier pairs is managed by at least two centres. Qualitatively such a decay process can be described by instantaneous lifetimes ti , determined by simultaneous recombination and trapping events [7]. The role of trapping increases during the decay of carrier density, and the trapping effect appears as a long-tail component within MWA transients at relatively low excitation intensity. Trapping is suppressed with enhanced excess carrier density, varied via either excitation intensity or bias illumination, as observed in our experiments. The observed lifetime dependence on temperature (Figs. 2b, c) can also be explained by the simultaneous interplay of recombination and trapping centres, when the trapping effect is resolvable for trapping coefficients b1. The trapping coefficient is determined mainly by the concentration of trapping centres M. Therefore, in the starting material wafers with small M this trapping peak in the lifetime temperature dependence was non-resolvable for the excitations applied. The effective values of trap activation energy, 0.56, 0.23 and 0.1 eV, can be attributed to
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di-vacancy and thermal donors [4,9] after irradiations in the range of moderate fluences. In the n-Si pad detectors irradiated with high proton fluences of 4 1014 and 1015 cm2, the dominant trapping level is characterized by an activation energy of 0.3 eV which may be associated with interstitials of Sii and vacancy complexes [9]. In p-type material, extracted trap activation energy values of 0.4, 0.19 and 0.1 eV can be attributed to di-vacancy, VO complex and thermal donors, respectively. Nevertheless, the ti–T peaks in the irradiated material are nearly independent of BI, and the position of the peaks on the T scale, attributed to deep levels, is shifted to lower temperatures. Thus, these results require a deeper consideration of the recombination characteristics. Inter-centre recombination [10], barriers at defects, clusters of defects and configurational multi-stability of defects [11] will be taken into account in a more comprehensive analysis of these characteristics.
4. Conclusions The inverse lifetime of the fast recombination, estimated from MWA decays, exhibited the nonlinear variation with proton irradiation fluence, demonstrating differences in defect formation rate and defect types in the ranges of moderate and highest irradiation fluences. Activation factors of the fast recombination centres and of the slow trapping ones extracted from the carrier lifetime variations with temperature in irradiated samples, correspond well with conventional vacancy attributed radiation defects. The trapping level activa-
tion energy in n-Si irradiated with high proton fluences in the range of 1015 cm2 has been shown to be different from that obtained with moderate fluences.
Acknowledgements The Department of Bioelectrochemistry and Biospectroscopy at the Institute of Biochemistry, Vilnius, is acknowledged for the possibility to exploit the FTIR spectrometer. This work was performed in the framework of the CERN RD50 Project and was partly supported by the Lithuanian State Science and Studies Foundation.
References [1] Z. Li, et al., IEEE Trans. Nucl. Sci. NS-39 (1992) 1730. [2] G. Lindstroem, et al., (RD48 Coll.), Nucl. Instr. and Meth. A 466 (2001) 308. [3] A. Sassella, A. Borghesi, T. Abe, J. Electrochem. Soc. 145 (1998) 1715. [4] E. Simoen, C. Claeys, R. Loo, et al., Mater. Sci. Eng. B B102 (2003) 207. [5] E. Verbitskaya, et al., Nucl. Instr. and Meth. A 514 (2003) 47. [6] E. Gaubas, Lith. J. Phys. 43 (2003) 145. [7] S.M. Ryvkin, Photoelectronic Effects in Semiconductors, Consulting Bureau, New York, 1964. [8] Annual book of ASTM standards, F 1188-93 (1993). [9] M.-A. Trauwaert, Radiation and impurity related deep levels in Si, Ph.D. Thesis, IMEC KUL, Leuven, 1995. [10] W.M. Chen, B. Monemar, E. Janzen, Phys. Rev. Lett. 61 (1991) 1914. [11] A. Chantre, Appl. Phys. A 32 (1989) 3.
Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112 www.elsevier.com/locate/nima
Characterization of carrier recombination and trapping processes in proton irradiated silicon by microwave absorption transients E. Gaubasa,, J. Vaitkusa, G. Niaurab, J. Ha¨rko¨nenc, E. Tuovinenc, P. Luukkac, E. Fretwurstd a
Institute of Materials Science and Applied Research, Vilnius University, Lithuania b Institute of Chemistry, Lithuania c Helsinki Institute of Physics, Helsinki University, Finland d Institute for Experimental Physics, Hamburg University, Germany Available online 18 April 2005
Abstract Carrier lifetime variations dependent on proton irradiation with fluences in the range from 5 1012 to 1015 cm2 were investigated in high resistivity oxygenated silicon wafers and pad detectors. The fast recombination and slow trapping constituents within recombination transients have been distinguished by combining analyses of the excess carrier decay dependence on the excitation intensity, bias illumination and temperature, measured using the technique of microwave absorption by free carriers. Differences in the rate of formation and type of defects in the ranges of moderate and highest proton irradiation fluences have been revealed from the inverse lifetime dependence on irradiation fluence. The activation factors of the capture centres have been evaluated from carrier lifetime variations in the range of low and elevated temperatures. r 2005 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 61.72.Ji; 61.82.Fk Keywords: Silicon; Microwave absorption; Carrier lifetime; Recombination; Trapping; Radiation defects
1. Introduction
Corresponding
author. Tel.: +370 2 769503; fax: +370 2 769313. E-mail address: [email protected] (E. Gaubas).
One of the prospects to improve silicon material radiation hardness is linked to the application of oxygenated high-resistivity (HR) silicon for the fabrication of particle detectors [1,2]. Formation of oxygen–radiation defect complexes and of
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.03.106
ARTICLE IN PRESS E. Gaubas et al. / Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112
oxygen associated thermal donors allows the deactivation of dopants and manipulation of the density of active recombination centres. In recent years, strong interest has appeared in the improvement of heat treatment technologies and in control and profiling of oxygen concentrations in differently doped Si, attaining temperature de-activation of the harmful defects [3–5]. In this work, carrier lifetime variations in starting and irradiated material wafers and diode structures, fabricated on oxygenated silicon processed by a high-temperature diffusion step and by low-temperature formation of thermal donors, were investigated. The aim is to understand the transformations of the electrically active radiation defects in silicon detectors. These high-resistivity (HR) oxygenated silicon wafers and diode structures were irradiated with protons at various radiation fluences in the range from 5 1012 to 1015 p cm2. The fast recombination and slow trapping constituents within recombination transients were distinguished by combining analyses of the excess carrier decay dependence on excitation intensity, bias illumination and temperature, measured using the technique of the microwave absorption by free carriers (MWA). The inverse lifetime dependence on proton irradiation fluence exhibited differences in defect formation rate and defect types in the range of moderate to highest proton irradiation fluences.
2. Samples and experimental techniques Two batches of samples were investigated. The first was composed of homogeneous n- and p-type magnetic Czochralski (M-Cz) Si wafers with dimensions of 20 20 mm2 and 300 mm thickness. Twin samples of thermally treated and non-heated wafers were fabricated to control the role of thermal donors. These twin sample pairs were simultaneously irradiated with a proton beam of 25 mm spot diameter. 10(50) MeV proton irradiations with fluences of 5 1012 and 1013 p cm2 (9 1012 and 2 1013 p cm2) were utilized. Thus, part of the wafer area was irradiated, allowing a comparison of the characteristics of irradiated and non-irradiated material as well as monitoring the
109
homogeneity of the irradiation. The wafer surfaces of the starting material were passivated with thick thermal oxide films. Thermal diffusion (TD) processing of one of the twin samples was carried out by heating at 450 1C for 30 min in an inert N2 atmosphere. The second batch of samples consisted of pad-detectors fabricated on oxygenated n-type DOFZ Si, with oxygen introduced by a 24 h high-temperature diffusion step, and non-processed FZ Si diodes for comparison. These detectors were irradiated by 24 GeV/c protons to fluences of 4 1014 and 1015 p cm2. A 2 mm diameter optical window in the centre of the detector and the 1 mm wide boundary of the detector area were left non-metallized, and utilized for microwave absorption (MWA) investigations. Using a Perkin-Elmer GX FTIR spectrometer, room temperature FTIR spectroscopy was employed to measure the concentration of interstitial oxygen (Oi), and to track transformations of oxygen related defects. Excess carrier recombination transients were examined by combining analyses of the dependence of the excess carrier decays on the excitation intensity, bias illumination and temperature, measured using the microwave absorption technique [6]. Excess carriers were generated in the bulk of the samples by light of 1064 nm wavelength from a 10 ns pulse length YAG: Nd3+ laser and probed by MWA (at 10 GHz). Continuous wave broad band bias illumination (BI) was employed to suppress trapping by emptying the capture levels. The samples were placed on a cold/hot finger to measure the temperature dependence of the lifetime characteristics. From these the trap activation factors were deduced.
3. Characteristics of carrier recombination and trapping processes MWA transients in the TD processed, unheated p- and n-type M-Cz Si wafers, measured at relatively low excitation without CW bias illumination exhibited a two-component decay characterized by very long lifetimes (up to tens ms), manifesting competition of carrier recombination and trapping processes. The recombination and
ARTICLE IN PRESS E. Gaubas et al. / Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112
102
mm
20 mm
25
trapping lifetimes decrease with excitation intensity, and the trapping constituent almost disappears at the highest excitation intensities. Bias illumination (BI) partially or fully suppresses the trapping long-tail component, which is more pronounced in the TD treated material. Trapping is also more conspicuous in the p-type material. These observations are consistent with FTIR spectra, where the differential spectrum, comparing the untreated and TD processed material, shows more intense peaks in the p-type starting material than those in n-Si. The interstitial oxygen concentration estimated from FTIR spectra at 1106 cm1, employing a baseline method [3,4] and using the IOC88 standard calibration factor F RT ¼ 3:14 1017 cm2 [8], is less than 9 1017 cm3 and nearly the same in oxygenated M-Cz n-Si and p-Si starting material. TD processing creates more changes of Oi concentration in the p-Si. Together with Oi, a peak at 1220 cm1 observable in FTIR spectra is attributed to precipitates of SiOx. Pronounced changes of peaks attributed to Oi and SiOx were observed in the differential FTIR spectra for both n- and p-type, comparing proton irradiated wafers with unirradiated starting material. At room temperature, the carrier lifetime decreases dramatically in the irradiated sample part even at the lowest fluences applied, and the decay shows only one component. This lifetime change is illustrated in Fig. 1. The steep increase of lifetime correlates well with the geometrical dimensions of the irradiation spot. Within the area of the proton beam spot, sketched in the inset of Fig. 1, nearly the same lifetime values were measured. In the non-irradiated sample area, values of carrier lifetime are close to those obtained in the starting material at the same excitation density. Significant carrier lifetime variations with irradiation fluence were obtained at room temperature (RT). Fig. 2a shows the reciprocal lifetime ðt1 Þ as a function of proton irradiation fluence. Such a plot is useful to extract defect introduction rates and to monitor radiation defect concentrations, since t1 is roughly proportional to the radiationdefect density. Different slopes can be determined from a linear approximation of segments of this
τ (µs)
110
101
20 mm n-Si TD processed 10 MeV 5x1012 p/cm2
100
10-1
0
5
10 x (mm)
15
20
Fig. 1. Lateral lifetime variation due to irradiation with a proton beam spot of 25 mm. Inset: circle—proton beam spot, square—the sample.
characteristic in various fluence ranges. Here, the t1 dependence on fluence is generalized for both n- and p-type wafers and detector samples. The slope variation in different fluence ranges implies that not only the defect concentrations, R, but also their type can vary with fluence. In the range of low fluences the slope appeared to depend on the distance between recombination centres, when 1=3 t1 . Defect conglomerates or clusters act R / R as extended centres characterized by a linear slope at higher fluences. This is partially corroborated by the different activation factors found for the trapping levels (0.23 eV and 0.3 eV) estimated for n-type wafers and pad detectors in the moderate and high fluence ranges, respectively. Nevertheless, the carrier lifetime in the samples irradiated with a fixed fluence appeared to depend on temperature, varying in the range from 90 to 450 K. These dependencies are illustrated in Figs. 2b and c. Two peak values of carrier lifetime were obtained in the ranges of low and elevated (above RT) temperature (Figs. 2b, c). The first, lowtemperature lifetime peak, is formed by the longtail trapping decay component, which appears together with an initial short component when cooling the sample, and is nearly independent of BI (in the range of cw BI intensities applied). This lifetime peak is inherent for all the irradiated samples, while the absolute lifetime values vary in samples irradiated by different fluences, as shown in Fig. 2b. In the starting material, these lifetime
ARTICLE IN PRESS E. Gaubas et al. / Nuclear Instruments and Methods in Physics Research A 546 (2005) 108–112
τeff-1 (µs-1)
10
diodes fabricated on oxygenated Si, 10 MeV
2
M-Czn-Si wafers TD treated, protons 10 and 50 MeV M-Czp-Si wafers TD treated, protons 10 and 50 MeV pad detectors DOFZ Si, protons 24 GeV/c
101 100
10-1 9
10
10
10
(a)
11
12
13
14
10 10 10 10 Irradiation fluence (p/cm2)
15
10
irradiated M-Cz n-Si
10
2
3
τ (µs)
2
1
10 MeV protons 1013 cm-2
2
10 MeV protons 5x1012 cm-2
3
50 MeV protons 9x1012 cm-2
0.23 eV
101
0.56 eV 0.1 eV
100
1
10-1 100 150 200 250 300 350 400 T (K)
(b)
M-Cz p-Si TD processed
101
1
50 MeV protons 2x1013 cm-2
2
10 MeV protons 5x1012 cm-2
τ (µs)
0.4 eV 1
0.19 eV
100 0.1 eV
2
10-1
(c)
100 150 200 250 300 350 400 T (K)
Fig. 2. (a) Reciprocal lifetime vs. proton irradiation fluence, at RT; (b) carrier lifetime temperature variations in n-type and (c) p-type irradiated wafers due to trapping and recombination.
111
variations with temperature were not resolvable. Variations of the short decay component, which prevails in the irradiated samples at elevated temperatures, form the high-temperature peak, shown for one sample of both n- and p-type M-Cz Si, in Figs. 2b and c, respectively. The trap activation factors were deduced by using plots of the lifetime as a function of 1/kT. In n-type wafers, the dominant slopes exhibited effective trap activation energies of 0.56, 0.23 and 0.1 eV. In the n-type DOFZ pad detectors irradiated with fluences of 4 1014 and 1015 p cm2, the dominant trapping level is characterized by an activation energy of 0.3 eV. In p-type M-Cz Si material, effective trap activation energy values of 0.4, 0.19 and 0.1 eV were extracted. The oxygenation and heat treatments (TD process) probably introduce shallow levels: characteristic activation energies of 0.07 and 0.15 eV have been measured in TD samples [4]. Trapping centres with activation energy close to 0.1 eV, determined from the lifetime temperature dependences (Figs. 2b, c), can therefore be attributed to TD. The two-component decay in the starting material wafers demonstrates that the relaxation process of excess carrier pairs is managed by at least two centres. Qualitatively such a decay process can be described by instantaneous lifetimes ti , determined by simultaneous recombination and trapping events [7]. The role of trapping increases during the decay of carrier density, and the trapping effect appears as a long-tail component within MWA transients at relatively low excitation intensity. Trapping is suppressed with enhanced excess carrier density, varied via either excitation intensity or bias illumination, as observed in our experiments. The observed lifetime dependence on temperature (Figs. 2b, c) can also be explained by the simultaneous interplay of recombination and trapping centres, when the trapping effect is resolvable for trapping coefficients b1. The trapping coefficient is determined mainly by the concentration of trapping centres M. Therefore, in the starting material wafers with small M this trapping peak in the lifetime temperature dependence was non-resolvable for the excitations applied. The effective values of trap activation energy, 0.56, 0.23 and 0.1 eV, can be attributed to
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di-vacancy and thermal donors [4,9] after irradiations in the range of moderate fluences. In the n-Si pad detectors irradiated with high proton fluences of 4 1014 and 1015 cm2, the dominant trapping level is characterized by an activation energy of 0.3 eV which may be associated with interstitials of Sii and vacancy complexes [9]. In p-type material, extracted trap activation energy values of 0.4, 0.19 and 0.1 eV can be attributed to di-vacancy, VO complex and thermal donors, respectively. Nevertheless, the ti–T peaks in the irradiated material are nearly independent of BI, and the position of the peaks on the T scale, attributed to deep levels, is shifted to lower temperatures. Thus, these results require a deeper consideration of the recombination characteristics. Inter-centre recombination [10], barriers at defects, clusters of defects and configurational multi-stability of defects [11] will be taken into account in a more comprehensive analysis of these characteristics.
4. Conclusions The inverse lifetime of the fast recombination, estimated from MWA decays, exhibited the nonlinear variation with proton irradiation fluence, demonstrating differences in defect formation rate and defect types in the ranges of moderate and highest irradiation fluences. Activation factors of the fast recombination centres and of the slow trapping ones extracted from the carrier lifetime variations with temperature in irradiated samples, correspond well with conventional vacancy attributed radiation defects. The trapping level activa-
tion energy in n-Si irradiated with high proton fluences in the range of 1015 cm2 has been shown to be different from that obtained with moderate fluences.
Acknowledgements The Department of Bioelectrochemistry and Biospectroscopy at the Institute of Biochemistry, Vilnius, is acknowledged for the possibility to exploit the FTIR spectrometer. This work was performed in the framework of the CERN RD50 Project and was partly supported by the Lithuanian State Science and Studies Foundation.
References [1] Z. Li, et al., IEEE Trans. Nucl. Sci. NS-39 (1992) 1730. [2] G. Lindstroem, et al., (RD48 Coll.), Nucl. Instr. and Meth. A 466 (2001) 308. [3] A. Sassella, A. Borghesi, T. Abe, J. Electrochem. Soc. 145 (1998) 1715. [4] E. Simoen, C. Claeys, R. Loo, et al., Mater. Sci. Eng. B B102 (2003) 207. [5] E. Verbitskaya, et al., Nucl. Instr. and Meth. A 514 (2003) 47. [6] E. Gaubas, Lith. J. Phys. 43 (2003) 145. [7] S.M. Ryvkin, Photoelectronic Effects in Semiconductors, Consulting Bureau, New York, 1964. [8] Annual book of ASTM standards, F 1188-93 (1993). [9] M.-A. Trauwaert, Radiation and impurity related deep levels in Si, Ph.D. Thesis, IMEC KUL, Leuven, 1995. [10] W.M. Chen, B. Monemar, E. Janzen, Phys. Rev. Lett. 61 (1991) 1914. [11] A. Chantre, Appl. Phys. A 32 (1989) 3.