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Deep-level transient spectroscopy measurements of majority carrier traps in neutron irradiated n-type silicon detectors
277
Nuclear Instruments and Methods in Physics Research A279 (1989) 277-280 North-Holland, Amsterdam
DEEP-LEVEL TRANSIENT SPECTROSCOPY MEASUREMENTS OF MAJORITY CARRIER TRAPS IN NEUTRON-IRRADIATED n-TYPE SILICON DETECTORS E. BORCHI University of Florence and INFN, Florence, Italy C . BERTRAND and C . LEROY McGill University, Montreal, Canada M . BRUZZI, C . FURETTA *, R. PALUDETTO, P .G . RANCOITA and L . VISMARA INFN, Milan, Italy P . GIUBELLINO INFN, Turin, Italy The effect of damage produced in silicon detectors by neutron irradiation at room temperature is examined by using the experimental technique of deep-level transient spectroscopy. The production of three defects, the A centre, the E centre, and the divacancy, is reported . The divacancy is especially important in neutron damage in silicon. There is evidence of some defects generated during annealing. It has been found that the properties of the point defects outside the clusters are the main results obtained with DLTS methods . 1 . Introduction A great deal of interest has arisen, in recent years, in the application of silicon diodes as position and energy detectors in high-energy physics . These detectors are the ones best suited to face the challenge posed by the exceptional experimental conditions [1] that will prevail in the new generation of colliding beam machines, such as the Large Hadron Collider (LHC) in the LEP tunnel at CERN and the Superconducting Super Collider (SSC) in the USA, both involving multi-TeV proton beams . The radiation hardness of the active medium of the calorimeter is of the greatest importance in view of the high intensities that will particularly exist at these hadron colliders . Neutrons, produced by the interaction of the beam with the environment and the absorber part of the calorimeters, will be responsible for most of the damage caused to the detectors. As regards the SSC, studies on the radiation levels have indeed shown that the neutron fluences per unit area in the region of maximum fluence are of the order of 10 11 -10 13 n/cmz . It is known from experiment that neutron damage is at least two orders of magnitude greater than electron or photon damage, and one order of magnitude greater than that created by charged particles [2]. In silicon * Also at University of Rome, Italy. 0168-9002/89/$03 .50 © Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)
most of the neutron damage is caused by fast neutrons, interacting with the silicon atoms primarily by elastic scattering . Such direct collisions result in up to 10 3 displacements per event, with silicon interstitials moving outwards, leaving behind a vacancy-rich and somewhat disordered zone [3] . The vacancies may combine to form divacancies and larger clusters which may produce defect energy levels in the band gap somewhat different from those of isolated single-defect levels. Moreover, there is experimental evidence that the formation of other point defects, such as the E centre (vacancy-phosphorus complex) and the A centre (vacancy-oxygen complex), may be attributed also, to a large extent, to the vacancy release from defect clusters . In the present paper, point defects produced in several n-type silicon detectors by neutron irradiation are presented . The experimental studies have been carried out using the deep-level transient spectroscopy (DLTS) technique. As will become evident from the forthcoming discussion, DLTS measurements are able to reveal the point-defect character of neutron damage . Different measurement techniques, e .g. the Hall effect, would have to be used in order to observe the presence of defect clusters. We show the spectrum after neutron irradiation for a typical silicon sample . Furthermore, a simple mechanism of annealing due to Tokuda and Usami [4], which VI. RADIATION DAMAGE STUDIES
27 8
E. Borchi et al. / Deep-level transient spectroscopy measurements
is consistent with our data and those of previous experiments [4,5], is partially confirmed. The evidence of the formation of several defects during annealing is also reported .
pF
2. Experimental procedure and results For this investigation p +-n silicon diodes * were used with a nominal area 10 X 10 mm2, a thickness of 400 [tm (made by diffusing boron into phosphorusdoped floating-zone-grown silicon wafers), and resistivities of about 6 kQ cm . The concentrations of dopant in the samples (obtained from capacitance-voltage mea-
lo
surements) are between 6 X 10 11 and 8 X ll cm -3 . Neutron irradiation was performed at room temper252Cf ature using a isotopic neutron source . The emis252 sion spectrum of Cf is shown elsewhere [6]. The total neutron fluences per unit area were between 2 .7 X 10 1°
Fig. 1 . Typical DLTS spectrum after irradiation for the sample number BI ,, (temperature scale is in kelvin) .
and 2.1 X 10 11 n/cm2. The DLTS analysis [7] of the silicon samples was carried out using a DLTS system, DL 4600 * *. We measured the capacitance transient associated with the slow emission of the majority car-
riers after they had been captured in traps. The capture is generated by a repetitive bias-filling pulse (about 1
ms long), during a thermal scan . Various time intervals, called rate windows, are used for the sampling of the capacitance transient. The DLTS spectrum after irradiation, obtained with a time interval for the sampling of 1 ms, is shown in fig. 1 for the sample number Bt9 (hereafter called B) with a resistivity value of 6.61 kQ cm, a dopant concentration of 8.25 X 10 11 cm -3 , and a neutron fluence of 2.7 X 10 1° n/cm2.
The three peaks shown in the spectrum correspond to three different point defects : El , E2, and E3. The activation energy ET is related to the temperature of the peak of the DLTS spectrum, and to the thermal emission rate e by In(e /T 2 ) = -ET/kT+constant, where T is the temperature of the peak. Fig. 2 shows the Arrhenius plot for E1, E2 , and E3 , where e /T 2 is
given as a function of 1000/T. The data are measured using time intervals of 1, 2.5, 5, 12 .5, 20, and 50 ms . The values of the three slopes in the Arrhenius plot are the values of the three activation energies. The E1, E2 , and E3 energies are E. - (0 .14 ± 0.01) eV, E, - (0 .26 ± 0.03) eV, and E, - (0.40 ± 0.02) eV respectively, where E, is the energy of the conduction band . These values are
compared in table 1 with the DLTS results obtained in neutron-irradiated n-type silicon by Tokuda and Usami [4] and by Tulach et al . [5].
* Made by Ansaldo, Italy. * * Manufactured by Bio Rad, UK.
Fig. 2. Arrhenius plot for El , E2 , and E3. Substantial agreement in the behaviour of the pointdefect energies (and also of the relative amplitudes) is shown by this comparison . This indicates that damage is quite independent of the various conditions of irradiation as well as of the material parameters .
Table 1 Comparison of the energy levels (in eV) in n-type silicon detectors obtained with neutron irradiation by the present authors, by Tokuda and Usami [4] and by Tulach et al . [5] Defect code
This work
Tokuda and Usami
Tulach et al.
El E2 E3
E,-(0.14±0.01) E~-(0.26±0.03) E, - (0.40± 0.02)
E~-0 .15 E,-0 .21 E,-0 .39
E~-(0 .18±0.01) E~-(0 .25±0.01) E, - (0 .46 ±0.02)
E. Borchi et al. / Deep-level transient spectroscopy measurements
279
series of mixed isothermal and isochronal annealings were carried out on our silicon samples over the temperature range from 100 ° C to 300 ° C with an annealing time from 1 up to 24 h . However, the results were frequently a function of both the type of sample and the annealing procedures used. Furthermore, two centres, E4 and E 5 , where clearly observed in the DLTS spectra with energies not well determined but close to the energy of the annealed-out centers . In view of this multiplicity of effects, only the main features will be presented here together with a tentative interpretation of the experimental results . The DLTS spectrum of sample B after annealing at 250'C is shown in fig . 4 together with the DLTS spectrum obtained after neutron irradiation. this spectrum clearly shows the presence of the defects E 4 and E 5 with corresponding energy levels of E, - (0 .35 ± 0 .04) eV and E, - (0.48 ± 0 .05) eV, respectively . These values are in relatively good agreement with those estimated by Tokuda and Usami (Ec - 0 .31 eV and Ec - 0 .45 eV) [4].
2
3. Discussion of the results 0
5
10
15
20
mxlol l Fig . 3 . Ratio N/Nd obtained from irradiation of 10 samples as a function of the neutron fluence . The dependence is linear . Fig. 3 shows the ratio NlNd (where N is the trap concentration of defect E 3 , the deepest point defect, and Nd is the donor concentration) obtained from an irradiation of ten samples as a function of the neutron fluence : the plot shows the linear dependence of N/Nd on the fluence. In order to obtain information that may be used to predict silicon-device stability from annealing studies, a
15
05
250
Fig. 4 . The DLTS spectrum of sample B after 250'C annealing is shown and reveals the presence of the E4 and E 5 defects.
The ratio NT/fn (where NT is the defect concentration and fn the neutron fluence per unit area) had roughly the following behaviour during the annealing of sample B . The E 3 centre shows two annealing stages around 150 and 300'C : NT/f assumes the values = 1 .36 after neutron irradiation, = 1 between 100 and 150' C, and = 0 .8 at 250' C. For E Z , an initial value of about 0 .5 appears after neutron irradiation and an initial annealing at 100 ° C for 24 h . Successive isothermal and isochronal annealings produce an increase of Nrlf, Moderate recovery of the defects is observed from 150 to 250 ° C, whereas a rapid decrease of NTlfn is present for temperatures greater than 250 ° C . A similar behaviour is observed for the other silicon detectors under investigation . On this basis we conclude that the annealing features of our samples agree reasonably well with the results of Tokuda and Usami [4] . The energy level and annealing-out temperature of E l are in agreement with those of the A centre ; the energy levels of E Z and E 3 can be associated with the double- and single-minus charge states of divacancy, respectively . Alternatively, according to the suggestion of Tokuda and Usami [4], we could suppose that the E 3 defect contains two different level contributions : the E centre and the single-minus state charge of divacancy . The energy levels of the E centre (E. - 0 .44 eV) and of the divacancy (Ec - 0 .39 eV) are so close that it turns out to be very difficult to resolve them by means of DLTS analysis . Also the value obtained by Tulach et al . [5] for their E 3 energy level (Ec - 0 .46 eV) is consistent with this interpretation . VI . RADIATION DAMAGE STUDIES
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E. Borchi et al. / Deep-level transient spectroscopy measurements
The growth of the El defect, occurring in silicon detectors at room temperature during storage of a few days, appears to be somewhat independent of the presence of the E3 defect, and it is probably due to the vacancy release following the room-temperature annealing of the defect clusters . On the other hand, the subsequent growth of the Et defect during annealing occurs in the temperature range corresponding to the first stage of annealing of the E3 defect (divacancy) . It is thus highly probable that the Et defect growth upon annealing to about 250'C may be attributed to the vacancy liberation both from defect clusters and from divacancy release. This strongly supports the hypothesis that the El defect should be considered as an A centre . To conclude this section, we observe that our data mainly show point-defect contributions to the neutron damage of silicon detectors. However, as previously mentioned, fast neutron damage in silicon mainly occurs via defect clusters . A cluster model has been proposed many years ago by Gossick [8], in which the space-charge region due to defect clusters forms a potential difference between the cluster and the surrounding undamaged material . For this reason, Gregory et al . [9] have pointed out that a relevant reduction of the probability of the majority carrier capture by defects inside the clusters should characterize cluster zones. Since DLTS response is not observed if carriers do not fill defect energy levels, it appears improbable that DLTS measurements can reveal defects inside the clusters . 4. Conclusions The relevance of the production of divacancies following multiple displacement effects in the primary radiation event due to fast neutron scattering and, indirectly, recombination of two vacancies, appears in our experimental results on neutron-irradiated n-type silicon detectors with fluences up to -- 1011 n/cm2. It has
also appeared from the experimental observation that the high production of divacancies partly suppresses the formation of A centres. The subsequent formation of A centres, in contrast, is due to vacancies released by thermal annealing of defect clusters and divacancy centres. Further experimental investigations are needed, using higher values of the neutron fluence, more systematic annealing procedures and, if possible, making use of additional investigation techniques in order to understand more clearly the complex phenomena associated with radiation damage in silicon detectors due to fast neutrons .
Acknowledgements We wish to acknowledge here useful discussions with A. Penzo, F. Lemeilleur, H.W . Kraner and A. Seidman. We are grateful to R. Raffnsoe, G. Roubaud and J.W .N . Tuyn of the CERN TIS Division for their support in the experimental work .
References [1] E. Borchi et al ., these Proceedings (Int. Conf . on Advanced Technology and Particle Physics, Como, Italy, 1988) Nucl . Instr. and Meth . A279 (1989) 57 . [2] H.W . Kraner, Z. Li and K.U . Posnacker, ibid., p. 266. [3] R.C. Newman, Rep. Prog. Phys. 45 (1982) 1163 . [4] Y. Tokuda and A. Usami, IEEE Trans. Nucl . Sci. NS-28 (1981) 3564 . [5] L. Tulach, H. Frank, B. Sopkc and Z. Prasil, Phys . Status Solidi A100 (1987) K13. [6] E.A . Lorch, Int. J. Appl . Radiat . Isot . 24 (1973) 585. [7] G.L. Miller, D.V. Lang and L.C . Kimmerling, Ann. Rev. Mater. Sci. 30 (1977) 377. [8] B.R . Gossick, J. Appl. Phys . 30 (1959) 1214 . [9] B.L . Gregory, S.S . Naik and W.G . Oldham, IEEE Trans. Nucl . Sci . NS-18 (1971) 50 .
Nuclear Instruments and Methods in Physics Research A279 (1989) 277-280 North-Holland, Amsterdam
DEEP-LEVEL TRANSIENT SPECTROSCOPY MEASUREMENTS OF MAJORITY CARRIER TRAPS IN NEUTRON-IRRADIATED n-TYPE SILICON DETECTORS E. BORCHI University of Florence and INFN, Florence, Italy C . BERTRAND and C . LEROY McGill University, Montreal, Canada M . BRUZZI, C . FURETTA *, R. PALUDETTO, P .G . RANCOITA and L . VISMARA INFN, Milan, Italy P . GIUBELLINO INFN, Turin, Italy The effect of damage produced in silicon detectors by neutron irradiation at room temperature is examined by using the experimental technique of deep-level transient spectroscopy. The production of three defects, the A centre, the E centre, and the divacancy, is reported . The divacancy is especially important in neutron damage in silicon. There is evidence of some defects generated during annealing. It has been found that the properties of the point defects outside the clusters are the main results obtained with DLTS methods . 1 . Introduction A great deal of interest has arisen, in recent years, in the application of silicon diodes as position and energy detectors in high-energy physics . These detectors are the ones best suited to face the challenge posed by the exceptional experimental conditions [1] that will prevail in the new generation of colliding beam machines, such as the Large Hadron Collider (LHC) in the LEP tunnel at CERN and the Superconducting Super Collider (SSC) in the USA, both involving multi-TeV proton beams . The radiation hardness of the active medium of the calorimeter is of the greatest importance in view of the high intensities that will particularly exist at these hadron colliders . Neutrons, produced by the interaction of the beam with the environment and the absorber part of the calorimeters, will be responsible for most of the damage caused to the detectors. As regards the SSC, studies on the radiation levels have indeed shown that the neutron fluences per unit area in the region of maximum fluence are of the order of 10 11 -10 13 n/cmz . It is known from experiment that neutron damage is at least two orders of magnitude greater than electron or photon damage, and one order of magnitude greater than that created by charged particles [2]. In silicon * Also at University of Rome, Italy. 0168-9002/89/$03 .50 © Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)
most of the neutron damage is caused by fast neutrons, interacting with the silicon atoms primarily by elastic scattering . Such direct collisions result in up to 10 3 displacements per event, with silicon interstitials moving outwards, leaving behind a vacancy-rich and somewhat disordered zone [3] . The vacancies may combine to form divacancies and larger clusters which may produce defect energy levels in the band gap somewhat different from those of isolated single-defect levels. Moreover, there is experimental evidence that the formation of other point defects, such as the E centre (vacancy-phosphorus complex) and the A centre (vacancy-oxygen complex), may be attributed also, to a large extent, to the vacancy release from defect clusters . In the present paper, point defects produced in several n-type silicon detectors by neutron irradiation are presented . The experimental studies have been carried out using the deep-level transient spectroscopy (DLTS) technique. As will become evident from the forthcoming discussion, DLTS measurements are able to reveal the point-defect character of neutron damage . Different measurement techniques, e .g. the Hall effect, would have to be used in order to observe the presence of defect clusters. We show the spectrum after neutron irradiation for a typical silicon sample . Furthermore, a simple mechanism of annealing due to Tokuda and Usami [4], which VI. RADIATION DAMAGE STUDIES
27 8
E. Borchi et al. / Deep-level transient spectroscopy measurements
is consistent with our data and those of previous experiments [4,5], is partially confirmed. The evidence of the formation of several defects during annealing is also reported .
pF
2. Experimental procedure and results For this investigation p +-n silicon diodes * were used with a nominal area 10 X 10 mm2, a thickness of 400 [tm (made by diffusing boron into phosphorusdoped floating-zone-grown silicon wafers), and resistivities of about 6 kQ cm . The concentrations of dopant in the samples (obtained from capacitance-voltage mea-
lo
surements) are between 6 X 10 11 and 8 X ll cm -3 . Neutron irradiation was performed at room temper252Cf ature using a isotopic neutron source . The emis252 sion spectrum of Cf is shown elsewhere [6]. The total neutron fluences per unit area were between 2 .7 X 10 1°
Fig. 1 . Typical DLTS spectrum after irradiation for the sample number BI ,, (temperature scale is in kelvin) .
and 2.1 X 10 11 n/cm2. The DLTS analysis [7] of the silicon samples was carried out using a DLTS system, DL 4600 * *. We measured the capacitance transient associated with the slow emission of the majority car-
riers after they had been captured in traps. The capture is generated by a repetitive bias-filling pulse (about 1
ms long), during a thermal scan . Various time intervals, called rate windows, are used for the sampling of the capacitance transient. The DLTS spectrum after irradiation, obtained with a time interval for the sampling of 1 ms, is shown in fig. 1 for the sample number Bt9 (hereafter called B) with a resistivity value of 6.61 kQ cm, a dopant concentration of 8.25 X 10 11 cm -3 , and a neutron fluence of 2.7 X 10 1° n/cm2.
The three peaks shown in the spectrum correspond to three different point defects : El , E2, and E3. The activation energy ET is related to the temperature of the peak of the DLTS spectrum, and to the thermal emission rate e by In(e /T 2 ) = -ET/kT+constant, where T is the temperature of the peak. Fig. 2 shows the Arrhenius plot for E1, E2 , and E3 , where e /T 2 is
given as a function of 1000/T. The data are measured using time intervals of 1, 2.5, 5, 12 .5, 20, and 50 ms . The values of the three slopes in the Arrhenius plot are the values of the three activation energies. The E1, E2 , and E3 energies are E. - (0 .14 ± 0.01) eV, E, - (0 .26 ± 0.03) eV, and E, - (0.40 ± 0.02) eV respectively, where E, is the energy of the conduction band . These values are
compared in table 1 with the DLTS results obtained in neutron-irradiated n-type silicon by Tokuda and Usami [4] and by Tulach et al . [5].
* Made by Ansaldo, Italy. * * Manufactured by Bio Rad, UK.
Fig. 2. Arrhenius plot for El , E2 , and E3. Substantial agreement in the behaviour of the pointdefect energies (and also of the relative amplitudes) is shown by this comparison . This indicates that damage is quite independent of the various conditions of irradiation as well as of the material parameters .
Table 1 Comparison of the energy levels (in eV) in n-type silicon detectors obtained with neutron irradiation by the present authors, by Tokuda and Usami [4] and by Tulach et al . [5] Defect code
This work
Tokuda and Usami
Tulach et al.
El E2 E3
E,-(0.14±0.01) E~-(0.26±0.03) E, - (0.40± 0.02)
E~-0 .15 E,-0 .21 E,-0 .39
E~-(0 .18±0.01) E~-(0 .25±0.01) E, - (0 .46 ±0.02)
E. Borchi et al. / Deep-level transient spectroscopy measurements
279
series of mixed isothermal and isochronal annealings were carried out on our silicon samples over the temperature range from 100 ° C to 300 ° C with an annealing time from 1 up to 24 h . However, the results were frequently a function of both the type of sample and the annealing procedures used. Furthermore, two centres, E4 and E 5 , where clearly observed in the DLTS spectra with energies not well determined but close to the energy of the annealed-out centers . In view of this multiplicity of effects, only the main features will be presented here together with a tentative interpretation of the experimental results . The DLTS spectrum of sample B after annealing at 250'C is shown in fig . 4 together with the DLTS spectrum obtained after neutron irradiation. this spectrum clearly shows the presence of the defects E 4 and E 5 with corresponding energy levels of E, - (0 .35 ± 0 .04) eV and E, - (0.48 ± 0 .05) eV, respectively . These values are in relatively good agreement with those estimated by Tokuda and Usami (Ec - 0 .31 eV and Ec - 0 .45 eV) [4].
2
3. Discussion of the results 0
5
10
15
20
mxlol l Fig . 3 . Ratio N/Nd obtained from irradiation of 10 samples as a function of the neutron fluence . The dependence is linear . Fig. 3 shows the ratio NlNd (where N is the trap concentration of defect E 3 , the deepest point defect, and Nd is the donor concentration) obtained from an irradiation of ten samples as a function of the neutron fluence : the plot shows the linear dependence of N/Nd on the fluence. In order to obtain information that may be used to predict silicon-device stability from annealing studies, a
15
05
250
Fig. 4 . The DLTS spectrum of sample B after 250'C annealing is shown and reveals the presence of the E4 and E 5 defects.
The ratio NT/fn (where NT is the defect concentration and fn the neutron fluence per unit area) had roughly the following behaviour during the annealing of sample B . The E 3 centre shows two annealing stages around 150 and 300'C : NT/f assumes the values = 1 .36 after neutron irradiation, = 1 between 100 and 150' C, and = 0 .8 at 250' C. For E Z , an initial value of about 0 .5 appears after neutron irradiation and an initial annealing at 100 ° C for 24 h . Successive isothermal and isochronal annealings produce an increase of Nrlf, Moderate recovery of the defects is observed from 150 to 250 ° C, whereas a rapid decrease of NTlfn is present for temperatures greater than 250 ° C . A similar behaviour is observed for the other silicon detectors under investigation . On this basis we conclude that the annealing features of our samples agree reasonably well with the results of Tokuda and Usami [4] . The energy level and annealing-out temperature of E l are in agreement with those of the A centre ; the energy levels of E Z and E 3 can be associated with the double- and single-minus charge states of divacancy, respectively . Alternatively, according to the suggestion of Tokuda and Usami [4], we could suppose that the E 3 defect contains two different level contributions : the E centre and the single-minus state charge of divacancy . The energy levels of the E centre (E. - 0 .44 eV) and of the divacancy (Ec - 0 .39 eV) are so close that it turns out to be very difficult to resolve them by means of DLTS analysis . Also the value obtained by Tulach et al . [5] for their E 3 energy level (Ec - 0 .46 eV) is consistent with this interpretation . VI . RADIATION DAMAGE STUDIES
280
E. Borchi et al. / Deep-level transient spectroscopy measurements
The growth of the El defect, occurring in silicon detectors at room temperature during storage of a few days, appears to be somewhat independent of the presence of the E3 defect, and it is probably due to the vacancy release following the room-temperature annealing of the defect clusters . On the other hand, the subsequent growth of the Et defect during annealing occurs in the temperature range corresponding to the first stage of annealing of the E3 defect (divacancy) . It is thus highly probable that the Et defect growth upon annealing to about 250'C may be attributed to the vacancy liberation both from defect clusters and from divacancy release. This strongly supports the hypothesis that the El defect should be considered as an A centre . To conclude this section, we observe that our data mainly show point-defect contributions to the neutron damage of silicon detectors. However, as previously mentioned, fast neutron damage in silicon mainly occurs via defect clusters . A cluster model has been proposed many years ago by Gossick [8], in which the space-charge region due to defect clusters forms a potential difference between the cluster and the surrounding undamaged material . For this reason, Gregory et al . [9] have pointed out that a relevant reduction of the probability of the majority carrier capture by defects inside the clusters should characterize cluster zones. Since DLTS response is not observed if carriers do not fill defect energy levels, it appears improbable that DLTS measurements can reveal defects inside the clusters . 4. Conclusions The relevance of the production of divacancies following multiple displacement effects in the primary radiation event due to fast neutron scattering and, indirectly, recombination of two vacancies, appears in our experimental results on neutron-irradiated n-type silicon detectors with fluences up to -- 1011 n/cm2. It has
also appeared from the experimental observation that the high production of divacancies partly suppresses the formation of A centres. The subsequent formation of A centres, in contrast, is due to vacancies released by thermal annealing of defect clusters and divacancy centres. Further experimental investigations are needed, using higher values of the neutron fluence, more systematic annealing procedures and, if possible, making use of additional investigation techniques in order to understand more clearly the complex phenomena associated with radiation damage in silicon detectors due to fast neutrons .
Acknowledgements We wish to acknowledge here useful discussions with A. Penzo, F. Lemeilleur, H.W . Kraner and A. Seidman. We are grateful to R. Raffnsoe, G. Roubaud and J.W .N . Tuyn of the CERN TIS Division for their support in the experimental work .
References [1] E. Borchi et al ., these Proceedings (Int. Conf . on Advanced Technology and Particle Physics, Como, Italy, 1988) Nucl . Instr. and Meth . A279 (1989) 57 . [2] H.W . Kraner, Z. Li and K.U . Posnacker, ibid., p. 266. [3] R.C. Newman, Rep. Prog. Phys. 45 (1982) 1163 . [4] Y. Tokuda and A. Usami, IEEE Trans. Nucl . Sci. NS-28 (1981) 3564 . [5] L. Tulach, H. Frank, B. Sopkc and Z. Prasil, Phys . Status Solidi A100 (1987) K13. [6] E.A . Lorch, Int. J. Appl . Radiat . Isot . 24 (1973) 585. [7] G.L. Miller, D.V. Lang and L.C . Kimmerling, Ann. Rev. Mater. Sci. 30 (1977) 377. [8] B.R . Gossick, J. Appl. Phys . 30 (1959) 1214 . [9] B.L . Gregory, S.S . Naik and W.G . Oldham, IEEE Trans. Nucl . Sci . NS-18 (1971) 50 .