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IBIC analysis of gallium arsenide Schottky diodes
Nuclear Instruments and Methods in Physics Research B 158 (1999) 470±475
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IBIC analysis of gallium arsenide Schottky diodes E. Vittone a,b,*, F. Fizzotti a,b, K. Mirri a, E. Gargioni a,b, P. Polesello a,b, A. LoGiudice a,b, C. Manfredotti a,b, S. Galassini c, P. Rossi d, P. Vanni e, F. Nava a
e
Dipartimento di Fisica Sperimentale, Universita di Torino, INFN-Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy b INFM-Unit a di Torino Universit a, via P. Giuria 1, 10125 Torino, Italy c Facolt a di Scienze, Universit a di Verona, Str. Le Grazie, C a Vignal, Borgo Roma, Verona, Italy d Dipartimento di Fisica, Universit a di Padova, via Marzolo 8, 35131 Padova, Italy e Dipartimento di Fisica, Universit a di Modena, via Campi 213/A, 41100 Modena, Italy
Abstract Semi-insulating (SI) gallium arsenide (GaAs) devices operating as a reverse biased Schottky diode oer an attractive choice as radiation detector at room temperature both in high energy physics experiments and as X-ray image sensors. However, SI GaAs devices contain a high concentration of traps, which decreases the charge collection eciency (cce), and aects the energy resolution of such detectors working as nuclear spectrometers. In this paper we present a detailed investigation of the spatial uniformity of the cce carried out by analysing ion beam induced charge (IBIC) space maps obtained by scanning a focused 2 MeV proton microbeam on a SI n-GaAs Schottky diode. The microbeam irradiated both the front (Schottky) and back (ohmic) contacts in order to evaluate the transport properties of both electrons and holes generated by ionisation. The IBIC space maps show a clear non-uniformity of the cce. The poor energy resolution previously observed in such detectors working as alpha particle spectrometers is ascribed to the presence of two dierent ``phases'' in the material, which produce two distinct collection eciency spectra. Such ``phases'' show dierent behaviour as a function of the applied bias voltage which is most likely due to the dierent electric ®eld dependence of the relevant capture cross sections of the trapping centres for both charge carriers. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 41.75.Ak; 72.20.Jv Keywords: GaAs nuclear detectors; Ion beam induced charge
1. Introduction Semi-insulating (SI) n-GaAs operating as a reverse bias Schottky barrier diode oers an attractive choice as a radiation detector for high energy * Corresponding author. Tel.: +39-0116707317; fax: +39116691104; e-mail: [email protected]
physics experiments [1] and as a room temperature X-ray and c-ray spectroscope for medical applications [2]. A factor which limits the performance of such a detector regards the inhomogeneity of the base SI GaAs material which has a great impact on the energy resolution of the detector [3]. Micro-IBIC by means of a low energy (2 MeV) proton
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 4 - 5
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
microbeam, represents a powerful tool in this case since it allows the distribution of the charge collection eciency (cce) to be mapped [4] and, consequently, it allows the morphological structure of the material used for detector realisation to be analysed. In this paper we present cce maps of a SI GaAs detectors obtained at dierent bias voltages. A 2 MeV proton microbeam was scanned over both the front (Schottky) and back (ohmic) electrodes in order to study the transport properties of electrons and holes, respectively. The IBIC maps were analysed in a rigorous way in order to evaluate the homogeneity of the material and to study the evolution of the cce spatial distribution as a function of the applied bias voltage. 2. Experimental The detectors studied in the present work were made on commercially available SI LEC undoped á1 0 0ñ oriented GaAs substrates with n-type resistivity q 107 X cm, supplied by Sumitomo. The detectors investigated are (100 6) lm thick with: (i) circular Schottky contacts (2 mm in diameter) realised on the front side by Ti/Pt/Au metallisation and (ii) uniform ohmic contacts realised on the whole back surface by e-beam deposited Au/Ge/Ni metallisation. A 2 mm hole on the sample holder was made in order to allow the back contact to be irradiated as shown in Fig. 1.
Fig. 1. Schematic representation of IBIC sample geometry. The trapezoid represents the active region whose thickness (depletion region) is a function of bias voltage.
471
The performance of the detector as a spectrometer has been tested by measuring the FWHM energy resolution and the cce with a 2 MeV focused proton microprobe at the Laboratori Nazionali di Legnaro [5,6]. The focused beam was scanned over the Schottky and ohmic contacts, with a shaping time of 500 ns. The relevant experimental set-ups are described in Refs. [1,4]. The ion beam induced charge (IBIC) measurements were performed with the proton beam intensity maintained as low as possible (less than 300 protons/s) in order to avoid saturation and pile-up of the electronic chain and space charge creation [1]. The dimensions of the 2 MeV proton microbeam spot were about 8 lm2 and the charge signal was recorded as a function of the beam position. The cce measured for protons impinging on the front electrode is primarily due to the electron drift in the detector, whereas for the back side exposure, the cce is primarily due to hole drift. The cce was evaluated by normalising the pulse height to the response, obtained in the same experimental conditions, of a Si surface barrier detector whose cce was assumed equal to 100%. 3. Results and discussion The cce spectra in the case of frontal irradiation are shown in Fig. 2a. As the bias increases the median of the spectra moves towards higher values of cce. The electric ®eld displays a rectangular shape in the depletion region [7], full depletion occurring at a threshold voltage Vd of about 100 V [1]. This explains both the low cce measured at a bias voltage of 50 V and the absence of signals when protons hit the back surface at low bias voltages as shown in Fig. 2b. The range of 2 MeV protons in the GaAs sample is about 30 lm and, if the depletion region edge penetrates less than 70 lm from the Schottky contacts, carriers are not generated within the active region. An important remark concerns the spectral distribution width of cces. In Fig. 3 the cces spectrum relevant to frontal irradiation at a bias voltage of 300 V is shown with the IBIC map obtained dividing the spectrum in two regions of interest (ROIs): ROI1 which contains the long,
472
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
Fig. 2. Total cce spectra of 2 MeV protons scanned on the Schottky contact (a) and on the ohmic contact (b) at dierent bias voltages.
low cce, tail and ROI2 which contains the peaked structure. Pulses belonging to ROI1 are generated in regions close to the ``white'' big spot, which corresponds to the electrical contact, made by silver paste and in regions located at the border of the electrode. The former are clearly due to the degrading thickness of the silver paste which makes such regions partially opaque to proton transmission. The latter are more likely due to the direction of the electric ®eld which is not perfectly perpendicular to the top electrode because of the complete metallisation of the back electrode (see Fig. 1), and the active region spreads laterally as discussed in [8]. In order to study the homogeneity of the GaAs detector, we have considered only the peaked spectral distribution which was divided into four ROIs corresponding to the four quartiles of the
Fig. 3. Total cce spectrum of 2 MeV protons scanned on the Schottky contact at 300 V bias and IBIC map obtained considering two ROIs relevant to the low cce tail and to the peaked structure.
distributions (the pth quartile is de®ned as the value n of cce such that if F is the cumulative distribution of the cce spectrum, F(n) p ´ 0.25). This criterion is rigorous and allows an unbiased comparison among various maps to be carried out. Fig. 4 shows four maps for each bias voltage relevant to the four quartiles of the spectral distributions illustrated in Fig. 2a (frontal irradia-
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
473
Fig. 4. IBIC maps at dierent bias voltages relevant to the frontal irradiation obtained by considering the four quartiles of the spectral distributions shown in Fig. 2a. The quartiles are indicated in Table 1.
tion). The lower boundaries of such ROIs are listed in Table 1. The maps show the inhomogeneity of the electronic response of such a GaAs sample. This is more evident at low bias voltages in correspondence to the relatively large cces spectra. In particular, the ®rst and forth quartile of the spectral distributions are almost complementary. At high bias voltages, i.e. at peaked cce spectra close to 100%, the maps appear to be slightly more uniform as can be seen in maps relevant to the two central quartiles. This is probably due to the electric ®eld dependence of the cross section of some traps [9]. It is worth noting that the local increase of the electric ®eld due to the presence of the electrode border is responsible for the circular
pro®le visible in the maps relevant to the fourth quartiles. A similar inhomogeneity can be observed in maps relevant to the back irradiation as shown in Fig. 5. This corroborates the hypothesis that the inhomogeneity in the spectral response is inherent to the bulk material and does not concern the surface or the contact formation. Such results indicate the presence of two spatially distinct ``electronic phases'', in which the charge collection is dierent and which are the responsible of the poor spectral resolution of such a detector at low bias voltages. Furthermore, the hypothesis that such ``electronic phases'' are spatially distinct and dominated by dierent kinds of
474
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
Table 1 Lower limits of the quartiles of the spectral distributions shown in Fig. 2a and b Bias voltage
Quartiles 1
2
3
4
Frontal irradiation 50 100 200 300
0 78.0 85.0 91.0
27.2 84.1 91.1 95.8
28.8 85.7 91.8 96.5
30.8 87.0 92.3 97.0
Back irradiation 200 300
83.0 87.0
88.2 92.4
89.2 93.2
90.4 94.0
Fig. 5. IBIC maps at dierent bias voltages relevant to the back irradiation obtained by considering the four quartiles of the spectral distributions shown in Fig. 2b. The quartiles are indicated in Table 1.
traps is corroborated by ``local spectra'' obtained by scanning the proton beam over small regions. If we consider only small squares of (70 ´ 70) lm2 centred at pixels with co-ordinates (103,125) (zone A) and (118,111) (zone B) with the relevant spectra as shown in Fig. 6, then zone A carriers are collected in a more ecient way than in zone B: in fact, zones A and B belong to the maps relevant to the 1st and 4th quartiles maps in Fig. 4, respectively. Moreover, the spectra are much narrower than the spectra relevant to the total device, i.e. zones A and B contain a single ``electronic phase''. Such phases are likely characterised by dierent
trap levels, and not by dierent trap concentrations, since the behaviour of the cce response as a function of electric ®eld is quite dierent for the two zones, as can be observed at the bottom of Fig. 6. 4. Conclusions A SI n-GaAs operating as a reverse bias Schottky barrier diode has been analysed by means of the micro-IBIC technique in order to determine the eect of inhomogeneities on the
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
475
structure whose median increases as the bias voltage increases and whose width decreases at high electric ®eld. The IBIC maps prove that the spectral resolution is strongly in¯uenced by the material inhomogeneity. The analysis of such maps leads us to conclude that the material behaves as a ``mixture'' of two ``electronic phases'' which probably contain dierent kinds of traps. This conclusion is corroborated by very sharp spectra obtained by studying the response of very small regions, which show a dierent dependence on the applied electric ®eld.
References
Fig. 6. cce spectra relevant to zone A and zone B at dierent bias voltages. Frontal irradiation. Bottom: behaviours of the mean cces as function of bias voltage.
performances of such a detector as a nuclear spectrometer. The cce spectra from 2 MeV protons show a long tail, extending throughout the spectrum, which is due to border eects, and a peaked
[1] K.H. Smith, Nucl. Instr. and Meth. A 383 (1996) 343. [2] C.M. Buttar, Nucl. Instr. and Meth. A 395 (1997) 1 and references therein. [3] F. Nava, P. Vanni, C. Canali, G. Apostolo, C. Manfredotti, P. Polesello, E. Vittone, IEEE Trans. Nucl. Sci. 45 (3) (1998) 609. [4] C. Manfredotti, F. Fizzotti, P. Polesello, E. Vittone, M. Truccato, A. LoGiudice, M. Jaksic, P. Rossi, Nucl. Instr. and Meth. B 136±138 (1998) 1333 and references therein. [5] D. Bollini, F. Cervellera, G.P. Egeni, P. Mazzoldi, G. Moschini, P. Rossi, V. Rudello, Nucl. Instr. and Meth. A 328 (1993) 173. [6] P. Boccaccio, D. Bollini, D. Ceccato, G.P. Egeni, P. Rossi, V. Rudello, M. Viviani, Nucl. Instr. and Meth. B 109/110 (1996) 94. [7] A. Castaldini, A. Cavallini, L. Polenta, C. Canali, C. del Papa, F. Nava, Phys. Rev. B 56 (15) (1997) 9201. [8] A. Cola, Nucl. Instr. and Meth. A 410 (1993) 85. [9] V. Ya Prinz, S.N. Rechkunov, Phys. Stat. Sol. B 118 (1983) 159.
www.elsevier.nl/locate/nimb
IBIC analysis of gallium arsenide Schottky diodes E. Vittone a,b,*, F. Fizzotti a,b, K. Mirri a, E. Gargioni a,b, P. Polesello a,b, A. LoGiudice a,b, C. Manfredotti a,b, S. Galassini c, P. Rossi d, P. Vanni e, F. Nava a
e
Dipartimento di Fisica Sperimentale, Universita di Torino, INFN-Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy b INFM-Unit a di Torino Universit a, via P. Giuria 1, 10125 Torino, Italy c Facolt a di Scienze, Universit a di Verona, Str. Le Grazie, C a Vignal, Borgo Roma, Verona, Italy d Dipartimento di Fisica, Universit a di Padova, via Marzolo 8, 35131 Padova, Italy e Dipartimento di Fisica, Universit a di Modena, via Campi 213/A, 41100 Modena, Italy
Abstract Semi-insulating (SI) gallium arsenide (GaAs) devices operating as a reverse biased Schottky diode oer an attractive choice as radiation detector at room temperature both in high energy physics experiments and as X-ray image sensors. However, SI GaAs devices contain a high concentration of traps, which decreases the charge collection eciency (cce), and aects the energy resolution of such detectors working as nuclear spectrometers. In this paper we present a detailed investigation of the spatial uniformity of the cce carried out by analysing ion beam induced charge (IBIC) space maps obtained by scanning a focused 2 MeV proton microbeam on a SI n-GaAs Schottky diode. The microbeam irradiated both the front (Schottky) and back (ohmic) contacts in order to evaluate the transport properties of both electrons and holes generated by ionisation. The IBIC space maps show a clear non-uniformity of the cce. The poor energy resolution previously observed in such detectors working as alpha particle spectrometers is ascribed to the presence of two dierent ``phases'' in the material, which produce two distinct collection eciency spectra. Such ``phases'' show dierent behaviour as a function of the applied bias voltage which is most likely due to the dierent electric ®eld dependence of the relevant capture cross sections of the trapping centres for both charge carriers. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 41.75.Ak; 72.20.Jv Keywords: GaAs nuclear detectors; Ion beam induced charge
1. Introduction Semi-insulating (SI) n-GaAs operating as a reverse bias Schottky barrier diode oers an attractive choice as a radiation detector for high energy * Corresponding author. Tel.: +39-0116707317; fax: +39116691104; e-mail: [email protected]
physics experiments [1] and as a room temperature X-ray and c-ray spectroscope for medical applications [2]. A factor which limits the performance of such a detector regards the inhomogeneity of the base SI GaAs material which has a great impact on the energy resolution of the detector [3]. Micro-IBIC by means of a low energy (2 MeV) proton
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 4 - 5
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
microbeam, represents a powerful tool in this case since it allows the distribution of the charge collection eciency (cce) to be mapped [4] and, consequently, it allows the morphological structure of the material used for detector realisation to be analysed. In this paper we present cce maps of a SI GaAs detectors obtained at dierent bias voltages. A 2 MeV proton microbeam was scanned over both the front (Schottky) and back (ohmic) electrodes in order to study the transport properties of electrons and holes, respectively. The IBIC maps were analysed in a rigorous way in order to evaluate the homogeneity of the material and to study the evolution of the cce spatial distribution as a function of the applied bias voltage. 2. Experimental The detectors studied in the present work were made on commercially available SI LEC undoped á1 0 0ñ oriented GaAs substrates with n-type resistivity q 107 X cm, supplied by Sumitomo. The detectors investigated are (100 6) lm thick with: (i) circular Schottky contacts (2 mm in diameter) realised on the front side by Ti/Pt/Au metallisation and (ii) uniform ohmic contacts realised on the whole back surface by e-beam deposited Au/Ge/Ni metallisation. A 2 mm hole on the sample holder was made in order to allow the back contact to be irradiated as shown in Fig. 1.
Fig. 1. Schematic representation of IBIC sample geometry. The trapezoid represents the active region whose thickness (depletion region) is a function of bias voltage.
471
The performance of the detector as a spectrometer has been tested by measuring the FWHM energy resolution and the cce with a 2 MeV focused proton microprobe at the Laboratori Nazionali di Legnaro [5,6]. The focused beam was scanned over the Schottky and ohmic contacts, with a shaping time of 500 ns. The relevant experimental set-ups are described in Refs. [1,4]. The ion beam induced charge (IBIC) measurements were performed with the proton beam intensity maintained as low as possible (less than 300 protons/s) in order to avoid saturation and pile-up of the electronic chain and space charge creation [1]. The dimensions of the 2 MeV proton microbeam spot were about 8 lm2 and the charge signal was recorded as a function of the beam position. The cce measured for protons impinging on the front electrode is primarily due to the electron drift in the detector, whereas for the back side exposure, the cce is primarily due to hole drift. The cce was evaluated by normalising the pulse height to the response, obtained in the same experimental conditions, of a Si surface barrier detector whose cce was assumed equal to 100%. 3. Results and discussion The cce spectra in the case of frontal irradiation are shown in Fig. 2a. As the bias increases the median of the spectra moves towards higher values of cce. The electric ®eld displays a rectangular shape in the depletion region [7], full depletion occurring at a threshold voltage Vd of about 100 V [1]. This explains both the low cce measured at a bias voltage of 50 V and the absence of signals when protons hit the back surface at low bias voltages as shown in Fig. 2b. The range of 2 MeV protons in the GaAs sample is about 30 lm and, if the depletion region edge penetrates less than 70 lm from the Schottky contacts, carriers are not generated within the active region. An important remark concerns the spectral distribution width of cces. In Fig. 3 the cces spectrum relevant to frontal irradiation at a bias voltage of 300 V is shown with the IBIC map obtained dividing the spectrum in two regions of interest (ROIs): ROI1 which contains the long,
472
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
Fig. 2. Total cce spectra of 2 MeV protons scanned on the Schottky contact (a) and on the ohmic contact (b) at dierent bias voltages.
low cce, tail and ROI2 which contains the peaked structure. Pulses belonging to ROI1 are generated in regions close to the ``white'' big spot, which corresponds to the electrical contact, made by silver paste and in regions located at the border of the electrode. The former are clearly due to the degrading thickness of the silver paste which makes such regions partially opaque to proton transmission. The latter are more likely due to the direction of the electric ®eld which is not perfectly perpendicular to the top electrode because of the complete metallisation of the back electrode (see Fig. 1), and the active region spreads laterally as discussed in [8]. In order to study the homogeneity of the GaAs detector, we have considered only the peaked spectral distribution which was divided into four ROIs corresponding to the four quartiles of the
Fig. 3. Total cce spectrum of 2 MeV protons scanned on the Schottky contact at 300 V bias and IBIC map obtained considering two ROIs relevant to the low cce tail and to the peaked structure.
distributions (the pth quartile is de®ned as the value n of cce such that if F is the cumulative distribution of the cce spectrum, F(n) p ´ 0.25). This criterion is rigorous and allows an unbiased comparison among various maps to be carried out. Fig. 4 shows four maps for each bias voltage relevant to the four quartiles of the spectral distributions illustrated in Fig. 2a (frontal irradia-
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
473
Fig. 4. IBIC maps at dierent bias voltages relevant to the frontal irradiation obtained by considering the four quartiles of the spectral distributions shown in Fig. 2a. The quartiles are indicated in Table 1.
tion). The lower boundaries of such ROIs are listed in Table 1. The maps show the inhomogeneity of the electronic response of such a GaAs sample. This is more evident at low bias voltages in correspondence to the relatively large cces spectra. In particular, the ®rst and forth quartile of the spectral distributions are almost complementary. At high bias voltages, i.e. at peaked cce spectra close to 100%, the maps appear to be slightly more uniform as can be seen in maps relevant to the two central quartiles. This is probably due to the electric ®eld dependence of the cross section of some traps [9]. It is worth noting that the local increase of the electric ®eld due to the presence of the electrode border is responsible for the circular
pro®le visible in the maps relevant to the fourth quartiles. A similar inhomogeneity can be observed in maps relevant to the back irradiation as shown in Fig. 5. This corroborates the hypothesis that the inhomogeneity in the spectral response is inherent to the bulk material and does not concern the surface or the contact formation. Such results indicate the presence of two spatially distinct ``electronic phases'', in which the charge collection is dierent and which are the responsible of the poor spectral resolution of such a detector at low bias voltages. Furthermore, the hypothesis that such ``electronic phases'' are spatially distinct and dominated by dierent kinds of
474
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
Table 1 Lower limits of the quartiles of the spectral distributions shown in Fig. 2a and b Bias voltage
Quartiles 1
2
3
4
Frontal irradiation 50 100 200 300
0 78.0 85.0 91.0
27.2 84.1 91.1 95.8
28.8 85.7 91.8 96.5
30.8 87.0 92.3 97.0
Back irradiation 200 300
83.0 87.0
88.2 92.4
89.2 93.2
90.4 94.0
Fig. 5. IBIC maps at dierent bias voltages relevant to the back irradiation obtained by considering the four quartiles of the spectral distributions shown in Fig. 2b. The quartiles are indicated in Table 1.
traps is corroborated by ``local spectra'' obtained by scanning the proton beam over small regions. If we consider only small squares of (70 ´ 70) lm2 centred at pixels with co-ordinates (103,125) (zone A) and (118,111) (zone B) with the relevant spectra as shown in Fig. 6, then zone A carriers are collected in a more ecient way than in zone B: in fact, zones A and B belong to the maps relevant to the 1st and 4th quartiles maps in Fig. 4, respectively. Moreover, the spectra are much narrower than the spectra relevant to the total device, i.e. zones A and B contain a single ``electronic phase''. Such phases are likely characterised by dierent
trap levels, and not by dierent trap concentrations, since the behaviour of the cce response as a function of electric ®eld is quite dierent for the two zones, as can be observed at the bottom of Fig. 6. 4. Conclusions A SI n-GaAs operating as a reverse bias Schottky barrier diode has been analysed by means of the micro-IBIC technique in order to determine the eect of inhomogeneities on the
E. Vittone et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 470±475
475
structure whose median increases as the bias voltage increases and whose width decreases at high electric ®eld. The IBIC maps prove that the spectral resolution is strongly in¯uenced by the material inhomogeneity. The analysis of such maps leads us to conclude that the material behaves as a ``mixture'' of two ``electronic phases'' which probably contain dierent kinds of traps. This conclusion is corroborated by very sharp spectra obtained by studying the response of very small regions, which show a dierent dependence on the applied electric ®eld.
References
Fig. 6. cce spectra relevant to zone A and zone B at dierent bias voltages. Frontal irradiation. Bottom: behaviours of the mean cces as function of bias voltage.
performances of such a detector as a nuclear spectrometer. The cce spectra from 2 MeV protons show a long tail, extending throughout the spectrum, which is due to border eects, and a peaked
[1] K.H. Smith, Nucl. Instr. and Meth. A 383 (1996) 343. [2] C.M. Buttar, Nucl. Instr. and Meth. A 395 (1997) 1 and references therein. [3] F. Nava, P. Vanni, C. Canali, G. Apostolo, C. Manfredotti, P. Polesello, E. Vittone, IEEE Trans. Nucl. Sci. 45 (3) (1998) 609. [4] C. Manfredotti, F. Fizzotti, P. Polesello, E. Vittone, M. Truccato, A. LoGiudice, M. Jaksic, P. Rossi, Nucl. Instr. and Meth. B 136±138 (1998) 1333 and references therein. [5] D. Bollini, F. Cervellera, G.P. Egeni, P. Mazzoldi, G. Moschini, P. Rossi, V. Rudello, Nucl. Instr. and Meth. A 328 (1993) 173. [6] P. Boccaccio, D. Bollini, D. Ceccato, G.P. Egeni, P. Rossi, V. Rudello, M. Viviani, Nucl. Instr. and Meth. B 109/110 (1996) 94. [7] A. Castaldini, A. Cavallini, L. Polenta, C. Canali, C. del Papa, F. Nava, Phys. Rev. B 56 (15) (1997) 9201. [8] A. Cola, Nucl. Instr. and Meth. A 410 (1993) 85. [9] V. Ya Prinz, S.N. Rechkunov, Phys. Stat. Sol. B 118 (1983) 159.