- Email: [email protected]
An optical-beam-induced-current study of active region and charge collection efficiency of GaAs particle detectors
Nuclear Instruments and Methods in Physics Research A 355 (1995) 420-424
& *H ELSBVIER
NUCLEAR INSTRUMENTS a METHODS IN PMYSw=S RESEARCH SectlooA
optical-beam-induced-current study of active region and charge collection efficiency of GaAs particle detectors An
M. Alietti d, L. Berluti ‘, C. Canali f, A. Castaldini h, A. Cavallini b, A. Cetronio a, S. D’Auria by1,C. de1 Papa bq*, C. Lanzieri a, F. Nava e,‘, M. Proia ‘, P. Rinaldi b, A. Zichichi ’ ADirezione Ricerca, Alenia SPA, Rome, It& ’ Dipartimenio di Fisica dell’u~il~er~it~ and INFN Bologna, Iialy ’ CERN? Genesa, Switzerlund ’ Dipartimenlo di Fisica dell’llnillersitb Modena, Italy e Dipartimento di Scienze dell ‘Ingegneria dell’Unir~er.sitirdi Modena. Italy f Dipartimento di Scienze dell’lngegneria dell ‘lJnil,ersit2 di Modena, Italy Received 18 August 1994
Abstract In a recent paper, it was described
how semi-insulating (SI) liquid encapsulated Czochraiski (LEC) gallium arsenide particle detectors made by Alenia SpA have been tested and found to be understandable in terms of one function: the local charge collection efficiency. We have now measured, always within the context of the RD8 experiment, the local charge collection efficiency for these diodes using a technique called OBIC (optical beam induced current) and derived the active region width (W) as a function of bias in a range of values lower than those obtainable with particles. Using data both from OBIC and from the previous paper, W is found to behave approximately linearly as a function of bias larger than about 50 V. Below this value, W follows a square root law as a function of bias.
1. introduction Semi-insulating (SI) liquid encapsulated Czochralski (LEC) GaAs partieie detectors were originally proposed for use at the Large Hadron Collider (LHC) experiments, where radiation hardness is of primary concern; until now they have been shown to have the following properties: (i) full detection efficiency for minimum ionizing particles (mips) [ll, (ii) radiation hardness to high doses of y and neutrons [21, (iii) partial charge collection efficiency (ccc), i.e. only a fraction of the charge released by a particle gets collected [1,2]. We define three global ccc’s for 01 and p particles and X-rays (E”, E@, cx) as the fraction of the charge produced in the detector by the particle that is actually coilected and measured [3].
* Corresponding author. ’ Now at the University of Glasgow, UK. ’ Also at INFN Bologna, Italy. 0168-9002/95/$09.50
Point (iii) is due to two facts: (a) The electric collection field inside the detector extends through the detector only at high voltages; in general the active region is a function of bias and grows with it. This fact was unexpected owing to the high resistivity of the bulk material. (b) There is trapping even where there is field [4]. Deep energy levels near the middle of the forbidden gap, recently measured on our detectors [5], cause these two problems by getting ionized when the bias is applied to the detector, thus preventing the collection field from extending through the detector [6,7] and acting as traps, in addition to the shallower levels. In a previous paper [3], we have experimentally investigated the behaviour of the width (W) of the active region (i.e. the region where the charge collection is nonzero) as a function of the appiied bias greater than 75 V, for detectors built by Atenia SpA on Sumitomo SI LEC material. We have also shown that the local charge collection, E(X), i.e. the fraction of the charge of an eh pair produced at a coordinate x inside the substrate that is actually collected, is the function that correlates the global ccc’s, cu. F@, 8’. We have now measured E(X) directly with the OBIC
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(94)01123-O
M. Alietti et al. / Nucl. Instr. and Meth. in Phys. Rex A 355 (1995) 420-424
421
lock-in amplifier and then measured with an ADC, read out by a computer. Further details can be found in Ref. [9].
3. Results
Fig. 1. Optical beam induced current setup. The beam scans the sample’s cleaved surface in the x direction and the phot~u~ent signal is amplified and then computer-stored.
technique [8,9] for the same Alenia detectors and shown experimentally that W can be calculated from E(X).
2. Experimenta! method Detectors, built by Alenia SpA on Sumitomo SI LEC material, are 3 mm diameter, 215 p,m thick dots with Schottky metall~ation; the ohmic metallization covers the entire back of the chip (4 roundels per 1 cm2 chip). A description of the manufacturing technique and metals used is given in Ref. [5]. A detector can be cleaved (Fig. 1) through the contact so as to have an edge illuminated by a narrow (1 pm diameter) beam of light. Scanning the edge with the beam, starting from the Schottky (n = 0) to the ohmic contact (x = t), a charge is produced and therefore a current is induced in the field inside the detector. The value of the current depends on the flux and spot size of the beam and the electric field, i.e. the bias applied, and finally on e(x). In fact, E(X) = Z(x)/(e#), where e is the electron charge, # is the beam flux (photons cmP2 s-t), and S is the beam area; Z=Z(x) is the current measured when the beam hits the edge of the GaAs at coordinate x. In general d, is not well known, since it represents the number of photons actually penetrating the material: that is the incident beam, easily measured, minus the reflected part, difficult to measure. Moreover the reflected fraction of the light may change with position so that CF,is a weak function of x. We assume here, however, that (It, is constant; Z = Z(x) thus represents the charge collection efficiency (ccc) on an arbitrary scale. The light is generated by a halogen lamp. By varying a filter, we can change the wavelength of the light from A = 500 nm to A = 900 nm, thus changing its abso~tion length in GaAs from a fraction of a micron to many microns. We have used A = 700 nm in the reported me~~ements (penetration depth was 0.8 pm) in order to optimize the ratio between spatial resolution and generation volume [8]. The beam is chopped by a mechanical chopper at a frequency of 72 Hz. The long current pulse is amplified with a
To test the technique, we have decided to first do an exercise using a commercial 370 pm thick Si detector. Fig. 2 shows the measured photocurrent in the Si detector as a function of the position of the light beam; this gives the behaviour of the function &= E(I), i.e. the local charge collection as a function of the depth inside the diode. Since the photocurrent goes to zero once a value of x = W is exceeded, i.e. where the electric field becomes zero, from these current profiles we can measure W as a function of bias; W is the width at half height. Fig. 3 shows W as a function of bias. The resuit is consistent with the expected square root behaviour, the fit to the data is calculated with the net ionized charge carrier concentration: No -N = 9 X IO” cme3, in good agreement with the electricaliropetiies of our material. With the same method for the Si detector as used to get Fig. 3 we can now measure the active region width W of GaAs detectors from the photocurrent curves of Fig. 4. Before plotting W as a function of bias, a correction is necessary: in fact the voltage applied to the diode is not equal to the voltage across the active region. Given the very high resistivity of the base material, p = lo7 fI cm far higher than that of the best silicon, the reverse current (Fig. 5) of the diode produces a sizeable voltage drop across the non-active volume of the diode (see the scheme in Fig. 6). Since we have measured W, we know t- W (t = thickness of the diode = 215 km), we can calculate the serial resistance Z?,, from p, t - W and the diode
Fig. 2. The photocurrentprofiles I = Z(x) as measured at various biases with OBIC for a commercial Si detector.
422
M. Alietti et al. /Nucl. Instr. and Meth. in Phys. Res. A 355 llW5f 420-424
Si#FN24
0
v, WI
Fig. 5. The I-V
characteristic
curve of the 215 pm diode under
test.
Fig. 3. Depleted region width of the Si detector of Fig. 2 as a function of bias, as measured from the photocurrent profiles. The
overlapped curve is the fit to the data with a net ionized carrier density No - NA=9X 10” cm-3.
surface, and the voltage drop follows from the value of the reverse current. Fig, 6 shows the correction, V,, as a function of the overall applied voltage, V,. The general behaviour of the correction V, is easy to understand: once the detector is totally active the correction is zero; the correction is also zero when the applied bias is zero (no reverse current); the correction is therefore a bell-shaped curve with a maximum whose value depends on the reverse current. We can now plot W as a function of V,, = V, - V,; this is the effective voltage drop across the
space-charge region. Fig. 7 shows the results on W as a function of VT, and as a function of V,. As can be seen from Fig. 7, at low voltages, V,, < 50 V, W increases with the root square of V,, and with a corresponding density of ionized ND - NA = 1013 cmF3, whilst for V, > 50 V, W increases linearly with reverse bias voltage [9]. The result of the OBIC measurement is reported in Fig. 8 (points below 100 V), where we also show the results from Ref. [3] (points above 75 V), i.e. the measurements of the active width carried out with particles: counting rates of X-rays on the front and the back of the diode and the ratio between the globa cues of betas and X-rays multiplied by
28 >” 6
4
2
0 0
20
40
80
100
10
v*w
Fig. 4. The photocurrent profiles I = I(x) for Alenia GaAs detectors (21.5 km) as measured with OBIC for various biases.
Fig. 6. The voltage drop across the resistor R,, representing the resistance of the non-active thickness of GaAs as a function of the applied voltage. The electrical scheme used is also shown. R, is the equivalent resistance that takes into account the leakage current.
M. Atietti et al. / Nuci. Instr. and M&h. in Phys. Res. A 3.53(1995) 420-424
0
oi
0
,
,
I
20
40
60
,
a
/
I
100
so
Bias(V) Fig. 7. Active region width as a function of bias voltage. The black squares indicate W as a function of V,, whilst the open circles indicate W as a a unction of V,, = V, - V,, which is the voltage drop across the space-charge region. The dashed line has been obtained fitting the open circle curve with a net ionized carrier density, iv, - NA= lOi cmm3.
the detector thickness. The data of Ref. [3] have also been corrected for the voltage drop across the non-active part of the diode. The data appear to fall on straight lines with the same slope, only just displaced from each other: an overall straight line from a bias of 200 V down to about 50 V. The displacement of the two lines is irrelevant since it only
200 -
423
reflects the way W has been calculated from the photocurrent curves and particle data: what is relevant is the identical slope of the two measurements. A linear behaviour is, of course, a very different behaviour from that of Si detectors, which has to be understood on the basis of a model of the electric field [6]. In Fig. 4, we have shown the curves E = E(X) obtained for different values of the bias detector. Some comments on these curves are now necessary. First of ail, let us notice that the rise of these curves, both in Si and @As, is somewhat slow due to the actual width of the beam and the photo~u~ent induced when the optical beam approaches the interface between the Schottky metai~~ation and the semiconductor. The slow drop is due to charge carriers produced by the light outside the actual active region, diffusing back into the active region and being partially collected. Another important point shown by these curves is that E(X) in GaAs grows with increasing X, before dropping at the edge of the active region; apparently we collect charge better even if the electric field is not increasing: the reason can perhaps be ascribed to less trapping for holes than for electrons, so that as the weight of holes on the total collected charge increases with the distance from the Schottky barrier, so does E(X). We have to point out that quaIitativeiy these curves are in agreement with all we know on the global cces. The width of the active area is apparently linear in the bias [3]. The value of e(x), at x = 20 pm, i.e. Ed, since the range of 5.5 MeV ~1s is 20 pm in GaAs, saturates at higher voltage than in the case of silicon as it should 131. The average value of E(X) over W, i.e. 8’ = l/WJow E(X) dx (for a discussion on this point, see Ref. [3]) does not appear to be too different from Ed. The general shape of these curves has been compared with what we can deduce from previous data obtained with protons of different energy, i.e. penetration [IO] and found to be in qualitative agreement. It is worth remembering that protons of different energy are indeed a probe at different depths: their ccc can be considered the local charge collection at different depths.
150-
4. Conclusions
3 :
loo-
50-
o! 0
,
SO
I
100
,
150
I
1
200
v, WI Fig. 8. The active region width, W, calculated from the data of Fig. 4 (points below 100 V), plotted as a function of the corrected bias and compared with the results obtained in Ref. [3] (points above 75 V).
We have continued our experimental investigation of the detectors made by Alenia SpA on Sumitomo material encouraged by their large ccc for mips (75%). Their general behaviour is well understood in terms of the locai charge collection, now directly measured with the OBIC technique. The behaviour of the active region as a function of bias (approximately linear) is very different from the behaviour of the depleted region of a Si detector as a function of bias. It will also be very interesting to try to fit the experimental curves E = E(X), using the assumption of E-field dependent capture cross sections of the carriers inside the GaAs.
424
M. Alietti et al. /h&l.
Inutr. and Meth. in Phys. Rex A 355 1199.5) 4.?0-424
Acknowledgements We wish to thank Prof. Ken Smith (Glasgow University), Prof. K. Luebelsmeyer and Dr. M. Toporowsky (Aachen University) for very interesting discussions.
References [ll R. Bertin et al., Nucl. Instr. and Meth. A 294 (1990) 211; K.W. Benz, Nucl. Instr. and Meth. A 322 (1992) 493; D.S. McGregor et al., Nucl. Instr. and Meth. A 322 (1992) 483; D.S. McGregor et al., IEEE Trans. Nucl. Sci. NS-3Y f 1992) 1226. @I S.P. Beaumont et al., Nucl. Instr. and Meth. A 322 (1992) 472. [31L. Berluti et al., Nucl. Instr. and Meth. A 354 (1995) 364. I41C. de1 Papa and S. D’Auria, Charge collection and electric field in GaAs particle detectors, Proc. 20th Eloisatron Work-
shop on GaAs Detectors and Electronics for High-Energy Physics (World Scientific, Singapore, 1993) p. 246; K. Berwick et at., Imaging of high field in SI GaAs particle detectors, Proc. 5th Int. Conf. on Defect Recognition and Image Processing in Semiconductors and Devices, Santander, Spain, September 1993 (Institute of Physics Conference Series No. 135, London, 1994) p. 306. @I C. Canali et al., Influence of electron traps on charge coliection efficiency in GaAs radiation detectors, preprint CERNPPE/Y4-50, Nucl. Instr. and Meth. A 349 (1994) 156. 161Th. Kubicki et al., NucI. Instr. and Meth. A 345 (1994) 468. and energy [71A. Castaldini et al., On the trap concentration level evaluation in LEC semi-insulating gallium arsenide, submitted to J. Appl. Phys. 181 7’. Wiison and C. Sheppard, Scanning optical microscopy (Academic Press, London, 1984). Dl A. Castaldini et al., Bias dependence of the depletion layer width in semi-insulating GaAs by charge collection scanning microscopy. Submitted to Scanning Microscopy. [lOI S.P. Beaumont et al., Nucl. Instr. and Meth. A 326 (313) 1993.
& *H ELSBVIER
NUCLEAR INSTRUMENTS a METHODS IN PMYSw=S RESEARCH SectlooA
optical-beam-induced-current study of active region and charge collection efficiency of GaAs particle detectors An
M. Alietti d, L. Berluti ‘, C. Canali f, A. Castaldini h, A. Cavallini b, A. Cetronio a, S. D’Auria by1,C. de1 Papa bq*, C. Lanzieri a, F. Nava e,‘, M. Proia ‘, P. Rinaldi b, A. Zichichi ’ ADirezione Ricerca, Alenia SPA, Rome, It& ’ Dipartimenio di Fisica dell’u~il~er~it~ and INFN Bologna, Iialy ’ CERN? Genesa, Switzerlund ’ Dipartimenlo di Fisica dell’llnillersitb Modena, Italy e Dipartimento di Scienze dell ‘Ingegneria dell’Unir~er.sitirdi Modena. Italy f Dipartimento di Scienze dell’lngegneria dell ‘lJnil,ersit2 di Modena, Italy Received 18 August 1994
Abstract In a recent paper, it was described
how semi-insulating (SI) liquid encapsulated Czochraiski (LEC) gallium arsenide particle detectors made by Alenia SpA have been tested and found to be understandable in terms of one function: the local charge collection efficiency. We have now measured, always within the context of the RD8 experiment, the local charge collection efficiency for these diodes using a technique called OBIC (optical beam induced current) and derived the active region width (W) as a function of bias in a range of values lower than those obtainable with particles. Using data both from OBIC and from the previous paper, W is found to behave approximately linearly as a function of bias larger than about 50 V. Below this value, W follows a square root law as a function of bias.
1. introduction Semi-insulating (SI) liquid encapsulated Czochralski (LEC) GaAs partieie detectors were originally proposed for use at the Large Hadron Collider (LHC) experiments, where radiation hardness is of primary concern; until now they have been shown to have the following properties: (i) full detection efficiency for minimum ionizing particles (mips) [ll, (ii) radiation hardness to high doses of y and neutrons [21, (iii) partial charge collection efficiency (ccc), i.e. only a fraction of the charge released by a particle gets collected [1,2]. We define three global ccc’s for 01 and p particles and X-rays (E”, E@, cx) as the fraction of the charge produced in the detector by the particle that is actually coilected and measured [3].
* Corresponding author. ’ Now at the University of Glasgow, UK. ’ Also at INFN Bologna, Italy. 0168-9002/95/$09.50
Point (iii) is due to two facts: (a) The electric collection field inside the detector extends through the detector only at high voltages; in general the active region is a function of bias and grows with it. This fact was unexpected owing to the high resistivity of the bulk material. (b) There is trapping even where there is field [4]. Deep energy levels near the middle of the forbidden gap, recently measured on our detectors [5], cause these two problems by getting ionized when the bias is applied to the detector, thus preventing the collection field from extending through the detector [6,7] and acting as traps, in addition to the shallower levels. In a previous paper [3], we have experimentally investigated the behaviour of the width (W) of the active region (i.e. the region where the charge collection is nonzero) as a function of the appiied bias greater than 75 V, for detectors built by Atenia SpA on Sumitomo SI LEC material. We have also shown that the local charge collection, E(X), i.e. the fraction of the charge of an eh pair produced at a coordinate x inside the substrate that is actually collected, is the function that correlates the global ccc’s, cu. F@, 8’. We have now measured E(X) directly with the OBIC
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(94)01123-O
M. Alietti et al. / Nucl. Instr. and Meth. in Phys. Rex A 355 (1995) 420-424
421
lock-in amplifier and then measured with an ADC, read out by a computer. Further details can be found in Ref. [9].
3. Results
Fig. 1. Optical beam induced current setup. The beam scans the sample’s cleaved surface in the x direction and the phot~u~ent signal is amplified and then computer-stored.
technique [8,9] for the same Alenia detectors and shown experimentally that W can be calculated from E(X).
2. Experimenta! method Detectors, built by Alenia SpA on Sumitomo SI LEC material, are 3 mm diameter, 215 p,m thick dots with Schottky metall~ation; the ohmic metallization covers the entire back of the chip (4 roundels per 1 cm2 chip). A description of the manufacturing technique and metals used is given in Ref. [5]. A detector can be cleaved (Fig. 1) through the contact so as to have an edge illuminated by a narrow (1 pm diameter) beam of light. Scanning the edge with the beam, starting from the Schottky (n = 0) to the ohmic contact (x = t), a charge is produced and therefore a current is induced in the field inside the detector. The value of the current depends on the flux and spot size of the beam and the electric field, i.e. the bias applied, and finally on e(x). In fact, E(X) = Z(x)/(e#), where e is the electron charge, # is the beam flux (photons cmP2 s-t), and S is the beam area; Z=Z(x) is the current measured when the beam hits the edge of the GaAs at coordinate x. In general d, is not well known, since it represents the number of photons actually penetrating the material: that is the incident beam, easily measured, minus the reflected part, difficult to measure. Moreover the reflected fraction of the light may change with position so that CF,is a weak function of x. We assume here, however, that (It, is constant; Z = Z(x) thus represents the charge collection efficiency (ccc) on an arbitrary scale. The light is generated by a halogen lamp. By varying a filter, we can change the wavelength of the light from A = 500 nm to A = 900 nm, thus changing its abso~tion length in GaAs from a fraction of a micron to many microns. We have used A = 700 nm in the reported me~~ements (penetration depth was 0.8 pm) in order to optimize the ratio between spatial resolution and generation volume [8]. The beam is chopped by a mechanical chopper at a frequency of 72 Hz. The long current pulse is amplified with a
To test the technique, we have decided to first do an exercise using a commercial 370 pm thick Si detector. Fig. 2 shows the measured photocurrent in the Si detector as a function of the position of the light beam; this gives the behaviour of the function &= E(I), i.e. the local charge collection as a function of the depth inside the diode. Since the photocurrent goes to zero once a value of x = W is exceeded, i.e. where the electric field becomes zero, from these current profiles we can measure W as a function of bias; W is the width at half height. Fig. 3 shows W as a function of bias. The resuit is consistent with the expected square root behaviour, the fit to the data is calculated with the net ionized charge carrier concentration: No -N = 9 X IO” cme3, in good agreement with the electricaliropetiies of our material. With the same method for the Si detector as used to get Fig. 3 we can now measure the active region width W of GaAs detectors from the photocurrent curves of Fig. 4. Before plotting W as a function of bias, a correction is necessary: in fact the voltage applied to the diode is not equal to the voltage across the active region. Given the very high resistivity of the base material, p = lo7 fI cm far higher than that of the best silicon, the reverse current (Fig. 5) of the diode produces a sizeable voltage drop across the non-active volume of the diode (see the scheme in Fig. 6). Since we have measured W, we know t- W (t = thickness of the diode = 215 km), we can calculate the serial resistance Z?,, from p, t - W and the diode
Fig. 2. The photocurrentprofiles I = Z(x) as measured at various biases with OBIC for a commercial Si detector.
422
M. Alietti et al. /Nucl. Instr. and Meth. in Phys. Res. A 355 llW5f 420-424
Si#FN24
0
v, WI
Fig. 5. The I-V
characteristic
curve of the 215 pm diode under
test.
Fig. 3. Depleted region width of the Si detector of Fig. 2 as a function of bias, as measured from the photocurrent profiles. The
overlapped curve is the fit to the data with a net ionized carrier density No - NA=9X 10” cm-3.
surface, and the voltage drop follows from the value of the reverse current. Fig, 6 shows the correction, V,, as a function of the overall applied voltage, V,. The general behaviour of the correction V, is easy to understand: once the detector is totally active the correction is zero; the correction is also zero when the applied bias is zero (no reverse current); the correction is therefore a bell-shaped curve with a maximum whose value depends on the reverse current. We can now plot W as a function of V,, = V, - V,; this is the effective voltage drop across the
space-charge region. Fig. 7 shows the results on W as a function of VT, and as a function of V,. As can be seen from Fig. 7, at low voltages, V,, < 50 V, W increases with the root square of V,, and with a corresponding density of ionized ND - NA = 1013 cmF3, whilst for V, > 50 V, W increases linearly with reverse bias voltage [9]. The result of the OBIC measurement is reported in Fig. 8 (points below 100 V), where we also show the results from Ref. [3] (points above 75 V), i.e. the measurements of the active width carried out with particles: counting rates of X-rays on the front and the back of the diode and the ratio between the globa cues of betas and X-rays multiplied by
28 >” 6
4
2
0 0
20
40
80
100
10
v*w
Fig. 4. The photocurrent profiles I = I(x) for Alenia GaAs detectors (21.5 km) as measured with OBIC for various biases.
Fig. 6. The voltage drop across the resistor R,, representing the resistance of the non-active thickness of GaAs as a function of the applied voltage. The electrical scheme used is also shown. R, is the equivalent resistance that takes into account the leakage current.
M. Atietti et al. / Nuci. Instr. and M&h. in Phys. Res. A 3.53(1995) 420-424
0
oi
0
,
,
I
20
40
60
,
a
/
I
100
so
Bias(V) Fig. 7. Active region width as a function of bias voltage. The black squares indicate W as a function of V,, whilst the open circles indicate W as a a unction of V,, = V, - V,, which is the voltage drop across the space-charge region. The dashed line has been obtained fitting the open circle curve with a net ionized carrier density, iv, - NA= lOi cmm3.
the detector thickness. The data of Ref. [3] have also been corrected for the voltage drop across the non-active part of the diode. The data appear to fall on straight lines with the same slope, only just displaced from each other: an overall straight line from a bias of 200 V down to about 50 V. The displacement of the two lines is irrelevant since it only
200 -
423
reflects the way W has been calculated from the photocurrent curves and particle data: what is relevant is the identical slope of the two measurements. A linear behaviour is, of course, a very different behaviour from that of Si detectors, which has to be understood on the basis of a model of the electric field [6]. In Fig. 4, we have shown the curves E = E(X) obtained for different values of the bias detector. Some comments on these curves are now necessary. First of ail, let us notice that the rise of these curves, both in Si and @As, is somewhat slow due to the actual width of the beam and the photo~u~ent induced when the optical beam approaches the interface between the Schottky metai~~ation and the semiconductor. The slow drop is due to charge carriers produced by the light outside the actual active region, diffusing back into the active region and being partially collected. Another important point shown by these curves is that E(X) in GaAs grows with increasing X, before dropping at the edge of the active region; apparently we collect charge better even if the electric field is not increasing: the reason can perhaps be ascribed to less trapping for holes than for electrons, so that as the weight of holes on the total collected charge increases with the distance from the Schottky barrier, so does E(X). We have to point out that quaIitativeiy these curves are in agreement with all we know on the global cces. The width of the active area is apparently linear in the bias [3]. The value of e(x), at x = 20 pm, i.e. Ed, since the range of 5.5 MeV ~1s is 20 pm in GaAs, saturates at higher voltage than in the case of silicon as it should 131. The average value of E(X) over W, i.e. 8’ = l/WJow E(X) dx (for a discussion on this point, see Ref. [3]) does not appear to be too different from Ed. The general shape of these curves has been compared with what we can deduce from previous data obtained with protons of different energy, i.e. penetration [IO] and found to be in qualitative agreement. It is worth remembering that protons of different energy are indeed a probe at different depths: their ccc can be considered the local charge collection at different depths.
150-
4. Conclusions
3 :
loo-
50-
o! 0
,
SO
I
100
,
150
I
1
200
v, WI Fig. 8. The active region width, W, calculated from the data of Fig. 4 (points below 100 V), plotted as a function of the corrected bias and compared with the results obtained in Ref. [3] (points above 75 V).
We have continued our experimental investigation of the detectors made by Alenia SpA on Sumitomo material encouraged by their large ccc for mips (75%). Their general behaviour is well understood in terms of the locai charge collection, now directly measured with the OBIC technique. The behaviour of the active region as a function of bias (approximately linear) is very different from the behaviour of the depleted region of a Si detector as a function of bias. It will also be very interesting to try to fit the experimental curves E = E(X), using the assumption of E-field dependent capture cross sections of the carriers inside the GaAs.
424
M. Alietti et al. /h&l.
Inutr. and Meth. in Phys. Rex A 355 1199.5) 4.?0-424
Acknowledgements We wish to thank Prof. Ken Smith (Glasgow University), Prof. K. Luebelsmeyer and Dr. M. Toporowsky (Aachen University) for very interesting discussions.
References [ll R. Bertin et al., Nucl. Instr. and Meth. A 294 (1990) 211; K.W. Benz, Nucl. Instr. and Meth. A 322 (1992) 493; D.S. McGregor et al., Nucl. Instr. and Meth. A 322 (1992) 483; D.S. McGregor et al., IEEE Trans. Nucl. Sci. NS-3Y f 1992) 1226. @I S.P. Beaumont et al., Nucl. Instr. and Meth. A 322 (1992) 472. [31L. Berluti et al., Nucl. Instr. and Meth. A 354 (1995) 364. I41C. de1 Papa and S. D’Auria, Charge collection and electric field in GaAs particle detectors, Proc. 20th Eloisatron Work-
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