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Spectroscopic performance of semi-insulating GaAs detectors for digital radiography
Nuclear Instruments and Methods in Physics Research A 422 (1999) 247—251
Spectroscopic performance of semi-insulating GaAs detectors for digital radiography E. Bertolucci , M. Conti *, G. Mettivier , P. Russo , A. Cola, F. Quaranta, L. Vasanelli, M.G. Bisogni, U. Bottigli, M.E. Fantacci, A. Stefanini Physics Department, Universita+ di Napoli Federico II and INFN, Napoli, Italy Material Science Department, Universita+ di Lecce and CNR-IME, Lecce, Italy Physics Department, Universita+ di Pisa and INFN, Pisa, Italy
Abstract We studied pixel radiation detectors for X-ray radiography based on semi-insulating GaAs: in particular, we investigated both annealed and non-annealed contact deposition techniques for the ohmic contact and both ringguarded and non-guarded Schottky contact, in order to reduce the leakage current and to increase the maximum applied electric field. Spectroscopic characterization with a 60 keV 241 Am source has been performed. Among these different detectors, the CCE can reach 99$6%, while the energy resolution *E/E can go down to 4.1$0.2%. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In the frame of a research work aimed to define the parameters of the bulk material, contact deposition, geometrical and electrical characteristics of GaAs pixellated detectors which could be optimal for digital radiography, we studied a set of 200 lm thick GaAs detectors which received different treatment on the ohmic contact, and have different pixel size. The ohmic contact was in some samples treated with annealing and in other samples not treated. The annealing process is supposed to influence the
* Corresponding author. e-mail: [email protected]
minority charge injection through the ohmic contact, therefore showing different breakdown characteristics. The optimized detector structure should have a high breakdown voltage, in order to work safely at full depletion and reach a saturation value from the point of view of CCE and energy resolution: low breakdown voltage due to unsuitable contact treatment would make it impossible to reach the best performances of each detector. The behavior of single pixels and pixel arrays was also studied with a guard-ring: the main reason for the presence of a guard-ring is that it limits the surface leakage current and corrects the electric field under the outer pixels in arrays, which, because of border effects, could have a different behavior, in particular regarding energy resolution (*E/E) and charge collection efficiency (CCE).
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 1 0 3 - 6
III. DETECTORS
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E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
2. Material preparation For all the detectors presented in this paper, we used 2 in diameter, n-type, 11 0 02 VGF grown semiinsulating GaAs wafers 200 lm thick. Both sides of the wafer are polished and the values of resistivity, mobility and etch pit density, as declared by the factory, are (1—6);10 ) cm, 5500—7400 cm/(V s) and (5000 cm\, respectively. On the front sides of the wafers we create, using standard photolithographic techniques, different geometry of single square pixels and matrices. The size of the pixels are 2,1 mm, 500, 300 and 200 lm. Some of them are guarded with a guard-ring 100 lm wide placed 50 lm apart from the pixel. The 6;6 matrices have a pixel size of 140,150 and 160 lm and an interpixel distance of 30, 20, 10 lm, respectively. Some of the matrices are also guarded with a guard-ring 100 lm wide, placed at a distance equal to the inter-pixel distance. The pixels are Schottky contacts made of a Ti, Pd and Au multilayer deposited by an electron beam system at a base pressure of 10\ mbar. The thickness of the Ti/Pd/Au multilayer is 500 A> /750 A> /2000 A> . After the lift-off process on the front side, the backside of the wafers is completely covered with a 300 A> Ge/600 A> Au/300 A> Ni/2000 A> Au multilayer evaporated by the same deposition equipment. After the depositions the wafers are cleaved in 5;5 mm slices, each containing simple pixels, guarded pixels, matrices or guarded matrices. Some of the slices are then annealed in a RTA apparatus at 370°C for 60 s in a nitrogen atmosphere.
3. Current—voltage characteristics The electrical characterization of the pixels has been carried out in a probe station using a 237 Keithley Source monitor unit and, for the measurements performed on the guarded pixels, a 617 Keithley electro-meter has also been utilized. We have carried out current measurements under reverse bias on squared Schottky diodes of different size, from 2 to 0.2 mm. Figs. 1 and 2 report the current density curves as a function of the reverse voltage for the 2 mm and 200 lm diodes, respectively. From the figures it can be seen that
Fig. 1. Current density vs. reverse bias voltage (diodes with 2 mm pixel size).
Fig. 2. Current density vs. reverse bias voltage (diodes with 200 lm pixel size).
diodes with annealed back-contacts show well defined break-down at much lower voltage than the corresponding non-annealed ones, and this phenomenon is more evident in smaller size pixels (i.e. 0.2 mm, with a breakdown around 200—300 keV). The systematic increase of the break-down voltage with the decrease of the size of the isolated diodes has been related [1] to the non-uniformity of the electric field and to the injection of minority carriers from the back contact. For semi-insulating GaAs detectors, the break-down is due to the injection of holes from the back-contact. As can be seen from the Figs. 1 and 2, the annealing procedure of the back contacts drastically affects the current flowing at high voltages.
E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
In Figs. 1 and 2 one can also notice that the reduction of the current due to the guard-ring is quite relevant for the small diodes (Fig. 2), where the slope of the J—» curve also decreases in the
Fig. 3. Leakage current vs. reverse bias voltage for angular pixels, in arrays with and without guard-ring.
249
presence of the guard-ring. On the other hand, in the case of the large diodes (Fig. 1), the current reduction is less pronounced, probably because the current flow is not simply proportional to the area and perpendicular to the contacts. Surface leakage [2], spreading of the lines of force of the electric field inside the bulk due to the geometric difference of the contacts, and edge contact effect can all contribute as parallel mechanisms [3]. These effects, being related to the contact perimeter, will be more important as the pixel size decreases. Measurements have also been carried out on pixels in matrices, with and without a guard-ring surrounding the matrices. Internal pixels are not affected by the presence of the guard-ring, while lateral, and even more, angular pixels show greater, voltage-dependent currents: the guard-ring is effective in reducing the current of all the outer pixels. In Fig. 3 we present the measured leakage current as a function of bias voltage for two angular
Fig. 4. CCE vs. reverse bias voltage for pixel sizes of (a) 0.2 mm, (b) 0.3 mm, (c) 0.5 mm and (d) 2 mm.
III. DETECTORS
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E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
pixels, belonging to arrays with and without guard-ring.
4. Spectroscopy All the samples have been mounted on a SMA connector: the ohmic side has been glued by a conductive paste to ground and the Schottky contact has been microbonded (Aluminum wire, UltraSound Kulicke & Soffa) and connected via a SMABNC adapter to a 142A Ortec pre-amplifier (sensitivity 0.16 lV/e—h pair) which had a high voltage input for detector bias. An Ortec Spectroscopy Amplifier 673 (amplication set at 1500, 1 ls peaking time) was the final part of the amplification chain, and the amplified signal was fed into a Emca-plus Silena MCA on PC board for the signal amplitude spectrum.
A 60 keV Am point source was placed at 4 mm from the samples and data were acquired for 20 min. We studied the charge signal as a function of the applied bias voltage, increasing the voltage up to the breakdown point. On each pulse height spectrum we performed a gaussian fit on the photoelectric peak, obtaining the average photoelectric deposited charge 1Q2 (or energy 1E2), and the standard deviation p: we can therefore obtain for each measurement a value of energy resolution p/E (or *E/E). GaAs average ionization energy being about 4.2 eV, injecting via a test input a charge corresponding to 60 keV, we obtain the position of the peak for 100% charge collection, and by comparison we obtain the charge collection efficiency (CCE) for 60 keV photons. In Fig. 4 we present the results of CCE as a function of the applied voltage for 0.2, 0.3, 0.5, 2 mm pixel size. As expected, the CCE values (in principle
Fig. 5. Energy resolution vs. reverse bias voltage for pixel sizes of (a) 0.2 mm, (b) 0.3 mm, (c) 0.5 mm and (d) 2 mm.
E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
depending on the intensity of the collection electric field, detector thickness and bulk material properties) are not much influenced by the annealing process or the pixel dimension at fixed bias voltage, and all curves overlap well within the error bars. Nevertheless, the annealing process is important because unannealed samples can reach higher bias voltage before breakdown, allowing us to work at conditions closer to the total collection (100% CCE). In Fig. 5 we present the *E/E as a function of the applied voltage for 0.2, 0.3, 0.5, 2 mm pixel size. We notice that while at large pixel size no difference can be appreciated between pixels with different Schottky contact structure, at small pixel size the presence of a guard-ring that corrects the electric field shape at the border increases the energy resolution performance. In Fig. 5a and b (0.2 and 0.3 mm) the *E/E values for samples with guardring are systematically lower than the corresponding ones without guard-ring. It should be noticed that from the point of view of energy resolution major contributions to the change in *E/E come from detector capacitance and leakage current, both growing as the square of the pixel side, and both contributing to the electronic noise. Moreover, we have observed a reduction in the break-down voltage of the diodes when passing from I—» probe station to nuclear spectroscopic measurements. In the last case, we have micro-bonded aluminum wires as electrical connection on the evaporated Schottky contact: we
251
attribute to micro-bonding process a worsening of performance, which prevented us from reaching full depletion bias voltage in some samples.
5. Conclusions We investigated the effect of annealing on ohmic contact and of guard-ring on the Schottky contacts in SI-GaAs pixel detectors. While the annealing process, lowering the breakdown voltage, certainly worsens the performance of such detectors, a guard-ring external to an array of pixels appears to help reduce the leakage current in outer pixels. Spectroscopic characterization with a 60 keV Am source showed that excellent charge collection efficiency can be obtained (up to 99$6%), while the energy resolution *E/E can go down to 4.1$0.2%.
Acknowledgements Thanks are due to Consortium OPTEL for the kind help in processing the detectors.
References [1] A. Cola, Nucl. Instr. and Meth. A 410 (1998) 85. [2] S.J. Pearton et al., Appl. Phys. Lett. 44 (1984) 684. [3] E.H. Rhoderick, R.H. Williams, Metal—Semiconductor Contacts, Clarendon Press, Oxford, 1988.
III. DETECTORS
Spectroscopic performance of semi-insulating GaAs detectors for digital radiography E. Bertolucci , M. Conti *, G. Mettivier , P. Russo , A. Cola, F. Quaranta, L. Vasanelli, M.G. Bisogni, U. Bottigli, M.E. Fantacci, A. Stefanini Physics Department, Universita+ di Napoli Federico II and INFN, Napoli, Italy Material Science Department, Universita+ di Lecce and CNR-IME, Lecce, Italy Physics Department, Universita+ di Pisa and INFN, Pisa, Italy
Abstract We studied pixel radiation detectors for X-ray radiography based on semi-insulating GaAs: in particular, we investigated both annealed and non-annealed contact deposition techniques for the ohmic contact and both ringguarded and non-guarded Schottky contact, in order to reduce the leakage current and to increase the maximum applied electric field. Spectroscopic characterization with a 60 keV 241 Am source has been performed. Among these different detectors, the CCE can reach 99$6%, while the energy resolution *E/E can go down to 4.1$0.2%. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In the frame of a research work aimed to define the parameters of the bulk material, contact deposition, geometrical and electrical characteristics of GaAs pixellated detectors which could be optimal for digital radiography, we studied a set of 200 lm thick GaAs detectors which received different treatment on the ohmic contact, and have different pixel size. The ohmic contact was in some samples treated with annealing and in other samples not treated. The annealing process is supposed to influence the
* Corresponding author. e-mail: [email protected]
minority charge injection through the ohmic contact, therefore showing different breakdown characteristics. The optimized detector structure should have a high breakdown voltage, in order to work safely at full depletion and reach a saturation value from the point of view of CCE and energy resolution: low breakdown voltage due to unsuitable contact treatment would make it impossible to reach the best performances of each detector. The behavior of single pixels and pixel arrays was also studied with a guard-ring: the main reason for the presence of a guard-ring is that it limits the surface leakage current and corrects the electric field under the outer pixels in arrays, which, because of border effects, could have a different behavior, in particular regarding energy resolution (*E/E) and charge collection efficiency (CCE).
0168-9002/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 1 0 3 - 6
III. DETECTORS
248
E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
2. Material preparation For all the detectors presented in this paper, we used 2 in diameter, n-type, 11 0 02 VGF grown semiinsulating GaAs wafers 200 lm thick. Both sides of the wafer are polished and the values of resistivity, mobility and etch pit density, as declared by the factory, are (1—6);10 ) cm, 5500—7400 cm/(V s) and (5000 cm\, respectively. On the front sides of the wafers we create, using standard photolithographic techniques, different geometry of single square pixels and matrices. The size of the pixels are 2,1 mm, 500, 300 and 200 lm. Some of them are guarded with a guard-ring 100 lm wide placed 50 lm apart from the pixel. The 6;6 matrices have a pixel size of 140,150 and 160 lm and an interpixel distance of 30, 20, 10 lm, respectively. Some of the matrices are also guarded with a guard-ring 100 lm wide, placed at a distance equal to the inter-pixel distance. The pixels are Schottky contacts made of a Ti, Pd and Au multilayer deposited by an electron beam system at a base pressure of 10\ mbar. The thickness of the Ti/Pd/Au multilayer is 500 A> /750 A> /2000 A> . After the lift-off process on the front side, the backside of the wafers is completely covered with a 300 A> Ge/600 A> Au/300 A> Ni/2000 A> Au multilayer evaporated by the same deposition equipment. After the depositions the wafers are cleaved in 5;5 mm slices, each containing simple pixels, guarded pixels, matrices or guarded matrices. Some of the slices are then annealed in a RTA apparatus at 370°C for 60 s in a nitrogen atmosphere.
3. Current—voltage characteristics The electrical characterization of the pixels has been carried out in a probe station using a 237 Keithley Source monitor unit and, for the measurements performed on the guarded pixels, a 617 Keithley electro-meter has also been utilized. We have carried out current measurements under reverse bias on squared Schottky diodes of different size, from 2 to 0.2 mm. Figs. 1 and 2 report the current density curves as a function of the reverse voltage for the 2 mm and 200 lm diodes, respectively. From the figures it can be seen that
Fig. 1. Current density vs. reverse bias voltage (diodes with 2 mm pixel size).
Fig. 2. Current density vs. reverse bias voltage (diodes with 200 lm pixel size).
diodes with annealed back-contacts show well defined break-down at much lower voltage than the corresponding non-annealed ones, and this phenomenon is more evident in smaller size pixels (i.e. 0.2 mm, with a breakdown around 200—300 keV). The systematic increase of the break-down voltage with the decrease of the size of the isolated diodes has been related [1] to the non-uniformity of the electric field and to the injection of minority carriers from the back contact. For semi-insulating GaAs detectors, the break-down is due to the injection of holes from the back-contact. As can be seen from the Figs. 1 and 2, the annealing procedure of the back contacts drastically affects the current flowing at high voltages.
E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
In Figs. 1 and 2 one can also notice that the reduction of the current due to the guard-ring is quite relevant for the small diodes (Fig. 2), where the slope of the J—» curve also decreases in the
Fig. 3. Leakage current vs. reverse bias voltage for angular pixels, in arrays with and without guard-ring.
249
presence of the guard-ring. On the other hand, in the case of the large diodes (Fig. 1), the current reduction is less pronounced, probably because the current flow is not simply proportional to the area and perpendicular to the contacts. Surface leakage [2], spreading of the lines of force of the electric field inside the bulk due to the geometric difference of the contacts, and edge contact effect can all contribute as parallel mechanisms [3]. These effects, being related to the contact perimeter, will be more important as the pixel size decreases. Measurements have also been carried out on pixels in matrices, with and without a guard-ring surrounding the matrices. Internal pixels are not affected by the presence of the guard-ring, while lateral, and even more, angular pixels show greater, voltage-dependent currents: the guard-ring is effective in reducing the current of all the outer pixels. In Fig. 3 we present the measured leakage current as a function of bias voltage for two angular
Fig. 4. CCE vs. reverse bias voltage for pixel sizes of (a) 0.2 mm, (b) 0.3 mm, (c) 0.5 mm and (d) 2 mm.
III. DETECTORS
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E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
pixels, belonging to arrays with and without guard-ring.
4. Spectroscopy All the samples have been mounted on a SMA connector: the ohmic side has been glued by a conductive paste to ground and the Schottky contact has been microbonded (Aluminum wire, UltraSound Kulicke & Soffa) and connected via a SMABNC adapter to a 142A Ortec pre-amplifier (sensitivity 0.16 lV/e—h pair) which had a high voltage input for detector bias. An Ortec Spectroscopy Amplifier 673 (amplication set at 1500, 1 ls peaking time) was the final part of the amplification chain, and the amplified signal was fed into a Emca-plus Silena MCA on PC board for the signal amplitude spectrum.
A 60 keV Am point source was placed at 4 mm from the samples and data were acquired for 20 min. We studied the charge signal as a function of the applied bias voltage, increasing the voltage up to the breakdown point. On each pulse height spectrum we performed a gaussian fit on the photoelectric peak, obtaining the average photoelectric deposited charge 1Q2 (or energy 1E2), and the standard deviation p: we can therefore obtain for each measurement a value of energy resolution p/E (or *E/E). GaAs average ionization energy being about 4.2 eV, injecting via a test input a charge corresponding to 60 keV, we obtain the position of the peak for 100% charge collection, and by comparison we obtain the charge collection efficiency (CCE) for 60 keV photons. In Fig. 4 we present the results of CCE as a function of the applied voltage for 0.2, 0.3, 0.5, 2 mm pixel size. As expected, the CCE values (in principle
Fig. 5. Energy resolution vs. reverse bias voltage for pixel sizes of (a) 0.2 mm, (b) 0.3 mm, (c) 0.5 mm and (d) 2 mm.
E. Bertolucci et al. /Nucl. Instr. and Meth. in Phys. Res. A 422 (1999) 247—251
depending on the intensity of the collection electric field, detector thickness and bulk material properties) are not much influenced by the annealing process or the pixel dimension at fixed bias voltage, and all curves overlap well within the error bars. Nevertheless, the annealing process is important because unannealed samples can reach higher bias voltage before breakdown, allowing us to work at conditions closer to the total collection (100% CCE). In Fig. 5 we present the *E/E as a function of the applied voltage for 0.2, 0.3, 0.5, 2 mm pixel size. We notice that while at large pixel size no difference can be appreciated between pixels with different Schottky contact structure, at small pixel size the presence of a guard-ring that corrects the electric field shape at the border increases the energy resolution performance. In Fig. 5a and b (0.2 and 0.3 mm) the *E/E values for samples with guardring are systematically lower than the corresponding ones without guard-ring. It should be noticed that from the point of view of energy resolution major contributions to the change in *E/E come from detector capacitance and leakage current, both growing as the square of the pixel side, and both contributing to the electronic noise. Moreover, we have observed a reduction in the break-down voltage of the diodes when passing from I—» probe station to nuclear spectroscopic measurements. In the last case, we have micro-bonded aluminum wires as electrical connection on the evaporated Schottky contact: we
251
attribute to micro-bonding process a worsening of performance, which prevented us from reaching full depletion bias voltage in some samples.
5. Conclusions We investigated the effect of annealing on ohmic contact and of guard-ring on the Schottky contacts in SI-GaAs pixel detectors. While the annealing process, lowering the breakdown voltage, certainly worsens the performance of such detectors, a guard-ring external to an array of pixels appears to help reduce the leakage current in outer pixels. Spectroscopic characterization with a 60 keV Am source showed that excellent charge collection efficiency can be obtained (up to 99$6%), while the energy resolution *E/E can go down to 4.1$0.2%.
Acknowledgements Thanks are due to Consortium OPTEL for the kind help in processing the detectors.
References [1] A. Cola, Nucl. Instr. and Meth. A 410 (1998) 85. [2] S.J. Pearton et al., Appl. Phys. Lett. 44 (1984) 684. [3] E.H. Rhoderick, R.H. Williams, Metal—Semiconductor Contacts, Clarendon Press, Oxford, 1988.
III. DETECTORS