- Email: [email protected]
3-D GaAs radiation detectors
Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
3-D GaAs radiation detectors A.R. Meiklea,*, R.L. Batesa, K. Ledinghama, J.H. Marshb, K. Mathiesona, V. O’Sheaa, K.M. Smitha b
a Detector Development Group, Department of Physics and Astronomy, University of Glasgow, UK Optoelectronics Group, Department of Electrical and Electronic Engineering, University of Glasgow, UK
Abstract A novel type of GaAs radiation detector featuring a 3-D array of electrodes that penetrate through the detector bulk is described. The development of the technology to fabricate such a detector is presented along with electrical and radiation source tests. Simulations of the electrical characteristics are given for detectors of various dimensions. Laser drilling, wet chemical etching and metal evaporation were used to create a cell array of nine electrodes, each with a diameter of 60 mm and a pitch of 210 mm. Electrical measurements showed I–V characteristics with low leakage currents and high breakdown voltages. The forward and reverse I–V measurements showed asymmetrical characteristics, which are not seen in planar diodes. Spectra were obtained using alpha particle illumination. A charge collection efficiency of 50% and a S/N ratio of 3 : 1 were obtained. Simulations using the MEDICI software package were performed on cells with various dimensions and were comparable with experimental results. Simulations of a nine-electrode cell with 10 mm electrodes with a 25 mm pitch were also performed. The I–V characteristics again showed a high breakdown voltage with a low leakage current but also showed a full depletion voltage of just 8 V. r 2002 Published by Elsevier Science B.V.
1. Introduction Planarised GaAs radiation detectors have recently been used extensively in the research of High Energy Physics (HEP) and X-ray imaging. Normally, these detectors are hundreds of microns thick, increasing their X-ray efficiency and making their fabrication and operation simple. HEP radiation studies [1,2] have shown, however, that high radiation levels introduce deep level defects to the bulk which affect the main parameters of planar GaAs detectors, namely the leakage current *Corresponding author. E-mail address: [email protected] (A.R. Meikle).
density, mean free drift length and charge collection efficiency. A novel architecture [3] that uses a 3-D array of electrodes that penetrate all the way through the thickness of the detector bulk offers the possibility of overcoming these limitations. The electrode pitch would be sufficiently small to collect the majority of the charge after irradiation and the need for a detector bulk hundreds of microns thick would be satisfied. These cylindrical electrodes create an electric field that sweeps the electron and hole charge carriers laterally through the bulk for collection at oppositely biased electrodes, and charge collection will take place throughout the thickness of the bulk.
0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 8 9 6 - 4
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
2. Fabrication To fabricate a 3-D detector cell array, holes have to be created in a substrate and electrodes created within those holes. Semi-insulating, undoped Liguid Encapsulated Czochralski (SI-U LEC) was used as the substrate material for the fabrication of the detector. Fig. 1 shows the complete fabrication process as follows. 1. The substrate was thinned to a thickness of 200 mm using mechanical lapping and chemical polishing.
Fig. 1. 3-D detector fabrication process flow.
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2. Resist 5 mm thick was spun on both sides to protect the sample surface from subsequent processing, namely ejected debris from the hole drilling process, wet chemical etchant used to treat the internal walls of the holes and from the metallisation process used to create the electrodes. 3. Holes were drilled through the sample using a 10 ns pulse duration, 355 nm wavelength Nd : YAG laser with a 5 cm focal length lens, 1 mm iris, and by firing the laser for an additional 100 pulses immediately after the hole is created. This process created repeatable, round holes of 30 mm diameter, smooth internal walls and minimal tapering. A heat-affected zone of approximately 500 nm will be created in the bulk region immediately surrounding the holes. This drilling process is described in greater detail in Ref. [4]. 4. The internal walls were etched with a solution of sulphuric acid, hydrogen peroxide and water to remove the heat-affected zone and to extend the holes to a diameter of 60 mm. This diameter will give the electrode formation process a greater chance of success. 5. Schottky barrier electrodes were formed within the holes by evaporation of Ti/Pd/Au (33/30/ 150 nm) from both sides of the sample. This recipe has been used extensively in the past for fabricating Schottky contacts on planar diodes [5]. Before the metal evaporation was performed the hole internal walls were de-oxidised for 30 s in a 1 : 1 solution of ammonia and water. The evaporation was performed on both sides of the sample to maximise the possibility that the evaporated metal from both sides would extend as far as the middle of the hole. 6. The photoresist on both surfaces was removed using acetone under ultrasonic agitation. This removed excess metal and debris from previous processes. The sample was then cleaned with methanol and water. 7. Al wire 25 mm thick, tipped with silver-loaded epoxy adhesive, was inserted into the holes to form electrical connections. The completed cell consisted of a nine electrode array of Schottky barrier electrodes. The electrodes had 60 mm diameters and 120 mm pitches.
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3. Detector characterisation
limit mechanism on the Keithley was activated immediately.
3.1. Current–voltage characteristics 3.2. Charge collection Electrical characterisation was carried out using a Keithley-237 source measurement unit. The central electrode was biased while the surrounding eight electrodes were held at zero. All the electrodes in the cell are of the Schottky type so all measurements taken will be with a Schottky– Schottky configuration. Fig. 2 shows the forward and reverse characteristics for the nine electrode cell. The forward and reverse bias characteristics are asymmetrical, which is not seen in planar diodes with this configuration. This may be due to a difference in active volume, depending on how it is biased, caused by this particular electrode pattern i.e. under reverse bias, only the central electrode will have a reverse biased Schottky barrier, but under forward bias the eight surrounding electrodes will have reverse biased Schottky barriers. This inequality may be the cause of the asymmetric characteristics. The breakdown voltage in both regions is approximately 500 V and the leakage current is very low, being only approximately 5 nA at 500 V in the reverse region. The increase in current at breakdown was so great that the current
The response of the cell to alpha particle irradiation from a 241Am source under vacuum was determined. Due to surface contamination on the source, which reduced the energy of the alpha particles, the average energy was measured as 4.1 MeV [6]. The cell was biased and read-out with an EG&G Ortec-142 pre-amplifier. The signal was shaped with an Ortec-485 post-amplifier with a shaping time of 500 ns and sent to a PC-based multichannel analyser. Fig. 3 shows the spectrum obtained at an applied bias of 300 V at room temperature. The spectrum is clearly separated from the noise pedestal and shows the familiar Gaussian shape of an alpha spectrum obtained with a planar diode. Both types of charge carrier will contribute to the total charge. The charge collection efficiency (CCE) for this spectrum was 48%, although the rate of counting was low in comparison to standard planar diodes. A Gaussian curve was fitted to the above spectrum to obtain the FWHM. From this the signal-to-noise ratio (S=N) was found to be 3 : 1. This is a poor ratio and would have been due, in part, to the contaminated alpha source described earlier. The CCE measurements as a function of applied bias are shown in Fig. 4. The maximum CCE in this plot was 50%. This incomplete charge collection is a manifestation of charge trapping, which affects both types of charge carrier and is seen in planar diodes made from the same substrate material.
3.3. Simulation Electrical characteristics were calculated using the MEDIC1 software package [7]. The inputs to the model were:
Fig. 2. Nine-electrode cell I–V characteristics.
1. the cell dimensions, 2. Schottky barrier height=0.79 eV, 3. ND ¼ 1:5 1013 ;
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
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Fig. 3. 4.1 MeV alpha particle spectrum of nine-electrode cell at room temperature.
Fig. 4. CCE measurements of 4.1 MeV alpha particles at room temperature.
4. zero charge trapping, 5. impact ionisation turned on. Comparing simulation with experiment is an essential contribution to the validation of experimental results. The first simulation used identical cell dimensions to those of the fabricated cell i.e. nine electrodes of 60 mm diameter and 210 mm
pitch. The bias was applied to the central electrode while the eight surrounding electrodes were held at zero. The reverse biased I–V characteristic behaviour is similar to the reverse region shown experimentally in Fig. 3. The leakage current was very low and breakdown commenced between 450 and 500 V. Such an agreement between simulation
202
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
and experiment inspires confidence in the accuracy of the experimental results, especially since the experimental I–V curve showed a vastly different characteristic to that obtained with planar diodes made from the same substrate and electrode material. The simulated forward biased I–V characteristic differs from that of the reverse biased simulation, which again shows similarity with the experiment. The leakage current had a slightly higher average value than that seen in the reverse bias characteristic and began to increase rapidly at a lower value of applied bias. Again, this is similar to what was seen experimentally. Fig. 5 shows the potential distribution across the cell. It can be seen clearly that it does not extend all the way to the other electrodes (at the corners
and edges of the cuboid) and therefore the device is not fully depleted at 550 V. The undepleted area covers a sizeable percentage of the total area and this may go some way to explain the incomplete charge collection described earlier. With reverse breakdown occurring just below 500 V and incomplete depletion at 550 V, it seems it would not be possible to fully deplete a 3-D detector with these dimensions. It was therefore interesting to see what the response from a cell with different dimensions would be. The second simulated cell consisted of nine electrodes with 10 mm diameters and 25 mm pitches. The biasing conditions were the same as those for the previous cell. The potential distribution characteristics up to 20 V showed clearly that this device was fully depleted at this bias and, infact,
Fig. 5. Sixty micrometer diameter; 210 mm pitch electrode cell simulation of potential distribution.
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
the program showed the device was fully depleted at just 8 V. This is a dramatically different result from the first cell simulation, where the device had not fully depleted at 550 V. The reverse bias I–V characteristic for the second cell found the leakage current to be low and the breakdown region commenced between 200 and 250 V. Given that full depletion occurs at just 8 V, it was obvious that this device could function at full depletion without any dramatic increase in current. It would seem that a device with these dimensions could offer full depletion, low voltage operation.
203
breakdown voltage and this may have contributed to the incomplete charge collection observed. Simulations of a cell with 10 mm diameter electrodes with 25 mm pitches were also performed. The leakage current again was much lower than that of a planar diode and the breakdown voltage was approximately 250 V. However, the most dramatic observation was that this cell fully depleted at just 8 V. The simulations also showed that there were undepleted regions in this type of cell due to the layout of the electrodes. This problem could be overcome simply by designing a cell with six electrodes in a hexagonal array.
4. Conclusions Acknowledgements A 3-D detector has been fabricated and tested. A nine-electrode test cell consisting of electrodes with 60 mm diameters and 210 mm pitches was fabricated on a 200 mm thick SI-U GaAs substrate. Holes were created by drilling with a 10 ns pulse duration Nd : YAG laser with a 5 cm focal length objective lens. Electrodes were formed within these holes by metal evaporation from both sides. Current–voltage characteristics showed asymmetrical forward and reverse characteristics. The breakdown voltage in the reverse region was approximately 500 V with a corresponding leakage current of just 5 nA. Charge collection experiments were carried out using 4.1 MeV alpha particles. The CCE was seen to increase with applied bias before reaching a maximum value of 50%. A S=N ratio of 3 : 1 was obtained, although a contaminated source may have contributed to this low value. Computer modelling was performed using the MEDICI software package. A simulation of a cell with 60 mm electrodes with 210 mm pitches was performed to compare with the experimental results. The forward and reverse I–V characteristics were asymmetrical, the leakage current much lower than that seen in planar diodes and the reverse breakdown region commenced at approximately 500 V. These results were all similar to what was seen experimentally. However, the simulations showed that the cell was not fully depleted at the
The authors would like to thank Mr. T. McCanny of the Laser Ionisation Group in the Department of Physics and Astronomy at the University of Glasgow for assistance in the laser drilling of holes. One of usFK. MathiesonFgratefully acknowledges funding through a CASE studentship award from BNFL. The authors would also like to thank Mr. S. Passmore for some useful discussions.
References [1] R.L. Bates, C. DaVia, S. DAuria, V. O’Shea, C. Raine, K.M. Smith, Nucl. Instr. and Meth. A 395 (1997) 54. [2] M. Rogalla, M. Battke, N. Duda, R. Geppert, J. Ludwig, R. Irsigler, T. Schmid, K. Runge, A. SoldnerRembold, Nucl. Instr. and Meth. A 410 (1998) 41. [3] S. Parker, 3-DFa new architecture for solid state radiation detectors, University of Hawaii internal report, 1996, UH 511-839-96. [4] A.R. Meikle, K. Ledingham, J.H. Marsh, B. More, V. O’Shea, C. Raine, K.M. Smith, Nucl. Instr. and Meth. A 410 (1998) 115. [5] R.L. Bates, Gallium arsenide radiation detectors for the ATLAS experiment, Ph.D. Thesis, Department of Physics and Astronomy, University of Glasgow, 1997, RAL-TH-97010. [6] R.L. Bates, C. DaVia, V. O’Shea, C. Raine, K.M. Smith, R. Adams, Nucl. Instr. and Meth. A 410 (1998) 46. [7] MEDICI, Technology Modeling Associates Inc., 3950 Fabian Way, Palo Alto, CA, USA.
3-D GaAs radiation detectors A.R. Meiklea,*, R.L. Batesa, K. Ledinghama, J.H. Marshb, K. Mathiesona, V. O’Sheaa, K.M. Smitha b
a Detector Development Group, Department of Physics and Astronomy, University of Glasgow, UK Optoelectronics Group, Department of Electrical and Electronic Engineering, University of Glasgow, UK
Abstract A novel type of GaAs radiation detector featuring a 3-D array of electrodes that penetrate through the detector bulk is described. The development of the technology to fabricate such a detector is presented along with electrical and radiation source tests. Simulations of the electrical characteristics are given for detectors of various dimensions. Laser drilling, wet chemical etching and metal evaporation were used to create a cell array of nine electrodes, each with a diameter of 60 mm and a pitch of 210 mm. Electrical measurements showed I–V characteristics with low leakage currents and high breakdown voltages. The forward and reverse I–V measurements showed asymmetrical characteristics, which are not seen in planar diodes. Spectra were obtained using alpha particle illumination. A charge collection efficiency of 50% and a S/N ratio of 3 : 1 were obtained. Simulations using the MEDICI software package were performed on cells with various dimensions and were comparable with experimental results. Simulations of a nine-electrode cell with 10 mm electrodes with a 25 mm pitch were also performed. The I–V characteristics again showed a high breakdown voltage with a low leakage current but also showed a full depletion voltage of just 8 V. r 2002 Published by Elsevier Science B.V.
1. Introduction Planarised GaAs radiation detectors have recently been used extensively in the research of High Energy Physics (HEP) and X-ray imaging. Normally, these detectors are hundreds of microns thick, increasing their X-ray efficiency and making their fabrication and operation simple. HEP radiation studies [1,2] have shown, however, that high radiation levels introduce deep level defects to the bulk which affect the main parameters of planar GaAs detectors, namely the leakage current *Corresponding author. E-mail address: [email protected] (A.R. Meikle).
density, mean free drift length and charge collection efficiency. A novel architecture [3] that uses a 3-D array of electrodes that penetrate all the way through the thickness of the detector bulk offers the possibility of overcoming these limitations. The electrode pitch would be sufficiently small to collect the majority of the charge after irradiation and the need for a detector bulk hundreds of microns thick would be satisfied. These cylindrical electrodes create an electric field that sweeps the electron and hole charge carriers laterally through the bulk for collection at oppositely biased electrodes, and charge collection will take place throughout the thickness of the bulk.
0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 8 9 6 - 4
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
2. Fabrication To fabricate a 3-D detector cell array, holes have to be created in a substrate and electrodes created within those holes. Semi-insulating, undoped Liguid Encapsulated Czochralski (SI-U LEC) was used as the substrate material for the fabrication of the detector. Fig. 1 shows the complete fabrication process as follows. 1. The substrate was thinned to a thickness of 200 mm using mechanical lapping and chemical polishing.
Fig. 1. 3-D detector fabrication process flow.
199
2. Resist 5 mm thick was spun on both sides to protect the sample surface from subsequent processing, namely ejected debris from the hole drilling process, wet chemical etchant used to treat the internal walls of the holes and from the metallisation process used to create the electrodes. 3. Holes were drilled through the sample using a 10 ns pulse duration, 355 nm wavelength Nd : YAG laser with a 5 cm focal length lens, 1 mm iris, and by firing the laser for an additional 100 pulses immediately after the hole is created. This process created repeatable, round holes of 30 mm diameter, smooth internal walls and minimal tapering. A heat-affected zone of approximately 500 nm will be created in the bulk region immediately surrounding the holes. This drilling process is described in greater detail in Ref. [4]. 4. The internal walls were etched with a solution of sulphuric acid, hydrogen peroxide and water to remove the heat-affected zone and to extend the holes to a diameter of 60 mm. This diameter will give the electrode formation process a greater chance of success. 5. Schottky barrier electrodes were formed within the holes by evaporation of Ti/Pd/Au (33/30/ 150 nm) from both sides of the sample. This recipe has been used extensively in the past for fabricating Schottky contacts on planar diodes [5]. Before the metal evaporation was performed the hole internal walls were de-oxidised for 30 s in a 1 : 1 solution of ammonia and water. The evaporation was performed on both sides of the sample to maximise the possibility that the evaporated metal from both sides would extend as far as the middle of the hole. 6. The photoresist on both surfaces was removed using acetone under ultrasonic agitation. This removed excess metal and debris from previous processes. The sample was then cleaned with methanol and water. 7. Al wire 25 mm thick, tipped with silver-loaded epoxy adhesive, was inserted into the holes to form electrical connections. The completed cell consisted of a nine electrode array of Schottky barrier electrodes. The electrodes had 60 mm diameters and 120 mm pitches.
200
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
3. Detector characterisation
limit mechanism on the Keithley was activated immediately.
3.1. Current–voltage characteristics 3.2. Charge collection Electrical characterisation was carried out using a Keithley-237 source measurement unit. The central electrode was biased while the surrounding eight electrodes were held at zero. All the electrodes in the cell are of the Schottky type so all measurements taken will be with a Schottky– Schottky configuration. Fig. 2 shows the forward and reverse characteristics for the nine electrode cell. The forward and reverse bias characteristics are asymmetrical, which is not seen in planar diodes with this configuration. This may be due to a difference in active volume, depending on how it is biased, caused by this particular electrode pattern i.e. under reverse bias, only the central electrode will have a reverse biased Schottky barrier, but under forward bias the eight surrounding electrodes will have reverse biased Schottky barriers. This inequality may be the cause of the asymmetric characteristics. The breakdown voltage in both regions is approximately 500 V and the leakage current is very low, being only approximately 5 nA at 500 V in the reverse region. The increase in current at breakdown was so great that the current
The response of the cell to alpha particle irradiation from a 241Am source under vacuum was determined. Due to surface contamination on the source, which reduced the energy of the alpha particles, the average energy was measured as 4.1 MeV [6]. The cell was biased and read-out with an EG&G Ortec-142 pre-amplifier. The signal was shaped with an Ortec-485 post-amplifier with a shaping time of 500 ns and sent to a PC-based multichannel analyser. Fig. 3 shows the spectrum obtained at an applied bias of 300 V at room temperature. The spectrum is clearly separated from the noise pedestal and shows the familiar Gaussian shape of an alpha spectrum obtained with a planar diode. Both types of charge carrier will contribute to the total charge. The charge collection efficiency (CCE) for this spectrum was 48%, although the rate of counting was low in comparison to standard planar diodes. A Gaussian curve was fitted to the above spectrum to obtain the FWHM. From this the signal-to-noise ratio (S=N) was found to be 3 : 1. This is a poor ratio and would have been due, in part, to the contaminated alpha source described earlier. The CCE measurements as a function of applied bias are shown in Fig. 4. The maximum CCE in this plot was 50%. This incomplete charge collection is a manifestation of charge trapping, which affects both types of charge carrier and is seen in planar diodes made from the same substrate material.
3.3. Simulation Electrical characteristics were calculated using the MEDIC1 software package [7]. The inputs to the model were:
Fig. 2. Nine-electrode cell I–V characteristics.
1. the cell dimensions, 2. Schottky barrier height=0.79 eV, 3. ND ¼ 1:5 1013 ;
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
201
Fig. 3. 4.1 MeV alpha particle spectrum of nine-electrode cell at room temperature.
Fig. 4. CCE measurements of 4.1 MeV alpha particles at room temperature.
4. zero charge trapping, 5. impact ionisation turned on. Comparing simulation with experiment is an essential contribution to the validation of experimental results. The first simulation used identical cell dimensions to those of the fabricated cell i.e. nine electrodes of 60 mm diameter and 210 mm
pitch. The bias was applied to the central electrode while the eight surrounding electrodes were held at zero. The reverse biased I–V characteristic behaviour is similar to the reverse region shown experimentally in Fig. 3. The leakage current was very low and breakdown commenced between 450 and 500 V. Such an agreement between simulation
202
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
and experiment inspires confidence in the accuracy of the experimental results, especially since the experimental I–V curve showed a vastly different characteristic to that obtained with planar diodes made from the same substrate and electrode material. The simulated forward biased I–V characteristic differs from that of the reverse biased simulation, which again shows similarity with the experiment. The leakage current had a slightly higher average value than that seen in the reverse bias characteristic and began to increase rapidly at a lower value of applied bias. Again, this is similar to what was seen experimentally. Fig. 5 shows the potential distribution across the cell. It can be seen clearly that it does not extend all the way to the other electrodes (at the corners
and edges of the cuboid) and therefore the device is not fully depleted at 550 V. The undepleted area covers a sizeable percentage of the total area and this may go some way to explain the incomplete charge collection described earlier. With reverse breakdown occurring just below 500 V and incomplete depletion at 550 V, it seems it would not be possible to fully deplete a 3-D detector with these dimensions. It was therefore interesting to see what the response from a cell with different dimensions would be. The second simulated cell consisted of nine electrodes with 10 mm diameters and 25 mm pitches. The biasing conditions were the same as those for the previous cell. The potential distribution characteristics up to 20 V showed clearly that this device was fully depleted at this bias and, infact,
Fig. 5. Sixty micrometer diameter; 210 mm pitch electrode cell simulation of potential distribution.
A.R. Meikle et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 198–203
the program showed the device was fully depleted at just 8 V. This is a dramatically different result from the first cell simulation, where the device had not fully depleted at 550 V. The reverse bias I–V characteristic for the second cell found the leakage current to be low and the breakdown region commenced between 200 and 250 V. Given that full depletion occurs at just 8 V, it was obvious that this device could function at full depletion without any dramatic increase in current. It would seem that a device with these dimensions could offer full depletion, low voltage operation.
203
breakdown voltage and this may have contributed to the incomplete charge collection observed. Simulations of a cell with 10 mm diameter electrodes with 25 mm pitches were also performed. The leakage current again was much lower than that of a planar diode and the breakdown voltage was approximately 250 V. However, the most dramatic observation was that this cell fully depleted at just 8 V. The simulations also showed that there were undepleted regions in this type of cell due to the layout of the electrodes. This problem could be overcome simply by designing a cell with six electrodes in a hexagonal array.
4. Conclusions Acknowledgements A 3-D detector has been fabricated and tested. A nine-electrode test cell consisting of electrodes with 60 mm diameters and 210 mm pitches was fabricated on a 200 mm thick SI-U GaAs substrate. Holes were created by drilling with a 10 ns pulse duration Nd : YAG laser with a 5 cm focal length objective lens. Electrodes were formed within these holes by metal evaporation from both sides. Current–voltage characteristics showed asymmetrical forward and reverse characteristics. The breakdown voltage in the reverse region was approximately 500 V with a corresponding leakage current of just 5 nA. Charge collection experiments were carried out using 4.1 MeV alpha particles. The CCE was seen to increase with applied bias before reaching a maximum value of 50%. A S=N ratio of 3 : 1 was obtained, although a contaminated source may have contributed to this low value. Computer modelling was performed using the MEDICI software package. A simulation of a cell with 60 mm electrodes with 210 mm pitches was performed to compare with the experimental results. The forward and reverse I–V characteristics were asymmetrical, the leakage current much lower than that seen in planar diodes and the reverse breakdown region commenced at approximately 500 V. These results were all similar to what was seen experimentally. However, the simulations showed that the cell was not fully depleted at the
The authors would like to thank Mr. T. McCanny of the Laser Ionisation Group in the Department of Physics and Astronomy at the University of Glasgow for assistance in the laser drilling of holes. One of usFK. MathiesonFgratefully acknowledges funding through a CASE studentship award from BNFL. The authors would also like to thank Mr. S. Passmore for some useful discussions.
References [1] R.L. Bates, C. DaVia, S. DAuria, V. O’Shea, C. Raine, K.M. Smith, Nucl. Instr. and Meth. A 395 (1997) 54. [2] M. Rogalla, M. Battke, N. Duda, R. Geppert, J. Ludwig, R. Irsigler, T. Schmid, K. Runge, A. SoldnerRembold, Nucl. Instr. and Meth. A 410 (1998) 41. [3] S. Parker, 3-DFa new architecture for solid state radiation detectors, University of Hawaii internal report, 1996, UH 511-839-96. [4] A.R. Meikle, K. Ledingham, J.H. Marsh, B. More, V. O’Shea, C. Raine, K.M. Smith, Nucl. Instr. and Meth. A 410 (1998) 115. [5] R.L. Bates, Gallium arsenide radiation detectors for the ATLAS experiment, Ph.D. Thesis, Department of Physics and Astronomy, University of Glasgow, 1997, RAL-TH-97010. [6] R.L. Bates, C. DaVia, V. O’Shea, C. Raine, K.M. Smith, R. Adams, Nucl. Instr. and Meth. A 410 (1998) 46. [7] MEDICI, Technology Modeling Associates Inc., 3950 Fabian Way, Palo Alto, CA, USA.