- Email: [email protected]
Solid-state photo-detectors for both CT and PET applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338 www.elsevier.com/locate/nima
Solid-state photo-detectors for both CT and PET applications Danielle Moraes, Jan Kaplon, Pierre Jarron CERN, CH-1211 Geneva 23, Switzerland Available online 10 November 2006
Abstract New semiconductor detectors have recently gained a lot of attention for medical applications in general. Advances in CdZnTe-detector arrays might improve both energy resolution and spatial resolution of clinical X-ray systems. Alternative system designs based on TFA technology combining photo-detector arrays with CMOS electronics open a possibility for compact imaging cameras. This scenario allows for the use of alternative materials such as a-Si:H and HgI2 that can be applied alone or integrated with scintillators. Results obtained with such materials are presented. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 29.30.Kv; 81.05.Gc; 87.58.Fg; 87.59.Fm Keywords: Radiation detection; Hydrogenated amorphous silicon; Mercury iodide; CdZnTe; PET and CT
1. Introduction Modern clinical diagnosis is based on systems that convert X-ray images into visible images, enabling acquisition of single images, images sequences or even real-time display. Recent advances in the use of composite material for X-ray detection [1] have shown that Cadmium– Zinc–Telluride (CdZnTe) is a suitable material for detection of X-rays up to a range of 300 keV. CdZnTe detectors offer high stopping power, wide band gap and allows for operation at room temperature. The high effective atomic number ensures good conversion efficiency at 100 keV with a 3 mm thickness, making it an attractive material for Computed Tomography (CT) application. Positron Emission Tomography (PET) is widely recognized as the best available molecular non-invasive diagnosis technique. PET instrumentation made significant progress over the last years, benefiting from the ongoing development of new particle physics detector components like inorganic scintillators and new photo-detectors. Materials such as hydrogenated amorphous silicon (a-Si:H) and polycrystalline mercuric iodide (HgI2) have gained a lot of attention for their use as photo-detector and Corresponding author.
E-mail address: [email protected] (D. Moraes). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.094
photoconductor. These materials have a unique capability to be deposited directly on top of integrated circuits, are known to be radiation hard and can be used independently or integrated with scintillators. The main characteristics of the materials described in this work are summarized in Table 1.
2. Counting mode readout of CdZnTe sensors CdZnTe is a room temperature semiconductor that directly converts X-ray or g-ray photons into electrons. In addition to its good conversion efficiency at 100 keV, the Compton scattering cross-section at 50 keV range is 2 orders of magnitude lower than the photoelectric absorption Table 1 Material properties Property
CdZnTe
a-Si:H
HgI2
Density r (g.cm 3) Mobility gap EG (eV) Dielectric constant Resistivity r (O cm) Electron drift mobility me (cm2/Vs) Hole drift mobility mh (cm2/Vs) Pair creation energy (eV)
5.78 1.57 10.9 3 1010 1000 50–80 4.64
2.15 1.7–1.8 12 1011–1012 1–10 0.011–1 4–6
6.4 2.13 8.8 1013 100 4 4.2
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D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
Fig. 1. CERN_DxCTA channel basic architecture.
cross-section, ensuring that X-ray interactions depositing full energy are predominant [2]. Therefore, the development of counting mode electronics for CdZnTe detectors readout became an attractive option for applications such as hard X-ray astronomy (10–100 keV) and high-flux X-ray applications, such as CT. 2.1. Front-end electronics CERN–DxCTA is a front-end electronic, dedicated for the readout of small CdZnTe sensors, implemented in a commercial 0.25 mm CMOS process using a 2.5 V power supply. It has been developed at CERN in collaboration with Interon.1 The chip consists of 128 front-end channels, shown in Fig. 1. Each channel is formed by a transimpedance amplifier followed by a 20 ns peaking time shaper stage, two discriminators and two 18-bit static ripple-counters. The channel has two 5-bit Digital to Analog Converters (DAC) integrated to it. The double discriminator and counterstructure allows two independent threshold settings. The circuit gain is measured to 143.5 mV with a channelto-channel gain variation of 1.5% rms. The measured equivalent noise charge is 879e for the nominal sensor capacitance of 5 pF. The power consumption is 2.1 mW per channel. A detail description of the circuit and complete evaluation of the electronic properties is described in Ref. [3]. 2.2. CdZnTe measurement results The CdZnTe measurements are performed with crystals manufactured by eV Products2 with very thin metalized 1 2
Interon AS, Ringveien 14, N-1386 Asker, Norway. eV Products, 373 Saxonburg Blvd., PA 16056, USA.
Fig. 2. Threshold scan obtained with cobalt and americium.
electrode geometries. The CdZnTe crystal is 5 mm thick and it has a continuous cathode and a pixilated anode with 1 mm2 pixels. For the preliminary results, only one pixel is wire-bonded to the readout channel, while the others are connected to the ground. A voltage of about 240 V/mm is applied to the cathode. The radiation sources are placed above the cathode, in order to exploit the higher electron mobility in comparison to the holes. Fig. 2 shows the full channel front-end threshold scan obtained with cobalt (57Co) and americium (241Am) sources. For a discriminator threshold of 150 mV, the 59.5 keV peak from the americium source can be detected.
ARTICLE IN PRESS D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
3. Mercury iodide Mercury iodide (HgI2) is an attractive material for use as radiation detector. Its X-ray sensitivity is more than four times higher than that of other scintillators or photoconductors, currently used as X-ray detectors. The advantages of HgI2 for digital X-ray radiography and fluoroscopic medical imaging have been demonstrated in previous works [4–7].
335
Mercury iodide has a high atomic number and directly converts X-rays into electrical signals. It requires lowenergy to generate electron–hole pairs and it has a high mobility lifetime of the electrons carriers (majority charge carriers). In addition, it has low leakage current and is tolerant to radiation damage. HgI2 has a high potential for the use in very advanced 3D-PET and HEP calorimetry detector heads, which should operate in fast singlecounting mode with a modest energy resolution.
Fig. 3. Schematic view of mercury iodide planar (a) and ASIC (b) based detector.
ARTICLE IN PRESS 336
D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
Fig. 4. Dark current of a 350 mm HgI2 planar detector as a function of the applied bias.
In order to evaluate the properties of the HgI2 polycrystalline film, both planar and ASIC-based detectors are made. The HgI2 polycrystalline film described in this work is fabricated by Real-Time Radiography3 using Physical Vapour Deposition (PVD) technique. The planar detector consists of 20 strips of Indium Tin Oxide (ITO) deposited on a glass substrate. The strips are 10 mm long and have a pitch of 2.54 mm. A thick layer of mercury iodide is deposited on top of the strips, followed by a Platinum (Pt) layer, defining the top contact. The overall sample is coated with a polymer encapsulation layer. Fig. 3a shows a schematic view of the sample. The dark current is measured for planar detectors of different thickness. As shown in Fig. 4, the dark current obtained for a 350-mm-thick detector is below 20 pA/mm2 for a bias voltage of 1 V/mm. The dark current is an important parameter for the operation of such detectors at high electric field to minimize the carrier drift time. The detector, shown in Fig. 3b, is an ASIC-based detector. It consists of a 450 mm HgI2 film deposited directly on top of the MACROPAD chip. The MACROPAD circuit has 48 octagonal pads with 380 mm pitch. Each pad is connected to a transimpedance amplifier followed by a shaper stage. The circuit has a peaking time of 160 ns and a measured noise of 27 e rms [8]. A 50 mm layer of platinum is deposited on top of the HgI2, defining the bias electrode. The ASIC-based detector shows a higher dark current than a planar detector. This effect is directly related to the
non-flatness of the ASIC surface, due to the passivation layer steps. The high dark current limits the applied voltage. As a consequence, the carrier mobility is reduced and the generated charge from a radioactive source is not fully collected.
3 Real-Time Radiography Ltd., 1b Jerusalem Technology Park, Jerusalem 91487, Israel.
4 Institute of Microtechnology, Rue Breguet 2, CH-2000 Neuchaˆtel, Switzerland.
4. Hydrogenated amorphous silicon Hydrogenated amorphous silicon (a-Si:H) electrical properties were first reported in 1976 [9]. Since then, considerable research and development have shown that a-Si is the most suitable semiconductor for large-area devices. X-ray imagers based on a-Si and Cesium Iodide (CsI) are under development [10]. The a-SiH detectors evaluated in the present work are based on deposition of an a-Si:H film on top of an ASIC that performs both signal charge amplification and readout processing. The advantages are the extremely compact and low-cost detector design, together with fast and low-noise signal retrieval. The a-Si:H film is deposited by Institute of Micro-Technology of Neuchatel,4 using the Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF-PECVD). The detector is formed by three layers (n–i–p diode structure): a top p-doped layer, an intrinsic sensitive layer and a bottom n-doped layer. The n-doped layer is a 20 nm thin low conductive layer that provides pixel isolation and avoids additional ASIC patterning. An ITO electrode is used as top contact. Fig. 5 shows a schematic view of the detector.
ARTICLE IN PRESS D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
337
Fig. 5. Hydrogenated amorphous silicon (a-Si:H) detector.
This can be explained by a non-full depletion of the n–i–p diode. Various a-Si:H detectors containing different metal structures and amplifiers optimized for different speed and noise performance are produced and tested. The results are described in Refs. [11–14]. 5. Conclusion
Fig. 6. Spectrum of 5.9 keV X-rays from 55Fe source obtained with a 15 mm thick a-Si:H film deposited on a MACROPAD.
Direct detection of soft X-rays are carried out on a 15 mm thick film deposited on a MACROPAD chip [8]. Signals from the detector are readout on an oscilloscope in self-trigger mode. A maximum reverse bias of 145 V is applied to the a-Si:H film. The noise level of the readout electronics is measured equal to 40e rms. The spectrum obtained with 5.9 keV X-rays from an iron (55Fe) source is shown in Fig. 6. A broad peak is observed with a peak charge at about 650e . The signal corresponds to charges induced by the transport of both electrons and holes. However, due to the low drift mobility of holes, only part of the signal from holes is readout. No saturation of the peak charge is observed, as a peak charge of 510 and 582e are readout for applied biases of 110 and 130 V.
The CdZnTe, HgI2 and a-Si:H materials studied in the present work are potential candidates for medical imaging applications. A fast CdZnTe counting mode front-end electronic has been developed and tested with CdZnTe sensors. It has been shown that X-rays from 59 keV up to 122 keV can be detected with 1 mm2 pixels. The results obtained with mercury iodide photoconductor and hydrogenated amorphous silicon are preliminary and further studies have to be performed. The integration of a scintillator layer on top of the a-Si:H detectors is under investigation. Acknowledgments The authors are grateful to all colleagues and collaborators that contributed to the measurements and studies to the work presented. References [1] T. Takahasin, S. Watanabe, IEEE Trans. Nucl. Sci. NS-48 (4) (2001) 950. [2] B.L. Henke, E.M. Gullikson, J.C. Davis, At. Data Nuclear Data Tables 54–2 (1993) 181.
ARTICLE IN PRESS 338 [3] [4] [5] [6] [7] [8]
D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338 D. Moraes, E. Nygard, A. Rudge, Proc. IEEE-NSS 1 (2005) 348. M. Schieber, et al., Proc. SPIE-MI 3770 (1999) 146. R.A. Street, et al., Proc. SPIE-MI 4320 (2001) 1. G. Zentai, et al., Proc. SPIE-MI 5030 (2003) 77. G. Zentai, et al., Proc. SPIE 5368 (2004) 200. D. Moraes, et al., J. Non-Cryst. Solids 338 (2004) 729.
[9] [10] [11] [12] [13] [14]
W.E. Spear, P.G. LeComber, Philos. Mag. 33 (1976) 935. M. Hoheisel, et al., J. Non-Cryst. Solids 230 (1998) 1300. N. Wyrsch, et al., Phys. Stat. Sol. C (2004) 1284. M. Despeisse, et al., Nucl. Instr. and Meth. A 518 (2004) 357. M. Despeisse, D. Moraes, et al., Proc. IEEE-NSS 1 (2005) 1389. P. Jarron, D. Moraes, et al., Nucl. Inst. and Meth. A 560 (2006) 122.
Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338 www.elsevier.com/locate/nima
Solid-state photo-detectors for both CT and PET applications Danielle Moraes, Jan Kaplon, Pierre Jarron CERN, CH-1211 Geneva 23, Switzerland Available online 10 November 2006
Abstract New semiconductor detectors have recently gained a lot of attention for medical applications in general. Advances in CdZnTe-detector arrays might improve both energy resolution and spatial resolution of clinical X-ray systems. Alternative system designs based on TFA technology combining photo-detector arrays with CMOS electronics open a possibility for compact imaging cameras. This scenario allows for the use of alternative materials such as a-Si:H and HgI2 that can be applied alone or integrated with scintillators. Results obtained with such materials are presented. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 29.30.Kv; 81.05.Gc; 87.58.Fg; 87.59.Fm Keywords: Radiation detection; Hydrogenated amorphous silicon; Mercury iodide; CdZnTe; PET and CT
1. Introduction Modern clinical diagnosis is based on systems that convert X-ray images into visible images, enabling acquisition of single images, images sequences or even real-time display. Recent advances in the use of composite material for X-ray detection [1] have shown that Cadmium– Zinc–Telluride (CdZnTe) is a suitable material for detection of X-rays up to a range of 300 keV. CdZnTe detectors offer high stopping power, wide band gap and allows for operation at room temperature. The high effective atomic number ensures good conversion efficiency at 100 keV with a 3 mm thickness, making it an attractive material for Computed Tomography (CT) application. Positron Emission Tomography (PET) is widely recognized as the best available molecular non-invasive diagnosis technique. PET instrumentation made significant progress over the last years, benefiting from the ongoing development of new particle physics detector components like inorganic scintillators and new photo-detectors. Materials such as hydrogenated amorphous silicon (a-Si:H) and polycrystalline mercuric iodide (HgI2) have gained a lot of attention for their use as photo-detector and Corresponding author.
E-mail address: [email protected] (D. Moraes). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.094
photoconductor. These materials have a unique capability to be deposited directly on top of integrated circuits, are known to be radiation hard and can be used independently or integrated with scintillators. The main characteristics of the materials described in this work are summarized in Table 1.
2. Counting mode readout of CdZnTe sensors CdZnTe is a room temperature semiconductor that directly converts X-ray or g-ray photons into electrons. In addition to its good conversion efficiency at 100 keV, the Compton scattering cross-section at 50 keV range is 2 orders of magnitude lower than the photoelectric absorption Table 1 Material properties Property
CdZnTe
a-Si:H
HgI2
Density r (g.cm 3) Mobility gap EG (eV) Dielectric constant Resistivity r (O cm) Electron drift mobility me (cm2/Vs) Hole drift mobility mh (cm2/Vs) Pair creation energy (eV)
5.78 1.57 10.9 3 1010 1000 50–80 4.64
2.15 1.7–1.8 12 1011–1012 1–10 0.011–1 4–6
6.4 2.13 8.8 1013 100 4 4.2
ARTICLE IN PRESS 334
D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
Fig. 1. CERN_DxCTA channel basic architecture.
cross-section, ensuring that X-ray interactions depositing full energy are predominant [2]. Therefore, the development of counting mode electronics for CdZnTe detectors readout became an attractive option for applications such as hard X-ray astronomy (10–100 keV) and high-flux X-ray applications, such as CT. 2.1. Front-end electronics CERN–DxCTA is a front-end electronic, dedicated for the readout of small CdZnTe sensors, implemented in a commercial 0.25 mm CMOS process using a 2.5 V power supply. It has been developed at CERN in collaboration with Interon.1 The chip consists of 128 front-end channels, shown in Fig. 1. Each channel is formed by a transimpedance amplifier followed by a 20 ns peaking time shaper stage, two discriminators and two 18-bit static ripple-counters. The channel has two 5-bit Digital to Analog Converters (DAC) integrated to it. The double discriminator and counterstructure allows two independent threshold settings. The circuit gain is measured to 143.5 mV with a channelto-channel gain variation of 1.5% rms. The measured equivalent noise charge is 879e for the nominal sensor capacitance of 5 pF. The power consumption is 2.1 mW per channel. A detail description of the circuit and complete evaluation of the electronic properties is described in Ref. [3]. 2.2. CdZnTe measurement results The CdZnTe measurements are performed with crystals manufactured by eV Products2 with very thin metalized 1 2
Interon AS, Ringveien 14, N-1386 Asker, Norway. eV Products, 373 Saxonburg Blvd., PA 16056, USA.
Fig. 2. Threshold scan obtained with cobalt and americium.
electrode geometries. The CdZnTe crystal is 5 mm thick and it has a continuous cathode and a pixilated anode with 1 mm2 pixels. For the preliminary results, only one pixel is wire-bonded to the readout channel, while the others are connected to the ground. A voltage of about 240 V/mm is applied to the cathode. The radiation sources are placed above the cathode, in order to exploit the higher electron mobility in comparison to the holes. Fig. 2 shows the full channel front-end threshold scan obtained with cobalt (57Co) and americium (241Am) sources. For a discriminator threshold of 150 mV, the 59.5 keV peak from the americium source can be detected.
ARTICLE IN PRESS D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
3. Mercury iodide Mercury iodide (HgI2) is an attractive material for use as radiation detector. Its X-ray sensitivity is more than four times higher than that of other scintillators or photoconductors, currently used as X-ray detectors. The advantages of HgI2 for digital X-ray radiography and fluoroscopic medical imaging have been demonstrated in previous works [4–7].
335
Mercury iodide has a high atomic number and directly converts X-rays into electrical signals. It requires lowenergy to generate electron–hole pairs and it has a high mobility lifetime of the electrons carriers (majority charge carriers). In addition, it has low leakage current and is tolerant to radiation damage. HgI2 has a high potential for the use in very advanced 3D-PET and HEP calorimetry detector heads, which should operate in fast singlecounting mode with a modest energy resolution.
Fig. 3. Schematic view of mercury iodide planar (a) and ASIC (b) based detector.
ARTICLE IN PRESS 336
D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
Fig. 4. Dark current of a 350 mm HgI2 planar detector as a function of the applied bias.
In order to evaluate the properties of the HgI2 polycrystalline film, both planar and ASIC-based detectors are made. The HgI2 polycrystalline film described in this work is fabricated by Real-Time Radiography3 using Physical Vapour Deposition (PVD) technique. The planar detector consists of 20 strips of Indium Tin Oxide (ITO) deposited on a glass substrate. The strips are 10 mm long and have a pitch of 2.54 mm. A thick layer of mercury iodide is deposited on top of the strips, followed by a Platinum (Pt) layer, defining the top contact. The overall sample is coated with a polymer encapsulation layer. Fig. 3a shows a schematic view of the sample. The dark current is measured for planar detectors of different thickness. As shown in Fig. 4, the dark current obtained for a 350-mm-thick detector is below 20 pA/mm2 for a bias voltage of 1 V/mm. The dark current is an important parameter for the operation of such detectors at high electric field to minimize the carrier drift time. The detector, shown in Fig. 3b, is an ASIC-based detector. It consists of a 450 mm HgI2 film deposited directly on top of the MACROPAD chip. The MACROPAD circuit has 48 octagonal pads with 380 mm pitch. Each pad is connected to a transimpedance amplifier followed by a shaper stage. The circuit has a peaking time of 160 ns and a measured noise of 27 e rms [8]. A 50 mm layer of platinum is deposited on top of the HgI2, defining the bias electrode. The ASIC-based detector shows a higher dark current than a planar detector. This effect is directly related to the
non-flatness of the ASIC surface, due to the passivation layer steps. The high dark current limits the applied voltage. As a consequence, the carrier mobility is reduced and the generated charge from a radioactive source is not fully collected.
3 Real-Time Radiography Ltd., 1b Jerusalem Technology Park, Jerusalem 91487, Israel.
4 Institute of Microtechnology, Rue Breguet 2, CH-2000 Neuchaˆtel, Switzerland.
4. Hydrogenated amorphous silicon Hydrogenated amorphous silicon (a-Si:H) electrical properties were first reported in 1976 [9]. Since then, considerable research and development have shown that a-Si is the most suitable semiconductor for large-area devices. X-ray imagers based on a-Si and Cesium Iodide (CsI) are under development [10]. The a-SiH detectors evaluated in the present work are based on deposition of an a-Si:H film on top of an ASIC that performs both signal charge amplification and readout processing. The advantages are the extremely compact and low-cost detector design, together with fast and low-noise signal retrieval. The a-Si:H film is deposited by Institute of Micro-Technology of Neuchatel,4 using the Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF-PECVD). The detector is formed by three layers (n–i–p diode structure): a top p-doped layer, an intrinsic sensitive layer and a bottom n-doped layer. The n-doped layer is a 20 nm thin low conductive layer that provides pixel isolation and avoids additional ASIC patterning. An ITO electrode is used as top contact. Fig. 5 shows a schematic view of the detector.
ARTICLE IN PRESS D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338
337
Fig. 5. Hydrogenated amorphous silicon (a-Si:H) detector.
This can be explained by a non-full depletion of the n–i–p diode. Various a-Si:H detectors containing different metal structures and amplifiers optimized for different speed and noise performance are produced and tested. The results are described in Refs. [11–14]. 5. Conclusion
Fig. 6. Spectrum of 5.9 keV X-rays from 55Fe source obtained with a 15 mm thick a-Si:H film deposited on a MACROPAD.
Direct detection of soft X-rays are carried out on a 15 mm thick film deposited on a MACROPAD chip [8]. Signals from the detector are readout on an oscilloscope in self-trigger mode. A maximum reverse bias of 145 V is applied to the a-Si:H film. The noise level of the readout electronics is measured equal to 40e rms. The spectrum obtained with 5.9 keV X-rays from an iron (55Fe) source is shown in Fig. 6. A broad peak is observed with a peak charge at about 650e . The signal corresponds to charges induced by the transport of both electrons and holes. However, due to the low drift mobility of holes, only part of the signal from holes is readout. No saturation of the peak charge is observed, as a peak charge of 510 and 582e are readout for applied biases of 110 and 130 V.
The CdZnTe, HgI2 and a-Si:H materials studied in the present work are potential candidates for medical imaging applications. A fast CdZnTe counting mode front-end electronic has been developed and tested with CdZnTe sensors. It has been shown that X-rays from 59 keV up to 122 keV can be detected with 1 mm2 pixels. The results obtained with mercury iodide photoconductor and hydrogenated amorphous silicon are preliminary and further studies have to be performed. The integration of a scintillator layer on top of the a-Si:H detectors is under investigation. Acknowledgments The authors are grateful to all colleagues and collaborators that contributed to the measurements and studies to the work presented. References [1] T. Takahasin, S. Watanabe, IEEE Trans. Nucl. Sci. NS-48 (4) (2001) 950. [2] B.L. Henke, E.M. Gullikson, J.C. Davis, At. Data Nuclear Data Tables 54–2 (1993) 181.
ARTICLE IN PRESS 338 [3] [4] [5] [6] [7] [8]
D. Moraes et al. / Nuclear Instruments and Methods in Physics Research A 571 (2007) 333–338 D. Moraes, E. Nygard, A. Rudge, Proc. IEEE-NSS 1 (2005) 348. M. Schieber, et al., Proc. SPIE-MI 3770 (1999) 146. R.A. Street, et al., Proc. SPIE-MI 4320 (2001) 1. G. Zentai, et al., Proc. SPIE-MI 5030 (2003) 77. G. Zentai, et al., Proc. SPIE 5368 (2004) 200. D. Moraes, et al., J. Non-Cryst. Solids 338 (2004) 729.
[9] [10] [11] [12] [13] [14]
W.E. Spear, P.G. LeComber, Philos. Mag. 33 (1976) 935. M. Hoheisel, et al., J. Non-Cryst. Solids 230 (1998) 1300. N. Wyrsch, et al., Phys. Stat. Sol. C (2004) 1284. M. Despeisse, et al., Nucl. Instr. and Meth. A 518 (2004) 357. M. Despeisse, D. Moraes, et al., Proc. IEEE-NSS 1 (2005) 1389. P. Jarron, D. Moraes, et al., Nucl. Inst. and Meth. A 560 (2006) 122.