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A study of charge collection processes on polycrystalline diamond detectors
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
A study of charge collection processes on polycrystalline diamond detectors S. Mersia,*, E. Borchib,a, M. Bruzzib,a, R. D’Alessandroc,a, S. Lagomarsinob,a, S. Sciortinob,a a INFN, Sezione di Firenze, Via Sansone, 1, I-50019 Sesto Fiorentino, Florence, Italy Dipartimento di Energetica, Universita" di Firenze, Via Santa Marta, 3, I-50139 Florence, Italy c Dipartimento di Fisica, Universita" di Firenze, Via Sansone, 1, I-50139 Sesto Fiorentino, Florence, Italy b
Available online 11 June 2004
Abstract We performed a study of charge collection distance (CCD) on medium to high-quality prototypes of diamond sensors prepared by Chemical Vapor Deposition (CVD). We studied the Charge Collection Efficiency in these materials supposing that it is limited by the presence of a recombination level and a distribution of trap levels centered at 1:7 eV from the band-edge. We also supposed that the exposition to ionizing radiation can make the trap levels ineffective (pumping effect). We have shown that these assumptions are valid by correlating the CCD to the pumping efficiency. Moreover, we have shown that the pumping efficiency is bias-dependent. We have explained our experimental results assuming that trapped carriers generate an electric field inside the diamond bulk. r 2004 Published by Elsevier B.V. PACS: 07.77.Ka; 29.40.Wk; 72.20.Jv; 81.15.Gh Keywords: Charge collection; CVD diamond; Defect; Priming; Pumping effect
1. Introduction A major effort of research groups involved in new high-energy physics experiments (S-LHC), or in space applications in very harsh environments, consists in the development and characterization of sensors yielding superior radiation hardness and high-temperature limit of operation. The different approaches to this challenging experimental problem are mainly the radiation hardening of silicon *Corresponding author. E-mail address: mersi@fi.infn.it (S. Mersi). 0168-9002/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.nima.2004.05.063
(RD50 CERN Collaboration [1]) with oxygenated silicon characterization, materials engineering (3-D silicon detectors) and the quest for new materials. Silicon carbide (SiC) and diamond are presently being tested. Epilayer 4H SiC-based particle detectors are studied by RD50 [4] whereas, the great improvement of chemical vapor deposited (CVD) diamond trackers in recent years is due to the RD42 Collaboration [2,3]. CVD polycrystalline diamond now yields an average carrier drift length (charge collection distance or CCD) of about 250 mm: This figure
ARTICLE IN PRESS S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
147
of merit together with the extremely low leakage current has allowed preparing diamond devices with a fast electronics with a S/N ratio of 10/1. However, it seems that progresses in diamond efficiency improvement have slackened, mainly because of the polycrystalline nature of the material. Devices based on single-crystal high-pressure, high-temperature diamond (HPHT) are also investigated [5]. The growth of high-quality CVD layers on low-quality single-crystal HPHT diamond is also very promising [3], however, it is limited by the small area of the available substrates. Moreover, homoepitaxial growth does not rule out completely the presence of defects as in natural diamond or in other single crystal materials. The study of defects with many different solid state [6–9] and nuclear techniques [10,11] has given a deeper insight on the physics of the transport in CVD diamond, yet a thorough picture of defects and their influence on carriers’ lifetime in CVD diamond is lacking. The picture of a deep recombination band and a distribution of trapping levels close to the bandedge is widely accepted. In a previous work [12] we have reported a combined investigation of photoconductivity and charge collection efficiency, aimed to determine quantitatively the location of these two bands in the diamond gap. In this work, we present a detailed study on polarization in diamond bulk. We studied the hysteresis loop of CCD vs. bias field, and characterized pumping dynamics. We discuss our results in terms of a simple model that takes into account a space charge formation which leads to a non-uniform field inside the device.
the system was 225 e =mV and the noise level was ENC E350 e : The samples were medium to high-quality particle detectors provided by the RD42 Collaboration and the metallizations were fabricated at The Ohio State University in the framework of RD42. The samples were coated with a standard single pad metallization [13].
2. Experimental setup
3.2. Correlation of hysteresis effect with respect to CCD
The CCD measurements were carried out with a characterization setup reproduced from the one used at NIKHEF in Amsterdam [13]. The source used was a 0:1 mCi 90 Sr source. The so-called pumping effect, i.e. the increase in collection efficiency after exposure to a b source, was studied by use of a 10 mCi 90 Sr source. The sensitivity of
3. Results and discussion 3.1. Correlation of charge trapping with respect to CCD The fundamental assumption in most CVD diamond studies is that charge collection efficiency Z is the function of carriers trapping: Zpn1 ; where n is trap density. In order to enhance CCD, it is a common procedure to irradiate the diamond detector with b particles, thus generating an amount of charge carriers, which become trapped and passivate deep trap levels. This procedure is generally known as pumping; the CCD enhancement obtained this way is long lasting (up to some months [14]), until the sample is exposed to light or heated (depumped). We began our study by checking the consistency of this picture, showing the correlation of trap density (measured via CCD) with respect to pumping efficiency (defined as the measured ratio between CCD in the pumped state and in the depumped state). We followed this procedure with several diamond detectors with different characteristics, and we reported these measurements in Fig. 1. We can easily observe in this graphics an anticorrelation between CCD and pumping efficiency (assumed proportional to deep trap density).
Hysteresis is a well-known effect in CVD diamond: when the bias field is raised, the mean signal increases, but when bias field is decreased, the signal yields lower values. Thus by cycling the bias field between two opposite values, a hysteresis curve can be drawn (see Fig. 2). This can be
ARTICLE IN PRESS 148
S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
interpreted as the effect of differential trapping of charge carriers inside the diamond bulk; electron and holes have different lifetimes, and this results
in a space charge distribution, which forms as they drift apart. If this hypothesis is valid, then an anticorrelation between CCD and hysteresis effect should be shown. For this reason we measured the whole hysteresis curve in a few detector samples, with different quality; we report in Fig. 3 two hysteresis curves, measured on two samples. It is evident that higher quality samples feature a smaller hysteresis. 3.3. Pumping dynamics
Fig. 1. Pumping efficiency measured through CCD in the pumped ðLPUMP Þ and in the depumped state ðLDEPUMP Þ in a few samples (UTS1-P13, CDS71, CDS88 and CDS92-P3, respectively).
Fig. 2. A sample’s CCD vs. different applied fields. This is the typical S-curve widened by the well-known hysteresis effect.
We performed a detailed characterization of pumping effect, by measuring CCD on a few diamond samples after an exposure to different radiation doses; in this way we could build a curve representing the dependence of pumping on absorbed radiation. For each sample we made measurements both biased and unbiased. It is evident in Fig. 4 that an external field has a strong influence on pumping dynamics. In order to analyze our data we built a simple one deep trap model. We assumed uniform irradiation of the sample during pumping. Then we assumed that the chance for a trap to be filled is proportional to the free trap density and to the radiation flux. Finally, we introduced a constant fraction of traps as a fit parameter which cannot be passivated. A similar model was proposed by the RD42 collaboration [15]. According to this model we obtain the following expression: LðFÞ ¼
LðNÞ 1 þ a exp½gF
Fig. 3. Two hysteresis curves, measured on two different quality samples.
ð1Þ
ARTICLE IN PRESS S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
Fig. 4. A sample’s CCD after pumping without external field and with a bias field of 1 V=mm: In both cases the rate of particles crossing the bulk has been kept constant.
Table 1 Best fit of pumping parameters during exposure to a b source, for a sample either unbiased or with an external applied field close to saturation (see text)
a gðevents1 ) LðNÞ ðmmÞ
E ¼ 0 V=mm
E ¼ 1 V=mm
0:4770:02 ð1071Þ 106 23672
ð3:370:5Þ 106 21074
149
where LðFÞ is the sample’s CCD, F is the radiation fluence, a is the ratio: passivable traps/not passivable traps, and it is an intrinsic property of the sample, while LðNÞ and g are empirical constants. We report in Fig. 4 our measurements, together with a minimum squares fit, whose values are reported in Table 1. We note that a higher pumping time is required if an external field is applied. This is because drifting carriers remain inside the bulk for time lower than the tipical trapping time. However, the most important effect is that when the sample is biased the final value LðNÞ is significantly lower. This means that if one wants to optimize the pumping procedure, irradiation should be performed when the sample is unbiased. We ascribed this result to a polarization effect which will be discussed later. In order to further study pumping dynamics we also estimated the dose absorbed by the sample during pumping and we measured its CCD after pumping at several applied bias voltages. We report these measurements in Fig. 5. It seems that there is a very low threshold for the creation of a space charge distribution inside the material’s bulk. We also observe that some of the curves are not monotonic. We interpret this effect as the result of a competition between the sudden setting
Fig. 5. A sample’s CCD after various pumping doses, with different applied bias fields. In the inset: a detail of CCD at low doses.
ARTICLE IN PRESS 150
S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151 260 240
ccd (µm)
220 200 180 160
simulation
CCD
140 120 100 0
50
100
150
200
250
300
pumping time (s)
Fig. 6. A simulation of bias field inside a diamond, after pumping with a 1 V=mm bias voltage applied. A space charge distribution distorts the field.
Fig. 7. A comparison between CCD measured during pumping and our simulation.
In the picture outlined above, the lowering of CCD is due to a distortion of the local field inside the bulk (while the mean field is still VBIAS =thickness). This effect is due to a differential charge trapping inside the diamond bulk. In order to validate this hypothesis, we performed a MonteCarlo simulation of the charge trapping at different pumping doses in a biased diamond detector. In this calculation we assumed that there is no reinjection of carriers, i.e. a hole or electron with a lifetime as high to reach one of the metallizations is not injected again at the other electrode. Moreover, we assumed that the main contribution to CCD is given by one type of carrier only. This is consistent with the recent results obtained by lapping CVD diamond sensors [3]. As an outcome of our model, we report the electric field distribution along the thickness axis in Fig. 6. In Fig. 7 we report a simulation of the CCD trend that describes quite well our experimental data.
photoconductivity measurements, locating deep centers close to midgap and trap levels centered at 1:7 eV from the band-edge, with no identification of the band (conduction or valence band). In this work, we have started from the general assumption that the pumping effect is due to the filling of deep traps, so that pumping efficiency is related to the concentration of defects. We have shown an anticorrelation between CCD and pumping efficiency which is consistent with the above assumption. We also have shown that the pumping efficiency is bias-dependent and analyzed this effect with a first-order model. We have explained this effect in terms of a space charge forming as the induced carriers drift apart along the applied field and are trapped inside the bulk. The RD42 Collaboration has explained the wellknown effect of the hysteresis of the CCD vs. bias field in terms of polarization of the bulk. We have found that the hysteresis loop is less and less evident as the quality of the samples increases, i.e. as the concentration of traps decreases. This is consistent with our picture of the polarization of CVD diamond. Furthermore, we made a MonteCarlo simulation of the non-uniform applied field inside the diamond bulk, which allows to calculate the CCD trend during pumping with a good agreement with the experimental data.
4. Conclusions
Acknowledgements
We have previously characterized the trapping and recombination levels with combined CCD and
We wish to thank Dr. Fred Hartjes of NIKHEF Amsterdam and RD42 for helping us setting the
of a local field, which opposes to the carriers drift, and the passivation of the traps which increases the drift length. 3.4. Polarization dynamics
ARTICLE IN PRESS S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
CCD station and for useful discussion during his stay in Florence. We are indebted to The RD42 Collaboration for lending us some of their samples and in particular to Prof. Harris Kagan of The Ohio State University for metallizing them.
[6] [7] [8] [9]
References [1] M. Moll, (RD50 Collaboration), Nucl. Instr. and Meth. A 511 (2003) 97. [2] W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 511 (2003) 124. [3] W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 514 (2003) 79. [4] F. Nava, P. Vanni, M. Bruzzi, S. Lagomarsino, S. Sciortino, G. Wagner, C. Lanzieri, IEEE Trans. Nucl. Sci. 51 (1) (2004) 238. [5] E. Berdermann, et al., The Diamond Project at GSI - Perspectives, in: M. Barone, E. Borchi, J. Huston, C. Leroy, P.G. Rancoita, P. Riboni, R. Ruchti (Eds.), Proceedings of the Seventh International Conference on Advanced Technology & Particle Physics (ICATPP-7),
[10] [11]
[12]
[13] [14]
[15]
151
Como, October 2001, World Scientific, Singapore, p. 246. E. Vittone, et al., Diamond Relat. Mater. 8 (1999) 1234. K. Iakoubovskii, G.J. Adriaenssens, Diamond Relat. Mater. 9 (2000) 1349. M. Bruzzi, D. Menichelli, S. Sciortino, L. Lombardi, J. Appl. Phys. 91 (2002) 5765. C. Manfredotti, et al., Diamond Relat. Mater. 11 (2002) 446. D.N. Jamieson, et al., Nucl. Instr. and Meth. B 158 (1999) 628. C. Manfredotti, F. Fizzotti, A. Lo Giudice, C. Paolini, E. Vittone, R. Lu, Nucl. Instr. and Meth. B 187 (2002) 566. M. Bruzzi, F. Hartjes, S. Lagomarsino, D. Menichelli, S. Mersi, S. Miglio, M. Scaringella, S. Sciortino, Phys. Stat. Sol. (a) 199 (1) (2003) 138. W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 476 (2002) 686. F. Hartjes, et al., (RD42 collaboration), CVD Diamond as a Detector Material, Fourth International Conference on Radiation Effects on Semiconductor Materials Detectors and Devices, Florence, 11 July 2002. W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 447 (2000) 244.
Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
A study of charge collection processes on polycrystalline diamond detectors S. Mersia,*, E. Borchib,a, M. Bruzzib,a, R. D’Alessandroc,a, S. Lagomarsinob,a, S. Sciortinob,a a INFN, Sezione di Firenze, Via Sansone, 1, I-50019 Sesto Fiorentino, Florence, Italy Dipartimento di Energetica, Universita" di Firenze, Via Santa Marta, 3, I-50139 Florence, Italy c Dipartimento di Fisica, Universita" di Firenze, Via Sansone, 1, I-50139 Sesto Fiorentino, Florence, Italy b
Available online 11 June 2004
Abstract We performed a study of charge collection distance (CCD) on medium to high-quality prototypes of diamond sensors prepared by Chemical Vapor Deposition (CVD). We studied the Charge Collection Efficiency in these materials supposing that it is limited by the presence of a recombination level and a distribution of trap levels centered at 1:7 eV from the band-edge. We also supposed that the exposition to ionizing radiation can make the trap levels ineffective (pumping effect). We have shown that these assumptions are valid by correlating the CCD to the pumping efficiency. Moreover, we have shown that the pumping efficiency is bias-dependent. We have explained our experimental results assuming that trapped carriers generate an electric field inside the diamond bulk. r 2004 Published by Elsevier B.V. PACS: 07.77.Ka; 29.40.Wk; 72.20.Jv; 81.15.Gh Keywords: Charge collection; CVD diamond; Defect; Priming; Pumping effect
1. Introduction A major effort of research groups involved in new high-energy physics experiments (S-LHC), or in space applications in very harsh environments, consists in the development and characterization of sensors yielding superior radiation hardness and high-temperature limit of operation. The different approaches to this challenging experimental problem are mainly the radiation hardening of silicon *Corresponding author. E-mail address: mersi@fi.infn.it (S. Mersi). 0168-9002/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.nima.2004.05.063
(RD50 CERN Collaboration [1]) with oxygenated silicon characterization, materials engineering (3-D silicon detectors) and the quest for new materials. Silicon carbide (SiC) and diamond are presently being tested. Epilayer 4H SiC-based particle detectors are studied by RD50 [4] whereas, the great improvement of chemical vapor deposited (CVD) diamond trackers in recent years is due to the RD42 Collaboration [2,3]. CVD polycrystalline diamond now yields an average carrier drift length (charge collection distance or CCD) of about 250 mm: This figure
ARTICLE IN PRESS S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
147
of merit together with the extremely low leakage current has allowed preparing diamond devices with a fast electronics with a S/N ratio of 10/1. However, it seems that progresses in diamond efficiency improvement have slackened, mainly because of the polycrystalline nature of the material. Devices based on single-crystal high-pressure, high-temperature diamond (HPHT) are also investigated [5]. The growth of high-quality CVD layers on low-quality single-crystal HPHT diamond is also very promising [3], however, it is limited by the small area of the available substrates. Moreover, homoepitaxial growth does not rule out completely the presence of defects as in natural diamond or in other single crystal materials. The study of defects with many different solid state [6–9] and nuclear techniques [10,11] has given a deeper insight on the physics of the transport in CVD diamond, yet a thorough picture of defects and their influence on carriers’ lifetime in CVD diamond is lacking. The picture of a deep recombination band and a distribution of trapping levels close to the bandedge is widely accepted. In a previous work [12] we have reported a combined investigation of photoconductivity and charge collection efficiency, aimed to determine quantitatively the location of these two bands in the diamond gap. In this work, we present a detailed study on polarization in diamond bulk. We studied the hysteresis loop of CCD vs. bias field, and characterized pumping dynamics. We discuss our results in terms of a simple model that takes into account a space charge formation which leads to a non-uniform field inside the device.
the system was 225 e =mV and the noise level was ENC E350 e : The samples were medium to high-quality particle detectors provided by the RD42 Collaboration and the metallizations were fabricated at The Ohio State University in the framework of RD42. The samples were coated with a standard single pad metallization [13].
2. Experimental setup
3.2. Correlation of hysteresis effect with respect to CCD
The CCD measurements were carried out with a characterization setup reproduced from the one used at NIKHEF in Amsterdam [13]. The source used was a 0:1 mCi 90 Sr source. The so-called pumping effect, i.e. the increase in collection efficiency after exposure to a b source, was studied by use of a 10 mCi 90 Sr source. The sensitivity of
3. Results and discussion 3.1. Correlation of charge trapping with respect to CCD The fundamental assumption in most CVD diamond studies is that charge collection efficiency Z is the function of carriers trapping: Zpn1 ; where n is trap density. In order to enhance CCD, it is a common procedure to irradiate the diamond detector with b particles, thus generating an amount of charge carriers, which become trapped and passivate deep trap levels. This procedure is generally known as pumping; the CCD enhancement obtained this way is long lasting (up to some months [14]), until the sample is exposed to light or heated (depumped). We began our study by checking the consistency of this picture, showing the correlation of trap density (measured via CCD) with respect to pumping efficiency (defined as the measured ratio between CCD in the pumped state and in the depumped state). We followed this procedure with several diamond detectors with different characteristics, and we reported these measurements in Fig. 1. We can easily observe in this graphics an anticorrelation between CCD and pumping efficiency (assumed proportional to deep trap density).
Hysteresis is a well-known effect in CVD diamond: when the bias field is raised, the mean signal increases, but when bias field is decreased, the signal yields lower values. Thus by cycling the bias field between two opposite values, a hysteresis curve can be drawn (see Fig. 2). This can be
ARTICLE IN PRESS 148
S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
interpreted as the effect of differential trapping of charge carriers inside the diamond bulk; electron and holes have different lifetimes, and this results
in a space charge distribution, which forms as they drift apart. If this hypothesis is valid, then an anticorrelation between CCD and hysteresis effect should be shown. For this reason we measured the whole hysteresis curve in a few detector samples, with different quality; we report in Fig. 3 two hysteresis curves, measured on two samples. It is evident that higher quality samples feature a smaller hysteresis. 3.3. Pumping dynamics
Fig. 1. Pumping efficiency measured through CCD in the pumped ðLPUMP Þ and in the depumped state ðLDEPUMP Þ in a few samples (UTS1-P13, CDS71, CDS88 and CDS92-P3, respectively).
Fig. 2. A sample’s CCD vs. different applied fields. This is the typical S-curve widened by the well-known hysteresis effect.
We performed a detailed characterization of pumping effect, by measuring CCD on a few diamond samples after an exposure to different radiation doses; in this way we could build a curve representing the dependence of pumping on absorbed radiation. For each sample we made measurements both biased and unbiased. It is evident in Fig. 4 that an external field has a strong influence on pumping dynamics. In order to analyze our data we built a simple one deep trap model. We assumed uniform irradiation of the sample during pumping. Then we assumed that the chance for a trap to be filled is proportional to the free trap density and to the radiation flux. Finally, we introduced a constant fraction of traps as a fit parameter which cannot be passivated. A similar model was proposed by the RD42 collaboration [15]. According to this model we obtain the following expression: LðFÞ ¼
LðNÞ 1 þ a exp½gF
Fig. 3. Two hysteresis curves, measured on two different quality samples.
ð1Þ
ARTICLE IN PRESS S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
Fig. 4. A sample’s CCD after pumping without external field and with a bias field of 1 V=mm: In both cases the rate of particles crossing the bulk has been kept constant.
Table 1 Best fit of pumping parameters during exposure to a b source, for a sample either unbiased or with an external applied field close to saturation (see text)
a gðevents1 ) LðNÞ ðmmÞ
E ¼ 0 V=mm
E ¼ 1 V=mm
0:4770:02 ð1071Þ 106 23672
ð3:370:5Þ 106 21074
149
where LðFÞ is the sample’s CCD, F is the radiation fluence, a is the ratio: passivable traps/not passivable traps, and it is an intrinsic property of the sample, while LðNÞ and g are empirical constants. We report in Fig. 4 our measurements, together with a minimum squares fit, whose values are reported in Table 1. We note that a higher pumping time is required if an external field is applied. This is because drifting carriers remain inside the bulk for time lower than the tipical trapping time. However, the most important effect is that when the sample is biased the final value LðNÞ is significantly lower. This means that if one wants to optimize the pumping procedure, irradiation should be performed when the sample is unbiased. We ascribed this result to a polarization effect which will be discussed later. In order to further study pumping dynamics we also estimated the dose absorbed by the sample during pumping and we measured its CCD after pumping at several applied bias voltages. We report these measurements in Fig. 5. It seems that there is a very low threshold for the creation of a space charge distribution inside the material’s bulk. We also observe that some of the curves are not monotonic. We interpret this effect as the result of a competition between the sudden setting
Fig. 5. A sample’s CCD after various pumping doses, with different applied bias fields. In the inset: a detail of CCD at low doses.
ARTICLE IN PRESS 150
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ccd (µm)
220 200 180 160
simulation
CCD
140 120 100 0
50
100
150
200
250
300
pumping time (s)
Fig. 6. A simulation of bias field inside a diamond, after pumping with a 1 V=mm bias voltage applied. A space charge distribution distorts the field.
Fig. 7. A comparison between CCD measured during pumping and our simulation.
In the picture outlined above, the lowering of CCD is due to a distortion of the local field inside the bulk (while the mean field is still VBIAS =thickness). This effect is due to a differential charge trapping inside the diamond bulk. In order to validate this hypothesis, we performed a MonteCarlo simulation of the charge trapping at different pumping doses in a biased diamond detector. In this calculation we assumed that there is no reinjection of carriers, i.e. a hole or electron with a lifetime as high to reach one of the metallizations is not injected again at the other electrode. Moreover, we assumed that the main contribution to CCD is given by one type of carrier only. This is consistent with the recent results obtained by lapping CVD diamond sensors [3]. As an outcome of our model, we report the electric field distribution along the thickness axis in Fig. 6. In Fig. 7 we report a simulation of the CCD trend that describes quite well our experimental data.
photoconductivity measurements, locating deep centers close to midgap and trap levels centered at 1:7 eV from the band-edge, with no identification of the band (conduction or valence band). In this work, we have started from the general assumption that the pumping effect is due to the filling of deep traps, so that pumping efficiency is related to the concentration of defects. We have shown an anticorrelation between CCD and pumping efficiency which is consistent with the above assumption. We also have shown that the pumping efficiency is bias-dependent and analyzed this effect with a first-order model. We have explained this effect in terms of a space charge forming as the induced carriers drift apart along the applied field and are trapped inside the bulk. The RD42 Collaboration has explained the wellknown effect of the hysteresis of the CCD vs. bias field in terms of polarization of the bulk. We have found that the hysteresis loop is less and less evident as the quality of the samples increases, i.e. as the concentration of traps decreases. This is consistent with our picture of the polarization of CVD diamond. Furthermore, we made a MonteCarlo simulation of the non-uniform applied field inside the diamond bulk, which allows to calculate the CCD trend during pumping with a good agreement with the experimental data.
4. Conclusions
Acknowledgements
We have previously characterized the trapping and recombination levels with combined CCD and
We wish to thank Dr. Fred Hartjes of NIKHEF Amsterdam and RD42 for helping us setting the
of a local field, which opposes to the carriers drift, and the passivation of the traps which increases the drift length. 3.4. Polarization dynamics
ARTICLE IN PRESS S. Mersi et al. / Nuclear Instruments and Methods in Physics Research A 530 (2004) 146–151
CCD station and for useful discussion during his stay in Florence. We are indebted to The RD42 Collaboration for lending us some of their samples and in particular to Prof. Harris Kagan of The Ohio State University for metallizing them.
[6] [7] [8] [9]
References [1] M. Moll, (RD50 Collaboration), Nucl. Instr. and Meth. A 511 (2003) 97. [2] W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 511 (2003) 124. [3] W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 514 (2003) 79. [4] F. Nava, P. Vanni, M. Bruzzi, S. Lagomarsino, S. Sciortino, G. Wagner, C. Lanzieri, IEEE Trans. Nucl. Sci. 51 (1) (2004) 238. [5] E. Berdermann, et al., The Diamond Project at GSI - Perspectives, in: M. Barone, E. Borchi, J. Huston, C. Leroy, P.G. Rancoita, P. Riboni, R. Ruchti (Eds.), Proceedings of the Seventh International Conference on Advanced Technology & Particle Physics (ICATPP-7),
[10] [11]
[12]
[13] [14]
[15]
151
Como, October 2001, World Scientific, Singapore, p. 246. E. Vittone, et al., Diamond Relat. Mater. 8 (1999) 1234. K. Iakoubovskii, G.J. Adriaenssens, Diamond Relat. Mater. 9 (2000) 1349. M. Bruzzi, D. Menichelli, S. Sciortino, L. Lombardi, J. Appl. Phys. 91 (2002) 5765. C. Manfredotti, et al., Diamond Relat. Mater. 11 (2002) 446. D.N. Jamieson, et al., Nucl. Instr. and Meth. B 158 (1999) 628. C. Manfredotti, F. Fizzotti, A. Lo Giudice, C. Paolini, E. Vittone, R. Lu, Nucl. Instr. and Meth. B 187 (2002) 566. M. Bruzzi, F. Hartjes, S. Lagomarsino, D. Menichelli, S. Mersi, S. Miglio, M. Scaringella, S. Sciortino, Phys. Stat. Sol. (a) 199 (1) (2003) 138. W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 476 (2002) 686. F. Hartjes, et al., (RD42 collaboration), CVD Diamond as a Detector Material, Fourth International Conference on Radiation Effects on Semiconductor Materials Detectors and Devices, Florence, 11 July 2002. W. Adam, et al., (RD42 collaboration), Nucl. Instr. and Meth. A 447 (2000) 244.