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Study on the bias-dependent effects of proton-induced damage in CdZnTe radiation detectors using ion beam induced charge microscopy
Accepted Manuscript Title: Study on the bias-dependent effects of proton-induced damage in CdZnTe radiation detectors using ion beam induced charge microscopy Author: Yaxu Gu Wanqi Jie Caicai Rong Lingyan Xu Yadong Xu Haoyan Lv Hao Shen Guanghua Du Na Guo Rongrong Guo Gangqiang Zha Tao Wang Shouzhi Xi PII: DOI: Reference:
S0968-4328(16)30087-7 http://dx.doi.org/doi:10.1016/j.micron.2016.06.003 JMIC 2318
To appear in:
Micron
Received date: Revised date: Accepted date:
13-5-2016 22-6-2016 22-6-2016
Please cite this article as: Gu, Yaxu, Jie, Wanqi, Rong, Caicai, Xu, Lingyan, Xu, Yadong, Lv, Haoyan, Shen, Hao, Du, Guanghua, Guo, Na, Guo, Rongrong, Zha, Gangqiang, Wang, Tao, Xi, Shouzhi, Study on the bias-dependent effects of proton-induced damage in CdZnTe radiation detectors using ion beam induced charge microscopy.Micron http://dx.doi.org/10.1016/j.micron.2016.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study on the bias-dependent effects of proton-induced damage in CdZnTe radiation detectors using ion beam induced charge microscopy
Yaxu Gu1,2, Wanqi Jie1,2,*, Caicai Rong3, Lingyan Xu1,2, Yadong Xu1,2, Haoyan Lv3, Hao Shen3, Guanghua Du4, Na Guo3,*, Rongrong Guo1,2, Gangqiang Zha1,2, Tao Wang1,2, Shouzhi Xi1,2
1
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, P R China 2 Key Laboratory of Radiation Detection Materials and Devices of Ministry of Industry and Information Technology, Northwestern Polytechnical University, Xi'an 710072, P R China 3 Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 200433, P R China 4 Materials Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P R China *
Corresponding author: [email protected] (W. Jie). Tel.: (86)29 8849 5414. [email protected] (N. Guo), Tel.: (86)21 5566 4131.
Highlights
Microscopic behaviors of damage induced by protons in CdZnTe radiation detectors under different applied biases are investigated using IBIC microscopy.
Local radiation damage severely deteriorates the overall energy spectrum and CCE uniformity, in particular under low applied biases.
Experimental demonstration of improving CCE uniformity degraded by local radiation damage with increasing detector applied bias.
Abstract: The influence of damage induced by 2 MeV protons on CdZnTe radiation detectors is investigated using ion beam induced charge (IBIC) microscopy. Charge collection efficiency (CCE) in irradiated region is found to be degraded above a fluence of 3.3 × 1011 p/cm2 and the energy spectrum is severely deteriorated with increasing fluence. Moreover, CCE maps obtained under the applied biases from 50 V to 400 V suggests that local radiation damage results in 1
significant degradation of CCE uniformity, especially under low bias, i. e., 50 V and 100 V. The CCE nonuniformity induced by local radiation damage, however, can be greatly improved by increasing the detector applied bias. This bias-dependent effect of 2 MeV proton-induced radiation damage in CdZnTe detectors is attributed to the interaction of electron cloud and radiation-induced displacement defects. Keywords: proton; radiation damage; ion beam induced charge microscopy; CdZnTe; bias dependent;
1. Introduction II-VI compound CdZnTe (CZT) has been considered as an important material for fabricating X-ray and gamma-ray detectors as well as for the application in nuclear microprobe, i. e., the scanning transmission ion microscopy (STIM) (Overley et al., 1988), owing to its superior performance at room temperature (Owens and Peacock, 2004). 5.5 MeV α-particle spectrum measured using 10 mm-thick planar CdZnTe detectors shows excellent energy resolution of about 1% in atmosphere (Amman et al., 2009). This is mainly because MeV-grade charged particles have relatively short ranges in CdZnTe crystals compared to the thickness of detector, typically of several tens of micrometers. Hence, the preferred single charge carrier collection (Luke, 1994; Owens and Kozorezov, 2006) can be easily achieved with simple electrode geometry. Usually, the detection of charged particles requires detectors to be operated in a hostile highfluence environment. Therefore, the problem of radiation damage emerges inevitably. Previous studies show that charged particles severely degrade the device performance of CdZnTe and CdTe detectors (Kuvvetli et al., 2003; Hull et al., 1997; Veeramani et al., 2006). For example, Owens et al (Owens et al., 2009) accessed the radiation tolerance of CdZnTe detectors under simulated solar proton events, which suffer severe charge loss and energy resolution degradation compared to HgI2 detectors after irradiation (Owens et al., 2004). Pastuović et al (Pastuović and Jakšić, 2001) found that the degraded CCE area due to proton-induced damage is much larger than the irradiated area. Doyle et al (Doyle et al., 2000) studied the pattern of low CCE region induced by 5.5 MeV α-beam, which is attributed to the space charge accumulation resulting from the trapping of charge carriers. Fraboni et al (Fraboni et al., 2004; Fraboni et al., 2007) and Zanarini et al (Zanarini et al., 2004) investigated the effects of damage caused by different ionizing radiation on the performance of CdZnTe and CdTe detectors, and identified the trap levels responsible for the charge loss. The impinging ions have been proved to introduce displacement defects (Rischau et al., 2011), such as vacancy-interstitial pairs, dislocation loops, and to deteriorate the performance of radiation detectors (Srour and Palko, 2013; Eisen et al., 2002). However, the microscopic behaviors of radiation damage under different operation conditions, i. e., applied electrical field, temperature, are still not well understood, and advances on this subject may provide guidance for radiation hardening techniques.
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Ion beam induced charge (IBIC) microscopy, different from electron beam induced current (EBIC) technique (Vanzi et al., 2000), collects the individual charge pulses generated by incident MeV ions (Breese, 1993; Breese et al., 1992), and is proved to be a useful tool to study the carrier transportation and charge collection properties of optoelectronic devices quantitatively in micronscale (Breese et al., 2007; Sellin et al., 2004). The main scope of this work is to investigate the macroscopic and microscopic effects of damage induced by 2 MeV protons on the CCE uniformity and spectroscopy response of CdZnTe detectors under different applied biases using IBIC technique. 2. Experimental A detector-grade Cd0.9Zn0.1Te single crystal in size of 7 × 7 × 2 mm3 grown by the modified vertical Bridgman method in Imdetek was used in the experiment. Following the surface treatment described in Ref. (Gu et al., 2015), the crystal was fabricated into planar detector to perform IBIC analysis. IBIC measurements using 2 MeV protons were carried out on the nuclear microprobe facility in the Institute of Modern Physics at Fudan University (Zhong et al., 2007). In this work, the CdZnTe detector was placed in the vacuum chamber with its cathode perpendicular to the incident beam direction, and the vacuum therein kept at around 3.6 ± 0.2 × 10 5 Pa. The ion beam spot diameter was focused to about 3 μm with a beam current density of 1000 ± 50 proton/s through the triplet quadruple lens supplied by Oxford Microbeams Ltd. Stable pulse height spectrum (PHS) indicated that the space charge polarization was efficiently avoided under such dose rate. Proton-induced charge signals were passed on to a shaping amplifier (Ortec 572A), subsequently digitized and saved pulse by pulse with its respective beam position. Corresponding IBIC experiments were carried out as follows. First, a region of 1 × 1 mm2 on the cathode was scanned to evaluate the original CCE distribution. The scanning area covers 256 × 256 pixels, and 10 events are estimated for each pixel. Based on the pulse height spectrum collected over the investigated area from 20 V to 500 V, corresponding CCE maps can be obtained via dividing the peak centroid channel of each pixel by the theoretically full charge-collection channel calculated from Hecht fitting. Immediately after the acquisition of the first set of CCE maps, local radiation damage with different fluence was introduced by irradiating six spots labeled Point 1 - 6 with 3 × 3 μm2 in area, and their locations are shown in the subsequent CCE maps. The fluence at each spot, ranging from 2 × 1012 p/cm2 to 1.3 × 1013 p/cm2 for Point 1 to Point 6. To monitor the characteristic of pulse height spectrum in the region of proton-induced radiation damage, we recorded the spectrum of impinging protons every half minute. After the irradiation, the 1 × 1 mm2 investigated region was rescanned from 50 V to 400 V using IBIC probe and corresponding CCE maps were obtained in the same manner. 3. Results and discussion 3.1. Evolution of local pulse height spectrum in the proton damaged regions Fig. 1(a) shows the typical degradation process of local pulse height spectrum with increasing fluence, in which the CdZnTe detector worked under the applied bias of 200 V. It was shown that 3
the energy resolution of local PHS is unaffected and the peak centroid shifts little until the fluence reaches up to 3.3 × 1011 p/cm2. When the accumulated dose keeps growing, however, the fullenergy peak tends to be broadened and the peak centroid shifts to the low energy side. Fig. 1(b) shows the peak channel changes from 1545 to 1351 with the fluence increasing from 3.3 × 1011 p/cm2 to 1.3 × 1013 p/cm2, resulting in CCE deterioration. Moreover, counts on the left of fullenergy peak, attributed to the charge loss caused by radiation damage, remarkably arise above a fluence of 4.3 × 1012 p/cm2. The low CCE counts further extend to the left side and form a plateau at the low energy part, proving increasing charge loss with accumulating radiation damage. Generally, radiation damage induced by charged particles is attributed to the vacancy-interstitial pairs resulting from displacement atoms in collision cascade by primary knock-on atoms, that is, Frenkel defects. Xu et al (Xu et al., 2013) calculated the threshold displacement energy both for Te and Cd atoms, and found that Cd has lower threshold displacement energy, indicating most of the primary defects in CdZnTe may be Cd vacancy-interstitial pairs. Though most of these defects will be annihilated through recombination immediately, a notable fraction of them may also aggregate into clusters, or rearrange to form other defect complexes, such as divacancies or dislocations (Robinson, 1994). Radiation-induced point defects and corresponding defect complexes usually act as deep traps in semiconductors (Srour and Palko, 2013), thus further leading to charge loss by trapping free carriers. A small peak on the right side of full-energy peak, marked by a dashed ellipse at the bottom of Fig. 1(a), emerges above the fluence of 4.3 × 1012 p/cm2. Its peak centroid is close to that of initial pulse height spectrum and appears unaffected with the accumulation of radiation damage. This is mainly due to the fact though the ion beam is focused, there are some incident ions beyond the statistical diameter of ion beam spot due to the oscillating electric fields, thermal drift, slit scattering, etc, which forms the beam halo (Breese et al., 1993). Charge collection efficiency of carriers generated by these halo ions is hardly affected by the radiation damage, and thus may explain the origin of this peak. Fig. 1(c) schematically shows the typical distribution of incident ions, in which the dashed circle illustrates its statistical diameter as well as the possible range of radiation damage. 3.2. Variation of PHS and CCE uniformity after the local radiation damage After irradiation at the six spots, CCE maps was measured again to study the influence of local radiation damage on the overall detector performance by rescanning over the 1 × 1 mm2 region. Fig. 2(a) and (b) show the spectrum obtained from the above region before and after the local irradiation under different applied biases, respectively. Remarkably, the peak-to-valley ratio and energy resolution under applied biases less than 100 V are degraded significantly even though only local radiation damage is introduced. Fig. 2(b) shows that counts in low energy part of 50 V PHS grow significantly, attributed to electrical noise resulting from the increased dark current. However, pulse height spectrums at higher applied biases, in particular, at 400V, see little change compared to those before irradiation.
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Corresponding to spectrums in Fig. 2 (a) and (b), Fig. 3(a) and (b) show the CCE distribution before and after the local irradiation, respectively. As seen in Fig. 3(a), low CCE regions exist in CCE maps, where the areas decrease with increasing applied biases. These regions are presumably attributed to subsurface crystal defects, i.e., dislocation clusters. IBIC results by Buis et al (Buis et al., 2013) show that severe charge loss can be caused by dislocation walls for both electrons and holes in CdTe detectors. After the local irradiation, the CCE uniformity over the investigated region degrades profoundly, especially under low applied biases, i. e., 50 V and 100 V. One could see that charge collection efficiency in regions corresponding to crystal defects, as shown in Fig. 3(a), fluctuates significantly, indicating that these defects act as sources of electronic noise and are responsible for the low-energy pileup of spectrum in Fig. 2(b). The origin of electronic noise in the CCE map may be ascribed to the trapping and detrapping of charge carriers by these defects. Menichelli et al (Menichelli et al., 2004) observed anomalous dark current transients in gamma ray-irradiated Si diodes, attributed to discharge of deep traps. It is worth noting that these intrinsic crystal defects play an important role in detector performance degradation, and their negative effects are even more detrimental than radiation damage itself. As shown in Fig. 3(b), however, CCE uniformity can be greatly improved when the detector bias increases, which provides a suitable approach to suppress the negative effects of radiation damage on detector performance. 3.3. Analysis on the bias-dependent behavior of proton-damaged regions Additionally, CCE maps in Fig. 3(b) show that radiation damage induced by local irradiation leads to lower CCE regions in circular shapes. These circular-shaped regions can be further separated into two parts, one with lowest CCE located in the center of the region and the other with lower CCE surrounding it. As shown in Fig. 3(b), the diameter of the former changes only from 7 μm to 5 μm when applied bias increases from 100 V to 400 V, while that of the latter decreases from about 120 μm to 50 μm at the same time. Fig. 4(a) shows that there are three separated peaks in CCE histogram of the degraded regions at Point 1 under applied bias of 400 V (dotted box in Fig. 3(b)). Peak a is attributed to counts in the central area with lowest CCE, peak b to surrounding circular area with lower CCE and peak c to normal CCE regions around. Fig. 4(b) suggests that electron mobility-lifetime product in the central damaged region (3.81 ± 0.08 × 10-4 cm2/V) is much lower than that of surrounding damaged region (7.28 ± 1.22 × 10-4 cm2/V), indicating the electron lifetime in the central regions is dramatically degraded. Moreover, Fig. 4(c) demonstrates that even after local irradiation, corresponding CCE in regions without high fluence of incident ions is severely degraded under low applied biases. It is unlikely to attribute CCE degradation of the undamaged regions to the polarization effect observed in diamond detectors (Sellin et al., 2000; Liechtenstein et al., 2004; Lohstroh et al., 2004; Grilj et al., 2013), since the dose rate employed in this work is only about 1000 proton/s, which has been demonstrated to be low enough to avoid the polarization effect in CdZnTe radiation detectors by Sellin et al (Sellin et al., 2008). Moreover, the fluence due to IBIC probing in these regions is as low as 109 p/cm2, about two orders of magnitude lower than the typical damage threshold. Therefore, charge loss 5
resulting from IBIC probing-induced damage is negligible and cannot account for CCE degradation in these regions. Besides, the diameter of beam halo in our IBIC experiments is estimated to be 20 μm, much smaller than that of the radiation-degraded regions, and should not account for the CCE degradation of unirradiated regions. In our case, we attribute the observed CCE degradation of undamaged regions to the interaction of electron cloud with the central damaged regions. Since clear boundary can be observed in the circular-shaped region of radiation damage (Fig. 3(b)), the diameter of these regions under different applied biases are acquired, as shown in Fig. 4(d). Clearly, the diameters of degraded regions decrease with increasing bias voltages. To explain the bias-dependent scope of degraded regions induced by local radiation damage, the interaction between carrier transport and proton-induced defects is analyzed. Since impinging ions can produce amounts of displacement atoms, a high concentration of vacancy-interstitial pairs or Frenkel defects thus form in semiconductors (Was, 2007). As calculated by SRIM code (Ziegler, 2004) (Fig. 5(a)), the displacement defects induced by 2 MeV protons radiation are mainly distributed at the path of incident ions, more exactly, at the end of ion trajectory. These defects are believed to act as trapping or recombination centers and to decrease the carrier lifetime, which should be responsible for CCE deterioration in irradiated regions. Moreover, a large amount of electron-hole pairs generate along the trajectory of incident ions and move parallel to the external electrical field. At the same time, the radius of these moving electrons r, also called electron cloud radius, expands under the driving force of diffusion gradient or electrostatic repulsion effect following Eq. (1) (Iniewski et al., 2007) and Eq. (2) (Gatti et al., 1987), respectively. r 1.15
r
3
2kTd 2 NeV
3Ned 2 4 0 rV
(1) (2)
where d is drift distance of electron cloud, k is the Boltzmann's constant, T is the temperature, N is the number of electrons, ε0 is the permittivity of vacuum, εr is the relative permittivity and V is the applied bias. Since radiation damage are mainly distributed at the end of ion trajectory, the drift distance of electron cloud d when running into these defects is equal to the stopping range of 2 MeV protons in CdZnTe crystals, which is 36.4 ± 2.1 μm calculated by SRIM code (Ziegler, 2004). Then, part of electrons will be trapped, leading to CCE degradation. As illustrated in Fig. 5(b), the scope of these radiation defects influencing on moving electron cloud is mainly decided by its radius r. Given the drifting distance d, the radius r tends to decrease if we increase the applied bias V, according to Eq. (1) and Eq. (2). Therefore, the sizes of circular regions induced by radiation damage decrease with increasing bias voltage, as shown in Fig. 4(d). Furthermore, a fitting process based on Eq. (1) and Eq. (2) is carried out on the experimental data in Fig. 4(d). Fig. 5(c) shows the best fitting results on basis of diffusion and electrostatic repulsion effect, indicating that diffusion may play a dominant role in the size evolution of moving electron cloud. 6
Results in Fig. 4(a) and 5(c) both suggest that the diameters of circular low CCE regions caused by local radiation damage may reach up to several hundreds of micrometers at applied bias of 50 V. These low CCE regions may connect with each other and form a larger degraded region in CCE map, which explains the origin of the dramatic degradation of undamaged regions in 50 V CCE map in Fig. 3(b). On basis of analysis above, both the scope and charge loss due to radiation damage will decrease with the increase of applied bias. In this regard, CCE as well as its uniformity in CdZnTe detector will be improved with increasing bias voltage, as shown in Fig. 3(b). 4.
Conclusion
The effects of 2 MeV proton-induced radiation damage on the macroscopic and microscopic device performance of CdZnTe radiation detectors are studied using IBIC microscopy. It's shown that local radiation damage can lead to serious degradation of pulse height spectrum as well as CCE uniformity. Moreover, increasing the applied bias of detector is an efficient way to reduce the scope of low CCE regions induced by radiation damage. The bias-dependent behavior of radiation damage is ascribed to the interaction between electron cloud and proton-induced displacement defects. Acknowledgement This work was supported by the National Natural Science Foundations of China (Nos. 51202197 and 51372205). Project was also supported by the 111 Project of China (No. B08040), and the Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09). References Amman M., Lee J.S., Luke P.N., Chen H., Awadalla S.A., Redden R. and Bindley G. 2009 Evaluation of THM-grown CdZnTe material for large-volume gamma-ray detector applications IEEE Trans. Nucl. Sci. 56 795-799 Breese M.B.H. 1993 A theory of ion beam induced charge collection J. Appl. Phys. 74 3789-3799 Breese M.B.H., Grime G.W. and Watt F. 1993 Study of nuclear microprobe beam halo using IBIC Nucl. Instrum. Methods Phys. Res., Sect. B 77 243-246 Breese M.B.H., King P.J.C., Grime G.W. and Watt F. 1992 Microcircuit imaging using an ion‐ beam‐induced charge J. Appl. Phys. 72 2097-2104 Breese M.B.H., Vittone E., Vizkelethy G. and Sellin P.J. 2007 A review of ion beam induced charge microscopy Nucl. Instrum. Methods Phys. Res., Sect. B 264 345-360 Buis C., d'Aillon E. Gros, Lohstroh A., Marrakchi G., Jeynes C. and Verger L. 2013 Effects of dislocation walls on charge carrier transport properties in CdTe single crystal Nucl. Instrum. Methods Phys. Res., Sect. A 735 188-192 Doyle B.L., Vıź kelethy G. and Walsh D.S. 2000 Ion beam induced charge collection (IBICC) studies of cadmium zinc telluride (CZT) radiation detectors Nucl. Instrum. Methods Phys. Res., Sect. B 161–163 457-461 Eisen Y., Evans L.G., Floyd S., Schlemm C., Starr R. and Trombka J. 2002 Radiation damage of Schottky CdTe detectors irradiated by 200 MeV protons Nucl. Instrum. Methods Phys. Res., Sect. A 491 176-180 7
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Fig. 1. Radiation damage-induced pulse height spectrum degradation of CdZnTe detectors (a) variation of pulse height spectrum of 2 MeV protons at Point 6 under different fluence, (b) peak centroid channel shift due to radiation damage, (c) schematic diagram of typical distribution of focused ions.
Fig. 2. Pulse height spectrum obtained from the 1 × 1 mm2 investigated region from 50 V to 400 V (a) before and (b) after local radiation damage.
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Fig. 3. CCE maps obtained under applied biases from 50 V to 400 V (a) before and (b) after the introduction of local radiation damage.. Solid and dotted boxes in 400-V CCE map illustrate the normal charge collection regions and radiation damaged regions, respectively, in which the six irradiated spot are shown.
Fig. 4. Features of CCE in regions corresponding to damaged (dotted boxes) and undamaged (solid box) regions in Fig. 3(b) (a) histogram of CCE distribution in damaged region at Point 6. The inset shows corresponding 80 × 80 μm2 CCE map under 400 V applied bias, (b) variation of CCE in central regions (dotted lines) and surrounding regions (solid lines) at different locations, (c) change of CCE in undamaged region before and after the local radiation damage, (d) evolution of the diameters of damaged regions with increasing applied biases.
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Fig. 5. Schematic diagram of interaction between proton-induced defects and electron cloud (a) X-Z distribution of 2 MeV proton-induced atomic displacements in CdZnTe crystals with 1000 incident ions calculated by SRIM code, (b) plan-view on the interaction of electron cloud with radiation defects under different applied bias, (c) best fitting results of low CCE diameter in Fig. 4(d) on basis of the diffusion and electrostatic repulsion effect.
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S0968-4328(16)30087-7 http://dx.doi.org/doi:10.1016/j.micron.2016.06.003 JMIC 2318
To appear in:
Micron
Received date: Revised date: Accepted date:
13-5-2016 22-6-2016 22-6-2016
Please cite this article as: Gu, Yaxu, Jie, Wanqi, Rong, Caicai, Xu, Lingyan, Xu, Yadong, Lv, Haoyan, Shen, Hao, Du, Guanghua, Guo, Na, Guo, Rongrong, Zha, Gangqiang, Wang, Tao, Xi, Shouzhi, Study on the bias-dependent effects of proton-induced damage in CdZnTe radiation detectors using ion beam induced charge microscopy.Micron http://dx.doi.org/10.1016/j.micron.2016.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study on the bias-dependent effects of proton-induced damage in CdZnTe radiation detectors using ion beam induced charge microscopy
Yaxu Gu1,2, Wanqi Jie1,2,*, Caicai Rong3, Lingyan Xu1,2, Yadong Xu1,2, Haoyan Lv3, Hao Shen3, Guanghua Du4, Na Guo3,*, Rongrong Guo1,2, Gangqiang Zha1,2, Tao Wang1,2, Shouzhi Xi1,2
1
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, P R China 2 Key Laboratory of Radiation Detection Materials and Devices of Ministry of Industry and Information Technology, Northwestern Polytechnical University, Xi'an 710072, P R China 3 Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 200433, P R China 4 Materials Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P R China *
Corresponding author: [email protected] (W. Jie). Tel.: (86)29 8849 5414. [email protected] (N. Guo), Tel.: (86)21 5566 4131.
Highlights
Microscopic behaviors of damage induced by protons in CdZnTe radiation detectors under different applied biases are investigated using IBIC microscopy.
Local radiation damage severely deteriorates the overall energy spectrum and CCE uniformity, in particular under low applied biases.
Experimental demonstration of improving CCE uniformity degraded by local radiation damage with increasing detector applied bias.
Abstract: The influence of damage induced by 2 MeV protons on CdZnTe radiation detectors is investigated using ion beam induced charge (IBIC) microscopy. Charge collection efficiency (CCE) in irradiated region is found to be degraded above a fluence of 3.3 × 1011 p/cm2 and the energy spectrum is severely deteriorated with increasing fluence. Moreover, CCE maps obtained under the applied biases from 50 V to 400 V suggests that local radiation damage results in 1
significant degradation of CCE uniformity, especially under low bias, i. e., 50 V and 100 V. The CCE nonuniformity induced by local radiation damage, however, can be greatly improved by increasing the detector applied bias. This bias-dependent effect of 2 MeV proton-induced radiation damage in CdZnTe detectors is attributed to the interaction of electron cloud and radiation-induced displacement defects. Keywords: proton; radiation damage; ion beam induced charge microscopy; CdZnTe; bias dependent;
1. Introduction II-VI compound CdZnTe (CZT) has been considered as an important material for fabricating X-ray and gamma-ray detectors as well as for the application in nuclear microprobe, i. e., the scanning transmission ion microscopy (STIM) (Overley et al., 1988), owing to its superior performance at room temperature (Owens and Peacock, 2004). 5.5 MeV α-particle spectrum measured using 10 mm-thick planar CdZnTe detectors shows excellent energy resolution of about 1% in atmosphere (Amman et al., 2009). This is mainly because MeV-grade charged particles have relatively short ranges in CdZnTe crystals compared to the thickness of detector, typically of several tens of micrometers. Hence, the preferred single charge carrier collection (Luke, 1994; Owens and Kozorezov, 2006) can be easily achieved with simple electrode geometry. Usually, the detection of charged particles requires detectors to be operated in a hostile highfluence environment. Therefore, the problem of radiation damage emerges inevitably. Previous studies show that charged particles severely degrade the device performance of CdZnTe and CdTe detectors (Kuvvetli et al., 2003; Hull et al., 1997; Veeramani et al., 2006). For example, Owens et al (Owens et al., 2009) accessed the radiation tolerance of CdZnTe detectors under simulated solar proton events, which suffer severe charge loss and energy resolution degradation compared to HgI2 detectors after irradiation (Owens et al., 2004). Pastuović et al (Pastuović and Jakšić, 2001) found that the degraded CCE area due to proton-induced damage is much larger than the irradiated area. Doyle et al (Doyle et al., 2000) studied the pattern of low CCE region induced by 5.5 MeV α-beam, which is attributed to the space charge accumulation resulting from the trapping of charge carriers. Fraboni et al (Fraboni et al., 2004; Fraboni et al., 2007) and Zanarini et al (Zanarini et al., 2004) investigated the effects of damage caused by different ionizing radiation on the performance of CdZnTe and CdTe detectors, and identified the trap levels responsible for the charge loss. The impinging ions have been proved to introduce displacement defects (Rischau et al., 2011), such as vacancy-interstitial pairs, dislocation loops, and to deteriorate the performance of radiation detectors (Srour and Palko, 2013; Eisen et al., 2002). However, the microscopic behaviors of radiation damage under different operation conditions, i. e., applied electrical field, temperature, are still not well understood, and advances on this subject may provide guidance for radiation hardening techniques.
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Ion beam induced charge (IBIC) microscopy, different from electron beam induced current (EBIC) technique (Vanzi et al., 2000), collects the individual charge pulses generated by incident MeV ions (Breese, 1993; Breese et al., 1992), and is proved to be a useful tool to study the carrier transportation and charge collection properties of optoelectronic devices quantitatively in micronscale (Breese et al., 2007; Sellin et al., 2004). The main scope of this work is to investigate the macroscopic and microscopic effects of damage induced by 2 MeV protons on the CCE uniformity and spectroscopy response of CdZnTe detectors under different applied biases using IBIC technique. 2. Experimental A detector-grade Cd0.9Zn0.1Te single crystal in size of 7 × 7 × 2 mm3 grown by the modified vertical Bridgman method in Imdetek was used in the experiment. Following the surface treatment described in Ref. (Gu et al., 2015), the crystal was fabricated into planar detector to perform IBIC analysis. IBIC measurements using 2 MeV protons were carried out on the nuclear microprobe facility in the Institute of Modern Physics at Fudan University (Zhong et al., 2007). In this work, the CdZnTe detector was placed in the vacuum chamber with its cathode perpendicular to the incident beam direction, and the vacuum therein kept at around 3.6 ± 0.2 × 10 5 Pa. The ion beam spot diameter was focused to about 3 μm with a beam current density of 1000 ± 50 proton/s through the triplet quadruple lens supplied by Oxford Microbeams Ltd. Stable pulse height spectrum (PHS) indicated that the space charge polarization was efficiently avoided under such dose rate. Proton-induced charge signals were passed on to a shaping amplifier (Ortec 572A), subsequently digitized and saved pulse by pulse with its respective beam position. Corresponding IBIC experiments were carried out as follows. First, a region of 1 × 1 mm2 on the cathode was scanned to evaluate the original CCE distribution. The scanning area covers 256 × 256 pixels, and 10 events are estimated for each pixel. Based on the pulse height spectrum collected over the investigated area from 20 V to 500 V, corresponding CCE maps can be obtained via dividing the peak centroid channel of each pixel by the theoretically full charge-collection channel calculated from Hecht fitting. Immediately after the acquisition of the first set of CCE maps, local radiation damage with different fluence was introduced by irradiating six spots labeled Point 1 - 6 with 3 × 3 μm2 in area, and their locations are shown in the subsequent CCE maps. The fluence at each spot, ranging from 2 × 1012 p/cm2 to 1.3 × 1013 p/cm2 for Point 1 to Point 6. To monitor the characteristic of pulse height spectrum in the region of proton-induced radiation damage, we recorded the spectrum of impinging protons every half minute. After the irradiation, the 1 × 1 mm2 investigated region was rescanned from 50 V to 400 V using IBIC probe and corresponding CCE maps were obtained in the same manner. 3. Results and discussion 3.1. Evolution of local pulse height spectrum in the proton damaged regions Fig. 1(a) shows the typical degradation process of local pulse height spectrum with increasing fluence, in which the CdZnTe detector worked under the applied bias of 200 V. It was shown that 3
the energy resolution of local PHS is unaffected and the peak centroid shifts little until the fluence reaches up to 3.3 × 1011 p/cm2. When the accumulated dose keeps growing, however, the fullenergy peak tends to be broadened and the peak centroid shifts to the low energy side. Fig. 1(b) shows the peak channel changes from 1545 to 1351 with the fluence increasing from 3.3 × 1011 p/cm2 to 1.3 × 1013 p/cm2, resulting in CCE deterioration. Moreover, counts on the left of fullenergy peak, attributed to the charge loss caused by radiation damage, remarkably arise above a fluence of 4.3 × 1012 p/cm2. The low CCE counts further extend to the left side and form a plateau at the low energy part, proving increasing charge loss with accumulating radiation damage. Generally, radiation damage induced by charged particles is attributed to the vacancy-interstitial pairs resulting from displacement atoms in collision cascade by primary knock-on atoms, that is, Frenkel defects. Xu et al (Xu et al., 2013) calculated the threshold displacement energy both for Te and Cd atoms, and found that Cd has lower threshold displacement energy, indicating most of the primary defects in CdZnTe may be Cd vacancy-interstitial pairs. Though most of these defects will be annihilated through recombination immediately, a notable fraction of them may also aggregate into clusters, or rearrange to form other defect complexes, such as divacancies or dislocations (Robinson, 1994). Radiation-induced point defects and corresponding defect complexes usually act as deep traps in semiconductors (Srour and Palko, 2013), thus further leading to charge loss by trapping free carriers. A small peak on the right side of full-energy peak, marked by a dashed ellipse at the bottom of Fig. 1(a), emerges above the fluence of 4.3 × 1012 p/cm2. Its peak centroid is close to that of initial pulse height spectrum and appears unaffected with the accumulation of radiation damage. This is mainly due to the fact though the ion beam is focused, there are some incident ions beyond the statistical diameter of ion beam spot due to the oscillating electric fields, thermal drift, slit scattering, etc, which forms the beam halo (Breese et al., 1993). Charge collection efficiency of carriers generated by these halo ions is hardly affected by the radiation damage, and thus may explain the origin of this peak. Fig. 1(c) schematically shows the typical distribution of incident ions, in which the dashed circle illustrates its statistical diameter as well as the possible range of radiation damage. 3.2. Variation of PHS and CCE uniformity after the local radiation damage After irradiation at the six spots, CCE maps was measured again to study the influence of local radiation damage on the overall detector performance by rescanning over the 1 × 1 mm2 region. Fig. 2(a) and (b) show the spectrum obtained from the above region before and after the local irradiation under different applied biases, respectively. Remarkably, the peak-to-valley ratio and energy resolution under applied biases less than 100 V are degraded significantly even though only local radiation damage is introduced. Fig. 2(b) shows that counts in low energy part of 50 V PHS grow significantly, attributed to electrical noise resulting from the increased dark current. However, pulse height spectrums at higher applied biases, in particular, at 400V, see little change compared to those before irradiation.
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Corresponding to spectrums in Fig. 2 (a) and (b), Fig. 3(a) and (b) show the CCE distribution before and after the local irradiation, respectively. As seen in Fig. 3(a), low CCE regions exist in CCE maps, where the areas decrease with increasing applied biases. These regions are presumably attributed to subsurface crystal defects, i.e., dislocation clusters. IBIC results by Buis et al (Buis et al., 2013) show that severe charge loss can be caused by dislocation walls for both electrons and holes in CdTe detectors. After the local irradiation, the CCE uniformity over the investigated region degrades profoundly, especially under low applied biases, i. e., 50 V and 100 V. One could see that charge collection efficiency in regions corresponding to crystal defects, as shown in Fig. 3(a), fluctuates significantly, indicating that these defects act as sources of electronic noise and are responsible for the low-energy pileup of spectrum in Fig. 2(b). The origin of electronic noise in the CCE map may be ascribed to the trapping and detrapping of charge carriers by these defects. Menichelli et al (Menichelli et al., 2004) observed anomalous dark current transients in gamma ray-irradiated Si diodes, attributed to discharge of deep traps. It is worth noting that these intrinsic crystal defects play an important role in detector performance degradation, and their negative effects are even more detrimental than radiation damage itself. As shown in Fig. 3(b), however, CCE uniformity can be greatly improved when the detector bias increases, which provides a suitable approach to suppress the negative effects of radiation damage on detector performance. 3.3. Analysis on the bias-dependent behavior of proton-damaged regions Additionally, CCE maps in Fig. 3(b) show that radiation damage induced by local irradiation leads to lower CCE regions in circular shapes. These circular-shaped regions can be further separated into two parts, one with lowest CCE located in the center of the region and the other with lower CCE surrounding it. As shown in Fig. 3(b), the diameter of the former changes only from 7 μm to 5 μm when applied bias increases from 100 V to 400 V, while that of the latter decreases from about 120 μm to 50 μm at the same time. Fig. 4(a) shows that there are three separated peaks in CCE histogram of the degraded regions at Point 1 under applied bias of 400 V (dotted box in Fig. 3(b)). Peak a is attributed to counts in the central area with lowest CCE, peak b to surrounding circular area with lower CCE and peak c to normal CCE regions around. Fig. 4(b) suggests that electron mobility-lifetime product in the central damaged region (3.81 ± 0.08 × 10-4 cm2/V) is much lower than that of surrounding damaged region (7.28 ± 1.22 × 10-4 cm2/V), indicating the electron lifetime in the central regions is dramatically degraded. Moreover, Fig. 4(c) demonstrates that even after local irradiation, corresponding CCE in regions without high fluence of incident ions is severely degraded under low applied biases. It is unlikely to attribute CCE degradation of the undamaged regions to the polarization effect observed in diamond detectors (Sellin et al., 2000; Liechtenstein et al., 2004; Lohstroh et al., 2004; Grilj et al., 2013), since the dose rate employed in this work is only about 1000 proton/s, which has been demonstrated to be low enough to avoid the polarization effect in CdZnTe radiation detectors by Sellin et al (Sellin et al., 2008). Moreover, the fluence due to IBIC probing in these regions is as low as 109 p/cm2, about two orders of magnitude lower than the typical damage threshold. Therefore, charge loss 5
resulting from IBIC probing-induced damage is negligible and cannot account for CCE degradation in these regions. Besides, the diameter of beam halo in our IBIC experiments is estimated to be 20 μm, much smaller than that of the radiation-degraded regions, and should not account for the CCE degradation of unirradiated regions. In our case, we attribute the observed CCE degradation of undamaged regions to the interaction of electron cloud with the central damaged regions. Since clear boundary can be observed in the circular-shaped region of radiation damage (Fig. 3(b)), the diameter of these regions under different applied biases are acquired, as shown in Fig. 4(d). Clearly, the diameters of degraded regions decrease with increasing bias voltages. To explain the bias-dependent scope of degraded regions induced by local radiation damage, the interaction between carrier transport and proton-induced defects is analyzed. Since impinging ions can produce amounts of displacement atoms, a high concentration of vacancy-interstitial pairs or Frenkel defects thus form in semiconductors (Was, 2007). As calculated by SRIM code (Ziegler, 2004) (Fig. 5(a)), the displacement defects induced by 2 MeV protons radiation are mainly distributed at the path of incident ions, more exactly, at the end of ion trajectory. These defects are believed to act as trapping or recombination centers and to decrease the carrier lifetime, which should be responsible for CCE deterioration in irradiated regions. Moreover, a large amount of electron-hole pairs generate along the trajectory of incident ions and move parallel to the external electrical field. At the same time, the radius of these moving electrons r, also called electron cloud radius, expands under the driving force of diffusion gradient or electrostatic repulsion effect following Eq. (1) (Iniewski et al., 2007) and Eq. (2) (Gatti et al., 1987), respectively. r 1.15
r
3
2kTd 2 NeV
3Ned 2 4 0 rV
(1) (2)
where d is drift distance of electron cloud, k is the Boltzmann's constant, T is the temperature, N is the number of electrons, ε0 is the permittivity of vacuum, εr is the relative permittivity and V is the applied bias. Since radiation damage are mainly distributed at the end of ion trajectory, the drift distance of electron cloud d when running into these defects is equal to the stopping range of 2 MeV protons in CdZnTe crystals, which is 36.4 ± 2.1 μm calculated by SRIM code (Ziegler, 2004). Then, part of electrons will be trapped, leading to CCE degradation. As illustrated in Fig. 5(b), the scope of these radiation defects influencing on moving electron cloud is mainly decided by its radius r. Given the drifting distance d, the radius r tends to decrease if we increase the applied bias V, according to Eq. (1) and Eq. (2). Therefore, the sizes of circular regions induced by radiation damage decrease with increasing bias voltage, as shown in Fig. 4(d). Furthermore, a fitting process based on Eq. (1) and Eq. (2) is carried out on the experimental data in Fig. 4(d). Fig. 5(c) shows the best fitting results on basis of diffusion and electrostatic repulsion effect, indicating that diffusion may play a dominant role in the size evolution of moving electron cloud. 6
Results in Fig. 4(a) and 5(c) both suggest that the diameters of circular low CCE regions caused by local radiation damage may reach up to several hundreds of micrometers at applied bias of 50 V. These low CCE regions may connect with each other and form a larger degraded region in CCE map, which explains the origin of the dramatic degradation of undamaged regions in 50 V CCE map in Fig. 3(b). On basis of analysis above, both the scope and charge loss due to radiation damage will decrease with the increase of applied bias. In this regard, CCE as well as its uniformity in CdZnTe detector will be improved with increasing bias voltage, as shown in Fig. 3(b). 4.
Conclusion
The effects of 2 MeV proton-induced radiation damage on the macroscopic and microscopic device performance of CdZnTe radiation detectors are studied using IBIC microscopy. It's shown that local radiation damage can lead to serious degradation of pulse height spectrum as well as CCE uniformity. Moreover, increasing the applied bias of detector is an efficient way to reduce the scope of low CCE regions induced by radiation damage. The bias-dependent behavior of radiation damage is ascribed to the interaction between electron cloud and proton-induced displacement defects. Acknowledgement This work was supported by the National Natural Science Foundations of China (Nos. 51202197 and 51372205). Project was also supported by the 111 Project of China (No. B08040), and the Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09). References Amman M., Lee J.S., Luke P.N., Chen H., Awadalla S.A., Redden R. and Bindley G. 2009 Evaluation of THM-grown CdZnTe material for large-volume gamma-ray detector applications IEEE Trans. Nucl. Sci. 56 795-799 Breese M.B.H. 1993 A theory of ion beam induced charge collection J. Appl. Phys. 74 3789-3799 Breese M.B.H., Grime G.W. and Watt F. 1993 Study of nuclear microprobe beam halo using IBIC Nucl. Instrum. Methods Phys. Res., Sect. B 77 243-246 Breese M.B.H., King P.J.C., Grime G.W. and Watt F. 1992 Microcircuit imaging using an ion‐ beam‐induced charge J. Appl. Phys. 72 2097-2104 Breese M.B.H., Vittone E., Vizkelethy G. and Sellin P.J. 2007 A review of ion beam induced charge microscopy Nucl. Instrum. Methods Phys. Res., Sect. B 264 345-360 Buis C., d'Aillon E. Gros, Lohstroh A., Marrakchi G., Jeynes C. and Verger L. 2013 Effects of dislocation walls on charge carrier transport properties in CdTe single crystal Nucl. Instrum. Methods Phys. Res., Sect. A 735 188-192 Doyle B.L., Vıź kelethy G. and Walsh D.S. 2000 Ion beam induced charge collection (IBICC) studies of cadmium zinc telluride (CZT) radiation detectors Nucl. Instrum. Methods Phys. Res., Sect. B 161–163 457-461 Eisen Y., Evans L.G., Floyd S., Schlemm C., Starr R. and Trombka J. 2002 Radiation damage of Schottky CdTe detectors irradiated by 200 MeV protons Nucl. Instrum. Methods Phys. Res., Sect. A 491 176-180 7
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Fig. 1. Radiation damage-induced pulse height spectrum degradation of CdZnTe detectors (a) variation of pulse height spectrum of 2 MeV protons at Point 6 under different fluence, (b) peak centroid channel shift due to radiation damage, (c) schematic diagram of typical distribution of focused ions.
Fig. 2. Pulse height spectrum obtained from the 1 × 1 mm2 investigated region from 50 V to 400 V (a) before and (b) after local radiation damage.
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Fig. 3. CCE maps obtained under applied biases from 50 V to 400 V (a) before and (b) after the introduction of local radiation damage.. Solid and dotted boxes in 400-V CCE map illustrate the normal charge collection regions and radiation damaged regions, respectively, in which the six irradiated spot are shown.
Fig. 4. Features of CCE in regions corresponding to damaged (dotted boxes) and undamaged (solid box) regions in Fig. 3(b) (a) histogram of CCE distribution in damaged region at Point 6. The inset shows corresponding 80 × 80 μm2 CCE map under 400 V applied bias, (b) variation of CCE in central regions (dotted lines) and surrounding regions (solid lines) at different locations, (c) change of CCE in undamaged region before and after the local radiation damage, (d) evolution of the diameters of damaged regions with increasing applied biases.
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Fig. 5. Schematic diagram of interaction between proton-induced defects and electron cloud (a) X-Z distribution of 2 MeV proton-induced atomic displacements in CdZnTe crystals with 1000 incident ions calculated by SRIM code, (b) plan-view on the interaction of electron cloud with radiation defects under different applied bias, (c) best fitting results of low CCE diameter in Fig. 4(d) on basis of the diffusion and electrostatic repulsion effect.
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