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Silicon carbide for UV, alpha, beta and X-ray detectors: Results and perspectives
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 583 (2007) 157–161 www.elsevier.com/locate/nima
Silicon carbide for UV, alpha, beta and X-ray detectors: Results and perspectives Francesco Moscatellia,b, a
IMM-CNR of Bologna, via Gobetti 101, 40129 Bologna, Italy b INFN of Perugia, via Pascoli 1, 06121 Perugia, Italy Available online 31 August 2007
Abstract Silicon carbide (SiC) is one of the candidates for the next generation semiconductor materials. Due to the wide bandgap (4H-SiC 3.2 eV), high critical breakdown voltage, high saturated drift velocity, high thermal conductivity, the detector fabricated with SiC has very low dark current, significantly high operating temperature and for UV sensors, insensitivity to visible/IR backgrounds compared to conventional Si detectors. In this paper, recent results on UV, alpha, beta and X-ray 4H-SiC detectors will be presented. High-energy resolution and full charge collection efficiency (CCE) have successfully been demonstrated before irradiation. After irradiation, the alpha and beta detectors continue to be operative, with a high CCE until fluences of the order of some 1014 n/cm2. The results for UV detectors show that the 4H-SiC photodetectors are capable of operating as solar-blind UV detector. SiC pixel detectors show leakage currents of a few femptoamperes at room temperature, which means a noise contribution of less than 1 e r.m.s. High performance for X-ray can be achieved with SiC detectors even above room temperature without using any cooling system. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 85.60.Gz; 61.80.Hg; 68.60.Dv Keywords: Silicon carbide; SiC detectors; Alpha detector; Beta detector; UV detector; X-ray detector
1. Introduction
2. Alpha and beta SiC detectors
The excellent physical and chemical stability of silicon carbide (SiC), its superior thermal conductivity, carrier saturation velocity and breakdown field, its very low leakage current justify the interest on this semiconductor and the outstanding performance achieved in power and high-frequency SiC devices. During recent years, some radiation detectors have been manufactured and tested demonstrating excellent results in neutron, alpha, gamma, ultraviolet (UV) and X-ray detection. The aim of this work is to summarize the most important results on alpha, beta, UV and X-ray SiC detectors, presenting the perspectives of this material and its issues and limitations.
Recent calculation of the electron–hole pair generation energy for 4H-SiC gave a value of 7.7870.05 eV [1,2], which is lower than the values reported in previous papers [3,4]. From these data, the authors of Ref. [5] calculated that a minimum ionizing particle (MIP) crossing the device generates 55 electron–hole (e–h) pairs per micron. This value is about 23 of that found in silicon (80–90 e–h pairs per micron). Finally, the high atomic binding energies within the material and the consequently high atomic displacement threshold of 35 eV for Si and 20 eV for C [6] indicate a potentially more radiation hard material than silicon. Nonetheless, the potential radiation hardness must be checked since, as a compound material, SiC is different from Si. Recent epitaxial SiC is characterized by a very good crystalline quality. Many tests with a- and b-particles [5,7,8] on SiC Schottky diodes provide the evidence that a 100% of charge collection efficiency (CCE) can be achieved
Corresponding author. IMM-CNR of Bologna, via Gobetti 101, 40129 Bologna, Italy. Tel.: +39 0516399110; fax: +39 0516399216. E-mail address: [email protected]
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on unirradiated devices. Recently [9,10], it has been shown that even p+n implanted junction have a 100% of CCE, indicating that the implantation process, if properly annealed, does not worsen the crystalline quality of SiC. In a recent work [11], the radiation hardness of SiC Schottky diodes has been analyzed after 24 GeV proton and 1 MeV neutron irradiation. These diodes have been tested with a- and b-particles. Fig. 1 shows the CCE for 5.48 MeV a-particles vs. bias voltage, at different neutron irradiations, from 2 1013 to 7 1015 n/cm2. A strong decrease in the CCE is obtained for fluences of the order of 1015 n/cm2. For b-particles, a CCE of the order of 30% has been achieved after a fluence of 1.4 1016 protons/cm2. After neutron irradiation of the order of 7 1015 n/cm2, only 300 e were collected at 600 V. A technological process based on only Schottky barriers is unsuitable for the manufacture of complex radiation detectors featuring an integrated electronic readout on board of the detector chip. In the latter case, pn junctions are needed for building electronic switches like MOSFETs or JFETs. In a recent work [12], the radiation hardness after 1 MeV neutron fluences in the range 1014–1016 n/cm2 has been analyzed for p+n diodes manufactured at CNRIMM of Bologna, Italy, used as MIP detectors. The leakage current remained very low even after neutron fluence of the order of 1016 cm2. After neutron irradiation of 1 1014 cm3, the lightly doped n-type epilayer of Ref. [12] (ND ¼ 2 1014 cm3) became intrinsic, and the detector continued to be operative. Fig. 2 shows the collected charges (CC) of these devices tested with b particles. The detector continues to be operative, with a high CC until fluences of the order of some 1014 n/cm2. Over fluences of the order 3 1015 n/cm2, the collection is very low. After a fluence of 1016 (1 MeV) n/cm2, the
Fig. 1. CCE of a-particles as a function of the fluence for Schottky diodes produced by Alenia Marconi Systems and published in Ref. [11].
Fig. 2. Collected charges as a function of the fluence for p+n diodes manufactured at IMM-CNR of Bologna. The diode diameter is 1 mm and the diode is not annealed. CC is measured at 950 V.
estimated collected charges are of the order of 100 e, with a CCD of the order of only 2 mm. Several papers [13–16] showed that neutron, proton and electron irradiations introduce deep levels in the bandgap of SiC, producing free-charge-carrier compensation. In particular, in a recent work [17] about neutron irradiation it has been observed that some deep levels in the range 0.7–1.5 eV below the conduction band are probably responsible for the compensation of the doping and the degradation of the SiC detectors. It has been proven that relatively low temperature annealing [13,16] causes some deep levels to disappear or to rearrange. It is important to analyze if this effect produces a recovery of the radiation damage in terms of collected charges and leakage current. Annealing cycles at 80 and 400 1C have been carried out on these p+n junction diodes irradiated with neutrons. For the devices irradiated at 3 1014 and 7 1014 n/cm2, the reverse current density decreased after 30 min at 80 1C at a very low value (8 and 15 nA/cm2, respectively, at 900 V) and then remained almost constant even after many hours of annealing. CCE measurements have been repeated after every thermal cycle at 80 1C. For the sample irradiated at 3 1014 n/cm2, after many hours of annealing, a slight increase of the collected charge can be observed, but in the range of the experimental error. This result has been confirmed even for the sample irradiated at 7 1014 n/cm2. After annealing at 80 1C, no damage recovery occurs. After 30 min at 400 1C, the reverse current density for the device irradiated at 7 1014 n/cm2 further decreases (8.5 nA/cm2 at 900 V). Moreover, the CC at 900 V increases at 1900 e with respect to 1400 e before annealing (Fig. 3). It is concluded that after an annealing at 400 1C, a partial recovery of the damage takes place. In Ref. [17], a correlation between traps and CCE through thermal activation energy evaluation has been
ARTICLE IN PRESS F. Moscatelli / Nuclear Instruments and Methods in Physics Research A 583 (2007) 157–161
Fig. 3. Collected charge at 900 V as a function of the annealing time for the p+n diode manufactured by IMM-CNR of Bologna irradiated with a fluence of 7 1014 n/cm2.
shown. The observed CCE degradation has to be ascribed to the induced deepest levels (ET41 eV). At 80 1C, the concentration of the deepest levels probably does not change. It is necessary to increase the temperature up to 400 1C to obtain a reduction of the deepest level concentration. Defects with an energy level near the midgap are related to carbon vacancy, VC (ECET ¼ 1.5 eV), and a carbon and silicon vacancy, VC+VSi (ECET ¼ 1.1 eV). These defects are structural defects and they are not related to impurities. It is important to understand why neutrons produce a so high concentration of defects even if the displacement energy of 4H-SiC (20–35 eV) is high. An important fact is that to rearrange the crystal damage is very difficult in SiC because there is a very low diffusion of defects [18]. It should be important to understand the mechanism of production of these defects in order to try to neutralize them. Doing it and considering the very low reverse current of SiC, it could be possible to manufacture a high radiation hardness SiC detector, with a very high signal-to-noise ratio.
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4H-SiC visible blind UV avalanche photodiode has been realized 7 years ago [22] with a breakdown voltage of 96 V. It showed a strongly desirable positive temperature coefficient for avalanche breakdown. When illuminated by a focused light from a 150 W UV lamp, the photocurrent was about one order higher than the dark current. The response spectrum biased at 90 V showed that the maximum responsivity of the 4H-SiC avalanche photodiode, located at 270 nm, was 106 A/W. The responsivity drops rapidly with increasing wavelength and is comparable to the AC noise limit at 370 nm. The ratio of the responsivity at 270 nm to that at 370 nm is higher than 20. Recently [24,25], Schottky barrier detectors for EUV detection have been realized. In particular 5 mm 5 mm Ni/4H-SiC Schottky photodiodes [25] have been realized with very good performances in EUV–UV range. The measured quantum efficiency (QE) at 0 V bias is higher than 50% from 230 to 295 nm and reaches the peak of 65% at 275 nm, corresponding to an internal QE close to 100%. Comparing with this peak QE, the rejection ratio of UV to visible light is 2 103. The QE in the EUV range is higher than 14%, exceeds 100% at the wavelength less than 50 nm, and finally becomes higher than 30 e/photon at 3 nm (Fig. 4). The spectral detectivity D* is one of the most commonly used figure of merits to evaluate the sensitivity of detectors. In this case D* ¼ 3.4 1015 cm Hz1/2/W at the wavelength of 275 nm. This value is two orders of magnitude higher than the D* of Si photodiodes and three orders of magnitude higher than the D* of the Si CCD [23,26,27]. The D* of SiC Schottky photodiodes is still about one order of magnitude lower than s20 photo multipliers tubes (PMT). Moreover, recently a 4H-SiC UV single photon counting avalanche detectors (Fig. 5) has been designed, fabricated
3. UV SiC detectors Ultraviolet, vacuum ultraviolet (VUV), and extreme ultraviolet (EUV) (l ¼ 5–380 nm) photodetectors have been widely used in the field of astronomy, EUV lithography, X-ray microscopy and plasma physics. Many applications require detectors with high responsivity, ultralow dark current, solar blindness and good radiation hardness. In recent years, tremendous progress in epitaxial growth technologies and processing of wide bandgap semiconductors, including SiC, has been made, useful for fabricating p–i–n [19], avalanche [20–22] and Schottky barrier [23,24] devices for UV detection. In particular a
Fig. 4. Photoresponse spectra in external QE from 3–400 nm of two Ni/4H-SiC Schottky photodiodes and a Si AXUV-100 photo detector published in Ref. [25].
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Fig. 5. 4H-SiC UV single photon counting avalanche photodiode [28].
and characterized [28]. The breakdown voltage (VBD) is 78 V. The peak QE is 20% for a wavelength between 270 and 280 nm at a bias voltage of 50% VBD. At 78 V, the probability of the photon to be counted is 75%. The corresponding photon counting efficiency is determined to be 2.6% at 353 nm. The photon count rate is 4.5 MHz (89 pulses in 20 ms) while the dark count rate is 650 kHz, more than one order of magnitude lower than that of InP/InGaAsP and GaN SPADs.
4. X-ray SiC detectors Silicon carbide has been studied recently as a material for X-ray detectors [1]. It has an absorption coefficient higher than that of silicon for X-ray energies up to 2 keV and identical to silicon for higher energies. Its wide bandgap, EG ¼ 3.3 eV, for the 4H-SiC polytype, gives a relatively high electron–hole pair generation energy e ¼ 7.7870.05 eV [1,2,29]; with a consequent factor of two smaller charge signal amplitude with respect to silicon. On the other hand, the wide bandgap makes the thermally generated current almost negligible. In fact, current density values at 100 V as low as 1–10 pA/cm2 [30–32] have been measured at room temperature, implying that the noise level of SiC detectors due to the leakage current is a factor of 30 lower than for other semiconductor detectors. Moreover, at a temperature of the order of 100 1C, the current density remains very low [30–32] with values at 100 kV/cm of the order of 0.1–4 nA/cm2. It has been demonstrated that in many conditions SiC detectors can reach signal/noise ratios higher than silicon detectors [1]. Fig. 6 shows the 241Am spectra acquired with a SiC pixel detector at room temperature and at 100 1C. The measured noise levels are 17 and 43 e r.m.s., respectively, corresponding to 315 and 797 eV full-width at half-maximum (FWHM). It is important to underline that the noise is fully limited by the front-end electronics [1,30,33]. An ultra-low noise integrated charge preamplifier in 0.35 mm CMOS technology has been designed and realized for 0.5 pF detectors [34]. The intrinsic equivalent noise charge is 7.6 e r.m.s. with noise slope of 18 e/pF at a power consumption as low as 41 mW. At 2.3 mW of power consumption, the best measured noise performance was obtained (3.9 e r.m.s.+6.2 e/pF), opening new
Fig. 6. 241Am spectra acquired at 27 and 100 1C with a SiC pixel detector manufactured by Alenia Marconi System and published in Ref. [30].
possibilities in the application of semiconductor detectors for soft X-ray spectroscopic imaging. 5. Conclusions SiC detectors for alpha, beta, UV and soft X-ray detection have been presented. For a- and b-particle, 100% of CCE has been demonstrated. After neutron irradiation, the alpha and beta detectors continue to be operative, with a high CCE until fluences of the order of some 1014 n/cm2. UV solar blind detectors have been manufactured with a very high QE even in the EUV range. Recently, a 4H-SiC UV single photon counting avalanche photodiode has been fabricated and characterized. Single photon counting at room temperature has been demonstrated. Soft X-ray detectors have been realized with ultralow noise level. In many conditions, SiC detectors can reach signal/noise ratios higher than silicon detectors and by designing suitable ultra-low noise front-end electronics, it will be possible to open new possibilities in the application of semiconductor detectors for soft X-ray spectroscopic imaging. Acknowledgments I wish to thank Prof. A. Scorzoni, Dott. A. Poggi and Dott. R. Nipoti for their advice during all of my research activities on SiC; Dott. S. Sciortino for his support and for the CC measurements on SiC devices and Prof. M. Bruzzi and Prof. G. Bertuccio, for their support and for the fruitful collaboration. I acknowledge the financial support of the Italian INFN SiCPOS Project and of the CERN RD50 collaboration. I wish to thank the IKZ institute of Berlin, Germany, for the epilayer growth, the clean room staff of ‘‘CNR-IMM Sezione di Bologna’’ for the device
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realization and the Jozef Stefan Institute, Ljubljana, Slovenia, for the neutron irradiation on SiC p+n junction devices. References [1] Bertuccio, R. Casiraghui, IEEE Trans. Nucl. Sci. NS-50 (2003) 175. [2] A. Lo Giudice, F. Fizzotti, C. Manfredotti, E. Vittone, F. Nava, Appl. Phys. Lett. 87 (2005) 222105. [3] M. Rugalla, K. Runge, A. Soldner-Rembold, Nucl. Phys. B 78 (1999) 516. [4] T. Dubbs, et al., IEEE Trans. Nucl. Sci. NS-46 (1999) 839. [5] F. Nava, P. Vanni, M. Bruzzi, S. Lagomarsino, S. Sciortino, G. Wagner, C. Lanzieri, IEEE Trans. Nucl. Sci. NS-51 (1) (2004) 238. [6] F. Gao, W.J. Weber, R. Devanathan, Nucl. Instr. and Meth. Phys. Res. B 191 (2002) 487. [7] M. Bruzzi, S. Lagomarsino, F. Nava, S. Sciortino, Diamond Relat. Mater. 12 (2003) 1205. [8] M. Bruzzi, F. Hartjes, S. Lagomarsino, F. Nava, S. Sciortino, P. Vanni, Proc. IEEE Nucl. Sci. Symp. 1 (2002) 14. [9] F. Moscatelli, A. Scorzoni, A. Poggi, M. Bruzzi, S. Lagomarsino, S. Mersi, S. Sciortino, R. Nipoti, Nucl. Instr. and Meth. Phys. Res. A 546 (2005) 218. [10] F. Moscatelli, A. Scorzoni, A. Poggi, M. Bruzzi, S. Lagomarsino, S. Mersi, S. Sciortino, M. Lazar, A. Di Placido, R. Nipoti, Mater. Sci. Forum 483 (2005) 1021. [11] S. Sciortino, F. Hartjes, S. Lagomarsino, F. Nava, M. Brianzi, V. Cindro, C. Lanzieri, M. Moll, P. Vanni, Nucl. Instr. and Meth. Phys. Res. A 552 (2005) 138. [12] F. Moscatelli, A. Scorzoni, A. Poggi, M. Bruzzi, S. Sciortino, S. Lagomarsino, G. Wagner, I. Mandic, R. Nipoti, IEEE Trans. Nucl. Sci. NS-53 (3) (2006) 1557. [13] A. Castaldini, A. Cavallini, L. Rigutti, F. Nava, Appl. Phys. Lett. 85 (17) (2004) 3780. [14] A. Castaldini, A. Cavallini, L. Rigutti, F. Nava, S. Ferrero, F. Giorgis, J. Appl. Phys. 98 (2005) 053706. [15] A.A. Lebedev, A.I. Vger, D.V. Davydov, V.V. Zozlovski, N.S. Savkina, A.M. Strel’chuk, J. Appl. Phys. 88 (2000) 6265.
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[16] X.D. Chen, S. Fung, C.C. Ling, C.D. Beling, M. Gong, J. Appl. Phys. 94 (2003) 3004. [17] A. Cavallini, A. Castaldini, F. Nava, P. Errani, V. Cindrol, Presented at MRS 2006 Spring Meeting, San Francisco, USA, 17–21 April 2006. [18] P. Musumeci, L. Calcagno, M.G. Grimaldi, G. Foti, Nucl. Instr. and Meth. Phys. Res. B 116 (1996) 327. [19] J.A. Edmond, H. Kong, C.H. Carter Jr., Physica B 185 (1993) 453. [20] F. Yan, C. Qin, J.H. Zhao, M. Weiner, B.K. Ng, J.P. David, R.C. Tozer, Electron. Lett. 38 (2002) 335. [21] X. Guo, A. Beck, B. Yang, J.C. Campbell, Electron. Lett. 39 (2003) 1673. [22] F. Yan, Y. Luo, J.H. Zhao, G.H. Olsen, Electron. Lett. 35 (1999) 929. [23] F. Yan, X. Xin, S. Aslam, Y. Zhao, D. Franz, J.H. Zhao, M. Weiner, IEEE J. Quantum Electron. 40 (2004) 1315. [24] X. Xin, F. Yan, T.W. Koeth, C. Joseph, J. Hu, J. Wu, J.H. Zhao, Electron. Lett. 41 (2005) 1192. [25] J. Hu, X. Xin, J.H. Zhao, F. Yan, B. Guan, J. Seely, B. Kjornrattanawanich, Optics Lett. 31 (2006) 1591. [26] S. Donati, PhotoDetectors-Devices, Circuits, and Applications, Prentice-Hall, Englewood Cliffs, NJ, 2000. [27] The Book of Photon Tools, Oriel Instruments, Stratford, CT, 2003, pp. 6.1–6.140; Group of Keithley Instruments Inc., Cleveland, OH. [28] X. Xin, F. Yan, X. Sun, P. Alexandrove, C.M. Stahle, J. Hu, M. Matsumura, X. Li, M. Weiner, H.J. Zhao, Electron. Lett. 41 (2005) 212. [29] B.F. Phlips, K.D. Hobart, F.J. Kub, R.E. Stahlbush, M.K. Das, B.A. Hull, G. De Geronimo, P. O’Connor, Nucl. Sci. Symp. Conf. Rec., 2005 IEEE 3 (2005) 1236. [30] G. Bertuccio, Nucl. Instr. and Meth. Phys. Res. A 546 (2005) 232. [31] G. Bertuccio, S. Caccia, R. Casiraghi, C. Lanzieri, IEEE Trans. Nucl. Sci. NS-53 (4) (2006) 2421. [32] R. Nipoti, F. Bergamini, F. Moscatelli, A. Poggi, M. Canino, G. Bertuccio, Mater. Sci. Forum 527–529 (2006) 815. [33] G. Bertuccio, R. Casiraghi, A. Cetronio, C. Lanzieri, F. Nava, Nucl. Instr. and Meth. Phys. Res. A 522 (2004) 413. [34] G. Bertuccio, S. Caccia, Progress in ultra low noise ASIC’s for radiation detectors, Nucl. Instr. and Meth. Phys. Res. A 579 (2007) 243–246.
Nuclear Instruments and Methods in Physics Research A 583 (2007) 157–161 www.elsevier.com/locate/nima
Silicon carbide for UV, alpha, beta and X-ray detectors: Results and perspectives Francesco Moscatellia,b, a
IMM-CNR of Bologna, via Gobetti 101, 40129 Bologna, Italy b INFN of Perugia, via Pascoli 1, 06121 Perugia, Italy Available online 31 August 2007
Abstract Silicon carbide (SiC) is one of the candidates for the next generation semiconductor materials. Due to the wide bandgap (4H-SiC 3.2 eV), high critical breakdown voltage, high saturated drift velocity, high thermal conductivity, the detector fabricated with SiC has very low dark current, significantly high operating temperature and for UV sensors, insensitivity to visible/IR backgrounds compared to conventional Si detectors. In this paper, recent results on UV, alpha, beta and X-ray 4H-SiC detectors will be presented. High-energy resolution and full charge collection efficiency (CCE) have successfully been demonstrated before irradiation. After irradiation, the alpha and beta detectors continue to be operative, with a high CCE until fluences of the order of some 1014 n/cm2. The results for UV detectors show that the 4H-SiC photodetectors are capable of operating as solar-blind UV detector. SiC pixel detectors show leakage currents of a few femptoamperes at room temperature, which means a noise contribution of less than 1 e r.m.s. High performance for X-ray can be achieved with SiC detectors even above room temperature without using any cooling system. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 85.60.Gz; 61.80.Hg; 68.60.Dv Keywords: Silicon carbide; SiC detectors; Alpha detector; Beta detector; UV detector; X-ray detector
1. Introduction
2. Alpha and beta SiC detectors
The excellent physical and chemical stability of silicon carbide (SiC), its superior thermal conductivity, carrier saturation velocity and breakdown field, its very low leakage current justify the interest on this semiconductor and the outstanding performance achieved in power and high-frequency SiC devices. During recent years, some radiation detectors have been manufactured and tested demonstrating excellent results in neutron, alpha, gamma, ultraviolet (UV) and X-ray detection. The aim of this work is to summarize the most important results on alpha, beta, UV and X-ray SiC detectors, presenting the perspectives of this material and its issues and limitations.
Recent calculation of the electron–hole pair generation energy for 4H-SiC gave a value of 7.7870.05 eV [1,2], which is lower than the values reported in previous papers [3,4]. From these data, the authors of Ref. [5] calculated that a minimum ionizing particle (MIP) crossing the device generates 55 electron–hole (e–h) pairs per micron. This value is about 23 of that found in silicon (80–90 e–h pairs per micron). Finally, the high atomic binding energies within the material and the consequently high atomic displacement threshold of 35 eV for Si and 20 eV for C [6] indicate a potentially more radiation hard material than silicon. Nonetheless, the potential radiation hardness must be checked since, as a compound material, SiC is different from Si. Recent epitaxial SiC is characterized by a very good crystalline quality. Many tests with a- and b-particles [5,7,8] on SiC Schottky diodes provide the evidence that a 100% of charge collection efficiency (CCE) can be achieved
Corresponding author. IMM-CNR of Bologna, via Gobetti 101, 40129 Bologna, Italy. Tel.: +39 0516399110; fax: +39 0516399216. E-mail address: [email protected]
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on unirradiated devices. Recently [9,10], it has been shown that even p+n implanted junction have a 100% of CCE, indicating that the implantation process, if properly annealed, does not worsen the crystalline quality of SiC. In a recent work [11], the radiation hardness of SiC Schottky diodes has been analyzed after 24 GeV proton and 1 MeV neutron irradiation. These diodes have been tested with a- and b-particles. Fig. 1 shows the CCE for 5.48 MeV a-particles vs. bias voltage, at different neutron irradiations, from 2 1013 to 7 1015 n/cm2. A strong decrease in the CCE is obtained for fluences of the order of 1015 n/cm2. For b-particles, a CCE of the order of 30% has been achieved after a fluence of 1.4 1016 protons/cm2. After neutron irradiation of the order of 7 1015 n/cm2, only 300 e were collected at 600 V. A technological process based on only Schottky barriers is unsuitable for the manufacture of complex radiation detectors featuring an integrated electronic readout on board of the detector chip. In the latter case, pn junctions are needed for building electronic switches like MOSFETs or JFETs. In a recent work [12], the radiation hardness after 1 MeV neutron fluences in the range 1014–1016 n/cm2 has been analyzed for p+n diodes manufactured at CNRIMM of Bologna, Italy, used as MIP detectors. The leakage current remained very low even after neutron fluence of the order of 1016 cm2. After neutron irradiation of 1 1014 cm3, the lightly doped n-type epilayer of Ref. [12] (ND ¼ 2 1014 cm3) became intrinsic, and the detector continued to be operative. Fig. 2 shows the collected charges (CC) of these devices tested with b particles. The detector continues to be operative, with a high CC until fluences of the order of some 1014 n/cm2. Over fluences of the order 3 1015 n/cm2, the collection is very low. After a fluence of 1016 (1 MeV) n/cm2, the
Fig. 1. CCE of a-particles as a function of the fluence for Schottky diodes produced by Alenia Marconi Systems and published in Ref. [11].
Fig. 2. Collected charges as a function of the fluence for p+n diodes manufactured at IMM-CNR of Bologna. The diode diameter is 1 mm and the diode is not annealed. CC is measured at 950 V.
estimated collected charges are of the order of 100 e, with a CCD of the order of only 2 mm. Several papers [13–16] showed that neutron, proton and electron irradiations introduce deep levels in the bandgap of SiC, producing free-charge-carrier compensation. In particular, in a recent work [17] about neutron irradiation it has been observed that some deep levels in the range 0.7–1.5 eV below the conduction band are probably responsible for the compensation of the doping and the degradation of the SiC detectors. It has been proven that relatively low temperature annealing [13,16] causes some deep levels to disappear or to rearrange. It is important to analyze if this effect produces a recovery of the radiation damage in terms of collected charges and leakage current. Annealing cycles at 80 and 400 1C have been carried out on these p+n junction diodes irradiated with neutrons. For the devices irradiated at 3 1014 and 7 1014 n/cm2, the reverse current density decreased after 30 min at 80 1C at a very low value (8 and 15 nA/cm2, respectively, at 900 V) and then remained almost constant even after many hours of annealing. CCE measurements have been repeated after every thermal cycle at 80 1C. For the sample irradiated at 3 1014 n/cm2, after many hours of annealing, a slight increase of the collected charge can be observed, but in the range of the experimental error. This result has been confirmed even for the sample irradiated at 7 1014 n/cm2. After annealing at 80 1C, no damage recovery occurs. After 30 min at 400 1C, the reverse current density for the device irradiated at 7 1014 n/cm2 further decreases (8.5 nA/cm2 at 900 V). Moreover, the CC at 900 V increases at 1900 e with respect to 1400 e before annealing (Fig. 3). It is concluded that after an annealing at 400 1C, a partial recovery of the damage takes place. In Ref. [17], a correlation between traps and CCE through thermal activation energy evaluation has been
ARTICLE IN PRESS F. Moscatelli / Nuclear Instruments and Methods in Physics Research A 583 (2007) 157–161
Fig. 3. Collected charge at 900 V as a function of the annealing time for the p+n diode manufactured by IMM-CNR of Bologna irradiated with a fluence of 7 1014 n/cm2.
shown. The observed CCE degradation has to be ascribed to the induced deepest levels (ET41 eV). At 80 1C, the concentration of the deepest levels probably does not change. It is necessary to increase the temperature up to 400 1C to obtain a reduction of the deepest level concentration. Defects with an energy level near the midgap are related to carbon vacancy, VC (ECET ¼ 1.5 eV), and a carbon and silicon vacancy, VC+VSi (ECET ¼ 1.1 eV). These defects are structural defects and they are not related to impurities. It is important to understand why neutrons produce a so high concentration of defects even if the displacement energy of 4H-SiC (20–35 eV) is high. An important fact is that to rearrange the crystal damage is very difficult in SiC because there is a very low diffusion of defects [18]. It should be important to understand the mechanism of production of these defects in order to try to neutralize them. Doing it and considering the very low reverse current of SiC, it could be possible to manufacture a high radiation hardness SiC detector, with a very high signal-to-noise ratio.
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4H-SiC visible blind UV avalanche photodiode has been realized 7 years ago [22] with a breakdown voltage of 96 V. It showed a strongly desirable positive temperature coefficient for avalanche breakdown. When illuminated by a focused light from a 150 W UV lamp, the photocurrent was about one order higher than the dark current. The response spectrum biased at 90 V showed that the maximum responsivity of the 4H-SiC avalanche photodiode, located at 270 nm, was 106 A/W. The responsivity drops rapidly with increasing wavelength and is comparable to the AC noise limit at 370 nm. The ratio of the responsivity at 270 nm to that at 370 nm is higher than 20. Recently [24,25], Schottky barrier detectors for EUV detection have been realized. In particular 5 mm 5 mm Ni/4H-SiC Schottky photodiodes [25] have been realized with very good performances in EUV–UV range. The measured quantum efficiency (QE) at 0 V bias is higher than 50% from 230 to 295 nm and reaches the peak of 65% at 275 nm, corresponding to an internal QE close to 100%. Comparing with this peak QE, the rejection ratio of UV to visible light is 2 103. The QE in the EUV range is higher than 14%, exceeds 100% at the wavelength less than 50 nm, and finally becomes higher than 30 e/photon at 3 nm (Fig. 4). The spectral detectivity D* is one of the most commonly used figure of merits to evaluate the sensitivity of detectors. In this case D* ¼ 3.4 1015 cm Hz1/2/W at the wavelength of 275 nm. This value is two orders of magnitude higher than the D* of Si photodiodes and three orders of magnitude higher than the D* of the Si CCD [23,26,27]. The D* of SiC Schottky photodiodes is still about one order of magnitude lower than s20 photo multipliers tubes (PMT). Moreover, recently a 4H-SiC UV single photon counting avalanche detectors (Fig. 5) has been designed, fabricated
3. UV SiC detectors Ultraviolet, vacuum ultraviolet (VUV), and extreme ultraviolet (EUV) (l ¼ 5–380 nm) photodetectors have been widely used in the field of astronomy, EUV lithography, X-ray microscopy and plasma physics. Many applications require detectors with high responsivity, ultralow dark current, solar blindness and good radiation hardness. In recent years, tremendous progress in epitaxial growth technologies and processing of wide bandgap semiconductors, including SiC, has been made, useful for fabricating p–i–n [19], avalanche [20–22] and Schottky barrier [23,24] devices for UV detection. In particular a
Fig. 4. Photoresponse spectra in external QE from 3–400 nm of two Ni/4H-SiC Schottky photodiodes and a Si AXUV-100 photo detector published in Ref. [25].
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Fig. 5. 4H-SiC UV single photon counting avalanche photodiode [28].
and characterized [28]. The breakdown voltage (VBD) is 78 V. The peak QE is 20% for a wavelength between 270 and 280 nm at a bias voltage of 50% VBD. At 78 V, the probability of the photon to be counted is 75%. The corresponding photon counting efficiency is determined to be 2.6% at 353 nm. The photon count rate is 4.5 MHz (89 pulses in 20 ms) while the dark count rate is 650 kHz, more than one order of magnitude lower than that of InP/InGaAsP and GaN SPADs.
4. X-ray SiC detectors Silicon carbide has been studied recently as a material for X-ray detectors [1]. It has an absorption coefficient higher than that of silicon for X-ray energies up to 2 keV and identical to silicon for higher energies. Its wide bandgap, EG ¼ 3.3 eV, for the 4H-SiC polytype, gives a relatively high electron–hole pair generation energy e ¼ 7.7870.05 eV [1,2,29]; with a consequent factor of two smaller charge signal amplitude with respect to silicon. On the other hand, the wide bandgap makes the thermally generated current almost negligible. In fact, current density values at 100 V as low as 1–10 pA/cm2 [30–32] have been measured at room temperature, implying that the noise level of SiC detectors due to the leakage current is a factor of 30 lower than for other semiconductor detectors. Moreover, at a temperature of the order of 100 1C, the current density remains very low [30–32] with values at 100 kV/cm of the order of 0.1–4 nA/cm2. It has been demonstrated that in many conditions SiC detectors can reach signal/noise ratios higher than silicon detectors [1]. Fig. 6 shows the 241Am spectra acquired with a SiC pixel detector at room temperature and at 100 1C. The measured noise levels are 17 and 43 e r.m.s., respectively, corresponding to 315 and 797 eV full-width at half-maximum (FWHM). It is important to underline that the noise is fully limited by the front-end electronics [1,30,33]. An ultra-low noise integrated charge preamplifier in 0.35 mm CMOS technology has been designed and realized for 0.5 pF detectors [34]. The intrinsic equivalent noise charge is 7.6 e r.m.s. with noise slope of 18 e/pF at a power consumption as low as 41 mW. At 2.3 mW of power consumption, the best measured noise performance was obtained (3.9 e r.m.s.+6.2 e/pF), opening new
Fig. 6. 241Am spectra acquired at 27 and 100 1C with a SiC pixel detector manufactured by Alenia Marconi System and published in Ref. [30].
possibilities in the application of semiconductor detectors for soft X-ray spectroscopic imaging. 5. Conclusions SiC detectors for alpha, beta, UV and soft X-ray detection have been presented. For a- and b-particle, 100% of CCE has been demonstrated. After neutron irradiation, the alpha and beta detectors continue to be operative, with a high CCE until fluences of the order of some 1014 n/cm2. UV solar blind detectors have been manufactured with a very high QE even in the EUV range. Recently, a 4H-SiC UV single photon counting avalanche photodiode has been fabricated and characterized. Single photon counting at room temperature has been demonstrated. Soft X-ray detectors have been realized with ultralow noise level. In many conditions, SiC detectors can reach signal/noise ratios higher than silicon detectors and by designing suitable ultra-low noise front-end electronics, it will be possible to open new possibilities in the application of semiconductor detectors for soft X-ray spectroscopic imaging. Acknowledgments I wish to thank Prof. A. Scorzoni, Dott. A. Poggi and Dott. R. Nipoti for their advice during all of my research activities on SiC; Dott. S. Sciortino for his support and for the CC measurements on SiC devices and Prof. M. Bruzzi and Prof. G. Bertuccio, for their support and for the fruitful collaboration. I acknowledge the financial support of the Italian INFN SiCPOS Project and of the CERN RD50 collaboration. I wish to thank the IKZ institute of Berlin, Germany, for the epilayer growth, the clean room staff of ‘‘CNR-IMM Sezione di Bologna’’ for the device
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