IBIC analysis of SiC detectors developed for fusion applications

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ Esta es la versión aceptada del artículo publicado en: This is an accepted manuscript of a paper published in: Radiation Physics and Chemistry (2020): 22/01/2025 DOI: https://doi.org/10.1016/j.radphyschem.2020.109100 Copyright: © 2020 Elsevier Ltd. All rights reserved. El acceso a la versión publicada del artículo puede requerir la suscripción de la revista. Access to the published version may require subscription. “This is an Accepted Manuscript of an article published by Elsevier in [Radiation Physics and Chemistry] on [2020], available at https://doi.org/10.1016/j.radphyschem.2020.109100” IBIC ANALYSIS OF SiC DETECTORS DEVELOPED FOR FUSION APPLICATIONS M.C. Jimenez-Ramos1*, J. García López1,2, A. García Osuna1, M. Rodriguez-Ramos1, A. Villalpando Barroso1, M. García Munoz1,2, E. Andrade3, G. Pellegrini4, S. Otero Ugobono4. P. Godignon4, J.M. Rafí4, G. Rius4 1Centro Nacional de Aceleradores (U. Seville, CSIC, J. de Andalucia), Seville, Spain 2Dept. of Atomic, Molecular and Nuclear Physics, University of Seville, Spain 3Instituto de Física, Universidad Nacional Autónoma de México, Ciudad de México, Mexico 4Instituto de Microelectrónica de Barcelona - Centro Nacional de Microelectrónica, IMB-CNM-CSIC, Bellaterra (Barcelona), Spain. *[email protected] Abstract In this work, we consider a 4H-SiC detector as a plasma diagnostic system for the detection of fusion-born alpha particles in future nuclear fusion reactors. A nuclear microprobe was used to locally irradiate micrometer-sized regions of the detector with 3.5 MeV He ions to fluences from 5x109 to 5x1011 cm-2. Ion Beam Induced Charge (IBIC) microscopy was employed to study its degradation in Charge Collection Efficiency (CCE) and energy resolution after irradiation. At high reverse-bias voltages, both parameters remain practically unaffected for fluences up to 1x1011 cm-2, while a significant deterioration of the spectroscopic performance was observed above 3x1011 cm-2. A theoretical drift-diffusion model, in combination with Shockley-Read-Hall recombination statistics, was used to obtain the holes lifetime from the fitting of the experimental CCE values measured at different reverse voltages. Holes lifetime was found to strongly decrease with increasing particle fluence, changing from 57 ns in pristine detectors to 0.2 ns after irradiation with a fluence of 1x1011 cm-2. Keywords Radiation damage, SiC detectors, Ion Beam Induced Charge, Charge Collection Efficiency, minority carrier lifetime Highlights Radiation hardness of SiC diodes is studied for the detection of 3.5 MeV He ions. The CCE and energy resolution are not degraded for fluences up to 1x1011 cm-2 Degradation of the spectrometric properties is observed at fluences ≥ 3x1011 cm2 Minority carrier lifetime strongly deteriorate with increasing particle fluence. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1. Introduction In nuclear fusion plasma devices, fast (i.e. suprathermal) ions generated by heating systems and fusion-born alpha-particles must be kept well confined until they transfer their energy to the background plasma. This is especially important in large future fusion devices such as ITER (International Thermonuclear Experimental Reactor) and DEMO (DEMOnstration Power Station), where even a loss of a small fraction of energetic ions must be prevented [1]. In ITER, the monitoring of escaping 3.5 MeV alphas produced in the deuterium-tritium (D-T) reaction is still a requirement of the outmost importance [1, 2]. In present fusion devices, based on magnetic confinement of deuterium-deuterium (D-D) plasmas, the main diagnostic used to study the velocityspace of the escaping ions is the scintillator-based Fast Ion Loss Detector (FILD) [3]. Although FILD systems are currently used on virtually all large fusion devices worldwide, the foreseen harsh operation conditions of ITER (high neutron, gamma and heat fluxes) [1] will compromise, or even invalidate, the use of some of the diagnostic and measurement systems that are employed today. For example, if employing FILD, the temperature of the inner side of the probe head, located near the plasma edge, should not exceed the quenching temperature of the scintillator material, which is ~350º C for most of the relevant scintillator materials [4]. An attractive alternative for lost-alpha particle monitoring in ITER would be the use of a semiconductor material as the active component of the probe head. Semiconductorbased detectors provide an intrinsic energy resolution by pulse height analysis, which implies that they have to be used in counting mode. They could have an additional advantage from the integration point of view. Indeed, a transmission line for the light delivery from the detection point to the port plug, in the case of a scintillator-based detector, may occupy more space than the electrical signal delivery by cables, in the case of a semiconductor-based system. This would allow installing a larger number of detectors in each poloidal cross section. Due to its wide bandgap (3.27 eV for the 4H polytype) and large thermal conductivity (3.7 W/cm·K), silicon carbide (SiC) is one of the most attractive semiconductor materials for the development of, e.g., high operating-temperature electronic devices [5]. Moreover, because of its high saturated drift velocity (2x107 cm/s) and high atomic displacement energy (Ed(Si)=35 eV , Ed(C)=22 eV), SiC devices have demonstrated much higher radiation tolerance compared to silicon [6]. For instance, during gamma-ray irradiation, no significant change in the electrical characteristics of SiC transistors was observed up to 105 Gy, whereas the Si MOSFETs have shown clear degradation [6]. On the other hand, the decrease in Charge Collection Efficiency (CCE) during irradiation with a 17 MeV proton beam was considerably larger for Si detectors in comparison to SiC Schottky barrier diodes (SBD) [7]. A comprehensive review on the use of SiC as radiation detector can be found in [8]. The behavior of 4H-SiC SBDs against irradiation with alpha particles has been recently studied by Pastuovic et al. [9]. In that work, the detectors were bombarded with 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 2 and 4 MeV He ions (with the detectors unbiased) and the deep traps created were characterized using Deep Level Transient Spectroscopy (DLTS). A significant degradation of the CCE, studied by the Ion Beam Induced Charge (IBIC) technique [10], was observed for He fluences above 1011 cm-2. In this paper, we have extended the previous studies, focusing on the possible use of a 4H-SiC pn junction diode (PND) as a plasma diagnostic system for the detection of alpha-born particles, which will be the main product of the D-T fusion reaction in the future ITER reactor. In order to assess the actual response of the detector in realistic operation conditions, the PND was submitted to 3.5 MeV He bombardment, with the detector biased at its usual polarization voltage. In the same way, IBIC analysis was carried out using also 3.5 MeV He to evaluate the degradation of the spectrometric and transport properties of the PND, such as CCE, energy resolution, and the minoritycarriers diffusion length. In this preliminary study, both irradiation and the IBIC measurements were performed at room temperature, which has permitted the validation of the device and the employed methodology. Further analysis at high temperature is being performed presently and will be reported in a forthcoming paper. 2. Experimental details 2.1. SiC diode fabrication and electrical characterization Fig. 1 shows a schematic cross section of the 4H-SiC PND fabricated in the micro/nanofabrication clean room of the IMB-CNM. Basically, the detector consists in a 3 inches 4H-SiC substrate from Si Crystal with a N-type 45 m thick 4H-SiC epilayer doped at 1.5x1015 cm-3 grown in Ascatron. A p+ layer was implanted in the top surface of the epilayer to create a pn junction. The p+ layer was formed by Al implantation performed at 300ºC with a total dose of 8x1014cm-2 and a maximum energy of 180 keV, resulting in a junction depth of approximately 420 nm. Dopant activation annealing was carried out in an Ar ambient at 1650 ºC during 20 minutes. Fig. 1. Schematic cross section (not drawn to scale) of the 4H-SiC PND. 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 The top contact, with lateral dimensions of 3 x 3 mm2, was formed by a multilayer structure created by 15 nm Ti + 90 nm Al + 30 nm Ti + 100 nm W followed by a 50 nm SiO2 + 100 nm Si3N4 passivation layer. Bonding pads were opened in the passivation layer, in specific areas of the device, but most of the active area was covered by the passivation layer. The ohmic contact at the substrate back side consists in a stack of Ti, Ni, and Au thin films. The electrical characterization of the pristine detector was carried out in a lightproof and electrically shielded probe station Summit 11000B-M by using a Keithley 2470 SourceMeter Unit (SMU) for the Current-Voltage (I-V) measurements and an Agilent 4284A Precision LCR Meter to determine the Capacitance-Voltage (C-V) characteristics. In order to facilitate the handling of the PND for the experiments with ion beams, the diode was connected to a PCB with a Microdot plug (Fig. 2). Fig. 2. Image of the PND detector after preparation for experiments with ion beams 2.2 Irradiation conditions The irradiation of the detector was carried out at the microprobe of the 3 MV Tandem accelerator of the CNA using the same experimental procedure explained in [910]. A 3.5 MeV He beam was focused to ~3x5 m2 and nine different regions of an area of 100x100 m2 were homogeneously swept using the scan system of the microbeam with fluences between 5x109 and 5x1011 cm-2 (Fig. 3). The beam intensity, set in the range from 10 to 27 kHz (~2-5 fA), was measured from the input count rate TTL signal of the amplifier (shaping time = 1.5 s) connected to a ratemeter. The largest fluence was achieved in about 30 minutes under these conditions. The number of ions arriving at the device was determined from the amplifier output signal connected to a Single Channel Analyzer (SCA) with the window set above the noise level and fed into a scaler. The ion fluence was calculated by dividing the number of pulses into the scaler by the area of 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 Fig. 3. Micrometric-sized regions of the PND irradiated with 3.5 MeV He. The letters correspond to the different fluences (in units of 1x1011 cm-2): a= 0.05, b= 0.13, c= 0.3, d= 0.5, e= 0.75, f = 1, g = 3, h = 5, *= 0.3 (irradiated at zero bias). Optical image of the PND (a) and CCE maps measured at (b) 20 V and (c) 60 V. a) c) b) 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 the scanned region. The uncertainty of the calculated fluence values is mainly dominated by the calibration of the scanning system (in the order of 1%). The irradiation was performed at room temperature with the PND biased in reverse conditions to V = 200 V to reproduce the standard polarization conditions during the diode operation. In similar studies, the devices are usually irradiated without any applied voltage [9-10]. Thus, to check whether there is any influence of the polarization state on the created defects, one region (marked with an asterisk in Fig. 3a) was irradiated with the PND unbiased. However, in that case the maximum fluence that could be measured using our methodology was 3x1010 cm-2, as the amplitude of the pulses produced at higher doses lies below the low level discriminator of the SCA and therefore cannot be counted into the scaler. 2.3. IBIC microscopy The PND was kept at room temperature for several days before the IBIC technique was employed to evaluate the degradation of its spectrometric properties. For these measurements, a focused (6x5 m2) 3.5 MeV He beam at low rate (< 200 Hz) was employed to scan an area 800x800 m2 covering the nine different damaged zones of the PND (Fig. 3a). The maps were recorded in List Mode for reverse voltages between 250 and 20 V. Fig. 3b and 3c show the CCE maps measured at 20 V and 60 V, respectively. To avoid possible edge effects, the spectra were obtained from the central part of the irradiated areas (62x62 m2) and the reported CCE values were calculated from the centroids of the peaks normalized to the signal resulting from the pristine material measured at 250 V, which was supposed to have 100% CCE. Selected IBIC spectra recorded at different reverse voltages for a pristine zone of the detector and for the areas irradiated to the three largest fluences are displayed in Fig. 4. It is apparent that, for a given voltage, at higher fluences the peaks widen and appear in lower channels (i.e. lower CCE), which is especially notable for the region irradiated to 5x1011 cm-2. 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 Fig. 4. IBIC spectra extracted from the List Mode files for the pristine material and for irradiated fluences  = 1, 3, and 5x1011 He cm-2 at reverse voltages a) V = 200 V; b) V= 100 V; and c) V = 40 V 3. Results and discussion 3.1. I-V and C-V measurements of non-irradiated PND Fig. 5 (a) shows the I-V characteristics near turn-on voltage and for high reverse bias conditions (inset), where the magnitude of the measured current is in the range of the resolution of the experimental set-up. The C-V characteristic of the diode is depicted in Fig. 5 (b), where the estimated error for the measured capacitance values is below 1 pF. From the C-V measurements, the thickness of the depletion region (w) as a function of the applied bias (V) (Fig. 5(c)) was calculated using the formula (1), where  𝑤 = 𝜖𝐴 𝐶 is the static permittivity of the material and A is the device area. Fig. 5. Electrical characterization of the non-irradiated PND. (a) I-V and (b) C-V curves, (c) depletion width vs applied voltage calculated with Eq. (1). 3.2. Spectrometric properties The CCE values obtained as a function of the reverse voltage for different fluences are represented in Fig. 6. The region irradiated to 0.3x1011 cm-2 at zero bias (not shown) presents the same CCE values as the one irradiated to the same fluence at 200 V, indicating that at least, up to this range of fluences, the polarization state of the PND during the irradiation has no influence on the structural defects formed in the diode. For the pristine material, the CCE value saturates for voltages ≥ 150 V, reinforcing our assumption that the CCE is 100% at V = 200-250 V. Note that for V ≥ 150 V the 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 depletion layer is thicker than the projected range of 3.5 MeV alpha particles in SiC (9.67 μm, calculated with SRIM code (2008) [11] taking SiC = 3.21 g/cm3) so, for the non-irradiated detector, all the generated carriers in the high electric field region are collected without recombination. Similar behavior is observed for fluences up to 1x1011 ions/cm2, which sets the limit in the maximum number of 3.5 MeV alpha particles that can be measured without visible CCE degradation. This threshold fluence value is considerably lower than the values for the irradiation of SiC detectors with 1 MeV neutrons (~1x1014 cm-2) [12,13], 60Co gamma rays (~7x1016 cm-2) [14,15], 8.3 MeV electrons (~1x1015 cm-2) [14], 24 GeV/c protons (~1x1014 cm-2) [14] and 17 MeV protons (~1x1012 cm-2) [7], but analogous to previous studies on Schottky barrier diodes with 2 and 4 MeV He (~1x1011 cm-2) [9]. The fact that the SiC is highly radiation hard to neutrons, -rays, and electrons is fundamental for the development of an alpha particle detector that can work in the harsh radiation environment of future nuclear fusion reactors. A more detailed comparison of the radiation damage in SiC detectors upon irradiation with electrons, alpha particles, protons, and -ray photons beyond these threshold fluence values can be found in [8]. Fig. 6. Experimental CCE vs. reverse bias for PND diode irradiated with 3.5 MeV alpha particles at different fluences. 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Volume 83, Issue 1, January 2006, Pages 30-33. 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: M.C. Jimenez Ramos: Conceptualization, Investigation, Writing – Original Draft J. García López: Conceptualization, Methodology, Investigation, Writing - Review & Editing A. García Osuna: Formal analysis, software M. Rodriguez Ramos: Software A. Villalpando Barroso: Software M. García-Muñoz: Formal analysis E. Andrade: Formal analysis G. Pellegrini: Conceptualization S. Otero-Ugobono: Formal analysis P. Godignon: Methodology JM Rafí: Formal analysis, Writing - Review & Editing G. Rius: Methodology, Writing - Review & Editing